高強(qiáng)塑性中錳鋼的組織演變及力學(xué)性能的研究
發(fā)布時(shí)間：2018-09-11 12:04
【摘要】：輕質(zhì)高強(qiáng)汽車用鋼因其兼具出色的強(qiáng)度和塑性,且能滿足節(jié)能環(huán)保的要求而成為研究熱點(diǎn)。強(qiáng)塑積(抗拉強(qiáng)度與總伸長率的乘積)是衡量汽車用鋼綜合性能的指標(biāo)。傳統(tǒng)的汽車用鋼的強(qiáng)塑積為15±10GPa%,代表性鋼種有無間隙原子(Interstitial Free, IF)鋼和雙相鋼等,為第一代汽車用鋼；而以奧氏體鋼為代表的第二代汽車用鋼的強(qiáng)塑積為60±10GPa%。這兩代汽車用鋼在工業(yè)使用上都存在限制,如第一代鋼強(qiáng)塑積過低,而第二代鋼太昂貴。近年來,很多關(guān)于中錳鋼(Mn含量為4～12%)的研究中發(fā)現(xiàn)其強(qiáng)塑積為30GPa%,是理想的第三代汽車用鋼。 本文主要研究Mn含量為8%和11%的TRIP鋼的組織演變和力學(xué)性能。研究表明TRIP鋼的力學(xué)性能取決于奧氏體的含量和穩(wěn)定性。優(yōu)化的淬火+回火的工藝使實(shí)驗(yàn)鋼獲得了大量的奧氏體,從而保證了拉伸過程中發(fā)生明顯的TRIP效應(yīng)。此外,本文還研究了奧氏體穩(wěn)定性的影響因素,如晶粒尺寸、形貌、化學(xué)成分等。本文獲得的實(shí)驗(yàn)結(jié)果歸納如下： (1)熱軋態(tài)實(shí)驗(yàn)鋼在750～800℃淬火并回火后獲得的力學(xué)性能優(yōu)于或與其他冷軋低合金TRIP鋼和中錳鋼相當(dāng),但本實(shí)驗(yàn)鋼未經(jīng)冷軋工序且需要的熱處理時(shí)間較短。8Mn熱軋鋼能獲得810～1000MPa的抗拉強(qiáng)度和32～39%的伸長率；11Mn熱軋鋼能獲得880～1100MPa的抗拉強(qiáng)度和34-40%的伸長率；11Mn-Nb熱軋鋼能獲得960～1160MPa的抗拉強(qiáng)度和28-40%的伸長率。 (2)通過比較熱軋實(shí)驗(yàn)鋼在800℃淬火并回火和未回火試樣的拉伸性能可知,回火可以顯著地提高實(shí)驗(yàn)鋼的塑性,主要是由于在回火過程中6鐵素體的碳原子向臨近的奧氏體擴(kuò)散,提高了奧氏體的穩(wěn)定性,從而表現(xiàn)出更好的伸長率。對于在850～900℃淬火后的試樣,回火顯著提高塑性,降低強(qiáng)度,這主要是因?yàn)榛鼗瘃R氏體的生成使內(nèi)應(yīng)力降低。 (3)冷軋實(shí)驗(yàn)鋼經(jīng)過淬火后均能獲得優(yōu)秀的力學(xué)性能。8Mn冷軋鋼在730℃C淬火后能獲得873MPa的抗拉強(qiáng)度和57%的伸長率；11Mn冷軋鋼在750℃淬火后能獲得998MPa的抗拉強(qiáng)度和67%的伸長率；11Mn-Nb冷軋鋼在750℃C淬火后能獲得979MPa的抗拉強(qiáng)度和63%的伸長率。11Mn鋼和11lMn-Nb的綜合力學(xué)性能是目前所報(bào)道的中錳鋼中最好的。 (4)通過研究11Mn熱軋實(shí)驗(yàn)鋼拉伸過程中的變形行為首次觀察到不連續(xù)TRIP效應(yīng),并闡明了其產(chǎn)生的主要原因：第一,馬氏體相變產(chǎn)生體積膨脹導(dǎo)致6鐵素體和臨界鐵素體的變形,最終引起局部應(yīng)力松弛和轉(zhuǎn)移；第二,奧氏體具有不同等級的穩(wěn)定性使得只有達(dá)到某一臨界應(yīng)力時(shí)TRIP效應(yīng)才能發(fā)生。而且,研究發(fā)現(xiàn)臨界鐵素體的分割使奧氏體由塊狀變成不同厚度和長度的薄膜狀,因而具有不同等級的穩(wěn)定性。 (5)通過對11Mn熱軋實(shí)驗(yàn)鋼拉伸前后的組織進(jìn)行EBSD分析可知,奧氏體的晶粒取向在一定程度的影響奧氏體的穩(wěn)定性,具有大施密特因子的晶粒能優(yōu)先發(fā)生相變；但是,施密特因子并不是決定奧氏體穩(wěn)定性的決定性因素,形貌對奧氏體穩(wěn)定性的影響更大。 (6)通過研究不同溫度淬火后的拉伸試樣的應(yīng)變硬化行為發(fā)現(xiàn),拉伸過程中鐵素體的優(yōu)先變形能有效地推遲TRIP效應(yīng)的發(fā)生,使奧氏體在較大的應(yīng)變下發(fā)生TRIP效應(yīng),從而使實(shí)驗(yàn)鋼獲得優(yōu)秀的伸長率。此外,實(shí)驗(yàn)獲得的應(yīng)變硬化行為與Crussard-Jaoul (C-J)分析的結(jié)果相一致。 (7)通過對1lMn冷軋實(shí)驗(yàn)鋼的研究表明,其應(yīng)變硬化中第三階段的鋸齒波動(dòng)行為主要是由于不連續(xù)TRIP效應(yīng)的作用。而不連續(xù)TRIP效應(yīng)主要是由于錳元素的不均勻分布使得奧氏體具有不同等級的穩(wěn)定性。研究發(fā)現(xiàn),延長熱處理時(shí)間可以使得錳元素分布更加均勻,但是提高熱處理的溫度的作用剛好相反。晶粒尺寸是影響冷軋實(shí)驗(yàn)鋼奧氏體穩(wěn)定性最主要的因素。
[Abstract]:Light-weight and high-strength automotive steel has become a research hotspot because of its excellent strength and plasticity, and can meet the requirements of energy-saving and environmental protection. Strength-plasticity product (product of tensile strength and total elongation) is an index to measure the comprehensive performance of automotive steel. Free, IF steel and dual-phase steel are the first generation automotive steel, while the second generation automotive steel represented by austenitic steel has a Strength-plasticity product of 60 6550 It is found that its strong plastic product is 30GPa%, which is the ideal third generation automobile steel.
The microstructure evolution and mechanical properties of TRIP steel with Mn content of 8% and 11% were studied in this paper. The results show that the mechanical properties of TRIP steel depend on the content and stability of austenite. The optimized quenching and tempering process makes the experimental steel obtain a large amount of austenite, which ensures the TRIP effect in the tensile process. The factors affecting the stability of austenite, such as grain size, morphology and chemical composition, were investigated.
(1) The mechanical properties of hot-rolled as-cast steel quenched and tempered at 750-800 C are superior to or equal to those of other cold-rolled low alloy TRIP steel and medium manganese steel, but the experimental steel has not been cold-rolled and needs shorter heat treatment time. The tensile strength of 810-1000 MPa and the elongation of 32-39% can be obtained by hot-rolled 11Mn steel. Tensile strength of 80-1100 MPa and elongation of 34-40%; 11Mn-Nb hot-rolled steel can obtain 960-1160 MPa tensile strength and 28-40% elongation.
(2) By comparing the tensile properties of the hot-rolled steel quenched and tempered at 800 C with that of the non-tempered steel, it can be seen that tempering can remarkably improve the plasticity of the test steel, mainly because the carbon atoms of 6 ferrite diffuse to the adjacent austenite during tempering, thus improving the stability of the austenite and thus showing better elongation. Tempering significantly increases plasticity and decreases strength of quenched specimens at 0-900 C, mainly because the formation of tempered martensite reduces internal stress.
(3) Cold-rolled steel can obtain excellent mechanical properties after quenching. 8Mn cold-rolled steel can obtain 873 MPa tensile strength and 57% elongation after quenching at 730 C; 11Mn cold-rolled steel can obtain 998 MPa tensile strength and 67% elongation after quenching at 750 C; 11Mn-Nb cold-rolled steel can obtain 979 MPa tensile strength and 97 9 MPa tensile strength after quenching at 750 C. The mechanical properties of.11Mn steel and 11lMn-Nb at 63% elongation are the best among reported medium manganese steels.
(4) Discontinuous TRIP effect was observed for the first time by studying the deformation behavior of 11Mn hot-rolled experimental steel during tensile process, and its main causes were clarified. Firstly, volume expansion of martensitic transformation resulted in the deformation of 6 ferrite and critical ferrite, which eventually led to local stress relaxation and transfer. Secondly, austenite had different grades. The TRIP effect can only occur when a critical stress is reached, and it is found that the separation of critical ferrite makes the austenite change from lump to thin film with different thickness and length, thus resulting in different levels of stability.
(5) EBSD analysis of the microstructure of 11Mn hot-rolled experimental steel before and after tensile shows that the grain orientation of austenite affects the stability of austenite to a certain extent, and the grain with large Schmidt factor can have preferred phase transformation; however, Schmidt factor is not the decisive factor determining the stability of austenite, and the morphology of austenite is stable. Qualitative impact is even greater.
(6) By studying the strain hardening behavior of the tensile specimens quenched at different temperatures, it is found that the preferential deformation of ferrite can effectively postpone the TRIP effect and make the austenite produce the TRIP effect under larger strain, so that the experimental steel can obtain excellent elongation. The results of d-Jaoul (C-J) analysis are consistent.
(7) The study of 1Mn cold-rolled steel shows that the sawtooth fluctuation in the third stage of strain hardening is mainly due to discontinuous TRIP effect. The discontinuous TRIP effect is mainly due to the uneven distribution of manganese elements, which makes the austenite have different stability grades. The distribution of manganese is more uniform, but the effect of increasing the heat treatment temperature is just the opposite. Grain size is the most important factor affecting the Austenite Stability of cold-rolled experimental steel.
【學(xué)位授予單位】：東北大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG142.1
本文編號：2236642
[Abstract]:Light-weight and high-strength automotive steel has become a research hotspot because of its excellent strength and plasticity, and can meet the requirements of energy-saving and environmental protection. Strength-plasticity product (product of tensile strength and total elongation) is an index to measure the comprehensive performance of automotive steel. Free, IF steel and dual-phase steel are the first generation automotive steel, while the second generation automotive steel represented by austenitic steel has a Strength-plasticity product of 60 6550 It is found that its strong plastic product is 30GPa%, which is the ideal third generation automobile steel.
The microstructure evolution and mechanical properties of TRIP steel with Mn content of 8% and 11% were studied in this paper. The results show that the mechanical properties of TRIP steel depend on the content and stability of austenite. The optimized quenching and tempering process makes the experimental steel obtain a large amount of austenite, which ensures the TRIP effect in the tensile process. The factors affecting the stability of austenite, such as grain size, morphology and chemical composition, were investigated.
(1) The mechanical properties of hot-rolled as-cast steel quenched and tempered at 750-800 C are superior to or equal to those of other cold-rolled low alloy TRIP steel and medium manganese steel, but the experimental steel has not been cold-rolled and needs shorter heat treatment time. The tensile strength of 810-1000 MPa and the elongation of 32-39% can be obtained by hot-rolled 11Mn steel. Tensile strength of 80-1100 MPa and elongation of 34-40%; 11Mn-Nb hot-rolled steel can obtain 960-1160 MPa tensile strength and 28-40% elongation.
(2) By comparing the tensile properties of the hot-rolled steel quenched and tempered at 800 C with that of the non-tempered steel, it can be seen that tempering can remarkably improve the plasticity of the test steel, mainly because the carbon atoms of 6 ferrite diffuse to the adjacent austenite during tempering, thus improving the stability of the austenite and thus showing better elongation. Tempering significantly increases plasticity and decreases strength of quenched specimens at 0-900 C, mainly because the formation of tempered martensite reduces internal stress.
(3) Cold-rolled steel can obtain excellent mechanical properties after quenching. 8Mn cold-rolled steel can obtain 873 MPa tensile strength and 57% elongation after quenching at 730 C; 11Mn cold-rolled steel can obtain 998 MPa tensile strength and 67% elongation after quenching at 750 C; 11Mn-Nb cold-rolled steel can obtain 979 MPa tensile strength and 97 9 MPa tensile strength after quenching at 750 C. The mechanical properties of.11Mn steel and 11lMn-Nb at 63% elongation are the best among reported medium manganese steels.
(4) Discontinuous TRIP effect was observed for the first time by studying the deformation behavior of 11Mn hot-rolled experimental steel during tensile process, and its main causes were clarified. Firstly, volume expansion of martensitic transformation resulted in the deformation of 6 ferrite and critical ferrite, which eventually led to local stress relaxation and transfer. Secondly, austenite had different grades. The TRIP effect can only occur when a critical stress is reached, and it is found that the separation of critical ferrite makes the austenite change from lump to thin film with different thickness and length, thus resulting in different levels of stability.
(5) EBSD analysis of the microstructure of 11Mn hot-rolled experimental steel before and after tensile shows that the grain orientation of austenite affects the stability of austenite to a certain extent, and the grain with large Schmidt factor can have preferred phase transformation; however, Schmidt factor is not the decisive factor determining the stability of austenite, and the morphology of austenite is stable. Qualitative impact is even greater.
(6) By studying the strain hardening behavior of the tensile specimens quenched at different temperatures, it is found that the preferential deformation of ferrite can effectively postpone the TRIP effect and make the austenite produce the TRIP effect under larger strain, so that the experimental steel can obtain excellent elongation. The results of d-Jaoul (C-J) analysis are consistent.
(7) The study of 1Mn cold-rolled steel shows that the sawtooth fluctuation in the third stage of strain hardening is mainly due to discontinuous TRIP effect. The discontinuous TRIP effect is mainly due to the uneven distribution of manganese elements, which makes the austenite have different stability grades. The distribution of manganese is more uniform, but the effect of increasing the heat treatment temperature is just the opposite. Grain size is the most important factor affecting the Austenite Stability of cold-rolled experimental steel.
【學(xué)位授予單位】：東北大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG142.1
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