JPH04268016A - Production of high tensile strength steel sheet for door guide bar having excellent crushing characteristic - Google Patents

Abstract

PURPOSE:To manufacture a steel sheet for door guide bar for a car having high compressive characteristic and shock absorbing energy by applying processing after executing continuous annealing to the specific composition of a cold-rolled sheet and executing heat treatment and overaging treatment under the specific condition. CONSTITUTION:After executing the continuous annealing to the cold-rolled steel sheet of composition containing by wt.% of 0.1-0.3% C, 0.2-3.0% Mn, 0.01-0.1% Sol.Al or further, if necessary, one or more kinds among 0.2-2.5% Si, <=0.15% P, <=0.04% Ti, <=0.04% Nb, <=0.1% V, <=0.5% Mo, <=0.5% Ni, <=0.5% Cr, <=0.5% Cu, <=0.5% W and <=0.005% B, this is heated at Ac1 point-1200 deg.C and rapidly cooled from this heating temp. to the room temp. at >=100 deg.C/sec cooling speed and successively, the overaging treatment is executed at 150-450 deg.C for 1sec-10min. To the steel sheet composed of 20vol% martensite and the balance ferrite in the structure, 1-30% of working strain is further given, and after making a pipe, quenched hardening treatment is applied to manufacture the excellent pipe for door guide bar for car.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a member for reinforcing automobile doors, and more specifically, the present invention relates to a member for reinforcing automobile doors. The present invention also relates to a method for manufacturing a steel plate for door guard bars that has high crushing properties and excellent impact absorption energy by subjecting it to a bake hardening treatment.

0002

[Background Art and Problems to be Solved by the Invention] In order to improve the fuel efficiency of automobile bodies and improve the safety in the event of impact, efforts are being made to increase the strength and reduce the weight of automobile reinforcing materials. In particular, pressed products of 100 kgf/mm2 class have conventionally been mainly used for door reinforcing members, but recently,
A pipe material with higher strength, such as that disclosed in Japanese Patent No. 37167, has come into use because it is advantageous in terms of weight reduction.

[0003] In order to obtain the same absorbed energy as a pressed product with such a pipe product, the conventional 60 kgf/mm
A pipe is produced by welding thin steel plates of approximately 200 mm by electric resistance welding, followed by high-frequency heating, and quenching from the austenite temperature range. However, a region with a low carbon content called a white band (see Figure 5) occurs in the welded part of the pipe, the strength of the welded part is low (see Figure 6), and the deformation in the heat affected zone becomes large during crushing. There was a drawback that buckling occurred during crushing, making it impossible to obtain the required absorbed energy.

[0004] Normally, the crushing characteristics of such a steel material formed into a pipe shape are determined by the yield strength when the strength is the same, and the higher the strength, the greater the crushing load and absorbed energy. Therefore, it is necessary to increase this yield strength.

[0005] The present invention was made in response to such a demand, and an object of the present invention is to provide a steel plate and a method for manufacturing the pipe that can yield a high-strength pipe for reinforcing doors with high yield strength. It is something.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have conducted intensive research on measures to improve the crushing characteristics of pipes made by electric resistance welding of thin steel plates.
The present invention has now been completed.

[0007] That is, in the present invention, C: 0.1 to 0.3
%, Mn: 0.2-3.0% and sol. Al: 0.0
Contains 1 to 0.1%, and if necessary, further Si: 0.2
~2.5%, P≦0.15%, Ti≦0.04%, Nb
≦0.04%, V≦0.1%, Mo≦0.5%, Ni≦
0.5%, Cr≦0.5%, Cu≦0.5%, W≦0.
5% and B≦0.005%, and the remainder is iron and unavoidable impurities. After hot rolling and/or cold rolling by a conventional method, continuous annealing is performed. After heating to 1200℃ or below Ac1 point,
From this temperature, it is rapidly cooled to room temperature at a cooling rate of 100℃/s or more, and then in the temperature range of 150 to 450℃ for 1 second to 10 minutes.
Crushing characteristics characterized by producing a low-temperature transformation product containing ferrite and martensite with a volume fraction of 20% or more by applying an over-aging treatment for 1 minute, and then further processing to 1 to 30%. The gist of this paper is a method for manufacturing a high-strength steel plate for a door guard bar that provides an excellent door guard bar.

[0008] Another aspect of the present invention is a method for manufacturing a pipe for a door guard bar having excellent crushing properties, which comprises subjecting the steel plate obtained by the above processing to a baking hardening treatment after forming the pipe. It is.

The present invention will be explained in more detail below.

0010

[Operation] In short, the present invention uses a steel type having the above-mentioned components, cools it to room temperature at a cooling rate of 100°C/s or more from a temperature range of 1,200°C or below Ac1 point, and then cools it to room temperature at a cooling rate of 100°C/s or more. By performing over-aging treatment for 1 second to 10 minutes in the range, the structure becomes a structure containing martensite with a volume fraction of 20% or more, which increases the yield strength due to tempered martensite and has bake hardenability. It turns out. By increasing the yield strength of this tempered martensite and using bake hardening, it is possible to obtain a steel plate with even higher yield strength than conventional ones, and thereby a door guard bar with excellent crushing properties can be obtained. .

First, the reasons for limiting the chemical components in the present invention will be explained.

C: C is an extremely important element for increasing the strength of steel sheets, but if the amount of C is less than 0.1%, the
A tensile strength of m2 or more cannot be obtained, nor can a sufficient yield strength be obtained. On the other hand, if it is added in excess of 0.3%, the welded part becomes brittle and cracks occur during crushing, making it impossible to obtain the desired absorbed energy. Therefore, the amount of C is 0.1~0
．． The range shall be 3%.

Mn: Mn is an element that increases the strength, stabilizes the austenite phase, and promotes the formation of martensite during the cooling process. In order to obtain this effect, it is necessary to add at least 0.2% or more, but if it is added in excess of 3.0%, Mn segregation occurs and a layered structure tends to occur. Therefore, the amount of Mn is set in the range of 0.2 to 3.0%.

[0014] sol. Al: Al is necessary for deoxidizing molten steel, and for this purpose at least sol. An Al content of 0.01% or more is required. However, if the sol. Al amount is 0.01~0
．． The range is 1%.

[0015] Although the above elements are essential components, in the present invention, at least one of the following elements may be added as necessary.
A suitable amount of seeds or more can be contained.

Si: Si is an element that increases the strength of steel without deteriorating its ductility, and also expands the ferrite-austenite temperature range. It is also an element that increases the amount of solid solution C in ferrite, and is useful for increasing bake hardenability. In order to exhibit such an effect, at least 0.2% or more is required. However, if it is added in excess of 2.5%, manufacturing costs will increase. Therefore, the amount of Si is 0.
The content should be in the range of 2 to 2.5%.

P: Like Si, P is an effective element for increasing the yield strength of steel, but if it is added in excess of 0.15%, the weld will become brittle and cracks will occur during crushing. shall be 0.15% or less.

B: B is an element that increases hardenability, and therefore has the effect of preventing a decrease in the strength of the welded part, but if it exceeds 0.005%, the effect is saturated, so the amount of B is 0.005%. 0.005% or less.

Ti, Nb, and V: Ti, Nb, and V are elements that form carbon and nitrides, strengthen steel, and increase the yield ratio. If it exceeds, 0.1% in case of V
above, such effects become saturated. therefore,
Ti amount is 0.04% or less, Nb amount is 0.04% or less, V
The amount shall be 0.1% or less.

Cr: Cr improves the hardenability of the welded part and prevents the strength of this part from decreasing. However, C
The amount of r is 0.5% or less.

Cu: Cu precipitates in steel as ε-Cu during tempering and has the effect of improving its strength. Further, it has the effect of lowering the Ac3 point of the white band layer of the welded part, remaining in this part to prevent a decrease in the strength of the welded part, and preventing destruction from this part at the time of crushing. However, if added in excess of 0.5%, such effects become saturated, so the amount of Cu is set to 0.5% or less.

Mo: Mo not only improves the hardenability of steel, but also exists in large amounts in the white band layer after welding, and has the effect of increasing the strength of this layer. However, even if it is added in an amount exceeding 0.5%, the effect is saturated, so the amount of Mo is set to be 0.5% or less.

Ni: Ni has the effect of improving the hardenability of steel, lowering the Ac3 point of the white band layer of the welded part, and preventing a decrease in the strength of this part. However, even if Ni is added in an amount exceeding 0.5%, such an effect is saturated, so the amount of Ni is set to be 0.5% or less.

W: W forms carbon and nitrides to increase the yield strength, and has the effect of preventing a decrease in the strength of the white band after welding. However, even if it is added in an amount exceeding 0.5%, the effect is saturated, so the amount of W is set to be 0.5% or less.

[0025] If Ca and Zr are added in addition to the above elements, cracking due to MnS inclusions can be prevented and elongation can be improved. Furthermore, the addition of REM (rare earth element) does not reduce the effects of the present invention, and may be added without any problem.

Next, the manufacturing method of the present invention will be explained. Steel having the above chemical composition is hot rolled and/or cold rolled and continuously annealed by a conventional method. In order to impart bake hardenability to such a steel plate, as shown in FIG.
Martensite with a volume fraction of 0% or more is required.

[0027] To do this, first, increase the heating temperature to 1200℃.
It is necessary to set the temperature to Ac1 point or higher below .degree. C., and to perform rapid cooling at 100.degree. C./s or more from this temperature. However, if the heating temperature is lower than the Ac1 point, low-temperature transformation products such as martensite cannot be obtained. On the other hand, if the heating temperature is too high, surface oxidation and shape deformation are likely to occur due to oxidation and softening of the steel surface, so it is preferably 1200° C. or lower. Furthermore, if the cooling rate is less than 100° C./s, low-temperature transformation products containing martensite will not be produced.

[0028] Next, 1
By performing an overaging treatment for 10 seconds to 10 minutes, martensite with a volume fraction of 20% or more is obtained as described above, and excellent bake hardenability is imparted (see FIG. 3).

Of course, by quenching and tempering under the same conditions as the heat treatment described above, a structure containing tempered martensite is obtained, which also increases the yield strength (YR) of the steel sheet.

[0030] Baking hardening (painting) treatment is a process that is normally carried out at a temperature of 150 to 250°C after the pipe is installed, but by adding a processing strain of 1 to 30% before this treatment, baking Approximately 5 to 20 kgf/mm after painting treatment
The yield strength of No. 2 can be improved, and large crushing loads and absorbed energy can be obtained (see Fig. 4). Processing strain is given by cold working.

Next, examples of the present invention will be shown.

[0032]

【Example】

[Table 1] Steel having the chemical composition shown in the table was vacuum melted, hot-rolled, cold-rolled, and annealed in a conventional manner. After heat treatment of quenching and tempering, processing strain was applied by cold rolling. After this, after forming the pipe,
BH treatment (baking coating treatment) was performed at 200°C for 20 minutes. The final shape of the pipe is 31.8mm in diameter and 2mm thick.
It is. The crush test was conducted with a span of 750 mm as shown in Figure 1.
The indentation was performed using an indenter with a curvature of 150 mmR. test results

[Table 2] as well as

[Table 3] Shown below.

From Tables 2 and 3, the following considerations can be made. Test No. 1~No. In pipe No. 2, since the tempering temperature was low, sufficient bake-hardening properties were not produced, and sufficient crushing properties were not obtained. Also, test no. 3~No. Pipe No. 8 shows the difference in characteristics depending on the processing rate after tempering, and when the processing rate is within the range of the present invention (1 to 30%) (No.
．． 5, No. 6) shows good crushing properties. Also, test no. 9~No. No. 10 shows the effect of the volume fraction of martensite in the steel, and if the volume fraction of martensite is less than 20%, the amount of hardening that is sufficient to change the crushing properties cannot be obtained. Test No. 11~No. No. 28 indicates differences due to added elements or differences in manufacturing conditions, and excellent crushing properties were obtained under conditions within the range of the present invention.

[0034]

[Effects of the Invention] As detailed above, according to the present invention,
A steel plate with high yield strength is obtained, and therefore, a pipe for a door guard bar having high crushing properties and excellent impact absorption energy can be obtained by baking hardening treatment after pipe forming.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the outline of a pipe crush test method.

FIG. 2 is a diagram showing the relationship between the martensite volume fraction and the amount of bake hardening.

FIG. 3 is a diagram showing the effect of overaging temperature on YP, TS, and BH characteristics.

FIG. 4 is a diagram showing the relationship between pipe absorption energy and crushing load due to differences in processing rates.

FIG. 5 is a photograph showing the metal structure (microstructure) of a welded portion of a pipe.

FIG. 6 is a diagram showing the hardness distribution of a welded part.

Claims (3)

[Claims] [Claim 1] In weight% (the same applies hereinafter), C: 0.1
~0.3%, Mn: 0.2-3.0% and sol. Al
: 0.01 to 0.1%, the balance being iron and unavoidable impurities. After hot rolling and/or cold rolling by a conventional method, continuous annealing is carried out at 1200°C or below A.
After heating to the c1 point or higher, it is rapidly cooled from this temperature to room temperature at a cooling rate of 100°C/s or more, and then over-aged for 1 second to 10 minutes in a temperature range of 150 to 450°C, thereby reducing the ferrite and volume. A low-temperature transformation product containing 20% or more martensite is produced, and further 1 to 30% martensite is produced.
A method for manufacturing a high-strength steel plate for door guard bars, which is a door guard bar with excellent crushing properties and is characterized by being processed up to %. 2. A method for manufacturing a pipe for a door guard bar having excellent crushing properties, which comprises subjecting the steel plate obtained by the processing according to claim 1 to a bake hardening treatment after forming the pipe. 3. The steel further contains Si: 0.2 to 2.5%.
, P≦0.15%, Ti≦0.04%, Nb≦0.04
%, V≦0.1%, Mo≦0.5%, Ni≦0.5%,
Cr≦0.5%, Cu≦0.5%, W≦0.5% and B
The method according to claim 1 or 2, comprising at least one or more of ≦0.005%.