JPH0463227A - Manufacture of hot rolled steel for resistance welded steel tube for reinforcing car body - Google Patents

Abstract

PURPOSE:To provide a steel with high strength, high toughness and high ductility by subjecting steel stock in which the amt. of C is specified, the amounts of Mn, Si, Ti and N are regulated and a trace amt. of B is added to hot rolling and coiling it at a specified temp. CONSTITUTION:A steel contg., by weight, 0.15 to 0.25% C, <=1.5% Mn, <=0.5% Si, <=0.04% Ti, 0.0003 to 0.O035% B, <=0.0080% N and the balance Fe with inevitable impurities is used as stock. This steel is hot-rolled and is thereafter coiled at >=600 deg.C. If required, the above steel is incorporated with one or >=two kinds among <=0.5% Ni, <=0.5% Cr and <=0.5% Mo. In this way, a hot rolled steel which is the stock for a high strength resistance welded steel tube excellent in elongation and toughness, having about >=120kgf/mm<2> tensile strength and useful for reinforcing car bodies as well as in which working loads applied thereto when it is formed into a resistance welded steel tube are same as those applied to a conventional low strength material can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to steel pipes for reinforcing car bodies that require particularly high strength, such as steel pipes for reinforcing doors to ensure the safety of drivers during side collisions of automobiles. The present invention relates to a method for manufacturing hot-rolled steel material for car body reinforcement electric resistance welded steel pipes that require high strength, such as door impact bars or core materials for bumpers.

(Prior art) In order to ensure the safety of occupants in the event of a collision, materials used for automobile body reinforcing members, such as impact beams, must not only have high strength but also resist rupture even if subjected to large plastic deformation during a collision. It is necessary to ensure that the strength does not suddenly decrease due to this phenomenon, and to ensure this property even at low temperatures. Thus, strength, ductility, and low temperature toughness are important properties.

As a manufacturing method for high-strength electric resistance welded steel pipes,
A method for manufacturing high-tensile resistance welded steel pipes is known, which is described in Japanese Patent No. 6538. In this method, tempering treatment is performed to ensure ductility, and generally, quenching and tempering treatment is necessary to restore the toughness and ductility of the steel pipe. However, when tempering is applied, the strength decreases significantly, so for example, 120k
It has been difficult to obtain a steel pipe with a high strength of gf/- or more. In order to obtain such a high-strength steel pipe, it is preferable to use the pipe as-quenched, but in this case the toughness deteriorates. Therefore, if you try to improve the toughness while still quenching,
It would be better to try to reduce carbon, but in this case, the variation in strength becomes extremely large depending on the cooling rate during quenching, causing problems in terms of practical performance.

(Problems to be Solved by the Invention) As described above, the present invention aims at reducing the chemical composition of steel pipes that require high strength, high toughness, and high ductility, such as steel pipes for impact beams, and using the same as quenched. This was done in order to solve problems such as difficulty in hardening and large variations in strength in such cases.

(Means for solving problems) The gist of the present invention is as follows.

(1) C: 0.15 to 0.25% (weight%, same below) Mn≦1.5% Si≦1.5% Ti≦1.04% B: 0.0003 to 0.0035% N≦ 0.008
1. A method for producing a hot-rolled steel material for car body reinforcement electric resistance welded steel pipes, which is made of steel containing 0% Fe and unavoidable impurities, and the method comprises hot rolling and then winding at 600°C or higher.

(2) C: 0.15 to 0.25% (weight%, same below) Mn≦1.5% Si≦1.5% Ti≦1.04% B: 0.0003 to 0.0035% N≦ 0.008
0% and further contains one or more of Ni≦0.5%, Cr≦0.5%, Mo≦1.5%, and the remainder consists of Feb and unavoidable impurities. A method for producing a hot-rolled steel material for car body reinforcement electric resistance welded steel pipes, which comprises winding at 600° C. or higher after rolling.

(Operation) The present invention has been made to solve the above problems,
By selecting the composition and hot-rolling conditions, the pipe-formability when making ERW pipes is equivalent to that of conventional low-strength mesh pipes, and by quenching after making ERW pipes, good elongation and
Provided is a hot-rolled steel material that can be made into a high-strength steel pipe exhibiting toughness.

The reasons for limiting the hot-rolled steel manufacturing conditions in the present invention will be described below.

First, regarding the component system, the present invention aims to strengthen the final product of the ERW steel pipe for car body reinforcement by a quenched martensitic structure, and the strength of the as-quenched martensitic structure depends on the C content. determined by This is thought to be due to the amount of solid solute C introduced into supersaturation through the use of transformation being a controlling factor. Preferable for reinforcing automobile bodies 1
In order to ensure a strength of 20 kgf/- or more, C must be 0.15% or more as shown in FIG. on the other hand,
As the amount of C increases, the deterioration of ductility becomes remarkable.

To ensure growth of about 10% or more, C should be 0.25%.
It is necessary to do the following. Furthermore, Fig. 2 shows the toughness of the quenched material relative to the carbon content. Toughness can be maintained high at C0.25% or less.

As described above, in the present invention, by examining the effect of carbon content in detail, it is possible to achieve high strength, high toughness, and high ductility after quenching in the range of 0.15%≦C≦0.25%. This makes it possible to obtain properties that are effective as steel pipes for reinforcing car bodies.

Mn is an element that lowers the martensitic transformation temperature of the steel, improves hardenability, and also has the effect of avoiding self-tempering after transformation during the hardening process and maintaining high strength. However, assuming that a steel pipe is manufactured by electric resistance welding, for example, Mn tends to cause welding defects, and the upper limit of its content is 1.50%.

Ni, Cr + Mo are very expensive compared to Mn, but adding these Ni, Cr, and Mo alone or in combination in addition to Mn lowers the martensitic transformation temperature, avoids self-tempering, and provides high strength. It is effective by changing the In order to ensure weldability, the upper limit is 0.5% for each.

Si, together with Mn, is a very important element in maintaining the integrity of the weld when manufacturing steel pipes by electric resistance welding. The upper limit of Si is set to 0.0 to prevent the formation of oxides called penetrators in the welding area.
5% or less. Balance the Mn/Si ratio from 3 to 10.
It is desirable to do so.

B is an element that dramatically improves hardenability, and the mesh type of the present invention is characterized by the addition of B in order to obtain a martensite fraction of 90% or more at a relatively low C. 000
If it is less than 3%, the effect of improving hardenability cannot be obtained, and if it is 0,0
If it exceeds 0.035%, not only will the cost increase, but it will also likely cause surface flaws and toughness deterioration. Therefore, the amount of B added was 0.0003 to 0.0035%.

Since the hardenability improvement effect of 20B is lost when 0.003% or more of N is present, Ti is added for the purpose of fixing this N. If the amount of Ti added exceeds 0.04%, quality problems such as occurrence of scratches and deterioration of machinability are likely to occur.
Therefore, Ti is regulated to 0.04% or less.

Note that N inevitably exists in steel, forms BN, and reduces the effect of B. Therefore, it is desirable to reduce N as much as possible, and the upper limit is set to 0.0080%.

Next, regarding the reason for limiting the hot rolling conditions, we conducted a detailed study of the coiling temperature in the present invention, and as a result, the results shown in FIG. 3 were obtained. The horizontal axis is the coiling temperature, and the vertical axis is the strength characteristics of steel pipes when ERW tubes are made from hot-rolled steel and then quenched. At a coiling temperature of less than 600°C, the strength varies greatly. growing. This is an example where all components are the same and the same hardening process is performed.The strength when hardened sufficiently is almost constant regardless of the winding temperature, but when the winding temperature is 60
Below 0°C, a partially insufficiently quenched structure is formed,
The strength variation is large, making it inappropriate for stably securing high strength. On the other hand, at a winding temperature of 600℃ or higher,
A relatively uniform and coarse ferrite-pearlite structure is formed at the hot-rolled steel stage, and when the tube is hardened after tube formation, sufficient hardening occurs and strength properties with little variation can be obtained.

Furthermore, from the viewpoint of pipe forming properties when hot-rolled steel pipes are made,
The winding temperature shall be 0°C or higher. Here, the pipe formability refers to the ease of handling, forming, and electric resistance welding of hot-rolled steel. The starting material of the present invention is
Although the carbon content is reduced as much as possible, the hardenability is increased by adding B, etc., so it is possible to easily obtain high-strength hot-rolled steel by lowering the hot-rolling temperature. Put it away. When it comes to preparing high-strength materials, the life of the cutter during the shearing process from hot-rolled steel to belts for ERW pipes is shortened, handling is difficult due to increased winding and unwinding force, and the strength of the material when folding increases. Due to the rise, the forming reaction force increases, the amount of hackling increases, making it difficult to form, and the shape of the ERW welding power supply part becomes unstable, which affects the ERW welding quality as well as the difficulty of forming, making it difficult to stabilize welding quality. .

On the other hand, if the coiling temperature is set to 600°C or higher, as shown in Figure 4 (the strength level of general ERW steel pipe is 40kg/
It becomes a hot-rolled wire material weighing from - to 60 kg/-, and is
It is possible to perform electric resistance welding in the same condition as for ll pipes.

Furthermore, a factor that adversely affects pipe formability is variation in material strength. Many materials for impact beams are thin-walled, and the temperature drop after hot rolling is relatively fast. As a result, slight differences in cooling conditions tend to affect the winding temperature.
When the temperature is less than 00°C, the strength of the material changes greatly in response to changes in the winding temperature, which adversely affects the stability of subsequent pipe forming and furthermore the stability of electric resistance welding. As shown in FIG. 3, at a temperature of 600° C. or higher, the range of variation in material strength with respect to the coiling temperature is very small, and by setting the coiling temperature to 600° C. or higher, good pipe formability can be obtained.

The hot-rolled steel material manufactured with the above ingredients and hot-rolling conditions is
It is easy to make an ERW pipe, and after making it into an ERW steel pipe, it can be hardened to have a tensile strength of 120.
kg/- or more, it has excellent ductility and toughness, and exhibits good performance as an electric resistance welded steel pipe for car body reinforcement.

Here, the quenching heat treatment method after forming the electric resistance welded tube is not particularly limited, but includes, for example, induction hardening.

(Example) Table 1 shows examples and comparative examples of the present invention.

The JIS No. 5 tensile properties of the hot rolled steel sheet produced by the manufacturing method of the present invention and the hot rolled steel sheet having an outer diameter of 31.8 mm and a wall thickness of 2.0 mm.
Table 1 also shows the heat treatment method after making the am electric resistance welded steel pipe, the JIS No. 11 tensile properties after the heat treatment, and the Charpy absorbed energy. Here, Charpy absorbed energy indicates data obtained from a full-sized test piece prepared exclusively for toughness evaluation.

In all of Examples A to G, the hot-rolled steel had a tensile strength of about 60 kgf/- or less, and no major problems occurred during pipe production. By performing quenching treatment after making it into an electric resistance welded tube, in both cases, the weight is 120 kgf/- or more and the elongation is 10%.
A steel pipe with a uniform structure was obtained, in which an absorbed energy of 2 kgfm/cm or more was obtained, and the variation in tensile strength was less than a few kgf/cm.

Comparative Example H is a case in which the C content is lower than the range of the components of the present invention,
The final target strength cannot be obtained due to heat treatment after making the ERW pipe.

Comparative Example (3) is a case where the amount of C is higher than the range of the components of the present invention, and after the heat treatment of the electric resistance welded tube, sufficient strength can be achieved, but elongation is very low.

Comparative example J-N is a case where low-temperature winding was performed at less than 600°C, but the characteristics of the final ERW pipe are relatively high strength and
Although high ductility and high toughness can be achieved, the strength variation is close to 20 kgf/-, making it difficult to ensure stable properties as a steel pipe for reinforcing car bodies.

In addition, it has high strength at the hot-rolled steel stage and is inferior in pipe formability.

Comparative examples J, L, and M are difficult to maintain good shearing quality due to the breakage of the round blade during the shearing process when they are made into electric resistance welded pipes on a normal line, and special measures are required when making them into electric resistance welded pipes. . In addition, in the comparative example, although the strength of the material of N was slightly lowered and there was a problem with the life of the shearing round blade, relatively good new shearing quality was obtained. However, it is difficult to handle the leading and trailing ends of the steel strip, the reaction force during the forming of the electric resistance welded tube is high, the load for adjustment, etc. increases, and the productivity decreases more markedly than with ordinary strength materials.

(Effect of the invention) As explained above, according to the present invention, the car body reinforcement mold W
Excellent elongation and toughness, useful as E-pipe, tensile strength 12
It is possible to obtain a hot-rolled steel material that is a material for high-strength electric resistance welded steel pipes with a strength of 0 kgf/- or more, and whose work load when forming an electric resistance welded mesh pipe is no different from that of conventional low-strength materials.

[Brief explanation of the drawing]

Figure 1 shows the effect of the carbon content of the hot-rolled steel wire on the tensile properties of the final ERW tube after quenching. Figure 3 shows the effect of winding temperature conditions on the strength characteristics after quenching. FIG. 2 is a diagram showing the effect of coiling temperature on the tensile strength of hot rolled steel. gl, 5 &, Su5 Higashiyuki (kpl/n; evaluation is good, figure 2, C combined quantity (wtZ) \ \ \ \ \ (〜 -existent and 0) \ \ Cll to C111 too

Claims (2)

[Claims] (1) C: 0.15-0.25% (weight%, same below)
Mn≦1.5% Si≦0.5% Ti≦0.04% B: 0.0003 to 0.0035% N≦0.0080%, the balance being Fe and unavoidable impurities. A method for producing a hot-rolled steel material for car body reinforcement ERW steel pipes, which comprises hot-rolling and then winding at 600°C or higher. (2) C: 0.15-0.25% (weight%, same below)
Contains Mn≦1.5% Si≦0.5% Ti≦0.04% B: 0.0003 to 0.0035% N≦0.0080%, and further includes Ni≦0.5% Cr≦0.5 %Mo≦0.5%, the balance being Fe and unavoidable impurities, and is characterized by being coiled at 600°C or higher after hot rolling. Method of manufacturing hot rolled steel for use.