TMCP特厚止裂钢板的显微组织和抗解理断裂性能

Microstructure and anti-cleavage fracture properties of ultra-thick crack arrest steel plate prepared by TMCP

为开发超大集装箱船用止裂钢板,提出了一种低碳微合金SiMnCrNiCuMo钢,使用控轧控冷工艺(TMCP)试制出最大厚度为80 mm特厚钢板,使用埋弧焊接技术对最厚钢板进行了焊接评定,分析了钢板和焊接接头的显微组织。通过系列示波Charpy冲击试验研究了钢板及焊接热影响区粗晶区(CGHAZ)的韧脆转变行为,获得了各部位的韧脆转变温度(DBTT)及其解理断裂强度σf;分别测试了钢板及CGHAZ区-10℃的全厚度裂纹张开位移(CTOD),并且估算其启裂韧性(Kc);通过系列温度梯度宽板双重拉伸试验,获得了钢板在-10℃的止裂韧性(Kca)。结果表明:钢板的显微组织由不规则铁素体、针状铁素体、细化上贝氏体和弥散分布的马氏体奥氏体组成,平均晶粒尺寸小于7.0μm,其屈服强度大于500 MPa,韧脆转变温度低于-78℃。高的止裂性能导致冲击韧性的提高;提高σf值可降低钢板的DBTT及改善其低温韧性;在低温发生解理断裂的Charpy冲击试样和在低温区发生解理断裂的双重拉伸试样具有相同的临界事件。在焊接状态下,焊接接头的启裂韧性值低于母材钢板的止裂韧性值,因此,如果裂纹在低韧性的CGHAZ区起始,则该裂纹的扩展将在母材钢板处停止。

A kind of low carbon microalloyed SiMnCrNiCuMo steel was presented in this paper for the development of super-large container ship anti-crack steel plate. The steel plate with maximum thickness of 80 mm was manufactured by thermomechanical control process(TMCP). The weld-ability of the 80 mm thick steel plate was evaluated by using submerged arc welding technique, and microstructure of the steel plate and welded joint was analyzed. The ductile-to-brittle transition temperature(DBTT) and its cleavage fracture stresses(σf) of the steel plate and welding coarse-grained heat affected zone(CGHAZ) were obtained by Charpy V notch(CVN) impact tests. The full-thickness crack tip open displacement(CTOD) of the steel plate and CGHAZ at-10 ℃ was tested and its crack-initiation toughness(Kc) was estimated, and the crack-arrest toughness(Kca) of the steel plate at-10 ℃ was obtained by a full scale temperature-gradient double tension technique. The results show that the microstructure of the steel plate is composed of quasispolygonal ferrite(QPF), acicular ferrite(AF), degenerated upper bainite(DUB) and martensite-austenite with dispersion distribution, and the average grain size is smaller than 7.0 μm. The yield strength of the steel plate is greater than 500 MPa, and its DBTT is lower than-78 ℃. The high cracking resistance leads to the improvement of impact toughness, and the increase of σf value leads to low DBTT and the improvement of low temperature toughness of the steel plate. Charpy impact specimens with cleavage fracture at low temperature and double tension specimens with cleavage fracture at low temperature have the same critical events. In the welding state, the crack-initiation toughness value of the welded joint is lower than the crack arrest toughness of the steel plate, and hence, if the crack starts in the CGHAZ, the propagation of this crack will stop at the steel plate.