Study On Microstructure And Properties Of Ultra-high Strength Boronsteel Plate For Automobile

With the development of automobile steel to the third generation,energy saving and safety are new directions for the development of automobile steel.Ultra-high strength steel is a typical representative of the third generation of automobile steel,which is widely used in automobile structural parts and reinforcing parts at abroad.At present,there are few complete ultra-high strength steel production lines for automobiles in China.In order to enrich the varieties of automobile steel,to promote the development of the market,and develop the third generation of low-cost super high-strength automobile steel plates with good comprehensive mechanical properties have been the research focus of automobile steel in recent years.The purpose of this paper is to study the microstructure and mechanical properties of ultra-high strength steel plate for automobiles.The microstructure of the test material is analyzed by optical microscope,SEM,EDS,XRD and TEM.The mechanical properties of the test material are measured by hardness and strength tests.The main tests are as follows:In order to determine the phase transition temperature and heat treatment performance of ultra-high strength steel plates for automobiles,CCT curve tests were carried out on the materials.Research shows that: For the CCT curves of 22MnB5-5V and 26MnB5-5V steels,the carbon content of 26MnB5-5V is higher,and the CCT curves move to the right and down obviously compared with the CCT curves of 22MnB5-5V.The upper transition temperature Bs of 22MnB5-5V bainite is about 580 ℃,the martensitic transition temperature Ms is about 412 ℃,while the upper transition temperature Bs of 26MnB5-5V bainite is about 530 ℃,the martensitic transition temperature Ms is about 379 ℃,which is quite different.Both 22MnB5-5V and 26MnB5-5V generate ferrite and pearlite structure at cooling rate lower than 0.5 ℃/s,generate full bainite structure at cooling rate ranging from 3 to 5 ℃/s,and generate full martensite structure at cooling rate exceeding 20 ℃/s.With the increase of cooling rate,the strength and hardness of the material show an overall upward trend.When the cooling rate is low,the material generates obvious banded structure.Carbon content and cooling rate are important factors affecting the banded structure of the material.In the actual production process,the cooling rate can be appropriately increased to eliminate the banded structure of the test material.In order to determine the corresponding process for obtaining the specific structure of ultra-high strength steel plates for automobiles,six different heat treatment methods of materials,including furnace-following,furnace-starting,air cooling,air cooling,oil cooling and water cooling,were studied to study the effects of these six different heat treatment methods on the structure and properties of materials.The research shows that the test materials 22MnB5-5V and 26MnB5-5V steel produce banded structures under furnace cooling conditions,equiaxed ferrite and pearlite structures under furnace cooling conditions,bainite structures under air cooling conditions,martensite structures under air cooling conditions,and single lath martensite structures under oil cooling and water cooling conditions,which reach the mechanical properties of ultra-high strength steel and meet the standards of the third generation of automobile steel.Tempering temperature has great influence on the microstructure and properties of the material.In order to determine the best tempering temperature of the material,the quenched 22MnB5-5V steel is subjected to four groups of tempering temperatures of 150 ℃,200 ℃,250 ℃ and 300 ℃ on the microstructure and properties of the material.Research shows that the tempered structure is mainly tempered martensite.Under the test conditions,150 ℃ is the best tempering temperature,and the tensile strength and hardness are the largest,1583 MPa and 467 HV;.Under TEM,it was observed that high density dislocations were formed in martensite lath bundles after quenching and tempering,and obvious second phase structure was precipitated and distributed dispersedly.The size of the second phase was about 25 ~ 50 nm.The material had high mechanical properties,but the plastic toughness had no obvious change.In order to explore whether the material has the possibility of cold stamping forming,and to study the effect of cold deformation on the microstructure and properties of ultra-high strength steel for automobiles,the effect of deformation pressure of 22MnB5-5V steel for automobiles at 120 kN,140 kN,160 kN,180 kN and 200 kN on the microstructure and properties of the material is studied.the research shows that the material has good cold deformation properties,and the deformation amount can reach 30% under the test conditions.With the increase of the deformation rate of the material,the greater the pressure on the test material,the greater the deformation of the test material.When the deformation is 30%,the hardness increases the most,and the maximum hardness is 537.5 HV.With the increase of punching force and deformation,the deformation hardening effect of the material will be enhanced,and the deformation of the material can improve the safety in the actual use process of the material.