JPH04218618A - Production of resistance welded tube for automobile use excellent in baking hardenability and workability - Google Patents

Abstract

PURPOSE:To provide sufficient workability at the time of attaching a resistance welded tube for automobile use to an automobile and to obtain the prescribed strength by means of treatment after attachment for the purpose of avoiding the occurrence of deterioration in workability at the time of making the resis tance welded tube for automobile use high strength and the occurrence of troubles at the time of tube making and incorporation into the automobile. CONSTITUTION:The method is a manufacturing method for a resistance welded tube for automobile use excellent in baking hardenability and workability characterized by subjecting a low carbon steel having a composition consisting of, by weight, 0.01-0.30% C, 0.10-2.0% Mo, 0.010-0.15% Nb and/or 0.010-0.30% V, and the balance Fe with inevitable impurities to hot rolling by the prdinary process, coiling the resulting hot rolled plate at <550 deg.C, and performing tube making by the user of the above hot rolled plate.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing electric resistance welded steel pipes for automobiles which have excellent bake hardenability and workability. In particular, by adding Mo and Nb or V in combination, it has excellent bake hardenability and workability.
The present invention relates to a method of manufacturing an electric resistance welded steel pipe for automobiles exhibiting a strength level of kgf/mm2 or higher.

0002

BACKGROUND OF THE INVENTION In recent years, from the viewpoint of resource and energy conservation, legislation regarding the fuel efficiency of automobiles has been enacted. In response, automobile companies are aiming to reduce the weight of automobiles, and the performance required of steel materials is thin walls and high strength. However, there is a strict condition that no deterioration in workability is observed.

Generally, methods for increasing the strength of ERW steel pipes include: (1) increasing the strength by increasing the Ceq of the material or adding precipitation strengthening elements; (2) increasing the strength by cold rolling a hot-rolled sheet to increase its strength; How to make high-strength ERW steel pipes, ■How to increase the strength by cold drawing after making ERW pipes, ■
A method of making a high-strength steel pipe by heat treatment such as quenching treatment after making it into an ERW pipe. For example, as described in JP-A No. 61-272318, a high-strength steel plate is made by rapid cooling during hot rolling and low-temperature rolling. After that, how to make high-strength ERW steel pipe,
However, in all cases, the disadvantage is that the higher the strength, the more the workability of the material deteriorates significantly.

[0004] In the cases of (1) to (4), the steps such as cold rolling, cold drawing, and steel pipe heat treatment are complicated, leading to high costs. In the case of (2) as well, operational troubles caused by low-temperature winding lower the yield and result in higher costs. Furthermore, since the material is made to have high strength and is used to form pipes, this may cause problems during pipe making.

Furthermore, it is quite difficult to install these high-strength steel pipes into automobiles because of their high strength. Steel pipes are also desired, but of course such steel pipes have not been developed at present.

[0006]

[Problems to be Solved by the Invention] The present invention aims to avoid deterioration in workability when high strength electric resistance welded steel pipes for automobiles are produced, and troubles during pipe manufacturing and installation into automobiles.
It is an object of the present invention to provide a method that has sufficient workability when installing an electric resistance welded steel pipe for an automobile into an automobile, and obtains a predetermined strength by processing after installation.

[0007]

[Means for Solving the Problems] As a result of detailed studies, the present inventors have considered utilizing the bake hardening properties used in cold rolled steel sheets for automobiles in order to increase the strength after pipe making. Ta. A steel plate with bake-hardening properties means that it has excellent workability during processing, and that it can be baked and painted after processing (e.g., 170°C x 20 min).
), it is possible to harden the steel plate and reduce the thickness corresponding to the increase in strength, making it an ideal steel plate for automobile steel plates, etc., and its industrial value is extremely high.

This phenomenon of bake hardening is so-called age hardening, which occurs when dislocations introduced by working are trapped in solid solution C, but the necessary condition for this is that the dislocations introduced by working and solid solution C are trapped. be. Since ERW steel pipes are manufactured by forming hot coils, they meet the requirements from the perspective of introducing processing dislocations, and we considered applying this to ERW steel pipes for automobiles.

[0009] However, when using the bake hardening property in cold-rolled steel sheets for automobiles, the strength increase is at most 3 to 5 kgf/mm.
2, the increase in strength is too small to be used for the purpose of the present invention. Therefore, 10kgf/mm2
It is necessary to have bake hardening characteristics of a certain level or higher, and it is thought that considerable efforts will be required to achieve this. Therefore, in order to apply bake hardening characteristics to electric resistance welded steel pipes, the present inventors conducted numerous experiments and detailed studies, and found that the addition of Mo as an alloying element and the combined addition of Mo, Nb, and V are effective. I found that it was true.

Furthermore, by using this method,
It has been discovered that it is possible to obtain a bake hardening amount of 10 kgf/mm 2 or more, which was previously at the level of 3 to 5 kgf/mm 2 , which was previously unthinkable. Regarding the components, as mentioned above, the addition of Mo is essential, but it has been found that the effect is increased when Nb and V are added in combination.

Furthermore, when Mo, Nb, or V is added, depending on the rolling temperature, these elements and C combine to form precipitates, reducing solid solution C. Therefore, the temperature at which no precipitates are formed is set. I discovered that I needed to choose. in particular,
Since precipitates are formed when the winding temperature is 550°C or higher, in the present invention, the range of the winding temperature is set to be less than 550°C.

[0012] It has been found that among the manufacturing conditions of the material, there is no need to particularly specify hot rolling conditions. Regarding finish rolling conditions, rolling just above the transformation point and rolling at a high temperature will result in a difference in crystal grains, which will slightly affect the material quality of the material itself, but will have almost no effect on the bake hardening characteristics. I found out that it doesn't give. Furthermore, regarding the rolling conditions, in addition to the difference in the formation of precipitates mentioned above, when rolling at a low temperature, martensite and bainite are mixed in the second phase, resulting in a so-called two-phase steel state and the yield ratio. decreases. For this reason, the yield ratio increases due to work hardening during steel pipe manufacturing, that is, during pipe making. In addition, when the winding temperature is high, Mo carbide precipitates and the tensile strength and yield ratio increase. Therefore, there is almost no increase in yield ratio during steel pipe manufacturing. In other words, differences due to the winding temperature include the formation of precipitates, the level of tensile strength, and the yield ratio of the material, and are considered to have a slight impact on the ease of manufacturing steel pipes.

Based on this knowledge, the present invention has made it possible to apply bake hardening characteristics to electric resistance welded steel pipes for automobiles, and the gist thereof is that C: 0.01 in weight %.
~0.30%, Mo: 0.10~2.0%, and Nb: 0.010~0.15%, V: 0.010~0.
It is manufactured using a hot-rolled sheet obtained by hot-rolling a low carbon steel containing 30% of one or two types and the balance consisting of Fe and unavoidable impurities in a normal process and coiling at a temperature below 550°C. The present invention provides a method for manufacturing an electric resistance welded steel pipe for automobiles which has excellent bake hardenability and workability.

[0014]

[Operation] In the present invention, after forming an electric resistance welded steel pipe from a material with specified components, we have successfully achieved remarkable bake hardening characteristics. Next, the components, hot rolling conditions, and steel pipe manufacturing conditions of the present invention will be described. In the method of the present invention, C: 0.01 to 0.
30% and Mo: 0.10 to 2.0%, and Nb
:0.010~0.15% and V:0.010~0.30
Good results can be obtained by applying it to low carbon steel containing one or two of %. Other preferred component compositions include: Si: 0.02-0.50%, Mn: 0.10-1.00%, Al: 0.001-0.100%, N: 0.0005-0.0100. %, and depending on the required properties of the strength steel, Cu: 2.0%
One or two of the following: Ni: 9.5% or less, Cr: 5.5% or less, Ti: 0.15% or less, B: 0.0003 to 0.0030%, Ca: 0.0080% or less The above may be added as necessary.

C is essential for age hardening as solid solution C, and it is sufficient if C>0. However, since C < 0.01% leads to high manufacturing costs, the lower limit of the C amount is set at 0.01%.
%. Further, C is useful for increasing the strength level of the material, but if added in a large amount, the workability deteriorates, so the upper limit was set at 0.30%. Mo is known as an element that has a very strong interaction with dislocations. In other words, since Mo exists as clusters in steel near dislocations, C attracted to Mo is used to fix dislocations, resulting in a significant increase in age hardening compared to the case where Mo is not added. It is thought to improve bake hardenability. For the above reasons, the addition of Mo is indispensable to the present invention, but if the amount added is too large, weldability will be inhibited, so the upper limit of the content was set at 2.0%. In addition, the content is 0.10%
The lower limit is set at 0.10 because the effect is not shown below.
%.

[0016] Nb is added to promote the formation of carbides of Mo, that is, to increase the force that attracts C, and indirectly contributes to increasing age hardening. The upper limit of the content was set to 0.15%. Moreover, since the effect is not exhibited when the content is less than 0.010%, the lower limit is set to 0.010%. V is N
Similar to b, it is added to promote the formation of Mo carbides, but if it becomes too large, weldability will be inhibited, so the content should be 0.
．． The upper limit was set at 30%. Moreover, since the effect is not exhibited if the content is less than 0.010%, the lower limit is set to 0.010%.

[0017] In addition, S defined as a preferable component range
Regarding i, Mn, Al, and N, commonly used component ranges may be used. Si has the effect of improving the strength and ductility of steel materials due to its solid solution strengthening effect, and is required to be present in an amount of 0.02% or more, but if it is contained in excess of 0.50%, the toughness of the steel material will deteriorate. Its content is 0.02~
It was set at 0.50%.

[0018] Mn is an element necessary for strength, so 0.10
% or more, but in order to ensure weldability and toughness, the upper limit was set at 1.00%. Al is necessary for deoxidation in the steel manufacturing stage, and the lower limit was set to 0.001%. However, if the content is too large, the absolute value of the inclusions themselves increases, so the upper limit was set at 0.100%.

[0019] If the content of N is too large, it will deteriorate the toughness of the steel material, so the upper limit is set at 0.0100%, but considering the cost increase due to steel manufacturing capacity, the lower limit is set at 0.000%.
It was set at 5%. We will also discuss the components added with the aim of increasing strength. First, Cu is added because it is useful for increasing strength and improving corrosion resistance, but even if it is added in excess of 2.0%, there is almost no increase in strength, so the upper limit of the content is set at 2.0%.
And so.

Ni is added because it is useful for improving low-temperature toughness, but since it is an expensive element, the upper limit of the content was set at 9.5%. Cr is added because it is useful for increasing strength and improving corrosion resistance, but since too much Cr impedes low-temperature toughness and weldability, the upper limit of the content is 5.5%. Ti is added because it is useful for refining austenite grains, but if it increases, it inhibits weldability, so the upper limit of the content was set at 0.15%.

B has the effect of significantly increasing the hardenability of steel when added in a small amount. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added in excess, B compounds will be generated and the toughness will deteriorate, so the upper limit was set at 0.0030%. Ca
is added because it is useful for controlling the form of sulfide-based inclusions, but if its content increases, it forms inclusions in the steel and deteriorates the properties of the steel, so the upper limit was set at 0.0080%.

[0022] Hot rolling conditions are regulated by winding conditions. In other words, when the coiling temperature reaches 550° C. or higher, Mo carbides begin to precipitate, solid solution C is used for precipitation, and there is no solid solution C for fixing dislocations, which loses the meaning of the present invention. Therefore, the winding temperature is set to Mo
It is necessary to specify the temperature below 550°C so that no precipitation occurs.

There are no particular regulations regarding other hot rolling conditions. However, it is necessary to determine the manufacturing conditions by considering the required properties of the final product, the grain size based on finish rolling conditions, and the tensile strength level and yield ratio based on winding conditions. There are no particular regulations regarding steel pipe manufacturing conditions. In order to exhibit bake hardening characteristics, the amount of strain during forming a hot coil into a steel pipe is sufficient. After manufacturing the steel pipe, secondary processing such as drawing the steel pipe can be performed without any problem.

[0024]

[Example] Table 1 shows the chemical composition of the test materials, and Table 2 shows the heat treatment conditions and the mechanical properties obtained. Steel pipe No. in Table 2
．． A1, B1, C1, D1, J1, K1, L1, M1,
N1, O1, P1, Q1, R1, S1, T1, U1, V
1 is an example of the present invention, and the target of the present invention is 10k.
It shows bake hardening characteristics of gf/mm2 or more.

Furthermore, since Mo was not added to E1 and F1, almost no bake hardening occurred. Also,
In G1, although Mo is added, neither Nb nor V is added, so the bake hardening is small and 10
less than kgf/mm2. In addition, H1 and I1 are Mo
Precipitates were formed, solid solution C was not secured, and almost no bake hardening occurred.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

Effects of the Invention As explained in detail above, the present invention can achieve a speed of 30kgf/m without using any particularly expensive alloying elements.
This makes it possible to manufacture low-cost electrical resistance welded steel pipes for automobiles that have a strength of more than m2 and have excellent workability, and have great industrial effects.

Claims (1)

[Claims] [Claim 1] C: 0.01 to 0.30 in weight%
%, Mo: 0.10 to 2.0%, and Nb: 0.
010-0.15%, V: 0.010-0.30% 1
A low carbon steel containing one or two species, the balance being Fe and unavoidable impurities, is hot rolled in a normal process and heated to 550°C.
1. A method for manufacturing an electric resistance welded steel pipe for automobiles having excellent bake hardenability and workability, the method comprising forming the pipe using a hot-rolled sheet obtained by winding the pipe with a hot rolled sheet.