High-strength and high-toughness seamless tube for automobile safety bag and preparation method of high-strength and high-toughness seamless tube

Abstract

The invention aims to provide the high-strength and high-toughness seamless tube for the automobile safety bag and the preparation method of the high-strength and high-toughness seamless tube, and the purpose that the steel tube has high strength, high plasticity and toughness and excellent low-temperature blasting resistance is achieved. The steel for the seamless tube for the automobile safety bag comprises the following chemical components in percentage by weight: 0.02-0.04% of C; 0.2 to 0.4 percent of Si; mn: 1.5 to 1.7%; s < = 0.005%; p is less than or equal to 0.01%; 0.04% to 0.06% of Nb; 0.02 to 0.04 percent of V; 0.01 to 0.02 percent of Ti; 0.7%-1.0% of Cu; n < = 0.002%; o < = 0.002%; al is less than or equal to 0.03%; and the balance of Fe and inevitable impurities. The steel pipe is made of microalloyed steel and has the advantages of being high in strength, good in plasticity and toughness, excellent in blasting resistance, good in cold machining performance and low in cost. The optimization of the steel pipe for the automobile safety bag from material selection to a specific cold drawing process is achieved, and the yield strength of the steel pipe is larger than 860 MPa, the tensile strength is larger than 1000 MPa, and the ductility is larger than 25%; and according to the GB / T241-2007 standard, the steel pipe is placed at the temperature of minus 40 DEG C for 24 hours and then subjected to a blasting experiment, and the pressure of the blasting steel pipe is larger than or equal to 130 MPa.

Description

Technical Field

[0001] The invention relates to a steel pipe for automobiles and a preparation process thereof, and in particular to a seamless pipe with high strength and high toughness for automobile safety bags and a preparation method thereof. Background Art

[0002] As people's living standards improve, the number of small family cars increases, and traffic accidents occur frequently. Therefore, the safety of cars is becoming more and more important. In addition to seat belts, the automobile airbag system is an important passive safety device in modern cars. It can quickly inflate when the vehicle collides to provide protection for passengers. The steel tube in the airbag is part of the airbag generator. Its main function is to store compressed inert gas. When the vehicle collides, the sensor detects that the impact force reaches a certain level, which triggers the ignition coil to stimulate the gas-generating agent to explode. The high pressure generated will open the sealing plug at the end of the steel tube, so that the stored gas will quickly fill the airbag, reduce the impact force on the passengers during the collision, and thus protect the passengers from injury. Therefore, the steel tube in the airbag needs to have high strength, high toughness, excellent explosion resistance, high precision and reliability to ensure that it can work normally at critical moments.

[0003] Patent document CN104046924A discloses the invention of a high-strength and tough seamless steel tube for airbags, whose tensile strength is ≥850MPa, -60℃ low-temperature impact absorption energy is ≥15J, and elongation is ≥18%. From the perspective of strength and toughness indicators, there is still room for further improvement in the strength and toughness of steel tubes, and higher strength and toughness can ensure greater safety. Patent document CN1791694 discloses the invention of a steel tube for an airbag system, which has a strength of 1000MPa, but its toughness is not clear and the manufacturing process is complicated. Summary of the invention

[0004] Based on the above requirements for high strength and high toughness of steel tubes used in airbags, the purpose of the present invention is to provide a high-strength, high-toughness seamless tube for automobile airbags, and at the same time invent a method for manufacturing the steel tube. The purpose is to obtain a steel tube for automobile airbags with higher strength and toughness, better plasticity, better explosion resistance and better safety.

[0005] The technical solution of the present invention is: A high-strength and high-toughness seamless tube for automobile safety bags, the chemical composition of the steel tube material is as follows by weight percentage: C: 0.02~0.04%; Si: 0.2~0.4%; Mn: 1.5~1.7%; S≤0.005%; P≤0.01%; Nb: 0.04~0.06%; V: 0.02~0.04%; Ti: 0.01~0.02%; Cu: 0.7~1.0%; N≤0.002%; O≤0.002%; Al≤0.03%; the balance is Fe and unavoidable impurities.

[0006] The high-strength and high-toughness seamless tube for automobile safety bags provides a preparation process thereof, comprising the following steps: (1) Surface treatment of the initial blank seamless steel pipe; (2) Lubricating the inner and outer surfaces of the steel pipe after the surface treatment in step (1) with lubricating oil; (3) cold drawing the steel pipe after lubrication treatment in step (2); (4) annealing the steel tube after cold drawing in step (3); (5) Straightening and polishing the steel pipe after annealing in step (4).

[0007] In particular, the structure of the initial blank seamless steel tube selected for the high-strength and high-toughness seamless tube for automobile safety bags is acicular ferrite, and the average effective grain size is 2-10 μm.

[0008] The surface treatment of the preparation process step (1) of the high-strength and high-toughness seamless tube for automobile safety bags includes inner and outer wall sandblasting, inner and outer wall grinding or a combination thereof to remove the oxide scale on the surface of the steel tube.

[0009] The cold drawing process in step (3) of the preparation process of the high-strength and high-toughness seamless tube for automobile safety bags is: multiple passes of large deformation cold drawing, the number of cold drawing passes is not less than 4 times, the deformation of each pass is not less than 12%, and an annealing and recrystallization treatment is performed between two consecutive passes.

[0010] The annealing process of step (4) of the preparation process of the high-strength and high-toughness seamless tube for automobile safety bags adopts a walking beam heat treatment furnace for annealing, and the annealing heat treatment is completed in a nitrogen protective atmosphere furnace, including: (1) The annealing temperature between passes is 700~750℃, and the annealing time is 5~15 minutes; (2) The annealing temperature of the finished tube is 600~650℃, and the annealing time is 10~20 minutes; (3) After annealing, the steel is cooled in the furnace to 200-230°C and then air-cooled.

[0011] According to the high-strength and high-toughness seamless tube for automobile safety bags and the preparation process thereof, the microstructure of the steel tube is ferrite, and the ferrite grain size is 2-10 μm.

[0012] According to the high-strength and high-toughness seamless tube for automobile safety bags and its preparation process, the yield strength of the steel tube is greater than 860MPa, the tensile strength is greater than 1000MPa, and the elongation is greater than 25%.

[0013] According to the above-mentioned high-strength and high-toughness seamless tube for automobile safety bag and its preparation process, referring to GB / T241-2007 standard, a bursting test is carried out after being placed at -40°C for 24 hours, and the bursting pressure of the steel tube is ≥130MPa.

[0014] The main design concept of the present invention is that the steel pipe material is ultra-low carbon micro-alloy steel. The steel pipe material composition uses a lower C content to obtain excellent toughness, and uses cheap Mn and Cu elements to increase the hardenability of the steel on the one hand, and obtain higher cold working performance on the other hand, while the raw material cost is low. Another role of Cu in steel is precipitation strengthening. When the finished pipe is annealed, the annealing temperature is regulated, and the precipitation strengthening of the nano Cu-rich phase can make up for the loss of strength during annealing while relieving stress and reducing strength. Therefore, the steel of the present invention can maintain a higher strength under the premise of high toughness.

[0015] The structural characteristics of the steel used in the steel pipe material are another design concept of the present invention. In addition to ultra-low C, the present invention also adds Nb, V, and Ti microalloying elements. They are strong carbide-forming elements, which combine with a small amount of C to form nanocarbides during the initial steel pipe rolling hot working process. This nanocarbide can effectively hinder the growth of grain size, and cooperate with the thermomechanical rolling process to obtain an acicular ferrite structure with excellent toughness and fine grain size. Therefore, the present invention clearly states that the structure of the initial blank seamless steel pipe used for the high-strength and high-toughness seamless pipe for automobile safety bags should be acicular ferrite, and the average effective grain size should be limited (2~10μm).

[0016] The present invention combines innovation of steel tube materials with the organization limitation of the initial cold-drawn steel tube and the formulation of a specific cold-drawn process, thereby integrating three aspects to obtain a seamless steel tube for automobile safety bags with high strength, high plasticity and excellent comprehensive performance.

[0017] The key elements and contents of the composition design of the materials used in the steel pipe of the present invention are described as follows: C: Carbon (C) is the main element that determines the strength of steel. The composition of traditional steel tube materials for automobile safety bags generally adopts measures to increase the C content in an appropriate amount to improve the strength of the steel tube. Therefore, the C content is relatively high. The purpose of reducing the C content in the present invention is to obtain excellent plasticity and toughness, and the loss of strength is compensated by the refinement of the grain size and the precipitation strengthening of the nano-Cu-rich phase. The purpose of the C content in the steel tube designed by the present invention being 0.02~0.04% is mainly to form nano-sized carbides with the micro-alloying elements Ti, Nb and V in the steel during hot working and annealing. These nano-sized carbides can inhibit the coarsening of the structure while improving the strength, and at the same time play the role of fine grain strengthening and precipitation strengthening.

[0018] Mn: Manganese (Mn) is the main alloying element in steel pipe materials, mainly because the addition of Mn causes solid solution strengthening. Mn can also reduce the γ→α phase transformation temperature of steel, and the reduction of the γ→α phase transformation temperature has a refining effect on the α grains, and can also change the microstructure after the phase transformation. Adding 1.0-1.5% Mn can reduce the γ→α phase transformation temperature by 50°C, thereby refining the ferrite grains and maintaining the polygonal ferrite matrix. When the M content is increased to 1.5-2.0%, the phase transformation temperature can be further reduced, and austenite is transformed into acicular ferrite. While increasing the strength, Mn also increases the toughness of the steel and reduces the tough-brittle transition temperature of the steel. Therefore, the Mn content added to the steel of the present invention is 1.5-1.7%.

[0019] Cu: Copper (Cu) is another main alloying element in steel pipe materials. When the Cu element exists in a solid solution state in steel, it can improve the cold working performance; after annealing, the supersaturated solid solution Cu precipitates in the form of a nano-sized Cu-rich phase, which plays a role in precipitation strengthening. Therefore, the Cu in the material of the steel pipe of the present invention improves the processing performance of the steel pipe during the cold drawing process on the one hand, and improves the precipitation strengthening effect on the other hand. For the selection of the Cu content, the solid solubility of Cu in ferritic steel is taken into consideration. When the Cu content is less than 0.7%, Cu mainly exists in the steel in a solid solution state, and there is no precipitation strengthening effect; if the Cu content is higher than 1.0%, it may have an adverse effect on the hot working performance of the steel. Therefore, in general, the Cu content in the steel pipe material of the present invention is selected to be 0.7%~1.0%.

[0020] Description of the cold drawing process of the steel pipe in the present invention: The present invention adopts a multi-pass cold drawing process with a large deformation amount, and annealing is performed once after two consecutive cold drawing passes, in order to increase the deformation energy storage inside the steel pipe, anneal and recrystallize the steel pipe at a lower temperature, and finally form a structure with a fine grain size.

[0021] The annealing temperature is different. The annealing between the intermediate passes adopts a higher temperature. The purpose is to avoid excessive precipitation of nano-Cu-rich phase while recrystallizing, so as to produce more precipitation strengthening prematurely to enhance deformation resistance. The annealing temperature of the finished tube is reduced. The purpose is to recrystallize at a lower temperature, and the structure is finer. At the same time, the saturated solid solubility of Cu decreases with temperature, and nano-precipitation can also be precipitated, thereby achieving higher strength.

[0022] (1) The steel tube for automobile safety bags of the present invention is made of micro-alloy steel, which has the advantages of high strength, good plasticity and toughness, excellent explosion resistance, good cold working performance and low cost; (2) The present invention solves the optimization of the steel tube for automobile safety bags from material selection to specific cold drawing process. The yield strength of the steel tube is greater than 860MPa, the tensile strength is greater than 1000MPa, and the elongation is greater than 25%. According to the GB / T241-2007 standard, after being placed at -40°C for 24 hours, a bursting test is carried out, and the bursting pressure of the steel tube is ≥130MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The microstructure of the initial steel pipe of the embodiment Figure 2 The microstructure of the finished steel pipe of the embodiment is Figure 3 The macroscopic appearance of the finished pipe after blasting in the embodiment DETAILED DESCRIPTION

[0024] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a detailed description of a high-strength and high-toughness seamless tube for automobile safety bags and a preparation method thereof proposed by the present invention in combination with embodiments.

[0025] The chemical composition of the microalloyed steel used in the high-strength and high-toughness seamless tube for automobile safety bags is as follows: C: 0.03%; Si: 0.22%; Mn: 1.66%; S: 0.002%; P: 0.004%; Nb: 0.05%; V: 0.03%; Ti: 0.013%; Cu: 0.82%; N: 0.002; O: 0.0011%; Al: 0.01%.

[0026] Figure 1 This is the organizational morphology of the initial blank tube, which is acicular ferrite with an average grain size of about 8μm.

[0027] The preparation process is as follows: (1) A blank tube with an outer diameter of 60 mm and a wall thickness of 4.0 mm was selected for sandblasting and grinding of the inner and outer walls. The outer diameter of the treated steel tube was 58 mm and the wall thickness was 3.8 mm; (2) Lubricate the inner and outer surfaces of the treated steel pipe with lubricating oil; (3) The steel pipe is subjected to the first cold drawing, and the outer diameter after cold drawing is 52 mm, the wall thickness is 3.2 mm, and the deformation amount of each pass is 13%; the steel pipe is subjected to the second cold drawing, and the outer diameter after cold drawing is 45 mm, the wall thickness is 2.6 mm, and the deformation amount of each pass is 15%; after the two cold drawing, intermediate annealing is performed, the annealing temperature is 730 ° C, and the annealing time is 13 minutes; (4) The steel pipe is subjected to the third cold drawing, and the outer diameter after cold drawing is 38 mm, the wall thickness is 2.25 mm, and the deformation amount of each pass is 15%; the steel pipe is subjected to the fourth cold drawing, and the outer diameter after cold drawing is 30 mm, the wall thickness is 1.85 mm, and the deformation amount of each pass is 17%. After the two cold drawing, intermediate annealing is performed, the annealing temperature is 710 ° C, and the annealing time is 7 minutes; (5) The steel pipe is subjected to the fifth cold drawing, and the outer diameter after cold drawing is 26 mm, the wall thickness is 1.6 mm, and the deformation amount of each pass is 16%; the steel pipe is subjected to the sixth cold drawing, and the outer diameter after cold drawing is 22 mm, the wall thickness is 1.4 mm, and the deformation amount of each pass is 15%; (6) The steel pipe is annealed at a temperature of 620°C for 15 minutes. After annealing, it is cooled to 220°C in the furnace and then air-cooled.

[0028] (7) Straighten and polish the annealed steel pipe.

[0029] The steel pipe tissue sample prepared by the present invention is cut, and the tissue morphology is observed after grinding, polishing and corrosion.

[0030] Figure 2 The steel pipe structure obtained by the preparation process of the present invention is ferrite, and the average particle size is 8 μm.

[0031] The steel pipe sample prepared by the present invention is cut to carry out a mechanical property test.

[0032] Table 1 shows the mechanical property values ​​of the steel pipe obtained by the preparation process of the present invention. It can be seen that the yield strength of the steel pipe is greater than 860 MPa, the tensile strength is greater than 1000 MPa, and the elongation is greater than 25%.

[0033] Table 1 Mechanical properties of the examples

[0034] The steel pipe prepared by the present invention was cut to conduct a low-temperature explosion test. According to GB / T241-2007 standard, the steel pipe samples were placed at -40°C for 24 hours and then subjected to an explosion test. The pressures of the two explosion steel pipes were 139.6MPa and 131.3MPa, respectively. Figure 3 This is the macroscopic morphology of one of the steel pipes after explosion.

[0035] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-strength and high-toughness seamless tube for automobile safety bags, characterized in that: The chemical composition of the steel pipe material by weight percentage is: C: 0.02~0.04%; Si: 0.2~0.4%; Mn: 1.5~1.7%; S≤0.005%; P≤0.01%; Nb ：0.04~0.06%； V：0.02~0.04%； Ti: 0.01~0.02%; Cu: 0.7~1.0%; N≤0.002%; O≤0.002%; Al≤0.03%; the balance is Fe and unavoidable impurities.

2. According to the high-strength and high-toughness seamless tube for automobile safety bags according to claim 1, a preparation process thereof is provided, characterized in that it comprises the following steps: (1) Surface treatment of the initial blank seamless steel pipe; (2) Lubricating the inner and outer surfaces of the steel pipe after the surface treatment in step (1) with lubricating oil; (3) cold drawing the steel pipe after lubrication treatment in step (2); (4) annealing the steel tube after cold drawing in step (3); (5) Straightening and polishing the steel pipe after annealing in step (4).

3. The process for preparing the high-strength and high-toughness seamless tube for automobile safety bags according to claim 2, characterized in that: The structure of the initial blank seamless steel pipe is acicular ferrite with an average effective grain size of 2~10μm.

4. The process for preparing the high-strength and high-toughness seamless tube for automobile safety bags according to claim 2, characterized in that: The surface treatment in step (1) includes inner and outer wall sandblasting, inner and outer wall grinding or a combination thereof to remove the oxide scale on the surface of the steel pipe.

5. The process for preparing the high-strength and high-toughness seamless tube for automobile safety bags according to claim 2, characterized in that: The cold drawing process in step (3) is: multiple passes of large deformation cold drawing, the number of cold drawing passes is not less than 4 times, the deformation of each pass is not less than 12%, and an annealing and recrystallization treatment is performed between two consecutive passes.

6. The process for preparing the high-strength and high-toughness seamless tube for automobile safety bags according to claim 2, characterized in that: The annealing process of step (4) is performed in a step-beam heat treatment furnace, and the annealing heat treatment is completed in a nitrogen protective atmosphere furnace: (1) The annealing temperature between passes is 700~750℃, and the annealing time is 5~15 minutes; (2) The annealing temperature of the finished tube is 600~650℃, and the annealing time is 10~20 minutes; (3) After annealing, the steel is cooled in the furnace to 200-230°C and then air-cooled.

7. The high-strength and high-toughness seamless tube for automobile safety bags and its preparation process according to any one of claims 1 to 6, characterized in that: The microstructure of the steel pipe is ferrite, and the ferrite grain size is 2~10μm.

8. The high-strength and high-toughness seamless tube for automobile safety bags and its preparation process according to any one of claims 1 to 6, characterized in that: The yield strength of the steel pipe is greater than 860MPa, the tensile strength is greater than 1000MPa, and the elongation is greater than 25%.

9. The high-strength and high-toughness seamless tube for automobile safety bags and its preparation process according to any one of claims 1 to 6, characterized in that: According to GB / T241-2007 standard, the explosion test is carried out after being placed at -40℃ for 24 hours, and the pressure of the explosion steel pipe is ≥130MPa.

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