Effect of laminar flow cooling process on microstructure and mechanical properties of hot rolled DP600 steel
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摘要: 测定了DP600 钢的 CCT 曲线，在不同层流冷却工艺下对该钢进行工业试生产，研究了冷却速率、空冷温度和卷取温度对其组织性能的影响。结果表明：当冷却速率 ＜ 10 ℃/s 时，DP600 钢发生珠光体相变；当冷却速率为 ≥10 ℃/s 时，DP600 钢发生贝氏体相变；当冷却速率 ＞ 20 ℃/s 时，DP600 钢发生马氏体相变。随着空冷温度由 700 ℃ 降低到 620、500 ℃，试验钢的显微组织分别为铁素体加马氏体、珠光体、贝氏体，抗拉强度先降低后升高。随着卷取温度由 500 ℃ 降低到 200 ℃ 以下，试验钢的组织由铁素体加珠光体变为铁素体加马氏体，且卷取温度低于100 ℃以下时，马氏体呈条带状偏聚分布，进而导致延伸率和冷弯性能不合格。空冷温度 700 ℃、卷取温度 200 ℃ 时，DP600 钢的力学性能满足要求，此时组织为75% 的铁素体加 25% 的马氏体，平均晶粒尺寸 （8.5 ± 0.3）μm，屈服强度 （397 ± 9）MPa，抗拉强度（ 659 ± 13）MPa，延伸率 27% ± 1.1%。Abstract: The CCT curve of DP600 steel was measured and the steel was industrially produced under different laminar flow cooling processes. The effects of cooling rate, air cooling temperature, and coiling temperature on the microstructures and mechanical properties of the steel were studied. The results show that when the cooling rate is less than 10 °C/s, pearlite transformation occurs in DP600 steel. Bainite transformation occurs in DP600 steel when the cooling rate is ≥ 10 °C/s. And martensite transition occurs in DP600 steel when the cooling rate is higher than 20 ℃/s. As the air cooling temperature decreased from 700 ℃ to 620℃ and 500 ℃, the microstructures of the tested steel were ferrite/martensite, pearlite and bainite respectively, and the tensile strength decreased first and then increased. When the coiling temperature reduced from 500 ℃ to below 200 ℃, the microstructures changed from ferrite/pearlite to ferrite/martensite. When the coiling temperature was below 100 ℃, the morphology of martensite changed from diffusing distribution to banded segregation distribution, which in turn leads to the unqualified elongation and cold bending properties. When the air cooling temperature was 700 ℃ and coiling temperature was 200 ℃, the mechanical properties of DP600 steel met the requirements, with the microstructure of 75% ferrite and 25% martensite in volume fraction, the average grain size of (8.5±0.3) μm, the yield strength of (397±9) MPa, the tensile strength of (659±13) MPa, and the elongation rate of 27%±1.1%.
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图 3 DP600钢在不同冷却速率下的显微组织
Figure 3. Microstructures of the DP600 steel cooling at different cooling rates
(a) 0.05 ℃/s； (b) 0.1 ℃/s； (c) 0.3 ℃/s；(d) 0.5 ℃/s；(e) 1 ℃/s；(f) 3 ℃/s；(g) 5 ℃/s；(h) 10 ℃/s；(i) 20 ℃/s；(j) 30 ℃/s；(k) 50 ℃/s
图 4 DP600 钢在不同空冷温度和 200 ℃ 卷取温度条件下的组织形貌、组织百分比和晶粒尺寸
（a）700 ℃；（b）620 ℃；（c）500 ℃；（d）组织百分比和晶粒尺寸
Figure 4. Microstructures, phase volume fractions and grain sizes of the DP600 steel produced at 200 ℃ coiling temperature and different air cooling temperatures
图 5 DP600 钢在 700 ℃ 空冷温度和不同卷取温度条件下的组织形貌、组织百分比和晶粒尺寸
（a）500 ℃；（b）200 ℃；（c）100 ℃；（d）组织百分比和晶粒尺寸
Figure 5. Microstructures, phase volume fractions and grain sizes of the DP600 steel produced at 700 ℃ air cooling temperature and different coiling temperatures
表 1 DP600 钢的化学成分及力学性能要求
Table 1. Chemical composition and mechanical properties requirements of the DP600 steel
化学成分/% 力学性能 C Si Mn P S Cr Als 屈服强度/MPa 抗拉强度/MPa 延伸率/% 180°冷弯 ≤0.12 ≤0.20 ≤1.70 ≤0.10 ≤0.01 ≤1.20 ≥0.015 330～470 580～700 ≥24.0 d=0.5a 
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表 2 DP600 钢的层流冷却工艺参数
Table 2. Laminar flow cooling process parameters of the DP600 steel
工艺 温度/℃ 冷却速率/(℃·s−1) 终轧 空冷 卷取 第一段 第二段 1 820 700 200 15.9 49.7 2 820 500 200 42.4 29.8 3 820 700 500 15.9 19.9 4 820 700 100 15.9 59.6 5 820 620 200 26.5 41.7
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表 3 不同空冷温度、卷取温度条件下DP600试验钢的力学性能
Table 3. Mechanical properties of the DP600 tested steel produced at different air cooling temperatures and coiling temperatures
空冷温度
/℃卷取温度
/℃屈服强度
/MPa抗拉强度
/MPa伸长率
/%180°冷弯 700 200 397±9 659±13 27.0±1.1 合格 620 200 411±9 597±11 26.5±1.1 合格 500 200 541±12 623±12 25.0±1.0 合格 700 500 384±8 588±11 24.5±1.0 合格 700 200 397±9 659±13 27.0±1.1 合格 700 100 426±9 709±13 21.5±0.9 不合格 要求值 330～470 580～700 ≥24.0 d=0.5a合格
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