【摘要】：隨著橋梁建造、公路運(yùn)輸?shù)陌l(fā)展,Q345鋼的應(yīng)用也越來越廣泛。這就要求Q345鋼即使在嚴(yán)苛的服役環(huán)境下也能保持良好的性能,而鑄坯往往伴隨著表面橫裂紋等缺陷,雖然橫裂紋可以通過熱吹掃的方法予以消除,但熱吹掃工序費(fèi)時(shí)費(fèi)力,嚴(yán)重影響生產(chǎn)效率。目前,國(guó)內(nèi)某鋼廠生產(chǎn)的260mm×2070mm規(guī)格的Q345鋼連鑄坯存在嚴(yán)重的表面橫裂紋,本文針對(duì)Q345鋼表面橫裂紋問題,從鑄坯凝固行為入手,以調(diào)整連鑄工藝參數(shù)為技術(shù)手段,深入分析鑄坯表面橫裂紋產(chǎn)生的原因、影響因素和控制措施,從根本上控制鑄坯表面裂紋。針對(duì)以上問題,利用金相顯微鏡、掃描電子顯微鏡等對(duì)Q345鋼連鑄坯典型裂紋試樣進(jìn)行分析發(fā)現(xiàn),裂紋主要分布在鑄坯角部附近表面,并沿著晶界上的先共析鐵素體薄膜向鑄坯內(nèi)部延伸,在顯微鏡下對(duì)先共析鐵素體薄膜進(jìn)行統(tǒng)計(jì)分析,發(fā)現(xiàn)其厚度偏大;同時(shí)在奧氏體晶界上有第二相質(zhì)點(diǎn)析出,呈鏈狀分布,脆化了晶界,在應(yīng)力作用下易形成空穴,不利于控制裂紋,對(duì)裂紋內(nèi)部取點(diǎn)進(jìn)行成分分析,發(fā)現(xiàn)不含保護(hù)渣成分,推斷裂紋應(yīng)該是在鑄坯出結(jié)晶器彎曲矯直的過程中受應(yīng)力作用形成的。對(duì)鑄坯全厚度試樣進(jìn)行低倍檢驗(yàn),發(fā)現(xiàn)其等軸晶率較低,柱狀晶發(fā)達(dá);鑄坯窄面振痕深度較大,有可能導(dǎo)致鑄坯窄面?zhèn)鳠岵痪鶆?易產(chǎn)生裂紋。通過金相顯微鏡觀察鑄坯微觀組織發(fā)現(xiàn),其表面為一定厚度的激冷層,內(nèi)部為魏氏組織,對(duì)鑄坯表面激冷層厚度進(jìn)行統(tǒng)計(jì)分析,發(fā)現(xiàn)在鑄坯窄面區(qū)域,激冷層厚度很薄或者不存在激冷層,內(nèi)部的魏氏組織在應(yīng)力作用下就容易導(dǎo)致開裂;先共析鐵素體在鑄坯窄面呈膜狀分布的現(xiàn)象也比較明顯,這些連續(xù)分布的軟相也有利于裂紋的擴(kuò)展;同時(shí)在鑄坯外弧角部發(fā)現(xiàn)索氏體組織區(qū)域,很可能是由于結(jié)晶器錐度過大造成的,過大的結(jié)晶器錐度還會(huì)使鑄坯所受摩擦力增大,在拉坯過程中出現(xiàn)表面橫裂紋。綜合以上研究結(jié)果,參考目前連鑄結(jié)晶器錐度參數(shù)(1.1%-1.2%),制訂了結(jié)晶器錐度分別為0.9%、1.0%和1.1%的三種不同工藝方案,對(duì)不同工藝參數(shù)下的鑄坯質(zhì)量進(jìn)行研究,對(duì)比發(fā)現(xiàn),當(dāng)結(jié)晶器錐度為1.0%時(shí),鑄坯激冷層厚度最大,鑄坯組織也相對(duì)更加均勻;用維氏硬度計(jì)進(jìn)行測(cè)量,其硬度數(shù)據(jù)分布也相對(duì)集中,組織過渡比較均勻;說明針對(duì)Q345鋼連鑄坯,當(dāng)結(jié)晶器錐度為1.0%時(shí),能夠較好地補(bǔ)償坯殼凝固收縮帶來的氣隙,保證鑄坯表面?zhèn)鳠崃己谩Ｗ罱K研究結(jié)果表明,該廠連鑄過程中原結(jié)晶器錐度參數(shù)設(shè)置偏大,導(dǎo)致鑄坯出現(xiàn)表面橫裂紋,工藝改進(jìn)后,結(jié)晶器錐度參數(shù)設(shè)置為1.0%時(shí)鑄坯質(zhì)量明顯改善。
[Abstract]:With the construction of bridge, the application of Q345 steel is more and more extensive. This requires that Q345 steel be able to maintain good performance even in severe service conditions, and the billet is often accompanied by defects such as transverse cracks on the surface. Although transverse cracks can be eliminated by means of hot sweep, the hot sweep process is time-consuming and laborious. The production efficiency is seriously affected. At present, there are serious surface transverse cracks in Q345 steel continuous casting billet of 260mm 脳 2070mm specification produced by a domestic steel factory. Aiming at the problem of surface transverse crack of Q345 steel, this paper starts with the solidification behavior of the billet and takes adjusting the technological parameters of continuous casting as the technical means. The causes, influencing factors and control measures of the transverse cracks on the billet surface are analyzed in depth to control the surface cracks of the billet fundamentally. In view of the above problems, the typical crack samples of Q345 continuous casting billet were analyzed by metallographic microscope and scanning electron microscope. It was found that the cracks mainly distributed on the surface near the corner of the billet. The proeutectoid ferrite film extends to the inner part of the slab along the grain boundary. The statistical analysis of the proeutectoid ferrite film under the microscope shows that the thickness of the proeutectoid ferrite film is too large, and at the same time, the second phase particle precipitates on the austenite grain boundary, which is distributed in chain shape. When the grain boundary is embrittlement, holes are easily formed under stress, which is not conducive to controlling the crack. The composition analysis of the point taken from the crack is carried out, and it is found that there is no composition of the mold powder. It is inferred that the crack should be formed by stress during the bending and straightening of the casting mould. It is found that the full thickness specimen has low equiaxed crystal ratio and developed columnar crystal, and that the depth of vibration marks on the narrow surface of the billet is large, which may lead to uneven heat transfer on the narrow surface of the billet and easily produce cracks. The microstructure of the billet was observed by metallographic microscope. It was found that the surface of the billet was a chilling layer with a certain thickness and the internal structure was Weiss's structure. The statistical analysis of the thickness of the quench layer on the surface of the billet was carried out. The thickness of the chilled layer is very thin or there is no chilling layer, the internal Weiss structure is easy to crack under the stress, and the phenomenon of preeutectoid ferrite distribution in the narrow surface of the billet is also obvious. The continuous distribution of soft phases is also beneficial to the crack propagation, and at the same time, it is found in the outer arc corner of the billet that the structure area of the Soxhlet body is probably caused by the excessive taper of the mould, and that too large the mold taper will increase the friction force of the billet. Transverse surface cracks appear in the process of billet drawing. Based on the above research results and referring to the taper parameters of continuous casting mould (1.1- 1.2%), three different technological schemes of mold taper of 0.9g 1.0% and 1.1% are worked out. The quality of billet under different technological parameters is studied and compared. When the mold taper is 1.0, the thickness of the quench layer is the largest and the structure of the billet is relatively more uniform. The hardness data distribution is relatively concentrated and the microstructure transition is more uniform when measured by Vickers hardness meter. When the mold taper is 1.0, the air gap caused by the solidification shrinkage of the shell can be compensated well, and the heat transfer on the surface of the billet can be ensured. The results show that the original mold taper parameter is too large during the continuous casting process, which leads to the appearance of surface transverse cracks. After the process is improved, the casting quality is obviously improved when the mold taper parameter is set to 1.0.
【學(xué)位授予單位】：鄭州大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TF777

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