JP2684184B2 - Manufacturing method of high strength ERW steel pipe for automobile - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a high-strength electric resistance welded steel pipe for automobiles, which has a tensile strength of 120 kgf / mm ² or more in the as-formed state, such as a door impact beam used as a reinforcing material. The present invention relates to a method for manufacturing a steel pipe, such as a core material for a bumper, which requires a small amount of secondary processing and requires high strength.

(Prior Art) A high-strength electric resistance welded steel pipe with a tensile strength of more than 120 kgf / mm ² can be manufactured by performing heat treatment such as quenching and tempering of the tubular body after electric resistance welding of annealed hot rolled steel sheet. However, this method has the following three drawbacks.

(1) Performing heat treatment after pipe making complicates the process.

(2) The heat treatment cost is high.

(3) A shape change is caused by heat treatment.

Further, as a method for obtaining high strength in the as-formed state,
Although a method for producing a high-strength ERW steel pipe for oil country tubular good is disclosed in Japanese Patent No. 61-272318, this method is also characterized by performing heating and quenching only on the ERW portion, and further performing tempering. However, some heat treatment steps remain, and an electric resistance weld heat treatment device and a tube annealing heat treatment device are required.

(Problems to be Solved by the Invention) The present invention omits a heat treatment process of a steel pipe, which causes a problem such as a change in shape of the high strength electric resistance welded steel pipe for an automobile, which results in a high cost.

(Means for Solving the Problem) The present invention contains C: 0.08 to 0.23% (weight%, the same applies hereinafter), Mn ≦ 2.0%, Si ≦ 0.8%, Ti ≦ 0.04%, B: 0.0003 to 0.0035%. However, the balance remains by adjusting the degree of deoxidation.
A material slab consisting of sol.Al, Fe and unavoidable impurities is finished rolled just above the α + γ two-phase region temperature, then cooled at a cooling rate of 15 ° C / sec or more, and wound at 200 ° C or less, and martens High strength hot-rolled steel sheet with a site fraction of 80% or more, and then electric resistance welding is performed, and a method for manufacturing high strength electric resistance welded steel pipe for automobiles, and C: 0.08 to 0.23%, Mn ≤ 2.0% , Si ≤ 0.8%, Ti ≤ 0.04%, B: 0.0003 to 0.0035%, and in addition to these, either Nb ≤ 0.10%, Cr ≤ 1.00%, and the balance to adjust the deoxidation degree. After finishing rolling the raw material slab consisting of sol.Al, Fe and unavoidable impurities that remain due to the finish rolling at a temperature just above the α + γ two-phase region, cool at a cooling rate of 15 ° C / sec or more and wind at 200 ° C or less. Characterized by performing high-strength hot-rolled steel sheet with a martensite fraction of 80% or more and then performing electric resistance welding This is a method for manufacturing high strength ERW steel pipes for automobiles.

(Operation) The present invention has been made to solve the above problems, and obtains a martensitic structure of 80% or more by selecting components, rolling conditions, and cooling conditions, and at the stage of hot-rolled material for ERW steel pipe. Along with obtaining a tensile strength of 120 kgf / mm ² or more, the workability required for ERW pipe molding is obtained at the same time, and a tensile strength of 120 kgf / mm ² or more can be obtained as-is by ERW welding after ERW pipe fabrication. As a result, the subsequent heat treatment, which is conventionally required, can be omitted altogether.

The reasons for limiting the chemical composition of the ERW steel pipe material in the method of the present invention will be described.

C is the most stable element that increases strength, but C
Increasing the amount generally leads to a decrease in toughness. When C exceeds 0.23%, the toughness is remarkably deteriorated and the formability at the time of manufacturing the electric resistance welded pipe is significantly deteriorated. In addition, although the metallic structure aimed at by this method is martensite, in order to obtain this, the lower limit was made 0.08% after considering the cooling rate as a hardenability improving element.

Si is an element effective as a deoxidizing element in steel, but if it is contained in excess, penetrator defects due to the SiO ₂ component are likely to occur during electric resistance welding. Therefore, the content is limited to 0.8% or less.

Like C, Mn is an element that increases the strength of the steel pipe and is also effective in improving toughness, but if it is contained in excess, penetrator defects due to MnO components are likely to occur during electric resistance welding,
Therefore, it is limited to 2.0% or less.

B, which is the most important point in the present invention, dramatically improves the hardenability of steel and is an element indispensable for forming low C martensite for obtaining high strength and high toughness. . B exhibits its effect in a very small amount, but if it is less than 0.0003%, the effect of improving the hardenability cannot be expected, and the lower limit value was made 0.0003%. Further, if it exceeds 0.0035%, not only the cost becomes high, but also it tends to cause surface defects and deterioration of toughness, so the upper limit was made 0.0035%.

The effect of improving the hardenability of B is lost when N is present in an amount of 0.003% or more. However, N is usually unavoidable and may exceed 0.003% in some cases. Therefore, Ti is added for the purpose of fixing the N. However, if Ti exceeds 0.04%, defects such as defects and machinability are likely to occur, so Ti is restricted to 0.04%.

Further, Cr and Nb added in some cases are elements for securing strength, and the upper limits thereof are 0.1% and 1.0% from the viewpoint of weldability and cost.

Next, rolling / cooling conditions performed to obtain high strength and high toughness using the above slab will be described.

Grain boundaries and sub-grain boundaries are the minimum units at the time of fracture, so grain refinement is effective for improving toughness. Therefore, in hot rolling, rolling is performed at a temperature as low as possible. However, as shown in Fig. 1, if the rolling finishing temperature is lowered too much, the α + γ two-phase region will be rolled, and the toughness will rather decrease due to the formation of processing α. To finish. This temperature range varies greatly depending on the composition, but is approximately 750 to 850 ° C.
Target.

Next, regarding cooling, rapid cooling is performed for the purpose of completing the martensitic transformation to the center of the plate thickness. As shown in FIG. 2, if a cooling rate of 15 ° C./sec or more is secured, a high hardness can be obtained, and a martensitic structure of 80% or more can be obtained structurally throughout the plate thickness.

Further, as shown in FIG. 3, when the winding temperature is higher than 200 ° C., even if a martensite structure is obtained by quenching, martensite is tempered by self-sensible heat in the coil cooling process after winding. However, since the strength will be reduced, the cooling stop temperature must be 200 ° C or lower.

The material for electric resistance welded pipes thus manufactured exhibits high strength as in the example, and continuous formation of electric resistance welded pipes is possible, and a tensile strength of 120 kgf / mm ² can be obtained as it is by electric resistance welding.

Although a tempered softening portion of martensite occurs in the heat affected zone of the electric resistance welding of the electric resistance welded tube thus obtained, the area is very narrow, and heat treatment of the electric resistance welded portion as disclosed in JP-A-61-272318 is performed. On the contrary, a large softened region is formed when the heat treatment is performed, so that the heat treatment is not performed (FIG. 4).

(Examples) Table 1 shows Examples and Comparative Examples of the present invention. The finish rolling temperature of each sample was 800 ° C, and the product thickness of the hot rolled sheet was
3.5mm, the dimensions of the electric resistance welded tube were 38.1mmφ in outer diameter and 3.5mm in wall thickness. As a result, in Examples A to F, a martensite structure of 80% or more was obtained over the entire wall thickness. The strength of the hot-rolled sheet was 131-152 kgf / mm ² , although it depended on the amount of the components.
Further, although C to F are obtained by adding either Nb or Cr, it is confirmed that the addition of these improves tensile strength by about 10 kgf / mm ² without deterioration of elongation by comparison with B. It The strength after pipe forming was further increased by ERW pipe forming, and a tensile strength of 141 to 162 kgf / mm ² was obtained as-is by ERW welding.

(Effects of the Invention) As described above, according to the present invention, a tensile strength of 120 kgf which could be obtained only in the conventional heat-treated steel pipe.
It is possible to obtain a strength of / mm ² or more with an as-pipe-machined ERW steel pipe. As a result, a large number of steps for obtaining a high-strength steel pipe can be omitted, and productivity can be improved and cost can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the hot rolling finish stop temperature and the tensile properties, FIG. 2 is a diagram showing the relationship between the cooling rate and the hardness at the center of the plate thickness, and FIG. 3 is the winding temperature and the tensile properties. FIG. 4 is a diagram showing a relationship between the softened state of the electric-welded portion and the softened state near the electric-welded portion when the electric-welded portion is heat-treated.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C22C 38/00 301 C22C 38/00 301Z 38/14 38/14 38/38 38/38 (72) Inventor Akeda Takazawa 5-3 Tokai-cho, Tokai-shi, Aichi Inside Nippon Steel Corporation (56) References JP-A 61-279623 (JP, A) JP-A 57-104623 (JP, A)

Claims (2)

(57) [Claims] 1. C: 0.08 to 0.23% (weight%, the same applies hereinafter), Mn ≤ 2.0%, Si ≤ 0.8%, Ti ≤ 0.04%, B: 0.0003 to 0.0035%, the balance adjusting the deoxidation degree. Survive by doing
A material slab consisting of sol.Al, Fe and unavoidable impurities is finished rolled just above the α + γ two-phase region temperature, then cooled at a cooling rate of 15 ° C / sec or more and wound at 200 ° C or less, and martens A method for manufacturing a high-strength electric resistance welded steel pipe for automobiles, which comprises performing electric strength welding after forming a high strength hot rolled steel sheet having a site fraction of 80% or more. 2. C: 0.08 to 0.23% (weight%, the same applies hereinafter), Mn ≤ 2.0%, Si ≤ 0.8%, Ti ≤ 0.04%, B: 0.0003 to 0.0035%, and in addition to these, Nb A material slab containing ≦ 0.10% or Cr ≦ 1.00%, the rest of which is sol.Al, Fe and unavoidable impurities remaining by adjusting the degree of deoxidation, immediately above the α + γ two-phase region temperature. After finishing rolling, it is cooled at a cooling rate of 15 ° C / sec or more, wound at 200 ° C or less, made into a high-strength hot-rolled steel sheet with a martensite fraction of 80% or more, and then electric resistance welded. And a method for producing a high strength ERW steel pipe for automobiles.