A production method of a preheating-free welded NM450 wear-resistant steel plate

Abstract

A method for producing NM450 wear-resistant steel without preheating welding, wherein the chemical composition of the steel is as follows: C = 0.19%–0.20%, Si = 0.25%–0.45%, Mn = 1.10%–1.20%, P ≤ 0.012%, S ≤ 0.003%, Cr = 0.65%–0.70%, Nb = 0.015%–0.020%, Ti = 0.008%–0.016%, Al T = 0.070%~0.090%, B = 0.0014%~0.0020%, H≤0.0015%, N≤0.0050%, O≤0.0015%, the remainder being Fe and unavoidable impurities, CEV≤0.54%. The process steps include converter smelting, LF refining, RH or VD vacuum degassing, continuous casting, heating, rolling, and heat treatment. The produced NM450 wear-resistant steel plate has a thickness ranging from 6 to 20 mm, a surface hardness of 420~480 HBW, a yield strength ≥1180 MPa, a tensile strength ≥1400 MPa, an elongation ≥16%, meets the requirements for 6-year diameter 90° cold bending, and an impact resistance of -40℃ AKV ≥50J; it exhibits good strength and toughness after welding without preheating.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy and relates to a preheating-free welded NM450 wear-resistant steel plate and a production method thereof. BACKGROUND

[0002] Wear-resistant steel is widely used in various fields such as metallurgy, mining, building materials, and railways. It plays an important role in key wear-resistant parts of equipment such as mine dump trucks, coal mine scraper conveyors, bulldozers, excavators, mixers, and loaders. The service environment of wear-resistant materials is relatively harsh, so it is required to have extremely high strength, hardness, wear resistance, and low-temperature impact toughness to prolong the service life of equipment and shorten the replacement cycle of parts, greatly improving the operation time. With the birth of the design demand for lightweight and complex mechanical equipment, higher requirements are put forward for the welding forming performance in the processing and manufacturing process of wear-resistant steel, that is, the service life of the equipment using wear-resistant steel is longer, and maintenance and downtime caused by maintenance are reduced, thereby reducing capital investment. The higher the hardness of wear-resistant steel, the better the wear resistance, but it also makes the welding processing more difficult. 450HB grade wear-resistant steel belongs to a medium-high grade wear-resistant variety, and the surface Brinell hardness is 420-480HB. The content of alloy elements such as C, Cr, Mo, and Ni is relatively high in the composition design, and the carbon equivalent of the steel plate is generally above 0.50%. The problem caused thereby is that welding processing is difficult, which leads to a significant tendency of cold cracking of the material, and the softening and embrittlement of the welding heat-affected zone easily causes brittle fracture of the welded joint.

[0003] In order to avoid such problems, manufacturers usually adopt a combination of increasing the preheating temperature and strictly controlling the welding process; however, preheating before welding increases energy consumption and cost and greatly reduces the welding processing forming efficiency. Chinese patent CN114734125B discloses a preheating-free welding method suitable for 500HB wear-resistant steel. The invention is directed to a preheating-free welding method for 500HB grade wear-resistant steel plate. By designing the groove of the welded joint, 50kg grade welding wire, 80kg grade welding wire, and wear-resistant steel welding wire are used to form a gradient transition, complete the welding of wear-resistant steel, and compared with traditional wear-resistant steel welded joints, the preheating-free welding method does not require preheating before welding and post-welding heat treatment, the welding process is simpler and more efficient, the welding deformation is controllable, and the generation of welding cracks is reduced. At the same time, the use of welding wire with a gradient transition in strength is more cost-effective than the use of wear-resistant steel welding wire throughout the process. The patent focuses on the matching of preheating-free welding wire for 500HB wear-resistant steel and the welding method, and is not suitable for preheating-free welding of NM450 wear-resistant steel plates with different strength levels.

[0004] Chinese patent CN116586816A discloses a 1000MPa high-strength steel preheating-free welding arc electrode and a preparation method. The arc electrode is used for welding 1000MPa high-strength steel, and the welding joint strength is not less than 1000MPa, while ensuring that no cracks are generated during room temperature welding. The invention focuses on the 1000MPa high-strength steel preheating-free welding arc electrode and the preparation method, and cannot meet the strength level of the preheating-free welding NM450 wear-resistant steel plate. SUMMARY

[0005] The purpose of the present application is to provide a production method of preheating-free welding NM450 wear-resistant steel plate. The produced wear-resistant steel plate is of NM450 grade, with a thickness of 6-20mm, a surface hardness of 420-480HBW, a yield strength of ≥1180MPa, a tensile strength of ≥1400MPa, and an elongation of ≥16%. The produced wear-resistant steel plate meets the requirements of 6a diameter 90° cold bending, -40℃ impact AKV≥50J, and good strength and toughness after preheating-free welding.

[0006] The technical solution of the present application is as follows:

[0007] A production method of high-strength and high-toughness wear-resistant steel, the chemical composition of the steel is as follows: C=0.19%-0.20%, Si=0.25%-0.45%, Mn=1.10%-1.20%, P≤0.012%, S≤0.003%, Cr=0.65%-0.70%, Nb=0.015%-0.020%, Ti=0.008%-0.016%, Al T =0.070%-0.090%, B=0.0014%-0.0020%, H≤0.0015%, N≤0.0050%, O≤0.0015%, and the rest is Fe and unavoidable impurities, CEV≤0.54%. The key process steps include:

[0008] (1) Converter smelting: top and bottom combined blowing converter, control the tapping temperature to be 1575-1592℃, the P content of the converter tapping is ≤0.012%, and deoxidizing agent and alloy are added during the tapping process for deoxidization and alloying. The alloy is low-P alloy to avoid P rising in the molten steel;

[0009] (2) Refining: the molten steel in the ladle is sent to the LF furnace for electric heating and then the chemical composition is accurately adjusted, and then the RH furnace or VD furnace is entered for vacuum treatment. The gas content of the molten steel is measured to be H≤0.0015%, N≤0.0050%, and O≤0.0015%;

[0010] (3) Continuous casting: the overheat degree of the tundish is ≤20℃, the overheat degree of the continuous casting furnace is ≤25℃, the dynamic soft reduction or heavy reduction technology is used to improve the internal quality of the continuous casting billet, the thickness size of the continuous casting billet is 180-260mm, and the width size is 2000-2300mm;

[0011] (4) Heating: the temperature of the preheating section is 650-800℃, the temperature of the heating section is 1100-1240℃, the temperature of the soaking section is 1150-1230℃, and the time in the furnace is 180-300min;

[0012] (5) Rolling: two-stage controlled rolling is adopted, the rolling temperature of rough rolling is 1150-980℃, the cumulative compression ratio of rough rolling is ≥2.2, the thickness of the intermediate billet is 60-80mm, the rolling temperature of 12mm fine rolling is ≤960℃, the rolling temperature of 20mm fine rolling is ≤900℃, the cumulative compression ratio of fine rolling is ≥3.5, and the final rolling temperature of fine rolling is 760-830℃;

[0013] (6) Heat treatment: quenching + tempering process is adopted, the heating temperature of quenching is 880-900℃, the heating speed is 1.6-2.2min / mm, the holding time is 10-30min, the quenching cooling rate is 20-40℃ / s, the heating temperature of tempering is 200-220℃, the heating speed is 3.5-4.0min / mm, and the holding time is 10-25min.

[0014] Invention principle:

[0015] The wear-resistant steel has high strength, high hardness, low-temperature high toughness, good tensile and impact properties after preheating-free welding, and good bending processing performance. The roles and mechanisms of the main alloying elements in the chemical composition of the steel are described below.

[0016] The surface hardness range of the steel plate is ensured to hit the target requirement by controlling the C range, the hardenability of the steel plate is improved to ensure uniform and consistent microstructure by adding Mn, B, Si and Cr elements, the strength performance of the steel plate is improved by combining microstructure strengthening and alloy strengthening, the grain is refined to improve the strength and toughness performance and hardness by the Nb, Ti and Al precipitates combined with a reasonable rolling and heat treatment process, and the P, S, H, N and O of the molten steel are strictly controlled. The steel plate has excellent mechanical properties, welding performance and processing performance by the effective combination of scientific alloy element ratio and reasonable rolling and heat treatment process. The design reasons of the main added elements are as follows:

[0017] The carbon element can effectively improve the strength of the steel, increase the hardenability, and significantly improve the surface hardness of the wear-resistant steel plate; but too high carbon has adverse effects on the toughness, cold forming and welding performance of the steel. Based on the above comprehensive consideration, the carbon content of the present application is controlled at 0.19%-0.20%.

[0018] Manganese is the most effective alloying element to improve hardenability, has solid solution strengthening effect in ferrite, and can improve the heat treatment performance of steel, refine pearlite grain, and improve the strength and hardness of steel. It is beneficial to add a certain amount of manganese in steel, but the content should not be too high; this is because Mn is an element that is easy to segregate, and during the solidification process of molten steel, Mn elements will gather at the end of solidification to form central segregation, resulting in coarse cementite in the core of the steel plate, which is not conducive to the core performance. At the same time, Mn can reduce the martensite transformation temperature in the material, and when the Mn accumulates in the segregation zone to a certain proportion, high-hardness microstructure such as martensite and bainite will be produced during the post-weld cooling process, and the toughness will decrease significantly. Based on the above comprehensive consideration, the manganese content of the present application is controlled at 1.10% to 1.20%.

[0019] Silicon is a good reducing agent and deoxidizer in the steelmaking process, and can easily form solid solution in ferrite, so that the strength of the steel, especially the yield strength, is improved. However, when the Si content is too high, the low-temperature toughness decreases, and at the same time, the steel plate surface iron oxide scale is difficult to remove. The Si content of the present application is controlled in the range of 0.25 to 0.45%, which is beneficial to the comprehensive performance.

[0020] Phosphorus has a strong solid solution strengthening effect, which can significantly increase the strength and hardness of steel. However, phosphorus is an element that is easy to segregate, increases temper brittleness, and is very harmful to the low-temperature toughness of steel. Therefore, the content of phosphorus in steel should be strictly controlled, and the P content in the present application is controlled below 0.012%.

[0021] Sulfur is a harmful element in steel, which has a tendency to crack, and sulfide inclusions significantly reduce the toughness of steel, so the S content should be as low as possible. The S content in the present application is controlled within 0.003%.

[0022] Niobium has a strong affinity with nitrogen and carbon in steel, and can form a very stable Nb(C, N) compound. The dispersion of Nb(C, N) particles along the austenite grain boundary can greatly increase the original austenite grain coarsening temperature, thereby refining the ferrite grain and improving the impact toughness and strength. The Nb content in the present application is controlled at 0.015% to 0.020%.

[0023] Titanium is a strong carbide-forming element, and has a strong binding force with carbon and nitrogen. During the solidification process of steel, Ti and N produce stable TiN, which can strongly hinder the migration of austenite grain boundaries, thereby refining the austenite grain. Ti and C combine to form TiC, which can have a precipitation strengthening effect. The Ti content in the present application is controlled at 0.008% to 0.016%.

[0024] Chromium is one of the basic elements of wear-resistant steel, which can improve the hardenability, solid solution strengthening matrix, refine the grain, and improve the strength, hardness and wear resistance of the steel; and can significantly improve the oxidation resistance of the steel and improve the corrosion resistance of the steel. The Cr content of the present application is controlled at 0.65% to 0.70%.

[0025] Aluminum can be used as a deoxidizer in steel, which can refine the grain. The Al content of the present application is controlled at 0.070% to 0.090%.

[0026] Boron is a key element for improving the hardenability of steel plate. A small amount of B (0.0008-0.0030%) added to the steel can significantly improve the hardenability. Therefore, a small amount of B added to the quenched and tempered steel can effectively replace some expensive hardenability alloy elements. However, the B content in wear-resistant steel should not exceed 0.0040%, otherwise non-soluble borides may be produced to cause boron brittleness. The B content of the present application is controlled at 0.0014% to 0.0020%.

[0027] The main innovation of the present application is to use low CEV composition design, combined with strict controlled rolling process, quenching + low temperature tempering heat treatment process, and to carry out preheating-free welding process-performance evaluation during the development process, to develop a preheating-free NM450 wear-resistant steel plate with excellent base material performance and welding performance. The beneficial effects of the present application are as follows: the NM450 wear-resistant steel produced by the method of the present application has excellent strength and toughness, wear resistance and preheating-free welding forming performance. The mechanical properties are as follows: surface hardness 420-480 HBW, yield strength ≥1120 MPa, tensile strength ≥1370 MPa, elongation ≥11.5%, 6a diameter 90° cold bending, -40℃ impact AKV ≥32J. The tensile strength of the butt joint after preheating-free welding is ≥780 MPa, and the Charpy V-notch test at -5℃ of the weld, fusion line and heat affected zone impact energy is respectively: 83J / 75J / 97J, 109J / 94J / 117J, 138J / 156J / 185J. The steel plate produced by the method of the present application can be used for wear-resistant parts and structural parts of excavators, bulldozers, loaders, dump trucks, scraper conveyors, etc., and has excellent mechanical properties and excellent welding and easy forming performance, which is a promising green steel product. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 、 Figure 2 are the metallographic structure photos of the steel produced by example 1 and example 2, respectively;

[0029] Figure 3 is the hardness distribution diagram of the preheating-free welded steel plate of the example. DETAILED DESCRIPTION

[0030] Example 1:

[0031] A production method of preheating-free welding NM450 wear-resistant steel plate, smelting 1 furnace NM450 wear-resistant steel to roll 12mm thickness steel plate, the chemical composition of the steel is shown in Table 1; including the following key process steps:

[0032] (1) Converter smelting: top and bottom combined blowing converter smelting is adopted, the tapping temperature is 1581℃, the P content at the end of converter tapping is 0.010%, alloying is carried out during tapping process, deoxidizers such as aluminum iron and aluminum block are added, low-carbon chromium iron and metal manganese and other alloys are added;

[0033] (2) Refining: the LF furnace is heated and the composition is adjusted, the composition is adjusted to 1600℃ after heating, the VD furnace is vacuumized, the vacuum time is 17min, the soft blowing is placed for 19min after breaking the vacuum, the sampling detects H=0.00012%, N=0.0031%, O=0.0011%;

[0034] (3) Continuous casting: the liquidus of the molten steel is about 1509℃, the tundish temperature is controlled at 1513-1526℃, the cross section thickness x width of the casting blank is 260x2090mm;

[0035] (4) Heating: the heating system is: preheating section 710℃, heating section 1120-1240℃, soaking section temperature 1170-1219℃, slab out of furnace temperature 1230℃, in-furnace time 210min;

[0036] (5) Rolling: rolling 12*2100mm(thick*wide) specification steel plate, adopting two-stage controlled rolling, the rolling process parameters are shown in Table 2;

[0037] (6) Heat treatment: quenching heating target temperature 890℃, single row arrangement; tempering heating target temperature 210℃, the detailed heat treatment parameters are shown in Table 3.

[0038] Preheating-free welding experiment: the specific process parameters of preheating-free welding are shown in Table 4. The mechanical property test results of the obtained steel plate base material are shown in Table 5; the tensile and impact performance results after preheating-free welding are shown in Table 6; the hardness distribution chart is shown in Figure 3 , the test results are shown in Table 7.

[0039] Example 2:

[0040] A production method of preheating-free welding NM450 wear-resistant steel plate, smelting 1 furnace NM450 wear-resistant steel to roll 20mm thickness steel plate, the chemical composition of the steel is shown in Table 1; including the following key process steps:

[0041] (1) Converter smelting: top and bottom combined blowing converter smelting is adopted, the tapping temperature is 1578℃, the P content at the end of converter tapping is 0.009%, alloying is carried out during tapping process, deoxidizers such as aluminum iron and aluminum block are added, low-carbon chromium iron and metal manganese and other alloys are added;

[0042] (2) Refining: LF furnace was heated and composition was adjusted, and after the adjustment was completed, the temperature was raised to 1602°C and the steel was discharged, VD furnace was vacuumized, vacuum time was 16 min, after breaking the vacuum, soft blowing was performed for 17 min, sampling detection showed H = 0.00013%, N = 0.0037%, O = 0.0010%;

[0043] (3) Continuous casting: the liquidus of the molten steel was about 1510°C, the tundish temperature was controlled at 1517-1528°C, and the cross section of the slab was 260*2090 mm;

[0044] (4) Heating: the heating system was as follows: preheating section 700°C, heating section 1100-1230°C, soaking section temperature 1165-1220°C, slab discharge temperature 1216°C, and in-furnace time 238 min;

[0045] (5) Rolling: 20*2200 mm (thickness* width) steel plate was rolled, two-stage controlled rolling was adopted, and the rolling process parameters are shown in Table 2;

[0046] (6) Heat treatment: the quenching heating target temperature was 890°C, and the materials were arranged in a single row; the tempering heating target temperature was 210°C, and the detailed heat treatment parameters are shown in Table 3.

[0047] Preheating-free welding experiment: the specific preheating-free welding process parameters are shown in Table 4. The mechanical property test results of the base material of the obtained steel plate are shown in Table 5; the tensile and impact performance results after preheating-free welding are shown in Table 6; the hardness distribution diagram is shown in Figure 3 , and the test results are shown in Table 7.

[0048] Table 1 Chemical composition (wt. %)

[0049] Example C Si Mn P S AlT Cr Nb Ti B Pcm CEV 1 0.20 0.31 1.12 0.010 0.0006 0.077 0.66 0.016 0.016 0.0014 0.31 0.52 2 0.20 0.28 1.13 0.009 0.0016 0.079 0.65 0.018 0.013 0.0018 0.31 0.52

[0050] Table 2 Rolling process parameters

[0051] Example Thickness mm Blank thickness mm Intermediate blank mm Finish rolling opening temperature °C Finish rolling temperature °C 1 12 260 70 950 830 2 20 260 75 890 811

[0052] Table 3 Heat treatment process parameters

[0053]

[0054]

[0055] Table 4 Preheating-free welding process parameters

[0056]

[0057] Table 5 Mechanical property test results of the base material of the example

[0058]

[0059] Table 6 Mechanical property results after preheat-free welding

[0060]

[0061] Table 7 Hardness results after preheat-free welding

[0062]

Claims

1. A method for producing NM450 wear-resistant steel plates without preheating welding, characterized in that: A 12mm thick steel plate was rolled. The chemical composition of the steel by weight percentage was C=0.20, Si=0.31, Mn=1.12, P=0.010, S=0.0006, Alt=0.077, Cr=0.66, Nb=0.016, Ti=0.016, B=0.0014, Pcm=0.31, CEV=0.52, with the remainder being Fe and unavoidable impurities. The key process steps include the following: (1) Converter smelting: Top and bottom blowing converter is used for smelting. The tapping temperature is 1581℃. The P content at the end of the converter tapping is 0.010%. Alloying is carried out during the tapping process, and aluminum iron, aluminum block deoxidizer, low carbon ferrochrome and metallic manganese are added. (2) Refining: The LF furnace is heated and the composition is adjusted. After the composition adjustment is completed, the temperature is raised to 1600℃ and the furnace is sent out. It is then vacuumed in the VD furnace for 17 minutes. After the vacuum is broken, it is soft blown and left to stand for 19 minutes. Samples are taken for testing. H=0.00012%, N=0.0031%, O=0.0011%; (3) Continuous casting: The liquidus of molten steel is 1509℃, the temperature of the tundish is controlled at 1513~1526℃, and the cross-section of the billet is 260×2090mm (thickness×width). (4) Heating: The heating regime is as follows: preheating section 710℃, heating section 1120~1240℃, soaking section temperature 1170~1219℃, slab exit temperature 1230℃, and furnace time 210min; (5) Rolling: Rolling steel plates with a thickness × width of 12mm × 2100mm, using two-stage controlled rolling, with a billet thickness of 260mm, an intermediate billet thickness of 70mm, a finishing rolling start temperature of 950℃, and a finishing rolling temperature of 830℃. (6) Heat treatment: Quenching target temperature is 890℃, single row of fabric, holding time is 19min, and cooling method is water cooling; tempering target temperature is 210℃, holding time is 25min, and cooling method is air cooling. The mechanical properties of the wear-resistant steel were obtained as follows: transversely, yield strength Rp0.2 is 1220 MPa, tensile strength Rm is 1450 MPa, elongation A50 is 17%, average impact energy at -40℃ is 61 J, and average surface hardness is 468 HBW; longitudinally, yield strength Rp0.2 is 1190 MPa, tensile strength Rm is 1420 MPa, elongation A50 is 19.5%, average impact energy at -40℃ is 56 J, and average surface hardness is 468 HBW.

2. A method for producing NM450 wear-resistant steel plates without preheating welding, characterized in that: A 20mm thick steel plate was rolled. The chemical composition of the steel by weight percentage was C=0.20, Si=0.28, Mn=1.13, P=0.009, S=0.0016, AlT=0.079, Cr=0.65, Nb=0.018, Ti=0.013, B=0.0018, Pcm=0.31, CEV=0.52, with the remainder being Fe and unavoidable impurities. The key process steps include the following: (1) Converter smelting: Top and bottom blowing converter is used for smelting. The tapping temperature is 1578℃. The P content at the end of the converter tapping is 0.009%. Alloying is carried out during the tapping process, and aluminum iron, aluminum block deoxidizer, low carbon ferrochrome and metallic manganese are added. (2) Refining: The LF furnace is heated and the composition is adjusted. After the composition adjustment is completed, the temperature is raised to 1602℃ and the furnace is discharged. It is then put into the VD furnace for vacuum treatment and the vacuum time is 16min. After the vacuum is broken, it is soft blown and left to stand for 17min. Sampling is taken and tested for H=0.00013%, N=0.0037%, and O=0.0010%. (3) Continuous casting: The liquidus of molten steel is 1510℃, the temperature of the tundish is controlled at 1517~1528℃, and the cross-section of the billet is 260×2090mm (thickness×width). (4) Heating: The heating regime is as follows: preheating section 700℃, heating section 1100~1230℃, soaking section temperature 1165~1220℃, slab exit temperature 1216℃, and furnace time 238min; (5) Rolling: Rolling steel plates with a thickness × width of 20mm × 2200mm, using two-stage controlled rolling, with a billet thickness of 260mm, an intermediate billet thickness of 75mm, a finishing rolling start temperature of 890℃, and a finishing rolling temperature of 811℃. (6) Heat treatment: Quenching target temperature is 893℃, single-row fabric, holding time is 21min, and cooling method is water cooling; tempering target temperature is 208℃, holding time is 22min, and cooling method is air cooling. The mechanical properties of the wear-resistant steel are as follows: transversely, yield strength Rp0.2 is 1211 MPa, tensile strength Rm is 1439 MPa, elongation A50 is 15.5%, average impact energy at -40℃ is 55 J, and average surface hardness is 471 HBW; longitudinally, yield strength Rp0.2 is 1190 MPa, tensile strength Rm is 1420 MPa, elongation A50 is 19.5%, average impact energy at -40℃ is 57 J, and average surface hardness is 469 HBW.

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