JP2013060652A - Hollow member for vehicle reinforcement - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hollow member for vehicle reinforcement capable of being advantageously produced in each respect of reliability in junction, energy cost, surface properties and a product shape.SOLUTION: In the hollow member for vehicle reinforcement, an electric resistance welded tube is used for a raw pipe, the electric resistance welded tube including 0.05-0.20 mass% of C, 0.5-2.0 mass% of Si, 1.0-3.0 mass% of Mn, 0.1 mass% or less of P and 0.01 mass% or less of S, with a balance being Fe and inevitable impurities, having a two-phase structure of a ferrite phase and a martensite phase or a three-phase structure of the two-phase and a retained austenite phase, and having a tensile strength of 980 MPa or more. Heat treatment for cooling down to room temperature after heating up to 500-750°C is subjected to an entire area or a part in the pipe circumferential direction at a portion of the raw pipe in the pipe length direction. A tensile strength of a part 3 (or 3A) subjected to the heat treatment is lower by 200 MPa or more than that of a portion 4 not subjected to the heat treatment yet, and a product of the tensile strength and an elongation of the part subjected to the heat treatment is 14,000 MPa% or more.

Description

  The present invention relates to a vehicle reinforcing hollow member. The hollow member for vehicle reinforcement referred to in the present invention is a material for a vehicle reinforcement part having a bend in the part length direction, particularly a part such as an A-pillar reinforcement among automobile frame parts, and is bent and processed. It is a hollow member that is a part, and the part to be bent in the member length direction is softer than other parts and has better bending workability, and the other parts have sufficient member strength (and thus part strength). This is a hollow member that ensures the above.

  As a method of obtaining the vehicle reinforcing hollow member, there is a method (Patent Document 1) in which a tailored tube is formed by using a steel material having different strengths tailored as a pipe material. Moreover, although it is not a steel pipe, there exists a method (patent document 2) which quenches for every partial area | region in order to make an intensity | strength difference in a part of the whole and another part.

JP 2004-314102 A Special table 2010-539326

In the method of obtaining the tailored tube, since the tailored welded portion is provided as the joint portion in addition to the electric seam welded portion, the joint reliability is not sufficient. In addition, the method by partial quenching requires heating to a high temperature region (austenite region), and has problems such as high energy cost, large skin roughness due to oxide scale, and deformation due to thermal strain. These points were problems.
An object of the present invention is to provide a vehicle reinforcing hollow member that can be advantageously manufactured in terms of joint reliability, energy cost, surface properties, and product shape.

The present invention that has solved the above problems is as follows.
(1) C: 0.05-0.20 mass%, Si: 0.5-2.0 mass%, Mn: 1.0-3.0 mass%, P: 0.1 mass% or less, S: 0.01% by mass or less, the balance being Fe and inevitable impurities, forming a two-phase structure of a ferrite phase and a martensite phase or a three-phase structure of the two phases and a retained austenite phase, and having a tensile strength of Using an ERW steel pipe of 980 MPa or more as a raw pipe, it is subjected to a heat treatment that is heated to 500 to 750 ° C. and then cooled to room temperature with respect to the whole or part of the pipe circumferential direction in a part of the pipe length direction, A vehicle-reinforcing hollow member characterized in that the tensile strength of the heat-treated portion is 200 MPa or more lower than that of the non-heat-treated portion, and the product of tensile strength and elongation of the heat-treated portion is 14000 MPa ·% or more.

  According to the present invention, for example, a vehicle reinforcing hollow member as a material for a vehicle reinforcing component such as an A pillar reinforcement is advantageous in terms of joint reliability, energy cost, surface properties, and product shape. Can be manufactured. That is, in the present invention, since a heat treatment of heating at 500 to 750 ° C. is performed on a part or the whole of the pipe circumferential direction in a part of the length direction of the base pipe which is an electric resistance steel pipe, an extra joint such as a tailored weld As much as it does not contain, it is excellent in the reliability of the joint, and the energy cost is reduced and the adverse effects of oxide scale and thermal deformation are reduced because heat treatment is performed at a lower temperature compared to quenching strengthening.

It is the schematic which shows Example 1 of this invention. It is the schematic which shows the (a) bending process point in Example 1, and the (b) tension test collection position. It is the schematic which shows Example 2 of this invention. It is the schematic which shows the (a) bending process point in Example 2, and the (b) tension test collection position.

In the present invention, the above configuration is adopted in order to satisfy the required performance of the material of the vehicle reinforcing part having a bend in the part length direction. Hereinafter, the reasons for limiting each requirement according to the present invention will be described.
[Chemical composition]
(C: 0.05-0.20 mass%)
C has the effect | action which produces | generates the retained austenite phase required in order to give the outstanding workability while improving the intensity | strength of an ERW steel pipe. If the C content is less than 0.05% by mass, these effects cannot be obtained. On the other hand, if it exceeds 0.20% by mass, the strength of the ERW steel pipe will increase excessively and the workability will deteriorate. Therefore, C: 0.05 to 0.20 mass%.

(Si: 0.5-2.0 mass%)
Si has the effect of improving the balance between TS and elongation (abbreviated as EL) of the ERW steel tube by solid solution strengthening, promoting ferrite transformation to generate a ferrite phase, and concentrating C in the residual austenite phase. . The residual austenite phase is stabilized by C enrichment. When the Si content is less than 0.5% by mass, these effects cannot be obtained. On the other hand, if it exceeds 2.0% by mass, scale is likely to be generated by hot rolling during the production process of the steel strip, and as a result, rough skin after cold rolling is caused, and the surface properties of the ERW steel pipe are deteriorated. Therefore, Si: 0.5 to 2.0 mass%.

(Mn: 1.0 to 3.0% by mass)
Mn has the effect | action which improves the hardenability of the cold rolled steel plate used for a pipe | tube raw material, and stabilizes a retained austenite phase. If the Mn content is less than 1.0% by mass, these effects cannot be obtained. On the other hand, if it exceeds 3.0 mass%, the strength of the ERW steel pipe will increase excessively and the workability will deteriorate. Therefore, Mn: 1.0 to 3.0% by mass.

(P: 0.1% by mass or less)
P has an action of promoting ferrite transformation to generate a ferrite phase. However, if the P content exceeds 0.1% by mass, the ductility of the ERW steel pipe is lowered and workability is deteriorated. Therefore, P: 0.1 mass% or less.
(S: 0.01% by mass or less)
S combines with other elements to generate sulfides. If the S content exceeds 0.01% by mass, the sulfide aggregates in the structure of the ERW steel pipe and becomes an inclusion, causing the strength of the ERW steel pipe to decrease. Therefore, S: 0.01 mass% or less.

The balance excluding the above components is Fe and inevitable impurities.
[TS of pipe ≥ 980 MPa]
If the TS of the raw tube is less than 980 MPa, the strength characteristics of the vehicle reinforcing hollow member are insufficient, and therefore TS ≧ 980 MPa.
[Organization]
The structure of the raw tube is preferably a two-phase structure of a ferrite phase and a martensite phase in order to ensure a high strength of TS980 MPa class or more and sufficient ductility, and in order to further increase the ductility, the two phases are used. A three-phase structure in which a residual austenite phase is added is preferable. The phase ratio of the two to three phases is preferably 20-60% by volume of ferrite phase, 40-80% by volume of martensite phase, and 0-15% by volume of retained austenite phase.

In addition, the ERW steel pipe used as a base pipe was formed by subjecting a cold-rolled steel strip having the above composition to a quenching treatment in a continuous annealing process, and using the obtained steel strip of TS 980 MPa or more as a raw material, and roll forming this into a tubular shape. It is manufactured by a method in which the welded abutting end portions are electro-welded (that is, a normal electro-sealing welding method).
[Tube heat treatment]
(Heating temperature: 500-750 ° C)
If the heating temperature is less than 500 ° C., it is difficult to achieve a softening of Δ200 MPa (200 MPa in terms of TS decrease) by heating for a short time of several seconds or less. On the other hand, if it exceeds 750 ° C. When the cooling rate is high, not only is it hardened and it becomes difficult to soften, but the TS × EL balance becomes less than 14000 MPa ·%, and ductility decreases. Further, when the heating temperature is high, the heating and holding time is long, and the cooling rate is slow, the softening region other than the predetermined portion increases due to heat conduction, and the softening region cannot be clearly distinguished. Therefore, it is set as heating temperature: 500-750 degreeC.

The heating and holding time is set to several seconds or less, and it is desirable to heat the predetermined portion (heat treatment application portion) so that the temperature of the region (unheat-treated portion) where TS980 MPa or more is secured does not rise to 450 ° C. or more due to heat conduction.
The heating means is preferably means that has high energy density such as induction heating or laser heating and can heat only a predetermined portion in a short time.

By heating while cooling with gas or liquid, it is possible to suppress the temperature rise other than the predetermined portion.
When the scale generated by heating becomes a problem, it is removed by shot blasting, pickling or the like in a later step. Alternatively, generation is suppressed by heating in an inert gas atmosphere.

[Softening: Δ200 MPa or more]
If it is less than Δ200 MPa, the function as a vehicle reinforcing hollow member is small with respect to the heat treatment cost. In other words, there is no advantage in cost / performance comparison with a solid steel pipe.
The upper limit is desirably about Δ600 MPa. When softening over Δ600 MPa, it is necessary to take at least one of the measures of increasing the heating temperature, increasing the heating and holding time, and increasing the cooling rate. The softened area increases and it becomes difficult to clearly distinguish the softened area.

[TS × EL ≧ 14000MPa%]
When TS × EL is less than 14000 MPa ·%, the ductility of the softened portion (heat treatment portion) is insufficient and the degree of freedom in designing the parts is reduced. TS × EL ≧ 14000 MPa ·% can be achieved by subjecting the ERW steel pipe having the composition and structure defined in the present invention to the heat treatment defined in the present invention.

  Two types of ERW steel pipes showing composition, structure, TS, and size are shown in Table 1 as elementary pipes A and B, respectively. The base tubes A and B are made of a steel strip of the structure and TS level shown in Table 1 obtained by quenching a cold-rolled steel strip having the composition shown in Table 1 in a continuous annealing process. Manufactured by the law.

  In Example 1, partial heat treatment was performed on the raw tube A by the method shown in FIG. 1 with the entire region in the tube circumferential direction in a portion in the tube length direction as a target site for heat treatment. That is, the pipe body 10 is fed using the induction heating coil 1 (arrangement form surrounding the entire circumference of the pipe body 10) and the water-cooling nozzle 2 arranged on the path for feeding the pipe body 10 in the pipe axis direction (pipe length direction). As shown in FIG. 1, the water-cooling nozzle 2 is kept on (cooling water sprayed onto the tube body 10) while the induction heating coil 1 is powered on (otherwise it is off) only when passing through the heat treatment target part. The heat treatment part 3 and the non-heat treatment part 4 which is the other part were formed. At this time, various heating conditions were changed, and the heating temperature (heat treatment temperature) T1 during heating of the heat treatment section 3 was measured with a radiation thermometer on the exit side of the induction heating coil 1. In addition, although the temperature was measured also about the unheat-treated part 4, the temperature was less than 100 degreeC.

A tensile test was conducted using the JIS 14A tensile test piece 7 taken from the heat-treated portion 3 and the unheat-treated portion 4 of the partially heat-treated tube 10 as shown in FIG. 2 (b), and the tensile strength TS1 of the heat-treated portion and The tensile strength TS2 of the unheat treated part was measured, and the strength difference Δ = TS2−TS1 was obtained. Moreover, EL of the heat treatment part was measured to obtain TS × EL.
Further, bending workability was evaluated by bending the pipe body 10 that had been partially heat-treated as a test piece in the manner shown in FIG. 2 (a).

  These results are shown in Table 2. From the table, the example of the present invention shows that the strength difference Δ = TS2-TS1 and TS × EL both satisfy the present invention and have good bending workability and have sufficient performance as a vehicle reinforcing hollow member. I understand.

  In Example 2, partial heat treatment was performed on the raw tube B by using the method shown in FIG. 3 with a quarter circumference (90 °) portion in the pipe circumferential direction in a part of the pipe length direction as a target site for heat treatment. . That is, the induction heating coil 1A (arrangement configuration facing only a quarter circumference (90 °) portion of the tube body 10) and the water-cooled nozzle 2 disposed on a path for sending the tube body 10 in the tube axis direction (tube length direction). The water-cooling nozzle 2 is kept on (cooling water sprayed onto the tube body 10) while the tube body 10 is being sent, and the induction heating coil 1A is powered on only when passing through the heat treatment target part (otherwise it is off) ), As shown in FIG. 1, the heat-treated portion 3 </ b> A and the non-heat-treated portion 4, which is the other portion, were formed. At this time, various heating conditions were changed, and the heating temperature (heat treatment temperature) T1 of the heat treatment part 3 being heated with the radiation thermometer was measured on the exit side of the induction heating coil 1A. In addition, although the temperature was measured also about the unheat-treated part 4, the temperature was less than 100 degreeC.

Using the JIS 14A tensile test piece 7 collected as shown in FIG. 4 (b) from the heat-treated portion 3A and the unheat-treated portion 4 of the partially heat-treated tube 10, the tensile strength TS1 of the heat-treated portion and The tensile strength TS2 of the unheat treated part was measured, and the strength difference Δ = TS2−TS1 was obtained. Moreover, EL of the heat treatment part was measured to obtain TS × EL.
Moreover, bending workability evaluation was performed by bending the pipe body 10 that had been partially heat-treated as a test piece in the manner shown in FIG. 4 (a).

  These results are shown in Table 3. From the table, the example of the present invention shows that the strength difference Δ = TS2-TS1 and TS × EL both satisfy the present invention and have good bending workability and have sufficient performance as a vehicle reinforcing hollow member. I understand.

1 Induction heating coil (heating the entire circumference (360 °) of the tube)
1A induction heating coil (heating 1/4 turn (90 °) part of the tube)
2 Water-cooled nozzle 3 Heat treatment part (all circumference (360 °) part in the length direction of the tube)
3A Heat treatment part (1/4 circumference (90 °) part in a part in the length direction of the tube)
4 Unheated part 7 JIS14A tensile specimen 10 Tube

Claims (1)

  C: 0.05-0.20 mass%, Si: 0.5-2.0 mass%, Mn: 1.0-3.0 mass%, P: 0.1 mass% or less, S: 0.01 The balance is Fe and unavoidable impurities, and has a two-phase structure of a ferrite phase and a martensite phase or a three-phase structure of the two phases and a retained austenite phase, and has a tensile strength of 980 MPa or more. Using a certain ERW steel pipe as a raw pipe, the whole or part of the pipe circumferential direction in a part of the pipe length direction is subjected to a heat treatment that is heated to 500 to 750 ° C. and then cooled to room temperature. The vehicle reinforcing hollow member is characterized in that the tensile strength of the heat treated portion is 200 MPa or more lower than that of the unheat treated portion, and the product of the tensile strength and elongation of the heat treated portion is 14000 MPa ·% or more.

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