Steel sheet and method for manufacturing steel sheet

• US 10,030,280 B2
• Filed: 10/21/2011
• Issued: 07/24/2018
• Est. Priority Date: 10/22/2010
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel & Sumitomo Metal Corporation (Nippon Steel Corporation)
• Inventors: Hayashi, Kunio, Aso, Toshimitsu, Tomokiyo, Toshimasa
• Primary Examiner(s): Walck, Brian
• Application Number:
  US13/879,049
• Publication Number:
  US 20130220490A1
• Time in Patent Office: 2,468 Days
• Field of Search: 148330, 148333, 148603
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 8/005 of ferrous alloys C21D8/02 - C21D8/12 take precedence
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0236 Cold rolling
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C21D 8/0273 Final recrystallisation annealing
• CPC Class Codes: C21D 9/46 for sheet metals
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/20 with copper
• CPC Class Codes: C22C 38/24 with vanadium
• CPC Class Codes: C22C 38/26 with niobium or tantalum
• CPC Class Codes: C22C 38/28 with titanium or zirconium
• CPC Class Codes: C22C 38/32 with boron
• CPC Class Codes: C22C 38/38 with more than 1.5% by weight of manganese
• CPC Class Codes: C22C 38/40 with nickel
• CPC Class Codes: C22C 38/42 with copper
• CPC Class Codes: C22C 38/44 with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 with vanadium
• CPC Class Codes: C22C 38/48 : with niobium or tantalum
• CPC Class Codes: C22C 38/50 : with titanium or zirconium
• CPC Class Codes: C22C 38/54 : with boron
• CPC Class Codes: C22C 38/58 : with more than 1.5% by weight of manganese

Abstract

The present invention provides a steel sheet with chemical components including, by mass %, 0.18-0.35% of C, 1.0%-3.0% of Mn, 0.01%-1.0% of Si, 0.001%-0.02% of P, 0.0005%-0.01% of S, 0.001%-0.01% of N, 0.01%-1.0% of Al, 0.005%-0.2% of Ti, 0.0002%-0.005% of B, and 0.002%-2.0% of Cr, and the balance of Fe and inevitable impurities, wherein: by volume %, a fraction of the ferrite is 50% or more, and a fraction of a non-recrystallized ferrite is 30% or less; and Cr_θ/Cr_M is 2 or less, where Cr_θ is a concentration of Cr subjected to solid solution in iron carbide and Cr_M is a concentration of Cr subjected to solid solution in a base material, or Mn₀/Mn_M is 10 or less, where Mn₀ is a concentration of Mn subjected to solid solution in an iron carbide, and Mn_M is a concentration of Mn subjected to solid solution in a base material.

Claims

1. A steel sheet with chemical components which include, by mass %, 0.18% to 0.35% of C, 1.0% to 3.0% of Mn, 0.01% to 1.0% of Si, 0.001% to 0.02% of P, 0.0005% to 0.01% of S, 0.001% to 0.01% of N, 0.01% to 1.0% of Al, 0.005% to 0.2% of Ti, 0.0002% to 0.005% of B, and 0.002% to 2.0% of Cr, and a balance of Fe and inevitable impurities, wherein:

by volume %, a fraction of a ferrite is equal to or more than 50%, and a fraction of a non-recrystallized ferrite is equal to or less than 30%;

a value of a ratio Cr_θ

/Cr_M is equal to or less than 2, where Cr_θ

is a concentration of Cr subjected to solid solution in an iron carbide and Cr_M is a concentration of Cr subjected to solid solution in a base material, or a value of a ratio Mn_θ

/Mn_M is equal to or less than 10, where Mn_θ

is a concentration of Mn subjected to solid solution in an iron carbide, and Mn_M is a concentration of Mn subjected to solid solution in a base material; and

a tensile strength average value is not more than 685 MPa.

2. The steel sheet according to claim 1, wherein the chemical components further include one or more from 0.002% to 2.0% of Mo, 0.002% to 2.0% of Nb, 0.002% to 2.0% of V, 0.002% to 2.0% of Ni, 0.002% to 2.0% of Cu, 0.002% to 2.0% of Sn, 0.0005% to 0.0050% of Ca, 0.0005% to 0.0050% of Mg, and 0.0005% to 0.0050% of REM.

3. The steel sheet according to claim 1, wherein a DI_inch value which is an index of a hardenability is equal to or more than 3.

4. The steel sheet according to claim 1, wherein a fraction of a non-segmentalized pearlite is equal to or more than 10%.

5. A method for manufacturing a steel sheet for hot stamping, the method comprising:

hot-rolling a slab containing chemical components according to claim 1 or 2, to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet which is subjected to hot-rolling;

cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and

continuously annealing the cold-rolled steel sheet which is subjected to cold-rolling, wherein the continuous annealing includes;

heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac₁°

C. and lower than Ac₃°

C.;

cooling the heated cold-rolled steel sheet from a highest heating temperature to 660°

C. at a cooling rate of equal to or less than 10°

C./s; and

holding the cooled cold-rolled steel sheet in a temperature range of 550°

C. to 660°

C. for 1 minute to 10 minutes.

6. The method for manufacturing a steel sheet for hot stamping according to claim 5, the method further comprising performing any one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating process, an alloyed molten aluminum plating process, and an electroplating process, after the continuous annealing.

7. A method for manufacturing a steel sheet for hot stamping, the method comprising:

hot-rolling a slab containing chemical components according to claim 1 or 2, to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet which is subjected to hot-rolling;

cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and

continuously annealing the cold-rolled steel sheet which is subjected to cold-rolling to obtain a steel sheet for hot stamping,

wherein, in the hot-rolling, in finish-hot-rolling configured with a machine with 5 or more consecutive rolling stands, rolling is performed by setting a finish-hot-rolling temperature F_iT in a final rolling mill F_i in a temperature range of (Ac₃−

80)°

C. to (Ac₃+40)°

C., by setting time from start of rolling in a rolling mill F<sub>i−

3</sub> which is a previous machine to the final rolling mill F_i to end of rolling in the final rolling mill F_i to be equal to or longer than 2.5 seconds, and by setting a hot-rolling temperature F<sub>i−

3</sub>T in the rolling mill F<sub>i−

3</sub> to be equal to or lower than F_iT+100°

C., and after holding in a temperature range of 600°

C. to Ar₃°

C. for 3 seconds to 40 seconds, coiling is performed, and

the continuous annealing includes;

heating the cold-rolled steel sheet to a temperature range of equal to or higher than (Ac₁−

40)°

C. and lower than Ac₃°

C.;

cooling the heated cold-rolled steel sheet from a highest heating temperature to 660°

C. at a cooling rate of equal to or less than 10°

C./s; and

holding the cooled cold-rolled steel sheet in a temperature range of 450°

C. to 660°

C. for 20 seconds to 10 minutes.

8. The method for manufacturing a steel sheet for hot stamping according to claim 7, the method further comprising performing any one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating process, an alloyed molten aluminum plating process, and an electroplating process, after the continuous annealing.