550 MPa级海洋工程用钢板低温韧性波动原因分析

doi:10.13228/j.boyuan.issn0449-749x.20220178

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Abstract The impact energy of 550 MPa Marine engineering steel fluctuates greatly at low temperature. In order to further explore the cause of low temperature toughness fluctuation,impact tests were carried out on the test steel at different temperatures (-100 ℃-room temperature). The impact fracture,microstructure,second phase and inclusions were analyzed with optical microscope,scanning electron microscope and transmission electron microscope. The reason of low temperature toughness fluctuation was analyzed and discussed by thermodynamics calculation. The results show that the strength of the tested steel meets the requirements of grade,the impact absorption energy decreases with decreasing temperature,and the toughness and brittleness transition temperature is about -50 ℃. At -60 ℃,the impact energy fluctuates greatly,and an extremely low value of 18 J appears. The fracture is quasi-cleavage fracture,the shear section rate is 8%,and there are compound inclusions of Ti,Nb(C,N) and MnS at the crack source. At the same temperature,the shear section of the test steel with impact energy of 122 J is 34%,and there are obvious dimples in the fracture. The microstructure of the test steel is mainly tempered bainite and very little ferrite. There are high density dislocations in bainite slat. There are (Fe,Cr)₃C alloy cementite and a small amount of NbC and Cu-rich precipitates on grain boundary. The test steel mainly has small Angle grain boundary,and large Angle grain boundary is relatively low. There are a small amount of (Ca,Al,Mg,Mn,S) inclusions in the matrix,which are nearly round and polygonal in size between 1-3 μm,accounting for 85.87% of the total amount detected. The highest proportion of cas-oxide-MNS inclusions was 31.2%. Thermodynamic calculation results show that TiN precipitates earlier than MnS during solidification. Many factors,such as coarse precipitated phase of grain boundary and ingrain,inclusions,high proportion of small Angle grain boundary and immobile dislocation of plugging,have adverse effects on the impact toughness at low temperature. The existence of large titanium-containing precipitated phase is the key reason for the large fluctuation of impact toughness.

Key words： marine engineering steel low-temperature toughness the second phase inclusion small angle crystal boundary

Received: 17 March 2022

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	Articles by authors
	ZHU Kang-feng
	MA Heng
	SONG Xin-li
	JIA Juan
	LIANG Xiao-kai
	SUN Xin-jun

Cite this article:

ZHU Kang-feng,MA Heng,SONG Xin-li等. Cause analysis of low temperature toughness fluctuation of 550 MPa steel plate for offshore engineering[J]. Iron and Steel, 2022, 57(10): 178-187.

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http://www.chinamet.cn/Jweb_gt/EN/10.13228/j.boyuan.issn0449-749x.20220178 OR http://www.chinamet.cn/Jweb_gt/EN/Y2022/V57/I10/178

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