High strength and high toughness cast steel material and method for producing the same

1. A cast steel product comprising a cast steel material, which has a composition containing 0.14 to 0.20% by mass of C, 0.10 to 0.50% by mass of Si, 0.40 to 1.20% by mass of Mn, 2.42 to 3.00% by mass of Ni, 0.40 to 0.70% by mass of Cr, less than 0.004% by mass of Al, and 0.10 to 0.50% by mass of Mo, and further containing Fe and unavoidable impurities, wherein the cast steel material further comprises more than 90 ppm by mass and 150 ppm or less by mass of N as a compositional ingredient, and wherein the cast steel product has absorbed energy of 75 J or more measured by a Charpy impact test carried out using a test piece of V-notch according to JIS (Japanese Industrial Standard) No. 4 Test Piece and at a temperature of 0° C. and

wherein the cast steel material has a tensile strength of 620 MPa to 696 MPa.

2. The cast steel product according to claim 1 ,

wherein the product mass is 1 metric ton or more.

3. The cast steel product according to claim 1 , wherein the cast steel material further contains 0.05% by mass or less of V as a compositional ingredient.

4. The cast steel product according to claim 3 , wherein the cast steel material contains V in an amount of 0.02% by mass or more and 0.05% by mass or less.

5. The cast steel product according to claim 1 ,

wherein the cast steel material contains less than 0.01% by mass of Ti, 0.025% by mass or less of Sn, less than 0.015% by mass of P, and less than 0.015% by mass of S as the unavoidable impurities.

6. The cast steel product according to claim 1 , wherein Mn is present in a range of 0.40-0.54% by mass.

7. A method for producing a cast steel material, comprising:

a step of casting an ingot having a composition containing 0.14 to 0.20% by mass of C, 0.10 to 0.50% by mass of Si, 0.40 to 1.20% by mass of Mn, 2.20 to 3.00% by mass of Ni, 0.20 to 0.70% by mass of Cr, less than 0.005% by mass of Al, and 0.10 to 0.50% by mass of Mo, and further containing Fe and unavoidable impurities, wherein the composition of the ingot further comprises more than 80 ppm by mass and 150 ppm or less by mass of N,

an annealing step of performing a heat treatment at 1,000 to 1,100° C. on said ingot having said composition;

then a quenching step of performing a heat treatment at 850 to 950° C. on said ingot having said composition; and

then a tempering step of performing a heat treatment at 610 to 670° C. on said ingot having said composition,

wherein the annealing step and the quenching step each comprises a cooling step,

wherein in both cooling steps, cooling is performed at a cooling rate lower than that of cooling by liquid immersion,

wherein in the quenching step, cooling is performed at a cooling rate of about 300° C./hour, and

wherein the method does not include performing liquid cooling.

8. The method for producing a cast steel material according to claim 7 , further comprising a stress-relief annealing step of performing a heat treatment at less than 610° C. after the tempering step.

9. The method for producing a cast steel material according to claim 7 ,

wherein the composition of the ingot further satisfies the requirement that the ingot contains 0.05% by mass or less of V.

10. The method according to claim 9 , wherein the ingot contains 0.02% by mass or more and 0.05% by mass or less of V.

11. The method according to claim 7 , wherein the cast steel material has a tensile strength of 620 MPa or more.

12. The method according to claim 1 , wherein the ingot mass is 1 metric ton or more.

13. The method for producing a cast steel material according to claim 7 , wherein Mn is present in a range of 0.40-0.54% by mass.