Manufacturing method for very high-strength, cold-rolled, dual-phase steel sheets

1. A manufacturing method for a cold-rolled and annealed Dual-Phase steel sheet, comprising the steps of:

supplying a steel sheet having a composition comprising:

0.055%≤C≤0.095%;

2%≤Mn≤2.6%;

0.005%≤Si≤0.35%;

S≤0.005%;

P≤0.050

0.1≤Al≤0.3%;

0.05%≤Mo≤0.25%;

0.2%≤Cr≤0.5%;

Cr+2Mo≤0.6%;

Ni≤0.1%;

0.010≤Nb≤0.040%;

0.010≤Ti≤0.050%;

0.0005≤B≤0.0025% and

0.002%≤N≤0.007%,

a remainder of the composition comprising iron and the inevitable impurities resulting from smelting;

casting the steel sheet as a semi-finished product;

bringing the semi-finished product to a temperature T R in which 1150° C. ≤T R ≤1250° C.;

hot-rolling the semi-finished product with an end-of-rolling temperature TFL ≥A r3 to obtain a hot-rolled product;

coiling the hot-rolled product at a temperature T bob in which 500° C. ≤T bob ≤570° C.;

descaling the hot-rolled product;

cold-rolling with a reduction from 30 to 80% to obtain a cold-rolled product;

heating the cold-rolled product at a rate 1° C./s≤Vc ≤5° C/s to an annealing temperature T M in which Ac1+40° C. ≤T M ≤Ac3-30° C. at which the product is held for a time 30s≤t M ≤300s so as to obtain a heated and annealed product with a structure comprising austenite and a non-recrystallized ferrite fraction less than or equal to 15%;

cooling the product to a temperature less than a temperature Ms at a rate V high enough for all of the austenite to transform to martensite;

the steel sheet having a tensile strength between 980 and 1100 MPA and a microstructure consisting of 40 to 65% ferrite, 35 to 50% martensite and 0 to 10% bainite.

2. The manufacturing method as recited in claim 1 , further comprising the steps of:

cooling the heated and annealed product at a rate V R high enough to prevent transformation of the austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T zn is reached;

continuously galvanizing the product by immersion in a bath of zinc or Zn alloy at a temperature 450° C. ≤T zn ≤480° C. to obtain a galvanized product;

cooling the galvanized product to ambient temperature at a rate V′ R greater than 4° C./s to obtain a cold-rolled, annealed and galvanized steel sheet.

3. The manufacturing method as recited in claim 1 , further comprising the steps of:

cooling the heated and annealed product at a rate V R high enough to prevent transformation of the austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T zn is reached;

continuously galvanized the product by immersion in a bath of zinc or Zn alloy at a temperature 450° C. ≤T zn ≤480° C. to obtain a galvanized product;

heating the galvanized product to a temperature TG from 490 to 550° C. for a time t G from 10 to 40s to obtain a galvannealed product;

cooling the galvannealed product to ambient temperature at a rate V″ R greater than 4° C./s, to obtain a cold-rolled and galvannealed steel sheet.

4. The manufacturing method as recited in claim 1 wherein the temperature T M is from 760 to 830° C.

5. The manufacturing method as recited in claim 2 , wherein the rate of cooling V R is greater than or equal to 15° C./s.

6. The manufacturing method as recited in claim 3 , wherein the rate of cooling V R is greater than or equal to 15° C./s.

7. A method for manufacturing a structural or safety part for a motor vehicle comprising:

the manufacturing method recited in claim 1 .

8. The manufacturing method as recited in claim 1 , wherein the microstructure of the cold-rolled and annealed dual-phase steel sheet consists of ferrite and martensite.

9. The manufacturing method as recited in claim 1 , wherein the composition of the steel sheet consists essentially of:

0.055%≤C≤0.095%;

2%≤Mn≤2.6%;

0.005%≤Si≤0.35%;

S≤0.005%;

P≤0.050%;

0.1≤Al≤0.3%;

0.05%≤Mo≤0.25%;

0.2%≤Cr≤0.5%;

Cr+2Mo≤0.6%;

Ni≤0.1%;

0.010≤Nb≤0.040%;

0.010≤Ti≤0.050%;

0.0005≤B≤0.0025% and

0.002%≤N≤0.007%,

a remainder of the composition consisting of iron and the inevitable impurities resulting from smelting.