Development Of High-Strength Automotive Structure Pickled Sheet QSTE420

The pickling plate is an intermediate product between the hot rolled plate and the cold rolled plate.It is mainly used in automobile,compressor,water heater and hardware machinery industry.As the replacement of cold rolled plate in some applications,it can save costs for the enterprises.In order to produce high-quality QSTE series pickled sheet,the phase transition kinetic behavior of test steels with Nb content of 0.015%,0.030%and 0.060%were researched.Finally,the QSTE420 smelting process,hot rolling and pickling processes were developed.And a large number of trial production work was carried out.A reasonable process was worked out by long-term tracking and process optimization of actual production process.The goal of stable batch production was aslo achieved.The specific research conclusions were:(1)The phase transformation kinetics study results of QSTE420 steel showed:the best finishing temperature should be higher than 820℃,otherwise it may be in two-phase zone rolling,causing uneven microstructure and resulting in performance deterioration.When the hot rolling coiling temperature was between 520～660 ℃,it was beneficial for phase transformation by reducing the coiling temperature.It can refine the microstructure of the test steel and increase the strength of the steel sheet by increasing the cooling rate and adding Nb.(2)The results of the pilot test showed that with the increase of Nb content in the steel plate,the effect of grain refinement became more and more obvious,which brought a very significant strengthening effect.As adding 0.03%Nb content,the tensile performance of hot rolled plate with the best impact performance closed to that of QSTE420.Combined with the results of laboratory research,the trial production process of QSTE420 pickling plate was formulated.Through the long-term tracking of the actual production process,the reason for the non-conforming product was found to add low Nb content.By adjusting the chemical composition,the lower limit content of Nb element was increased,and the Nb content was controlled to be more than 0.035%,and finally a reasonable production process was obtained,so that the comprehensive pass rate of the pickled plate reached 95%or more.