Technological Practice for Manufacturing 20MnTiB Quenched

Described in the assay are the manufacturing process and key control techniques for the 20MnTiB quenched-tempered seamless steel tube for making of hi-strength， medium-heavy wall cylinder casing， and quality of the tube itself. Thanks to employment of the combined process of converter/electric steelmaking + LF refining + tuberolling + quenching & tempering treatment， the said seamless steel tube stays with high steel cleanness， and all the performance target values and dimensional accuracy up to the technical requirements as specified by the customer. The comprehensive properties of the said tube is better than that of the 27SiMn quenched and tempered seamless steel tube.
Key words： seamless steel tube for cylinder casing service； high strength； 20MnTiB； quenching and tempering treatment

by 27SiMn has been widely used in various hydraulic cylinders , hydraulic supports and other cylinders . After quenching and tempering treatment, the material has good performance, the yield strength index can reach more than 830 MPa , and the U- shaped impact energy at room temperature is greater than 39 J , which can meet the requirements for the use of seamless steel pipes for general cylinders . In recent years, with the rapid development of China's construction machinery industry, higher requirements have been put forward for the strength, toughness and welding performance of seamless steel pipes for cylinder blocks . Type impact energy requirements greater than 55 J . If the traditional
27SiMn material is used to produce such high-demand seamless steel pipes for medium-thick-walled high-strength cylinders, and the performance indicators will not fully meet the technical requirements . In order to meet the needs of the construction machinery industry for new high-strength seamless steel pipe materials for cylinder blocks , in 2008, Panzhihua Iron and Steel Group Chengdu Steel Vanadium Co., Ltd. (hereinafter referred to as Pancheng Steel Company) cooperated with a company to develop a new type of 20MnTiB hot-rolled seamless steel pipe for cylinder blocks. Steel pipe, after the steel pipe is quenched and tempered by the user, the performance index can meet the use requirements .

2 Determine the main technical indicators and formulate the technical agreement of 20MnTiB quenched and tempered seamless steel pipe .
According to the service conditions of the oil cylinder and the specific requirements of the user, the chemical composition of the 20MnTiB quenched and tempered seamless steel pipe and the mechanical performance requirements of the relevant specifications of the GB/T 3077-1999 " Alloy Structural Steel " standard after quenching and tempering treatment are shown in the table. 1 and Table 2 .
Table 1 Chemical composition (mass fraction) of 20MnTiB quenched and tempered seamless steel pipe %

C	Si	mn	P	S	Ti	B	Cr	Mo	Ni	Cu
0.17~0.24	0.17~0.37	1.30 ~1.60	W 0.020	W 0.035	0.04~0.10	0.000 5~0.003 5	W 0.30	W0.10	W 0.30	W 0.25

Table 2 Mechanical property requirements of 20MnTiB seamless steel pipe after quenching and tempering treatment

Tensile strength /MPa	Yield strength /MPa	Elongation /%	Shrinkage /%	Normal temperature impact energy A kU ( 2 longitudinal /J	Hardness HBW
=1 050	=930	=10	=45	=55	300~340

• The key control technology in the process of production implementation
• Smelting Process Control Technology

20MnTiB cylinder blocks are generally medium-thick wall pipes. In order to ensure that the performance indicators of the pipe body after quenching and tempering treatment meet the requirements of high strength and high toughness, the B element in the steel plays a key role. How to obtain effective B element is the key to smelting process control technology .
B element has the ability to improve the hardenability of steel, which is much higher than the alloy element with the strongest hardenability effect calculated by unit content . B element is a very active element. It is easy to combine with 0 and N elements in steel to form stable inclusions and lose its effect of improving hardenability. Only the effective B element dissolved in steel can significantly improve hardenability The role of sex . The effective w ( B in low-alloy structural steel can be controlled at 0.0010 % - 0.0015 % , but B , N and Ti in 20MnTiB steel have interactive effects on hardenability, so it must be considered comprehensively .
w ( Ti ) / w ( N M 4.0 in the steel , the proportion of effective B in the steel can reach more than 80% of the total B , so w ( Ti and w ( B in 20MnTiB steel are 0.08% and 0.001 5% respectively ~0.002 0% control can ensure that the B element in the steel can fully play the role of improving hardenability, and can also increase the austenite coarsening temperature of 20MnTiB steel .
B- containing steel must be based on good deoxidation , refining and reasonable B addition methods during smelting . In the smelting process of 20MnTiB steel, the more critical operations are deoxidation , nitrogen fixation and B addition.

• Process Control Technology of Tube Blank for Hot Rolling Cylinder

The production of the oil cylinder body is generally to sandblast the outer surface of the seamless steel pipe first, and then machine the inner surface to reach the size of the oil cylinder body, which requires that the seamless steel pipe used for the cylinder body has a higher quality. dimensional accuracy . In order to make the hot-rolled billet meet the needs of seamless steel pipes for cylinders after treatment, the following key processes should be well controlled during the rolling of the billet:
( 1 ) The heating temperature of the control blank should not be too high . Because the seamless steel pipe used to make the cylinder body is generally a medium-thick wall pipe, the elongation coefficient of rolling is relatively small when the continuous casting slab or steel ingot is used for production. , Especially the impact toughness brings adverse effects. In the rolling process, the maximum heating temperature of the billet should not exceed 12707. It can be fine-tuned according to the different production units .
( 2 ) In order to ensure that the size of the seamless steel pipe after quenching and tempering can meet the machining requirements of the oil cylinder block, the tooling and mold for rolling the pipe should be reasonably configured during the rolling process, and the tolerance matching of the outer diameter and wall thickness should be controlled well , to ensure that the inner hole size of the steel pipe meets the final processing requirements .

• Pipe quenching and tempering process control technology
• Determination of heat treatment temperature

the 20MnTiB seamless steel pipe after quenching and tempering treatment reaches high strength and high toughness and meet the requirements of the cylinder block, it is first necessary to determine the appropriate quenching and tempering temperature for this steel type . For this reason, the phase transition point of 20MnTiB steel is measured, and the chemical composition of the steel is selected (mass fraction is:
C : 0.20% ,
Si : 0.30% ,
Mn : 1.53% ,
P : 0.012% ,
S ： 0.004% ，
Ti ： 0.07% ，
B : 0.0018 % .

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