Bainite steel and preparation method therefor

Abstract

Disclosed is a bainite steel comprising the following chemical components in percentages by mass: 0.10-0.19% of C, 0.05-0.45% of Si, 1.5-2.2% of Mn, 0.001-0.0035% of B, 0.01-0.05% of Al, 0.05-0.40% of Cr, 0.05-0.40% of Mo, and more than or equal to 90% of Fe. By rationally controlling the contents of C, Si, Mn, B, Al, Cr, Mo, and the other elements in the steel, the steel can spontaneously form a phase having a structural gradient during the preparation process. In addition, the hardenability of the steel is also improved, such that the strength and forming performance of the bainite steel can be improved. Further disclosed in the present invention is a method for preparing the bainite steel. By using the preparation method of the present invention, the bainite steel having a structural gradient in the thickness direction can be prepared, and the bainite steel has good formability

1 . A bainite steel comprising the following chemical elements in mass percentages: C: 0.10˜0.19%, Si: 0.05˜0.45%, Mn: 1.5˜2.2%, B: 0.001˜0.0035%, Al: 0.01˜0.05%, Cr: 0.05˜0.40%, Mo: 0.05˜0.40%, Fe≥90%; and optionally at least one of Ti and Nb, wherein Nb is ≤0.1%, Ti is ≤0.15%. 
     
     
         2 . (canceled) 
     
     
         3 . The bainite steel according to  claim 1 , comprising the following chemical elements in mass percentages: C: 0.100.19%, Si: 0.05˜0.45%, Mn: 1.5˜2.2%, B: 0.001˜0.0035%, Al: 0.01˜0.05%, Cr: 0.05˜0.40%, Mo: 0.05˜0.40%, with a balance of Fe and unavoidable impurities. 
     
     
         4 . The bainite steel according to  claim 3 , wherein in the unavoidable impurities, P is ≤0.015%, S is ≤0.004%. 
     
     
         5 . The bainite steel according to  claim 1 , wherein the mass percentages of the chemical elements meet the following relationships:
 R=(Mn+Si)/(12*C+160*B), wherein 0.9≤R≤1.2, where each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element; optionally the mass percentages of chemical elements in the bainite steel meet the following relationship:   Q=(C+Cr+Mo+Mn/2)/R, wherein 1.15≤Q≤1.5, where the value in front of the percentage sign in the mass percentage of each element is used for calculation.   
     
     
         6 . (canceled) 
     
     
         7 . The bainite steel according to  claim 1 , wherein the bainite steel comprises two layers of surface layer structure, one layer of core structure, wherein the core structure is between the two layers of surface layer structure; optionally, the bainite steel further comprises two multi-phase layers, wherein the and the two layers of surface layer structure and the one layer of core structure form an intermediate layer that is between the two multi-phase layers. 
     
     
         8 . The bainite steel according to  claim 7 , wherein in the bainite steel, the volume of the core structure accounts for 20%˜50% of the volume of the bainite steel, and the rest is the surface layer structure. 
     
     
         9 . The bainite steel according to  claim 7 , wherein the surface layer structure comprises needle-like bainite and granular carbide precipitate phase, wherein the core structure comprises massive bainite and granular carbide precipitate phase. 
     
     
         10 . The bainite steel according to  claim 9 , wherein the needle-like bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the surface layer structure, and the massive bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the core structure. 
     
     
         11 . (canceled) 
     
     
         12 . The bainite steel according to  claim 7 , wherein in the bainite steel, the volume of the multi-phase layers accounts for 2%˜10% by volume of the bainite steel, and the rest is the intermediate layer. 
     
     
         13 . The bainite steel according to  claim 12 , wherein the multi-phase layer comprises polygonal ferrite, needle-like bainite and granular carbide precipitate phase, wherein the polygonal ferrite accounts for no more than 50% by volume of the multi-phase layer, and the polygonal ferrite, the needle-like bainite and the granular carbide precipitate phase account for no less than 99% by volume of the multi-phase layer. 
     
     
         14 . The bainite steel according to  claim 1 , wherein the bainite steel has a tensile strength of ≥1000 MPa, a yield strength of ≥800 MPa, a hole expansion ratio of ≥40%, and an elongation at break of ≥12%. 
     
     
         15 . A manufacturing method for the bainite steel according to  claim 1 , comprising steps of:
 smelting and casting;   hot rolling;   post-rolling cooling and coiling;   pickling and cold rolling;   annealing.   
     
     
         16 . The manufacturing method for the bainite steel according to  claim 15 , wherein the annealing step comprises a heating stage, a slow cooling stage, a fast cooling stage, a controlled cooling stage and an air cooling stage in sequence, wherein the cooling rates at the slow cooling stage, the fast cooling stage, and the controlled cooling stage are controlled to satisfy: the controlled cooling stage < the slow cooling stage < the fast cooling stage. 
     
     
         17 . The manufacturing method for the bainite steel according to  claim 16 , wherein:
 (1) the bainite steel is cooled to a slow cooling temperature of 720˜800° C. at a slow cooling rate of Q˜10*Q ° C./s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤Q≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element, or   (2) the bainite steel is cooled to a slow cooling temperature of 620˜700° C. at a slow cooling rate of Q˜10*Q ° C./s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤0≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element.   
     
     
         18 . The manufacturing method for the bainite steel according to  claim 17 , wherein:
 in (1), the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel, wherein the cooling gas injection pressure is controlled at 0.2*Q˜Q kPa, and the holding time of the cooling gas injection is controlled at 5˜20 seconds; and   in (2), the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel, wherein the cooling gas injection pressure is controlled at 0.05*Q˜0.15*Q kPa, and the holding time of the cooling gas injection is controlled at 5˜15 seconds.   
     
     
         19 .- 20 . (canceled) 
     
     
         21 . The manufacturing method for the bainite steel according to  claim 17 , wherein the bainite steel is cooled to a fast cooling temperature of 400˜540° C. at a fast cooling rate of 10*Q˜20*Q ° C./s at the fast cooling stage. 
     
     
         22 . The manufacturing method for the bainite steel according to  claim 21 , wherein the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel twice, wherein the first injection pressure of the cooling gas is controlled at 0.3*Q˜1.5*Q kPa, and the first holding time of the cooling gas is controlled at 1˜7 seconds; the second injection pressure of the cooling gas is controlled at 0.08*Q˜0.2*Q kPa, and the second holding time of the cooling gas is controlled at 5˜10 seconds. 
     
     
         23 . The manufacturing method for the bainite steel according to  claim 18 , wherein the cooling gas is a mixture of a reducing gas and an inert gas, wherein the reducing gas is hydrogen with a volume fraction of 1%˜8%, and the temperature of the cooling gas is controlled at 5˜50° C. 
     
     
         24 . The manufacturing method for the bainite steel according to  claim 16 , wherein at the controlled cooling stage, the controlled cooling rate is controlled at ≤Q ° C./s, the holding time of controlled cooling is 100˜200 seconds, and the controlled temperature of the bainite steel is ≥350° C. at the end of the controlled cooling stage; and/or at the heating stage, the bainite steel is heated at a heating rate of ≤50° C./s to the soaking temperature of 840˜950° C. and then held for a holding time of 60˜180 s. 
     
     
         25 . (canceled) 
     
     
         26 . The manufacturing method for the bainite steel according to  claim 15 , wherein the process parameters of the manufacturing method are controlled to meet at least one of the following:
 in the step of hot rolling, a heating temperature is controlled at 1100˜1230° C.; an initial rolling temperature of finishing rolling is controlled at 1050˜1180° C.; and a final rolling temperature of finishing rolling is controlled at 870˜930° C.;   in the step of post-rolling cooling and coiling, a cooling rate is controlled at 30˜150° C./s, and a coiling temperature is controlled at 540˜620° C.;   in the step of cold rolling, a cold rolling reduction rate is controlled at ≥30%.