Fundamental Research On Stainless Steel Manufacture By CSP Technology

To produce stainless steel using CSP process may greatly increase the market competition ability of steel casting products. But in our country, the testing procedure and experienced data for CSP procedure were almost nothing. There was also little report about this in international journal. Based on the demand of techniques of the stainless steels production and the status of thin slab continuous casting development in this paper, according to numerical simulation with off-line experiment, the feasibility of production of stainless steels applying CSP process were first investigated.Firstly, on the basis of detailed derivation of the fluid flow and heat transfer coupled finite-difference turbulent equation, a three-dimensional coupled model has been formulated, using the commercial code ANSYS-CFX4 with secondary development, to first describe the fluid flow and heat transfer in the funnel mold and secondary cooling segment of CSP stainless steel casting process. In this model, the latent heat release is treated by the enthalpy method and the each effect of fluid flow and the convection heat transfer of molten steel is effectively considered. Traditionally, the effective thermal conductivity model is used to simulate the surface temperature distribution and solidification shell growth, but the effect of fluid flow on the convection heat transfer of molten steel is artificially considered with the effective conductivity coefficient, which is somewhat too simple to describe the flow and thermal field and the interaction of transport phenomena. Very little detailed information is available on solidification process of spray cooling zone and the effect of transport characteristics on the final solidification in new developed coupled fluid flow and heat transfer model. The model in this paper features are as follows: a) A large amount of data from the plant manufacture practice are used to establish and verify the model, which make the model more accuracy and trusty. b) Supposed to meet the demand of calculation, we simplify the condition with the minimum degree, for example: the complex geometrical surface of funnel shape mold is fit by the equations, and a analogous submerged entrance nozzle (SEN) structure with a two big exit ports design is using, and functionally heat flux in mould and functionally heat convection coefficient in secondary cooling segment are applied. c) The calculation domain of the model is extended to spray cooling zone of CSP casting. d) The effect of fluid flow in liquid and mushy zone on solidification process has been considered.Secondly, on condition that it is no capable of directly studying CSP stainless steel casting process, the solidification microstructure and properties of the stainless steels under variation cooling conditions are studied by the off-line experiment. The solidification and continuous casting characteristics of the stainless steels by CSP technology are investigated;Finally, a systemic comparative study has been made for the qualities of the SPA-H slabs and steel strips produced by LCR and Non-LCR in CSP. The microstructure, performance as well as the defects and inclusions during CSP are studied. The reasons for grain refinement and composition segregation in CSP process are discussed. In addition, the influences of CSP stainless steels product's quality by LCR technology are analyzed.According to above these study work, for stainless steel manufacture by CSP technology, we can draw these conclusions:(1) The fluid field character in CSP mold is similar to the one in the conventional slab continuous caster mold, which forms two violent circumfluence fields in vertical direction. The distinction lies in that the lower recirculation zone is compressed, while the upper recirculation zone is expanded accordingly. These recirculation zones are all under control in the mold cavity, when leaving the mold exit, the steel fluid enveloped in the solidification shell has the character of piston flow. Temperature distribution is consistent with flow field. There exists a high temperature region due to the superheat of steel under the exit ports of nozzle. With the violent convection and the diffusion of turbulence, the superheat is removed in the impinging jet zone from the port of SEN to the narrow face of the mold. The serration shape of the isotherms occurs at the top of liquid core, the unevenness of solidification shell can disappear gradually at the solidification end. In addition, both in water model and in numerical model, the secondary vortex phenomenon are discovered at the juncture of the narrow face and meniscus level. It was also little report about this in such studies. The formation condition of the secondary vortex, metallurgy characteristic and the influences need further research.(2) Comparison with carbon steel CSP casting technology, with the same casting speed, the stainless steel strand flow of casting SEN was weaker than that of carbon steel due to the larger viscosity, which brings to a smaller upward flow speed and a slower fluctuating of melt surface; With the same superheat temperature, the slab liquid core temperature decreases slowly, and the liquid core length of stainless steel is longer than that of carbon steel due to the lower material thermal conductivity, which lead to a higher temperature gradient and difficultly nucleate. Therefore, columnar crystals zone were broader in CSP stainless steel slab. (3) The solidifying shell thickness is getting thinner with casting speed and superheat increasing. However, it is getting thicker with secondary cooling segment water amount increasing. The variety of SEN submergence depth is not obvious with solidifying shell thickness, while it is significant with flow field. The primary reason for the influenceable factor of solidifying shell thickness is casting speed. Cooling segment water amount is of secondary importance and superheat is something nonsignificant. Casting speed must match uo to cooling segment water amount. On condition that assure solidifying shell adequate thickness at mold exit and not bring out the crack of thin-slab, we can increase casting speed to some extent and increase secondary cooling segment water amount in proportion. In addition, increasing superheat makes casting process unhindered, but high inlet temperature also let the quality of the slab down. The length of liquid core is getting longer as casting speed increased or superheat increased or secondary cooling segment water amount decreased. The variety of superheat is no longer apparent, while casting speed has an obvious influence for the length on liquid core.(4) A close relationship exists between the solidification microstructure and properties of the stainless steels and cooling conditions. Refined solidification microstructures are obtained with increasing cooling rate in 1Cr18Ni9Ti steel, and so as the tensile strength. The respective tensile strength and elongation are 990MPa and 33% at a cooling rate of 10°C/s. Further increasing the cooling rate leads to a reduction in the extent of grain refinement and tensile strength. The solidified morphologies of stainless steels are relatively complex, which have several embrittlement temperature regions during cooling process. In high-temperature embrittlement region, the surface of the slab is under tensile stress and the center of the slab is under compressive stress, while the situation is just the opposite in moderate temperature embrittlement region. The structure of stainless steel gotten by CSP technics is mainly the slim columnar dendrite and rarely has center equiaxed dendrite. Because the cooling rate of CSP is much greater than the conventional slab continuous casting, the structure with CSP process is refined and distributed uniformly. This is good for the formation of fine structure of the final strip. Furthermore, when the casting speed is faster, the time in the embrittlemet temperature ranges andγ+δdouble phrase are decreased, and thus, the brittle of stainless steel is reduced. At the same time, the opportunity of formation of crack is decreased.(5) On the basis of assumption to crystal grain shape, the token expression of three-dimension size of columnar is established along with crystal grain in microstructure of the CSP finished strip according to quantitative metallurgical analysis and probability theory. And quantitative description about three-dimension growth feature of crystal grain in microstructure of the CSP finished strip is established. The average size of axis length and length are about 9μm and 12μm, respectively. The grain size ranges in every dimension is about 5-15μm, which is about 90% of the overall grains. The tensile strength and elongation of strip reaches about 525MPa and 33%, respectively. In comparison with Non-LCR process, the surface microstructure of the slab differs remarkably from the interiors in LCR process. Moreover, there are some differences in properties of slab and strip in these two processes. In the CSP production, the microstructure of steel strip consists of refined grains with excellent properties. Due to a single pass and small reduction, there is no significant difference in microstructures and properties between finished steel strips after six-pass hot rolling. In terms of the CSP process, few macro-segregations in slab, which are mainly carbon and phosphors segregations, are observed, whereas the extent of segregation in CSP process is much smaller. The result of Magnetic Powder Test (MPT) indicates that the a few shallow cracks appear on the surface of casting billets. Ultrasonic Test (UT) results show that the defects and inclusions are distributed centrosymmetrically, and loose are present in hole-shape in the center of slabs. Moreover, the inclusions mainly consist of spherical aluminates with majority of less than 5μm after calcium treatment, which has little influence on the steel quality. It is anticipated that if LCR technology could be reasonably adopting, it would improve the solidification structure, eliminate or decrease the forming defects in casting process; and finally improve CSP stainless steels product's quality.According to above experimental and simulated results, it is showed that, with the reasonable continuous casting production institutions and relative process parameters, the application of CSP process is necessary and feasible for stainless steel productin of steel plane in our country.