Hot forming method and a hot formed member

• US 20060185774A1
• Filed: 11/28/2005
• Published: 08/24/2006
• Est. Priority Date: 05/28/2003
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation, Toyoda Iron Works Company Limited, Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Original Assignee: Sumitomo Metal Industries Limited (Nippon Steel Corporation), Toyoda Iron Works Company Limited, Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Inventors: Nakata, Masahiro, Nishibata, Toshinobu, Suda, Shuntaro, Obayashi, Akira, Ichikawa, Masanobu
• Granted Patent: US 7,559,998 B2
• Time in Patent Office: Days
• US Class Current: 148/653
• CPC Class Codes: C21D 1/02 Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• CPC Class Codes: C21D 1/19 by interrupted quenching
• CPC Class Codes: C21D 7/13 by hot working
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/28 with titanium or zirconium
• CPC Class Codes: C22C 38/38 with more than 1.5% by weight of manganese
• CPC Class Codes: C22C 38/42 with copper

Abstract

A hot press-formed member having stable strength and toughness is manufactured from a high strength steel sheet by hot press forming. In the cooling stage during hot press forming, the cooling rate is at least the critical cooling rate until the Ms point is reached and it is then in the range of 25-150° C./s in the temperature range from the Ms point to 200° C. The Vickers hardness of the hot pressed member is less than the value of (maximum quenching hardness −10) and at east the value of (maximum quenching hardness −100).

Claims

1. A hot forming method comprising heating a steel sheet having a steel composition consisting essentially of, in mass percent, C:

0.15-0.45%;

Mn;

0.5-3.0%;

Cr;

0.1-0.5%;

Ti;

0.01-0.1%;

B;

0.0002-0.004%;

Si;

at most 0.5%;

P;

at most 0.05%;

S;

at most 0.05%;

Al;

at most 1%;

N;

at most 0.01%;

one or more of Ni;

at most 2%, Cu;

at most 1%, Mo;

at most 1%, V;

at most 1%, and Nb;

at most 1%; and

a remainder of Fe and unavoidable impurities to a temperature of the Ac₃ point or higher, holding it at that temperature, and then forming the heated steel sheet to the shape of a finished member, wherein the formed member is quenched by cooling from the forming temperature during forming or after forming in such a manner that the cooling rate to the Ms point is at least the critical cooling rate and that the average cooling rate from the Ms point to 200°

C. is in the range of 25-150°

C./s.

2. A hot forming method as set forth in claim 1 wherein the forming is carried out using a pair of press forming dies.

3. A hot forming method as set forth in claim 1 wherein the forming is carried out using a pair of press forming dies and after forming, cooling is carried out by injecting water between the dies.

6. A hot forming method as set forth in claim 2 wherein the forming is carried out using a pair of press forming dies and after forming, cooling is carried out by injecting water between the dies.

4. A hot formed member made from a steel sheet having a steel composition consisting essentially of, in mass percent, C:

0.15-0.45%;

Mn;

0.5-3.0%;

Cr;

0.1-0.5%;

Ti;

0.01-0.1%;

B;

0.0002-0.004%;

Si;

at most 0.5%;

P;

at most 0.05%;

S;

at most 0.05%;

Al;

at most 1%;

N;

at most 0.01%;

one or more of Ni;

at most 2%, Cu;

at most 1%, Mo;

at most 1%, V;

at most 1%, and Nb;

at most 1%; and

a remainder of Fe and unavoidable impurities, characterized in that the hardness after hot forming expressed in Vickers hardness is less than the value of (maximum quenching hardness −

10) and at least the value of (maximum quenching hardness −

100).

5. A hot formed member as set forth in claim 4 wherein the hot forming is hot press forming.