Study On New Type High Strength And Ultra High Strength Transformation Induced Plasticity Steels

Transformation Induced Plasticity (TRIP) steel is one of low alloy and high strength steels which are characterized by high elongation and formability, of which the plasticity is originated from strain induced phase transformation. The micro structure of TRIP steels is characterized by three main components:ferrite, bainite and retained austenite. TRIP steels are mainly used as vehicle’s structure and safety parts, in order to realize lightweighting and increase the safety due to the high strength, good formability and high absorbing crash energy. However, the mass production and application of such materials was hindered by some relevant technical problem. For example, the typical TRIP steels with a chemical composition of 0.40%C-1.50%Si-0.80%Mn shows high strength and good elongation but poor weldabilty due to the high content of C, whereas the TRIP steel with a chemical composition of 0.15%C-1.50%Si-1.50%Mn exhibits good weldability but the high content of Si seriously increases the propensity for surface oxidation and makes it difficult to hot dip galvanization and surface coating. The problems relating to welding and surface quality was solved in a new type of TRIP steels with a typical chemical composition of 0.15%C-1.50%Al-1.50%Mn. However, the high content of Al easily induces puzzle clogging during continuous casting. Besides, the increase of Al content will also increase the temperature of phase transformation from austenite to martensite, leading to unstable retained austenite and low strength.In order to solve the above problems, series high strength and ultrahigh strength TRIP steels and the corresponding production and process technologies were developed, under the supporting of National Science and Technology Supporting Program "Development of automotive cold rolling steel plate by means of ASP-cold rolling process (Grant No.: 2007BAE51B08)". The materials design was based on the ASP-cold rolling process, and sufficiently considered the characteristics of ASP production line in Ansteel. By means of the scientific calculation and advanced testing and analyzing techniques of laboratory scale and pilot production line, the relationship between chemical composition, processing, microstructures and mechanical properties was built. The main achievements include:(1) the new type high strength TRIP steel of 590 MPa grade (low content of Si, P but without Al) and 780 MPa grade (low content of Si, P and V, but without Al) were put into mass production on ASP-cold rolling process The products are up to the national standards, and exhibit good formability, weldability and coating ability. Such materials were used commercially in Shanghai Huizhong’s and FAW’s vehicles. The research results win the first prize of China Metallurgical Science and Technology Award. (2) A new type ultrahigh strength TRIP steels of 980 MPa grade containing low Si and Al bearing V were developed and produced commercially in Ansteel plant. This TRIP steel show superior mechanical properties and were stamped into vehicle’s B pillar. (3) The influences of grain refinement of TRIP steel with different chemical compositions on their microstructures and properties were studied. (4) The influence of TRIP effect on micro structure and properties of TWIP steel was studied by means of Thermodynamic calculation. The new type ultrahigh strength TWIP steel was designed according to the influence of TRIP effect and stacking fault energy on the deformation mechanism and delayed fracture. The TWIP steel with chemical composition of Fe-18%Mn-0.60%C-0.30%Si-1.50%Al was firstly and successfully produced commercially through BOF - Continuous casting - hot rolling - cold rolling - continuous annealing process in China. The product of strength and elongation of TWIP steel reaches 70GPa% and the TWIP steel sheet was stamped into high strain vehicle part spring base.The main achievements in this doctoral dissertation are listed as follows:(1) Solving the rusty on steel surface and the difficulty of coating in conventional TRIP steel with high content of Si, through replacing of Si by P or P + V. The new type high strength TRIP steels with strength grade of 590MPa and 780MPa possess good mechanical properties and surface quality. The mass production and widely application of such TRIP steels show that the surface quality meets the requirements of super automotive steel sheet. Meanwhile the hot-dip galvanizing test showed that the new type high strength TRIP steel is easily coated and exhibits good surface quality after Zn coating.(2) Solving the easily cracking of low carbon alloy steel in peritectic reaction area during the continuous casting process on ASP-cold rolling production line. A new type high strength TRIP steel with strength grade of 590 MPa and 780 MPa were designed by optimizing the alloy content of C, Si, Mn etc. Such materials do not crack during continuous casting and continuous rolling.(3) Solving the puzzle clogging of high Al TRIP steel during the continuous casting process through replacing Al by P or P + V. High strength TRIP steel with strength grade of 590 MPa and 780 MPa were developed. No strong oxides were precipitated during continuous casting, which avoids puzzle clogging and reduces inclusions. It is proved that the production process of this new type high strength TRIP steel is stable and high efficiency, and realizes sequence casting.(4) The new type high strength TRIP steel with strength grade of 590MPa and 780MPa replaced high price Al and Si by low price P in the alloy design, adopted hot delivery and hot charging technology on the middle-thin slab continuous casting and continuous rolling production line, which lower the cost of TRIP steel production, save the energy and realize "minimization" production of advanced steel series.(5) The new type high strength TRIP steel with strength grade of 590 MPa and 780 MPa realized mass production on ASP-cold rolling production line, and the products meet the property requirement of national standard. These TRIP steels show good formability, weldability and coating ability and were used in large scale in Shanghai Huizhong’s and FAW’s vehicles.(6) The influence of continuous annealing process on the microstructure of new type high strength TRIP steel was studied. The new type TRIP590 and TRIP780 steel are composed of ferrite, bainite and retained austenite. The bainite and retained austenite are mainly located in ferrite grain boundaries. The bainite is characterized by plate-shape, while the retained austenite island and film-like shape. The volume fraction of retained austenite in TRIP590 and TRIP780 is 13.5% and 12.0%, respectively.(7) Solving the technical problems to enhance the strength of TRIP steel containing Al, a new type ultrahigh strength TRIP steel containing low Si, low Al and bearing V with 980 MPa strength grade were studied and produced commercially in Ansteel plant. The TRIP 980 steels show high strength and good elongation, and were stamped into vehicle’s B pillar.(8) The influence of grain refinement on the microstructure and properties of high strength TRIP steel with different alloy systems were studied. For TRIP steel with the low Si no Al containing P the yield strength and tensile strength has been enhanced through grain refinement to 625 MPa and 859 MPa respectively, and the same time, the elongation reached 22.5%. For the Si-Mn and low Si high Al TRIP steels, the yield strength can be as high as 695MPa and 635MPa, respectively, and the tensile strength reaches 885 MPa and 859 MPa, whereas the elongation reached 23.5% and 23.0%, respectively.(9) The influence of TRIP effect and stacking fault energy on the microstructure and deformation mechanism was studied, and the thermodynamic model on the stacking fault energy was set up. The new type ultrahigh strength TWIP steel were designed according to the influence of TRIP effect on properties and the interacting among C, Mn, Si and Al. The ultrahigh strength TWIP steel with Fe-18%Mn-0.60%C-0.30%Si-1.50%Al were firstly produced industrially on the BOF-continuous casting-hot rolling-cold rolling-continuous annealing process in China by breaking through technological bottleneck of producing high Mn content steel on ASP middle-thin slab continuous casting production line. The TWIP steel sheet exhibits a product of strength and elongation of 70GPa%, and was stamped into vehicle part firstly such as spring base.