Claims
- 1. A high-tensile-strength steel with a tensile strength of not less than 900 MPa, consisting essentially of, by weight percent:C: 0.02% to 0.1%; Si: not greater than 0.6%; Mn: 0.2% to 2.5%; Ni: greater than 1.2% but not greater than 2.5%; Nb: 0.01% to 0.1%; Ti: 0.005% to 0.03%; N: 0.001% to 0.006%; Al: not greater than 0.1%; B: 0% to 0.0004%; Cu: 0% to 0.4%; Cr: 0% to 0.8%; Mo: 0% to 0.6%; V: 0% to 0.1%; and Ca: 0% to 0.006%; and balance Fe and incidental impurities; wherein the condition (a) and (b) below is satisfied, and P and S among unavoidable impurities are contained in an amount of not greater than 0.015% and not greater than 0.003%, respectively:(a): the carbon equivalent value Ceq defined by equation 1) below being 0.53% to 0.7%; Ceq=C+(Mn/6)+{(Cu+Ni)/15}+{(Cr+Mo+V)/5} 1): wherein each atomic symbol represents the content (wt. %) of the corresponding element;(b): a mixed structure of martensite and lower bainite occupying at least 90 vol. % in the microstructure; lower bainite occupying at least 2 vol. % in the mixed structure; and the aspect ratio of prior austenite grains being not less than 3.
- 2. A high-tensile-strength steel according to claim 1, wherein the value of Vs defined by equation 2) below is 0.10% to 0.42%:Vs=C+(Mn/5)+5P−(Ni/10)−(Mo/15)+(Cu/10), 2): wherein each atomic symbol represents its content (wt %).
- 3. A method of manufacturing a high-tensile-strength steel according to claim 1, comprising the steps of: heating a steel slab to a temperature of 1000° C. to 1250° C.; rolling the steel slab into a steel plate such that the accumulated reduction ratio of the steel plate in the non-recrystallization temperature zone of γ becomes not less than 50%; terminating the rolling at a temperature above the Ar3 point; and cooling the steel plate from the temperature above the Ar3 point to a temperature of not greater than 500° C. at a cooling rate of 10° C./sec to 45° C./sec as measured at the center in the thickness direction of the steel plate.
- 4. A method of manufacturing a high-tensile-strength steel according to claim 3, further comprising a step of tempering at a temperature of not higher than the Ac1 point.
- 5. A high-tensile-strength steel according to claim 1, wherein the steel is V-free.
- 6. A high-tensile-strength steel according to claim 1, wherein the steel is Mo-free.
- 7. A high-tensile-strength steel according to claim 1, wherein the steel is Cr-free.
- 8. A high-tensile-strength steel according to claim 1, wherein the steel is Cu-free.
Priority Claims (1)
Number |
Date |
Country |
Kind |
9-40839 |
Feb 1997 |
JP |
|
Parent Case Info
This application is a continuation, of application Ser. No. 09/028,574, now U.S. Pat. No. 6,045,630 filed Feb. 24, 1998.
US Referenced Citations (3)
Number |
Name |
Date |
Kind |
5454883 |
Yoshie et al. |
Oct 1995 |
|
5876521 |
Koo et al. |
Mar 1999 |
|
6066212 |
Koo et al. |
May 2000 |
|
Foreign Referenced Citations (6)
Number |
Date |
Country |
62-149814 |
Jul 1987 |
JP |
8-104922 |
Apr 1996 |
JP |
8-209287 |
Aug 1996 |
JP |
8-209288 |
Aug 1996 |
JP |
8-209290 |
Aug 1996 |
JP |
8-209291 |
Aug 1996 |
JP |
Continuations (1)
|
Number |
Date |
Country |
Parent |
09/028574 |
Feb 1998 |
US |
Child |
09/482527 |
|
US |