Method for producing a high strength steel piece

Patent metadata

Field	Value
Publication number	US-10415112-B2
Application number	US-201515322869-A
Country	US
Kind code	B2
Filing date	Jul 23, 2015
Priority date	Jul 30, 2014
Publication date	Sep 17, 2019
Grant date	Sep 17, 2019

Abstract

A method for producing a high strength steel piece having desired mechanical properties, obtainable by a reference heat treatment comprising a first reference treatment and a final reference treatment comprising at least an overaging, The method comprising a step of heat treating on an equipment comprising at least an overaging means for which it is possible to set at least one operating point, the final treatment comprising an overaging for which it is possible to calculate two final treatment parameters OAP1 and OAP2 depending on an operating point of the overaging means. A minimum OAP1 min and a maximum OAP2 max final treatment parameters are determined in order to obtain the desired properties, at least one operating points of the overaging section means is determined such that OAP1≥OAP1 min and OAP2≤OAP2 max. The piece is heat treated accordingly. The parameters are with T (t) the temperature at QT time t Formula (I).

First claim

The invention claimed is:

1. A method for producing a high strength steel piece having desired mechanical properties, comprising determining a reference heat treatment able to obtain the desired mechanical properties, the reference heat treatment including a first reference treatment conferring to a steel piece a defined structure and a final reference treatment comprising at least an averaging, the reference heat treatment being defined by an annealing temperature AT, a quenching temperature QT, an averaging temperature PTo and a holding duration Pto at the averaging temperature;
said method for producing a high strength steel piece comprising a step of heat treating a steel piece on an equipment including at least averaging means in order to obtain the desired mechanical properties for the high strength steel piece, the step of heat treating including at least a final treatment made on a steel having a structure similar to the defined structure resulting from said first reference treatment, the final treatment including at least an averaging step made on said averaging means for which it is possible to set at least one operating point, for which it is possible to calculate two final treatment parameters OAP1 and OAP2 depending on said at least one operating point of the averaging means,
wherein the steel piece is a steel sheet produced on a continuous line and the averaging means is an averaging section of a continuous annealing line, and before entering in the averaging section, the sheet is annealed at the annealing temperature AT of the first reference treatment and quenched to the quenching temperature OT of the first reference treatment, the sheet moving at a speed V;
and the method comprises the steps of:

determining a minimum first final treatment parameter OAP1 min and a maximum second final treatment parameter OAP2 max respectively, in order to obtain the desired mechanical properties, by performing a plurality of experiments with overagings consisting in a heating from the quenching temperature OT up to a holding temperature Th at a heating speed of more than 10° C./s, a holding step at the holding temperature Th for a plurality of durations tm and a cooling down to the room temperature at a cooling speed higher than 10° C./s and selected so as not to form untempered martensite in the structure of the sheet,
determining the at least one operating point of the overaging section such that the first final treatment parameter OAP1 and the second final treatment parameter OAP2 resulting from the at least one operating point fulfill:
OAP 1> OAP 1 min and
OAP 2< OAP 2 max,

the operating points which are determined comprising at least one of the following operating points: the speed V of the sheet, a heat power and the overaging temperature, and
heat treating the piece on the equipment according to the determined operating points,
wherein, if T(t) is the temperature in ° C. of the steel sheet at the time t, to the time of the beginning of the final treatment and t f the time of the end of the final treatment: the corresponding first overaging parameter OAP1 is
OAP 1=∫ t 0 tf exp(− Q|R ( T ( t )+273)) dt,

wherein Q=activation energy of the diffusion of carbon and R=ideal gas constant, and the second averaging parameter OAP2 is:

OAP
⁢

⁢
2

=

a
*

T
0

+

b
*

(

∫

t
0

t
f

⁢

T
⁡

(
t
)

2

⁢

⁢
d
⁢

⁢
t

)

1
2

T 0 being the temperature at time t 0 ,
wherein Q=148000 J/mol, R=8.314 J/(mol·K), a=b=0.016 and t is in seconds.

2. The method according to claim 1 , wherein the desired mechanical properties are minimum values for at least a traction property and for at least a ductility property.

3. The method according to claim 1 , wherein the first reference treatment comprises an annealing at a temperature higher than the Ac1 transformation point of the steel in order to obtain, before quenching, a structure containing at least 50% of austenite, and a quenching down to the quenching temperature QT lower than the Ms transformation point of the steel in order to obtain a structure comprising just after quenching at least martensite and austenite, and the overaging is made at a temperature not less than the quenching temperature QT and lower than the Ac1 transformation point of the steel.

4. The method according to claim 3 , wherein the annealing is made at a temperature higher than Ac3 in order to obtain before quenching a structure fully austenitic.

5. The method according to claim 3 , wherein the quenching temperature QT is such that the final treatment results in a structure containing at least 10% of austenite.

6. The method according to claim 1 , wherein the final treatment comprises further to the overaging step, a hot dip coating step.

7. The method according to claim 1 , wherein, to determine the minimum first final treatment parameter and the maximum second final treatment parameter, the experiments are performed on a continuous annealing line.

8. The method according to claim 1 , wherein the steel has a chemical composition comprising in weight %:
0.1%≤C≤0.5%;
0.5%≤Si≤2%;
1%≤Mn≤7%;
Al≤2%;
P≤0.02%;
S≤0.01%;
N≤0.02%;
optionally one or more elements selected from Ni, Cr, Mo, Cu, Nb, V, Ti, Zr and B, the contents of which being such that:
Ni≤0.5%;
0.1%≤Cr≤0.5%;
0.1%≤Mo≤0.03%;
Cu≤0.5%;
0.02%≤Nb≤0.05%;
0.02%≤V≤0.05%;
0.001%≤Ti≤0.15%;
0.2%≤Zr≤0.3%;
0.0005%≤B≤0.005%;
with: Nb+V+Ti+Zr/2≤0.2%; and
a remainder being Fe and unavoidable impurities.

9. A method for producing a high strength steel piece having desired mechanical properties, comprising determining a reference heat treatment able to obtain the desired mechanical properties, the reference heat treatment comprising a first reference treatment conferring to a steel piece a defined structure and a final referen

Assignees

• Arcelormittal

Inventors

• Arlazarov Artem

Classifications

• C21D11/00

  Process control or regulation for heat treatments · CPC title

• C21D9/573

  with cooling · CPC title

• C21D1/22

  Martempering · CPC title

• C22C38/00

  Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00) · CPC title

• C21D6/005

  containing Mn · CPC title