Research On Mechanism Of Strength And Toughness Of Cu Bearing High Strength Low Alloy Steel

In the 1980s,the high-strength low-alloy(HSLA)steels with Cu bearing(1.0～1.5 wt.%)and low C concentration(<0.06 wt.%)have been developed in the US.Compared with traditional carbide strengthened CrNiMo high strength steels,the Cubearing HSLA steels display a better combination of strength,toughness and weldability,which have been widely used in ship building,offshore structures,etc.However,when the Cu-bearing HSLA steels obtain the peak strength,the impact energy is low which seriously limits their application in different fields.In order to improve the combination property of strength and toughness at peak strength condition,it is necessary to study the effect of Cu on toughness for the Cu-bearing HSLA steels.Therefore,the steels with different contents of Cu are prepared and the OM,SEM,TEM and APT have been used to characterize the microstructures and Cu-riced clusters for the tempered samples,respectively.The main research works and conclusions are as follows:1.The HSLA steels without Cu(0Cu)and with 1.4 wt.%Cu(1.4Cu)additon are prepared and the effect of Cu-riched clusters on strength and toughness have been studied systemically.After tempering at 450℃ for 2 h,a great number of Cu-riched clusters precipitate in the 1.4Cu steel,which can induce a strong precipitation strengthening and the maximum yield strength is about 1053 MPa.However,the Curiched clusters at the lath boundary could aggravate the stress concentration and then promote the crack initiation.Also,the segregation of Mo element at the lath boundary is restrained and the Mn and Si are segregated at the lath boundary,which will decrease the boundary bonding energy and promote the crack propagation along the lath boundary.Thus,the fracture stress will be reduced and the crack will prefer to propagate by cleavage fracture mode and the impact energy at-50℃ is only 7 J.When the tempering temperature increases to 550℃,the number density of Cu-riched clusters at lath boundary decreases remarkably,which will lead to the segregation of Mo at lath boundary.As a result,the toughness of 1.4Cu steel is improved,with the mixed fracture surface exhibits a composed of dimple and river poattern.2.By reducing the Mn content from 1 wt.%(1Mn)to 0.47 wt.%(0.47Mn),the effect of Mn on the precipitation of Cu-riched clusters,strength and toughness are investigated.With the reduction of Mn contents,the precipitation of Cu-riched clusters is retarded.Compared with the 1Mn steel,the Cu-riched clusters is smaller and it takes longer tempering time to obtain the maximum number density of Cu-riched clusters for the 0.47Mn steel tempered at 450 ℃ for 1～100 h.The yield strength attains the maximum value of about 1076 and 1088 MPa for the 1Mn and 0.47Mn steel tempered at 450℃ for 5 h,respectively.When the tempering time increases to 50 h,the yield strength of 1Mn and 0.47Mn steel decreases slightly,at about 1006 and 1028 MPa,respectively.Reducing of Mn and Si contents and prolonging the tempering time can reduce the peak concentration of Mn and Si and promote the segregation of Mo at lath boundary,which is conducive to the improvement of toughness.As a result,the impact energy at-50℃ increases from 37 J at tempering time of 5 h to 162 J at 50 h of 0.47Mn steel and that of the 1Mn steel is less than 12 J in the later condition.Apparently,the 0.47Mn steel exhibits excellent combination of strength and toughness when tempered at 450℃ for 50 h.3.By adding Al to the Cu-bearing HSLA steel,the precipitation behavior of CuNiAl nanoscale particles and their effect on mechanical properties are studied systemically.Based on thermal dynamic consideration,the Cu-riched zone will nucleate firstly at the early tempering stage due to the low critical nucleation energy.With the extension of tempering time,the Cu-riched zones will transform into Curiched clusters or act as a nucleation core to promote the heterogeneous nucleation of NiAl particles.With the proceeding of precipitation reaction,the Cu-riched clusters and NiAl particles will transformed into Cu-NiAl particles.The precipitation sequence in the studied HSLA steel can be expressed as:supersaturated solid solution Cu-riched zones→Cu-riched clusters+isolated NiAl→Cu-NiAl.The mechanical test results show that the yield strength increases significantly by the precipitation strengthening of Cu-NiAl and Iso-NiAl particles,while the elongation and impact energy are decreased significantly.