Patent Description:
Pipeline transportation is the most economical and reasonable transportation mode of oil and natural gas. The long transmission pipeline not only needs to pass through different temperature areas, but also needs to pass through the stratum movement areas caused by natural disasters such as earthquake tundra, debris flow and landslide. Therefore, in addition to meeting the requirements of high strength and high toughness, the pipeline also needs to have relatively high anti deformation ability to adapt to the transmission geological environment.

Large deformation resistant pipeline steel is one of the most challenging research fields in the development of pipeline steel, which requires pipeline steel to have higher compression and tensile strain resistance. A large number of studies have proved that in addition to the basic strength plasticity performance parameters, such as yield strength, tensile strength and elongation, the main indexes that can measure its resistance to large deformation are "uniform plastic deformation elongation Ue ≥ <NUM>%, yield strength ratio Rt0. <NUM> / Rm ≤ <NUM>", etc..

For the requirement of "resistance to a large deformation ", pipeline steels disclosed in patent documents such as application No. <CIT>, <CIT> and <CIT> involve obtaining ferrite + bainite dual phase structure by relaxation and other methods, which has good resistance to a large deformation characteristics. However, because the structure is two-phase structure, see <FIG>. Moreover, the two-phase structure has obvious strip along the rolling direction, so the HIC resistance is not ideal. The two-phase structure is easy to accumulate hydrogen at boundary, and the strip structure can also induce hydrogen accumulation. For the pipeline steel with two-phase structure of ferrite + bainite, the HIC resistance is tested according to the corresponding NACE standards. The steel plate has many HIC cracks along different directions of thickness, and the HIC resistance is not ideal. Further prior art is disclosed in <CIT>.

Aiming at the above prior art, the invention provides a pipeline steel with both HIC resistance and resistance to a large deformation and a production method thereof, which can adapt to the product development of pipeline steel plate of Grade X80 and below. The pipeline steel not only has the characteristics of resistance to a large deformation such as low yield strength ratio, high uniform elongation and high stress ratio, but also shows good HIC resistance.

The technical scheme adopted by the invention to solve the above problems is: a pipeline steel with both HIC resistance and resistance to a large deformation, which is characterized in that the alloy constituents adopted are C: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, S: ≤ <NUM>%, P: ≤ <NUM>%, Al: <NUM>- <NUM>%, Cr: <NUM>-<NUM>%, <NUM> ≤ Nb + V + Ti ≤ <NUM>%, Nb, V and Ti are not <NUM>, Ni: <NUM>-<NUM>%, Cu: <NUM>-<NUM>%, Ca: ≤ <NUM>%, N: ≤ <NUM>%, Mo: <NUM>-<NUM>%, and the balance is Fe and unavoidable impurity elements.

The content of Nb is determined as per the content of C and niobium carbide, and the content of Ti is determined as per the stoichiometric ratio of Ti / N of <NUM>.

Further, the product is bainite single-phase structure, and the grain size of bainite is grade <NUM> to grade <NUM>.

The transverse yield strength of the product of the invention Rt0. <NUM>: <NUM> - 550MPa, transverse tensile strength Rm: ≥ 710MPa, transverse yield ratio Rt0. <NUM> / Rm ≤ <NUM>, - <NUM> Charpy impact energy ≥ 350J, - <NUM> drop weight shear area SA% ≥ <NUM>%;Longitudinal yield strength <NUM>-530MPa; Longitudinal tensile strength ≥ 690MPa, longitudinal uniform elongation Uel ≥ <NUM>%, longitudinal yield ratio ≤ <NUM>; Longitudinal stress ratio Rt1. <NUM>≥<NUM>, Rt2. <NUM>≥<NUM>; And the HIC resistance of the product: after soaking in a solution according to NACE TM0284-<NUM> A for <NUM> hours, the crack length rate %: <NUM>, the crack width rate %: <NUM>, and the crack sensitivity rate %: <NUM>.

The design basis for chemical constituents of the pipeline steel of the invention are as follows:.

The production method of pipeline steel with both HIC resistance and resistance to a large deformation in the application: first smelt the molten steel conforming to the chemical constituents design, cast the continuous casting billet with the chemical constituents consistent with the chemical constituents of the finished steel plate, heat the continuous casting billet to <NUM>-<NUM> for <NUM>-<NUM> hours, and then discharge the furnace; After descaling with high-pressure water, two-stage rolling is carried out: the first stage is recrystallization zone rolling, and the start rolling temperature is <NUM>-<NUM>. After multi pass rolling, the final rolling temperature is controlled at <NUM>-<NUM> , and the rolling deformation rate of two passes of rough rolling is controlled to be ≥ <NUM>%; The second stage is rolled in the non-recrystallization zone. The start rolling temperature is <NUM>-<NUM> , the final rolling temperature is controlled at <NUM>-<NUM>, and the rolling cumulative deformation rate in the second stage is ≥ <NUM>%; After rolling, according to the change of austenite microstructure, the steel plate is sent to the cooling system through a <NUM>-<NUM> long roller table at a conveying speed of V = a * H, H is the steel plate thickness mm, a = <NUM>-<NUM> / (s * mm);.

In the cooling system, the billet is directly quenched, air cooled to Ar<NUM> temperature after direct quenching, and then cooled rapidly. The final cooling temperature is controlled below <NUM>, straightened with temperature, and finally air cooled to room temperature to obtain X80 pipeline steel plate with both HIC and resistance to a large deformation.

The conveying speed of billet roller table after rolling shall take into account the microstructure dislocation movement of steel plate after sufficient deformation of austenite to obtain microstructure with different dislocation density at different grain positions, so as to provide growth conditions for obtaining very fine bainite structure.

Compared with the prior art, the invention has the advantages that according to the HIC resistance principle and hydrogen trap theory, in order to achieve good HIC resistance performance, it is prefer to have a relatively single and uniform structure. While according to the resistance to a large deformation principle, the structure needs to have excellent cooperative deformation ability in deformation in order to have excellent resistance to a large deformation ability. It is confirmed by research, some low carbon bainites have the ability to combine these two properties. According to the deformation principle, the bainite needs to be very small in order to give play to the good cooperative deformation effect between grains in the deformation process, so as to obtain excellent resistance to a large deformation. In order to obtain this very fine bainite, it is necessary to design the constituents and process. The pipeline steel developed by the invention has a uniform microstructure of very fine bainite, and the microstructure grain size reaches more than grade <NUM>. Compared with the two-phase structure, H is not easy to aggregate, so it shows good HIC resistance.

The invention is described in further detail below in combination with the embodiments of the attached drawings. The embodiments described below with reference to the attached drawings are exemplary and are intended to explain the invention and cannot be understood as limitations on the invention.

The following embodiments take pipeline steel of X80 steel grade as an example. The performance and production difficulty of steel grades below X80 steel grade, such as X70 and X60, are lower than those of X80, so they are not listed one by one in this application.

The continuous casting billet with thickness no more than <NUM> is produced by continuously casting the molten steel consistent with the chemical constituents of the prepared pipeline steel plate through the continuous casting machine. The chemical constituents of the obtained continuous casting billet includes: C: <NUM>%, Si: <NUM>%, Mn: <NUM>%, S ≤ <NUM>%, P ≤ <NUM>%, Al: <NUM>%, Cr: <NUM>%, Nb + V + Ti: <NUM>%, Ni: <NUM>%, Cu: <NUM>%, Ca: ≤ <NUM>%, N: ≤ <NUM>%, Mo: <NUM>%. The balance is Fe and unavoidable impurity elements.

The continuous casting billet is heated to <NUM> for <NUM> hours, discharged from the furnace, descaled by 20MPa high-pressure water, and then rolled in two stages: The first stage is rolled in the recrystallization zone, the start rolling temperature is <NUM>, and rolled in seven passes, in which the deformation rate of two passes is ≥ <NUM>%. The final rolling temperature is <NUM>, and the thickness of the intermediate billet obtained after rolling in the recrystallization zone is <NUM>; The second stage is rolled in the non-recrystallization zone. The start rolling temperature is <NUM>, the final rolling temperature is <NUM>, the cumulative deformation rate of rolling in the non-recrystallization zone is ≥ <NUM>%, and the thickness of the finished pipeline steel plate is <NUM>;After rolling, the steel plate is sent to the cooling system through a <NUM> long roller table at the conveying speed of <NUM>/s. First, it is directly quenched in water, then air cooled to Ar<NUM> temperature after water is discharged, then is cooled rapidly by ACC, the final cooling temperature is <NUM>, and finally air cooled to room temperature. The microstructure of the obtained pipeline steel is very fine bainite with grain size of <NUM>. The microstructure morphology in the thickness direction is shown in <FIG>. Compared with X80 pipeline steel in the ferrite + bainite dual phase structure, as prepared by conventional relaxation air cooling shown in <FIG>, the microstructure is more uniform and the bainite grain is finer. After testing, the strength and plasticity indexes are as follows: transverse yield strength Rt0. <NUM>: 540MPa; Tensile strength Rm: 740MPa, transverse yield ratio Rt0. <NUM>/Rm=<NUM>; Longitudinal yield strength 510MPa, - <NUM> Charpy impact energy = 450J, SA% (- <NUM>) = <NUM>%; Longitudinal tensile strength Rm: 730MPa longitudinal uniform elongation Uel = <NUM>%; Longitudinal yield ratio = <NUM>; Longitudinal Rt1. <NUM>=<NUM>, Rt2. <NUM> = <NUM>. The HIC resistance test results are shown in Table <NUM>.

The continuous casting billet with thickness no more than <NUM> is produced by continuously casting the molten steel consistent with the chemical constituents of the prepared pipeline steel plate through the continuous casting machine. The chemical constituents of the obtained continuous casting billet includes: C: <NUM>%, Si: <NUM>%, Mn: <NUM>%, S≤ <NUM>%, P ≤ <NUM>%, Al: <NUM>%, Cr: <NUM>%, Nb + V + Ti: <NUM>%, Ni: <NUM>%, Cu: <NUM>%, Ca: ≤ <NUM>%, N: ≤ <NUM>%, Mo: <NUM>%. The balance is Fe and unavoidable impurity elements.

The continuous casting billet is heated to <NUM> for <NUM> hours, discharged from the furnace, descaled by 20MPa high-pressure water, and then rolled in two stages: The first stage is rolled in the recrystallization zone, the start rolling temperature is <NUM>, and rolled in seven passes, in which the deformation rate of two passes is ≥ <NUM>%. The final rolling temperature is <NUM>, and the thickness of the intermediate billet obtained after rolling in the recrystallization zone is <NUM>;The second stage is rolled in the non-recrystallization zone. The start rolling temperature is <NUM>, the final rolling temperature is <NUM>, the cumulative deformation rate of rolling in the non-recrystallization zone is ≥ <NUM>%, and the thickness of the finished pipeline steel plate is <NUM>;After rolling, the steel plate is sent to the cooling system through a <NUM> long roller table at the conveying speed of <NUM>/s. First, it is directly quenched in water, then air cooled to Ar<NUM> temperature after water is discharged, then is cooled rapidly by ACC, the final cooling temperature is <NUM>, and finally air cooled to room temperature. The microstructure of the obtained pipeline steel is very fine bainite with grain size of <NUM>. The microstructure morphology in the thickness direction is shown in <FIG>. Compared with X80 pipeline steel in the ferrite + bainite dual phase structure, as prepared by conventional relaxation air cooling shown in <FIG>, the microstructure is more uniform and the bainite grain is finer. After testing, the strength and plasticity indexes are as follows: transverse yield strength Rt0. <NUM>: 535MPa; Tensile strength Rm: 735MPa, transverse yield ratio Rt0. <NUM>/Rm=<NUM>; Longitudinal yield strength 500MPa, - <NUM> Charpy impact energy = 450J, SA% (- <NUM>) = <NUM>%; Longitudinal tensile strength Rm: 730MPa longitudinal uniform elongation Uel = <NUM>%; Longitudinal yield ratio = <NUM>; Longitudinal Rt1. <NUM>=<NUM>, Rt2. <NUM> = <NUM>, the HIC resistance test results are shown in Table <NUM>.

The continuous casting billet with thickness no more than <NUM> is produced by continuously casting the molten steel consistent with the chemical constituents of the prepared pipeline steel plate through the continuous casting machine. The chemical constituents of the obtained continuous casting billet includes: C: <NUM>%, Si: <NUM>%, Mn: <NUM>%, S≤<NUM>%, P≤<NUM>%, Al: <NUM>%, Cr: <NUM>%, Nb+V+Ti: <NUM>%, Ni: <NUM>%, Cu: <NUM>%, Ca: ≤<NUM>%, N: ≤<NUM>%, Mo: <NUM>%. The balance is Fe and unavoidable impurity elements.

The continuous casting billet is heated to <NUM> for <NUM> hours, discharged from the furnace, descaled by 20MPa high-pressure water, and then rolled in two stages: The first stage is rolled in the recrystallization zone, the start rolling temperature is <NUM>, and rolled in five passes, in which the deformation rate of two passes is ≥ <NUM>%. The final rolling temperature is <NUM>, and the thickness of the intermediate billet obtained after rolling in the recrystallization zone is <NUM>; The second stage is rolled in the non-recrystallization zone. The start rolling temperature is <NUM>, the final rolling temperature is <NUM>, the cumulative deformation rate of rolling in the non-recrystallization zone is ≥ <NUM>%, and the thickness of the finished pipeline steel plate is <NUM>;After rolling, the steel plate is sent to the cooling system through a <NUM> long roller table at the conveying speed of <NUM>/s. First, it is directly quenched in water, then air cooled to Ar<NUM> temperature after water is discharged, then is cooled rapidly by ACC, the final cooling temperature is <NUM>, and finally air cooled to room temperature. The microstructure of the obtained pipeline steel is very fine bainite. After testing, the strength and plasticity indexes are as follows: Transverse yield strength Rt0. <NUM>: 510MPa; Tensile strength RM: 705MPa, Transverse yield ratio Rt0. <NUM>/Rm=<NUM>; Longitudinal yield strength 505MPa, - <NUM> Charpy impact energy=380J, SA% (-<NUM> )=<NUM>%; Longitudinal tensile strength Rm: 700MPa longitudinal uniform elongation Uel=<NUM>%; Longitudinal yield ratio = <NUM>; Longitudinal Rt1. <NUM>=<NUM>, Rt2. <NUM>=<NUM>, the HIC resistance test results are shown in Table <NUM>.

The continuous casting billet with a thickness of no more than <NUM> is produced by continuously casting the molten steel consistent with the chemical constituents of the prepared pipeline steel plate through the continuous casting machine. The chemical constituents of the obtained continuous casting billet includes: C: <NUM>%, Si: <NUM>%, Mn: <NUM>%, S ≤ <NUM>%, P ≤ <NUM>%, Al: <NUM>%, Cr: <NUM>%, Nb + V + Ti: <NUM>%, Ni: <NUM>%, Cu: <NUM>%, Ca: ≤ <NUM>%, N: ≤ <NUM>%, Mo: <NUM>%. The balance is Fe and unavoidable impurity elements.

The continuous casting billet is heated to <NUM> for <NUM> hours, discharged from the furnace, descaled by 20MPa high-pressure water, and then rolled in two stages: The first stage is rolled in the recrystallization zone, the start rolling temperature is <NUM>, and rolled in five passes, in which the deformation rate of two passes is ≥ <NUM>%. The final rolling temperature is <NUM>, and the thickness of the intermediate billet obtained after rolling in the recrystallization zone is <NUM>; The second stage is rolled in the non-recrystallization zone. The start rolling temperature is <NUM>, the final rolling temperature is <NUM>, the cumulative deformation rate of rolling in the non-recrystallization zone is ≥ <NUM>%, and the thickness of the finished pipeline steel plate is <NUM>; After rolling, the steel plate is sent to the cooling system through a <NUM> long roller table at the conveying speed of <NUM>/s. First, it is directly quenched in water, then air cooled to Ar<NUM> temperature after water is discharged, then is cooled rapidly by ACC, the final cooling temperature is <NUM>, and finally air cooled to room temperature. The microstructure of the obtained pipeline steel is very fine bainite. After testing, the strength and plasticity indexes are as follows: Transverse yield strength Rt0. <NUM>: 485MPa; Tensile strength Rm: 710MPa, transverse yield ratio Rt0. <NUM>/Rm=<NUM>; Longitudinal yield strength 475MPa, - <NUM> Charpy impact energy=420J, SA% (- <NUM>)=<NUM>%; Longitudinal tensile strength Rm: 695MPa longitudinal uniform elongation Uel=<NUM>%; Longitudinal yield ratio = <NUM>; Longitudinal Rt1. <NUM>=<NUM>, Rt2. <NUM>=<NUM>, The HIC resistance test results are shown in Table <NUM>.

Claim 1:
A pipeline steel with both HIC resistance and resistance to a large deformation, which is characterized in that the alloy constituents are C: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, S: ≤ <NUM>%, P: ≤ <NUM>%, Al: <NUM>-<NUM>%, Cr: <NUM>-<NUM>%, <NUM> ≤ Nb + V + Ti ≤ <NUM>%, Nb, V and Ti are not <NUM>, Ni: <NUM>-<NUM>%, Cu: <NUM>-<NUM>%, Ca: ≤ <NUM>%, N: ≤ <NUM>%, Mo: <NUM>-<NUM>%, and the balance is Fe and unavoidable impurity elements;
wherein the pipeline steel has a bainite single-phase structure, and the grain size of bainite is grade <NUM> to grade <NUM>;
wherein transverse yield strength of the pipeline steel is Rt0.<NUM>: <NUM>-550MPa, transverse tensile strength Rm: ≥ 710MPa, transverse yield ratio Rt0.<NUM> / Rm ≤ <NUM>, - <NUM> Charpy impact energy ≥ 350J, - <NUM> drop weight shear area SA% ≥ <NUM>%; longitudinal yield strength <NUM>-530MPa; longitudinal tensile strength ≥ 690MPa, longitudinal uniform elongation Uel ≥ <NUM>%, longitudinal yield ratio ≤ <NUM>; longitudinal stress ratio Rt1.<NUM>/Rt0.<NUM>≥<NUM>, Rt2.<NUM>/Rt1.<NUM>≥<NUM>;
and the HIC resistance of the pipeline steel: after soaking in a solution according to NACE TM0284-<NUM> A for <NUM> hours, the crack length rate % is <NUM>, the crack width rate % is <NUM>, and the crack sensitivity rate % is <NUM>.