Production method of high-toughness NM500 wear-resistant steel plate

Abstract

The invention discloses a production method of high-toughness NM500 wear-resistant steel. The steel comprises the following chemical components in percentage by weight: 0.25%-0.26% of C, 0.15%-0.25% of Si, 0.88%-0.95% of Mn, less than or equal to 0.010% of P, less than or equal to 0.002% of S, 0.25%-0.30% of Cr, 0.015%-0.022% of Nb, 0.008%-0.016% of Ti, 0.020%-0.050% of Alt, 0.0014%-0.0020% of B, less than or equal to 0.00015% of H, less than or equal to 0.0040% of N, less than or equal to 0.0015% of O and the balance of Fe and inevitable impurities. CEV is not more than 0.48%, and Pcm is not more than 0.34%. The process comprises the steps of KR deS, converter smelting, LF refining, RH vacuum degassing, continuous casting, heating, rolling, slow cooling and heat treatment. According to the produced NM500 wear-resistant steel plate, the thickness covers 6-25 mm, the surface hardness ranges from 480 HB to 520 HB, the yield strength is larger than or equal to 1300 MPa, the tensile strength is larger than or equal to 1600 MPa, the elongation is larger than or equal to 14%, and the requirements that cold bending is conducted with the 6a diameter being 90 degrees, and the impact Akv at the temperature of-40 DEG C is larger than or equal to 45 J are met.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a method for producing low-cost, high-toughness NM500 wear-resistant steel plates. Background Technology

[0002] Wear-resistant steel is a type of wear-resistant material widely used in metallurgy, mining, building materials, railways, and other fields under abrasive working conditions. It is mainly used in key wear-prone parts of equipment such as mining dump trucks, coal mine scraper conveyors, bulldozers, excavators, mixers, and loaders, and is also a crucial component requiring high wear resistance. Wear-resistant materials operate in harsh environments, thus requiring extremely high strength, hardness, wear resistance, and low-temperature impact toughness to extend equipment lifespan, shorten component replacement cycles, and significantly improve operating time. With the emergence of lightweight and complex mechanical equipment design requirements, higher demands are placed on the welding and forming performance of wear-resistant steel during processing and manufacturing. This means that the application of wear-resistant steel results in longer equipment lifespans, reduced maintenance and downtime, and consequently, reduced capital investment.

[0003] Higher hardness in wear-resistant steel results in better wear resistance, but also increases the difficulty of welding. 500HB grade wear-resistant steel is a high-level wear-resistant variety, with a surface Brinell hardness of 470-540HB. To cope with extreme cold conditions, manufacturers often improve the impact toughness of steel plates by adding alloys. This involves using C and Mn as base elements in the composition design, combined with certain amounts of alloying elements such as Cr, Mo, and Ni. The carbon equivalent of the steel plate is generally ≥0.56%, and the welding cold cracking sensitivity coefficient Pcm ≥0.38%. This leads to difficulties in welding, a significant tendency for cold cracking, and the softening and embrittlement of the weld heat-affected zone, which easily causes brittle fracture of the weld joint.

[0004] Chinese patent CN102943212B discloses an NM500 high-strength wear-resistant steel plate and its heat treatment process. The steel plate produced by this invention has a tensile strength ≥1584MPa, elongation ≥7.0%, and impact energy ≥32J at -20℃. The steel plate has high strength, good wear resistance, impact toughness, and good weldability. However, the P and S content of the steel plate in this invention is P≤0.020% and S≤0.010%, which is significantly high, increasing the risk of embrittlement and cold cracking. In addition, it contains 0.10%~0.30% Mo, resulting in high manufacturing costs.

[0005] CN116555678B discloses an NM500 grade wear-resistant steel with good low-temperature toughness and its manufacturing method. The method employs in-line quenching (DQ) to produce NM500 wear-resistant steel with good low-temperature toughness. The steel plate produced by this invention has high strength, good low-temperature toughness, and cold bending performance. However, the high Mn content (1.20~1.35%) easily causes segregation cracks; the significantly high levels of harmful elements N (100~140ppm), H≤2ppm, and O≤2ppm easily lead to poor plasticity and toughness of the steel plate, increasing the crack risk rate; and the presence of 0.17%~0.27% Mo results in high manufacturing costs.

[0006] CN105385951B discloses a method for producing NM500 wear-resistant steel plates that combine high hardness and high toughness. This invention employs a low-carbon equivalent design, appropriately adds elements to improve hardenability and toughness, strictly controls the Si content, and strictly controls the Mn content to reduce banded structure. The resulting NM500 steel plate possesses both high hardness and high toughness. However, the P and S contents of this invention's steel plate are significantly high (P≤0.015%, S≤0.005%), increasing the risk of embrittlement and cold cracking. Furthermore, it contains 0.2%~0.5% Mo and 0.3%~0.5% Ni, resulting in significantly higher manufacturing costs. Summary of the Invention

[0007] This invention aims to provide a low-cost, high-toughness NM500 wear-resistant steel plate production method. The technical problem to be solved is to improve the overall performance of the steel plate, such as cold forming and welding, by using a low-cost, low-CEV, and low-Pcm composition design while ensuring that the 500HB grade wear-resistant steel has a good strength-toughness balance. The produced wear-resistant steel plate is NM500 grade, with a thickness ranging from 6 to 25 mm, a surface hardness of 480 to 520 HB, a yield strength ≥1300 MPa, a tensile strength ≥1600 MPa, and an elongation ≥14%. It meets the requirements of 6-year diameter 90° cold bending and -40℃ impact Akv ≥45 J. It exhibits excellent strength, toughness, cold forming, and welding performance at a low cost.

[0008] The technical solution of this invention: A method for producing high-toughness NM500 wear-resistant steel plate, wherein the chemical composition of the steel by weight percentage is: C=0.25%~0.26%, Si=0.15%~0.25%, Mn=0.88%~0.95%, P≤0.010%, S≤0.002%, Cr=0.25%~0.30%, Nb=0.015%~0.022%, Ti=0.008%~0.016%, Alt=0.020%~0.050%, B=0.0014%~0.0020%, H≤0.00015%, N≤0.0040%, O≤0.0015%, with the remainder being Fe and unavoidable impurities, CEV≤0.48%, Pcm ≤0.34%; the steel's yield strength ≥1300MPa, tensile strength ≥1600MPa, elongation ≥14%, surface hardness 480~520HB, meets 6a diameter 90° cold bending, -40℃ impact Akv ≥45J; key process steps include: (1) Smelting: Top and bottom blowing converter is used for smelting. The molten iron is treated by KR desulfurization and dephosphorization. 200m pure calcium wire is fed into the LF furnace when it leaves the station. Then it enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is ≥15 min. Soft blowing is carried out on the platform for ≥17 min. 200m pure calcium wire is fed in one go after the molten steel leaves the station. Deoxidizer and alloy are added during the tapping process for deoxidation and alloying. Low P alloy is used to avoid P expansion in the molten steel. The chemical composition of the molten steel is adjusted in the LF furnace. Then it enters the RH furnace for vacuum treatment. When leaving the station, the gas content of the molten steel is measured to be H≤0.00015%, N≤0.0040%, and O≤0.0015%. (2) Continuous casting: The superheat of the ladle in the continuous casting furnace is ≤20℃ and the superheat of the ladle in the open casting furnace is ≤25℃. Dynamic light pressure is adopted to improve the internal quality of the continuous casting billet. The cross-section thickness × width of the continuous casting billet is 180~260mm × 2070mm. (3) Heating: Preheating section temperature 700~810℃, heating section temperature 1100~1230℃, soaking section temperature 1130~1220℃, furnace time 190~260min; (4) Rolling: Two-stage controlled rolling is adopted. The initial rolling temperature of the roughing stage is ≥1050℃, the final rolling temperature is ≥980℃, the cumulative compression ratio of the roughing stage is ≥2.6, and the thickness of the intermediate billet is 60~75mm; the initial rolling temperature of the finishing stage is ≤830~960℃, and the cumulative compression ratio of the finishing stage is ≥3. (5) Heat treatment: Quenching and low-temperature tempering processes are adopted. The target temperature for quenching is 850℃, the heating rate is 1.6~2.2min / mm, and the holding time is 10~20min. The target temperature for tempering is 190℃, the heating rate is 3.5~4.0min / mm, and the holding time is 10~25min.

[0009] Invention Principle: The 500HB high-grade wear-resistant steel of this invention possesses high strength, high toughness at low temperatures, good cold forming and welding performance, and is also low-cost. The NM500 wear-resistant steel involved is based on C and Mn elements, with appropriate amounts of Si, Cr, and B added to improve hardenability; combined with precipitation strengthening of Nb, Ti, and Al, and rigorous rolling and heat treatment processes to refine grains, thereby improving mechanical properties; simultaneously, the P and S content of the molten steel is strictly controlled to ensure that the steel plate has excellent mechanical, weldable, and cold forming properties. The proportions of each main element added and the design rationale are as follows: Carbon content of 0.25%~0.26% increases hardenability and is a solid solution strengthening element, which can effectively improve the strength and hardness of steel plates; however, excessive carbon content is detrimental to the low-temperature impact toughness, cold forming performance and weldability of steel.

[0010] Mn (0.88%~0.95%) plays a role in solid solution strengthening and lowering the phase transformation temperature, thereby improving the strength, hardness, and wear resistance of steel. However, Mn is prone to central segregation and can significantly lower the martensitic transformation initiation temperature (Ms point), leading to an increase in retained austenite at room temperature, which is detrimental to the strength, hardness, and wear resistance of steel.

[0011] Cr (0.25%~0.30%) improves hardenability, promotes the low-temperature transformation of martensite, and significantly improves the oxidation resistance and corrosion resistance of steel. Increased Cr significantly improves the strength, hardness, and wear resistance of steel, but reduces toughness and weldability.

[0012] Boron (B), with a content of 0.0014% to 0.0020%, is one of the important elements for improving the hardenability of quenched and tempered steel. Appropriate addition of B can effectively reduce the need for precious alloys such as Mo and Ni, significantly lowering manufacturing costs. However, when the B content in steel exceeds 0.005%, it forms borides, causing segregation and leading to a deterioration in the toughness and hardenability of the steel plate.

[0013] Si = 0.15%~0.25% plays a solid solution strengthening role in the steel of this invention, significantly improving the strength and hardness of the steel; however, it has the unfavorable tendency to increase grain coarsening and temper brittleness, and when it exceeds 0.50%, it will be detrimental to the low-temperature toughness and weldability of the steel, and at the same time, it will make it difficult to remove the iron oxide scale on the surface of the steel plate.

[0014] P ≤ 0.010%, S ≤ 0.002%. P, a harmful element in steel, easily causes segregation, worsening the steel's low-temperature impact toughness and weldability; this invention controls this. S easily combines with Mn to form MnS inclusions, which are detrimental to the steel's low-temperature impact toughness. Therefore, this invention minimizes the adverse effects of P and S elements on steel properties.

[0015] O ≤ 0.0015%, N ≤ 0.0040%. O and N are harmful gaseous elements in steel; high content leads to more inclusions and reduces the plasticity and toughness of the steel plate. Since Ti is extremely reactive, it preferentially precipitates with O and N, reducing the amount of TiC precipitation and thus reducing the wear resistance of the steel plate. Therefore, this invention strictly controls these concentrations.

[0016] H ≤ 0.00015%. H is a harmful gaseous element; high content will reduce the plasticity and toughness of steel plates and will lead to hydrogen-induced cracking.

[0017] The main innovations and beneficial effects of this invention are as follows: It employs a low CEV and low Pcm composition design, strictly controlling P and S elements that negatively affect low-temperature toughness without adding Mo and Ni precious alloys, combined with rigorous rolling and heat treatment processes; ensuring the steel plate possesses excellent strength, toughness, cold forming performance, and weldability. The produced NM500 wear-resistant steel plate has a thickness ranging from 6 to 25 mm, a surface hardness of 480 to 520 HB, a yield strength ≥1300 MPa, a tensile strength ≥1600 MPa, and an elongation ≥14%. It meets the requirements for 6-year diameter 90° cold bending and -40℃ impact Akv ≥45 J. The steel plate produced by this invention can be used in wear-resistant parts of mining dump trucks, excavators, bulldozers, scraper conveyors, and other engineering machinery equipment. It possesses both excellent mechanical properties and superior weldability and formability, and can replace low-grade wear-resistant steel to achieve lightweight mechanical assembly. It is a highly promising low-cost green steel product. Attached Figure Description

[0018] Figure 1 Metallographic image of the steel produced in Example 1.

[0019] Figure 2 Metallographic image of the steel produced in Example 2. Detailed Implementation Example 1

[0020] A method for producing low-cost, high-toughness NM500 wear-resistant steel plates involves smelting one heat of NM500 wear-resistant steel and rolling it into steel plates with thicknesses of 6 mm and 12 mm. The chemical composition by weight percentage of the steel is shown in Table 1. The method includes the following key process steps: (1) Smelting: Top and bottom blowing converter was used for smelting. The P content at the end of the converter tapping was 0.008%. Alloying was carried out during the tapping process, and deoxidizers such as aluminum iron and aluminum blocks were added. Low carbon ferrochrome and metallic manganese were added. The LF furnace was heated and the composition was adjusted. The vacuum time of the RH furnace was 17 min. After the vacuum was broken, it was soft blown and left to stand for 18 min. Sampling and testing were performed to find that H=0.00012%, N=0.0039%, and O=0.0012%. (2) Continuous casting: Full-process protective casting is adopted, and the tundish superheat is 13~18℃; the billet cross section (thickness*width): 180*2070mm; (3) Heating: Preheating section 711~806℃, heating section 1123~1227℃, soaking section temperature 1160~1218℃, slab exit temperature 1191~1212℃, furnace time 190~208min; (4) Rolling: Steel plates with thickness × width of 6mm × 2500mm and 12mm × 2500mm are rolled in two stages, of which 6mm is hot rolled (AR) and 12mm is controlled rolled (CR). The rolling process parameters are shown in Table 2. (5) Heat treatment: Quenching target temperature is 850℃, and tempering target temperature is 190℃. Example 2

[0021] A method for producing low-cost, high-toughness NM500 wear-resistant steel plates involves smelting one heat of NM500 wear-resistant steel and rolling it into steel plates with thicknesses of 15mm and 25mm. The chemical composition by weight percentage of the steel is shown in Table 1. The method includes the following key process steps: (1) Smelting: Top and bottom blowing converter was used for smelting. The P content at the end of the converter tapping was 0.006%. Alloying was carried out during the tapping process, and deoxidizers such as aluminum iron and aluminum blocks were added. Low carbon ferrochrome and metallic manganese were added. The LF furnace was heated and the composition was adjusted. The RH furnace was kept under vacuum for 16 minutes. After breaking the vacuum, it was soft blown and left to stand for 19 minutes. Sampling and testing showed that H=0.00014%, N=0.0032%, and O=0.0011%. (2) Continuous casting: Full-process protective casting is adopted; the superheating degree of the tundish is 14~18℃; the cross section of the billet (thickness*width) is 260*2070mm; (3) Heating: Preheating section 707~804℃, heating section 1112~1227℃, soaking section temperature 1152~1216℃, slab exit temperature 1188~1210℃, furnace time 223~260min; (4) Rolling: Steel plates with thickness × width of 15mm × 2500mm and 25mm × 2500mm are rolled using two-stage rolling, both of which are controlled rolling (CR). The rolling process parameters are shown in Table 2. (5) Heat treatment: Quenching target temperature is 850℃, and tempering target temperature is 190℃.

[0022] The rolling process parameters for Examples 1 and 2 are shown in Table 2, and the heat treatment process parameters are shown in Table 3. The mechanical property test results of the steel are shown in Table 4, where the 6mm impact test specimens were thinned specimens with a size of 5×10mm; and the ≥12mm impact test specimens were standard specimens with a size of 10×10mm. The metallographic structures of the steels produced in Examples 1 and 2 are shown in Table 4. Figure 1 , Figure 2 .

[0023] Table 1. Chemical composition (wt.%) of the steel in the examples .

[0024] Table 2 Rolling process parameters of the embodiment .

[0025] Table 3 Heat treatment process parameters for the examples .

[0026] Table 4. Test results of mechanical properties of the steel in the examples. .

Claims

1. A method for producing high-toughness NM500 wear-resistant steel, characterized in that: The steel's chemical composition by weight percentage is: C = 0.25%~0.26%, Si = 0.15%~0.25%, Mn = 0.88%~0.95%, P ≤ 0.010%, S ≤ 0.002%, Cr = 0.25%~0.30%, Nb = 0.015%~0.022%, Ti = 0.008%~0.016%, Alt = 0.020%~0.050%, B = 0.0014%~0.0020%, H ≤ 0.00015%, N ≤ 0.0040%, O ≤ 0.0015%, with the remainder being Fe and unavoidable impurities. CEV ≤ 0.48%, Pcm ≤0.34%; the yield strength of the steel is ≥1300MPa, the tensile strength is ≥1600MPa, the elongation is ≥14%, the surface hardness is 480~520HB, and it meets the requirements of 6a diameter 90° cold bending and -40℃ impact Akv≥45J; Key process steps include: (1) Smelting: Top and bottom blowing converter is used for smelting. The molten iron is treated by KR desulfurization and dephosphorization. 200m pure calcium wire is fed into the LF furnace when it leaves the station. Then it enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is ≥15 min. Soft blowing is carried out on the platform for ≥17 min. 200m pure calcium wire is fed in one go after the molten steel leaves the station. Deoxidizer and alloy are added during the tapping process for deoxidation and alloying. Low P alloy is used to avoid P expansion in the molten steel. The chemical composition of the molten steel is adjusted in the LF furnace. Then it enters the RH furnace for vacuum treatment. The gas content of the molten steel is measured at the station: H≤0.00015%, N≤0.0040%, O≤0.0015%. (2) Continuous casting: The superheat of the ladle in the continuous casting furnace is ≤20℃ and the superheat of the ladle in the open casting furnace is ≤25℃. Dynamic light pressure is adopted to improve the internal quality of the continuous casting billet. The cross-section thickness × width of the continuous casting billet is 180~260mm × 2070mm. (3) Heating: Preheating section temperature 700~810℃, heating section temperature 1100~1230℃, soaking section temperature 1130~1220℃, furnace time 190~260min; (4) Rolling: Two-stage controlled rolling is adopted. The initial rolling temperature of the roughing stage is ≥1050℃, the final rolling temperature is ≥980℃, the cumulative compression ratio of the roughing stage is ≥2.6, and the thickness of the intermediate billet is 60~75mm; the initial rolling temperature of the finishing stage is ≤830~960℃, and the cumulative compression ratio of the finishing stage is ≥3. (5) Heat treatment: Quenching and low-temperature tempering processes are adopted. The target temperature for quenching is 850℃, the heating rate is 1.6~2.2min / mm, and the holding time is 10~20min. The target temperature for tempering is 190℃, the heating rate is 3.5~4.0min / mm, and the holding time is 10~25min.

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