| 430 ferritic stainless steels are the delegate of medium-chromium type,the domestic brand is 1Cr17.Owing to their unique properties,such as small linear expansion coefficient,large thermal conductivity,good resistance chloride stress corrosion resistance and cold working performance,they are wildely used in building,decoration,food machinery,home appliances,kitchen appliances and other industries.However,in the processing of 430 ferritic stainless steels,edge cracks occur frequently,which reduce the product quality and increase the production costs.Based on this,the present paper studied edge cracks mechanism of 430 ferritic stainless steels by means of heat treatment experiment,high temperature tensile experiment and hot compression deformation experiment;The behavior of edge cracks in the subsequent process was simulated by hot rolling experiment of pre-fabricated cracks in different processes;The resaon and influence factor of edge cracks on products were summarized by analyzing edge cracks on product.And the appropriate control measures were proposed.The main results are present as follows:(1)The effects of heating temperature,holding time and heating rate on the second phase are studied by means of optical microscope.The results show that,430 ferritic stainless steel has ferrite/austenite duplex phases at 850-1200?.When the holding time is 30min,the ratios of austenite in the columnar crystal zone and the equiaxed crystal region increase gradually with the heating temperature from 850? to 950?,reaching the highest at 950?(-46%).Then the ratios of austenite decrease and reache lowest at 1200?(?10%).It should be noted that the ratio of austenite in the equiaxed crystal region is smaller than that in the columnar crystal zone.When the heating temperature is 1170?,the ratio of austenite decreases with the prolonging of holding time and slowing of heating rate.The thermodynamic calculation for the phase diagram of test steel at high temperature is carried out by Thermo-calc software and the calculation results are in good agreement with the experiments.(2)The plasticity of 430 ferritic stainless steel in the temperature range of 750-1150? is studied by hot tensile test.The results show that the percentage reduction of area are all larger than 60%at 750-1150?,indicating 430 ferritic stainless steel has a good high temperature plasticity.Due to the embrittlement of ferrite/austenite duplex microstructure,the columnar crystal zone samples show the plastic trough at 850?.No obvious differences are shown in the horizontal sample and the longitudinal sample as the percentage reductions of area are both 68%.However,the plastic trough of the equiaxed crystal samples occurs at 900?.The percentage reductions of area for the horizontal sample and the longitudinal sample are 64.6%and 70.8%,respectively.(3)The flow stress curves of 430 ferritic stainless steels at different deformation temperature and deformation rate are obtained by high temperature uniaxial compression tests.The hot processing map for 430 ferritic stainless steels is then established.The results show that,unsafe hot working regime exists in hot deformation processing at low strain rates.At strain 0.2,the optimum deformation temperature is 925-986? and 1040-1116?.(4)By hot rolling experiment of pre-fabricated cracks,the effects of original crack depth,reduction rate,rolling temperature and side press on the expansion of cracks is studied.The results show that,with the increasing of initial crack depth,the cracks expand greater after rolling.When the initial crack depth is 3mm,with the increasing of reduction rate from 48%-66%,the crack width increases and depth decreases,which may heal the crack.In addition,The crack expand greatest at 920? when the rolling temperature are 1050?,920?and 820?.Side press at the reduction rate of 1.5%and 2.5%prevent the expansion of crack effectively.(5)The evolution of crack shape is studied by hot rolling experiment of pre-fabricated cracks.The results show that,after rolling,the pre-fabricated cracks show 3 types,i.e."V"type,"Y" type and "U" type.When the original depth is small,the crack will be "V" type at small reduction rate.With the increasing of reduction rate,the crack will change to "U" type.However,oxidation will promote the crack expansion,retarding the formation of the "U" type crack.When the original depth is large,it is easy to form "Y" type crack.The oxidation and heavy reduction promote the formation of "Y" type cracks.(6)The characteristics of cracks formed at different processes after hot rolling are studied by observing the morphology of the cracks and analyzing the quantitative analysis using EDS.The cracks formed during reheating process exhibit loose and easily broken oxide layer..The thickness of the crack is as large as 100 ?m,and the microstructure around the crack is obviously rough.The cracks formed during rolling process remain normal after rolling,and the crack oxide layer is thin.Some circular pits and small crack can be seen around the main cracks,which are caused by SiO2 inclusion.(7)The edge cracks on product can be classified into 3 types,i.e."V" type,"Y" type,and "U" type.There are many defects,such as oxidation holes,large cracks,and longitudinal minor cracks.The main ingredients of these defects are Fe and Cr oxide.The billet has defects such as subcutaneous air bubbles.The inhomogeneous deformation in the rolling process causes cracks formation in the defects and the cracks will expanded gradually,forming the edge cracks finally.The appropriate control measures are proposed in two aspects,the control of the original casting billet defects and suppression of crack expansion. |
PDF Full Text Request