Microstructure Optimization And Performance Research Of Pre-Hardened Bainitic Plastic Mold Steel

With the development of the automotive industry, the demand for plastic mold blooms with large thicknesses(particularly 700 mm to 1100 mm) has grown rapidly. Such large mold blooms, which are usually used for bumpers and dashboards, have mainly been manufactured with quench-tempered(QT) steel grade DIN 40 Cr Mn Ni Mo(1.2738). Recently, direct-cooled and tempered(DT) bainitic mold steel with medium carbon content has been developed. Compared with the conventional QT steel, the DT mold steel grade with medium carbon content does not require the addition of large amounts of expensive alloying elements like Ni, and the production process is significantly shortened by reducing the reheating process. These advantages give this grade of DT steel great application potential in many areas, but it still can not be applied in the production of plastic mould with large cross-section.Because the special production and application conditions of large steel bloom, some conventional strategies of bainite design can not be applied to effectively control the microstructures, therefore it is important to develop the relatively new design ideas of bainitic mold steel. In this paper, as a starting point, by the adjusting of cheap alloy elements Mn and Si, an old steel grade, which is suit for the bloom with the cross section less than 600 mm, are optimized to product large steel bloom for plastic mould. The optimization goals mainly include the decreasing of critical cooling rates of the proeutectoid ferrite and the volume of granular bainite; the increasing of the volume of the lower bainite; and the decreasing of the stability of retained austensite corresponding to the bainitic transformation. In this study, the pre-hardened bainite steel grade named SDP1 has been developed as an alternative to DIN 40 Cr Mn Ni Mo, the new steel grade is proved to meet the need of large steel bloom witn the cross section up to 1050 mm in the factory production.In this study, the scanning and transmission electron microscopy, XRD, dilatometric test and color metallography are used to analyse the material characters, which mainly include the phase transformation, the fracture morphology and the surface integrity. The current study can be divided into three parts. First, the poor toughness of a type of DT steel, SDFT600, was analyzed, and the relation between its toughness and microstructures during a certain process was explored. The aim is to find the key phases in which to control the toughness of this steel grade. Then based on the established relationship, the bainite transformation theory was used to optimize the alloy design. Thus a new steel grade named SDP1 was developed, and a large steel bloom of SDP1 was manufactured and tested. The results indicate that even when the cross section thickness of the SDP1 bloom reaches 1050 mm, its mechanical properties are better than those of a SDFT600 bloom with a cross section thickness of 660 mm. In the next part, the flow stress constitutive equation during the process of hot compression was obtained through a study of the thermal deformation of SDP1 steel, and the effects of processing parameters on the grain size after recrystallization were investigated. The result can help to better control the microstructures of the steel. The last part of the study is about the performance ability of SDP1 steel grade, including milling tests at different grain sizes, wear resistance and weld ability tests, aiming at further promoting the application of bainitic steel grades. The main findings are as follows:1. For the SDFT600 steel bloom of 660 mm×1210 mm×2700 mm, the toughness of the core samples decreases due to the increase of the volume of proeutectoid ferrite and the coarsening of tempered martensite/austenite constituents resulted from the slow cooling rate at the core of the mold bloom. The proeutectoid ferrite is conducive to carbide precipitation, the formation of martensite/austenite constituents, and granular bainite transformation; the relatively large tempered martensite/austenite constituents reduce the intergranular and intragranular fracture resistance. Thermal deformation will promote the transformation from austenite to bainite, presenting as a raise of the bainite start temperature at a certain cooling rate. Without the thermal deformation, the volume of proeutectoid ferrite decreases with the increase of grain, accompanying/leading to the increase of the start temperature of bainite transformation.2. The phase transformation theories, especially the T0 concept of bainite transformation, are employed to optimize bainitic steel with carbides used for large cross section molds. An alloy optimization scheme, by reducing silicon from 0.5 wt% to 0.2 wt% and increasing manganese from 1.4 wt% to 1.9 wt%, is proposed to manufacture the large cross section plastic mold of SDP1. The critical cooling rates of proeutectoid ferrite are 0.1oC·s-1 in alloy SDFT and 0.05oC·s-1 in alloy SDP1. At the same cooling rate, the proeutectoid ferrite content in alloy SDP1 is less than that of alloy SDFT600. It can be attributed to the decrease of both nucleation and growth rate of proeutectoid ferrite. For the optimized alloy SDP1, by promoting carbide precipitation and decreasing the carbon capacity of austenite, the new lower bainite transformation starts from 370oC·s-1 to 380oC·s-1 when the cooling rate is below 0.1oC s-1. Although the thickness of the bloom made of alloy SDP1 reaches 1050 mm, its mechanical properties are still better than that of alloy SDFT with a thickness of 660 mm.3. The temperature is the most important parameter in the dynamic recrystallization. The test results of single-pass compression experiment show that at 850 oC, there are no obvious dynamic recrystallization at all strain rates; at 1150 oC, the dynamic recrystallization occurs at each strain rate. At the strain rate of 0.0015 s-1, the static recrystallization occurs in the interval of the double-pass compression deformation of 850 oC, and the metadynamic recrystallization occurs in the interval of the double-pass compression deformation of 1100 oC pass deformation.4. The test results of the milling experiments show that the grain size significantly affects the residual stress of machined surface, but has little effect on the surface roughness and cutting force. As the grain size increases, the residual tensile stress of the machining plane increases from 223 MPa to 478 MPa and the surface roughness decreases from Ra 732 nm to Ra 621 nm. Another effect of the increasing of grain size is that the chip length increases; chips of different samples are composed of parallel lathes, with widths ranging from 0.5 μm to 1 μm. Different samples possess the similar value and fluctuation of cutting force.