JPH05302119A - Production of high strength automotive parts - Google Patents

Abstract

PURPOSE:To manufacture the automotive parts good in dimensional accuracy and having high strength by subjecting a hot rolled steel sheet having a specified compsn. constituted of C, Si, Mn, Cr, Al and Fe to softening, thereafter subjecting it to forming and welding into a steel pipe, furthermore subjecting it to cold working to form its shape into a prescribed one, subjecting it to heating treatment to a specified temp. and thereafter executing air cooling. CONSTITUTION:Steel contg., by weight 0.15 to 0.40% C, 0.1 to 0.7% Si, 0.5 to 2.5% Mn, 0.2 to 2.5% Cr and 0.01 to 0.05% sol.Al and furthermore contg., at need, one or more kinds among 0.05 to 1.0% Mo, 0.02 to 0.1% V, 0.2 to 2.5% Ni, 0.02 to 0.10% Ti, 0.02 to 0.10% Nb and 0.0005 to 0.005% B, and the balance Fe with inevitable impurities is used as stock and is subjected to hot rolling. This hot rolled steel sheet is subjected to softening to facilitate the subsequent forming and is subjected to forming and welding to form its shape into a pipelike one. The obtd. steel pipe is subjected to cold working to form its shape into a prescribed one and is thereafter heated at 850 to 1050 deg.C for 0.5 to 30min to form its structure into a fine austenitic one. After that, the steel pipe is air-cooled to uniformly impart desired strength thereto.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an automobile structural part made of steel pipe, and in particular, it is manufactured by plastic working such as bending and flattening typified by stabilizers, bumper beams and the like, and has high strength. This method is suitable for manufacturing required parts.

[0002]

2. Description of the Related Art As a prior art relating to a steel pipe stabilizer, there is "a method for manufacturing a hollow stabilizer" described in JP-A-58-197218. This technology "provides a lightweight stabilizer that has practically sufficient strength as a stabilizer at a low cost, and also allows easy management of the work environment and safety in the manufacturing process, contributing to resource and energy saving." In order to achieve the above, "a carbon steel pipe for machine structure or a carbon steel pipe for general structure containing boron is formed into a predetermined shape by bending and then water-quenched, and then the hardness is within the range of H _R C36 to 43. It is characterized by performing tempering so that

In addition, in the manufacture of torsion bars, JIS SU
A method is generally adopted in which a steel pipe of a component system corresponding to P is plastically worked into a predetermined shape and then oil-quenched and tempered. As described above, according to the conventional technique, the strength required for an automobile structural component is secured by plasticizing a steel pipe having a specific component and then quenching it with water (or oil).

[0004]

However, water (or oil) quenching causes a large strain,
There is a problem that an appropriate product size cannot be secured as an automobile part. This dimensional deviation cannot be corrected by the subsequent tempering treatment, and is finally corrected by performing plastic working such as bending again in the cold. In such plastic working at the final stage, the product itself already has high strength due to quenching and tempering heat treatment, so a large working force is required, and its adjustment is difficult, resulting in a large number of man-hours and costs. It depends.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems of the prior art, the present inventors have studied the quenching strain reduction and obtained the following findings. 1) The cause of quenching strain is that the cooling rate is different in each part in the same part.

2) In order to make the cooling rates of the respective parts uniform, for example, stirring the cooling liquid will bring about some effects, but the effects are not yet sufficient. 3) This is because it is extremely difficult to completely homogenize the cooling rate by stirring the cooling liquid because the cooling rate is basically too fast in water or oil quenching. 4) Therefore, in order to make the cooling rate uniform, it is necessary to reduce the quenching cooling rate, and the steel composition is adjusted so that the strength required for automobile structural parts can be secured even at a slow cooling rate. There is a need. The present invention has been made based on the above findings, and its method is as follows.

(1) C: 0.15 to 0.40 by weight
%, Si: 0.1 to 0.7%, Mn: 0.5 to 2.5%, C
r: 0.2 to 2.5%, Sol.Al: 0.01 to 0.05%, the balance being steel consisting of Fe and unavoidable impurities, hot-rolled steel sheet by hot rolling, and softening After annealing, the steel pipe formed into a tubular shape and welded is cold-worked so as to have a predetermined part shape, and then heated at 850 to 1050 ° C. for 0.5 to 30 minutes and then air-cooled. Manufacturing method of high strength automobile parts.

(2) The raw material steel further contains M in weight%.
o: 0.05 to 1.0%, V: 0.02 to 0.1%, Ni: 0.0.
2 to 2.5%, Ti: 0.02 to 0.10%, Nb: 0.02
~ 0.10%, B: 0.0005 ~ 0.005%, one or more kinds are contained, The manufacturing method of the high strength automobile parts of (1) characterized by the above-mentioned.

[0009]

[Operation] First, the reasons for limiting the components of the steel material used in the present invention will be described below. C: C is a component added to obtain the strength required for automobile parts at low cost. However, if the amount is less than 0.15%, sufficient strength cannot be obtained, and if it exceeds 0.4%, it becomes difficult to weld during pipe making and the toughness decreases, so the content is 0.15%. ˜0.40%.

Si: Si is an element necessary for deoxidation, and
If it is less than 1%, the deoxidation becomes insufficient and the toughness decreases. On the other hand, if it exceeds 0.7%, welding defects are likely to occur during welded pipe manufacturing. Therefore, the range of Si is set to 0.1 to 0.7%.

Mn: Mn is effective for improving the hardenability, and is added to secure the strength even at a slow cooling rate. This effect is insufficient at less than 0.5%. On the other hand, if it exceeds 2.5%, welding defects are likely to occur at the time of welded pipe manufacturing, and the toughness decreases. Therefore, the range of Mn is set to 0.5 to 2.5%.

Cr: Cr is added to improve hardenability and toughness. This effect is not sufficient if less than 0.2% is added, and it is necessary to add 0.2% or more. However, if it exceeds 2.5%, welding defects become prominent during welding pipe manufacturing, so the upper limit is made 2.5%.

Sol.Al:Al is an effective component for deoxidizing steel and refining the structure, but when the amount of Sol.Al is less than 0.01%, the effect is not sufficiently obtained, and 0.05 %, The toughness of the welded pipe is adversely affected, so the content is set to 0.01 to 0.05%.

By limiting the ingredients as described above,
Sufficient strength and toughness can be secured for automobiles in the manufacturing process described later, but if it is desired to further improve this, in addition to the above components, Mo, V, Ni, Ti, Nb,
It is effective to further add one or more of B. The reasons for specifying the contents of these additive components will be described below.

Mo: Mo has the effects of increasing the strength by solid solution strengthening and improving hardenability. This effect is
If it is less than 0.05%, it is not sufficient, and conversely, if it is 1.0% or more, the toughness of the welded pipe is deteriorated. Therefore, the content should be 0.0
5 to 1.0%.

V: V has the effect of forming precipitates and improving the strength, and it is advisable to add 0.02% or more. However, if the content exceeds 0.1%, the toughness of the welded pipe is deteriorated, so the content is made 0.1% or less.

Ni: Ni has the effects of improving hardenability and toughness, and is added in an amount of 0.2% or more. If the content exceeds the upper limit of the present invention, the effect is exerted, but since it is expensive, it is set to 2.5% or less.

Ti, Nb: Both Ti and Nb have the effect of improving toughness by refining the structure, and are added in an amount of 0.02% or more. However, if it is added in excess of 0.10%, the toughness is decreased, so that the addition of 0.10%
% Or less. B: B is an element effective for improving hardenability, but 0.00
If less than 05%, the desired effect cannot be obtained, while 0.005
%, The toughness deteriorates, so 0.0005 to 0.0
The range is 05%.

In the present invention, a steel material having the components adjusted as described above is used as a raw material to form a hot-rolled steel sheet having a desired thickness by hot rolling, and after softening annealing, formed into a tube and welded to obtain a welded steel tube. At this time, the method of welding the hot-rolled steel sheet into a steel pipe is limited, as compared with the production of a seamless steel pipe such as a Mannesmann pipe or a Eugene pipe, this method is economical,
Furthermore, it is due to the excellent dimensional accuracy of the outer diameter and wall thickness of the steel pipe.

Further, the softening and annealing of the steel sheet before the welded pipe making is performed by this heat treatment, 1) facilitating the forming into a tubular body, and 2) keeping the hardness of the steel pipe at a low level after the pipe making at a predetermined level. It is for facilitating the cold working for molding into the shape of automobile parts. At this time, it is necessary to pay attention to the following in order to achieve the above 2). That is, since the steel material of the present invention has high hardenability, it is necessary not to apply excessive heat at the time of welded pipe manufacturing.
TIG welding, submerged arc welding, electric resistance welding and the like are preferably used, and electric resistance welding is most preferable. However, even in electric resistance welding, the welded part is locally heated to a high hardness because it is heated to a temperature higher than the transformation point and then cooled.

For this reason, it is conceivable to carry out an annealing treatment in order to reduce the hardness of the welded portion after the welded pipe is manufactured. Since this treatment has the same purpose as the above 2), it is substantially included in the method of the present invention. In addition, as a method for producing a hot-rolled steel sheet by hot hot-rolling, one usually used in this technical field may be used.

In the present invention, the welded steel pipe manufactured as described above is cold-worked so as to have a predetermined automobile part shape. At this time, as the cold working, three-dimensional bending, pipe expanding, drawing, flattening, etc. are typical. Such processing is performed so as to satisfy the dimensional accuracy finally required for automobile parts, and the hardness of the steel pipe is maintained at a low level as described above, so that it can be performed relatively easily. ..

In the present invention, after performing such processing,
The final product is obtained by heating at 850 to 1050 ° C for 0.5 to 30 minutes and then air cooling. The reason why the heating temperature and time are limited as described above is as follows. That is, in order to sufficiently convert a steel material into austenite, heating at 850 ° C. or higher is required, but if it exceeds 1050 ° C., it is not possible to suppress coarsening of austenite grains even if Ti, Nb, etc. are added, This is because the toughness deteriorates. Further, it is because heating for less than 0.5 minutes cannot uniformly heat the whole, while heating for more than 30 minutes causes coarsening of austenite grains at a high heating temperature close to 1050 ° C.

Further, the reason why the air is cooled after heating and soaking is as follows. That is, when rapidly cooled in water or oil, it is extremely difficult to equalize the cooling rate of the entire part because the cooling rate is extremely fast, and this non-uniformity causes a large quenching strain, ensuring the dimensional accuracy of the part. Can not. Further, since the steel material used in the present invention has high hardenability, quenching may cause quench cracking, resulting in loss of soundness as a product. Further, in slow cooling in the heating furnace, the cooling rate is extremely slow, and the strength required as a product cannot be secured.

[0025]

EXAMPLES Next, the present invention will be described in more detail based on examples. The slab materials of the component steels shown in Table 1 were hot rolled into hot-rolled steel sheets, and after softening annealing, electric resistance welded steel pipes were manufactured. The manufacturing conditions are as follows.

1. Hot rolling 1) Heating temperature: 1200 to 1250 ° C 2) Final thickness: 1.6 to 3.2 mm 3) Coil winding temperature: 500 to 550 ° C 2. Softening annealing 1) Heating temperature: 550 to 750 ° C 2) Heating time: 1 to 10 hours 3. Steel pipe dimensions 1) Outer diameter: 20 to 50 mm 2) Wall thickness: 1.6 to 3.2 mm

This steel pipe was cut into a length of 1 m, and the center portion thereof was bent by 90 ° with a pipe bender and cold-worked (bending radius:
2DR), each was heated and cooled at the temperature and time shown on the right side of the chemical composition column in Table 1, and the tensile strength and the heat treatment deformation amount were examined to obtain the data in Table 1. Here, the heat treatment deformation amount is the amount of displacement of the end portion of the steel pipe after heating / cooling measured with reference to before heating / cooling. If the value is 5 mm or less, it is "○", and if it is more than 5 mm, it is "×". ".

[0028]

[Table 1]

As can be seen from Table 1, in the examples of the present invention, a high tensile strength of 100 Kgf / mm ² or more was obtained and the heat treatment deformation was extremely small. On the other hand, in the comparative example, there is a problem that the strength is insufficient, or the strength is high, but the heat treatment deformation amount is large or the toughness is extremely low. Incidentally, Comparative Example B4 is an experimental result of those exemplified in JP-A-58-197218, and Comparative Example B5 is an experimental result of one corresponding to JIS SUP9. In both cases, not only the deformation is large, but also when the tempering heat treatment is not carried out, the toughness becomes poor and it cannot be put to practical use.

[0030]

As described above, according to the method of the present invention, a steel pipe automobile part having high strength and less heat treatment deformation can be manufactured, and it is unnecessary to rectify heat treatment deformation which is conventionally performed.

Claims (2)

[Claims] 1. C: 0.15 to 0.40% by weight and S
i: 0.1 to 0.7%, Mn: 0.5 to 2.5%, Cr: 0.2
~ 2.5%, Sol.Al: 0.01-0.05%, with the balance being steel consisting of Fe and unavoidable impurities,
A hot-rolled steel sheet is formed by hot rolling, and after softening annealing, a steel tube produced by forming into a tubular shape and welding is cold-worked into a predetermined part shape, and then at 850 to 1050 ° C for 0.5 to 0.5.
A method for producing a high-strength automobile part, which comprises heating for 30 minutes and then air-cooling. 2. The raw material steel further comprises, in weight%, Mo: 0.0.
05-1.0%, V: 0.02-0.1%, Ni: 0.2-2.
5%, Ti: 0.02 to 0.10%, Nb: 0.02 to 0.1
The method for producing a high-strength automobile part according to claim 1, further comprising one or more of 0% and B: 0.0005 to 0.005%.