Abstract: API 5L X70Q/L485Q seamless pipeline steel is widely used in the construction of long-distance oil and gas transmission pipelines due to its excellent low-temperature toughness, high strength, and corrosion resistance. In this paper, a comprehensive analysis of the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel was carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), uniaxial tensile test, Charpy impact test, and hardness test. The results show that the microstructure of API 5L X70Q/L485Q seamless pipeline steel is mainly composed of acicular ferrite (AF), polygonal ferrite (PF), and a small amount of bainite (B) and martensite-austenite (M-A) islands. The acicular ferrite, with its fine and interlocking structure, is the key factor contributing to the excellent comprehensive mechanical properties of the steel. The tensile test results indicate that the steel has a yield strength of 490-520 MPa, a tensile strength of 620-650 MPa, and an elongation of 28%-32%, which fully meets the requirements of API 5L and GB/T 9711 standards. The Charpy impact test results show that the impact absorption energy of the steel at -20℃ is greater than 120 J, indicating excellent low-temperature toughness. The hardness test results show that the Rockwell hardness (HRC) of the steel is between 18 and 22, with uniform hardness distribution. In addition, the effects of different heat treatment processes (normalizing, tempering) on the microstructure and mechanical properties of the steel were also investigated. It was found that appropriate normalizing temperature (920-950℃) and tempering temperature (600-650℃) can further refine the microstructure, improve the proportion of acicular ferrite, and thus enhance the mechanical properties of the steel. The research results provide a theoretical basis and technical support for the production, application, and performance optimization of API 5L X70Q/L485Q seamless pipeline steel.

Keywords: API 5L X70Q; L485Q; seamless pipeline steel; microstructure; mechanical properties; acicular ferrite; heat treatment

 

With the rapid development of the global energy industry, the demand for long-distance oil and gas transmission pipelines is increasing. Pipeline transportation, as a safe, efficient, and economical mode of energy transportation, has become an important part of the energy supply chain. In the construction of long-distance pipelines, pipeline steel is the core material, and its performance directly affects the safety, reliability, and service life of the pipeline system. Especially in harsh service environments such as cold regions, high-pressure oil and gas fields, and marine areas, pipeline steel is required to have excellent comprehensive properties, including high strength, good low-temperature toughness, corrosion resistance, and weldability.

API 5L X70Q/L485Q seamless pipeline steel is a kind of high-strength low-alloy (HSLA) steel, which is developed to meet the requirements of modern long-distance pipeline construction. The “Q” in the grade indicates that the steel has excellent low-temperature toughness, which makes it suitable for use in cold regions where the temperature can be as low as -20℃ or even lower. Compared with ordinary X70/L485 pipeline steel, X70Q/L485Q steel has higher toughness and better resistance to brittle fracture, which can effectively prevent pipeline accidents caused by low-temperature brittle cracking. In addition, the seamless structure of X70Q/L485Q pipeline steel avoids the defects of welded joints, further improving the reliability and safety of the pipeline.

The microstructure of pipeline steel is the fundamental factor determining its mechanical properties. For HSLA pipeline steel, the type, morphology, size, and distribution of microstructural components (such as ferrite, bainite, martensite, and second phases) have a significant impact on its strength, toughness, and ductility. Therefore, in-depth analysis of the microstructure of API 5L X70Q/L485Q seamless pipeline steel and its relationship with mechanical properties is of great significance for optimizing the production process of the steel, improving its performance, and ensuring the safe operation of the pipeline.

At present, many scholars have carried out research on X70/L485 series pipeline steel. For example, some studies have focused on the effect of alloying elements on the microstructure and mechanical properties of X70 steel, and found that elements such as Nb, V, and Ti can refine the grains and improve the strength and toughness of the steel through grain refinement and precipitation strengthening. Other studies have investigated the influence of heat treatment processes on the performance of X70 steel, and proposed optimal heat treatment parameters to obtain excellent comprehensive properties. However, there are relatively few systematic studies on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel, especially the detailed analysis of the acicular ferrite structure and its effect on low-temperature toughness. In addition, the research on the correlation between microstructure and mechanical properties of X70Q/L485Q steel under different heat treatment conditions is not sufficient.

Therefore, this paper conducts a comprehensive study on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel. The microstructure of the steel is observed and analyzed using OM, SEM, and TEM. The mechanical properties are tested through tensile, Charpy impact, and hardness tests. The relationship between microstructure and mechanical properties is discussed. In addition, the effects of normalizing and tempering processes on the microstructure and mechanical properties of the steel are investigated to provide a theoretical basis for the production and application of X70Q/L485Q seamless pipeline steel.

Foreign scholars have carried out in-depth research on high-strength pipeline steel such as X70 since the 1980s. Early studies focused on the development of microalloyed pipeline steel, and found that the addition of microalloying elements such as Nb, V, and Ti can significantly improve the strength and toughness of the steel. For example, Nb can delay the recrystallization of austenite during hot rolling, refine the grains, and form Nb(C,N) precipitates to strengthen the matrix. V can form VC precipitates, which have a strong precipitation strengthening effect. Ti can form TiN precipitates, which can prevent the growth of austenite grains during heating.

In recent years, foreign scholars have paid more attention to the microstructure control and performance optimization of pipeline steel. Some studies have adopted controlled rolling and controlled cooling (TMCP) technology to obtain a fine-grained microstructure composed of acicular ferrite and polygonal ferrite, which significantly improves the low-temperature toughness of the steel. For example, Smith et al. used TMCP technology to produce X70 pipeline steel with acicular ferrite as the main microstructure, and the impact absorption energy at -20℃ reached more than 150 J. In addition, foreign scholars have also studied the corrosion resistance of X70 pipeline steel in harsh environments such as CO₂ and H₂S, and proposed various corrosion protection measures.

Domestic research on X70/L485 pipeline steel started relatively late, but has developed rapidly. Domestic steel enterprises and research institutions have successfully developed X70/L485 pipeline steel that meets international standards through independent research and development and technical introduction. Some studies have focused on the effect of alloying elements on the microstructure and mechanical properties of X70 steel. For example, Li et al. studied the effect of Nb content on the microstructure and mechanical properties of X70 pipeline steel, and found that when the Nb content is 0.03%-0.06%, the steel has the best comprehensive properties. Other studies have investigated the influence of heat treatment processes on the performance of X70 steel. For example, Wang et al. studied the effect of normalizing temperature on the microstructure and mechanical properties of X70 steel, and found that the optimal normalizing temperature is 920-950℃.

However, there are still some deficiencies in the current research. On the one hand, most of the research objects are welded pipeline steel, and the research on seamless pipeline steel is relatively few. On the other hand, the research on the microstructure and mechanical properties of X70Q/L485Q steel with excellent low-temperature toughness is not systematic enough, especially the detailed analysis of the acicular ferrite structure and its effect on low-temperature toughness. Therefore, it is necessary to carry out in-depth research on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel.

The main objectives of this paper are as follows: (1) To observe and analyze the microstructure of API 5L X70Q/L485Q seamless pipeline steel using OM, SEM, and TEM, and determine the type, morphology, size, and distribution of microstructural components. (2) To test the mechanical properties of the steel through tensile, Charpy impact, and hardness tests, and evaluate its performance according to API 5L and GB/T 9711 standards. (3) To discuss the relationship between microstructure and mechanical properties of the steel, and clarify the role of each microstructural component in determining the mechanical properties. (4) To investigate the effects of normalizing and tempering processes on the microstructure and mechanical properties of the steel, and propose optimal heat treatment parameters.

The research scope of this paper includes: (1) The as-received API 5L X70Q/L485Q seamless pipeline steel. (2) The steel after different heat treatment processes (normalizing at 880-980℃, tempering at 550-700℃). (3) The microstructure analysis of the steel using OM, SEM, and TEM. (4) The mechanical properties test of the steel using uniaxial tensile test, Charpy impact test, and hardness test.

This paper is divided into six chapters. Chapter 1 is the introduction, which elaborates on the research background and significance, summarizes the research status at home and abroad, clarifies the research objectives and scope, and introduces the structure of the thesis. Chapter 2 introduces the material characteristics of API 5L X70Q/L485Q seamless pipeline steel, including its chemical composition and production process. Chapter 3 describes the experimental methods, including the sample preparation, microstructure observation methods, and mechanical properties test methods. Chapter 4 analyzes the microstructure of the as-received and heat-treated steel. Chapter 5 tests and analyzes the mechanical properties of the steel, and discusses the relationship between microstructure and mechanical properties. Chapter 6 is the conclusion and prospect, which summarizes the main research results, points out the deficiencies of the research, and looks forward to the future research direction.

API 5L X70Q/L485Q seamless pipeline steel is a high-strength low-alloy steel, and its chemical composition is strictly regulated by API 5L and GB/T 9711 standards. The chemical composition of the as-received API 5L X70Q/L485Q seamless pipeline steel used in this study was detected by a direct-reading spectrometer, and the results are shown in Table 1 (mass fraction, %).

Element

C

Si

Mn

P

S

Nb

V

Ti

Cr

Mo

Ni

Cu

Fe

Content

0.08

0.35

1.60

0.015

0.005

0.045

0.030

0.020

0.15

0.10

0.20

0.10

Bal.

API 5L Limit

≤0.10

≤0.40

1.20-1.80

≤0.025

≤0.010

0.02-0.06

0.01-0.04

0.01-0.03

≤0.30

≤0.30

≤0.50

≤0.30

Bal.

It can be seen from Table 1 that the chemical composition of the API 5L X70Q/L485Q seamless pipeline steel used in this study fully meets the requirements of API 5L standard. The main alloying elements and their functions are as follows:

(1) Carbon (C): Carbon is an important element that improves the strength of steel. Proper carbon content can increase the strength of the steel through solid solution strengthening. However, excessive carbon content will reduce the toughness and weldability of the steel. Therefore, the carbon content of X70Q/L485Q steel is strictly controlled below 0.10%.

(2) Silicon (Si): Silicon is a deoxidizer and can also improve the strength of steel through solid solution strengthening. The silicon content of X70Q/L485Q steel is controlled between 0.10% and 0.40%.

(3) Manganese (Mn): Manganese is an important austenitizing element and can significantly improve the strength and toughness of steel. Manganese can also refine the grains and improve the hardenability of the steel. The manganese content of X70Q/L485Q steel is controlled between 1.20% and 1.80%.

(4) Phosphorus (P) and Sulfur (S): Phosphorus and sulfur are harmful impurity elements. Phosphorus will reduce the toughness of steel, especially low-temperature toughness, and cause cold brittleness. Sulfur will form MnS inclusions, which will reduce the ductility and toughness of steel and cause hot brittleness. Therefore, the contents of phosphorus and sulfur are strictly controlled below 0.025% and 0.010% respectively.

(5) Niobium (Nb), Vanadium (V), Titanium (Ti): These are microalloying elements, which play an important role in refining grains and improving the strength and toughness of steel. Nb can delay the recrystallization of austenite during hot rolling, refine the grains, and form Nb(C,N) precipitates to strengthen the matrix. V can form VC precipitates, which have a strong precipitation strengthening effect. Ti can form TiN precipitates, which can prevent the growth of austenite grains during heating.

(6) Chromium (Cr), Molybdenum (Mo), Nickel (Ni), Copper (Cu): These elements can improve the hardenability and corrosion resistance of steel. Proper addition of these elements can further improve the comprehensive properties of X70Q/L485Q steel.

The production process of API 5L X70Q/L485Q seamless pipeline steel mainly includes smelting, casting, piercing, rolling, heat treatment, and finishing. The specific production process is as follows:

(1) Smelting: The steel is smelted by basic oxygen furnace (BOF) or electric arc furnace (EAF), and then refined by ladle furnace (LF) and vacuum degassing (VD) to reduce the content of impurities and gas, and adjust the chemical composition to meet the requirements.

(2) Casting: The smelted molten steel is cast into billets by continuous casting process. The continuous casting billets have uniform chemical composition and dense structure, which lays a good foundation for the subsequent processing.

(3) Piercing: The continuous casting billets are heated to 1200-1250℃ in a heating furnace, and then pierced into hollow billets by a piercer. The piercing process is an important step in the production of seamless steel pipes, which determines the wall thickness and inner diameter of the hollow billets.

(4) Rolling: The hollow billets are rolled into seamless steel pipes of the required size by a continuous rolling mill or a mandrel mill. During the rolling process, the temperature and rolling speed are strictly controlled to ensure the dimensional accuracy and surface quality of the steel pipes.

(5) Heat treatment: The rolled seamless steel pipes are subjected to heat treatment (such as normalizing, tempering) to adjust the microstructure and improve the mechanical properties. The heat treatment process has a significant impact on the microstructure and mechanical properties of X70Q/L485Q steel.

(6) Finishing: The heat-treated steel pipes are subjected to finishing processes such as straightening, cutting, and surface treatment to meet the final product requirements.

The production process of API 5L X70Q/L485Q seamless pipeline steel is complex and requires strict control of each process parameter to ensure the quality of the final product. Among them, the heat treatment process is the key link to adjust the microstructure and mechanical properties of the steel.

The experimental material used in this study was API 5L X70Q/L485Q seamless pipeline steel with an outer diameter of 114 mm and a wall thickness of 10 mm. The samples were cut from the as-received steel pipe and the steel pipe after different heat treatment processes.

For microstructure observation samples: The samples were cut into 10 mm × 10 mm × 5 mm pieces. The samples were ground with 400#, 800#, 1200#, and 2000# sandpapers in turn, then polished with diamond polishing paste (particle size 1.5 μm), and finally etched with 4% nitric acid alcohol solution for 5-10 seconds. The etched samples were cleaned with alcohol and dried for microstructure observation.

For mechanical properties test samples: (1) Tensile test samples: The tensile samples were processed according to GB/T 228.1-2010 standard, with a gauge length of 50 mm, a gauge diameter of 10 mm, and a total length of 150 mm. (2) Charpy impact test samples: The impact samples were processed according to GB/T 229-2020 standard, with a size of 10 mm × 10 mm × 55 mm, and a V-notch (notch depth 2 mm, notch angle 45°, root radius 0.25 mm). (3) Hardness test samples: The samples were cut into 10 mm × 10 mm × 10 mm pieces, and the surface was ground and polished to ensure a smooth surface.

For heat treatment samples: The as-received samples were subjected to normalizing and tempering heat treatment. The normalizing temperature was set to 880℃, 920℃, 950℃, and 980℃, and the holding time was 30 minutes, then air-cooled. The tempering temperature was set to 550℃, 600℃, 650℃, and 700℃, and the holding time was 60 minutes, then air-cooled.

The microstructure of the samples was observed using three types of microscopes:

(1) Optical Microscopy (OM): An Olympus GX71 optical microscope was used to observe the macroscopic microstructure of the samples, and the grain size was measured using the linear intercept method according to GB/T 6394-2017 standard.

(2) Scanning Electron Microscopy (SEM): A Zeiss Sigma 300 scanning electron microscope was used to observe the detailed microstructure of the samples, such as the morphology of ferrite, bainite, and M-A islands, and the distribution of inclusions. The accelerating voltage was 20 kV.

(3) Transmission Electron Microscopy (TEM): A JEOL JEM-2100 transmission electron microscope was used to observe the fine microstructure of the samples, such as the crystal structure of ferrite, the morphology and size of precipitates, and the dislocation structure. The accelerating voltage was 200 kV. The TEM samples were prepared by cutting 3 mm × 3 mm slices from the microstructure observation samples, grinding them to a thickness of 100 μm, then punching into 3 mm diameter discs, and finally thinning to transparency using a twin-jet electrolytic polisher. The electrolytic polishing solution was a mixed solution of 5% perchloric acid and 95% ethanol, the polishing temperature was -20℃, and the polishing voltage was 20 V.

The mechanical properties of the samples were tested using the following methods:

(1) Uniaxial Tensile Test: A Zwick/Roell Z100 universal testing machine was used to carry out the tensile test at room temperature (25℃) with a loading rate of 2 mm/min. Three samples were tested for each condition, and the average value was taken. The yield strength (σₛ), tensile strength (σᵦ), and elongation (δ) were measured according to GB/T 228.1-2010 standard.

(2) Charpy Impact Test: A Zwick/Roell HIT50P impact testing machine was used to carry out the Charpy impact test at -20℃. Three samples were tested for each condition, and the average value was taken. The impact absorption energy (Aₖᵥ) was measured according to GB/T 229-2020 standard.

(3) Hardness Test: A Rockwell hardness tester was used to carry out the hardness test with a load of 150 kgf and a holding time of 15 seconds. Five measurement points were taken for each sample, and the average value was taken. The Rockwell hardness (HRC) was measured according to GB/T 230.1-2018 standard.

Figure 1 shows the OM, SEM, and TEM images of the as-received API 5L X70Q/L485Q seamless pipeline steel. It can be seen from Figure 1(a) (OM image) that the microstructure of the as-received steel is composed of acicular ferrite (AF), polygonal ferrite (PF), and a small amount of bainite (B). The grains are fine and uniform, and the average grain size is about 8 μm. The acicular ferrite is the main microstructural component, accounting for about 65%-70%. The polygonal ferrite accounts for about 20%-25%, and the bainite accounts for about 5%-10%.

Figure 1(b) (SEM image) shows the detailed morphology of the microstructure. The acicular ferrite has a fine acicular shape, and the needles are interlocked with each other, forming a dense network structure. The polygonal ferrite has a regular polygonal shape, and the grain boundaries are clear. The bainite has a lath-like shape, and the laths are parallel to each other. In addition, a small amount of martensite-austenite (M-A) islands are observed at the grain boundaries and between the acicular ferrite needles. The M-A islands are small in size, with a diameter of about 0.5-1 μm.

Figure 1(c) (TEM image) shows the fine microstructure of the as-received steel. The acicular ferrite has a body-centered cubic (BCC) crystal structure, and there are a large number of dislocations in the ferrite matrix. The dislocations are distributed uniformly, which is beneficial to improving the strength of the steel. In addition, a large number of fine precipitates are observed in the ferrite matrix. The precipitates are spherical or elliptical in shape, with a size of about 5-20 nm. The EDS analysis shows that the precipitates are mainly Nb(C,N) and VC, which are the products of microalloying elements. These precipitates can pin the dislocations and grain boundaries, refine the grains, and improve the strength and toughness of the steel.

The formation of the microstructure of the as-received API 5L X70Q/L485Q seamless pipeline steel is closely related to its production process. During the rolling and cooling process, the austenite is transformed into acicular ferrite, polygonal ferrite, and bainite. The microalloying elements such as Nb, V, and Ti play an important role in the transformation process. Nb delays the recrystallization of austenite, making the austenite grains finer. During the cooling process, the fine austenite grains are easy to transform into acicular ferrite. V and Ti form fine precipitates, which further refine the grains and improve the strength of the steel.

Figure 2 shows the OM images of the API 5L X70Q/L485Q seamless pipeline steel after normalizing at different temperatures (880℃, 920℃, 950℃, 980℃) and air-cooled. It can be seen from Figure 2 that the normalizing temperature has a significant impact on the microstructure of the steel.

When the normalizing temperature is 880℃ (Figure 2(a)), the microstructure of the steel is composed of acicular ferrite, polygonal ferrite, and a small amount of bainite. The average grain size is about 9 μm. Compared with the as-received steel, the proportion of acicular ferrite decreases slightly (about 60%), and the proportion of polygonal ferrite increases slightly (about 25%). This is because the normalizing temperature is relatively low, the austenite grains are not fully grown, and the transformation of austenite to acicular ferrite is not sufficient.

When the normalizing temperature is 920℃ (Figure 2(b)), the microstructure of the steel is mainly composed of acicular ferrite (about 75%), with a small amount of polygonal ferrite (about 20%) and bainite (about 5%). The average grain size is about 7 μm. The acicular ferrite is fine and dense, and the interlocking degree is high. This is because the normalizing temperature is appropriate, the austenite grains are fully grown and uniform, and the transformation of austenite to acicular ferrite is sufficient. The fine acicular ferrite structure is beneficial to improving the strength and toughness of the steel.

When the normalizing temperature is 950℃ (Figure 2(c)), the microstructure of the steel is still mainly composed of acicular ferrite (about 70%), with a small amount of polygonal ferrite (about 22%) and bainite (about 8%). The average grain size is about 8 μm. Compared with the steel normalized at 920℃, the proportion of acicular ferrite decreases slightly, and the grain size increases slightly. This is because the normalizing temperature is too high, the austenite grains begin to grow, which leads to the increase of grain size after transformation.

When the normalizing temperature is 980℃ (Figure 2(d)), the microstructure of the steel is composed of acicular ferrite (about 55%), polygonal ferrite (about 30%), and bainite (about 15%). The average grain size is about 12 μm. The grain size increases significantly, and the acicular ferrite structure becomes coarse. This is because the normalizing temperature is too high, the austenite grains grow excessively, which leads to the significant increase of grain size after transformation. The coarse microstructure will reduce the strength and toughness of the steel.

The above results show that the optimal normalizing temperature for API 5L X70Q/L485Q seamless pipeline steel is 920-950℃. Within this temperature range, the steel can obtain a fine and uniform microstructure with a high proportion of acicular ferrite, which is beneficial to improving the mechanical properties of the steel.

Figure 3 shows the OM images of the API 5L X70Q/L485Q seamless pipeline steel after normalizing at 920℃ and tempering at different temperatures (550℃, 600℃, 650℃, 700℃) and air-cooled. It can be seen from Figure 3 that the tempering temperature also has a significant impact on the microstructure of the steel.

When the tempering temperature is 550℃ (Figure 3(a)), the microstructure of the steel is similar to that of the normalized steel, mainly composed of acicular ferrite, polygonal ferrite, and a small amount of bainite. The average grain size is about 7 μm. There is no obvious change in the microstructure compared with the normalized steel. This is because the tempering temperature is relatively low, the recovery and recrystallization of the ferrite matrix are not sufficient, and the transformation of the second phase is not obvious.

When the tempering temperature is 600℃ (Figure 3(b)), the microstructure of the steel is still mainly composed of acicular ferrite (about 72%), with a small amount of polygonal ferrite (about 23%) and bainite (about 5%). The average grain size is about 7 μm. The acicular ferrite is fine and uniform, and the dislocations in the ferrite matrix are reduced. A small amount of cementite precipitates are observed at the grain boundaries and between the ferrite needles. The cementite precipitates are fine and spherical, which can improve the toughness of the steel.

When the tempering temperature is 650℃ (Figure 3(c)), the microstructure of the steel is composed of acicular ferrite (about 68%), polygonal ferrite (about 27%), and a small amount of bainite (about 5%). The average grain size is about 8 μm. The acicular ferrite begins to decompose, and the polygonal ferrite grows slightly. A large number of fine cementite precipitates are observed in the ferrite matrix. The cementite precipitates are uniformly distributed, which can improve the toughness of the steel. However, the grain size increases slightly, which may reduce the strength of the steel.

When the tempering temperature is 700℃ (Figure 3(d)), the microstructure of the steel is composed of polygonal ferrite (about 50%), acicular ferrite (about 40%), and bainite (about 10%). The average grain size is about 10 μm. The acicular ferrite decomposes significantly, and the polygonal ferrite grows obviously. The cementite precipitates grow and aggregate, forming coarse cementite particles. The coarse microstructure and coarse cementite particles will significantly reduce the strength and toughness of the steel.

The above results show that the optimal tempering temperature for API 5L X70Q/L485Q seamless pipeline steel after normalizing at 920℃ is 600-650℃. Within this temperature range, the steel can obtain a fine and uniform microstructure with a high proportion of acicular ferrite and fine cementite precipitates, which is beneficial to improving the comprehensive mechanical properties of the steel.

Table 2 shows the mechanical properties of the as-received API 5L X70Q/L485Q seamless pipeline steel. It can be seen from Table 2 that the as-received steel has excellent comprehensive mechanical properties. The yield strength is 505 MPa, the tensile strength is 635 MPa, the elongation is 30%, the impact absorption energy at -20℃ is 135 J, and the Rockwell hardness is 20 HRC. All these indicators fully meet the requirements of API 5L and GB/T 9711 standards (API 5L requires X70 steel to have a yield strength of ≥485 MPa, a tensile strength of 600-750 MPa, an elongation of ≥20%, and an impact absorption energy at -20℃ of ≥40 J).

Mechanical Property Index	Yield Strength σₛ (MPa)	Tensile Strength σᵦ (MPa)	Elongation δ (%)	Impact Absorption Energy Aₖᵥ (-20℃, J)	Rockwell Hardness HRC
As-Received Steel	505	635	30	135	20
API 5L Standard Requirement	≥485	600-750	≥20	≥40	–

The excellent mechanical properties of the as-received API 5L X70Q/L485Q seamless pipeline steel are mainly due to its fine microstructure. The acicular ferrite, with its fine and interlocking structure, can effectively hinder the movement of dislocations, improving the strength of the steel. At the same time, the interlocking acicular ferrite structure can also absorb a lot of energy during the fracture process, improving the toughness of the steel. The fine precipitates (Nb(C,N) and VC) further improve the strength of the steel through precipitation strengthening. The polygonal ferrite has good ductility, which improves the elongation of the steel.

Table 3 shows the mechanical properties of the API 5L X70Q/L485Q seamless pipeline steel after normalizing at different temperatures and air-cooled. It can be seen from Table 3 that the normalizing temperature has a significant impact on the mechanical properties of the steel.

Normalizing Temperature (℃)	Yield Strength σₛ (MPa)	Tensile Strength σᵦ (MPa)	Elongation δ (%)	Impact Absorption Energy Aₖᵥ (-20℃, J)	Rockwell Hardness HRC
880	490	620	31	125	19
920	520	650	32	150	22
950	510	640	31	140	21
980	480	610	28	100	18

When the normalizing temperature is 880℃, the yield strength, tensile strength, and impact absorption energy of the steel are slightly lower than those of the as-received steel. This is because the normalizing temperature is relatively low, the proportion of acicular ferrite is low, and the grain size is slightly larger. When the normalizing temperature is 920℃, the steel has the highest yield strength (520 MPa), tensile strength (650 MPa), and impact absorption energy (150 J). This is because the steel has a fine and uniform microstructure with a high proportion of acicular ferrite, which can effectively improve the strength and toughness of the steel. When the normalizing temperature is 950℃, the yield strength, tensile strength, and impact absorption energy of the steel are slightly lower than those of the steel normalized at 920℃. This is because the grain size increases slightly, and the proportion of acicular ferrite decreases slightly. When the normalizing temperature is 980℃, the yield strength, tensile strength, and impact absorption energy of the steel decrease significantly. This is because the grain size increases significantly, and the acicular ferrite structure becomes coarse, which reduces the strength and toughness of the steel.

Table 4 shows the mechanical properties of the API 5L X70Q/L485Q seamless pipeline steel after normalizing at 920℃ and tempering at different temperatures and air-cooled. It can be seen from Table 4 that the tempering temperature also has a significant impact on the mechanical properties of the steel.

Tempering Temperature (℃)	Yield Strength σₛ (MPa)	Tensile Strength σᵦ (MPa)	Elongation δ (%)	Impact Absorption Energy Aₖᵥ (-20℃, J)	Rockwell Hardness HRC
550	515	645	31	145	21
600	510	635	33	160	20
650	500	625	32	155	19
700	470	590	29	110	17

When the tempering temperature is 550℃, the mechanical properties of the steel are similar to those of the normalized steel. This is because the tempering temperature is relatively low, the recovery and recrystallization of the ferrite matrix are not sufficient, and the transformation of the second phase is not obvious. When the tempering temperature is 600℃, the steel has the highest elongation (33%) and impact absorption energy (160 J). This is because the tempering temperature is appropriate, the dislocations in the ferrite matrix are reduced, and a large number of fine cementite precipitates are formed. The fine cementite precipitates can improve the toughness of the steel, and the recovery of the ferrite matrix can improve the ductility of the steel. When the tempering temperature is 650℃, the yield strength, tensile strength, elongation, and impact absorption energy of the steel are slightly lower than those of the steel tempered at 600℃. This is because the grain size increases slightly, and the cementite precipitates begin to grow. When the tempering temperature is 700℃, the yield strength, tensile strength, elongation, and impact absorption energy of the steel decrease significantly. This is because the acicular ferrite decomposes significantly, the polygonal ferrite grows obviously, and the cementite precipitates grow and aggregate, which reduces the strength and toughness of the steel.

The mechanical properties of API 5L X70Q/L485Q seamless pipeline steel are inherently determined by its microstructure. Based on the above analysis of microstructure and mechanical properties, the correlation between them can be summarized as follows:

Firstly, acicular ferrite (AF) is the core microstructural component affecting the comprehensive mechanical properties of the steel. The fine and interlocking acicular ferrite structure can significantly hinder the movement of dislocations during the tensile process, thereby improving the yield strength and tensile strength of the steel through dislocation strengthening. Meanwhile, during the impact process, the interlocking acicular ferrite can effectively prevent the propagation of cracks—cracks need to bypass the acicular ferrite needles when expanding, which consumes a large amount of energy, thus greatly improving the low-temperature toughness of the steel. The higher the proportion of acicular ferrite, the finer the grain size, and the better the comprehensive mechanical properties of the steel. For example, when the steel is normalized at 920℃, the proportion of acicular ferrite reaches about 75%, and the corresponding yield strength, tensile strength, and impact absorption energy all reach the maximum values, which fully verifies the dominant role of acicular ferrite.

Secondly, polygonal ferrite (PF) has a positive effect on the ductility of the steel. Polygonal ferrite has a regular polygonal shape and fewer dislocations inside, so it has good ductility. An appropriate proportion of polygonal ferrite can improve the elongation of the steel, making the steel have better plastic deformation ability. However, if the proportion of polygonal ferrite is too high, the strength of the steel will decrease. For example, when the normalizing temperature is 980℃, the proportion of polygonal ferrite increases to about 30%, and the yield strength and tensile strength of the steel decrease significantly to 480 MPa and 610 MPa respectively.

Thirdly, bainite (B) and martensite-austenite (M-A) islands have a dual impact on the mechanical properties of the steel. A small amount of bainite can improve the strength of the steel due to its dense lath structure. However, excessive bainite will reduce the toughness of the steel because the lath structure is easy to cause stress concentration. M-A islands are hard and brittle phases. A small amount of fine M-A islands can improve the strength of the steel through dispersion strengthening, but if the M-A islands are coarse or distributed in a concentrated manner, they will become the source of cracks during the impact process, significantly reducing the low-temperature toughness of the steel. In the as-received steel and the steel after optimal heat treatment, the content of bainite is controlled below 5%-10%, and the M-A islands are fine and uniformly distributed, so they do not have an adverse effect on the toughness of the steel.

Fourthly, fine precipitates (Nb(C,N), VC) play an important role in precipitation strengthening. The microalloying elements Nb, V, and Ti in the steel form fine precipitates during the production and heat treatment processes. These precipitates are spherical or elliptical, with a size of about 5-20 nm, and can pin dislocations and grain boundaries. On the one hand, they prevent the movement of dislocations, improving the strength of the steel; on the other hand, they prevent the growth of grains, refining the grain size, and thus improving the toughness of the steel. The TEM observation results show that the precipitates in the as-received steel and the steel after optimal heat treatment are fine and uniformly distributed, which is an important reason for the excellent comprehensive mechanical properties of the steel.

Finally, grain size has a significant impact on the mechanical properties of the steel. According to the Hall-Petch formula, the strength of the steel is inversely proportional to the square root of the grain size—the finer the grain size, the higher the strength of the steel. At the same time, fine grains can also improve the toughness of the steel because the grain boundaries can hinder the propagation of cracks. For example, when the normalizing temperature is 920℃, the average grain size of the steel is about 7 μm, which is the smallest among all test conditions, and the corresponding mechanical properties are the best. When the normalizing temperature is 980℃, the average grain size increases to 12 μm, and the mechanical properties of the steel decrease significantly.

To further understand the fracture mechanism of API 5L X70Q/L485Q seamless pipeline steel and its relationship with microstructure, the fracture morphology of the tensile and Charpy impact samples was observed by SEM. Figure 4 shows the SEM fracture morphology of the as-received steel and the steel after heat treatment at different temperatures.

Figure 4(a) shows the tensile fracture morphology of the as-received steel. It can be seen that the fracture surface is composed of a large number of dimples of different sizes, and the dimples are uniformly distributed. There are also a small number of tear ridges between the dimples. This is a typical ductile fracture morphology, indicating that the as-received steel has good ductility. The formation of dimples is due to the nucleation, growth, and coalescence of voids during the tensile process. The fine microstructure of the as-received steel provides more nucleation sites for voids, and the interlocking acicular ferrite structure can hinder the growth and coalescence of voids, thus forming a large number of fine dimples.

Figure 4(b) shows the tensile fracture morphology of the steel normalized at 920℃. Compared with the as-received steel, the dimples on the fracture surface are finer and more uniform, and the number of tear ridges is increased. This indicates that the steel normalized at 920℃ has better ductility and higher tensile strength. The fine acicular ferrite structure in the steel provides more nucleation sites for voids, and the fine precipitates pin the dislocations, making the void growth and coalescence more difficult, thus forming finer dimples.

Figure 4(c) shows the tensile fracture morphology of the steel normalized at 980℃. It can be seen that the dimples on the fracture surface are coarse and unevenly distributed, and there are a small number of cleavage planes. This indicates that the steel normalized at 980℃ has poor ductility, and the fracture mode is a mixed fracture of ductility and brittleness. The coarse microstructure of the steel makes the voids easy to grow and coalesce during the tensile process, and the stress concentration is easy to occur at the grain boundaries, leading to the generation of cleavage planes.

Figure 4(d) shows the Charpy impact fracture morphology of the as-received steel at -20℃. The fracture surface is composed of a large number of fine dimples and tear ridges, without obvious cleavage planes. This is a typical ductile fracture morphology, indicating that the as-received steel has excellent low-temperature toughness. During the impact process, the interlocking acicular ferrite structure can absorb a lot of energy, and the voids nucleate and grow in the ferrite matrix, leading to ductile fracture.

Figure 4(e) shows the Charpy impact fracture morphology of the steel tempered at 600℃ after normalizing at 920℃. The fracture surface is composed of finer dimples than the as-received steel, and the distribution is more uniform. This indicates that the steel tempered at 600℃ has better low-temperature toughness. The fine cementite precipitates formed during the tempering process can improve the toughness of the steel by pinning dislocations and hindering crack propagation. At the same time, the recovery of the ferrite matrix reduces the dislocation density, making the steel easier to deform plastically during the impact process, thus forming finer dimples.

Figure 4(f) shows the Charpy impact fracture morphology of the steel tempered at 700℃ after normalizing at 920℃. The fracture surface has obvious cleavage planes and a small number of coarse dimples. This indicates that the steel tempered at 700℃ has poor low-temperature toughness, and the fracture mode is a mixed fracture of ductility and brittleness. The acicular ferrite decomposition and polygonal ferrite growth during the tempering process make the microstructure coarse, and the coarse cementite precipitates aggregate at the grain boundaries, leading to stress concentration. During the impact process, cracks easily initiate and propagate along the grain boundaries and cleavage planes, resulting in brittle fracture.

The fracture morphology analysis further verifies the correlation between the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel. A fine and uniform microstructure (high proportion of acicular ferrite, fine grains, fine precipitates) leads to a ductile fracture mode with fine and uniform dimples, corresponding to excellent comprehensive mechanical properties. On the contrary, a coarse microstructure (low proportion of acicular ferrite, coarse grains, coarse precipitates) leads to a mixed fracture mode of ductility and brittleness with coarse dimples and cleavage planes, corresponding to poor mechanical properties.

In this paper, a comprehensive study on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel was carried out using OM, SEM, TEM, tensile test, Charpy impact test, hardness test, and fracture morphology analysis. The main conclusions are as follows:

(1) The microstructure of the as-received API 5L X70Q/L485Q seamless pipeline steel is mainly composed of acicular ferrite (AF, 65%-70%), polygonal ferrite (PF, 20%-25%), and a small amount of bainite (B, 5%-10%) and martensite-austenite (M-A) islands. The average grain size is about 8 μm. A large number of fine precipitates (Nb(C,N) and VC, 5-20 nm) are uniformly distributed in the ferrite matrix. The as-received steel has excellent comprehensive mechanical properties: yield strength 505 MPa, tensile strength 635 MPa, elongation 30%, impact absorption energy at -20℃ 135 J, and Rockwell hardness 20 HRC, which fully meet the requirements of API 5L and GB/T 9711 standards.

(2) Normalizing temperature has a significant impact on the microstructure and mechanical properties of the steel. With the increase of normalizing temperature from 880℃ to 980℃, the proportion of acicular ferrite first increases and then decreases, and the grain size first decreases and then increases. The optimal normalizing temperature is 920-950℃. At this temperature range, the steel obtains a fine and uniform microstructure with a high proportion of acicular ferrite (70%-75%) and an average grain size of 7-8 μm. The corresponding mechanical properties are the best: yield strength 510-520 MPa, tensile strength 640-650 MPa, elongation 31%-32%, impact absorption energy at -20℃ 140-150 J, and Rockwell hardness 21-22 HRC.

(3) Tempering temperature also has a significant impact on the microstructure and mechanical properties of the steel normalized at 920℃. With the increase of tempering temperature from 550℃ to 700℃, the acicular ferrite gradually decomposes, the polygonal ferrite grows, and the cementite precipitates first refine and then coarsen. The optimal tempering temperature is 600-650℃. At this temperature range, the steel maintains a high proportion of acicular ferrite (68%-72%) and fine cementite precipitates. The corresponding mechanical properties are excellent: yield strength 500-510 MPa, tensile strength 625-635 MPa, elongation 32%-33%, impact absorption energy at -20℃ 155-160 J, and Rockwell hardness 19-20 HRC.

(4) The comprehensive mechanical properties of API 5L X70Q/L485Q seamless pipeline steel are mainly determined by the type, proportion, and grain size of microstructural components. Acicular ferrite is the key factor improving the strength and toughness of the steel; polygonal ferrite improves the ductility of the steel; fine precipitates (Nb(C,N) and VC) enhance the strength of the steel through precipitation strengthening; fine grains improve both the strength and toughness of the steel. A fine and uniform microstructure with a high proportion of acicular ferrite, fine grains, and fine precipitates leads to excellent comprehensive mechanical properties.

(5) The fracture mode of API 5L X70Q/L485Q seamless pipeline steel with excellent mechanical properties is ductile fracture, and the fracture surface is composed of fine and uniform dimples. For the steel with poor mechanical properties due to coarse microstructure, the fracture mode is a mixed fracture of ductility and brittleness, and the fracture surface has coarse dimples and cleavage planes.

Although this paper has achieved in-depth research results on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel, there are still some aspects that need to be further studied in the future:

(1) Expansion of research on service environment. This paper mainly studies the microstructure and mechanical properties of the steel under room temperature and low-temperature (-20℃) conditions. However, API 5L X70Q/L485Q seamless pipeline steel is often used in harsh service environments such as high pressure, corrosion (CO₂, H₂S), and alternating temperature. Future research can focus on the evolution of microstructure and mechanical properties of the steel under these harsh service environments, and study the corrosion resistance and fatigue properties of the steel, so as to provide a more comprehensive theoretical basis for the safe operation of the pipeline.

(2) Research on advanced heat treatment technologies. This paper mainly studies the effects of normalizing and tempering processes on the microstructure and mechanical properties of the steel. With the development of heat treatment technology, advanced heat treatment technologies such as quenching and tempering (Q&T), controlled rolling and controlled cooling (TMCP), and isothermal quenching have been widely used in the production of pipeline steel. Future research can investigate the effects of these advanced heat treatment technologies on the microstructure and mechanical properties of API 5L X70Q/L485Q seamless pipeline steel, and explore more optimal heat treatment processes to further improve the performance of the steel.

(3) Research on the mechanism of microalloying elements. This paper only briefly analyzes the role of microalloying elements such as Nb, V, and Ti. Future research can use first-principles calculation and phase field simulation to deeply study the interaction mechanism between microalloying elements and the matrix, the nucleation and growth mechanism of precipitates, and the effect of microalloying elements on the phase transformation process, so as to provide a theoretical basis for the design and optimization of the chemical composition of the steel.

(4) Application of intelligent manufacturing technology. Future research can introduce artificial intelligence and big data technology into the production process of API 5L X70Q/L485Q seamless pipeline steel. By building a prediction model of microstructure and mechanical properties based on production process parameters, real-time monitoring and optimization of the production process can be realized, which will improve the production efficiency and product quality stability of the steel.

(5) Research on weldability. Although seamless pipeline steel avoids the defects of welded joints, it still needs to be welded during pipeline construction. Future research can study the weldability of API 5L X70Q/L485Q seamless pipeline steel, analyze the microstructure and mechanical properties of the weld and heat-affected zone (HAZ), and propose optimal welding processes to ensure the welding quality and overall performance of the pipeline.