Hot-press formed product and method for manufacturing same

Patent metadata

Field	Value
Publication number	US-9611518-B2
Application number	US-201314372126-A
Country	US
Kind code	B2
Filing date	Mar 15, 2013
Priority date	Mar 15, 2012
Publication date	Apr 4, 2017
Grant date	Apr 4, 2017

Abstract

Provided is a hot-press molded article that can achieve a high level of balance between high strength and extension by region and has a region corresponding to an energy absorption site and a shock resistant site within a single molded article without applying a welding method by means of having first region having a metal structure containing both 80-97 area % of martensite and 3-20 area % of residual austenite, the remaining structure comprising no more than 5 area %, and a second region having a metal structure comprising 30-80 area % of ferrite, less than 30 area % (exclusive of 0 area %) of bainitic ferrite, no greater than 30 area % (exclusive of 0 area %) of martensite, and 3-20 area % of residual austenite.

First claim

The invention claimed is:

1. A hot-press formed product obtained by forming a thin steel sheet by a hot-press forming method, the product comprising:
a first region having a metal structure comprising:
80 to 97 area % martensite, and
3 to 20 area % retained austenite,
the remaining structure being 5 area % or less; and

a second region having a metal structure containing comprising:
30 to 80 area % ferrite,
less than 30 area % bainitic ferrite, exclusive of 0 area %,
30 area % or less martensite, exclusive of 0 area %, and
3 to 20 area % retained austenite.

2. The product according to claim 1 , wherein a chemical component composition of the product comprises, in mass %, with respect to the chemical component composition:
C: 0.1-0.3%;
Si: 0.5-3%;
Mn: 0.5-2%;
P: 0.05% or less, exclusive of 0%;
S: 0.05% or less, exclusive of 0%;
Al: 0.01-0.1%; and
N: 0.001-0.01% respectively, with the remainder comprising iron and inevitable impurities.

3. The product according to claim 2 , wherein the chemical component composition further comprises as other elements:
B: 0.01% or less, exclusive of 0%; and
Ti: 0.1% or less, exclusive of 0%.

4. The product according to claim 2 , wherein the chemical component composition further comprises as other elements:
at least one element selected from the group consisting of Cu, Ni, Cr and Mo in an amount of 1% or less, exclusive of 0%, in total.

5. The product according to claim 2 , wherein the chemical component composition further comprises as other elements:
V and/or Nb in an amount of 1% or less, exclusive of 0%, in total.

6. A method for manufacturing the hot-press formed product according to claim 1 by forming a thin steel sheet separatedly to a plurality of regions including at least first and second regions, the method comprising:
employing a hot rolled steel sheet having a metal structure with 50 area % or more of ferrite or a cold rolled steel sheet having been subjected to cold rolling with 30% or more of cold rolling rate as the thin steel sheet;
heating the thin steel sheet by simultaneously performing a plurality of heating treatments comprising a first heating treatment for heating the first forming region to a temperature of Ac 3 transformation point or above and 1,000° C. or below and a second heating treatment for heating the second forming region to Ac 1 transformation point or above and a temperature equivalent to (Ac 1 transformation point×0.3+Ac 3 transformation point×0.7) or below;
thereafter starting cooling with an average cooling rate of 20° C./s or more and forming by pressing jointly with a tool at least for the first forming region and the second forming region; and
finishing forming at a temperature of Ms point−50° C. or below with respect to the first forming region and the second forming region, wherein the temperature of Ms point−50° C. denotes a temperature lower than a martensitic transformation starting point by 50° C.

7. A method for manufacturing the hot-press formed product according to claim 1 by forming a thin steel sheet separatedly to a plurality of regions including at least first and second regions, the method comprising:
heating at least the first forming region and the second forming region to a temperature of Ac 3 transformation point or above and 1,000° C. or below;
maintaining the first forming region at the heating temperature and cooling the second forming region to a temperature of 700° C. or below and 500° C. or above with an average cooling rate of 10° C./s or less thereafter and before starting forming;
thereafter starting cooling with an average cooling rate of 20° C./s or more and forming by pressing jointly using a tool at least for the first forming region and the second forming region; and
finishing forming at a temperature of Ms point−50° C. or below with respect to the first forming region and the second forming region, wherein the temperature of Ms point−50° C. denotes a temperature lower than a martensitic transformation starting point by 50° C.

Assignees

• Kobe Steel Ltd

Inventors

• Naitou Junya
• Murakami Toshio
• Ikeda Shushi
• Okita Keisuke

Classifications

• C21D6/005

  containing Mn · CPC title

• C21D2211/002

  Bainite · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D9/0068 Primary

  for particular articles not mentioned below · CPC title

• C22C38/28

  with titanium or zirconium · CPC title