| Low alloy high strength steels are widely used in ships,bridges,offshore platforms,construction and pipelines.More efficient large heat input welding methods have been adopted to improve construction efficiency and reduce the cost.However,with the increase of heat inputs,the performance of the welding heataffected zone(HAZ)significantly deteriorates,thereby increasing the safety risk of the steel during service.The key poinrt of oxide metallurgy technology is to use fine particles in steel to promote acicular ferrite(AF)nucleation and pin austenite grain boundaries,which can effectively improve the performance of HAZ,so it was widely used in the fields of welding steel.This paper takes C-Mn steel as the research object,the effects on the characteristics(type,number and size distribution,etc.)of inclusions and the formation of AF in steel were systematically studied by changing the addition order and amount of Ca,Mg and La.By changing the heat treatment conditions,the influence of austenitization temperature on austenite grain size and the formation of AF was studied,and the temperature range suitable for AF formation was clarified.The influence of welding heat input on the microstructure and toughness of HAZ was discussed.The evolution of inclusions in molten steel after Ca,Mg and La addition was studied by analyzing process samples,and the effects of different treatment conditions(Ca-treatment,Ca-Mg-treatment and Ca-Mg-La-treatment)on the characteristics of inclusions and the formation of AF in steel were discussed.The results showed that inclusions in molten steel changed from Si-Mn-Al-O to Ca-AlO,Ca-Mg-Al-O and La-Mg-Ca-Al-O successively after the addition of Ca,Mg and La.Inclusions in the final quenched samples under different treatment conditions were of the same type,which mainly consisted of oxide cores and MnS shell.Oxide cores of Ca-treatment.Ca-Mg-treatment and Ca-Mg-La-treatment were Ca-Al-TiO,Ca-Mg-Al-Ti-O and La-Mg-Ca-Al-Ti-O,respectively.After Ca-Mg-La treatment,the average size of inclusions was the smallest,while the number density was the largest.There were a large number of effective inclusions(nucleus of acicular ferrite)and induced more acicular ferrite formation.The effect of Mg,Ca and La addition order on inclusions and microstructure was studied.It was found that the inclusion element types in the final quenched samples with different addition orders were almost unchanged,which were mainly composed of oxide cores and sulfide shells,but the layered structure of the inclusions was different.When the addition order was Mg→Ca→La or Ca→Mg→La,the core mainly consisted of Mg-containing oxide,while the outside layers were composed of Ca-containing and rare earth inclusions.When La was preferentially added,the oxide core was mainly rare earth inclusions,and there was MgO formed on the outside of the inclusions.Under the addition sequence of Mg→Ca→La,the average size of inclusions was the smallest,while the number density in the final quenched sample was the largest,and a large number of effective inclusions were obtained.which induced more acicular ferrite formation.The effects of Mg,Ca and La content on the characteristics of inclusions and the formation of acicular ferrite in steel were studied under the addition sequence of Mg→Ca→La,and it was found that the typical inclusions in steel mainly consisted of Mg-Ca-containing oxides and the outer layer of rare earth inclusions and sulfides.The Mg-containing oxide changed from Mg-Al-O to MgO with the increase of Mg content,and the excessive Mg content led to the collision and aggregation of inclusions.There were a large number of effective inclusions that induced more acicular ferrite formation under the suitable Mg content(0.001 wt%0.003wt%).With the increase of Ca content,the number density of inclusions increased first and then decreased,while the average size decreased first and then increased.The precipitation of MnS on the outside of inclusions changed to CaS+MnS and CaS gradually dominate with the increase of Ca content,which weakened the effectiveness of inclusions to induce the formation of acicular ferrite.The suitable Ca content for obtaining more acicular ferrite in steel was 0.0008wt%0.0018wt%.With the increase of La content,the number of inclusions increased first and then remained unchanged,and the average size decreased first and then increased.Excessive La content led to the formation of irregular large-size rare earth inclusions,and the ability of inclusions to induce the formation of acicular ferrite was reduced.The suitable La content to obtain a large number of effective inclusions and induce more acicular ferrite formation was 0.010wt%-0.023wt%.The effects of heat treatment and cooling rates on the formation of acicular ferrite were systematically investigated.The results showed that the austenization temperature had a great influence on the austenite grain size,which affected the formation of AF.And the austenite grain size conducive to AF formation was 123180μm.Under continuous cooling conditions,the cooling rate for acicular ferrite formation in Ca and Mg-Ca treated steels was 1℃/s-3℃/s,while the cooling rate for acicular ferrite formation was widened(1℃/s-5℃/s)after Mg-Ca-La treatment.The effect of the isothermal experiment on the formation of aciculate ferrite was studied.It was found that a large amount of aciculate ferrite was obtained in Ca treated and Mg-Ca treated steels at 550℃ isothermal 60 s.While the addition of La promoted the formation of acicular ferrite,and the suitable isothermal condition for more acicular ferrite formation was 500℃ to 600℃ for 60 s in Mg-Ca-La treated steel.The effects of different treatments and welding heat inputs on the microstructure and properties of the heat-affected zone were studied by thermal simulation experiments.It was found that the precipitates in steel mainly consisted of TiN,and TiN+oxygen/sulfide with high heat stability.After Mg-Ca-La treatment,the type and number of precipitates in steel increased,while the average size of precipitates decreased from 94 nm(Ca-treated steel)to 71 nm.And the austenite grain size in the heat-affected zone was the smallest and the impact toughness was the best(184 J).When the heat input was 80 kJ/cm and 150 kJ/cm,the austenite grain size in the heat-affected zone of Mg-Ca-La treated steel was 157μm and 160μm,respectively,and the difference in impact toughness of the heat-affected zone was not significant.The austenite grain size increased and the impact toughness decreased with a further increase in heat inputs. |
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