AU2022325314B2

AU2022325314B2 - High-toughness, cold-worked die steel and preparation method therefor

Info

Publication number: AU2022325314B2
Application number: AU2022325314A
Authority: AU
Inventors: Haoran Hu; Qin KANG; Xu LUO; Qiang XIAO
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2021-08-10
Filing date: 2022-07-20
Publication date: 2024-05-16
Anticipated expiration: 2042-07-20
Also published as: AU2022325314A1; CN113604744B; WO2023016219A1; CN113604744A; KR20230104949A

Abstract

The present invention provides a high-toughness, cold-worked die steel, the components of which are as follows: 0.9wt％-0.94wt％ of C, 1.35wt％-1.45wt％ of Si, 0.4wt％-0.6wt％ of Mn, ≤0.02wt％ of P, ≤0.003wt％ of S, 7.4wt％-7.8wt％ of Cr, 2.1wt％-2.3wt％ of Mo, 0.3wt％-0.35wt％ of V, 0.4wt％-0.5wt％ of Nb, ≤0.2wt％ of Cu, ≤0.2wt％ of Ni, 0.02％-0.03wt％ of Al; ≤2.5ppm of H; ≤0.002wt％ of O, and a trace amount of rare earth Re, the remainder being Fe. The present invention provides high-toughness, cold-worked die steel that has specific components and a certain preparation process. Said cold-worked die steel has good performance and prevents the problem of flaw detection caused by large-block carbides. The present invention further provides a preparation method for high-toughness, cold-worked die steel.

Description

HIGH-TOUGHNESS COLD-WORKED DIE STEEL AND PREPARATION METHOD THEREFOR TECHNICAL FIELD

The present invention belongs to the technical field of cold-worked die steel, and particularly relates to high-toughness cold-worked die steel and a preparation method therefor.

BACKGROUND

The use amount of cold stamping dies is increased year by year, and the dies have become important process equipment for producing various plate parts. Metal materials used as the cold stamping dies, especially male and female dies for important parts, are greatly affected by the cold extrusion forming and blanking force during usage, so that cold-worked die steel is required to have high strength, toughness and wear resistance. The common steel types for the cold stamping dies are generally Cr12MoV, SKD11 and D2, the chemical components of which are basically the same. In order to further improve the toughness of the cold stamping dies and keep the original strength unchanged, the Japan Daido Corporation developed DC53 (with the domestic brand of Cr8Mo2SiV) on the basis of Cr12MoV steel, and during design of the chemical components, the ratios of the content of carbon and the content of chromium are reduced, and then the precipitation of eutectic carbides is reduced to improve the toughness. However, actual production practices show that this type of steel still has the quality problem of failed flaw detection caused by the large-block eutectic carbides.

OBJECT

It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above problems, or to at least provide a useful alternative.

SUMMARY

One aspect of the present invention provides a high-toughness cold-worked die steel, comprising the following components: 0.9wt%-0.94wt% of C; 1.35wt%-1.45wt% of Si; 0.4wt%-0.6wt% of Mn; < 0.02wt% of P; < 0.003wt% of S; 7.4wt%-7.8wt% of Cr; 2.lwt%-2.3wt% of Mo; 0.3wt%-0.35wt% of V; o 0.4wt%-0.5wt% of Nb; < 0.2wt% of Cu; < 0.2wt% of Ni; 0.02%-0.03wt% of Al; < 2.5 ppm of H; < 0.002wt% of 0; and a trace amount of rare earth Re, the remainder being Fe. The present invention according to a preferred embodiment provides a preparation method for the above-mentioned high-toughness cold-worked die steel, the method comprising: performing electric furnace smelting, LF refining and VD vacuum refining on alloy raw materials in sequence to obtain molten steel; and pouring the molten steel to obtain the high-toughness cold-worked die steel. Preferably, the preparation method for the high-toughness cold-worked die steel comprises: using scrap steel as a raw material, and performing 40t electric furnace smelting, 40t LF refining and 40t VD furnace vacuum refining in sequence with a remelting method to obtain molten steel; and then pouring the molten steel to form an ingot to obtain a high-toughness cold-worked die steel slab ingot.

Preferably, the preparation method for the high-toughness cold-worked die steel comprises: using Cr12MoV scrap steel, Cr12 scrap steel and 4Cr13 scrap steel as steelmaking raw materials, and performing 40t electric furnace smelting, 40t LF refining and 40t VD furnace vacuum refining in sequence to obtain molten steel having the target components; and pouring the molten steel to form an ingot to obtain a high-toughness cold-worked die steel slab ingot. Preferably, the tapping temperature in the electric furnace smelting process is > 1600°C. Preferably, the charging temperature of the molten steel in the LF refining process is > 1510°C, and the slag thickness is 31-35 mm; an aluminum wire feeding operation is performed firstly in the LF refining process, and the feeding amount of aluminum wires is 120-150 m/furnace molten steel; and at the same time, argon is introduced to the furnace bottom at the flow of 80-100 NL/min. Preferably, slag lime and refining slag prepared in advance are added in the LF refining process, and then lime is added; and the addition amount of the slag lime is 500-580 kg/furnace molten steel, the addition amount of the refining slag is 120-140 kg/furnace molten steel, and the addition amount of the added lime is 180-200 kg/furnace molten steel. Preferably, power-on reduction is performed in the LF refining process, and carbon powder and a steel slag mixture are added in the power-on reduction process; the use amount of the carbon powder is 80-100 kg/furnace molten steel, and the use amount of the steel slag mixture is 100-150 kg/furnace molten steel; and the addition manner for the steel slag mixture is as follows: adding 100-150 kg/furnace molten steel of the steel slag mixture after power-on refining for 10-15 min, and adding the steel slag mixture as per 35-40 kg/furnace molten steel, 30-35 kg/furnace molten steel and 20-30 kg/furnace molten steel every 10 min for reduction during refining; and after complete reduction, the slag being white, performing sampling analysis (including total Al), returning a sample, and adjusting total Al to 0.05wt% according to analysis results. Preferably, the white slag holding time in the LF refining process is 20-25 min, and 10-15 kg/furnace molten steel of the carbon power is added in the later period of refining to maintain a reduction atmosphere. Preferably, in the LF refining process, according to the sampling analysis results, the chemical components are adjusted and Nb alloying is performed according to the specific chemical component control requirements, and Nb iron is added as per 0.05%-0.1% of the weight of the molten steel; and the total aluminum control requirement after LF furnace refining is as follows: [Al]: 0.02-0.04wt%. Preferably, after alloying is completed in the LF refining process, sampling for total analysis is performed, and when the chemical components are within the internal control range, [S] < 0.003wt% and the temperature is 1610-1640°C, a VD furnace operation is performed. Preferably, the ladle temperature in the VD vacuum refining process is 1610-1640°C; and the thickness of slag entering a VD furnace is < 80 mm. Preferably, the limit vacuum degree in the VD vacuum refining process is < 60 Pa, and the holding time under limit vacuum is > 20 min. Preferably, large-flow Ar blowing is kept under limit vacuum in the VD vacuum refining process, and the Ar blowing flow is > 130 NL/min; and 1-2 min before vacuum breakage, the Ar blowing flow is adjusted to 20-40 NL/min. Preferably, the VD vacuum refining process includes: performing vacuum breakage, temperature measurement and [H] sampling, and requiring [H] to be < 2.5 ppm; after vacuum breakage of the VD vacuum furnace, adding 13-17 kg/furnace molten steel of the rare earth Re immediately, putting the rare earth Re in an aluminum lunch box, and then directly throwing the box into a steel ladle; and after the rare earth is added, performing ladle pouring only after the soft argon blowing time is > 25 min, and if the soft blowing time exceeds 50 min, performing the [H] setting operation again. Preferably, the ladle temperature in the pouring process is 1505-1515°C, and the ingot is cast finally. The present invention according to a preferred embodiment provides a high-toughness cold-worked die steel that has the specific components and a certain preparation process, which can achieve the purpose of reducing or improving the eutectic carbides. Said cold-worked die steel has good performance and prevents the problem of failed flaw detection caused by large-block carbides. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a metallographic picture of eutectic carbides on an edge of flat steel prepared in Embodiment 1 of the present invention; FIG. 2 is a metallographic picture of eutectic carbides at the core of the flat steel prepared in Embodiment 1 of the present invention; FIG. 3 is a metallographic picture of eutectic carbides on an edge of flat steel prepared in Embodiment 2 of the present invention; and FIG. 4 is a metallographic picture of eutectic carbides at the core of the flat steel prepared in Embodiment 2 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The technical solutions in embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of embodiments of the present invention, not all of them. On the basis of the embodiments of the present invention, all other embodiments obtained by a person of ordinary skill in the art through improvements or embellishments should fall within the scope of protection of the present invention. It should be understood that the embodiments of the present invention are only used to illustrate the technical effects of the present invention, not to limit the scope of protection of the present invention. In the embodiments, methods used are conventional methods unless otherwise specified.

The present invention provides the high-toughness cold-worked die steel, the components of which are as follows: 0.90wt%-0.94wt% of C; 1.35wt%-1.45wt% of Si; 0.4wt%-0.6wt% of Mn; < 0.02wt% of P; < 0.003wt% of S; 7.4wt%-7.8wt% of Cr; 2.lwt%-2.3wt% of Mo; 0.3wt%-0.35wt% of V; 0.4wt%-0.5wt% of Nb; < 0.2wt% of Cu; < 0.2wt% of Ni; 0.02%-0.03wt% of Al; < 2.5 ppm of H; < 0.002wt% of 0; and a trace amount of rare earth Re, the remainder being Fe. In the present invention, the mass content of C is preferably 0.91%-0.93%, and more preferably 0.92%; the mass content of Si is preferably 1.38%-1.42%, and most preferably 1.4%; the mass content of Mn is preferably 0.5%; the mass content of P is preferably < 0.02wt%; the mass content of S is preferably < 0.003wt%; the mass content of Cr is preferably 7.5%-7.7%, and more preferably 7.6%; the mass content of Mo is preferably 2.2%; the mass content of V is preferably 0.31%-0.34%, and more preferably 0.32%-0.33%; the mass content of Nb is preferably 0.45%; the mass content of the Cu is preferably < 0.2wt%; the mass content of Ni is preferably < 0.2wt%; the mass content of the Al is preferably 0.025%; the mass content of H is preferably < 2.5 ppm; and the mass content of the 0 is preferably < 0.002wt%. The present invention provides a preparation method for the high-toughness cold-worked die steel in the above-mentioned technical solution, the method comprising: performing electric furnace smelting, LF refining and VD vacuum refining on alloy raw materials in sequence to obtain molten steel having the qualified components; and pouring the molten steel to obtain a high-toughness cold-worked die steel ingot. In the present invention, the preparation method for the high-toughness cold-worked die steel comprises: using scrap steel as a raw material, and performing 40t electric furnace smelting, 40t LF refining and 40t VD furnace vacuum refining in sequence with a remelting method to obtain molten steel; and then pouring the molten steel to form an ingot to obtain a high-toughness cold-worked die steel slab ingot. In the present invention, more preferably, the preparation method for the high-toughness cold-worked die steel comprises: using Cr12MoV scrap steel, Cr12 scrap steel and 4Cr13 scrap steel as steelmaking raw materials, and performing 40t electric furnace smelting, 40t LF refining and 40t VD furnace vacuum refining in sequence to obtain molten steel having the qualified components; and pouring the molten steel to form an ingot to obtain a high-toughness cold-worked die steel slab ingot. In the present invention, an electric furnace is preferably a 40t electric furnace; an LF is preferably a 40t LF; and a 40t vacuum refining furnace is preferred for VD vacuum refining. In the present invention, the alloy raw materials are not specially restricted, the alloy raw materials that are familiar to a person skilled in the art to prepare the cold-worked die steel are used for batching, preferably, the scrap steel is used, and more preferably, one or more of the Cr12MoV scrap steel, Cr12 scrap steel and 4Cr13 scrap steel are used. In the present invention, the components of the alloy raw materials are basically the same as those of the high-toughness cold-worked die steel in the above-mentioned technical solution. In the present invention, a reduction method is preferably used for smelting in the electric furnace smelting process; and the tapping temperature in the electric furnace smelting process is preferably > 1600°C, more preferably 1610-1630°C, and most preferably 1620°C. In the present invention, the temperature in molten steel charging conditions in the LF refining process is preferably > 1510°C, more preferably 1520-1540°C, and most preferably 1530°C; and the slag thickness is preferably 31-35 mm, more preferably 32-34 mm, and most preferably 33 mm. In the present invention, aluminum wire feeding is preferably performed in the LF refining process, and the addition amount of aluminum wires is preferably 130-140 m/furnace molten steel, more preferably 132-138 m/furnace molten steel, and most preferably 134-136 m/furnace molten steel. In the present invention, argon is preferably introduced at the furnace bottom at the same time of aluminum wire feeding in the LF refining process; and the flow of the argon is preferably 80-100 NL/min, more preferably 85-95 NL/min, and most preferably 90 NL/min. In the present invention, lime and refining slag are preferably added in the LF refining process, and then lime is added; and the addition amount of the lime is preferably 510-580 kg/furnace molten steel, more preferably 520-560 kg/furnace molten steel, more preferably 530-550 kg/furnace molten steel, and most preferably 540 kg/furnace molten steel. In the present invention, the addition amount of the refining slag is preferably 125-140 kg/furnace molten steel, more preferably 130-135 kg/furnace molten steel, and most preferably 132-133 kg/furnace molten steel. In the present invention, the addition amount of the added lime is preferably 185-200 kg/furnace molten steel, more preferably 190-195 kg/furnace molten steel, and most preferably 192-193 kg/furnace molten steel. In the present invention, power-on reduction is preferably performed in the LF refining process, carbon powder and a steel slag mixture are used for reduction in the power-on reduction process, and the use amount of the carbon powder is preferably 85-100 kg/furnace molten steel, more preferably 90-95 kg/furnace molten steel, and most preferably 92-93 kg/furnace molten steel. In the present invention, the use amount of the steel slag mixture is preferably 100-150 kg/furnace molten steel, more preferably 110-140 kg/furnace molten steel, and most preferably 120-130 kg/furnace molten steel. In the present invention, the addition method for the steel slag mixture preferably comprises: after 10-15 min of power-on refining, adding 100-150 kg/furnace molten steel of the steel slag mixture, preferably 110-140 kg/furnace molten steel, and most preferably 120-130 kg/furnace molten steel. In the present invention, the steel slag mixture is preferably added by stages in the refining period, and the steel slag mixture is added as per 35-40 kg/furnace molten steel, 30-35 kg/furnace molten steel and 20-30 kg/furnace molten steel every 10 min for reduction. In the present invention, in the LF refining process, preferably, after complete reduction, the slag is white, sampling analysis (including total Al) is performed, a sample in returned, the total Al in the molten steel is adjusted to 0.05wt% according to analysis results, and the holding time of the white slag is preferably 20-25 min, more preferably 21-24 min, and most preferably 22-23 min; the carbon powder is added in the later period of refining to maintain a reduction atmosphere; and the addition amount of the carbon powder is preferably 10-15 kg/furnace molten steel, more preferably 11-14 kg/furnace molten steel, and most preferably 12-13 kg/furnace molten steel. In the present invention, in the LF refining process, preferably, the chemical components are adjusted and Nb alloying is performed according to the chemical component control requirements of the specific steel type, and Nb iron is added as per 0.05%-0.1% of the weight of the molten steel, more preferably 0.06%-0.09%, and most preferably 0.07%- 0 .0 8 %.

In the present invention, after LF refining, the content of the total aluminum in the molten steel is preferably 0.02wt%-0.04wt%, and more preferably 0.03wt%; and the mass content of S in the molten steel is preferably < 0.003%, the temperature is 1610-1640°C, and VD vacuum refining is started after ladle deslagging. In the present invention, in the VD vacuum refining process, the thickness of the slag entering the VD furnace is preferably < 80 mm, more preferably 70-80 mm, and most preferably 75 mm. In the present invention, the limit vacuum degree in the VD vacuum refining process is preferably < 60 Pa, more preferably 50-60 Pa, and most preferably 55 Pa; and the holding time under limit vacuum is preferably > 20 min, more preferably 20-25 min, and most preferably 22-23 min. In the present invention, large-flow Ar blowing is preferably kept under limit vacuum in the VD vacuum refining process, and the Ar blowing flow is preferably > 130 NL/min, more preferably 130-150 NL/min, and most preferably 140 NL/min; and 1-2 min before vacuum breakage, the Ar blowing flow is preferably adjusted to 20-40 NL/min, more preferably 25-35 NL/min, and most preferably 30 NL/min. In the present invention, vacuum breakage, temperature measurement and [H] sampling are preferably performed in the VD vacuum refining process, and the content of [H] in the molten steel is preferably controlled to be < 2.5 ppm. In the present invention, in the VD vacuum refining process, after vacuum breakage of the VD vacuum furnace, 13-17 kg/furnace molten steel of the rare earth Re is preferably added immediately, more preferably 14-16 kg/furnace molten steel, and most preferably 15 kg/furnace molten steel. In the present invention, the addition method for the rare earth preferably comprises: putting the rare earth Re in an aluminum lunch box, and then directly throwing the box into a steel ladle. In the present invention, after the rare earth is added, preferably, ladle pouring can only be performed after the soft argon blowing time is > 25 min, and if the soft blowing time exceeds 50 min, the [H] setting operation needs to be performed again. In the present invention, the ladle temperature in the pouring process is preferably 1505-1515°C, more preferably 1508-1512°C, and most preferably 1510°C. The present invention provides the high-toughness cold-worked die steel that has the specific components and a certain preparation process, which can achieve the purpose of reducing or improving the eutectic carbides. Said cold-worked die steel has good performance and prevents the problem of failed flaw detection caused by large-block carbides. Embodiment 1 Cr12MoV scrap steel, Cr12 scrap steel and 4Cr13 scrap steel are used as steelmaking raw materials, and 40t electric furnace smelting, 40t LF refining and 40t VD vacuum refining are performed in sequence to obtain molten steel having the qualified chemical components; and the molten steel is poured to obtain a high-toughness cold-worked die steel slab ingot. In the electric furnace smelting process: smelting is performed with a reduction method according to the conventional smelting process, and the tapping temperature is 1620°C; in the LF refining process: the temperature of the molten steel entering an LF is 1550°C, and the slag thickness is 35 mm; aluminum wires are fed by 150 m, the argon flow is adjusted to be 90 NL/min, 570 kg of slag lime and 120 kg of refining slag prepared in advance are added, and 200 kg of lime is added; power-on reduction is performed, 100 kg of carbon powder in total and 100 kg of a steel slag mixture in total are used for reduction in the whole process, and the addition manner of the steel slag mixture is as follows: adding 100 kg of the steel slag mixture after power-on refining for 15 min, and adding the steel slag mixture as per 40 kg, 30 kg and 30 kg every 10 min for reduction during refining; after complete reduction, the slag is white, sampling analysis (including total

Al) is performed, a sample in returned, and the total Al is adjusted to 0.05wt% according to analysis results; the holding time of the white slag is 24 min in the refining process, and a small amount of C powder (10 kg/ furnace molten steel) is added in the later period of refining so as to maintain a reduction atmosphere; according to the sampling analysis results, the chemical components are adjusted according to the chemical component control requirements of the specific steel type, and Nb iron is added as per 0.06% of the total weight of the molten steel; after LF furnace refining, the total aluminum is as follows: [Al]: 0.04wt%; well alloying is ensured, sampling for total analysis is performed, and when the chemical components are within the internal control range, [S] < 0.003wt% and the temperature is 1640°C, a ladle after deslagging is hung to VD for vacuum treatment; in the VD vacuum refining process: the thickness of the slag entering a VD furnace is 70 mm; the VD furnace vacuum treatment requirements: the limit vacuum degree is 50 Pa, and the holding time under limit vacuum is 25 min; large-flow Ar blowing is kept under limit vacuum (with the Ar blowing flow being 140 NL/min), and 2 min before vacuum breakage, the Ar blowing flow is adjusted to 40 NL/min; vacuum breakage, temperature measurement and [H] sampling are performed, and [H] is 2.5 ppm; after vacuum breakage of the VD vacuum furnace, 15 kg/furnace molten steel of the rare earth Re is added immediately, the rare earth Re is put into an aluminum lunch box, and then the box is directly thrown into a steel ladle; after the rare earth is added, ladle pouring can only be performed after the soft argon blowing time of 28 min, and if the soft blowing time exceeds 50 min, the [H] setting operation needs to be performed again; and the ladle temperature in the pouring process is 1514°C. The components of the high-toughness cold-worked die steel prepared in Embodiment 1 of the present invention are detected according to the method of the

Carbon and Low-alloy steel-Determination of Multi-element Contents-Spark Discharge Atomic Emission Spectrometric Method (Routine Method) (GB/T 4336-2016), and detection results are shown in Table 1. Table 1 Detection results of components of high-toughness cold-worked die steel prepared in Embodiment 1 of present invention C Si Mn P S Cr Mo Remainde r 0.93wt 1.4wt 0.55wt 0.02wt 0.003wt 7.75wt 2.25wt% Fe

V Nb Cu Ni Al H 0 0.34wt 0.5wt 0.15wt 0.2wt% 0.03wt% 2.2ppm 0.002wt

After the slab ingot prepared in Embodiment 1 of the present invention is rolled into flat steel having the thickness of 70 mm, an edge and the core of a cross section are sampled to analyze the metallographic structures, which are as shown in FIG. 1 and FIG. 2, FIG. 1 shows the metallographic structure on the edge, and FIG. 2 shows the metallographic structure at the core; and the unevenness of eutectic carbides is graded according to the standard of the Eutectic Carbide of Steel-Micrographic Method Using Standard Diagrams (GB/T 14979-1994), and the results are grade 0.5. Embodiment 2 Cr12MoV scrap steel, Cr12 scrap steel and 4Cr13 scrap steel are used as steelmaking raw materials, and 40t electric furnace smelting, 40t LF refining and 40t VD vacuum refining are performed in sequence to obtain molten steel having the qualified chemical components; and the molten steel is poured to obtain a high-toughness cold-worked die steel slab ingot. In the electric furnace smelting process: smelting is performed with a reduction method according to the conventional smelting process, and the tapping temperature is 1610°C; in the LF refining process: the temperature of the molten steel entering an LF is 1560°C, and the slag thickness is 31 mm; aluminum wires are fed by 150 m, the argon flow is adjusted (95 NL/min), 510 kg of slag lime and 140 kg of refining slag prepared in advance are added, and 180 kg of lime is added; power-on reduction is performed, 80 kg of carbon powder in total and 130 kg of a steel slag mixture in total are used for reduction in the whole process, and the addition manner of the steel slag mixture is as follows: adding 130 kg of the steel slag mixture after power-on refining for 12 min, and adding the steel slag mixture as per 50 kg, 40 kg and 40 kg every 10 min for reduction during refining; after complete reduction, the slag is white, sampling analysis (including total Al) is performed, a sample in returned, and the total Al is adjusted to 0.05wt% according to analysis results; and the holding time of the white slag is 21 min in the refining process, and a small amount of C powder (10 kg/ furnace molten steel) is added in the later period of refining so as to maintain a reduction atmosphere. According to the sampling analysis results, the chemical components are adjusted according to the chemical component control requirements of the specific steel type, and Nb iron is added as per 0.1% of the total weight of the molten steel; after LF furnace refining, the total aluminum is as follows: [Al]: 0.03wt%; well alloying is ensured, and sampling for total analysis is performed; when the chemical components are within the internal control range, [S] < 0.003wt% and the temperature is 1615°C, a ladle after deslagging is hung to VD for vacuum treatment; in the VD vacuum refining process: the thickness of the slag entering a VD furnace is 72 mm; the VD furnace vacuum treatment requirements: the limit vacuum degree is 55 Pa, and the holding time under limit vacuum is 21 min; large-flow Ar blowing is kept under limit vacuum (with the Ar blowing flow being 130 NL/min), and 2 min before vacuum breakage, the Ar blowing flow is adjusted to 50 NL/min; vacuum breakage, temperature measurement and [H] sampling are performed, and [H] is 2 ppm; after vacuum breakage of the VD vacuum furnace, 15 kg/furnace molten steel of the rare earth Re is added immediately, the rare earth Re is put into an aluminum lunch box, and then the box is directly thrown into a steel ladle; after the rare earth is added, ladle pouring can only be performed after the soft argon blowing time of 25 min, and if the soft blowing time exceeds 50 min, the [H] setting operation needs to be performed again; and the ladle temperature in the pouring process is 1506°C. The components of the high-toughness cold-worked die steel prepared in Embodiment 2 of the present invention are detected according to the method in Embodiment 1, and detection results are shown in Table 2. Table 2 Detection results of components of high-toughness cold-worked die steel prepared in Embodiment 2 of present invention C Si Mn P S Cr Mo Remainde r 0.91wt 1.36wt 0.41wt 0.02wt 0.003wt 7.45wt 2.15wt Fe

V Nb Cu Ni Al H 0 0.32wt 0.4wt% 0.15wt 0.15wt 0.025wt 2 ppm 0.002

After the slab ingot prepared in Embodiment 2 of the present invention is rolled into flat steel having the thickness of 70 mm, an edge and the core of a cross section are sampled to analyze the metallographic structures, which are as shown in FIG. 3 and FIG. 4, the unevenness of eutectic carbides is graded according to the method in Embodiment 1, and the results are grade 1.0. The present invention provides the high-toughness cold-worked die steel that has the specific components and a certain preparation process, which can achieve the purpose of reducing or improving the eutectic carbides. Said cold-worked die steel has good performance and prevents the problem of failed flaw detection caused by large-block carbides. The above embodiments are only preferred embodiments of the present invention; and it should be pointed out that those of ordinary skill in the art can also make several improvements and embellishments on the premise of not departing from the principle of the present invention, and these improvements and embellishments should also fall within the scope of protection of the present invention.

Claims

WHAT IS CLAIMED IS:

1. High-toughness cold-worked die steel, comprising the following components: 0.9wt%-0.94wt% of C; 1.35wt%-1.45wt% of Si; 0.4wt%-0.6wt% of Mn; < 0.02wt% of P; < 0.003wt% of S; 7.4wt%-7.8wt% of Cr; 2.1wt%-2.3wt% of Mo; 0.3wt%-0.35wt% of V; 0.4wt % -0.5wt% of Nb; < 0.2wt% of Cu; < 0.2wt% of Ni; 0.02%-0.03wt% of Al; < 2.5 ppm of H; < 0.002wt% of 0; and a trace amount of rare earth Re, the remainder being Fe.

2. A preparation method for the high-toughness cold-worked die steel according to claim 1, the method comprising: performing electric furnace smelting, LF refining and VD vacuum refining on alloy raw materials in sequence to obtain molten steel; and pouring the molten steel to obtain the high-toughness cold-worked die steel.

3. The method according to claim 2, wherein the tapping temperature in the electric furnace smelting process is > 1600°C.

4. The method according to claim 2, wherein the charging temperature of the molten steel in the LF refining process is > 1510°C, and the slag thickness is 31-35 mm; and an aluminum wire feeding operation is performed firstly in the LF refining process, and the feeding amount of aluminum wires is 120-150 m/furnace molten steel.

5. The method according to claim 2, wherein slag lime and refining slag are added in the LF refining process, and then lime is added; and the addition amount of the lime is 500-580 kg/furnace molten steel, the addition amount of the refining slag is 120-140 kg/furnace molten steel, and the addition amount of the added lime is 180-200 kg/furnace molten steel.

6. The method according to claim 2, wherein power-on reduction is performed in the LF refining process, and carbon powder and a steel slag mixture are used in the power-on reduction process; and the use amount of the carbon powder is 80-100 kg/furnace molten steel, and the use amount of the steel slag mixture is 100-150 kg/furnace molten steel.

7. The method according to claim 2, wherein the ladle temperature in the VD vacuum refining process is 1610-1640°C; and the thickness of slag entering a VD furnace is < 80 mm.

8. The method according to claim 2, wherein the limit vacuum degree in the VD vacuum refining process is < 60 Pa, and the holding time under limit vacuum is > 20 min.

9. The method according to claim 2, wherein large-flow Ar blowing is kept under limit vacuum in the VD vacuum refining process, and the Ar blowing flow is > 130 NL/min; and 1-2 min before vacuum breakage, the Ar blowing flow is adjusted to 20-40 NL/min.

10. The method according to claim 2, wherein the ladle temperature in the pouring process is 1505-1515°C.