thick gauge E36 high strength ship plate

Abstract: In order to ensure that the pass rate of strength and impact toughness of E36 high-strength steel plate of thick specification ( > 50mm) is above 95% , the influence of different rolling processes on its properties was studied. The results show that a lower heating temperature ( 1200 ~ 124°C) and a lower final cooling temperature ( 600 ~ 650°C) can make the product meet the requirements of mechanical performance standards.
Key words: E36 high strength ship plate ; rolling process ; mechanical properties; low temperature toughness

The production of E36 high-strength ship plates generally adopts a two-stage controlled rolling process, that is, controlled rolling in the recrystallized area and controlled rolling in the non-recrystallized area. The most ideal way is that the cumulative rolling deformation in the recrystallized area is above 70 % . , The cumulative rolling deformation in the non-recrystallized area is more than 50% . Xinyu Iron and Steel Co., Ltd. produces E36 high-strength ship plates , generally using 250 mm X2300 mm billets . If the chemical composition is unchanged, it is difficult to produce ship plates with a thickness of more than 50mm to ensure that the performance pass rate is above 95% , mainly because the strength and impact toughness of the product cannot meet the requirements . For this reason ,
Table 2 Three different experimental process parameters

Craft category	Heating section temperature /°C	Temperature of soaking section /°C	Heating time /h	Thickness to be heated /mm	Finishing temperature /°c		Final cooling temperature /C
1 #	1250 ~ 1270	1240 ~ 1260	5. 0 ~ 6. 0	130	840 ~	800	720〜680
2＃	1200〜1240	1190〜1230	3. 5〜4. 0	130	840〜	800	720〜680
3＃	1200〜1240	1190〜1230	3. 5〜4. 0	130	840〜	800	650 ~ 600

Experimental results

Table 3 Mechanical properties of products obtained by three different process systems

Craft category	Yield strength /MPa	Tensile strength/ MPa	Elongation / %	-40C impact energy /J			Average impact energy/ J
	370	500	33.0 _	68	56	70	65
1 #	365	505	33.0 _	68	86	72	75
	395	525	29.0	41	45	46	44
	365	530	24.0	50	47	59	52
	320	465	34.5	244	252	220	239
2#	340	485	31.1	232	184	200	205
	340	480	30.5	252	228	260	247
	325	470	33.0	204	214	206	208
	405	530	28.0	168	155	172	165
3#	415	550	27.5	150	162	170	161
	405	535	33.5	208	195	198	200
	435	550	28.5	85	120	135	113

Influence of Heating Temperature on Properties of Steel Plate

Analyzing the three processes and the corresponding mechanical properties of steel plates , the one with the relatively higher heating temperature is 1 # The heating time of the rolling process is also longer , and the low-temperature impact energy is significantly lower as a result , but it still maintains a higher strength than the 2 # rolling process at the same final cooling temperature . According to literature [ 1 ] , as the heating temperature increases , the austenite grain grows , and the size of the austenite grain is relatively uniform near the two temperatures of 900C and 1150C . The steel grade in this test contains Nb microalloying elements. Nb nitrides are completely dissolved above 1250C , and part of Nb nitrides below 1250C are still undissolved particles . These undissolved particles can inhibit austenite crystals at high temperatures. Grain growth provides a basis for grain refinement after rolling , so as to obtain better low temperature toughness .
2 # and 3 # process is relatively lower than that of 1 # process , and the heating time is shorter , so the impact energy at low temperature is higher . On the other hand, due to the higher heating temperature, the more Nb nitrides dissolve, resulting in more precipitation strengthening and solid solution strengthening of the matrix structure after rolling, which improves the strength . Pogor Gers BH et al's research [1] showed that the heating temperature is particularly sensitive to the strength of Nb- containing steel, as shown in Figure 2 . Figure 2 illustrates the reason why the strength index of the steel plate produced by the 1 # process is high under the same final rolling temperature and final cooling temperature as the 1 # process and the 2 # process .
42 Finish Cooling Temperature on Properties of Steel Plate after Rolling
2 # process and the 3 # process are the same except for the final cooling temperature, and the rest of the process parameters are the same . With the 3 # process, the strength of the steel plate is relatively high . It can be seen from Figure 3 that with the decrease of the final cooling temperature, the strength of the steel plate increases gradually.
The post-rolling cooling of Xinyu Iron and Steel Co., Ltd. is carried out through the ACC cooling system. The degree of reduction of the final cooling temperature is mainly achieved by increasing the amount of water and adjusting the speed of the roller table . It includes two aspects : one is to use the water volume to control the cooling speed ; the other is to use the roller speed to control the cooling time . The reason for increasing the strength of the steel plate by accelerating the cooling rate is that the grains are refined , which increases the amount of bainite in the structure and the precipitation strengthening effect of trace element carbides ; Resilience exacerbated by other reinforcement mechanisms . Prolonging the cooling time can obtain a lower final cooling temperature and increase the strength of the steel plate . However , the final cooling temperature cannot be lower than 450°C, otherwise the toughness of the steel plate will deteriorate rapidly . This is the reason why the low-temperature toughness of the 3 # process is still good under the condition of high steel plate strength .
Through the comparison of the three processes, it is concluded that the 3 # process is more ideal , and it is applied to industrial production . The qualified rate of steel plate performance is 96.4 % .
5 Conclusion
When producing thicker E36 high-strength ship plates , With the chemical composition unchanged , It is feasible to meet the requirements of product performance standards by adjusting the rolling process. The 3 # process is a better process to meet the performance requirements of the thicker specification E36 high-strength steel plate .

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