CN112251672A - Low yield-strength ratio EH690 steel plate with excellent weldability and its manufacturing method - Google Patents

Abstract

The invention discloses a low yield ratio EH690 steel plate with excellent welding performance and a manufacturing method thereof. Steel contains: C 0.06%～0.09%, Si 0.05%～0.1%, Mn 1.2%～1.5%, P≤0.02%, S≤0.01%, Als 0.01%～0.03%, Ni 0.8%～1.5%, Cr 0.3%～0.7%, Mo 0.3%～0.5%, Cu 0.1%～0.3%, V 0.03%～0.05%, Ti 0.005%～0.01%, N 0.005%～0.009%, B 0.0005%～0.001%, balance for iron and inevitable impurities. The first stage rolling temperature is 1000～1100℃, the second stage rolling temperature is 800～850℃, the second stage rolling cumulative reduction rate is 40%～50%, and the single pass average reduction rate is ≥13%. Finish rolling temperature 750～800 ℃. The steel plate is subjected to high temperature quenching, low temperature quenching and tempering, the yield ratio of the steel plate is less than or equal to 0.93, and the maximum welding heat input is 100KJ/cm.

Description

Low yield-strength ratio EH690 steel plate with excellent weldability and its manufacturing method

technical field

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

Background technique

The 21st century is the century of the ocean. With the development of science and technology and the improvement of people's living standards, all countries in the world have focused their attention on the huge resources contained in the ocean. In recent years, the continuous and rapid development of the offshore equipment industry has also promoted the large demand for steel for offshore platforms and the upgrading of products. The market urgently needs ultra-high-strength and extra-thick offshore steel plates with good comprehensive properties.

The service environment of the offshore engineering platform is harsh. In addition to the conventional force, various factors such as strong wind, surge, tide, ice impact, earthquake, etc. must be considered, which determines the particularity of the steel used for the offshore platform. In terms of material selection, it must be able to adapt to various sea conditions. At the same time, the steel plate has been in a humid and high-salinity marine environment for a long time, and the adhesion of moist air, seawater and marine organisms will cause problems such as peeling of paint film, surface corrosion of the steel plate, corrosion fatigue and other problems, which will reduce the mechanical properties of the steel plate and shorten the service life. Affect the normal use of offshore engineering platforms. In addition, the offshore platform is far from the coast and cannot be regularly docked for repair and maintenance like a ship. In order to enable offshore engineering platforms to be used safely in complex environments, it is urgent to develop a high-quality ultra-high-strength steel for offshore engineering with excellent comprehensive properties. Yield-to-tensile ratio, high ductility, fatigue resistance, hydrogen-induced crack resistance, marine environment corrosion resistance, marine biological adhesion resistance, excellent welding performance, etc.

At present, the steel for marine engineering can meet most of the market needs in the field of marine engineering, but the special steel with high strength and excellent comprehensive properties is still the development goal of all countries in the world. The EH690 steel plate with large thickness and low yield ratio is difficult to scientific research. High, strict production process, high equipment requirements, and difficult development.

The patent "700MPa Grade High Toughness Low Yield Ratio Thick Steel Plate and Its Manufacturing Method" with the announcement number of CN100430507C proposes a low yield ratio ultra-high strength steel plate. The chemical composition of the invention steel plate is low, and it is impossible to produce large thickness and ultra-high strength. The steel plate has high Cu element but no Ni element in the steel, which will cause serious hot cracks that cannot be solved. At the same time, the TMCP process will generate serious internal stress inside the ultra-high strength steel plate. Severe warpage deformation occurs during thermal welding.

Patent Publication No. CN109536850A "A High-Strength-Toughness Low-Yield-Ratio Thick Steel Plate and Its Production Process" proposes a low-yield-ratio thick steel plate with a yield strength of 800 MPa and its manufacturing method, and its Ni content is 4.0% to 6.0% , the economy is poor, far beyond the scope of use of the alloy content of ships and marine engineering steel.

Patent Publication No. CN110846577A "690MPa Grade High Strength Low Yield Ratio Medium Manganese Steel and Manufacturing Method" proposes a 690MPa grade medium manganese steel with low yield ratio and its manufacturing method. The steel plate contains 4.1% ～4.7% of Mn element, the addition of a large amount of Mn element will bring great difficulties to the steelmaking and continuous casting process, and continuous casting production is very easy to cause accidents, and although medium manganese steel has high low temperature toughness, the rolled steel plate is extremely difficult. Problems such as unqualified flaw detection and corner cracks are prone to occur.

Patent Publication No. CN109983146A "Low-yield-ratio ultra-high strength steel and its manufacturing method" proposes a low-yield-ratio ultra-high-strength steel bar and a manufacturing method, which can only be used to produce steel bars of specific specifications, and cannot be used. It is used to produce wide and thick steel plates for ships and marine engineering, and the structure of the steel plate produced by this method is bainitic ferrite and a small amount of M-A islands. It is difficult for ultra-high-strength steels with this microstructure to ensure low temperature toughness. The examples show the impact temperature of the steel Only -5°C.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art, and to prepare a low yield ratio EH690 ultra-high strength steel plate with excellent welding performance suitable for the field of marine engineering. /cm large line energy welding, the service safety performance can reach the service conditions of marine engineering equipment. A set of specific low yield ratio EH690 ultra-high strength marine steel plates with excellent welding performance and corresponding production processes are formed.

In order to achieve the purpose of the present invention, the inventors have conducted a large number of systematic experimental studies through the selection and ratio of alloying elements, steel cleanliness control, high-efficiency rolling process optimization and parameter selection, etc. Purpose Alloying element ratio and rolling heat treatment process: The specific technical scheme is:

A large-thickness and low-yield-strength ratio EH690 marine steel plate with excellent welding performance, in terms of weight percentage, the steel contains: C 0.06%-0.09%, Si 0.05%-0.1%, Mn 1.2%-1.5%, P≤0.02 %, S≤0.01%, Als0.01%～0.03%, Ni 0.8%～1.5%, Cr 0.3%～0.7%, Mo 0.3%～0.5%, Cu 0.1%～0.3%, V 0.03%～0.05%, Ti 0.005% to 0.01%, N 0.005% to 0.009%, B 0.0005% to 0.001%, and the rest are Fe and inevitable impurities.

The design reasons for the chemical composition of steel grades are as follows:

(1) C element in steel can improve the strength through interstitial solid solution strengthening, and is the main element to ensure the strength in the solution of the present invention. At the same time, C element can improve the hardenability of the steel plate; too high C content affects the toughness and yield strength of the steel plate. In the welding process, a large amount of hardened structure is produced, which causes welding cracks. Therefore, the present invention precisely controls the content of C element in the steel, and controls the content of C within 0.06% to 0.09%.

(2) Si can improve the strength of the steel plate, and at the same time, as a deoxidizer, Si can reduce the O content. When the Si content is high, the structure will be coarsened, and the toughness, plasticity and yield-strength ratio will be significantly reduced. Therefore, the Si content in the present invention is 0.05%. ~0.1%.

(3) Mn atoms are similar to Fe atomic radius, and can be replaced by a large amount of solid solution in the Fe matrix to improve the strength of the steel plate. Due to the low C content of the steel plate, more Mn elements are needed to improve the strength of the steel plate. However, when the mass percentage of Mn elements is greater than 1.5%, the segregation of Mn elements will reduce the low-temperature toughness of the core of the thick plate and increase the yield ratio. , the performance of the welding heat affected zone deteriorates, so the Mn content in the present invention is 1.2% to 1.5%.

(4) P and S elements are not beneficial to the mechanical properties and welding properties of the steel plate. Considering the cost factor comprehensively, the present invention controls P and S to be P≤0.02% and S≤0.01%.

(5) Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidizing effect is not good, and micro-alloying elements such as Ti cannot achieve the purpose of refining grains and improving welding performance due to oxidation; on the contrary, the Al element is too high. Then, large inclusions are formed, so the Als content in the present invention is 0.01% to 0.03%.

(6) Ni can improve the low temperature toughness of the steel plate, and it is also an alloying element that can improve the yield ratio of the steel plate. Ni can reduce the sensitivity of the steel plate to notches, and the addition of Ni element can obtain a lower ductile-brittle transition temperature. Therefore, the Ni content in the present invention is 0.8% to 1.5%.

(7) Cr is an element that improves the hardenability and tensile strength of steel sheets. In the case of low C content, adding an appropriate amount of Cr element can improve the tensile strength of the steel plate and ensure that the steel plate reaches the required yield ratio, but adding excessive Cr element in the steel will reduce the toughness and weldability of the material. Therefore, in the present invention, the Cr content is 0.3% to 0.7%.

(8) The addition of Mo element can improve the hardenability of the steel sheet, and the addition of an appropriate amount of Mo element can also improve the temper brittleness of the steel sheet, but the addition of excessive Mo element is unfavorable to the yield ratio of the steel sheet, so the Mo content in the present invention is 0.3%～0.5%.

(9) The addition of Cu element can improve the strength and toughness of the steel plate, and can effectively form nano-phase precipitation in combination with the Ni element, improve the tensile strength of the steel plate, and reduce the yield ratio, but excessive addition will increase the yield ratio of the steel plate, and Produces Cu embrittlement phenomenon. Therefore, the Cu content in the present invention is 0.1% to 0.3%.

(10) V element can form V(C,N) particles in the matrix, which can refine and strengthen the grains. Working together with Ti and N elements, it can significantly improve the strength, toughness and welding performance of steel plates. The grain refinement effect of V element in low-carbon alloy steel can reduce the yield-strength ratio of the steel plate after heat treatment. Therefore, the V content in the present invention is 0.03% to 0.05%.

(11) Ti element is the key factor of the chemical composition of the present invention. Ti can produce strong precipitation strengthening effect, prevent the recrystallization of austenite from growing, and improve the tensile strength of steel by grain refinement. Elements such as Ti and V, N, and C can form fine and dispersed second phases of C and N compounds during the quenching process, which can effectively control the growth of the original austenite grains, thereby significantly improving the strength and toughness of the steel plate. Ti, V, and N elements are precipitated near the welding pool and the heat-affected zone to form a fine and dispersed N compound second phase, which can organize the growth of austenite grains in the weld and heat-affected zone, and improve the welding performance of the steel plate at high line energy. . Reasonable design of the content of Ti, V and N can reduce the content of N in the solid solution in the matrix and improve the comprehensive performance of the steel plate. Therefore, the content of Ti in the present invention is 0.005% to 0.01%.

(12) N element can act together with Ti, V and other elements in steel to form extremely stable nitrides, which can improve the welding and mechanical properties of steel plates. The N content in the present invention is 0.005% to 0.009%.

(13) B element can improve the hardenability of the steel plate, and a small amount of B element can significantly improve the hardenability effect. When the B element is excessive, the brittleness of the steel plate increases and the tendency of welding cracks increases. Therefore, the present invention controls the B element to 0.0005 %～0.001%.

A manufacturing method of EH690 marine steel plate with large thickness and low yield strength ratio with excellent welding performance, which adopts high cleanliness and alloying smelting + low temperature heating + controlled rolling + heat treatment (including high temperature quenching + sub-temperature quenching in critical zone + tempering) ), the yield strength of the produced steel plate is ≥690MPa, the tensile strength is 770～940MPa, the single value of Charpy impact energy at -40℃ is ≥120J, and the yield-strength ratio is ≤0.93. It is suitable for high-energy welding with a maximum welding heat input of 100KJ/cm. The maximum thickness of the EH690 marine steel plate with large thickness and low yield ratio is 80mm. Specifically include the following steps:

(1) High cleanliness and alloying smelting

The molten steel is refined through a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions. The obtained weight percentage composition is: C 0.06%-0.09%, Si 0.05%-0.1%, Mn 1.2%-1.5%, P≤0.02%, S≤0.01%, Als 0.01%-0.03%, Ni 0.8%-1.5% , Cr 0.3%～0.7%, Mo 0.3%～0.5%, Cu 0.1%～0.3%, V 0.03%～0.05%, Ti 0.005%～0.01%, N 0.005%～0.009%, B0.0005%～0.001% , and the rest are Fe and inevitable impurities. The whole process protects the casting, the superheat of the tundish is 15~30℃, the continuous casting billet drawing rate is less than or equal to 1.2m/min, and the billet goes into the slow cooling pit and the heat shield is slowly cooled to room temperature.

(2) rolling process

The billet is loaded into the heating furnace at a furnace temperature of 600-800 ℃, the purpose is to keep the internal and external temperature of the billet consistent in the low temperature stage, and prepare for the uniform structure of the high temperature section. After the temperature of the slab is uniform, the heating rate is controlled at 5-8°C/min, so as to avoid the uneven heating inside the slab due to the excessive heating of the slab. The soaking temperature is 1000~1100℃, and the holding time is 90~120min. The purpose of low temperature soaking and heat preservation is to ensure complete austenitization and avoid abnormal growth of the as-cast structure.

The rolling temperature of the first stage is 1000～1100℃, the rolling temperature of the second stage is 800～850℃, the cumulative rolling reduction rate of the second stage is 40%～50%, and the average rolling reduction rate of a single pass is ≥13%. Finish rolling temperature is 750～800 ℃. The purpose of high-temperature hot rolling at 1000-1100°C is to improve the as-cast structure of the slab, reduce the thickness of the slab to be warmed, and shorten the warm-up time of the steel plate. The purpose of the second-stage rolling process design is to increase the deformation accumulation in the recrystallization temperature region near the Ac3 temperature and promote the flattening and refinement of the austenite grains. During the steel plate rolling process, the pass reduction should be strictly controlled to prepare for the quenching and tempering treatment.

(3) Heat treatment process

The heat treatment process of the steel plate includes high temperature quenching, sub-temperature quenching and tempering in the critical zone. The sub-temperature quenching in the critical zone is 700～780℃, and the holding time is 1.0～1.5min/mm. The tempering temperature is 400～580℃, and the holding time is 2.5～3.5min/mm. By using high temperature quenching, sub-temperature quenching in critical zone and medium and low temperature tempering, a more refined effective grain size can be obtained, the number of large-angle grain boundaries can be increased, and bainitic ferrite + tempering can be obtained by combining Cu and Ni elements The soft-hard phase dual-phase structure of martensite reduces the yield ratio of the steel plate and greatly improves the toughness.

Beneficial effects:

Compared with the prior art, the present invention has the following beneficial effects:

(1) Combined with the composition design of Ni, Cr, Cu, V and Ti alloys and the key production technology of large-thickness and low-yield ratio steel plates, the austenite can be refined by recrystallization rolling of austenite in the critical temperature range. body tissue in preparation for final tissue refinement. Secondary quenching in the fully austenitized region and the two-phase region, plus low-temperature tempering, forms a microstructure with a combination of bainitic ferrite and tempered martensite soft and hard phases, where, by volume percentage, The content of tempered martensite is 60% to 80%, and the rest is bainitic ferrite, which can make the yield ratio of large thickness EH690 ultra-high strength steel plate ≤ 0.93.

(2) With the help of the precipitation effect of Ti, V, and N elements near the welding pool and the heat-affected zone, the large line energy welding of steel plates of 100KJ/cm can be realized.

(3) The innovative alloy composition system of the present invention can ensure that the yield strength of the steel plate after quenching and tempering treatment is greater than or equal to 690MPa, the tensile strength is 770-940MPa, and the single value of Charpy impact energy at -40°C is greater than or equal to 120J.

(4) Utilizing the composition design of Ni, Cr, Cu, V, and Ti alloys and the key production technology of large-thickness and low-yield-strength steel plates, ultra-high-strength EH690 steel plates with a maximum thickness of 80mm can be produced.

Description of drawings

Figure 1 shows the quenched and tempered metallographic structure at 1/4 of the thickness of the steel plate in Example 1, and the structure at 1/4 of the thickness of the steel plate is bainitic ferrite + tempered martensite;

Detailed ways

The following embodiments are used to specifically illustrate the content of the present invention, and these embodiments are only general descriptions of the content of the present invention, and do not limit the content of the present invention.

The chemical composition of the steel of the embodiment of the present invention is shown in Table 1, the heating and rolling process of the steel slab of the embodiment of the present invention is shown in Table 2, the quenching and tempering treatment process of the steel of the embodiment of the present invention is shown in Table 3, and the mechanical properties of the steel plate of the embodiment of the present invention are shown in the table. 4.

Table 1 The chemical composition wt% of the steel according to the embodiment of the present invention

Example 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, slab heating and rolling process according to the embodiment of the present invention

Table 3 steel heat treatment process of the embodiment of the present invention

Table 4 Mechanical properties of steel plates according to embodiments of the present invention

Table 5 High-energy welding performance of steel plates according to the present invention

Example Welding method Welding line energy (KJ/cm) Rm(MPa) -40℃ average impact energy (J) 1 Submerged arc welding 100 831 87 2 Submerged arc welding 100 852 98 3 Submerged arc welding 100 879 91 4 Gas-electric vertical welding 100 830 90 5 Gas-electric vertical welding 100 857 92 6 Gas-electric vertical 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 Gas-electric vertical welding 100 869 78 11 Gas-electric vertical welding 100 875 74 12 Gas-electric vertical welding 100 881 94

It can be seen from Tables 1 to 4 that the steel for marine engineering produced by the technical scheme of the present invention has a yield strength of ≥690 MPa, a tensile strength of 770 to 940 MPa, a single value of Charpy impact energy at -40°C ≥ 120 J, and a yield-to-strength ratio of ≤ 0.93. It can realize high-energy welding of steel plates up to 100KJ/cm.

Claims (6)

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 0.03 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.

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, having a low yield ratio and excellent weldability as claimed in claim 1 or 2 wherein the maximum thickness of the finished steel sheet is 80 mm.

4. The EH690 steel sheet with a low yield ratio excellent in weldability according to claim 1 or 2, wherein the structure of the finished steel sheet is bainitic ferrite + tempered martensite, and the tempered martensite content is 60% to 80% by volume, and the balance is bainitic ferrite.

5. The EH690 steel sheet with a low yield ratio and excellent weldability according to claim 3, wherein the structure of the finished steel sheet is bainitic ferrite + tempered martensite, the content of the tempered martensite is 60% to 80% by volume percentage, and the balance is bainitic ferrite.

6. A method for manufacturing the EH690 steel sheet with a low yield ratio excellent in weldability according to any one of claims 1 to 5, the production process of the steel sheet is as follows: smelting, continuous casting, heating, rolling and heat treatment, which is characterized in that,

(1) smelting and continuous casting

Refining the molten steel by a converter, an LF furnace and an RH or 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 750-800 ℃;

(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.

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