阅读说明：本技术 一种低温环境服役大应变管线钢的生产方法 (Production method of low-temperature environment service large-strain pipeline steel ) 是由 彭宁琦 汤伟 杨建华 周文浩 李红英 吉玲康 宋光鑫 李阳华 罗登 熊祥江 张勇 于 2021-08-30 设计创作，主要内容包括：一种低温环境服役大应变管线钢的生产方法,钢的化学组成质量百分比为：C=0.04%～0.05%,Si≤0.10%,Mn=1.40%～1.90%,P≤0.015%,S≤0.003%,Al=0.030%～0.060%,Nb=0.04%～0.08%,Ti=0.010%～0.015%,Cr≤0.3%,Mo≤0.3%,Ni≤0.3%,Cu≤0.3%,B≤0.0005%,N=0.005%～0.008%,H≤0.0002%,余量为Fe和不可避免的杂质。本发明通过合理的成分设计、轧制规程的优化、精准的相变控制、以及压平和矫直的合理利用,获得横纵向上晶粒细小且均匀的铁素体和贝氏体双相组织,使生产的管线钢具有优异的低温韧性和良好的抗大应变能力,同时具有良好的板形控制能力。(A production method of a large-strain pipeline steel in service in a low-temperature environment comprises the following chemical components in percentage by mass: 0.04-0.05% of C, less than or equal to 0.10% of Si, 1.40-1.90% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 0.030-0.060% of Al, 0.04-0.08% of Nb, 0.010-0.015% of Ti, less than or equal to 0.3% of Cr, less than or equal to 0.3% of Mo, less than or equal to 0.3% of Ni, less than or equal to 0.3% of Cu, less than or equal to 0.0005% of B, 0.005-0.008% of N, less than or equal to 0.0002% of H, and the balance of Fe and inevitable impurities. The invention obtains ferrite and bainite dual-phase structures with fine and uniform grains in the transverse and longitudinal directions through reasonable component design, optimization of rolling procedures, accurate phase change control and reasonable utilization of flattening and straightening, so that the produced pipeline steel has excellent low-temperature toughness, good large-strain resistance and good plate shape control capability.)

1. A production method of pipeline steel with large strain in service in a low-temperature environment is characterized by comprising the following steps: the chemical composition of the steel comprises, by mass, 0.04-0.05% of C, 0.10% or less of Si, 1.40-1.90% of Mn, 0.015% or less of P, 0.003% or less of S, 0.030% to 0.060% of Al, 0.04-0.08% of Nb, 0.010% to 0.015% of Ti, 0.3% or less of Cr, 0.3% or less of Mo, 0.3% or less of Ni, 0.3% or less of Cu, 0.0005% or less of B, 0.005% to 0.008% of N, 0.0002% or less of H, and the balance of Fe and inevitable impurities; the key process steps comprise:

(1) pouring the refined molten steel into a plate blank at a low superheat degree of 5-15 ℃, and stacking and cooling to room temperature;

(2) the heating temperature of the plate blank is 1220-1250 ℃, and the soaking time is 60-90 min;

(3) the rolling process is divided into a rough rolling stage and a finish rolling stage, and the rough rolling finishing temperature is 960-1000 ℃; after rough rolling, carrying out first temperature waiting, then carrying out finish rolling, wherein the start temperature of the finish rolling is less than or equal to 900 ℃, the finish temperature is 810-830 ℃, and then carrying out second temperature waiting; controlling the reduction rate of the finish rolling pass to be less than or equal to 15 percent and the time interval of the pass to be more than or equal to 12 s;

(4) when the temperature of the steel plate is reduced to 740-780 ℃, a finishing mill is adopted to carry out steel plate flattening operation, the roll gap in the flattening process is controlled to be consistent with the roll gap of the finish pass, and the flattening speed is less than or equal to 2.0 m/s; then carrying out third temperature waiting;

(5) when the temperature of the steel plate is cooled to 690-730 ℃, the steel plate is rapidly cooled to 300-500 ℃ at a cooling speed of 15-25 ℃/s;

(6) and (3) straightening twice, wherein the first straightening temperature is 250-400 ℃, then the second straightening is carried out after the interval time is more than or equal to 100s, the second straightening temperature is less than or equal to 250 ℃, and air cooling is carried out to the room temperature after straightening.

2. The production method of the pipeline steel with high service strain in the low-temperature environment as claimed in claim 1, wherein the production method comprises the following steps: in the step (3), the rolling total compression ratio is controlled to be between 8 and 15, and the expansion-width ratio is controlled to be more than or equal to 1.5.

3. The production method of the pipeline steel with high service strain in the low-temperature environment as claimed in claim 1, wherein the production method comprises the following steps: in the step (3), the total reduction rate in the rough rolling stage is more than or equal to 70%, wherein the reduction rate in at least 1 pass is more than or equal to 20%, and high-pressure water descaling is performed before the rolling in the pass.

4. The production method of the pipeline steel with high service strain in the low-temperature environment as claimed in claim 1, wherein the production method comprises the following steps: and (4) repeating the flattening operation for 2-3 times.

5. The production method of the pipeline steel with high service strain in the low-temperature environment as claimed in claim 1, wherein the production method comprises the following steps: in the step (6), air cooling is adopted after the first straightening to ensure that the cooling speed of the steel plate is more than or equal to 1 ℃/s.

Technical Field

The invention belongs to the technical field of metallurgy, and relates to a production method of high-strain pipeline steel in service in a low-temperature environment.

Background

The best mode of oil and gas long-distance transmission is to adopt pipelines, and the oil and gas pipeline transmission is always in the severe environment with severe climate, rare people smoke and extremely complex geological and topographic conditions, and is inevitably challenged by severe working conditions such as ice and snow lands, permafrost, marshland, debris flow, earthquake fracture zones, landslide zones, subsidence zones, collapse zones, mine goafs and the like. In order to ensure that the oil and gas transmission pipeline can still run at high speed, high efficiency, economy and safety in the complex and severe environments, very strict requirements are put forward on the performance of pipeline steel.

Chinese patents CN105112815A, CN104762461A, CN103952638A and CN102409224A all develop pipeline steel with excellent low-temperature toughness, can realize the crack arrest performance of a drop hammer of more than or equal to 85 percent at the temperature of 20 ℃ below zero or even lower, and are suitable for being used in a low-temperature environment; chinese patents CN105200336A, CN101456034A, CN101914723A, CN101906569A, etc. propose large strain pipeline steels used in strain design areas, which have low yield ratio, high uniform elongation, and other large deformation resistance capabilities. However, the structure of the pipeline steel with excellent low-temperature toughness is mainly an acicular ferrite structure, the yield ratio of the steel plate is often higher, and the steel plate does not have the capability of large strain; the pipeline steel with large strain capacity is usually designed by adopting a dual-phase structure, but the soft-phase ferrite generated by the general relaxation process is usually relatively coarse, a severe banded structure is easily formed by rolling in a two-phase region, and a large number of coarse M-A components are often attached to a hard phase generated by a rapid cooling process, which bring adverse effects on low-temperature toughness. Therefore, the development of the pipeline steel with excellent low-temperature toughness and good large-strain resistance is an important direction for the construction of future oil and gas transmission pipelines.

Disclosure of Invention

The invention aims to provide a production method of pipeline steel with high strain serving in a low-temperature environment, which obtains ferrite and bainite dual-phase structures with fine and uniform grains in the transverse direction and the longitudinal direction through component design and controlled rolling and controlled cooling processes, and realizes high drop weight performance with DWTT more than or equal to 85% at the transverse temperature of-20 ℃ and high strain capacity with longitudinal yield ratio less than or equal to 0.80 and uniform elongation more than or equal to 10%.

The technical scheme of the invention is as follows:

the production method of the steel for the pipeline with large strain serving in the low-temperature environment comprises the following steps of enabling the chemical composition of the steel to be C =0.04% -0.05%, Si is less than or equal to 0.10%, Mn is 1.40% -1.90%, P is less than or equal to 0.015%, S is less than or equal to 0.003%, Al is =0.030% -0.060%, Nb is 0.04% -0.08%, Ti is 0.010% -0.015%, Cr is less than or equal to 0.3%, Mo is less than or equal to 0.3%, Ni is less than or equal to 0.3%, Cu is less than or equal to 0.3%, B is less than or equal to 0.0005%, N is 0.005% -0.008%, H is less than or equal to 0.0002%, and the balance is Fe and inevitable impurities; the key process steps comprise:

(1) pouring the refined molten steel into a plate blank at a low superheat degree of 5-15 ℃, and stacking and cooling to room temperature;

(2) the heating temperature of the plate blank is 1220-1250 ℃, and the soaking time is 60-90 min;

(3) the rolling process comprises two stages of rough rolling and finish rolling, wherein the finish temperature of the rough rolling is 960-1000 ℃, the first temperature waiting is carried out after the rough rolling, then the finish rolling is carried out, the start temperature of the finish rolling is less than or equal to 900 ℃, the finish temperature is 810-830 ℃, and then the second temperature waiting is carried out; controlling the reduction rate of the finish rolling pass to be less than or equal to 15 percent and the time interval of the pass to be more than or equal to 12 s;

(4) when the temperature of the steel plate is reduced to 740-780 ℃, a finishing mill is adopted to carry out steel plate flattening operation, the roll gap in the flattening process is controlled to be consistent with the roll gap of the finish pass, and the flattening speed is less than or equal to 2.0 m/s; then carrying out third temperature waiting;

(5) when the temperature of the steel plate is cooled to 690-730 ℃, the steel plate is rapidly cooled to 300-500 ℃ at a cooling speed of 15-25 ℃/s;

(6) and (3) straightening twice, wherein the first straightening temperature is 250-400 ℃, then the second straightening is carried out after the interval time is more than or equal to 100s, the second straightening temperature is less than or equal to 250 ℃, and air cooling is carried out to the room temperature after straightening.

Preferably, in the step (3), the rolling total compression ratio is controlled to be between 8 and 15, and the aspect ratio is controlled to be more than or equal to 1.5.

Preferably, in the step (3), the total reduction rate of the rough rolling stage is more than or equal to 70%, wherein the reduction rate of at least 1 pass is more than or equal to 20%, and high-pressure water descaling is performed before the pass of rolling.

Preferably, in the step (4), the flattening operation is repeated for 2-3 times.

Preferably, in the step (6), air cooling is adopted after the first straightening to ensure that the cooling speed of the steel plate is more than or equal to 1 ℃/s.

The chemical composition and key process setting basis of the invention is as follows:

the carbon content is strictly controlled to obtain a soft phase ferrite structure with a relatively stable proportion, when the carbon content is 0.06%, the residual austenite amount is increased, the toughness is influenced to a certain extent, and when the carbon content is lower than 0.04%, the hard phase bainite strength is influenced, so that the carbon content is controlled to be 0.04% -0.05%. Silicon is a non-carbide forming element, has the function of strongly preventing the desolvation of supersaturated ferrite, can slow down the transformation of bainite, promotes the formation of M/A, and improves the size of granular bainite. Therefore, the invention uses a very small amount of silicon content to reduce the amount of M/A and reduce the size of M/A islands and granular bainite, which is beneficial to DWTT performance. According to the invention, N with higher content and Nb and Al with higher content are adopted, and the amount of Ti and B is controlled, so that more nitrides are formed, on one hand, enough TiN and NbN precipitates are still pinned at a crystal boundary in the heating process, and the phenomenon that low-carbon steel is easily subjected to high-temperature heating to cause uneven austenite grains is reduced; on the other hand, NbN, AlN, NbC and other particles are continuously separated out in the rolling process, and the function of inhibiting the austenite grains from growing is achieved; thereby contributing to obtaining a uniform and fine prior austenite structure.

The method is characterized in that low-superheat-degree steel casting and high-temperature heating are adopted to control and improve center segregation, meanwhile, a sufficient total compression ratio is preferably ensured, a sufficiently large aspect ratio and a sufficient rough rolling total reduction rate are preferably controlled, the reduction rate of at least 1 pass of rough rolling is increased, the temperature difference between the surface of a rolled piece before rolling and the core is increased by using high-pressure water so that deformation can easily penetrate into the core, and low pass reduction rate, long pass interval time and high finish rolling temperature are adopted for finish rolling. The high temperature heating can also ensure that the plate blank absorbs enough heat, thereby ensuring that the tensile strength is high enough to control the low yield ratio. The invention adopts two-stage rolling to avoid mixed crystal texture, and the air cooling is carried out after the rollingA _r3And flattening operation is carried out at a nearby temperature, so that compressive stress is generated in the steel plate, the austenite is promoted to be transformed to the ferrite, the phase transformation time is shortened, and the size of the transformed crystal grain is reduced. And then, continuing relaxation air cooling to obtain a soft phase ferrite structure with a sufficient proportion, and then rapidly cooling in time to avoid the generation of a pearlite structure. Considering that the rapid cooling and the bainite phase change bring larger residual stress, two times of straightening is carried out, and the straightening is kept after the first time of straighteningAfter a period of time, bainite phase transformation is promoted to be complete, and then second pass straightening is carried out at a lower temperature, so that the problem of poor plate shape caused by phase transformation structure stress is effectively solved, and meanwhile, the defects of dislocation and the like in soft-phase ferrite grains can be reduced and become purer, and the effect of reducing the yield ratio is also achieved.

The invention has the beneficial effects that: the pipeline steel produced by the production method of the invention consists of 50-70% of soft phase ferrite and 30-50% of hard phase bainite, has fine and uniform structure, strength level reaching X70 and plate thickness reaching 33.8mm, and has excellent low-temperature toughness: the charpy V-shaped impact energy at minus 20 ℃ is more than or equal to 300J, and the DWTT at minus 20 ℃ in the transverse direction is more than or equal to 85 percent. Simultaneously has good large strain resistance: the transverse yield strength of the steel plate is 500-590 MPa, the tensile strength is 650-750 MPa, the yield ratio is less than or equal to 0.80, and the elongation is more than or equal to 40%; the longitudinal yield strength of the steel plate is 485-550 MPa, the tensile strength is 620-720 MPa, the yield ratio is less than or equal to 0.80, the elongation is more than or equal to 40%, the uniform elongation is more than or equal to 10%, and the strain ratio R_t1.5/R_t0.5≥1.10，R_t2.0/R_t1.0≥1.06，R_t5.0/R_t1.0Not less than 1.10. The product can be used for pipeline laying in polar regions and high-cold regions with large displacement deformation sections such as earthquakes, landslides, subsidence, collapse and the like, and occasions such as construction of low-temperature deep sea pipelines. The invention ensures that the high-strain pipeline steel in service in a low-temperature environment has good comprehensive mechanical property and good plate shape control capability through reasonable component design, optimization of rolling schedule, accurate phase change control and reasonable utilization of flattening and straightening.

Drawings

FIG. 1 is a metallographic photograph showing a cross section at 1/4 thickness of a steel sheet in example 1.

FIG. 2 is a metallographic photograph of a longitudinal cross section taken at a thickness of 1/4 of the steel sheet in example 1.

FIG. 3 photograph of full thickness drop weight in the transverse direction at-20 ℃ of the steel sheet of example 1.

Detailed Description

The present invention will be further described with reference to the following examples.

Example 1: steel grade X70, plate thickness 33.8mm, plate width 4350mm

The chemical composition of the steel is, by mass, C =0.04%, Si =0.08%, Mn =1.78%, P =0.010%, S =0.001%, Al =0.045%, Nb =0.065%, Ti =0.012%, Cr =0.23%, Mo =0.12%, Ni =0.25%, Cu =0.15%, B =0.0003%, N =0.0060%, H =0.00012%, and the balance is Fe and unavoidable impurities; the key process steps comprise:

(1) casting the refined molten steel into a plate blank by adopting a low superheat degree, wherein the superheat degree is 8-12 ℃, and then stacking and cooling to room temperature; the thickness of the plate blank is 300mm, the width is 2280mm, and the plate blank is rolled into a steel plate with the thickness of 33.8mm and the width of 4350mm, so that the total compression ratio of the steel plate is 8.9, and the aspect ratio of the steel plate is 1.91;

(2) the heating temperature of the plate blank is 1240 +/-10 ℃, and the soaking time is 75 min;

(3) the rolling process comprises two stages of rough rolling and finish rolling, the total reduction rate of the rough rolling is 71.7%, the maximum pass of the reduction rate is the second pass after broadening, the reduction rate is 22.3%, and high-pressure water descaling is carried out before the pass of rolling; the finish temperature of rough rolling is 980 ℃, the first time of temperature waiting is carried out after rough rolling, then finish rolling is carried out, the start temperature of finish rolling is 880 ℃, the finish temperature is 820 ℃, and then the second time of temperature waiting is carried out; the maximum reduction rate of the finish rolling pass is 13.7%, and the minimum time of the pass interval is 14.6 s;

(4) when the temperature of the steel plate is reduced to 760 ℃, performing steel plate flattening operation for 1 time by using a finishing mill, and controlling the roll gap in the flattening process to be consistent with the roll gap of a finishing pass of finishing rolling, wherein the flattening speed is 1.0 m/s; then carrying out third temperature waiting;

(5) when the temperature of the steel plate is cooled to 720 ℃, the steel plate starts to be rapidly cooled, the cooling speed is about 22.0 ℃/s, and the steel plate is cooled to 360-420 ℃;

(6) two times of straightening are adopted, the first time of straightening is carried out at the temperature of 320 ℃, air cooling is adopted after straightening, the cooling time (interval time) is 120s, the cooling is carried out to 180 ℃, the cooling speed is about 1.17 ℃/s, then second time of straightening is carried out, and air cooling is carried out to the room temperature after straightening.

Metallographic structure observation was performed on the steel sheet of example 1, and the metallographic structure of the cross section at a thickness of 1/4 parts of the steel sheet is shown in fig. 1, and the metallographic structure of the longitudinal section is shown in fig. 2. As can be seen from the figure, the steel sheet had a uniform and fine structure in the transverse and longitudinal directions, and was a ferrite-bainite dual phase structure, with a ferrite fraction of about 58% in the cross-sectional structure, an average grain size of about 12 μm, a ferrite fraction of about 65% in the longitudinal-sectional structure, and an average grain size of about 10 μm. 2 samples were taken from the steel plate of example 1 and the drop weight crack arrest properties were examined, and FIG. 3 shows a photograph of a transverse full thickness drop weight fracture at-20 ℃. According to the national standard judgment of GB/T8363, the shear areas of the transverse full-thickness DWTT at-20 ℃ are 91 percent and 93 percent respectively, the average shear area is 92 percent, and the low-temperature toughness is excellent. The steel sheets of example 1 were sampled and subjected to tensile and impact tests, and the results are shown in Table 1. The results show that the steel plate in the embodiment 1 has uniform transverse and longitudinal properties, good strength and toughness matching and good large strain resistance in the longitudinal direction.

Example 2: steel grade X70, plate thickness 25.7mm, plate width 4370mm

The steel comprises the following chemical components in percentage by mass: c =0.05%, Si =0.06%, Mn =1.75%, P =0.009%, S =0.002%, Al =0.048%, Nb =0.065%, Ti =0.011%, Cr =0.21%, Mo =0.12%, Ni =0.24%, Cu =0.14%, B =0.0004%, N =0.0062%, H =0.00016%, the balance being Fe and unavoidable impurities; the key process comprises the following steps:

(1) casting the refined molten steel into a plate blank by adopting a low superheat degree, wherein the superheat degree is 10-13 ℃, and then stacking and cooling to room temperature; the thickness of the plate blank is 300mm, the width is 2280mm, and the plate blank is rolled into a steel plate with the thickness of 25.7mm and the width of 4370mm, so that the total compression ratio of the steel plate is 11.7, and the aspect ratio is 1.92;

(2) the heating temperature of the plate blank is 1235 +/-10 ℃, and the soaking time is 70 min;

(3) the rolling process comprises two stages of rough rolling and finish rolling, the total reduction rate of the rough rolling is 71.7%, the maximum pass of the reduction rate is the second pass after broadening, the reduction rate is 21.5%, and high-pressure water descaling is carried out before the pass of rolling; the finish temperature of rough rolling is 972 ℃, the first time of warming is carried out after rough rolling, then finish rolling is carried out, the start temperature of finish rolling is 890 ℃, the finish temperature is 815 ℃, and then the second time of warming is carried out; the maximum reduction rate of the finish rolling pass is 12.8%, and the minimum time of the pass interval is 12.5 s;

(4) when the temperature of the steel plate is reduced to 775 ℃, a finishing mill is adopted to carry out steel plate flattening operation for 3 times, the roll gap in the flattening process is controlled to be consistent with the roll gap of the pass after finishing the finish rolling, and the flattening speed of each pass is 2.0 m/s; then carrying out third temperature waiting;

(5) when the temperature of the steel plate is cooled to 700 ℃, the steel plate starts to be rapidly cooled, the cooling speed is about 18.7 ℃/s, and the steel plate is cooled to 390-450 ℃;

(6) two times of straightening are adopted, the first time of straightening is at the temperature of 360 ℃, air cooling is adopted after straightening, the cooling time (interval time) is 150s, the temperature is reduced to 160 ℃, the cooling speed is about 1.33 ℃/s, then second time of straightening is carried out, and air cooling is carried out to the room temperature after straightening.

2 transverse drop weight samples of the steel plate in the example 2 are taken to detect the full-thickness DWTT shearing area, the test temperature is-20 ℃, and the results are 94% and 95% respectively, so that the steel plate has excellent low-temperature toughness; the steel sheets of example 2 were sampled and subjected to tensile and impact tests, and the results are shown in Table 2. The results also show that the steel plate of the embodiment 2 has uniform transverse and longitudinal properties, good strength and toughness matching and good large strain resistance in the longitudinal direction.

Table 1 example 1 steel sheet samples were taken for tensile and impact test results

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Table 2 example 2 steel sheet sampling tensile and impact test results

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