Method for producing a steel part and steel part

Abstract

Method for producing a steel part including by weight: 0.05%≤C≤0.15%, 0.01%≤Si≤1%, 1.2%≤Mn≤2%, 0.1%≤Cr≤2%, 0.001≤Al≤0.1%, 0.003%≤N≤0.01%, 0≤S≤0.015%, 0≤P≤0.015%, 0%≤Ni≤1%, 0%≤B≤0.01%, 0%≤Mo≤1%, 0%≤Ti≤0.04%, 0%≤Nb≤0.1%, 0≤V≤0.5% the remainder consisting of iron and unavoidable impurities, annealing this semi-finished product at an annealing temperature strictly lower than the Ac1 temperature of the steel; cooling it down to room temperature; cold forming the semi-finished product into a cold formed product; subjecting the cold formed product to a heat treatment comprising heating the cold formed product to a heat treatment temperature greater than or equal to the full austenitisation temperature Ac3 of the steel; and quenching to room temperature; optionally reheating the product at a holding temperature from 180° C. to 400° C. for a time from 15 minutes to 2 hours.

What is claimed is: 
     
         1 - 16 . (canceled) 
     
     
         17 : A method for producing a steel part comprising:
 providing a semi-finished product made of a steel comprising, by weight:   0.05%≤C≤0.15%   0.01%≤Si≤1%   1.2%≤Mn≤2%   0.1%≤Cr≤2%   0.001≤Al≤0.1%   0.003%≤N≤0.01%   0≤S≤0.015%   0≤P≤0.015%   and optionally   0%≤Ni≤1%   0%≤B≤0.01%   0%≤Mo≤1%   0%≤Ti≤0.04%   0%≤Nb≤0.1%   0≤V≤0.5%   a remainder consisting of iron and unavoidable impurities,   annealing the semi-finished product at an annealing temperature strictly lower than the Ac1 temperature of the steel;   cooling the semi-finished product down to room temperature;   cold forming the semi-finished product into a cold formed product; and   subjecting the cold formed product to a heat treatment including:
 heating the cold formed product to a heat treatment temperature greater than or equal to a full austenitisation temperature Ac3 of the steel; and 
 quenching to room temperature. 
   
     
     
         18 : The method as recited in  claim 17  wherein, during the heating step of the heat treatment, the cold formed product is heated to a heat treatment temperature which is at least 50° C. greater than the full austenitisation temperature Ac3 of the steel. 
     
     
         19 : The method as recited in  claim 17  wherein the annealing temperature is greater than or equal to Ac1 minus 20° C. 
     
     
         20 : The method as recited in  claim 17  wherein the semi-finished product is a wire having diameter from 5 mm to 25 mm. 
     
     
         21 : The method as recited in  claim 17  further comprising, before the cold forming step, a preparation of the surface of the semi-finished product, including cleaning the surface of the semi-finished product and forming a lubricating coating on the surface thereof. 
     
     
         22 : The method as recited in  claim 21  wherein the step of forming a lubricating coating on the surface of the semi-finished product comprises performing a phosphate treatment and a soaping. 
     
     
         23 : The method as recited in  claim 17  wherein the carbon content of the steel is comprised from 0.08 to 0.14 wt %. 
     
     
         24 : The method as recited in  claim 17  wherein the manganese content of the steel is comprised from 1.3 to 1.9 wt %. 
     
     
         25 : The method as recited in  claim 17  wherein the chromium content of the steel is comprised from 0.2 to 1.6 wt %. 
     
     
         26 : The method as recited in  claim 17  wherein the cold forming step is a cold heading step. 
     
     
         27 : The method as recited in  claim 17  wherein, during the holding step, the product is held at the holding temperature by dipping in a molten salt bath. 
     
     
         28 : The method as recited in  claim 17  further comprising reheating the product at a holding temperature from 180° C. to 400° C. for a time from 15 minutes to 2 hours. 
     
     
         29 : A steel part made of an alloy comprising, by weight:
 0.05%≤C≤0.15%   0.01%≤Si≤1%   1.2%≤Mn≤2%   0.1%≤Cr≤2%   0.001≤Al≤0.1%   0.003%≤N≤0.01%   0≤S≤0.015%   0≤P≤0.015%   and optionally   0%≤Ni≤1%   0%≤B≤0.01%   0%≤Mo≤1%   0%≤Ti≤0.04%   0%≤Nb≤0.1%   0≤V≤0.5%   a remainder consisting of iron and unavoidable impurities,   wherein the steel part has a microstructure comprising, at least 80 area % of bainite and a cumulative presence of martensite and residual austenite from 1 area % to 25 area %, wherein the steel part has a tensile strength of 1100 MPa or more.   
     
     
         30 : The steel part as recited in  claim 29  wherein the martensite of the steel has iron carbides in stick form with sticks being 50 to 200 nm long. 
     
     
         31 : The steel part as recited in  claim 29  wherein the steel part has a hardness from 360 HV to 405 HV. 
     
     
         32 : The steel part as recited in  claim 29  wherein the steel part has a hydrogen embrittlement index of less than 0.09. 
     
     
         33 : The steel part as recited in  claim 29  wherein the steel part has a reduction in area of more than 58%.