| The application of advanced high-strength steel for automobiles not only improves the safety performance,but also makes a significant contribution to lightweight of the car.Lightweight of automobiles can effectively achieve energy conservation and emission reduction.Medium-Mn steel acting as the thirdgeneration advanced high-strength steel has become the best candidate for the manufacturing of body in white due to its excellent product of strength and elongation(PSE).However,due to the high tensile strength,the hydrogen embrittlement(HE)of medium-Mn steel become a serious limitation of its application.Fe-0.2C-11Mn-xAl(x=0,2,4)medium-Mn steels were used as the research object in this paper.Uncharged/pre-charged samples were subjected to slow strain rate tensile test(SSRT)method,and combined with optical microscope(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),Xray diffraction(XRD)and transmission electron microscopy(TEM)ethods,to systematically investigate the microstructure evolutions and HE mechanisms during deformation of the medium-Mn steels prepared by different processing and heat treatment conditions.The comprehensive mechanical properties and resistance to HE were improved by adjusting the composition and processing technologies.The main conclusions are as follows:(1)Fe-0.2C-11Mn-0/2Al mdium-Mn steels were subjected to warm+cold rolling,and then tempered(To and T2 samples)and annealed(A0 and A2),respectively.The four uncharged/charged samples before and after tensile were analyzed to study the effects of Al and heat treatment on the microstructure and HE susceptibility.The results showed that hydrogen mainly enriched in the BCC phase after precharging for 1 h.For tempered samples,the BCC phase of To was martensite which resulted into a serious hydrogen embrittlement(HE)susceptibility with the TEL loss(I?)of 91.9%and the reduction loss(I?)of 97.3%,respectively.Due to the stabilizing effect of Al on ferrite,the BCC phase of T2 steel was mainly ferrite phase and a small amount of martensite,the resistance to HE susceptibility was increased with a I? of 66.1%and I? of 63.9%,respectively.For annealed samples,the BCC phase was ferrite.The I? and I? of A0 samples were 49.6%and 61.8%,respectively,while for the A2 sample were 47.7%and 39.7%,respectively.with lower austenite stability occurred earlier than that of A2 steel.The more uniform microstructure and more stable of austenite in the A2 sample led to lowest HE susceptibility.(2)On the basis of the above research,three types of Fe-0.2C-11Mn-2Al medium-Mn steels were prepared,i.e.,HR-QT with hot rolling-750? annealing for 1 h-quenching-tempering at 200? for 1 h,and WR5-CA/WR1-CA with warm rolling-soft annealing(5 h for WR5-CA and 1 h for WR1-CA)-cold rolling-annealing at 650? for 30 min.The PSEs of them were 35.2 GPa·%,53.5 GPa·%and 57.6 GPa·%,respectively.The three samples with similar austenite volume fracitons(?-50%)were all pre-charged for 1 h,and the hydrogen concentrations were similar(1 ppm).HR-QT exhibited significant HE with the I? of 81.5%and the strength loss(I?)of 62.1%.The fracture surface was full of brittle area with mixed of intergranular cracking along prior austenite grain boundary and quasi-cleavage morphology of transgranular througn martensite.The hydrogen diffuse on rate is highest for HR-QT under loading.The warm and cold rolling process eliminated most of the original austenite grain boundaries.The fracture surface of WR5-CA and WR1-CA samples contained only a few quasi-cleavage morphologies,but without intergranular features,and the I? were 47.7%and 32.7%,while the I? were 3.2%and 3.8%,respectively.WR1-CA exhibited homogeneous microstructure for a high degree of recrystallization which contributed to the uniform of hydrogen distribution.Thus,WR1-CA sample possessed the lowest HE susceptibility.(3)In order to further increase the austenite volume fraction,on the basis of the above-mentioned as part(2),Fe-0.2C-11Mn-2Al medium-Mn steels with warm rolling-soft annealing(5 h for Steel A and 1 h for Steel B)-cold-rolling were then annealed at a high intercritical annealing temperature(700? 30 min)to obtain higher austenite volume fractions(-65%).The samples were marked as Steel A and Steel B.respectively.Compared with WR5-CA and WR1-CA samples,the annealing temperature increased resulted into increase of the austenite volume fraction,which effectively inhibited the entry of hydrogen.However,at the same time,the austenite grain size also inceased for annealing at higher temperatrure and accompanied with the decrease in stability.The phase transformation contributed more to the work hardening,and the tensile strengths reached?1500 MPa grade.Pre-charged for 1 h,the concentrations of hydrogen entering the sample were reduced to 0.2 ppm,which were significantly lower than those of WR5-CA and WR1-CA.The austenite of Steel A contained bimodal-grained microstructure(grain size span is large that the average grain size of bolcky austenite was 6.5 ?m2,and the grain size of lath austenite is 0.4 ?m2).The blocky austenite preferred to transform into martensite which led to stress concentration and then of large hydrogen-induced cracks formed,leading to higher I? of 61.5%and and I? of 27.2%.The austenite of Steel B exhibited as equiaxed grains(the average grain size was 1.3 ?m2),and overall austenite stability of Steel B was lower than that of Steel A.The transformation of equiaxed austenite for Steel B was uniform without uneven stress distribution,so that the resistance to HE was higher with the I? and I? of 28.8%and 15.9%,respectively.(4)In order to further improve the plasticity of the medium-Mn steels,the Al content was further increased to 4%.Two subsequent processes were explored for hot-rolled Fe-0.2C-11Mn-4Al medium-Mn steels:hot rolling+quenching and tempering(HR for short)and warm rolling+tempering(WR for short),respectively.Due to the coupling of multiple toughening mechanisms,i.e.,transformation induced plasticity(TRIP)effect+twinning-induced plasticity(TWIP)effect+strain anisotropy during deformation,HR sample showed the best PSE of 70 GPa·%(ultimate tensile strength(UTS):889 MPa,total elongation(TEL):79.6%).WR sample showed a PSE of 52.3 GPa·%(UTS:1336.9 MPa,TEL:39.1%).Furthermore,WR sample also exhibited an increased dislocation density and reduced of grain size of high temperature ferrite,leading to a high yield strength of 938.7 MPa.HR steel can be used in parts that require energy absorption such as front and rear anti-collision beams;WR steel with high yield strength can be used in parts that require strict intrusion prevention such as A/B pillars of the cockpit.(5)Three kinds of follow-up processes were explored for Fe-0.2C-1 1 Mn-4Al steels:warm rolling+tempering(WT),warm cold rolling+tempering(WC-T)and warm cold rolling+intercritical annealing(WC-A),respectively,to obtain different structures.After pre-charged for 1 h,the I? of the WT sample was 71.2%,while the fracture strength was 1052.0 MPa.The fracture occurred at the end of the Ludes band and there were more hydrogen-induced cracks on the surface,which appeared in the fresh martensite.WC-T with high volume fraction of pre-existing martensite showed extremely high HE susceptibility with I? of 92.0%,while the fracture strength was 990.8 MPa.The reverted film austenite of WC-A sample formed between lath martensite during intercritical annealing showed high stability.At the sme time,the annealing twins formed inside the blocky austenite to separate grain into smaller,further increased the austenite stability.There were a few surface cracks after fracture.Compared with WC-T,the I? of WC-A reduced to 57.2%.However,after annealing,a large number of ferrite grains with large-size and lowdislocation density were easy to occur transgranular crack,which led to a lower fracture strength(764.1 MPa). |
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