JPH0774382B2 - Manufacturing method of hot-rolled steel for body-reinforced ERW steel pipe manufactured by induction hardening - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a vehicle body reinforcing steel pipe that requires particularly high strength, for example, a door that is a door reinforcing steel pipe for ensuring the safety of the driver in the event of a side collision of an automobile. The present invention relates to a method for manufacturing a body-reinforced ERW steel pipe that requires high strength, such as an impact bar or a core material for a bumper, particularly a hot-rolled steel material for a body-reinforced ERW steel pipe manufactured by induction hardening.

(Prior Art) A car body reinforcement member, for example, a material used as an impact beam, has high strength in order to ensure the safety of an occupant at the time of a collision, and at the same time, rupture occurs even when subjected to a large plastic deformation at the time of a collision. It is necessary to prevent the strength from rapidly decreasing and to ensure this property even at low temperatures. As described above, strength, ductility, and low temperature toughness are important properties for automobile body reinforcing members.

As a method of manufacturing high strength ERW steel pipe, Japanese Patent Publication No.
A method of manufacturing a high-strength electric resistance welded steel pipe described in Japanese Patent No. 8 is known. In this method, tempering treatment is performed in order to secure ductility, and quenching and tempering treatment is generally necessary for recovering the toughness and ductility of the steel pipe. However, if tempering is applied, the strength will drop significantly, so for example 120 kgf / mm ²
It was difficult to obtain a high strength steel pipe as described above. In order to obtain such a high strength steel pipe, it is preferable to use it as it is by induction hardening, but in this case, the toughness deteriorates. Therefore, if it is attempted to improve toughness with induction hardening, it is necessary to reduce carbon, but at this time, the variation in strength becomes extremely large due to the cooling rate during induction hardening, which causes a problem in practical performance. .

(Problems to be Solved by the Invention) In the present invention, as described above, when manufacturing a steel pipe that requires high strength, high toughness, and high ductility, such as an impact beam steel pipe, its composition is reduced and it is used as it is by induction hardening. In such a case, it is made to solve problems such as difficulty in baking and large variation in strength.

(Means for Solving the Problems) The gist of the present invention is that C: 0.15 to 0.25% (weight%, the same applies hereinafter) Mn ≦ 1.5% Si ≦ 0.5% Ti ≦ 0.04% B: 0.0003 to 0.0035% N ≤ 0.0080%, Ni ≤ 0.5% Cr ≤ 0.5% Mo ≤ 0.5%, one or more, and the balance is Fe and unavoidable impurities. A method for producing a hot-rolled steel material for body-reinforced ERW steel pipe manufactured by induction hardening, which is characterized in that

(Operation) The present invention has been made to solve the above problems,
By selecting the components and hot rolling conditions, the pipe-forming property of the ERW pipe after that is the same as that of the conventional low strength steel pipe. -Providing a hot-rolled steel material capable of obtaining a high-strength body-reinforced ERW steel pipe exhibiting toughness.

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

First, regarding the component system, the present invention aims to strengthen by a quenched martensitic structure at the time of the body-reinforced ERW steel pipe of the final product, and the strength of the as-quenched martensitic structure depends on the C content. It is determined. It is considered that this is because the solute C introduced into supersaturation by utilizing the transformation is the dominant factor. Preferred as a car body reinforced ERW pipe
In order to secure a strength of 120 kgf / mm ² or more, C must be 0.15% or more as shown in FIG. On the other hand, when the C content is increased, the ductility is significantly deteriorated. In order to secure growth of 10% or more, C must be 0.25% or less. FIG. 2 shows the toughness of the induction hardened material with respect to the amount of carbon. C0.25% or less can maintain high toughness.

As described above, in the present invention, by examining the effect of the carbon content in detail, it is possible to achieve high strength, high toughness, and high ductility after induction hardening in the range of 0.15% ≦ C ≦ 0.25%. Therefore, it is possible to obtain effective characteristics as a body-reinforced ERW steel pipe.

Mn is an element capable of lowering the martensitic transformation temperature of steel, improving hardenability, avoiding self-tempering after transformation during the quenching treatment, and maintaining high strength. However, Mn is apt to cause a welding defect, for example, assuming that a steel pipe is manufactured by electric resistance welding, and the upper limit of its content is 1.50%.

Ni, Cr, Mo are much more expensive than Mn, but if Ni, Cr, Mo are added alone or in addition to Mn, the martensitic transformation temperature is lowered, self-tempering is avoided, and high strength is achieved. The effect is shown by the change. To ensure weldability, the upper limit is 0.5%.

Si is a very important element for maintaining the soundness of the welded part when producing a steel pipe by electric resistance welding together with Mn. The upper limit of Si is 0.5% or less in order to prevent the formation of an oxide called a penetrator in the weld. It is desirable that the balance of the Mn / Si ratio is 3 to 10.

B is an element that dramatically improves the hardenability, and in the case of the steel type of the present invention, the martensite fraction of 90% or more is obtained at a relatively low C, so it is characterized by the addition of B, but less than 0.0003%. Then, the effect of improving the hardenability cannot be obtained, and if it exceeds 0.0035%, not only the cost becomes high, but also the surface flaw and the deterioration of the toughness are easily caused. Therefore, the addition of B is 0.0003-0.
It was set to 0035%.

Since the effect of improving the hardenability of B is lost when N is present in an amount of 0.003% or more, Ti is added for the purpose of fixing N. If the amount of Ti to be added exceeds 0.04%, defects such as flaws and deterioration of machinability tend to occur in terms of quality, so Ti is 0.
Restrict to below 04%.

In addition, N inevitably exists in the 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 made 0.0080%.

Next, as a reason for limiting the hot rolling conditions, as a result of detailed examination of the winding temperature in the present invention, the results shown in FIG. 3 were obtained. The horizontal axis represents the winding temperature, and the vertical axis represents the strength characteristics of the steel pipe when induction-hardening treatment was performed after making ERW steel pipe from hot-rolled steel material. growing. In the case of performing the same induction hardening treatment with all the same components, the strength when quenching is sufficient is almost constant regardless of the winding temperature, but when the winding temperature is less than 600 ° C However, it is not suitable to stably secure high strength because it has a structure in which quenching is insufficient and the strength varies widely. On the contrary, at a coiling temperature of 600 ° C or higher, relatively uniform and coarse ferrite
When a pearlite structure is formed and baking is performed after pipe making, sufficient strength is achieved and strength characteristics with little variation are obtained.

Furthermore, in terms of pipe forming properties when using hot-rolled steel as ERW steel pipe,
The coiling temperature should be 600 ℃ or higher. The pipe-forming property here means the ease of handling, forming, and electric resistance welding of the hot rolled steel material. The starting material of the present invention is
Although the amount of carbon is reduced as much as possible, since the hardenability is increased by adding B, etc., a high strength hot rolled steel material can be easily obtained by lowering the coiling temperature of hot rolling. . High-strength steel material shortens the life of the blade during shearing from hot-rolled steel to steel strip for ERW pipes, difficulty in handling such as increase in winding / rewinding force, and increase in yield strength of material. As a result, the forming reaction force increases and the amount of buckling increases, which makes it difficult to perform a standard shape, and the forming becomes difficult, and at the same time, the shape of the electric resistance welding welding feed portion, which affects the quality of the electric resistance welding, becomes unstable.

When the winding temperature 600 ° C. or higher contrast, it becomes 60 kgf / mm ² about the hot-rolled steel from 40 kgf / mm ² is the intensity level of ordinary ERW pipe as shown in FIG. 4, the normal ERW ERW welding is possible in the same state as steel pipes.

Further, as a factor that adversely affects the pipe forming property, there is variation in material strength. Since many materials for impact beams are thin, the temperature drop after hot rolling is relatively fast. As a result, the winding temperature tends to be affected due to subtle differences in cooling conditions.
When the temperature is lower than 0 ° C, the material strength greatly changes according to the change of the coiling temperature, which adversely affects the stability of molding in the subsequent pipe forming and the stability of electric resistance welding. As shown in FIG.
At 600 ° C or higher, the variation range of the material strength with respect to the coiling temperature is extremely small, and at a coiling temperature of 600 ° C or higher, good pipe forming properties can be obtained.

The hot rolled steel materials manufactured under the above components and hot rolling conditions,
It is easy to make ERW steel pipe, and after making ERW steel pipe,
Tensile strength of 120kgf / m due to induction hardening
With m ² or more, it has excellent ductility and toughness, and shows good performance as a body-reinforced ERW steel pipe.

(Examples) Table 1 shows Examples and Comparative Examples of the present invention. JIS No. 5 tensile properties of hot rolled steel sheet by the manufacturing method of the present invention and heat treatment method after the hot rolled steel sheet is an electric resistance welded steel pipe having an outer diameter of 31.8 mm × wall thickness 2.0 mm, and JIS No. 11 tensile properties after heat treatment , Charpy absorbed energy is also shown in Table 1. Here, the Charpy absorbed energy is the data obtained from a full-size test piece prepared exclusively for toughness evaluation.

In each of Examples A to F, the tensile strength of the hot rolled steel material was
At about 60 kgf / mm ² or less, no major problems were caused during pipe making. In all cases, 120 kgf / mm ² or more, elongation of about 10%, absorbed energy of about 2 kgf-m / cm ² or more can be obtained by performing induction hardening treatment after using ERW steel pipe. ERW steel pipe with a uniform structure of several kgf / mm ² or less was obtained.

In Comparative Example G, when the C content is lower than the composition range of the present invention, the final target strength cannot be obtained by the induction hardening treatment after forming the electric resistance welded steel pipe.

In Comparative Example H, the C content is higher than the composition range of the present invention, and after the induction hardening treatment of the electric resistance welded steel pipe, the strength can be sufficiently achieved, but the elongation is very low.

Comparative Examples I to M are cases where low-temperature winding at a temperature of less than 600 ° C. was carried out.
Although high ductility and high toughness can be achieved, the variation in strength is close to 20 kgf / mm ^2, and it is difficult to secure stable properties as a body-reinforced steel pipe. Further, it has high strength in the hot rolled steel material stage and is inferior in pipe forming property. In Comparative Examples I, K, and L, when an electric resistance welded steel pipe is used in a normal line, it is difficult to maintain good shearing quality due to the loss of the round blade in the shearing process, and special measures are required when converting to electric resistance welded steel pipe. Further, in Comparative Examples J and M, the strength of the material was slightly lowered, and although there was a problem of the life of the shearing round blade, relatively good shearing quality was obtained. However, it is difficult to handle the front and rear ends of the steel strip, and the reaction force when forming ERW steel pipe is high,
The load such as adjustment increases, and the productivity is significantly lower than that of ordinary strength materials.

(Effects of the Invention) As described above, according to the present invention, a tensile strength of 120 kgf / m excellent in elongation and toughness, which is useful as a body-reinforced ERW steel pipe.
It is possible to manufacture a hot-rolled steel material that is a material for a high-strength electric resistance welded steel pipe of m ² or more and that has the same work load as that of the conventional low-strength material when used as an electric resistance welded steel pipe.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the carbon content of the hot rolled steel material on the tensile properties of the final electric resistance welded steel pipe after the induction hardening treatment,
Fig. 2 shows the effect of the carbon content of hot rolled steel on the ultimate tensile strength and Charpy impact value of the electric resistance welded steel pipe after induction hardening, and Fig. 3 shows the strength characteristics after induction hardening. Fig. 4 is a diagram showing the influence of the winding temperature condition on the, and Fig. 4 is a diagram showing the effect of the winding temperature on the tensile strength of the hot rolled steel.

Claims (1)

[Claims] 1. C: 0.15 to 0.25% (weight%, the same applies hereinafter) Mn ≦ 1.5% Si ≦ 0.5% Ti ≦ 0.04% B: 0.0003 to 0.0035% N ≦ 0.0080%, and Ni ≦ 0.5% Cr ≦ 0.5% Mo ≦ 0.5% containing one or more kinds, steel made of balance Fe and unavoidable impurities as raw material, manufactured by induction hardening characterized by being rolled at 600 ° C or more after hot rolling Of manufacturing hot rolled steel for body-reinforced ERW steel pipe.