CN112251672B

CN112251672B - Low yield ratio EH690 steel sheet with excellent weldability and method for manufacturing same

Info

Publication number: CN112251672B
Application number: CN202011059381.2A
Authority: CN
Inventors: 朱隆浩; 赵坦; 金耀辉; 李家安; 于浩男; 任子平; 王�华; 肖青松; 陈华; 应传涛
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2020-09-30
Filing date: 2020-09-30
Publication date: 2021-12-24
Anticipated expiration: 2040-09-30
Also published as: CN112251672A

Abstract

The invention discloses an EH690 steel sheet with a low yield ratio and excellent weldability and a method for manufacturing the same. The steel contains: 0.06 to 0.09 percent of C, 0.05 to 0.1 percent of Si, 1.2 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.01 to 0.03 percent of Als, 0.8 to 1.5 percent of Ni, 0.3 to 0.7 percent of Cr, 0.3 to 0.5 percent of Mo, 0.1 to 0.3 percent of Cu, 0.03 to 0.05 percent of V, 0.005 to 0.01 percent of Ti, 0.005 to 0.009 percent of N, 0.0005 to 0.001 percent of B, and the balance of Fe and inevitable impurities. The initial rolling temperature of the first stage of the casting blank is 1000-1100 ℃, the initial rolling temperature of the second stage is 800-850 ℃, the cumulative reduction rate of the second stage rolling is 40-50%, the average reduction rate of a single pass is more than or equal to 13%, and the final rolling temperature is 750-800 ℃. The steel plate is quenched at high temperature, quenched at low temperature and tempered, the yield ratio of the steel plate is less than or equal to 0.93, and the maximum welding heat input is 100 KJ/cm.

Description

Low yield ratio EH690 steel sheet with excellent weldability and method for manufacturing same

Technical Field

The invention belongs to the field of steel material preparation, and particularly relates to a component design and a manufacturing method of a large-thickness low-yield-ratio EH690 marine steel plate with excellent welding performance.

Background

The 21 st century is the ocean century, and with the development of science and technology and the improvement of the living standard of people, all countries in the world focus on huge resources stored in the ocean. In recent years, the maritime work equipment industry is continuously and rapidly developed, the massive demand of steel for maritime work platforms and the upgrading and updating of products are promoted, and the market urgently needs ultrahigh-strength and extra-thick maritime work steel plates with good comprehensive performance.

The ocean engineering platform has a severe service environment, besides conventional stress, the influence of various factors such as strong wind, surge, tide, ice impact, earthquake and the like is also considered, so that the specificity of the steel for the ocean platform is determined, and the ocean engineering platform can adapt to various sea conditions in the aspect of material selection of platform construction. Meanwhile, the steel plate is in a humid and high-salinity marine environment for a long time, and the problems of paint film falling, corrosion of the surface of the steel plate, corrosion fatigue and the like caused by adhesion of humid air, seawater and marine organisms are solved, the mechanical property of the steel plate is reduced, the service life is shortened, and the normal use of the ocean engineering platform is seriously influenced. In addition, the ocean platform is far away from the coast and cannot be periodically docked for maintenance like a ship. In order to ensure that the ocean engineering platform can be safely used in a complex environment, a high-quality ultrahigh-strength steel for ocean engineering with excellent comprehensive performance is urgently needed to be developed, and the ultrahigh-strength steel plate for ocean engineering has the advantages of high strength, high and low temperature toughness, low yield ratio, high ductility, fatigue resistance, hydrogen induced cracking resistance, marine environmental corrosion resistance, marine organism adhesion resistance, excellent welding performance and the like.

At present, steel for ocean engineering can meet most of requirements of the market in the field of maritime work, but special steel with high strength level and excellent comprehensive performance is still the target of development of all countries in the world, and an EH690 steel plate with large thickness and low yield ratio has high difficulty in scientific research, strict production process, high requirement on equipment and high development difficulty.

The patent '700 MPa grade high-toughness low-yield ratio thick steel plate and manufacturing method thereof' with the publication number of CN100430507C provides an ultrahigh-strength steel plate with low yield ratio, the chemical composition of the steel plate is lower, the ultrahigh-strength steel plate with large thickness can not be produced, the steel has higher Cu element but does not have Ni element, so that serious heat crack can not be solved, meanwhile, the TMCP process can generate serious internal stress in the ultrahigh-strength steel plate, the subsequent stress relief treatment is not carried out, and the steel plate can cause serious buckling deformation when being subjected to thermal welding.

Patent publication No. CN109536850A, a high-strength-toughness low-yield-ratio thick steel plate and a production process thereof, proposes a low-yield-ratio thick steel plate with yield strength of 800MPa and a manufacturing method thereof, wherein the Ni content is 4.0-6.0%, the economy is poor, and the application range of the steel alloy content for ships and marine engineering is far exceeded.

Patent ' 690MPa grade high-strength low-yield-ratio medium-thickness manganese steel with low yield ratio ' and manufacturing method ' with publication number CN110846577A proposes 690MPa grade medium-thickness manganese steel with low yield ratio and manufacturing method thereof, the steel plate contains 4.1% -4.7% of Mn element, the addition of a large amount of Mn element can bring huge difficulty to the steel-making and continuous casting process, the continuous casting production is easy to cause accidents, and the medium-thickness manganese steel has high low-temperature toughness, but the rolled steel plate is easy to have the problems of unqualified flaw detection, corner cracks and the like.

The patent publication No. CN109983146A entitled "ultra-high strength steel with low yield ratio" and manufacturing method thereof "proposes an ultra-high strength steel bar with low yield ratio and manufacturing method thereof, only steel bars with specific specifications can be produced by the patent, wide and thick steel plates for ships and maritime workers cannot be produced, the steel plate produced by the method has a structure of bainite ferrite and a small amount of M-A islands, the ultra-high strength steel with a microstructure is difficult to ensure low-temperature toughness, and the embodiment shows that the impact temperature of the steel is only-5 ℃.

Disclosure of Invention

The invention aims to overcome the defects in the prior art, and the EH690 ultrahigh-strength steel plate with the low yield ratio and the excellent welding performance, which is suitable for the field of ocean engineering, is prepared, has the yield ratio of less than or equal to 0.93, is suitable for large-linear energy welding with the maximum welding heat input of 100KJ/cm, and has the service safety performance reaching the service condition of ocean engineering equipment. Forming a set of specific EH690 ultrahigh-strength marine steel plate components with excellent welding performance and low yield ratio and a corresponding production process.

In order to realize the purpose of the invention, the inventor carries out a large amount of systematic experimental researches through screening and proportioning of alloy elements, steel cleanliness control, high-efficiency rolling process optimization, parameter selection and the like, and finally determines the alloy element proportioning and rolling heat treatment process which can meet the purpose of the invention: the specific technical scheme is as follows:

a large-thickness low-yield-ratio EH690 marine steel plate with excellent welding performance comprises the following components in percentage by weight: 0.06 to 0.09 percent of C, 0.05 to 0.1 percent of Si, 1.2 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.01 to 0.03 percent of Als, 0.8 to 1.5 percent of Ni, 0.3 to 0.7 percent of Cr, 0.3 to 0.5 percent of Mo, 0.1 to 0.3 percent of Cu, 0.03 to 0.05 percent of V, 0.005 to 0.01 percent of Ti, 0.005 to 0.009 percent of N, 0.0005 to 0.001 percent of B, and the balance of Fe and inevitable impurities.

The design reason of the chemical components of the steel grade is as follows:

(1) the C element can improve the strength in the steel by a method of interstitial solid solution strengthening, and is a main element for ensuring the strength in the scheme of the invention, and meanwhile, the C element can improve the hardenability of the steel plate; the C content is harmful to the toughness and yield ratio of the steel plate, and a large amount of hardened tissues are generated in the welding process to cause welding cracks, so the content of the C element in the steel is accurately controlled, and the content of the C is controlled to be 0.06-0.09%.

(2) Si can improve the strength of the steel plate, and simultaneously Si can be used as a deoxidizer to reduce the content of O, when the content of Si is higher, the structure is coarsened, and the toughness, plasticity and yield ratio are obviously reduced, so that the content of Si in the invention is 0.05-0.1%.

(3) The Mn atoms have similar radiuses to the Fe atoms, can be replaced and dissolved in the Fe matrix in a large amount, and improve the strength of the steel plate. Because the content of C in the steel plate is low, more Mn element is needed to improve the strength of the steel plate, but when the content of Mn element is more than 1.5 percent by mass, the segregation of Mn element can reduce the low-temperature toughness of the core part of the thick plate, the yield ratio is increased, and the performance of a welding heat affected zone is deteriorated, so the content of Mn in the invention is 1.2 to 1.5 percent.

(4) P, S elements have no benefit on the mechanical property and the welding property of the steel plate, and comprehensively consider the cost factor, the invention controls P, S to be less than or equal to 0.02 percent of P and less than or equal to 0.01 percent of S.

(5) Al is a main deoxidizing element in steel, when the content of Al is too low, the deoxidizing effect is poor, and micro-alloy elements such as Ti and the like cannot achieve the purposes of refining grains and improving welding performance due to oxidation; on the contrary, since a large inclusion is formed when the Al element is too high, the content of Als in the present invention is 0.01 to 0.03%.

(6) Ni is an alloying element that can improve the low-temperature toughness of the steel sheet and also can improve the yield ratio of the steel sheet. Ni can reduce the sensitivity of the steel plate to notches, and the lower ductile-brittle transition temperature can be obtained by adding Ni element, so that the Ni content in the invention is 0.8-1.5%.

(7) Cr is an element for improving the hardenability and tensile strength of the steel sheet. Under the condition of low C content, the addition of a proper amount of Cr element can improve the tensile strength of the steel plate and ensure that the steel plate achieves the required yield ratio, but the addition of excessive Cr element in the steel can reduce the toughness and welding performance of the material, so the Cr content is 0.3-0.7 percent.

(8) The addition of Mo element can improve the hardenability of the steel plate, and the addition of a proper amount of Mo element can also improve the temper brittleness of the steel plate, but the addition of excessive Mo element is unfavorable for the yield ratio of the steel plate, so the content of Mo in the invention is 0.3-0.5%.

(9) The addition of the Cu element can improve the strength and the toughness of the steel plate, and the Cu element can be matched with the Ni element to effectively form a nano phase for precipitation, improve the tensile strength of the steel plate and reduce the yield ratio, but the excessive addition can improve the yield ratio of the steel plate and generate the Cu brittleness phenomenon. Therefore, the Cu content in the invention is 0.1-0.3%.

(10) The V element can form V (C, N) particles in the matrix, and can play a role in refining and strengthening crystal grains. The Ti and the N act together, and the toughness and the welding performance of the steel plate can be obviously improved. The grain refining effect of the V element in the low-carbon alloy system steel can reduce the yield ratio of the steel plate after heat treatment. Therefore, the V content in the invention is 0.03-0.05%.

(11) Ti is a key factor of the chemical components of the invention, and can generate strong precipitation strengthening effect, prevent austenite from recrystallizing and growing, refine grains and improve the tensile strength of steel. Ti, V, N, C and other elements can form a fine and dispersed C, N compound second phase in the quenching process, so that the growth of original austenite grains is effectively controlled, and the toughness of the steel plate is further remarkably improved. Ti and V, N elements are separated out near a welding pool and a heat affected zone to form a fine and dispersed second phase of an N-compound, thereby organizing the growth of austenite grains in a welding seam and the heat affected zone and improving the large heat input welding performance of the steel plate. The reasonable design of Ti, V and N contents can reduce the N content dissolved in the matrix and improve the comprehensive performance of the steel plate, so that the Ti content in the invention is 0.005-0.01%.

(12) The N element can form extremely stable nitride under the combined action with elements such as Ti, V and the like in the steel, and the welding and mechanical properties of the steel plate are improved. The content of N in the invention is 0.005% -0.009%.

(13) The element B can improve the hardenability of the steel plate, a trace amount of the element B can obviously improve the hardenability, and the brittleness of the steel plate is increased and the welding crack tendency is increased when the element B is excessive, so the element B is controlled to be 0.0005-0.001 percent.

A manufacturing method of a large-thickness low-yield-ratio EH690 marine steel plate with excellent welding performance adopts high-cleanliness and alloying smelting, low-temperature heating, controlled rolling and heat treatment (including high-temperature quenching, critical zone sub-temperature quenching and tempering), the yield strength of the produced steel plate is more than or equal to 690MPa, the tensile strength is 770-940 MPa, the Charpy impact energy single value at minus 40 ℃ is more than or equal to 120J, and the yield ratio is less than or equal to 0.93. The method is suitable for large heat input welding with the maximum welding heat input of 100 KJ/cm. The maximum thickness of the EH690 maritime work steel plate finished product with large thickness and low yield ratio is 80 mm. The method specifically comprises the following steps:

(1) high cleanliness and alloying smelting

And refining the molten steel by a converter, an LF furnace and an RH or VD furnace to further reduce P, S and the content of nonmetallic inclusions. The obtained weight percentage composition is as follows: 0.06 to 0.09 percent of C, 0.05 to 0.1 percent of Si, 1.2 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.01 to 0.03 percent of Als, 0.8 to 1.5 percent of Ni, 0.3 to 0.7 percent of Cr, 0.3 to 0.5 percent of Mo, 0.1 to 0.3 percent of Cu, 0.03 to 0.05 percent of V, 0.005 to 0.01 percent of Ti, 0.005 to 0.009 percent of N, 0.0005 to 0.001 percent of B, and the balance of Fe and inevitable impurities. And (3) protecting and casting the whole process, wherein the superheat degree of a tundish is 15-30 ℃, the withdrawal speed of a continuous casting billet is less than or equal to 1.2m/min, and the billet is inserted into a slow cooling pit and is buckled with a heat insulation cover to be slowly cooled to room temperature.

(2) Rolling process

And (3) loading the casting blank into a heating furnace at the furnace temperature of 600-800 ℃, so that the inner temperature and the outer temperature of the steel blank are kept consistent in a low-temperature stage, and preparation is made for uniform structure of a high-temperature stage. After the temperature of the casting blank is uniform, the heating rate is controlled to be 5-8 ℃/min, and the condition that the interior of the steel blank is heated unevenly due to the fact that the steel blank is heated too fast is avoided. The soaking temperature is 1000-1100 ℃, the heat preservation time is 90-120 min, and the purpose of soaking and heat preservation at low temperature is to avoid abnormal growth of as-cast structure while ensuring complete austenitization.

The initial rolling temperature of the first stage is 1000-1100 ℃, the initial rolling temperature of the second stage is 800-850 ℃, the accumulated reduction rate of the second stage rolling is 40-50%, the average reduction rate of a single pass is more than or equal to 13%, and the final rolling temperature is 750-800 ℃. The high-temperature hot rolling at 1000-1100 ℃ aims to improve the cast structure of a slab, reduce the thickness of a billet to be heated and shorten the time of the steel plate to be heated. The purpose of the design of the second-stage rolling process is to increase deformation accumulation in a recrystallization temperature region near the Ac3 temperature so as to promote the flattening and the refining of austenite grains. Pass rolling reduction is strictly controlled in the steel plate rolling process, and organization preparation is made for hardening and tempering.

(3) Heat treatment process

The steel plate heat treatment process comprises high-temperature quenching, critical zone sub-temperature quenching and tempering, wherein the quenching temperature of the steel plate is 850-920 ℃, and the heat preservation time is 1.0-1.5 min/mm. The critical zone is subjected to sub-temperature quenching at 700-780 ℃, and the heat preservation time is 1.0-1.5 min/mm. The tempering temperature is 400-580 ℃, and the heat preservation time is 2.5-3.5 min/mm. By adopting high-temperature quenching, critical zone sub-temperature quenching and medium-low temperature tempering, more refined effective grain size can be obtained, the number of large-angle grain boundaries is increased, and simultaneously, the soft and hard phase double-phase structure of bainite ferrite and tempered martensite can be obtained by combining Cu and Ni elements, so that the yield ratio of the steel plate is reduced, and the toughness is greatly improved.

Has the advantages that:

compared with the prior art, the invention has the following beneficial effects:

(1) by combining the component design of Ni, Cr, Cu, V and Ti alloys and the key production technology of the large-thickness low-yield-ratio steel plate, austenite can be refined by carrying out recrystallization rolling on austenite in a critical temperature range, and preparation is made for final structure refinement. And carrying out secondary quenching in a complete austenitizing interval and a two-phase region, and carrying out low-temperature tempering to form a microstructure in which bainite ferrite and a tempered martensite soft-hard phase are combined, wherein the tempered martensite content is 60-80% by volume percentage, and the balance is bainite ferrite, so that the yield ratio of the large-thickness EH690 ultrahigh-strength steel plate is less than or equal to 0.93.

(2) The effect of Ti, V and N elements precipitation in the weld pool and the vicinity of the heat affected zone can realize the high heat input welding of 100KJ/cm steel plate.

(3) The innovative alloy component system can ensure that the yield strength of the quenched and tempered steel plate is more than or equal to 690MPa, the tensile strength is 770-940 MPa, and the Charpy impact energy single value at-40 ℃ is more than or equal to 120J.

(4) By utilizing the component design of Ni, Cr, Cu, V and Ti alloys and the key production technology of the steel plate with large thickness and low yield ratio, the ultrahigh-strength EH690 steel plate with the maximum thickness of 80mm can be produced.

Drawings

FIG. 1 shows the metallographic structure of the quenched and tempered steel sheet at 1/4 and the bainitic ferrite + tempered martensite at 1/4 in example 1;

Detailed Description

The following examples are intended to illustrate the invention in detail, and are intended to be a general description of the invention, and not to limit the invention.

The chemical components of the steel of the embodiment of the invention are shown in table 1, the heating and rolling process of the steel casting blank of the embodiment of the invention is shown in table 2, the quenching and tempering process of the steel of the embodiment of the invention is shown in table 3, and the mechanical properties of the steel plate of the embodiment of the invention are shown in table 4.

TABLE 1 chemical composition wt% of steel of examples of the invention

Examples	C	Si	Mn	P	S	Als	Ni	Cr	Mo	Cu	V	Ti	N	B
															1	0.064	0.06	1.33	0.01	0.01	0.024	1.37	0.33	0.42	0.11	0.034	0.011	0.0060	0.0006
2	0.062	0.08	1.45	0.01	0.007	0.021	1.12	0.47	0.44	0.27	0.045	0.012	0.0058	0.0009
															3	0.073	0.09	1.47	0.01	0.006	0.012	0.89	0.58	0.38	0.29	0.033	0.014	0.0054	0.001
4	0.082	0.07	1.38	0.02	0.01	0.017	0.93	0.69	0.47	0.22	0.031	0.015	0.0063	0.0005
															5	0.078	0.05	1.26	0.02	0.009	0.018	1.31	0.62	0.49	0.24	0.038	0.012	0.0088	0.0007
6	0.076	0.1	1.36	0.02	0.009	0.022	1.49	0.44	0.33	0.14	0.049	0.011	0.0051	0.0008
															7	0.087	0.06	1.21	0.01	0.004	0.021	1.43	0.37	0.31	0.21	0.05	0.006	0.0056	0.0006
8	0.089	0.09	1.24	0.02	0.004	0.029	1.07	0.31	0.36	0.17	0.032	0.008	0.0066	0.0005
															9	0.061	0.08	1.39	0.01	0.01	0.019	1.01	0.49	0.46	0.19	0.041	0.009	0.0075	0.001
10	0.069	0.07	1.28	0.02	0.006	0.023	1.18	0.54	0.41	0.26	0.046	0.013	0.0081	0.0008
															11	0.066	0.06	1.49	0.02	0.007	0.02	0.81	0.51	0.39	0.16	0.037	0.01	0.0086	0.0009
12	0.088	0.08	1.37	0.01	0.007	0.02	1.26	0.38	0.35	0.13	0.047	0.014	0.0084	0.0006

TABLE 2 Steel continuous casting, billet heating and rolling process of the examples of the present invention

TABLE 3 Heat treatment Process for steels of examples of the invention

TABLE 4 mechanical Properties of Steel sheets according to examples of the present invention

TABLE 5 high heat input weldability of steel plates according to the examples of the present invention

Examples	Welding method	Welding line energy (KJ/cm)	Rm(MPa)	Average work of impact (J) at-40 DEG C
					1	Submerged arc welding	100	831	87
2	Submerged arc welding	100	852	98
					3	Submerged arc welding	100	879	91
4	Electro-gas welding	100	830	90
					5	Electro-gas welding	100	857	92
6	Electro-gas welding	100	871	95
					7	Submerged arc welding	100	859	79
8	Submerged arc welding	100	818	70
					9	Submerged arc welding	100	860	86
10	Electro-gas welding	100	869	78
					11	Electro-gas welding	100	875	74
12	Electro-gas welding	100	881	94

As can be seen from tables 1-4, the ocean engineering steel produced by the technical scheme of the invention has the yield strength of more than or equal to 690MPa, the tensile strength of 770-940 MPa, the Charpy impact energy single value of more than or equal to 120J at minus 40 ℃, the yield ratio of less than or equal to 0.93, and can realize the large heat input welding of a steel plate with the maximum of 100 KJ/cm.

Claims

1. The EH690 steel plate with the low yield ratio and the excellent welding performance is characterized in that the steel comprises the following chemical components in percentage by mass: 0.06 to 0.09 percent of C, 0.05 to 0.1 percent of Si, 1.2 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.01 to 0.029 percent of Als, 0.8 to 1.5 percent of Ni, 0.3 to 0.58 percent of Cr, 0.35 to 0.46 percent of Mo, 0.1 to 0.19 percent of Cu, 0.03 to 0.05 percent of V, 0.005 to 0.01 percent of Ti, 0.005 to 0.009 percent of N, 0.0005 to 0.001 percent of B, and the balance of Fe and inevitable impurities;

the structure of the finished steel plate is bainite ferrite and tempered martensite, the content of the tempered martensite is 60-80% by volume percent, and the balance is bainite ferrite;

the manufacturing method of the EH690 steel plate with excellent welding performance and low yield ratio comprises the following production processes: smelting, continuous casting, heating, rolling and heat treatment,

(1) smelting and continuous casting

Refining the molten steel by a converter-LF furnace-RH/VD furnace; protecting casting in the whole continuous casting process, wherein the superheat degree of tundish molten steel is 15-30 ℃, the continuous casting blank drawing speed is less than or equal to 1.2m/min, and a steel billet is inserted into a slow cooling pit and is slowly cooled to room temperature by buckling a heat insulation cover;

(2) heating and rolling

Putting a casting blank into a heating furnace at the furnace temperature of 600-800 ℃, controlling the heating rate to be 5-8 ℃/min after the temperature of the casting blank is uniform, controlling the soaking temperature to be 1000-1100 ℃ and controlling the heat preservation time to be 90-120 min; the initial rolling temperature of the first stage is 1000-1100 ℃, the initial rolling temperature of the second stage is 800-850 ℃, the accumulated reduction rate of the second stage rolling is 40-50%, the average reduction rate of a single pass is more than or equal to 13%, and the final rolling temperature is 750-760 ℃;

(3) thermal treatment

The steel plate heat treatment process comprises high-temperature quenching, critical zone sub-temperature quenching and tempering, wherein the high-temperature quenching temperature is 850-920 ℃, and the heat preservation time is 1.0-1.5 min/mm; the sub-temperature quenching temperature of the critical zone is 700-780 ℃, and the heat preservation time is 1.0-1.5 min/mm; the tempering temperature is 400-580 ℃, and the heat preservation time is 2.5-3.5 min/mm.

2. The EH690 steel sheet with a low yield ratio excellent in weldability according to claim 1, wherein the steel sheet has a yield strength of 690MPa or more, a tensile strength of 770 to 940MPa, a Charpy impact energy single value of 120J or more at-40 ℃, a yield ratio of 0.93 or less, and a welding heat input of 100KJ/cm at maximum.

3. The EH690 steel sheet with low yield ratio excellent in weldability according to claim 1 or 2 wherein the maximum thickness of the finished steel sheet is 80 mm.