Effects Of Steelmaking And Rolling Process On Mechanical Properties Of Steels For Low Temperature Environment

High-strength low-alloy steel has been used for more than one hundred years.With the development of market,the demand for mechanical properties of high-strength low-alloy steel,especially the steel for low temperature environment,is becoming higher and higher by the end users.It is an important engineering problem for the development and research of high-strength low-alloy steels to obtain the appropriate second phase particles by oxide metallurgy to improve the mechanical properties.In this paper,in order to investigate the effect of Zr deoxidization process on mechanical properties of high-strength low-alloy steel,the industry trial steel plates were produced in a steelwork.Two steel plates were produced in this trial,Steel A was produced by Zr deoxidation,Steel B was produced by Al deoxidation.Further,in order to explore the effects of Ti-Zr microalloying on mechanical properties of high-strength low-alloy steel,the industry trial was conducted in a steelworks.Two steel plates were produced in this trial,Steel C was produced by feeding Ti-Zr microadditives,Steel D was produced by feeding conventional Si-Ca wire.In order to investigate the effects of low temperature rolling on mechanical properties of high-strength low-alloy steel,the industry trial was also conducted.Two steel plates were produced in this trial,Steel E was produced by conventional rolling,Steel F was produced by low temperature rolling.From the experiment results,the tensile strength of the first batch of two steel plates is similar,the yield ratio of Steel A is about 0.80 and for Steel B is about 0.89.Besides,Steel A has better ductility,more stable tensile strength and more excellent low temperature toughness,whereas Steel B showed obvious anisotropy in tensile properties.The average elongation ratio of Steel A in transversal direction?TD?is 25.5�0.5%,in longitudinal direction?LD?is 26.8�0.4%,while Steel B in TD is 22.0�0.5%,in LD is23.6�0.5%.It is obvious that the average elongation ratio of Steel A is better and more stable than Steel B.The impact absorbed energy at-80?for Steel A were 223�70J?TD?and 180�95J?LD?,while for Steel B were 153�68J?TD?and 132�77J?LD?.Therefore,the Steel A has better low temperature toughness than Steel B.For the second batch of steel plates,the yield strength and tensile strength of Steel C were a little higher and more stable than Steel D.The yield strength of Steel C is 380MPa�10MPa,for Steel D is 355MPa�15MPa.The tensile strength of Steel C is 520MPa�15MPa,for Steel D is480MPa�30MPa.Besides,Steel C has better ductility and more excellent low temperature toughness than Steel D.For the third batch of steel plates,the tensile strength is similar.Besides,the impact toughness absorbed power at-100?for Steel F was 153�54J,while for Steel E was 73�43J.Therefore,the Steel F has better low temperature impact toughness than Steel E.For the first batch of investigated plates,the impurity level?O+N+S?and inclusion density of Steel A is 118ppm and 83.7 incl./mm²,respectively.Those values are higher than Steel B.For Steel B,the impurity level is 82ppm and inclusion density is 78.8incl./mm².However,the inclusions with size more than 8?m have negative effect on the mechanical properties.The density of this kind of inclusions is 0.8 incl./mm² in Steel A and the main inclusions are weak anisotropy spherical complex oxides?xZrO₂-yCaO-zAl₂O₃ with MnS phases?,while in Steel B,this value is 1.1 incl./mm²and the majority inclusions are anisotropy elongated MnS,coarse TiN and stringer xCaO-yAl₂O₃.These differences are the key point to enhance the mechanical properties of Steel A.For the second batch of investigated plates,the inclusions in Steel C were obviously modified to spherality,those inclusions were finer and evener dispersed in steel matrix than Steel D.The center segregation was weaker in Steel C,which results the mechanical properties improving and more stable.For the third batch of steel plates,the majority structure of Steel F is acicular bainitic ferrite,while in Steel E is pearle+ferrite.In addition,the grain size distribution at different thickness in Steel F is evener and finer than that of Steel E.As the result of the similar lattice parameters between ZrO₂ and MnS,fine ZrO₂ is able to act as nucleation of MnS,and modify elongated MnS and stringer xCaO-yAl₂O₃to spherical complex oxides.Meanwhile,there are some Nb elements in plates,fine ZrO₂ can also act as nucleation sites for Nb?C,N?.Those fine oxides and Nb?C,N?are excellent second phase particles to pin grain boundary and refine grains.In the future,in order to improve the mechanical properties of steel,there are demands to decrease quantities of enlongated MnS,oxide stringers and coarse TiN in steel matrix or modify those inclusions to spherical inclusions by Zr-killed or Ti-Zr microalloy technology.In the meantime,it is possible to add some Nb to get Nb?C,N?to pin grain boundary and refine grain.It is also possible to adopt low temperature rolling to product steel plate to get finer grains and more homogenous microstructures.