Production method of low-alloy steel thick plate with good machinability

Abstract

The invention provides a production method of a low-alloy steel thick plate with good machinability. Steel comprises the following chemical components in percentage by mass: 0.10%-0.13% of C, 0.40%-0.50% of Si, 1.20%-1.60% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.06%-0.08% of Al, 0.04%-0.05% of V, 0.0040%-0.0070% of N and the balance of Fe and inevitable impurities. The carbon equivalent CEV of the steel is smaller than or equal to 0.43%, and the welding crack sensitivity index Pcm is smaller than or equal to 0.22%. The thickness of the steel plate ranges from 40 mm to 100 mm. By adopting the smelting, continuous casting, heating, controlled rolling and controlled cooling processes, the prepared low-alloy steel thick plate has good comprehensive mechanical properties: the yield strength is greater than or equal to 355MPa, the tensile strength is greater than or equal to 470MPa, the percentage elongation after fracture is greater than or equal to 25%, the impact absorbing energy KV2 at-20 DEG C is greater than or equal to 27J, and the surface hardness is less than or equal to 200HB. The steel plate is low in alloy cost, simple in production process, good in plate shape and low in residual stress.

Description

Technical Field

[0001] The invention belongs to the field of steel material preparation, and in particular relates to a production method for a low-alloy steel thick plate with good processability. Background Art

[0002] Low alloy high strength steel plates have good comprehensive mechanical properties and are widely used in steel structure fields such as buildings, bridges, towers, and engineering machinery. Usually, the manufacturing and processing of steel structures requires cutting, welding, drilling, bending, and straightening of steel plates, so low alloy high strength steel plates must not only have high strength and toughness, but also have good weldability and processing performance. The national standard GB / T 1591 has clear strength and impact toughness requirements for low alloy high strength structural steels, as well as provisions for carbon equivalent CEV and welding crack sensitivity index Pcm, but does not involve the hardness of steel, and the hardness of the material is also critical to the processing and use of the material. Therefore, how to control the low hardness of low alloy high strength steel plates while ensuring their strength, toughness and weldability has become a challenge for low alloy high strength structural steels. Summary of the invention

[0003] The present invention aims to provide a production method of a low alloy steel thick plate with good processability. By designing a composition of low C, low CEV and Pcm, and adding relatively high Si, Al, V and the like, and adopting reasonable smelting, continuous casting, heating, controlled rolling and controlled cooling processes, a 355MPa grade low alloy high strength steel thick plate with excellent processability and a surface hardness of the steel plate not higher than 200HB is prepared, and the thickness of the steel plate is 40-100mm.

[0004] The technical solution of the present invention: A method for producing a low-alloy steel thick plate with good processability, wherein the chemical composition of the steel is C=0.10%~0.13%, Si=0.40%~0.50%, Mn=1.20%~1.60%, P≤0.015%, S≤0.005%, Al=0.06~0.08%, V=0.04%~0.05%, N=0.0040%~0.0070%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.43%, welding crack sensitivity index Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B≤0.22%, and the remainder is Fe and unavoidable impurities; the thickness of the produced steel plate is 40~100 mm; and the key process steps include: 1) Smelting: Use converter + LF refining + RH vacuum treatment; control Al=0.01%~0.03% and O≤15ppm at the end of refining, then carry out calcium treatment and soft blowing, the soft blowing time is ≥5min; after vacuum treatment, the molten steel is sampled to detect the Al content, and aluminum wire is fed to increase aluminum so that the final Al content meets the target value, and then stand and bottom blow nitrogen soft blowing, control the stand + soft blowing time ≥12min; 2) Continuous casting: adopt full-process protection pouring; use weak cooling for secondary cooling, and control the surface temperature of the billet ≥850℃ during straightening; 3) Heating: directly load the ingot into the heating furnace for heating, and control the inlet temperature to ≥750℃; control the outlet temperature to 1100~1120℃, and the soaking time to ≥60min; 4) Controlled rolling: Control the total compression ratio to ≥3.5, with at least one pass reduction ratio ≥20% above 1000℃ and a rolling compression ratio ≥1+80 / below 880℃ t ,in t The thickness of the steel plate is in mm; the final rolling temperature is controlled at 780~820℃; 5) Controlled cooling: After rolling, the steel plate is air-cooled and straightened, and then hoisted to the cooling pit for slow cooling. The temperature of the steel plate before cooling is controlled to be ≥700℃.

[0005] The yield strength of the low alloy steel thick plate produced by the above method is ≥355MPa, the tensile strength is ≥470MPa, the elongation after fracture is ≥25%, the impact absorption energy KV2 at -20℃ is ≥27J, and the surface hardness is ≤200HB.

[0006] The principle of the present invention is as follows: a low C component design is adopted to effectively reduce the carbon equivalent CEV and the welding crack sensitivity index Pcm, so that the steel has good weldability and weldability. At the same time, low C combined with air cooling (slow cooling) generates a larger proportion of soft phase ferrite, thereby improving the plasticity and toughness of the steel. To compensate for the strength loss, the present invention adds more elements such as Si, Al, and V. Si has a good solid solution strengthening effect, and the precipitation of AlN and VC plays a precipitation strengthening role. At the same time, AlN has the effect of refining austenite grains, and the interphase precipitation of VC can also refine ferrite grains. In terms of technology, in order to improve the beneficial effect of AlN, the molten steel is treated with calcium after deoxidation to modify Al2O3 inclusions, and then aluminum wire is fed after vacuum treatment to increase the acid-soluble aluminum content; at the same time, during continuous casting and straightening, in order to avoid a large amount of AlN precipitation causing a significant decrease in the high-temperature plasticity of the steel and the generation of surface cracks, the surface temperature of the ingot during straightening is controlled to be ≥850°C; the present invention adopts direct loading to avoid the precipitation of AlN and VC on the ferrite and the appearance of a ferrite film after austenitization, and then adopts a lower heating temperature and a longer soaking time to control the subsequent rolling process at a lower temperature as a whole, especially A larger rolling reduction ratio is set below 880°C, which is very beneficial for refining the grain size throughout the thickness. At the same time, the temperature range of 780~880°C is just the range near the nose point temperature of the nucleation rate-temperature curve of AlN precipitation in austenite. Large deformation in this range can obtain the finest precipitate phase size, which is beneficial to improving the strength and toughness of steel. In addition, accumulating large deformation in the low temperature area is beneficial to the interphase precipitation of VC during subsequent air cooling and slow cooling, which also improves the strength and toughness of steel. The air cooling and slow cooling cooling methods are used to obtain low hardness structure for subsequent processing and use.

[0007] The beneficial effects of the present invention are as follows: the steel plate has good weldability and weldability by adopting the composition design of low C, low CEV and Pcm; the present invention makes full use of AlN and VC precipitates to improve the strength and toughness of steel, and even if air cooling and slow cooling are adopted, a thick plate of 40~100mm can be produced, and its yield strength is ≥355MPa, tensile strength is ≥470MPa, elongation after fracture is ≥25%, and impact absorption energy KV2 at -20℃ is ≥27J, and the surface hardness can be controlled below 200HB. It can be seen that the low alloy steel thick plate has good comprehensive mechanical properties: high strength and toughness, high plasticity, low hardness, and easy welding, so that the steel plate has good processing and use performance. In addition, the steel does not add precious metals such as Ni, Mo, Cr, Nb, Ti, etc., and the alloy cost of the steel is low. At the same time, the production process adopted is simple, and batch production can be carried out, and the shape of the steel plate is easy to control and the residual stress is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a microstructure morphology diagram of the steel plate produced in Example 1 of the present invention at 1 / 4 of the thickness. DETAILED DESCRIPTION

[0009] The present invention will be further described below in conjunction with specific embodiments.

[0010] Embodiment 1: A method for producing a low-alloy steel thick plate with good processability, wherein the chemical composition and mass percentage of the steel are C=0.12%, Si=0.46%, Mn=1.47%, P=0.010%, S=0.003%, Al=0.068%, V=0.043%, N=0.0062%, and the remainder is Fe and unavoidable impurities; the carbon equivalent CEV of the steel is 0.39%, and the welding crack sensitivity index Pcm is 0.22%; the thickness of the steel plate is 80 mm; and the key process steps include: 1) Smelting: converter + LF refining + RH vacuum treatment; at the end of refining, Al is 0.024% and O is 10ppm, then calcium treatment and soft blowing are carried out, and the soft blowing time is 6 minutes; after vacuum treatment, the molten steel is sampled and the Al content is 0.01% after vacuum treatment. At this time, aluminum wire is fed to increase aluminum, and then it is allowed to stand and bottom blow nitrogen soft blowing. The standing + soft blowing time is controlled to be 15 minutes, of which the soft blowing time is 10 minutes; 2) Continuous casting: full protection pouring is adopted; weak cooling is used for the second cooling, so that the surface temperature of the ingot is 870℃ during straightening; 3) Heating: The ingot is directly loaded into the heating furnace for heating, the furnace entry temperature is 760℃; the furnace exit temperature is 1110℃, and the soaking time is 80min; 4) Controlled rolling: The total reduction ratio of the steel plate is 3.75, of which one pass with a reduction ratio of 21.5% above 1000°C, and the rolling reduction ratio below 880°C is 2.0; the final rolling temperature is 800°C; 5) Controlled cooling: After rolling, the steel plate is air-cooled and straightened, and then hoisted to the cooling pit for slow cooling. The temperature of the steel plate before cooling is 715℃.

[0011] Observe the metallographic structure of the steel plate of Example 1. Figure 1 As shown in the figure, the steel plate at 1 / 4 thickness is mainly composed of ferrite and pearlite, with fine and uniform structure and grain size of 9~10.

[0012] The mechanical properties of the steel plate of Example 1 were tested, and the yield strength was 373 MPa, the tensile strength was 519 MPa, the elongation after fracture was 28%, the impact absorption energy KV2 at -20°C was 104 J, and the surface hardness was 174 HB.

[0013] Embodiment 2: A method for producing a low-alloy steel thick plate with good processability, wherein the chemical composition and mass percentage of the steel are C=0.11%, Si=0.42%, Mn=1.32%, P=0.012%, S=0.002%, Al=0.065%, V=0.045%, N=0.0047%, and the remainder is Fe and unavoidable impurities; the carbon equivalent CEV of the steel is 0.36%, and the welding crack sensitivity index Pcm is 0.20%; the thickness of the steel plate is 50 mm; and the key process steps include: 1) Smelting: converter + LF refining + RH vacuum treatment; at the end of refining, Al is 0.018% and O is 12ppm, then calcium treatment and soft blowing are carried out, and the soft blowing time is 5min; after vacuum treatment, the molten steel is sampled and the Al content is 0.008%, then aluminum wire is fed to increase aluminum, and then standing and bottom blowing nitrogen soft blowing are carried out, and the standing + soft blowing time is controlled to be 12min, of which the soft blowing time is 5min; 2) Continuous casting: full protection pouring is adopted; weak cooling is used for the second cooling, so that the surface temperature of the ingot is 855℃ during straightening; 3) Heating: The ingot is directly loaded into the heating furnace for heating, the furnace entry temperature is 760℃; the furnace exit temperature is 1100℃, and the soaking time is 70min; 4) Controlled rolling: The total reduction ratio of the steel plate is 5.2, of which the reduction ratios of the two passes above 1000°C are 21.3% and 23.6% respectively, and the rolling reduction ratio below 880°C is 2.8; the final rolling temperature is 790°C; 5) Controlled cooling: After rolling, the steel plate is air-cooled and straightened, and then hoisted to the cooling pit for slow cooling. The temperature of the steel plate before cooling is 700℃.

[0014] The mechanical properties of the steel plate of Example 2 were tested, and the yield strength was 395 MPa, the tensile strength was 539 MPa, the elongation after fracture was 28%, the impact absorption energy KV2 at -20°C was 208 J, and the surface hardness was 183 HB.

[0015] Embodiment 3: A method for producing a low-alloy steel thick plate with good processability, wherein the chemical composition and mass percentage of the steel are C=0.12%, Si=0.45%, Mn=1.51%, P=0.011%, S=0.003%, Al=0.072%, V=0.046%, N=0.0065%, and the remainder is Fe and unavoidable impurities; the carbon equivalent CEV of the steel is 0.40%, and the welding crack sensitivity index Pcm is 0.22%; the thickness of the steel plate is 100 mm; and the key process steps include: 1) Smelting: converter + LF refining + RH vacuum treatment; at the end of refining, Al is 0.028% and O is 9ppm, then calcium treatment and soft blowing are carried out, and the soft blowing time is 8 minutes; after vacuum treatment, the molten steel is sampled and the Al content is 0.016% after vacuum treatment. At this time, aluminum wire is fed to increase aluminum, and then it is allowed to stand and bottom blow nitrogen soft blowing. The standing + soft blowing time is controlled to be 15 minutes, of which the soft blowing time is 11 minutes; 2) Continuous casting: full protection pouring is adopted; weak cooling is used for the second cooling, so that the surface temperature of the ingot is 880℃ during straightening; 3) Heating: The ingot is directly loaded into the heating furnace for heating, the furnace entry temperature is 780℃; the furnace exit temperature is 1120℃, and the soaking time is 80min; 4) Controlled rolling: The total reduction ratio of the steel plate is 3.5, of which one pass reduction ratio is 20.4% above 1000°C, and the rolling reduction ratio is 2.0 below 880°C; the final rolling temperature is 810°C; 5) Controlled cooling: After rolling, the steel plate is air-cooled and straightened, and then hoisted to the cooling pit for slow cooling. The temperature of the steel plate before cooling is 720℃.

[0016] The mechanical properties of the steel plate of Example 3 were tested as follows: the yield strength was 362 MPa, the tensile strength was 500 MPa, the elongation after fracture was 30%, the impact absorption energy KV2 at -20°C was 90 J, and the surface hardness was 170 HB.

Claims

1. A method for producing a low alloy steel thick plate with good processability, characterized in that: The chemical composition of steel is C=0.10%~0.13%, Si=0.40%~0.50%, Mn=1.20%~1.60%, P≤0.015%, S≤0.005%, Al=0.06~0.08%, V=0.04%~0.05%, N=0.0040%~0.0070%, CEV≤0.43%, Pcm≤0.22%, and the balance is Fe and unavoidable impurities; The thickness of the produced steel plate is 40~100mm; The key process steps include: 1) Smelting: Use converter + LF refining + RH vacuum treatment; control Al=0.01%~0.03% and O≤15ppm at the end of refining, then carry out calcium treatment and soft blowing, the soft blowing time is ≥5min; after vacuum treatment, the molten steel is sampled to detect the Al content, and aluminum wire is fed to increase aluminum so that the final Al content meets the target value, and then stand and bottom blow nitrogen soft blowing, control the stand + soft blowing time ≥12min; 2) Continuous casting: adopt full-process protection pouring; use weak cooling for secondary cooling, and control the surface temperature of the billet ≥850℃ during straightening; 3) Heating: directly load the ingot into the heating furnace for heating, and control the inlet temperature to ≥750℃; control the outlet temperature to 1100~1120℃, and the soaking time to ≥60min; 4) Controlled rolling: Control the total compression ratio to ≥3.5, with at least one pass reduction ratio ≥20% above 1000℃ and a rolling compression ratio ≥1+80 / below 880℃ t ,in t The thickness of the steel plate is in mm; the final rolling temperature is controlled at 780~820℃; 5) Controlled cooling: After rolling, the steel plate is air-cooled and straightened, and then hoisted to the cooling pit for slow cooling. The temperature of the steel plate before cooling is controlled to be ≥700℃.

2. The method for producing a low alloy steel thick plate with good processability according to claim 1, characterized in that: The yield strength of the low alloy steel thick plate is ≥355MPa, the tensile strength is ≥470MPa, the elongation after fracture is ≥25%, the impact absorption energy KV2 at -20°C is ≥27J, and the surface hardness is ≤200HB.

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