Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-tempered steel

Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al2O3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.

Research into Grinding Hardening of Microalloyed Non-quenched and Tempered Steel

Grinding hardening is a new technology of hardening steel piece surfaces with grinding heat generated in the grinding process instead of with a high or medium frequency induction heating method,which can effectively integrate grinding and surface hardening. Experimental studies were carried out on grinding hardening of non-quenched and tempered steel. Through grinding experiments with variable depths of cut and feeding rate,the variation in the depth of the hardening layer was studied and the microstructure of the hardening zone of the test pieces was subsequently ana-lyzed. In the end,the hardening effect of non-quenched and tempered steel was compared with that of 40Cr steel,which revealed the superiority of non-quenched and tempered steel in grinding hardening technology.

Development of non-quenched and tempered bolt steel

The 8.8 grade non-quenched and tempered bolt steel was studied according to the process conditions of wire rod plant and customer requirments.Three types of experimental steel grades were selected.10MnSiTi Nb and 20Mn2VTi(N) were chosen as the formal steel after several experimemts.

Theoretical calculation of the impact work in the alloying non-quenched and tempered steel

Coupled with hot-continuous rolling technology and based on the calculation of the finishing rolling impact work in the non-quenched and tempered Si-Mn steel, the calculations of the finishing rolling impact work in the alloying non-quenched and tempered steel with the elements of Cr, Ni, Mo, W, Cu, V, Nb and Ti are studied with the covalent electron number nA of the strongest bond in alloying phases, the smallest electron density difference ?ρ of phase interfaces, and the number of atom states σ (σ′) which keep the interface electron density continuous. The calculated results show that the finishing rolling impact work of the alloying non-quenched and tempered steel intensely depends on strengthening mechanisms. The solution strengthening, interface strengthening, precipita- tion strengthening of pearlite, and dispersion strengthening will result in the decrease of the finishing rolling impact work; the refinement strengthening, the precipitation strength- ening of V, Nb and Ti in α-Fe-C-V(Nb, Ti), and the residual austenite containing Ni on the boundary of α-Fe-C-Ni will increase the impact work; and the increments or decrements can be calculated with nA, ?ρ, σ (σ′) and weights of alloying elements. The calculation formulas of the finishing rolling impact work in this paper are intergraded with the sug- gested ones of the finishing rolling tensile strength, yield strength, and elongation of the non-quenched and tempered steel. The calculated results agree well with the measured ones.

Theoretical calculation of the finishing rolling impact work in non-quenched and tempered Si-Mn steel

Based on the hot-continuous rolling technology, the finishing rolling impact work α k of the non-quenched and tempered Si-Mn steel is theoretically calculated with the covalent electron number nA of the strongest bond in alloying phases, and the smallest interface electron density difference Δρ of alloying phase interface and the number of atom states σ which keep the interface electron density continuous. Calculations show that the solution strengthening, the precipitation strengthening, and the interface strengthening will result in the decrease of the finishing rolling impact work α k, and the effects of the number of atom states σ which keep the interface electron density continuous on the finishing rolling impact work α k are different. Taking the impact work and the number of atom states σ 0 keeping the electron density continuous of the phase interface α-Fe/α-Fe-C between α-Fe and α-Fe-C as reference values, the impact work of the interface will increase when σ of some interface is larger than σ 0; otherwise, the impact will decrease. Therefore, the finishing rolling impact work α k can be calculated with the impact value of the refined α-Fe matrix and the influence amounts caused by the solution strengthening, the precipitation strengthening, the interface strengthening, and the number of atom states σ which keep the interface electron density continuous. The calculated results agree well with the measured ones. In this paper, the effect of S on the impact work is also discussed.

Effect of Nb-V Microalloying on Microstructures and Mechanical Properties of Medium Carbon Non-Quenched and Tempered Steel

Based on optical microscope(OM),transmission electron microscope(TEM) and mechanical performance measurement,the microstructures and mechanical properties of Nb-V micro-alloying non-quenched and tempered steels have been studied.The results showed that the microstructure consists of ferrite and pearlite,in which there exists a lot of intragranular ferrite.Niobium carbide is the main form of carbonitrides,Nb-enriched carbonitrides refine grains,V-enriched carbonitrides have precipitation strengthening effect,which promotes the toughness of the studied steel.The mechanical properties for steels 1,2 and 3 have met the standards required by high load automobile crankshaft,in which the comprehensive property for No.2 is the best.

38MnVTi非调质钢的TTT曲线的测定与分析

研究了38MnVTi非调质钢在等温冷却过程中的奥氏体转变产物及转变量与时间的关系。通过Gleeble-1 500D热模拟试验机获得不同等温温度的膨胀曲线。采用膨胀法,通过Gleeble-1 500D热模拟试验机获得不同等温温度的膨胀曲线,再计算和运用Origin软件拟合得到38MnVTi非调质钢的等温转变曲线(TTT曲线)。结果表明:38MnVTi非调质钢渗碳体的析出温度为750℃、铁素体析出温度为840℃、马氏体析出温度为360℃以及马氏体转变终止温度为212℃。在540、510、480、430℃等温冷却条件下贝氏体的最大转变量分别为16.9%、26.8%、38.5%、55.3%。

35CrMo钢蓄能器开裂原因

某公司生产的蓄能器在使用1 a后发生开裂现象。采用宏观观察、化学成分分析、力学性能测试、扫描电镜分析、金相检验等方法对蓄能器的开裂原因进行分析。结果表明:制做蓄能器壳体的35CrMo钢存在微裂纹;蓄能器工作时,内部介质气体和液压油产生的压力使微裂纹逐步扩展并贯穿管壁,导致蓄能器开裂。

一种汽车用微合金非调质钢的连续冷却转变

测定了一种汽车用微合金非调质钢的过冷奥氏体连续冷却转变曲线,研究了冷却速率对相变组织及显微硬度的影响。结果表明:试验钢的临界点A c3为838℃,A c1为732℃;当冷却速率小于0.2℃/s时,试验钢的连续冷却转变产物为铁素体、珠光体和贝氏体;当冷却速率为0.2℃/s时,转变产物中出现马氏体;当冷却速率为5℃/s时,铁素体、珠光体消失,转变产物为贝氏体和马氏体;随着冷却速率的增大,马氏体含量逐渐增多,贝氏体含量逐渐减少,甚至完全消失;当冷却速率增大至20℃/s时,转变产物均为马氏体;随着冷却速率的增大,试验钢的显微硬度呈先快速增长,后增长速率变缓的趋势。

8.8级非调紧固件用10B21盘条研究与开发

采用转炉→精炼炉→真空脱气→方坯连铸→加热→粗轧→中轧→精轧→减定径→吐丝→XDWP(Xingcheng Direct Water Patenting)冷却→斯太尔摩风冷冷却的方式试验生产基于10B21钢的8.8级非调紧固件用热轧盘条。通过控制减定径温度为813℃,吐丝温度为828℃,斯太尔摩辊道速率设置为0.75 m/s,在连续经过XDWP冷却和斯太尔摩风冷后获得的热轧盘条抗拉强度达到680 MPa,断后伸长率为23.6%,断面收缩率为75%,力学性能完全满足GB/T 29087―2012的要求。且经过1/4冷顶锻和25%的大减面率拉拔试验,其结果满足紧固件用户使用要求。

活塞杆电镀后表面缺陷原因分析

针对非调钢活塞杆电镀后表面缺陷,通过宏观、金相、扫描电镜等分析,发现表面缺陷为磨削工序中的铁屑经超声碱洗未清除,电镀后与基体结合力不良,在抛光工序中镀层脱落形成的镀层起皮缺陷。在活塞杆电镀前加强表面清洗、表面检查,对表面存在的氧化皮、粘附异物进行清理和去除,可有效减少活塞杆表面镀层起皮缺陷。

非调质钢1E1106“黑心”问题研究

对不同规格不同程度出现“黑心”超标试样进行低倍、成分偏析、晶粒度和金相组织分析,结果显示,由于成分C、Mn偏聚在钢坯中心,造成心部大颗粒珠光体产生,表明成分偏析是导致非调质钢1E1106冷蚀“黑心”的主要原因。

轧后控制冷却对非调质钢F45MnVS组织和性能的影响

对非调质钢F45MnVS进行轧后控制冷却试验,对比了轧后未冷却与控制冷却的钢材热轧态力学性能、脱碳层和显微组织。结果表明,轧后控制冷却对钢材下屈服强度、延伸率、断面收缩率和常温冲击功均有明显改善作用;钢材表面脱碳层在轧后控制冷却条件下可以得到有效控制,甚至脱碳层“消失”;在钢材轧后表面温度冷却至650℃时,表层区域出现少量粒状贝氏体,而在钢材轧后表面温度冷却至460℃时,表层区域出现大量粒状贝氏体。轧后控制冷却使钢材表层区域和1/2半径附近的铁素体和珠光体组织被细化,铁素体网状比例降低,长条铁素体转变为块状铁素体;因钢材尺寸较大,心部区域显微组织差异性较小。

汽车曲轴用非调质钢C38+N的热变形行为研究

以重型汽车曲轴用C38+N非调质钢为对象,对其热变形行为开展实验研究。对C38+N钢在850~1250℃保温不同时间时的奥氏体晶粒长大行为进行研究。得出模铸轧材和连铸轧材C38+N钢奥氏体晶粒长大激活能分别为7.49×10^(4)J/mol、7.87×10^(4)J/mol。计算并建立了不同实验状态下的奥氏体晶粒长大模型,分析了C38+N钢模铸轧材、连铸轧材奥氏体晶粒长大行为差异的原因是连铸轧材比模铸轧材的MnS分布更加弥散,MnS对奥氏体晶界有钉扎作用。

V(C,N)在V-N微合金非调质钢奥氏体中的析出动力学

在Gleeble-3500热模拟试验机上采用应力松弛法,研究了38MnSiVS非调质钢在750~950℃、10%~40%变形量条件下的析出-时间-温度(precipitation-time-temperature,PTT)曲线。基于Dutta和Sellars提出的应变诱导析出模型(DS模型),建立了等温析出动力学模型,计算得到理论PTT曲线。通过与试验PTT曲线比较,得出模型对钢的析出鼻尖温度、析出开始时间和结束时间的预测较为精准。采用“时间补偿法”建立了连续冷却析出动力学模型,计算出连续冷却过程析出相的体积分数和平均尺寸,与透射电子显微镜和能谱分析结果基本一致,而且冷速的增大可抑制钒的析出,细化析出相,表明模型可以准确地预测钒微合金非调质钢的连续冷却析出过程。最后,通过析出强化增量计算得出,V(C,N)析出强化对38MnSiVS非调质钢强度的贡献较大。

35MnVS非调质钢针状铁素体形核原位观察

非调质钢生产过程中由于省去了淬火、回火等热处理工序,符合倡导的高效节能钢的发展要求。然而,非调质钢产品仍会出现铁素体晶粒粗大等问题,导致组织和性能不均匀,严重影响其使用寿命。本研究利用高温激光共聚焦显微镜,采用样品表面熔化的方法,研究非调质钢中微观组织的演变过程及铁素体形核规律,以及铁素体异质形核的特点。结果表明:非调质试样钢凝固过程相的转变阶段主要分为:1480℃时液相开始向奥氏体转变,1420℃到1400℃为TiN-MnS夹杂物的形核析出温度区间,780℃为α-Fe相的开始形核温度;原位观察实验中,当样品温度下降到1420℃左右时,钢中开始析出尺寸细小的Al 2O 3-MgO夹杂物,而试样冷却至室温时,试验钢中的夹杂物主要成分为TiN-MnS;热力学计算结果表明,在785℃时奥氏体开始向铁素体发生转变,α-Fe含量不断增多,并在690℃时达到最大值,和原位观察实验结果基本一致。本研究对理解氧化物冶金过程,及进一步改进该工艺和应用效果,均具有重要意义。

含硫非调质钢中硫化锰夹杂物的控制

在非调质钢中加入硫元素用于改善钢材切削性能,但硫与锰结合形成MnS夹杂严重影响钢材的横向冲击性能,因此含硫钢中的MnS夹杂物的控制是该类钢种生产的主要难题之一。对含硫非调质钢中MnS进行研究发现,在铸坯的柱状晶区域,MnS夹杂物分布均匀、量多细小;在等轴晶区域,MnS夹杂物分布不均匀,量少粗大;MnS超标比例为45%。通过在连铸过程中降低拉速和过热度、调整冷却参数,控制铸坯冷却速度,MnS夹杂显著细化,主要以2.5~3.5为主;在热处理过程中,随着加热温度由800℃提高到1200℃,MnS分裂球化逐渐明显。调整连铸和加热工艺参数后,MnS超标比例降至10%,铸坯不合格率大大降低。

热规范对35MnVS非调质钢连杆锻件组织和硬度的影响

研究了35MnVS非调质钢在JX493发动机连杆中的应用。通过生产探讨了锻造加热温度和冷却速度对连杆锻件组织和性能的影响。结果表明,锻造加热温度和冷却速度均对产品组织和性能有较明显影响。当锻造温度为1200℃,冷却速度为80℃/min时,锻件硬度为285HB,组织为铁素体和珠光体。实际生产中连杆锻件按照此工艺生产,经过连续20批次批量生产验证,可获得组织和性能优良的产品,质量稳定可靠。

非调质钢加热过程的硫化物转变和形貌控制

非调质钢的要求高,除硫化物含量外,还需要控制其形貌、分布。对非调质钢硫化物高温形变机理进行了研究。结果表明,当钢加热时,钢中的硫化物随着温度的提高和保温时间的延长,开始发生形变,小的硫化物逐渐溶解到基体中,较大的硫化物发生延伸、溶解、断裂、球化等一系列的形貌转变,且钢奥氏体化后,硫化物变化更加明显。当加热温度达到1200℃时,形变更加活跃,随着保温时间延长,钢中长条形的硫化物断裂碎化加快,数量增多。

∅160 mm非调质钢棒材的显微组织和疲劳性能

采用控轧控冷技术生产了直径为160 mm,含0.41%C、0.60%Si、1.41%Mn、0.16%Cr、0.07%V和0.023%Nb(质量分数)的可直接机加工的非调质钢棒材。通过金相检验、疲劳试验和力学性能测定等研究了棒材的显微组织、力学性能和疲劳性能。结果表明:棒材表面至距表面40 mm处的晶粒度为8.5~10.0级,显微组织为珠光体和铁素体,珠光体片层间距为170~220 nm;棒材抗拉强度为1002 MPa,屈服强度为755 MPa,断后伸长率为18%,断面收缩率为48%,棒材距表面25 mm处的疲劳强度为502 MPa。