Development Of Hot Bake-hardening Steel Sheet

Hot bake-hardening steel is the latest developed new generation of auto-motive steel.Compared with traditional cold rolled bake-hardening steel,hot bake-hardening steel is improved obviously not only in its BH value,but also in the tensile strength before and after baking(BHT value).Besides,it still has many other advantages,such as low production cycle,low manufacturing cost and the use of scope widely etc.At present,Japan JFE company has dev-eloped the plate(called JFE steel as follows),while China is still bland in this field.To developing the hot bake-hardening steel,which is meet or exceed the properties of JFE steel,in this paper the components of experimental steel were designed and smelted steel by a vacuum furnace.The experiments was made at the Gleeble 1500 thermal simulator at 1100℃austenite on the undef-ormation and deformation temperature was 860℃and 920℃with 30%and 50%compressive deformation austenite.The CCT cures was drawed respecti-vely,and the tested specimen microstructure was observed.Then the hot rol-ling experiment was made on the hot rolling mills,and the microstructure and properties of experimental steel After baking and rolling was analyzed separ-ately by using OEM,SEM,TEM and tensile test machine,following are the main conclusions:1.The main metallurgical structure of experimental steel components are ferrite and pearlite.With the cooling rate and deformation increasing,the pha-se transition point of ferrite and pearlite has increased,ferrite grains become more smaller,and the distribution of pearlite become more dispersive.2.After the experimental steel is rolled,its average yield strength can re-ach more than 300MPa,the average tensile strength can reach more than 400 MPa,elongation has also reached more than 30%,n values is between 0.15 and 0.2,r value is above 1.6.After being baked,its yield strength can reach more than 400 MPa when predeformation is 2%and its tensile strength is clo-se to 500 MPa when predeformation is 10%.3.The BH value and the BHT value of hot bake-hardening steel increase continually with the increase of predeformation.When the predeformation is 2%,BH and BHT value could reach an average of 80;when prestrain beyond 7%,the increase of BH value and BHT value tends to be slow.4.Hot bake-hardening steel has good aging properties.After being maintained three months at room temperature,hot-rolled steel bake-hardening without losing efficacy;After a year,it's yield strength increases about 30 MPa,but tensile strength is almost unchanged.5.Hot bake-hardening steel and cold-rolled steel plate shares the same bake-hardening mechanism.6.Hot bake-hardening steel tensile fracture is tough fracture,and necking down is obvious.When the cooling rate reaches 19℃/S,the middle of fracture appears cracks and the crack will more deep with the increase of cooling rate.