高强塑积热轧双相钢组织调控研究

发布时间：2018-05-12 17:46

本文选题：临界区淬火 + 马氏体演化　；参考：《安徽工业大学》2017年硕士论文

【摘要】：本文以低碳、微合金钢为基础,结合组织调控思路利用热处理工艺匹配材料固态相变组织,充分发挥双相物理冶金结合组织组成(F及回火M)的各自力学性能优势,期冀获得具有高强塑积为特征的重载汽车大梁钢。针对设计低碳、微合金成分材料,本文研究内容如下。(1)利用Gleeble3500研究了试验钢过冷奥氏体连续相变转变。试样钢加热过程静态转变相变点分别为AC1=731.84℃、AC3=873.85℃。过冷奥氏体连续转变大致分为:F+P转变、B转变、M转变,其中B转变有着宽泛的冷速区间(3-20℃/s)。冷速为0.05-1℃/s时,试验钢相转变组织为F+P;冷速在3-10℃/s范围时,过冷奥氏体主要发生B转变,在5℃/s的冷速下得到典型的粒状贝氏体(GB),并于此冷速范围内,B形貌依据冷速的提高,逐渐由单一GB过渡到GB与LB(板条贝氏体)共存;冷速处于10℃/s~20℃/s范围时,相变组织构成为少量GB、LB和LM(板条马氏体);冷速≥30℃/s试验钢过冷奥氏体得到全部的LM组织。(2)IQ(Inter-critical Quenching,直接临界两相区淬火)及IQ'(固溶处理后迅速冷却至不同临界区温度保温一定时间后淬火)工艺条件下,不同临界两相区温度对试验钢马氏体及铁素体两相形貌演化的影响:a)IQ工艺临界两相区温度不仅与试验钢M相含量有关并且影响此温度下形成的马氏体的C、Mn等元素合金含量进而影响M的形态。M体积分数随着IQ温度升高而升高,800℃以下时M大体呈岛状分布于F基体,800℃及以上淬火时M开始成为基体相并且部分开始具有明显板条状形貌。b)IQ'工艺下,试验钢在较低的临界区温度(≤780℃)马氏体及铁素体交替分布并且呈现出明显带状组织特征(F晶粒呈多边形特征),在较高的临界区温度下马氏体与铁素体呈弥散分布组织特征(铁素体晶粒由多边形部分转变为片状形貌)。(3)经受IQ(760、780、800、810、820℃,1 h)+T(Temper:520℃,1 h)工艺试样的拉伸试验表明:Ys及Ts随着IQ温度增加而升高,硬质相M成为基体相时,强化作用明显;比例延伸随着IQ温度的升高而降低,软质相F成为次组成相时,比例延伸A5下降明显;屈强比与IQ温度的关系比较复杂出现“谷底”,在IQ温度为800℃时获得最低的屈强比(0.835)及符合指标强度(YS-568 MPa,TS-680 MPa)。目前,IQ+T组织调控工艺已成功应用于南钢工业化大生产。(4)对基于IQ+T工艺工业化大生产的20 mm成品板材的不同焊接热输入的CGHAZ(TH=1320℃,t8/5=10、20、30、60、120 s)冲击及硬度试验表明:a)CGHAZ不存在软化现象;b)撇除晶粒尺寸对冲击韧性的影响外,较短t8/5时间(≤20s)形成的位向不同的板条马氏体束(或贝氏体束)或岛状相呈弥散分布的粒状贝氏体,在裂纹扩展过程中板条束与原奥晶界可使裂纹在扩展过程中产生偏移(或偏向),损耗部分冲击势能,并且岛状相弥散分布对材料的冲击韧性影响较小;c)t8/5≥30 s时,冲击韧性剧烈下降,主要原因为岛状相(为可能的马氏体、贝氏体或M/A组元)粗化将减少与B铁素体基体界面的接触,导致塑性α相变形时滑移自由程减少,裂纹扩展容易,冲击韧性降低。
[Abstract]:In this paper, on the basis of low carbon and microalloy steel, combined with the idea of microstructure regulation, the solid phase transformation of materials is matched by heat treatment technology, and the mechanical properties of the biphasic physical metallurgical structure (F and tempering M) are fully played. The heavy load car beam steel with high strength plasticity is expected to be obtained. The contents of this paper are as follows. (1) the continuous transformation of the supercooled austenite in test steel is studied by Gleeble3500. The static transition phase transition point of the sample steel is AC1=731.84 C and AC3=873.85 C respectively. The continuous transformation of the supercooled austenite is roughly divided into F+P transformation, B transformation, and M transformation, in which the B transformation has a wide cold speed range (3-20). When the cooling rate is 0.05-1 C /s, the transformation structure of the steel phase is F+P, and the supercooled austenite is mainly B transition at 3-10 centigrade /s, and the typical granular bainite (GB) is obtained at the cold speed of 5 C /s, and in this cold speed range, the B morphology is gradually changed from single GB to GB and LB (lath bainite). At the range of 10 C /s~20 C /s, the phase transformation structure consists of a small amount of GB, LB and LM (lath martensite), and the supercooled austenite of the test steel at 30 degrees centigrade at cold speed is all LM tissue. (2) IQ (Inter-critical Quenching, direct critical two-phase quenching) and IQ'(quenching to a certain time after solid solution treatment to a certain time after a certain temperature for a certain time). The influence of different critical two phase region temperature on the evolution of martensite and ferrite morphology in test steel: a) the critical two phase temperature of IQ process is not only related to the M phase content of the test steel, but also affects the C of the martensite formed at this temperature, and the content of Mn and other elements, and then the form.M volume fraction of M increases with the increase of IQ temperature. At the height of 800, M is mostly island shaped in F matrix, and at 800 and above, M begins to become matrix phase and part of which begins to have clear strip shape.B) IQ'process. The test steel is distributed alternately at lower critical zone temperature (less than 780 degrees) and ferrite (F grain is polygonal). Characteristics) the dispersion distribution of martensite and ferrite at high critical temperature (the ferrite grain from polygon to sheet shape). (3) the tensile test of IQ (760780800810820 C, 1 h) +T (Temper:520, 1 h) shows that Ys and Ts increase with the increase of IQ temperature, and the hard phase M becomes the matrix phase When the IQ temperature increases, the proportion extension decreases with the increase of the IQ temperature. When the soft phase F becomes the sub component phase, the proportional extension A5 decreases obviously; the relationship between the ratio of the flexion ratio and the IQ temperature is more complex, and the lowest yield strength ratio (0.835) and the index strength (YS-568 MPa, TS-680 MPa) are obtained at the IQ temperature of 800 and IQ+T MPa, TS-680 MPa. Currently, IQ+T The tissue regulation technology has been successfully applied to the industrial production of Nansteel. (4) the impact and hardness test of different welding heat input of 20 mm finished sheet based on IQ+T process industrial production (TH=1320, t8/5=10,20,30,60120 s) shows that a) CGHAZ does not exist softening appearance; b) is shorter than the effect of grain size on impact toughness. The t8/5 time (less than 20s) forms a granular bainite with diffused distribution of the lath martensitic bundle (or bainite bundle) or the island phase. In the process of crack propagation, the crackle and the original crystal boundary can cause the crack to be offset (or biased) in the process of expansion, loss of partial impact potential energy, and the impact toughness of the island phase dispersion distribution to the material. The impact toughness of C) is less than that of t8/5. The impact toughness decreases sharply when t8/5 > 30 s. The main reason is that the coarsening of the island phase (for possible martensite, bainite or M/A) will reduce the contact with the interface of the B ferrite matrix, which leads to the reduction of the slip free range in the plastic alpha phase deformation, the easy crack propagation and the decrease of the impact toughness.

【学位授予单位】：安徽工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1

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