發(fā)布時間：2018-05-07 19:42

  本文選題：超高強(qiáng)度鋼 + 二次硬化　； 參考：《燕山大學(xué)》2016年博士論文

【摘要】：超高強(qiáng)度鋼廣泛運(yùn)用于航空航天領(lǐng)域。本文以多組元碳化物(Mo,Cr,W)_2C為主要強(qiáng)化相,設(shè)計(jì)并試制了一種強(qiáng)度級別為2200MPa級的鎢鉬復(fù)合二次硬化超高強(qiáng)度鋼,系統(tǒng)研究了熱處理工藝對其組織與性能的影響,從而確定了最佳熱處理工藝;分析并討論了主要強(qiáng)化相M_2C碳化物在回火過程中的析出長大機(jī)制;運(yùn)用第一性原理計(jì)算的方法預(yù)測了不同Cr、Mo、W含量條件下M_2C碳化物的結(jié)構(gòu)穩(wěn)定性、彈性性能以及電學(xué)性能;建立熱變形方程,提出了該鋼的最佳熱加工工藝參數(shù)。取得如下主要研究成果:鎢鉬復(fù)合二次硬化超高強(qiáng)度鋼經(jīng)950℃~1100℃保溫1h淬火,隨淬火溫度的升高,強(qiáng)度和韌性均先升高后降低,在1000℃~1050℃達(dá)到最大值。淬火溫度為950℃和1000℃時,存在未溶的M6C碳化物;淬火溫度為1050℃時,M6C碳化物全部溶解;淬火溫度高于1050℃時,晶粒發(fā)生異常長大。鎢鉬復(fù)合二次硬化超高強(qiáng)度鋼經(jīng)200℃~700℃范圍內(nèi)回火5小時,在200℃回火時,析出較多的ε-碳化物;在300℃~440℃回火時,析出大量粗大的層片狀滲碳體,導(dǎo)致強(qiáng)度和韌性不斷下降,在440℃達(dá)到最低值。回火溫度高于470℃時,馬氏體板條內(nèi)析出大量均勻彌散的M_2C碳化物及少量的Laves相,使實(shí)驗(yàn)鋼出現(xiàn)明顯的二次硬化現(xiàn)象,抗拉強(qiáng)度、屈服強(qiáng)度分別在490℃和530℃回火時達(dá)到最大值,沖擊功在510℃時達(dá)到最大值,此時具有較好的綜合力學(xué)性能�；鼗饻囟雀哂�490℃時,逆轉(zhuǎn)變奧氏體含量不斷增加�；鼗饻囟仍�560℃附近時,晶界上出現(xiàn)大量碳化物并富集了大量的N、P、S元素,引起沿晶斷裂,韌性下降。鎢鉬復(fù)合二次硬化超高強(qiáng)度鋼在510℃回火時,隨回火時間的延長,M_2C依次出現(xiàn)形核、長大及熟化三個過程。當(dāng)回火10min~30min時,有滲碳體存在,伴隨M_2C碳化物的析出,滲碳體逐漸溶解,晶內(nèi)M_2C碳化物處于形核階段,完成由G-P區(qū)到新相晶核的轉(zhuǎn)變;在晶界處聚集了大量的M_2C碳化物,且隨著回火時間的延長碳化物晶界聚集狀態(tài)逐漸減弱�；鼗�5h時,晶內(nèi)M_2C碳化物處于長大階段,滲碳體全部溶解,晶界M_2C碳化物聚集現(xiàn)象消失�；鼗�100h時,M_2C碳化物處于熟化階段,碳化物尺寸出現(xiàn)大小兩類,大尺寸碳化物繼續(xù)長大而小尺寸碳化物逐漸溶解。在形核和長大階段,隨回火時間的延長合金元素?cái)U(kuò)散距離逐漸變長,M_2C碳化物數(shù)量變多、尺寸增大,且碳化物內(nèi)部C、Cr、Mo、W元素含量逐漸上升。在熟化階段,M_2C碳化物尺寸及體積含量繼續(xù)增大,而數(shù)量減少,受合金元素?cái)U(kuò)散和碳化物穩(wěn)定性共同影響,與C原子結(jié)合能力更強(qiáng)的Mo和W元素逐漸取代Cr原子進(jìn)入M_2C碳化物,使碳化物心部C、Mo、W元素含量逐漸上升,而Cr元素含量則逐漸下降。運(yùn)用密度泛函理論,系統(tǒng)地研究了不同Mo、Cr、W含量的多組元(Mo,Cr,W)_2C型碳化物結(jié)構(gòu)穩(wěn)定性、彈性性能、熱物理性能以及電學(xué)性能等。發(fā)現(xiàn)Mo6W1Cr1C4擁有最高的穩(wěn)定性和體積模量、剪切模量、彈性模量及硬度,具有優(yōu)良的抗變形能力。Mo7Cr1C4的各向異性最強(qiáng),而Mo_8C_4的各向異性最弱。從電子性質(zhì)分析中發(fā)現(xiàn),摻雜Cr原子和W原子不僅能使自身軌道雜化,而且能增強(qiáng)Mo原子與C原子之間的軌道雜化,增強(qiáng)原子間相互作用。在變形溫度為850℃~1200℃、應(yīng)變速率為0.01 s-1~10s-1的變形條件下,鎢鉬復(fù)合二次硬化超高強(qiáng)度鋼的熱變形激活能為471.28kJ/mol,建立了熱變形本構(gòu)方程和動態(tài)再結(jié)晶晶粒平均晶粒尺寸D(um)與Z參數(shù)的定量關(guān)系。基于動態(tài)材料模型,建立了該鋼的熱加工圖,確定了其最佳熱加工工藝條件。
[Abstract]:Ultra high strength steel is widely used in the field of Aeronautics and Astronautics. In this paper, Mo, Cr, W _2C as the main strengthening phase, a tungsten molybdenum composite two hardening super strength steel with a strength grade of 2200MPa is designed and tested. The effect of heat treatment process on its microstructure and properties is studied systematically, and the optimum heat treatment process is determined. The mechanism of precipitation and growth of the main enhanced phase M_2C carbides in the process of tempering was analyzed and discussed. The structural stability, elastic properties and electrical properties of M_2C carbides in different Cr, Mo and W content were predicted with the method of first principle calculation, and the thermal deformation equation was established, and the optimum processing parameters of the steel were put forward. The main results are as follows: tungsten molybdenum composite two hardening super strength steel is quenched at 950 C ~1100 C for 1H. With the increase of quenching temperature, the strength and toughness increase first and then decrease and reach the maximum value at 1000 C ~1050 C. When the quenching temperature is 950 C and 1000 C, there is an undissolved M6C carbides; when the quenching temperature is 1050 C, all M6C carbides dissolve When the quenching temperature is higher than 1050 C, the grain has abnormal growth. The tungsten molybdenum composite two hardening ultra high strength steel is tempered for 5 hours in the range of 200 c ~700 C, and more epsilon carbides are precipitated when tempering at 200 c. A large amount of thick layer like carburizing body is precipitated when tempering at 300 centigrade, causing the strength and toughness to decrease and reach 440 C. Minimum value. When the tempering temperature is higher than 470, the martensitic slab can precipitate a large number of uniformly dispersed M_2C carbides and a small amount of Laves phase. The tensile strength and yield strength reached the maximum value at 490 and 530 C, and the impact work reached the maximum at 510 degrees C. When the tempering temperature is higher than 490, the reversed austenite content increases continuously. When the tempering temperature is near 560, a large number of carbides appear on the grain boundary and a large number of N, P, S elements have been enriched, causing the intergranular fracture and the toughness decrease. When the tungsten molybdenum compound two hardening ultra high strength steel is tempered at 510 C, with the tempering time prolonged, M_2C depends on the tempering time. There are three processes of nucleation, growth and maturation. When tempering 10min~30min, the carburized body exists, with the precipitation of M_2C carbide, the carburized body gradually dissolves, the intragranular M_2C carbides are in the nucleation stage, and the transition from the G-P zone to the new phase nucleation is completed; the large amount of M_2C carbides are gathered at the grain boundary, and the carbides are prolonged with the tempering time. The grain boundary aggregation state gradually weakened. When tempering 5h, the intragranular M_2C carbide was in the growing stage, the carburized body dissolved and the grain boundary M_2C carbide aggregation disappeared. When tempering 100h, the M_2C carbide was in the ripening stage, the size of the carbide appeared two classes, the large size carbides continued to grow and the small size carbides dissolved gradually. In the nucleation and in the nucleation, In the growing stage, the diffusion distance of alloy elements gradually grows with the time of tempering, the number of M_2C carbide becomes more and the size increases, and the content of C, Cr, Mo and W increases gradually. In the ripening stage, the size and volume content of the M_2C carbide continues to increase, and the quantity decreases, which is influenced by the diffusion of alloy elements and the stability of carbides. The Mo and W elements, which are stronger with the C atom, gradually replace the Cr atoms into the M_2C carbide, which makes the content of C, Mo, W elements in the carbide core gradually increasing, while the Cr element content decreases gradually. It is found that Mo6W1Cr1C4 has the highest stability and bulk modulus, the shear modulus, the modulus of elasticity and the hardness, the excellent anisotropy of the anti deformability of.Mo7Cr1C4, and the weakest anisotropy of Mo_8C_4. It is found that the doping of the Cr atom and the W atom can not only make its own orbit miscellaneous from the analysis of the electron properties. When the deformation temperature is 850 C and the strain rate is 0.01 s-1~10s-1, the thermal deformation activation energy of the tungsten molybdenum composite two hardening ultra high strength steel is 471.28kJ/mol. The thermal deformation constitutive equation and the dynamic recrystallized grain are established under the deformation temperature of 850 C ~1200 C and the strain rate of 0.01 s-1~10s-1. The relationship between average grain size D (UM) and Z parameters is established. Based on the dynamic material model, the hot working diagram of the steel is established, and the best hot working conditions are determined.

【學(xué)位授予單位】：燕山大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG142.1

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