Zinc-coated steel sheet with high resistance spot weldability

Patent metadata

Field	Value
Publication number	US-12054802-B2
Application number	US-202318195578-A
Country	US
Kind code	B2
Filing date	May 10, 2023
Priority date	Jun 20, 2017
Publication date	Aug 6, 2024
Grant date	Aug 6, 2024

Abstract

A method for producing a zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa, for the fabrication of resistance spot welds containing in average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 μm or more, with steps of providing a cold-rolled steel sheet, heating cold-rolled steel sheet up to a temperature T1 between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, so that the iron is not oxidized, then adding in the furnace atmosphere, water steam or oxygen with an injection flow rate Q higher than (0.07%/h×α), α being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T≥T1, so to obtain an atmosphere (A2) with a dew point DP2 between −15° C. and the temperature Te of the iron/iron oxide equilibrium dew point, then heating the sheet from temperature T 1 up to a temperature T 2 between 720° C. and 1000° C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10 −21 atm., wherein the duration to of heating of the sheet from temperature T 1 up to the end of soaking at temperature T 2 is between 100 and 500 s, soaking the sheet at T 2 , then cooling the sheet at a rate between 10 and 400° C./s, then coating the sheet with zinc or zinc-alloy coating.

First claim

What is claimed is:

1. A zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa for the fabrication of resistance spot welds containing io average not more than two Liquid Metal Embrittlement cracks having a depth of 100 μm or more per weld, comprising:
a steel substrate, a nominal composition of the steel substrate containing, in weight percent:
0.07%≤C≤0.5%
0.3%≤Mn≤5%
0.010%≤Al≤1%
0.010%≤Si≤2.45%
with 0.35%≤(Si+Al)≤2.5%,
0.001%≤Cr≤1.0%
0.001% Mo≤0.5%
and optionally
0.005%≤Nb≤0.1%
0.005%≤V≤0.2%
0.005%≤Ti≤0.1%
0.0001%≤B≤0.004%
0.001%≤Cu≤0.5%
0.001%≤Ni≤1.0%,

a remainder being iron and unavoidable impurities from the smelting; and
a zone D 100 immediately comprised between 0 and 100 micrometers under said zinc or zinc-alloy coating, wherein an average carbon content Cav (100) satisfies in said zone D 100 :
C av(100) /C nom <0.6,

C av(100) being the average content of C in said zone D 100 , C nom being the nominal C content of the steel substrate, and:
C av(100) +(Si av(100) )/32<0.21%,

C av(100) and Si av(100) being respectively the average content of C and Si in said zone D 100 , expressed in weight %.

2. The steel sheet according to claim 1 wherein:
C av(100) +(Si av(100) /32)+(Mn av(100) /14)<0.30%,

C av(100) , Si av(100) and Mn av(100) being respectively the average content of C, Si and Mn in said zone D 100 , expressed in weight %.

3. The steel sheet according to claim 1 , wherein:
C av(100) +(Si av(100) /32)+(Mn av(100) /14)−(Al av(100) /48)+(Cr av(100) /11)<0.34%,

C av(100) , Si av(100) , Mn av(100) , Al av(100) , Cr av(100) being respectively the average content of C, Si, Mn, Al, Cr in said zone D 100 , expressed in weight %.

4. The steel sheet according to claim 1 , wherein the Mn content is not constant in said zone D 100 and wherein:
d Mnmin >1 μm,

d Mnmin being the depth in D 100 at which the Mn content is equal to the minimum value Mn min in said zone and:
d Mnmin /(Mn min /Mn nom )>8,
Mn nom being the nominal Mn content of the steel.

5. The steel sheet according to claim 1 wherein the Si content is not constant in said zone D 100 and wherein:
d Simin >1 μm,

d Simin being the depth in D 100 at which the Si content is equal to the minimum value Si min in said zone and:
d Simin /(Si min /Si nom )>4,
Si nom being the nominal Si content of the steel.

6. The steel sheet according to claim 1 wherein the impurities include S<0.003%, P<0.02% and N<0.008%.

7. A method for the fabrication of a resistance spot weld containing not more than two Liquid Metal Embrittlement cracks having a depth of 100 μm or more, comprising the following successive steps of:
providing at least two zinc or zinc-alloy coated steel sheets according to claim 1 , a thickness of said zinc or zinc-coated steel sheets being between 0.5 and 2.5 mm; then
superposing partly at least said zinc or zinc-coated steel sheets, then
applying a force comprised between 350 and 500 daN via electrodes placed perpendicularly on outer sides of the superposed sheets; then
welding the steels sheets with an intensity I between Imin and 1.10 Imax, Imin being the minimum intensity above which pullout failure is observed when the resistance spot weld is submitted to shear tensile test, Imax being the intensity at which expulsion of liquid metal starts to be observed in resistance spot welding.

8. A structural part of an automotive vehicle comprising the steel sheet according to claim 1 .

9. A method for producing a zinc or zinc-alloy coated steel sheet as recited in claim 1 comprising the successive steps of:
providing a cold-rolled steel sheet, a nominal composition of the steel sheet containing, in weight percent:
0.07%≤C≤0.5%
0.3%≤Mn≤5%
0.010%≤Al≤1%
0.010%≤Si≤2.45%
with 0.35%≤(Si+Al)≤2.5%,
0.001%≤Cr≤1.0%
0.001%≤Mo≤0.5%
and optionally
0.005%≤Nb≤0.1%
0.005%≤V≤0.2%
0.005%≤Ti≤0.1%
0.0001%≤B≤0.004%
0.001%≤Cu≤0.5%
0.001%≤Ni≤1.0%,

a remainder being iron and unavoidable impurities from smelting;
heating said cold-rolled steel sheet up to a temperature T1 between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, the balance being nitrogen and unavoidable impurities, so that the iron is not oxidized, then
adding in the furnace atmosphere, at least one element selected from the group consisting of water steam and oxygen with an injection flow rate Q higher than (0.07%/h×α), α being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T≥T1, so to obtain an atmosphere (A2) with a dew point DP2 between −15° C. and the temperature Te of the iron/iron oxide equilibrium dew point, wherein the injection flow rate Q is the injected volume of water steam or oxygen per hour divided by the volume of the furnace between an injection location of water steam or oxygen, and an end of the furnace section heated at the soaking temperature T2;
heating the sheet from said temperature T 1 up to a temperature T 2 between 720° C. and 1000° C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen, more than 0.1% CO by volume, with an oxygen partial pressure higher than 10 −21 atm., wherein the duration t D of said heating of the sheet from temperature T 1 up to the end of soaking at temperature T 2 is between 100 and 500 s;
soaking the sheet at T 2 ; then
cooling the sheet at a rate between 10 and 400° C./s; and then
coating the sheet with zinc or zinc-alloy coating.

Assignees

• Arcelormittal

Inventors

• Pipard Jean-Marc
• Cremel Sebastien
• Chakraborty Anirban
• Panahi Damon
• Girina Olga
• Ghassemi-Armaki Hassan
• Challa Venkata Surya Pavan
• Benlatreche Yacine

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C23C2/02

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title

• C23C2/0222

  in a reactive atmosphere, e.g. oxidising or reducing atmosphere (C23C2/024 takes precedence) · CPC title

• C23C2/0224

  Two or more thermal pretreatments · CPC title

• C23C2/024

  by cleaning or etching · CPC title