JP2820784B2 - Method of manufacturing high-strength thin steel sheet for door reinforcement pipe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the manufacture of steel sheets for automobiles, and more particularly to a method of manufacturing a high-strength thin steel sheet for a door reinforcing pipe.

(Prior Art) Higher strength and lighter weight of automobile reinforcing members have been promoted in order to improve fuel efficiency of automobile bodies and safety in collisions. In particular, the door reinforcing member, conventional, but 100kgf / mm ² class of pressed product has been mainly used, recently, CAMP-ISIJ
As described in Vol. 2 (1989) -2023, higher strength pipe materials have been used because they are advantageous in terms of weight reduction. In order to obtain the same absorbed energy as that of a pressed product using such a pipe product, a high tensile strength of about 150 kgf / mm ² is required.

(Problems to be solved by the invention) Conventionally, to obtain such high-strength pipe material, 60 kg
A thin steel plate of about f / mm ² was manufactured by pipe welding by electric resistance welding, subsequently heated by high frequency heating, etc., and rapidly cooled from the austenite temperature range by water cooling or the like. However, pipe materials fused by ERW etc.
At the joint interface, a decarburized layer called a white band as shown in FIG. 1 is generated. This white band part,
It is a portion that is melted during welding and contains oxides, inclusions, etc., and originally has low workability. Further, since the hardness of the white band portion hardly increases even by quenching treatment for increasing the strength of the pipe, a softened zone is generated in the heat-affected zone as shown in FIG. Therefore, when the bending test (crush test) shown in FIG. 3 is performed, the deformation concentrates on this portion,
Before the pipe buckles, cracks occur in the white band portion, that is, the welded portion, and a predetermined absorbed energy cannot be obtained as in No. 4 in FIG. This behavior is more pronounced in low temperature tests.

It is almost impossible to prevent the formation of a white band portion when an iron alloy containing a carbon content lower than the eutectic point is melt-joined by electric resistance welding or the like of a pipe. Also, if the upset amount at the time of electric resistance welding is increased in order to reduce the width of the white band portion, joint failure such as cold welding is caused, and the weld strength is further reduced.

Further, when the pipe material having the white band portion is heated to the austenitic region and then quenched by water cooling or the like, the hardness of the white band portion becomes lower than that of the base material portion as described above. Therefore, when the crushing occurs, the deformation concentrates on the portion and the crack occurs, so that a predetermined absorbed energy cannot be obtained. On the other hand, if it remains welded, the hardness of the weld is high and the ductility is low, so if the amount of strain at the time of crushing is large, the weld will break in the direction perpendicular to the bead or in the white band, The absorbed energy is low.

A method for improving the workability of a welded portion of an electric resistance welded pipe is not conventionally known, but in a rim material for a wheel using a flash butt welding method which is a similar welding method, Japanese Patent Application Laid-Open No. It has been proposed to reduce the difference in hardness between the welded portion and the base material by making the structure of the base material bainite.

It is considered that even in the door reinforcing pipe material, if the hardness distribution is made like the wheel rim material, cracks at the welded portion can be prevented, and a predetermined absorbed energy can be obtained. However, the above proposal is based on a low C content of 60 kgf / mm ²
It is intended for high-grade steel sheets, and because the welding speed and cooling speed are different between flash butt welding and ERW welding, C, such as high-strength pipe material of 100 kgf / mm ² or more,
It is difficult to obtain the same hardness distribution in a pipe containing many elements having high quenching strengthening ability such as the amount of Mn. Although the softening zone in the weld heat affected zone in the composite structure steel sheet of comparative material in the proposed observed, similar to the above, at 100 kgf / mm ² or more pipes containing a large amount of high hardening strengthening capacity element, and welding It is unclear whether similar hardness distributions can be obtained with high-speed ERW welding, and furthermore, whether it affects the crushing of pipes.

The present invention solves the above-mentioned problems of the prior art, and manufactures a high-strength thin steel sheet for a door reinforcing pipe having high absorption energy without breaking a pipe formed by ERW welding at a welded portion at the time of crushing. It is intended to provide a method.

(Means for Solving the Problems) In order to solve the above problems, the present inventors have conducted intensive studies on the quality of welded portions of pipes formed by electric resistance welding of thin steel plates, and as a result, completed the present invention here. It is.

That is, the present invention provides C: 0.10 to 0.30% and Mn: 1.5 to 3.
0% and the C content is as follows: 0.367−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² ≧ C ≧ 0.248−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² However, σ _B : tensile strength (kgf / mm ² ) is satisfied, and if necessary, Si: 0.2 to 2.0%, P: 0.02
~ 0.15%, Cr: 0.1 ~ 1.0%, Mo: 0.1 ~ 1.0% and B: 0.0003
One or more of 0.005% and / or N
b, Ti, Zr and V are each in the range of 0.01 to 0.06%, each containing one or two or more steels, the balance being Fe and unavoidable impurities, hot-rolled by a conventional method or continuous annealing after cold rolling. And the low-temperature transformation product is 70% or more and the yield ratio is
A gist of the present invention is to provide a method of manufacturing a high-strength thin steel sheet for a door reinforcing pipe processed by electric resistance welding, which is characterized by obtaining a thin steel sheet for a pipe of 0.7 or more.

 Hereinafter, the present invention will be described in more detail.

(Action) In short, the method of the present invention comprises: C: 0.10 to 0.30% and Mn:
Steel containing 1.5 to 3.0% as an essential component is hot-rolled or cold-rolled and continuously annealed to obtain a thin steel sheet for pipes having a low-temperature transformation product of 70% or more and a yield ratio of 0.7 or more. It is manufactured.

Here, the low-temperature transformation product may be bainite, martensite, tempered martensite or a mixture thereof, and is not particularly limited.

In general, the higher the yield ratio, the higher the absorbed energy when crushing the pipe. Therefore, the more low-temperature transformation products, the more
It is preferable because the yield ratio is increased at the same strength, and particularly in the case of a fully tempered martensite material, the value is as high as 0.90 or more. Further, since the high yield ratio material has lower strength than the low yield ratio material having the same yield strength, cutting is easy and productivity is high. However, if a steel sheet having such a chemical composition, structure, and yield ratio is simply subjected to electric resistance welding, a crack occurs at the time of pipe crushing, and a predetermined absorbed energy cannot be obtained.

Therefore, the present inventors have conducted further studies on this point, and as a result, by controlling the amount of C within the range of the above chemical components within the range of the following formula, it is possible to obtain high absorption energy without cracking during crushing. I learned that I can do it.

0.367−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² ≧ C ≧ 0.248−3.0 × 10 ^-3 σ _B + 1.5 × 10 ⁻⁵ σ _B ² where σ _B : tensile strength (kgf / mm ² ) That is, it has been found that the deformation at the welded portion can be reduced by satisfying the above conditions to provide the heat affected zone with a softened region of appropriate hardness.

Minimum hardness Hv ₁ and the ratio of the hardness Hv ₂ of the base material of the weld heat affected zone, Hv
_{Formula 1} / Hv ₂ is shown as a function of the tensile strength sigma _B of the pipe, -
Higher than 0.001σ _B +1.05, because variations in the heat affected zone during crushing is small, the number of deformation of the weld portion,
It breaks due to a white band or the like at the welded portion, and the predetermined absorbed energy cannot be obtained. Hv ₁ / Hv ₂ is −0.003σ _B +
If it is smaller than 1.05, the deformation in the heat-affected zone becomes large, and buckling is likely to occur at the time of crush, so that a predetermined absorbed energy cannot be obtained. As already mentioned, the hardness ratio of the softening zone in the heat-affected zone, Hv ₁ / Hv ₂ is the proper range by the tensile strength sigma _B of the pipe. In a relatively low-strength pipe material having a σ _B of about 100 kgf / mm ² , the value of Hv ₁ / Hv ₂ may be relatively high. This is because the maximum hardness of the weld is not very high,
This is because it has a certain degree of workability. However, when the pipe strength σ _B is about 200 kgf / mm ² , it is important to increase the amount of deformation in the heat-affected zone, and unless the value of Hv ₁ / Hv ₂ is relatively low, cracking during crushing To prevent high energy absorption.

The present inventors have found that in order to obtain a proper value of Hv ₁ / Hv ₂ in response to the strength of such pipes, the C amount _{^{0.367-3.0 × 10 -3 σ B + 1.5}} × 10 -5 σ it was found that it is necessary to control the _B ² in the range of _{^{0.248-3.0 × 10 -3 σ B + 1.5}} × 10 -5 σ B 2.

When C amount is larger than this range, the higher the Hv ₁ / Hv _2,
Cracks in the weld tends to occur at the time of collapse, when the C amount is small, Hv ₁ / Hv ₂ is lowered, tends to occur buckling during crushing, not obtain a high absorbed energy in any case.

The minimum hardness Hv ₁ in the softened zone of the heat-affected zone needs to have an appropriate value as described above, but the width of the softened zone is not particularly limited. In the range obtained by Further, the width of the softened zone in the heat-affected zone varies depending on the thickness of the pipe, the structure of the pipe blank, and the chemical composition, but there is no particular problem as long as the range can be obtained by electric resistance welding.

Also, in the case of a pipe obtained by the present invention, which is obtained by electric resistance welding of a steel plate having a predetermined strength, unlike a pipe subjected to heat treatment after pipe formation, a welding position can be easily identified by a bead sensor or the like. Therefore, when mounted on an actual vehicle, the deformation of the welded portion is made smaller by controlling and setting the welded portion not just below the loaded point but in a horizontal direction perpendicular to the loaded point. Thus, the degree of safety against cracking can be further increased.

 Next, the reasons for limiting the chemical components in the present invention will be described.

C: C is an extremely important element for increasing the strength of steel sheets. However, if the C content is less than 0.10%, a tensile strength of 100 kgf / mm ² or more cannot be obtained, and excessive C content exceeds 0.30%. Then, the weld becomes brittle, cracks occur at the time of crushing, and a predetermined absorbed energy cannot be obtained. Therefore, the amount of C is 0.10-0.
The range is 30%.

However, the amount of C must satisfy the following expression within the above range for the above-mentioned reason.

0.367−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² ≧ C ≧ 0.248−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² Mn: Mn is a low temperature with high strengthening ability It is necessary to obtain a transformation product, and if the addition amount is less than 1.5%, the cooling rate at the time of winding by hot rolling to obtain a low-temperature transformation product or the quenching start temperature in continuous annealing is high. And the shape defect of the steel plate occurs. If it exceeds 3.0%, segregation increases,
The metal flow in the welded portion of the pipe deteriorates, cracks occur at the time of crushing, and a predetermined absorbed energy cannot be obtained. Therefore, the Mn content is in the range of 1.5 to 3.0%.

Although the above C and Mn are essential components, Si,
One or more of P, Cr, Mo and B, and / or one or more of Nb, Ti, Zr and V can be added in an appropriate amount.

Si: Si is an effective element to increase the yield strength of steel,
If it is less than 0.2%, the effect cannot be obtained, and if it exceeds 2.0%, defects in the welded portion increase and cracks occur at the time of crushing. Therefore, the Si content is in the range of 0.2 to 2.0%.

P: P, like Si, is an effective element for increasing the yield strength of steel. However, if it is less than 0.02%, the effect cannot be obtained, and if added over 0.15%, the weld becomes brittle. Cracks when crushed. Therefore, the P content is 0.02-0.15%
Range.

Cr: Like Mn, Cr is liable to generate low-temperature transformation products, can lower the cooling rate during winding in hot rolling or the quenching start temperature in continuous annealing, and improves the warpage of the steel sheet. But 0.1
%, The effect cannot be obtained, and even if added over 1.0%, the effect is saturated. Therefore, the Cr content is in the range of 0.1 to 1.0%.

Mo: Mo, like Mn, is an element that enhances the hardenability of steel,
When the amount is less than 0.1%, the effect is not obtained, and when the amount exceeds 1.0%, the effect is saturated. Therefore, the amount of Mo is set in the range of 0.1 to 1.0%.

B: B is an element that increases the hardenability and has the same effect as Mn, but if less than 0.0003%, it has no effect, and the content is 0.1%.
If the content exceeds 005%, the effect is saturated, so the B content is 0.000%.
The range is 3 to 0.005%.

Nb, Ti, Zr, V: Nb, Ti, Zr, and V are elements that form carbon and nitride in steel, strengthen the steel, and increase the yield ratio. But,
If any element is less than 0.01%, the effect cannot be obtained, and if it exceeds 0.06%, the effect is saturated. Therefore, the amounts of Nb, Ti, Zr, and V are each in the range of 0.01 to 0.06%.

Next, the manufacturing process and conditions of the high-strength thin steel sheet for a door reinforcing pipe of the present invention will be described.

Steel having the above chemical composition is hot-rolled, then pickled, cold-rolled, and continuously annealed by a normal method,
A thin steel sheet for pipes with a low-temperature transformation product amount of 70% or more and a yield ratio of 0.7 or more is obtained.

On the other hand, it is also possible to obtain a thin steel sheet for a pipe having a low-temperature transformation product amount of 70% or more and a yield ratio of 0.7 or more by hot rolling. For example, a method of setting the finishing temperature of hot rolling to a temperature containing austenite of 70% or more, and rapidly cooling to a bainite transformation point or less at a cooling rate of 30 ° C./second or more, and winding.

If the low-temperature transformation product is 70% or less or the yield ratio is 0.7 or less, the absorbed energy at the time of pipe crushing cannot be increased. In particular, when the low-temperature transformation product is 70% or less, it is difficult to obtain a high yield ratio.

The obtained thin steel plate may be formed into a pipe by electric resistance welding by a usual method. No heat treatment is required after pipe making.

 Next, examples of the present invention will be described.

(Examples) Steel having the chemical components shown in Table 1 was melted, hot-rolled by a usual method, pickled, cold-rolled, and continuously annealed to obtain a 2.0 mm thick steel sheet. A thin steel plate was obtained.

This thin steel plate was slit and subjected to electric resistance welding by high frequency induction heating to produce a 31.8 mmφ pipe.

In addition to examining the mechanical properties of the thin steel sheet, a crush test was performed on the obtained pipe, and the hardness and the like were examined. In addition,
The crush test was performed according to the procedure shown in FIG. 3 (span 950 mm), and as shown in FIG. 4, the absorbed energy at the time of pushing 150 mm was determined from the load-displacement curve.

 The results are shown in Table 2.

The following is considered from Table 2 and FIGS. 2 and 4.

Comparative Example No. 1 is insufficient in tensile strength because the amount of C is small.

Comparative Example No.2 is thin steel sheet material plate is a high yield ratio material consisting of martensite, lower than the value of the expression C amount is determined by the sigma _B, softened in the heat affected zone is larger Nv, Hv ₁ /
The value of Hv ₂ was small, and buckling was rapid at the time of crush.

Comparative Example No.4 is plain plates of the pipe is high yield ratio material from a composite structure of ferrite containing 70% tempered martensite at 600 ° C., of the formula where the C content is determined by sigma _B Value, the softening in the heat-affected zone is small and therefore
The value of Hv ₁ / Hv ₂ increases, cracks in the white band portion occurring during crushing.

In Comparative Examples No. 5 and No. 11, the low-temperature transformation products are small and the yield ratio is low, so that the predetermined absorbed energy cannot be obtained.

In Comparative Example No. 7, since the C amount was more than 0.3%, it broke from the weld at the time of crushing.

On the other hand, in each of the examples of the present invention, high absorbed energy was obtained, and no cracking or breaking occurred during crushing.

FIG. 2 shows the hardness distribution of the welded portion of the pipe for Comparative Examples No. 2 and No. 4 and Example No. 3 of the present invention. In the present invention, the proper value of Hv ₁ / Hv ₂ is shown. It can be seen that the values of Hv ₁ / Hv ₂ are too large or too small in the comparative example.

(Effects of the Invention) As described in detail above, according to the present invention, a high-strength thin steel sheet for a door reinforcing pipe having high absorbed energy can be provided without the pipe breaking at the welded portion during crushing. . Further, it is economical because no heat treatment is required after pipe forming by electric resistance welding.

[Brief description of the drawings]

FIG. 1 is a photograph showing the metal structure of the ERW weld, FIG. 2 is a diagram showing the hardness distribution of the ERW weld, FIG. 3 is a view for explaining the procedure of a pipe crush test, and FIG. It is a figure which shows the load-displacement curve in the crush test of a pipe.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/18 C22C 38/18 (72) Inventor Akifumi Fujiwara 4-4-2, Inagawa-cho, Nishinomiya-shi, Hyogo (56) References Kohei 7-42499 (JP, B2) (58) Fields surveyed (Int. Cl. ⁶ , DB name) C21D 9/46 C21D 8/02 C21D 8/10

Claims (3)

(57) [Claims] (1) C: 0.10 to 0.30% by weight (hereinafter the same)
And Mn: 1.5 to 3.0%, and the C content is as follows: 0.367−3.0 × 10 ⁻³ σ _B + 1.5 × 10 ⁻⁵ σ _B ² ≧ C ≧ 0.248 −3.0 × 10 ⁻³ σ _B +1. 5 × 10 ^-5 σ _B ² However, σ _B : steel that satisfies tensile strength (kgf / mm ² ) and the balance is made of Fe and unavoidable impurities
Hot-rolled or cold-rolled and continuously annealed by a conventional method to obtain a thin steel sheet for pipes with a low-temperature transformation product of 70% or more and a yield ratio of 0.7 or more. Of manufacturing high-strength thin steel sheets for door reinforcement pipes to be used. 2. The steel further comprises: Si: 0.2 to 2.0%, P: 0.02 to
0.15%, Cr: 0.1-1.0%, Mo: 0.1-1.0% and B: 0.0003-
2. The method according to claim 1, comprising one or more of 0.005%. 3. The method according to claim 1, wherein the steel further contains one or more of Nb, Ti, Zr and V in the range of 0.01 to 0.06%.