Analysis on the production process of HP295 hot-rolled steel strip in the ASP production line

This paper introduces the development and rolling of HP295 for bottle welding in the ASP production line of BBN and Steel Co., Ltd. , and explores the influence of the rolling process of the HP295 steel strip on the ASP production line on the yield ratio. The results show that the composition design idea of C0.13%~0.19% , Si £ 0.05% , Mn0.7%~1.0% used in the trial production steel and the final rolling temperature of 860 °;C , the coiling temperature of 640 °;C and the two-stage layer The flow cooling process can effectively reduce the yield ratio and meet the requirements of steel strength and plasticity.
Key words : HP295 welded bottle steel ; medium and thin plate continuous casting and rolling production line (ASP) ; yield strength ratio ; two-stage cooling ; coiling temperature;HP295 hot-rolled steel strip

HP295 is mainly used to make household LPG cylinders and environment-friendly motor vehicle LPG cylinders. With the development of China's petroleum and natural gas industry , the demand for steel for welding bottles is increasing , which has a broad market prospect. According to the current demand for liquefied petroleum gas in China, the demand and the specifications of gas cylinders , combined with the characteristics of BBN and Steel 's ASP hot rolling , in June 2006 , the rolled welded cylinder steel HP295 was developed, and the main specifications are (2.9~3.0 ) mm X 1 150 mm .

1. AS P medium and thin slab continuous casting and rolling production line

1700 medium and thin slab continuous casting and rolling production line of BBN and Steel Co., Ltd. adopts the Anshan Iron and Steel 1700 medium and thin slab continuous casting and rolling ( ASP) production process and technology , with an annual output of 3 million tons . The first phase of the project was completed and put into operation at the end of 2005 , and the second phase of the project will add the third continuous casting machine , 3 # walking beam heating furnace , steel coil flat

Table 6 Appearance quality inspection results
project	Side width /mm	Edge thickness /mm	Vertex angle
standard requirement	100 ± 1.8	10 ± 0.7	90 ± 35 '
detection value	98.7 ~ 100.3	9.5 ~ 10.1	89°;50 ~ 90°;05 '
	99.5	9.8	89°;56 '

It can be seen from the test results that the product quality index of marine angle steel is stable. The chemical composition is controlled within the design range , and the pass rate of mechanical properties and appearance quality inspection is 100% . Whole and sub-winding machines and other equipment. After the completion of the second phase of the project, the annual output will reach about 3.5 million tons .
The main equipment of the production line consists of 120t converter , LF refining furnace , 2 sets of thin slab continuous casting machines , 2 walking furnaces , 1 set of four-high reversible roughing mill with front vertical roll , heat preservation cover, cutting head flying shears , six It consists of rack finishing unit , laminar flow cooling device , 2 coilers and 1 coil transportation system. Adopting direct thermal charging, hydraulic AGC , hydraulic looper, bending roll, stringing roll device , so that the strip steel can obtain good shape and thickness control accuracy ; the coiler adopts hydraulic control technology and high-response speed automatic jumping of the coiling roll Control ( AJC) technology realizes precise centering and micro-tension coiling of the strip , reduces the tower shape , and eliminates the indentation of the strip head and the coiling roll at the same time ; the rolling process is controlled by a two-level computer to realize The automatic control of strip thickness, width, plate shape, finish rolling temperature and coiling temperature is realized.

1. HP 295 production process

In the early stage of trial rolling , the dynamic CCT curve and deformation resistance curve of HP295 were tested in the laboratory work .
Through formulating a reasonable production process and strict production organization, the CCSB marine angle steel with high technical requirements has been successfully developed. The physical quality of the product has reached a high level, meeting the needs of users, various technical and economic indicators have reached a high level, the finished product rate has reached 96.30% , and the standard coal and power consumption indicators have reached the level of Q235B ordinary angle steel. According to calculations, compared with ordinary Q235B angle steel, the profit has increased by more than 80 yuan per ton, which has increased the enterprise's profit, and at the same time, laid the foundation for the further development of high-strength marine angle steel.

1. formulation of chemical composition

The manufacture of steel cylinders is formed by one-time cold stamping and necking , annealed by heat treatment , and then welded into two half-bottles. In the production process, it is required that the stretching thinning amount is less than 10%, the hydraulic blasting force is less than 7.65 MPa, and the volume expansion rate is greater than 10% . To meet the above indicators , usually the hot-rolled steel plates for making liquefied petroleum gas cylinder steels are required to have sufficient strength, toughness, good cold stamping formability, small difference in welding performance and longitudinal and transverse mechanical properties, and high precision in size and shape . Therefore, the requirements for chemical composition and performance indicators are strict.
According to the requirements of GB6653-94 , the composition design of HP295 is carried out : the composition of welded gas cylinder steel adopts the appropriate design idea of reducing the content of C and Mn . Reducing Si , P and S can make the steel more pure , which can improve the cold forming performance and crack arrest performance of the steel. See Table 1 for specific composition design and Table 2 for performance requirements .

Table 2 Mechanical and process properties
standard	Yield point o s/MPa	Tensile strength o b/MPa	Elongation Yield Tensile Ratio	180 °; cold bending test
GB6653	$ 295	$ 440	$ 26 < 0.8	d=2a
3.2	process flow

KR hot metal pretreatment - 120t converter smelting-external refining
( CAS+LF) One-step continuous casting furnace heating - 1700 rough rolling one heat preservation cover heat preservation one six-stand continuous rolling laminar flow cooling one coiling one inspection one packaging and leaving the factory.

1. HP295 development trial production
2. The smelting process is based on the composition characteristics of this steel type . The smelting and continuous casting process adopted in this trial production is : molten iron [S] < 0.020% in the furnace , bottom blowing argon throughout the smelting , slag blocking and tapping , and the tapping time is about 8 minutes . The temperature at the end point of converter smelting is controlled at 1643-1672 °;C, and the end point carbon content is 0.046% ~ 0.087 % . Pre-deoxidation of steel core aluminum , alloying of metal manganese and high-carbon ferromanganese can reduce the peroxidation of molten steel and reduce the content of oxide inclusions. Each furnace of CAS is fed with 260 m of aluminum wire, and the argon blowing time is 10 to 11 minutes to reduce the content of oxide inclusions. Argon blowing is used throughout the continuous casting process to prevent secondary oxidation of molten steel and nitrogen absorption in molten steel . During casting, the casting speed is controlled at 1.85 to 2.35 m/min to ensure the surface and internal quality of the slab. Low-silicon mold powder and covering agent are used. The smelting of C and Mn is controlled according to the upper limit of internal control , the composition is controlled uniformly , and the pass rate is 100% .
3. The rolling process temperature is (1 220±20) C; the finish rolling start temperature is (1 020±30) C; the final rolling temperature is (860±20) C; the coiling temperature is 640±20) C; the laminar cooling method is Two stages of cooling.
4. Quality of trial rolled steel coils

3.4.1 Surface quality and dimensional accuracy Trial rolled HP295 steel strip has a good surface and size , no scale, air bubbles, cracks, scars, folds, scratches, inclusions, delamination and other defects ;
Except for partial narrowing , the rest were normal.
3.4.2 Performance The strength indicators of HP295 rolled in the test are all qualified , and the yield strength ratio of individual steel coils is high ( see Table 3) .

	Table 3	HP295 physical performance
o s/MPa 355 ~ 385	o b/MPa 465 ~ 495	s 5 /% 31.5 ~ 35.5	Yield ratio 0.75 ~ 0.83	180 °; cold bending test d=2a , intact
367.93	476.9	33.32	0.771

• Discussion on rolling process

1. Influence of Composition and Organization on Yield-strength Ratio

According to the problems in the trial rolling process , tracking the quality of the steel strip rolling , summarizing, analyzing and researching the process parameters and related data , rationally adjusting the production process system , and studying the improvement of the yield strength ratio of the HP295 steel strip from different angles Methods.
The Pickering [1-2] formula believes that both yield strength and tensile strength are functions of composition and tissue factors , which can be described by the following typical equations :
o s = 15.4|3.5+2.1(%Mn>+5.4(%Si>+23(%f 2 +1.13d -1/2 ], ob= 15.4[19.1+1.8(%Mn>+5.4(%Si> +0.25(%P>+0.5d -1/2 ], where %Nf The mass percentage of free nitrogen ;
%P - area percentage of pearlite ; d - - -Ferrite grain size , mm .
With the increase of C content , the pearlite content (%P) increases obviously , the Ob value increases , O s remains unchanged , and the yield ratio decreases ; with the grain refinement , the increase of O s is greater than that of ob , and the yield ratio increases. Therefore, it can be concluded that by taking measures such as increasing the C content of the steel , increasing the rolling temperature and coiling temperature , and slow cooling , a uniform and appropriate microstructure and ferrite grain size can be obtained , and the buckling of the welded bottle steel can be reduced. stronger than.
It is difficult to control the hot-rolled strip to ensure the yield strength ratio while the strength meets the requirements . In order to ensure the requirement of the yield strength ratio of welding bottle steel , according to the Pickering principle , the optimal composition design and rolling process are established. A series of key process controls are adopted , such as changing the rolling speed, coiling temperature, and increasing the carbon equivalent to meet the requirement that the yield ratio is less than 0.8 . According to the Pickering formula , the tensile strength increased by carbon addition is more obvious than the yield strength , and the yield ratio is reduced , so the carbon content is increased to 0.15% ~ 0.19% .

1. Influence of Laminar Cooling Mode on Mechanical Properties

In August 2006 , a laminar cooling method was tested in the laboratory. On this basis , in December 2006, the trial rolling adopted mid-section cooling. In the case of good effect , the front section, the back section and the two-stage cooling were also tested respectively , and the mechanical properties of the test rolled steel coils were compared , see Table 4 .
Table 4 Mechanical properties of different cooling methods

o		o b /MPa			Proportion /% of yield ratio > 0.8
cooling method	/MPa		/%	Yield ratio
Front section	381	485	34	0.786	19.6
middle section	360	465	35	0.784	12.0
back section	345	449	36	0.775	1.0
two-stage cooling	366	474	35	0.772	0

It can be seen from the data in Table 4 that the laminar flow cooling adopts the two-stage cooling method of the steel strip , the yield strength and tensile strength are relatively moderate compared with the front-stage cooling and the rear-stage cooling, and the elongation is relatively low , especially the yield strength ratio 100% pass rate . Therefore , the two-stage cooling has the effect of reducing the yield ratio , and the performance indicators can be guaranteed to be within the optimal range.

1. 1. coiling temperature (CT ) on properties

Coiling temperature is an important factor affecting the properties of HP295 steel strip , and has a significant impact on the ferrite grain size and microstructure of hot-rolled steel strip. As the coiling temperature decreases , the ferrite grains gradually become finer , and a higher strength index can be obtained.
Even under the same coiling temperature system , different ambient temperatures have a great influence on the strength index of steel coils. The laboratory conducted tests on coiling temperatures of 620 , 640 , 650 , 660 and 680 °;C , and found that the performance indicators of HP295 steel are very sensitive to coiling temperature. If the ambient temperature is high , the coiling temperature should be properly controlled at the lower limit of (640±20) C , the strength range is moderate , and the performance qualification rate is high ; when the temperature target is controlled at the upper limit , the strength will be inconsistent ; the CT temperature should be controlled above +20 C , some steel coils have a high failure rate due to too low strength . When the ambient temperature changes , the coiling temperature system should also be adjusted accordingly to meet the requirements of performance indicators.

1. 1. Influence of finish rolling temperature on properties

According to the performance and strength and toughness requirements of welding bottle steel , fine ferrite and pearlite are the best microstructures. It is found in the laboratory and production rolling that when the final rolling temperature of HP295 is controlled at 860±20) C , its microstructure and grain size are relatively suitable ( see Figure 1), and the strength and toughness indicators of steel coil products are relatively ideal.
of HP295 is improved , the chemical composition is optimized , and the laminar flow cooling method adopts a two-stage cooling method to ensure a certain final rolling temperature and coiling temperature. After taking the above measures , the qualification rate of HP295 has been greatly improved , and the performance qualification rate of the 1000 t steel coils produced has increased from 90% to 100% .

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