Reducing the Yield Ratio of HP295 Welded Bottle Steel Coil

Abstract : This paper introduces the production situation of HP295 welded bottle steel in the hot rolling production line. Production practice shows that by improving the finishing rolling Temperature, using two-stage cooling method, can make the yield strength ratio of HP295 lower than 08 , and the mechanical properties meet the requirements of the national standard GB6653-1994 .
close key Words : HP295 welded bottle steel; yield strength ratio ; two - stage cooling ; coiling temperature

With the rapid development of the domestic gas industry , welding bottle steel has shown a broad market prospect. HP295 welded cylinder steel is mainly used to manufacture liquefied petroleum gas cylinders, acetylene cylinders, liquefied chlorine gas cylinders, etc. Because it contains flammable and explosive liquids or gases, it has extremely strict requirements on stamping performance, mechanical properties, and welding performance. During the production of HP295 welded bottle steel in the hot rolling mill , the phenomenon of high yield strength ratio is common, and the pass rate of yield strength ratio is less than 95% . Combined with the production characteristics of 1 580 mm hot continuous rolling of new steel , without changing the chemical composition, heating system and rough rolling process, the effects of finishing rolling temperature, coiling temperature and cooling method on the microstructure, properties and yield strength ratio of HP295 steel were studied . According to the law of influence, it is concluded that the rolling process is beneficial to greatly reduce the yield ratio.

1 Test material and test plan
HP295 steel used in the test is shown in Table 1. The thickness of the blank is 230 mm . The temperature system of the roughing and finishing rolling units is: heating temperature 1 220 °C, rough rolling start temperature 1 150 °C, finish rolling temperature 1 050 °C, finish rolling start temperature 1 000C . Using different finish rolling temperature, coiling temperature and cooling method, the rolling process parameters are shown in Table 2 , and the trial rolling was carried out in 1580 mm hot continuous rolling mill.
Table 1 Chemical composition % of HP295 test steel
Composition w ( C ) w ( Mn ) w ( Si ) w ( P ) w ( S ) w ( Als )
National standard C0.20 C1.00 C0.35 C0.035 <0.035 >0.015
Internal control 0.16 0.87 <0.05 <0.015 <0.010 0.02 ~ 0.05

2 Analysis of test results
2 1 coiling temperature
The test conditions and on-site mechanical properties of steel b and c are shown in Table 2, where the coiling temperature of bc is different, and other rolling conditions are the same. It can be seen from table 2 that when the coiling temperature increases from 620 °C to 660 °C, the yield strength and tensile strength both decrease, and the yield ratio decreases from 077 to 0.76 . The metallographic structure is shown in Figure 11 and 1c . When the coiling temperature increases from 620 C to 660 C , the ferrite content increases slightly , and the grain size increases from 987 Mm to 10.76 4m . It can be seen that the main reason for increasing the coiling temperature and reducing the yield ratio is that the ferrite grains become larger, which is also the main reason for the decrease of the strength index.

Table 2 Industrial test conditions and mechanical properties of HP295 steel

test serial number	Product thickness / mm	Finishing temperature / C	cooling method	coiling temperature / C	Rel AlPa	R m A lPa	A A%	Yield ratio	ferrite A%	grain size AM m
a	3	870	Front section	620	410	525	31	0.78	73.4	8.67
,	3	870	back section	620	400	517	34	0.77	81.4	9.87
c	3	870	back section	660	390	510	35	0.76	84.6	10.76
d	3	890	back section	660	370	500	38	0.75	88.5	12.27
e	3	890	two sections	660	389	516	36	0.73	85.9	10.12

2 . 2 finishing rolling temperature
From the data in Table 2 , it can be seen that under the same rolling parameters for No. c and No. d steel, the finishing rolling temperature increased from 870 C to 890 C, and the yield ratio decreased from 0.76 to 0.75 . Strength is reduced. Comparing metallographic diagrams 1c and 1d , when the final rolling temperature is 890 C , the ferrite grains are coarse and irregular in shape. Due to the phase transformation effect induced by the change of final rolling temperature, the ferrite is coarse and the average grain size is 12.27 Mm . High temperature is beneficial to the diffusion of carbon atoms, thereby reducing the content of pearlite and increasing the content of ferrite, which is about 88.5% . When the final rolling temperature is 870 C , the ferrite grain size is relatively uniform, the ferrite content is 84.6%, and the average grain size is 1076 Mm . It can be seen that increasing the finish rolling temperature can effectively reduce the yield ratio, increase the residual deformation dislocations in the austenite, weaken the ferrite content in the high-shape structure, and increase the grain size.

2.3 Effect of cooling method on yield ratio
The effective cooling length of the laminar cooling system in Hot Rolling Plant is about 10488m. The whole cooling section has 22 groups of cooling sections, of which 19 are fine-tuning and 3 are fine-tuning. Each cooling section is connected with 4 headers by a cooling frame. Laminar cooling adopts front section, back section and two-stage comparative test. According to Table 2 , it can be seen that steel a and b adopt post-cooling, which can effectively reduce the yield strength ratio. Comparing Figures 1a and 1b , it can be observed that the ferrite grains obtained by pre-cooling of steel a are finer , but the grain size distribution is uneven . Because the proportion of small grains is larger , the average grain size is smaller. Small, about 867 Hm ; In addition, because the cooling process in the previous stage cannot provide enough growth time for the small grains , the influence of the ferrite precipitation process on the pearlite precipitation form is relatively weak , Therefore, the pearlite pellets obtained in the previous stage of cooling are relatively coarse, poor in dispersibility, high in area fraction, and the ferrite content is about 73.4% . The b steel adopts post-cooling , Due to the longer air cooling time at higher temperature after final rolling , It will restore the residual deformation dislocations in the austenite , Weaken the deformation-induced phase transition effect , lead to coarse ferrite grains and increased content , The average grain size is 987 Hm, the ferrite content is about 81.4%, and the strength index is reduced , but it can reduce the solid solution carbon content in the ferrite grains, increase the mobile dislocation density, and reduce the yield ratio , Cold forming performance is improved. The e- steel adopts a two-stage cooling method. After the final rolling of the strip steel, it is cooled to 720 C rapidly by opening the front group headers at a cooling rate greater than 30 °C/s ; after closing the middle group headers, the strip steel Under the condition of air cooling, slow cooling for 3 s , and finally turn on the rear array header to rapidly cool the strip to 660 C, and its performance parameters are shown in Table 2 . After two- stage cooling, the e- steel has small grains, the average grain size is 10.12 Hm, and the ferrite content is less, about 85.2% . It can be seen from Table 2 that the two-stage cooling intensity is greater than that of the latter stage cooling, and the yield strength Compared with other cooling methods, it is significantly lower. Therefore, the two-stage cooling method can not only increase the strength, but also reduce the yield ratio.
3 Conclusion
Through the above studies, increasing the finish rolling temperature, two-stage cooling and appropriate coiling temperature can effectively reduce the yield ratio of HP295 welding bottle steel. It has been proved by production practice that the final rolling temperature of 890C, two-stage cooling, and coiling temperature of 660C can make various mechanical properties meet the requirements of the national standard GB6653-1994 , and the pass rate of yield strength ratio has increased from less than 95% to 100% .

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