Hot-work die steel and a preparation method thereof

1. A hot-work die steel, comprising the following chemical constituents:

C: 0.20-0.32 wt %, Si: ≤0.5 wt %, Mn: ≤0.5 wt %, Cr: 1.5-2.8 wt %, Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt %, V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and

a balance of iron,

wherein an alloying degree is 5-7 wt %;

wherein a tensile strength of the hot-work die steel at 700° C. is 560-700 MPa;

wherein a value of hardness of the hot-work die steel at room temperature is 32-38 HRC after holding at 700° C. for 3-5 h; and

wherein the hot-work die steel has an elongation of 14% to 16% at room temperature, a percentage reduction of area of 48% to 65% at room temperature, and an impact toughness of 52-63 J at room temperature.

2. The hot-work die steel according to claim 1 , wherein the hot-work die steel further comprises at least one of the following chemical constituents:

Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B: 0.001-0.005 wt %, Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y: 0.01-0.1 wt %.

3. The hot-work die steel according to claim 1 , wherein the hot-work die steel comprises less than 0.02 wt % of S and less than 0.02 wt % of P.

4. The hot-work die steel according to claim 1 , wherein the hot-work die steel comprises a tempered sorbite structure that retains lath characteristics after the hot-work die steel is stretched at 700° C.

5. The hot-work die steel according to claim 1 , wherein the hot-work die steel comprises a nanoscale acicular alloy carbide after the hot-work die steel is stretched at 700° C.

6. The hot-work die steel according to claim 5 , wherein the nanoscale acicular alloy carbide is: V 0.5-0.8 Mo 0.5-0.6 Cr 0.15-0.3 W 0.06-0.14 Nb 0.01-0.02 C.

7. The hot-work die steel according to claim 1 , wherein the tensile strength of the hot-work die steel at 700° C. is 600-700 MPa.

8. A method for producing the hot-work die steel according to claim 1 , comprising the following steps:

a smelting step:

preparing a raw material according to the following mass percentages:

C: 0.20-0.32 wt %, Si: <0.5 wt %, Mn: <0.5 wt %, Cr: 1.5-2.8 wt %, Mo: 1.5-2.5 wt %,

W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt %, V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a balance of iron,

processing the raw material into an electrode rod by arc smelting, secondary refining, vacuum degassing, and forging in a forging furnace;

an electroslag remelting step:

removing an oxidized layer of the electrode rod, then introducing the electrode rod into a vacuum electroslag remelting device for secondary refining,

keeping a temperature of water in the water cooling system of the electroslag remelting device not higher than 70° C., and

obtaining an electroslag ingot by electroslag remelting from the electrode rod,

wherein a melting rate is 7-12 kg/min, and a temperature of a cooling water of a crystallizer is held at 40-50° C.;

a homogenizing annealing step:

heating the electroslag ingot to 1200-1250° C. and holding for 15-23 h;

a forging step:

cooling the electroslag ingot to a forging heating temperature of 1150-1200° C. and then forging to obtain an ingot,

wherein an initial forging temperature is 1130 to 1160° C., and a final forging temperature is >850° C.;

an annealing after forging step:

introducing the ingot into an annealing furnace after the temperature of the ingot is lower than 500° C., heating to 830-890° C. at a heating rate not more than 100° C./h, holding for [120 min+r (mm)×2 min/mm] or [120 min+d (mm)/2×2 min/mm],

lowering the temperature to below 500° C. at a cooling rate of 20-40° C./h,

taking the ingot out from the annealing furnace and air-cooling to obtain an annealed ingot;

a heat treatment of fine grain step:

heating the annealed ingot to 930-1150° C. and performing a first holding for a first holding time of [(15-40) min+r (mm)×2 min/mm] or [(15-40) min+d (mm)/2×2 min/mm],

water cooling to 400-500° C. within 1-2 min, then air cooling to 250-280° C. and performing a second holding for a second holding time of 5-10 h; and then holding at a temperature of 660-700° C. for 5-10 h;

a tempering treatment step:

heating the held ingot to 980-1100° C. and holding for [(15-40) min+r (mm)×2 min/mm] or [(15-40) min+d (mm)/2×2 min/mm],

then quenching to 50-150° C., and

then tempering at 580-660° C. for 6-16 h to obtain the hot-work die steel;

wherein r is a radius of the material and d is a thickness of the material.

9. The method for producing the hot-work die steel according to claim 8 , wherein the raw material further comprises at least one of the following constituents: Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B: 0.001-0.005 wt %, Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y: 0.01-0.1 wt %.

10. The method for producing the hot-work die steel according to claim 8 , wherein the forging step includes:

forming and forging by means of a precision forging machine, wherein the forging heating temperature is 900-1050° C., the initial forging temperature is 850-950° C., and the final forging temperature is >800° C.;

alternatively, forming and forging by a hydraulic hammer or oil hydraulic press, wherein the forging heating temperature is 1150-1200° C., the initial forging temperature is 1130-1160° C., and the final forging temperature is >850° C.

11. The method for producing the hot-work die steel according to claim 8 , wherein the holding time of the annealing after forging step is 6-8 h.