CN110055464A - Tough hot stamping die steel of a kind of fine grain height and preparation method thereof - Google Patents

Abstract

The invention relates to a fine-grained high-strength-toughness hot stamping die steel and a preparation method thereof. (1) The composition formula of the invention is as follows (element weight percentage): C 0.28-0.40%; Si 0.20-1.05%; Mn 0.25-0.85%; Cr 3.5-5.5%; Mo 2.2-3.4%; W 0.8-2%, V 0.1-1.0%; the remaining trace residual elements are S, P, N, O, H, etc., and Fe is the remainder. (2) the process of hot stamping die steel of the present invention is smelting after batching, casting into electrode rods, and after electroslag remelting, the electrode rods are cast into circular steel ingots, and the steel ingots are homogenized at high temperature, and then upsetting and Elongation, then stress relief annealing, and finally ultra-fine treatment and quenching and tempering treatment to obtain the finished product. The hot stamping die steel of the invention has a grain size as small as about 1-4 μm, and has excellent toughness, high temperature friction and wear performance, high temperature thermal strength, thermal stability, thermal fatigue performance and high thermal conductivity.

Description

A kind of fine-grained high-strength and toughness hot stamping die steel and preparation method thereof

technical field

The invention relates to a fine-grained high-strength-toughness hot stamping die steel and a preparation method thereof, belonging to the technical field of alloy steel manufacturing.

Background technique

The high temperature and high strength steel plate is stamped and quenched in the mold. The mold must have high strength, wear resistance, high thermal fatigue performance, high thermal conductivity, thermal stability, high sealing and corrosion resistance. The thermal conductivity of steel at 600°C is about 20W/(m·K), and the wear resistance and thermal fatigue performance are not good. Later, ASSAB introduced 8418 steel (DIEVAR steel) with silicon and molybdenum, which makes it have higher performance. impact toughness and wear resistance. However, the low thermal conductivity and thermal strength and toughness of 8418 steel still cannot meet the harsh conditions of current high-strength steel hot stamping, and it is urgent to develop mold materials that can meet the needs of hot stamping.

W and Mo are effective elements to comprehensively improve wear resistance, high temperature thermal strength and thermal conductivity, and the dispersion and precipitation of WC can prevent grain coarsening. The fine and dispersed WC wear-resistant precipitates are densely pinned around the fine grains and hinder the growth of the grains. Therefore, this kind of fine-grained hot stamping die steel will have better wear resistance, high temperature thermal toughness and thermal conductivity than 8418 steel.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a fine-grained high-strength-toughness hot stamping die steel.

Another object of the present invention is to provide a method for preparing a fine-grained, high-strength and tough hot stamping die steel.

The invention provides a fine-grained high-strength-toughness hot stamping die steel with the following composition formula: element weight percentage C: 0.28-0.40%; Si: 0.20-1.05%; Mn: 0.25-0.85%; Cr: 3.5-5.5%; Mo : 2.2-3.4%; W: 0.8-2%; V: 0.1-1.0%; Fe and trace residual elements S, P, N, O, H are the remainder.

The above-mentioned die steel is named SR2019, the S generation means "Shang", which means Shanghai University of Engineering Technology, the R generation means "Ri", which means Hubei Risheng Technology Co., Ltd., and 2019 means "2019".

The preparation method of the above-mentioned fine-grained high-strength and tough hot stamping die steel is characterized in that the following technological process and steps are adopted:

1) Smelting: put the ingredients into an electric arc furnace for smelting, cast into a mold at 1500℃-1650℃ to form an electrode steel rod, and remove the oxide scale and pit defects on the surface of the electrode rod after demoulding;

2) Electroslag remelting: Electroslag remelting is carried out on the electrode rod, most of the impure impurities in the electrode rod are filtered, and its shape distribution is changed, and finally it is cast into a round steel ingot;

3) High temperature homogenization: heat the round steel ingot to 1230-1265℃, keep the temperature for (0.2~0.4)×D hours, D is the diameter of the steel ingot in cm, so that the components in the steel diffuse evenly, and then cool to the forging temperature of 1180℃;

4) Upsetting: Upsetting the 1180℃±10℃ steel ingot on the press along the height direction of the ingot to 45% height, then finishing, returning to the furnace for heating for 2-4 hours, and then performing the second upsetting and finishing, Return to the furnace for 2-4 hours, and then carry out the third upsetting and finishing, and always keep the final forging temperature above 870 °C;

5) Lengthening: The steel ingot after repeated upsetting for three times is drawn and forged to the final size, the final forging temperature is kept above 870°C, and the pit is cooled to about 350°C after stretching;

6) Stress relief annealing: heat the module to 850℃ for 10-16 hours to relieve stress, and then cold cut with the furnace;

7) Ultra-fine treatment: heat the module to 1060-1100°C for 0.2×d hours, d is the effective size of the forging, cm, water quenched to room temperature, and then heated to 860°C isothermal (0.4～0.6)×d hours, d is the effective forging Size cm, then furnace cooled to 740 ℃ isothermal (0.9 ~ 1.2) × d hours d is the effective size of the forging cm, and then cooled to room temperature with the furnace.

8) Quenching and tempering treatment: heat the module to 1060°C for 2 hours, quench it to room temperature in vacuum, temper at 560°C for 10 hours, air-cool to room temperature, temper at 600°C for 10 hours, and air-cool the furnace.

Theoretical basis of the present invention: the high temperature friction and wear properties, thermal strength, thermal stability, thermal fatigue properties and thermal conductivity of 8418 steel (DIEVAR steel) still cannot meet the current harsh conditions of hot stamping of high-strength steels. Die material for hot stamping needs. The general idea of alloying is to reduce the chromium content to improve thermal conductivity, and increase the manganese content to improve wear resistance. However, chromium element is the core element of Cr5 series die steel, and reducing chromium will make it difficult to ensure oxidation resistance and corrosion resistance; Increasing manganese increases the risk of large module manganese segregation. In addition, the current hot work die steel is still dominated by the free forging of large modules, supplemented by the ultra-refining process, and the ultra-refined precipitation structure can be obtained by a benign combination of the two, but the essential grain size that determines the impact toughness and comprehensive performance is still not controlled. In addition, on the basis of grain refinement, whether high strength and thermal stability can be improved at the same time, and good impact toughness can be maintained, this is a major problem that puzzles current mold material researchers.

W and Mo are effective elements to comprehensively improve wear resistance, high temperature thermal strength and thermal conductivity, and the dispersion and precipitation of WC can prevent grain coarsening. The fine and dispersed WC wear-resistant precipitates are densely pinned around the fine grains and hinder the growth of the grains. Therefore, this kind of fine-grained hot stamping die steel will have better wear resistance, high temperature thermal toughness and thermal conductivity than 8418 steel.

The SR2019 hot stamping die steel of the present invention has a grain size as small as about 1-4 μm, and has excellent toughness, high temperature friction and wear performance, high temperature thermal strength, thermal stability, thermal fatigue performance and high thermal conductivity.

The beneficial effects of the present invention are:

(1) The impact toughness of the invention steel SR2019 will be maintained at the excellent level of 8418 steel (DIEVAR steel), which not only has excellent high temperature friction and wear properties, high temperature thermal strength, thermal stability, thermal fatigue properties, but also partially solid solution Mo and W elements will significantly improve thermal conductivity.

(2) Since the refinement of grain size is realized from the perspective of composition, it will be beneficial to realize the stable trial production of the invention steel in conventional mass production.

(3) The present invention keeps the grain size of the hot stamping die steel above ASTM grade 8, the hardness is in the range of 50-56 HRC, and the impact energy of the 7×10×55 unnotched style is greater than 280J.

Description of drawings

Fig. 1 is the 1000 times metallographic structure diagram of the die steel of the application

Fig. 2 is the test comparison chart of the thermal stability of the die steel of the present application and the 8418 steel tempered at 600°C

Fig. 3 is the test comparison diagram of the friction and wear coefficient of the die steel of the application and 8418 steel

Detailed ways

The technical solutions of the present invention will be described below with reference to specific embodiments.

Example 1

The invention provides a fine-grained high-strength-toughness hot stamping die steel SR2019 with the following composition formula (element weight percentage): C 0.45%; Si 1.05%; Mn 0.25%; Cr 3.5%; Mo 3.4%; W 2% V 1.0% ; Fe and trace residual elements S, P, N, O, H are the remainder.

In this embodiment, the technological process and steps of SR2019 steel are as follows:

a. Smelting: put the ingredients into the electric arc furnace to smelt, and then cast into φ290mm×2050mm electrode rods, remove the oxide scale and pits and other defects after the electrode rods are demolded.

b. Electroslag remelting: Electroslag remelting and refining is carried out on the electrode rod to remove most of the impure impurities in the electrode rod and change its shape distribution, and then cast into a 1 ton round steel ingot.

c. High temperature homogenization: heat 1 ton of steel ingot to 1245℃, keep for 11 hours, and then slowly cool to 1180℃ for forging.

e. Upsetting: Upsetting the 1180°C steel ingot along the height direction of the ingot to 40% height, then finishing, returning to the furnace for heating for 3 hours, and then performing the second upsetting and finishing, returning to the furnace for 3 hours, and then performing the third Secondary upsetting, finishing, keep the final forging temperature above 870 ℃.

f. Drawing length: Forging and drawing the steel ingot after repeated upsetting three times to the final size of 165mm×520mm×1550mm, keeping the final forging temperature above 870°C, and cooling the pit to about 350°C after drawing.

g. Stress relief annealing: heat the module to 850℃ for 10 hours to relieve stress, and then cool it with the furnace.

h. Ultra-fine treatment: heat the module to 1060°C for 3.5 hours, quench it with water to room temperature, then heat up to 860°C for 7 hours and then cool to 740°C for 15 hours, then cool to room temperature with the furnace.

i. Quenching and tempering treatment: heat the module to 1060℃ for 2 hours, vacuum air quench to room temperature, temper at 560℃ for 10 hours, air-cool to room temperature, temper at 600℃ for 10 hours, and air-cool the furnace.

Performance Testing

The performance test of the above SR2019 steel is carried out, and the results are as follows:

(1) The hardness after quenching and tempering is 53HRC;

(2) The impact energy of the unnotched 7×10×55 style is greater than 280J;

The impact energy of opening U-notch is 26J

(3) Thermal stability: SR2019 steel and 8418 steel are kept at 600 ℃ for different times, and the thermal stability is judged by the decreasing trend of hardness. The composition comparison is shown in Table 1 (wt.%), and the thermal stability data comparison is shown in Table 2 and FIG. 2 .

(4) Wear resistance: SR2019 steel and 8418 steel were subjected to room temperature friction and wear experiments, and the advantages and disadvantages of wear resistance were judged by comparing the fluctuation of friction coefficient. The lower the friction coefficient, the better the wear resistance, as shown in Figure 3.

Table 1 Composition comparison of 8418 and SR2019 steels

steel number C Si Mn Cr Mo W V S P 8418 0.36 0.25 0.45 5.2 2.25 0 0.55 0.002 0.006 SR2019 0.45 1.05 0.25 3.5 3.4 2 1 0.002 0.009

Table 2 Comparison of hardness of 8418 and SR2019 steels tempered at 600℃ for different times

steel number 5h 10h 15h 20h 25h 30h 35h 40h 8418 50 49 46 43 41 40 38 32 SR2019 53 53 50 49.5 48 47 45 42

Claims (2)

1. a kind of high tough hot stamping die steel of fine grain, it is characterised in that component prescription is as follows: according to element weight percent C 0.28-0.40%；Si 0.20-1.05%；Mn 0.25-0.85%；Cr 3.5-5.5%；Mo 2.2-3.4%；W 0.8- 2%, V 0.1-1.0%；Fe and micro residue element S, P, N, O, H are surplus.

2. a kind of preparation method of the high tough hot stamping die steel of fine grain according to claim 1, it is characterised in that use Technical process and step below:

1) melting: being put into melting in electric arc furnaces for ingredient, and formation electrode rod iron in mold is cast at 1500 DEG C -1650 DEG C, is taken off Clearing electrode stick surface scale and pit defect after mould；

2) electroslag remelting: electrode bar carries out electroslag remelting, most of not neat in filtering electrode stick, and changes its form point Cloth is cast into round steel ingot；

3) high temperature homogenization: being heated to 1230-1265 DEG C for round steel ingot, keep the temperature (0.2~0.4) × D hours, and D is that steel ingot is straight Diameter size cm, keeps component diffusion in steel uniform, is subsequently cooled to 1180 DEG C of forging temperature；

4) jumping-up: carrying out jumping-up to 45% height along steel ingot short transverse on press for 1180 DEG C of ± 10 DEG C of steel ingots, then smart It is whole, heating 2-4 hours is melted down, then carry out second of jumping-up, finishing melts down heat preservation 2-4 hours, then carries out third time jumping-up, essence It is whole, remain 870 DEG C of final forging temperature or more；

5) pull out: the steel ingot three times repeatedly after jumping-up pulled out and forged to final size, keep 870 DEG C of final forging temperature with On, hole is cooled to 350 DEG C or so after pulling；

6) stress relief annealing: module is heated to 850 DEG C of annealing 10-16 hours, stress is eliminated, then with furnace cold cut；

7) ultra fine: module is heated to 1060-1100 DEG C of heat preservation 0.2 × d hours, d is forging effective dimensions cm, water quenching To room temperature, 860 DEG C of isothermals (0.4~0.6) × d hours are then heated to, 740 DEG C of isothermal (0.9~1.2) × d are then furnace-cooled to Hour, then with being furnace-cooled to room temperature；

8) modifier treatment: being heated to 1060 DEG C for module and keep the temperature 2 hours, and vacuum air-quenching to room temperature, 560 DEG C are tempered 10 hours, empty After being cooled to room temperature, it is tempered 10 hours, comes out of the stove air-cooled at 600 DEG C.