【作者】 李宭；

【导师】 王天生； 荆天辅；

【作者基本信息】 燕山大学 ， 材料学， 2009， 博士

【摘要】 本文目的是研究碳钢的马氏体温变形行为及温变形过程中的超细晶组织演变与性能特征。在Gleeble3500热模拟机上利用热机械模拟方法对淬火态低碳钢(Q235)、中碳钢(45)和高碳钢(T8和T12)在Acl温度以下的马氏体温变形行为进行了研究,建立了温变形本构方程,分析了变形温度、变形速率和碳含量对变形抗力的影响；利用光学显微镜(OM)和透射电镜(TEM)研究了不同工艺温变形的超细晶组织演变、位错形态和碳化物变化；还对淬火碳钢温变形后的样品的超细晶钢力学性能、物理性能和化学性能进行了分析。结果表明,随着变形温度的提高或变形速率的降低,淬火碳钢温变形抗力下降。在一定的变形温度和变形速率范围内,随着碳含量的增加峰值应力先升高后下降。碳含量为0.45 wt.%时峰值应力最高。其中淬火态45钢在60℃以应变速率0.01s-1进行50%变形量的温变形处理后,变形抗力达357MPa；与其在室温下的变形抗力(2600MPa)相比较,温变形处理后变形抗力降低84%左右。实验发现,含C0.17-1.26wt.%的淬火碳钢在Acl以下温变形可以制备超细晶/亚微米等轴晶粒铁素体和纳米渗碳体颗粒组成的超细晶组织。随着碳含量的增加,超细晶铁素体晶粒尺寸先减小后增大,含C0.45wt.%时铁素体晶粒最小；变形量从30%增加到90%,铁素体晶粒尺寸也呈先减小后增大趋势,变形量达到50%时晶粒尺寸最小。研究显示,淬火45钢在600℃以应变速率0.01s-1进行50%压下量的温变形得到超细晶组织,用微拉伸试样获得的屈服强度达777MPa,延伸率达19%。与国家标准GB/T699-1999(840℃淬火+600℃回火)给出的屈服强度355 MPa和延伸率16%相比,屈服强度提高了近1.2倍,延伸率略高于该标准。同样,淬火Q235钢在600℃以0.001s-1的应变速率进行50%变形量的温变形处理,屈服强度达到420MPa,延伸率16%。屈服强度是热轧态的约1.8倍。淬火后温变形处理制备的超细晶Q235钢热膨胀分析表明,以5℃/min升温的淬火Q235钢在250℃开始发生碳原子的脱溶,550℃脱溶结束。变形量50%以上温变形试样,膨胀系数无明显变化,说明没有出现碳原子脱溶现象,即获得了完全再结晶的铁素体。这种超细晶Q235钢人工海水腐蚀以均匀腐蚀为主,而盐雾腐蚀出现了较多的均匀分布的腐蚀坑。人工海水腐蚀表面粗糙度随腐蚀时间呈S形变化,在35天以内腐蚀缓慢,在35-55天腐蚀最快,超过55天腐蚀又趋于缓慢。更多还原

【Abstract】 The aim of this dissertation is to investigate the warm deformation behavior of quenched carbon steels and the microstructure evolution and properties features during the warm deformation.The warm deformation behavior of quenched carbon steels with (0.17-1.26)C (wt.%), such as Q235,45, T8 and T12, at temperatures below Acl was studied by thermalmechanical simulation, and constitutive equations of the warm deformation were set up. Microstructure evolution during the warm deformation with different processes was examined by optical microscopy (OM) and transmission electron microscopy (TEM). In addition, the mechanical, physical and chemical properties of these steels subjected to quenching and warm deformation were also analyzed.Experimental results show that the resistance of the warm deformation of the quenched steels decreases with the deformation temperature elevating and the strain rate reducing. In certain rang of deformation temperature and strain rate, the peak stress of the warm deformation shows a trend from ascent to descent with the increase of carbon content, and the peak stress reaches maximum for 0.45C steel. Ultrafine microstructures with equiaxial ultrafine/submicron-grained ferrite and nano cementite particles can be fabricated in (0.17-1.26C) steels by quenching and warm deformation (QWD). With the increase of carbon content, the size of ferrite grain shows a trend from descent to ascent, and the finest grains can be obtained in 0.45C steel. With the increase of the reduction from 30% to 90%, the size of ferrite grain also shows a trend from descent to ascent, and the finest grains can be obtained at reduction of 50%. The yield strength and elongation are reached to 777 MPa and 19% for 0.45C steel by QWD with a reduction of 50% at 600℃and 0.01 s-1. Comparing to the yield strength (355 MPa) and elongation (16%) given in GB/T699-1999 for 0.45C steel treated by quenching at 840℃and tempering at 600℃, the yield strength is increased by～1.2 times and the elongation is slightly increased by QWD treatment. For the Q235 steel (0.17C) treated by quenching and warm deformation in reduction of 50% at 600℃and 0.001 s-1, the yield strength reaches 420 MPa which is higher than that of the as hot rolled sample by～80%, and the elongation is 16%. Thermal dilatometry measurement reveals that the precipitation of C from quenched Q235 steel starts at 250℃and completes at 550℃when heating at a rate of 5℃/min. However, for samples warmly deformed in reduction over 50%, the expansion coefficient is unchanged, suggesting that the pretipitation is not occurred, i.e. obtaining the completely recrystallized ferrite grains. The corrosion of this ultrafine grained Q235 steel in artificial sea water is mainly in mode of uniform corrosion, and the corrosion pits with uniform distribution ocuurs in the process of salt spray corrosion. The relationship between the surface roughness of the sample corroded in artificial sea water and corrosion time displays S-shape curve. The corrosion rate is slow for corrosion time less than 35 days, rapid for 35-55 days and slow for over 55 days.更多还原