JP2723013B2 - High strength ERW pipe manufacturing method with excellent delayed fracture resistance - Google Patents

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 a high-strength ERW steel pipe having a tensile strength of 1180 N / mm ² or more, which is used for a door impact beam for reinforcing automobile doors and the like, in which high strength is particularly required. thing.

[0002]

The vehicle door, such as an automobile, which secures such as in the event of a side collision, moreover to obtain a light weight, impact beam of door over reinforcement made of a high strength steel pipe is used. Impact beam of door over reinforcement, when under actual use environment load is added by the collision,
Although the plastic deformation with the body, important or can absorb how much energy before the door over inside the passenger are in contact, and a very high strength, a certain degree of plastic deformation, important to have a so-called elongation amount It is.

As a method of obtaining such an impact beam, C: 0.08 to 0.23%, Mn, Si, Nb
After hot rolling of steel containing one or more of Cr and Mo at a specific value or less, and then carrying out ERW welding on the pipe as it is, or performing strain relief annealing to obtain a tensile strength. 6
0 to 100 kgf / mm2 (71 to 85 kg in the example)
f / mm 2) (JP-A-2-19752)
No. 5), C: 0.15 to 0.23%, Mn: 1.5
% Or less, Si: 0.5% or less, Ti: 0.04% or less,
B: 0.0003-0.0035%, N: 0.0080
%, Or in addition to these,
i: 0.5% or less, Cr: 0.5% or less, Mo: 0.5
% Or less, and the remaining sol. A method of quenching a steel pipe composed of Al, Fe and unavoidable impurities (Japanese Patent Laid-Open No. 3-122219), C: 0.10 to 0.10
0.30%, Mn: 0.6 to 2.0%, Si: 0.03
To 1.20%, Cr: 0.3 to 1.5%, the balance being Fe and unavoidable impurities, a martensite phase ratio of 45 to 100%, C: 0.10 to 0.1%. 30
%, Mn: 0.6 to 2.0%, Si: 0.03 to 1.2
0%, Cr: 0.3 to 1.5%, the balance being Fe and unavoidable impurities, hot-rolled, slitted and then subjected to ERW welding (Japanese Patent Laid-Open No. 5-5949).
No. 3) has been proposed.

[0004]

The above-mentioned JP-A-2-197
The method disclosed in Japanese Patent No. 525 is a method in which high strength is used as it is in electric resistance welding from the viewpoint of cost and it is not broken by three-point bending, but this method alone may not always be preferable for reinforcing doors. Absent. Also, JP-A-3-1222
The method disclosed in 19 JP is a door over the reinforcing steel of the induction hardening Irigata as shown in FIG. 4, which is advantageous with respect to delayed fracture for induction hardened Irigata, very heat treatment cost Disadvantageous. Furthermore, the method disclosed in Japanese Patent Laid-Open No. 5-59493, since the as electric resistance welding, such as residual uneven internal stress caused by the hardness variation of the weld vicinity, is disadvantageous for delayed fracture, the door over the reinforcement It is not excellent in delayed fracture resistance for use.

Namely, the impact beam of the door over a reinforcement, since after being set to the vehicle body is left in a long period of time within the door over, delayed fracture is concerned, especially in high strength as a door over reinforcement In addition, it is extremely disadvantageous in terms of environmental conditions, such as infiltration of rainwater into the door, and when delayed fracture progresses, it does not serve as a door reinforcing material and becomes a serious problem. Let alone if left electric resistance welding, such as residual uneven internal stress caused by the hardness variation of the weld vicinity, is disadvantageous for delayed fracture, excellent steel especially delayed fracture resistance as door over reinforcement Needed.

An object of the present invention is to provide a method for producing high strength and resistance to delayed excellent electric resistance welded steel pipe fracture characteristics required for impact beam of door over reinforcement.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object. As a result, the relationship between the hardness of the weld and the base metal and the delayed fracture was clarified,
By specifying the components of the material and specifying the maximum hardness, an electric resistance welded steel tube for impact bars of 1180 N / mm ² or more with excellent not only three-point bendability but also delayed fracture resistance can be obtained at low cost. The present invention has been achieved.

That is, according to the present invention, C: 0.10-0.
20%, Si: 0.10 to 0.50%, Mn: 1.5 to
3.0%, P: 0.02% or less, S: 0.02% or less,
A hot-rolled steel sheet containing 0.5 to 2.0% of Cr and 1.0% or less of Mo and the balance of Fe and unavoidable impurities is hot-rolled and wound at 500 ° C or less. The maximum hardness of the ERW pipe obtained by ERW welding
v550 or less, which is a method for producing a high-strength ERW steel pipe having excellent delayed fracture resistance.

Further, the present invention provides a method for preparing C: 0.10 to 0.2.
0%, Si: 0.10 to 0.50%, Mn: 1.5 to
3.0%, P: 0.02% or less, S: 0.02% or less,
Cr: 0.5 to 2.0% and Mo: 1.0% or less, Nb: 0.015 to 0.05%, Ti:
0.015 to 0.10%, V: 0.1% or less, Ni:
1.0% or less, B: A steel containing at least one of 0.0005 to 0.0050%, and the balance consisting of Fe and inevitable impurities was hot-rolled and wound at 500 ° C or less. A method of manufacturing a high-strength ERW pipe with excellent delayed fracture resistance, characterized in that the maximum hardness of an ERW pipe obtained by ERW welding using a hot-rolled steel sheet is Hv550 or less including the welded part. is there.

[0010] Further, when the maximum hardness of the steel pipe including the welded portion exceeds Hv550 in the as-welded state, the ERW pipe after the ERW is heat-treated at a temperature of 600 ° C or less, and the steel pipe including the welded portion is subjected to heat treatment. Is set to Hv550 or less.

[0011]

In the present invention, the steel sheet used as the material of the electric resistance welded steel pipe must satisfy the tensile strength of 1180 N / mm ² or more without being subjected to a special heat treatment while being hot-rolled. For this reason, the reasons for limiting the chemical composition of steel are as follows. C is an important element for imparting strength to steel, but the tensile strength of hot-rolled hot-rolled steel sheet is 11%.
In order to obtain 80 N / mm ² or more, 0.10% or more is necessary. On the other hand, if it exceeds 0.20%, the hardness of the welded portion becomes HV 550 or more, so that 0.10 to 0.20% And Si is an element necessary for deoxidizing steel, but 0.10% or more is required to maintain its effect.
On the other hand, if the content exceeds 0.50%, defects tend to occur in the welded portion during electric resistance welding, so the content was set to 0.10 to 0.50%.
Mn is an element effective for improving the hardenability and is inexpensive, but is required to satisfy the tensile strength of 1180 N / mm ² or more without performing a special heat treatment while hot rolling. % Or more is required. On the other hand, if it exceeds 3.0%, a defect is likely to occur in the welded portion during electric resistance welding as in the case of Si. Therefore, the content is set to 1.5 to 3.0%.

P is an element that deteriorates the toughness of the welded portion and the base material portion. If the content exceeds 0.02%, the toughness decreases.
0.02% or less. S is an element that generates MnS as a nonmetallic inclusion and deteriorates the toughness and the soundness of the welded portion. If the content exceeds 0.02%, this tendency is particularly remarkable, so S is set to 0.02% or less. Cr is an element effective for improving the hardenability at a relatively low cost. In order to satisfy the tensile strength of 1180 N / mm ² or more without performing any special heat treatment while hot rolling, 0.5% Although it is necessary,
If it exceeds 2.0%, defects are likely to occur in the welded portion during electric resistance welding as in the case of Mn.
Mo is the most effective element for improving the hardenability, but if it exceeds 1.0%, the effect is saturated and it is economically disadvantageous.

Nb is effective in preventing the increase in strength due to the precipitation strengthening of carbides and the coarsening due to the precipitates, and is effective in improving the toughness. If less than 0.015%, however, the effect is not sufficient. If the content exceeds 0.05%, the effect is saturated. V is an element effective for increasing the strength due to precipitation strengthening of carbides.
If it exceeds 1%, the effect is saturated and it is economically disadvantageous. Ti is an element effective in fixing N dissolved in steel as nitride TiN to prevent coarsening and to improve strength and toughness, but the range of N normally included inevitably is included. In order to obtain this effect, 0.015% or more is required. On the other hand, if it exceeds 0.10%, coarse nitrides are formed and strength and toughness are deteriorated.
15 to 0.10%. Ni is an element effective for strengthening the material and improving the toughness. However, if it exceeds 1.0%, it is economically disadvantageous. B is an element having a large hardenability improving effect, but if it is less than 0.0005%, its effect is not sufficient, and if it exceeds 0.0050%, toughness is deteriorated.
50%.

In the present invention, C: 0.10-0.
20%, Si: 0.10 to 0.50%, Mn: 1.5 to
3.0%, P: 0.02% or less, S: 0.02% or less,
A hot-rolled steel sheet containing 0.5 to 2.0% of Cr and 1.0% or less of Mo and the balance of Fe and inevitable impurities is hot-rolled and wound at 500 ° C or less. As a result, the strength and toughness of the hot-rolled steel sheet can be obtained, and the maximum hardness of the ERW steel pipe obtained by ERW welding can be set to Hv55 including the welded portion.
By setting it to 0 or less, it is possible to obtain a high-strength ERW pipe excellent in delayed fracture resistance and satisfying a tensile strength of 1180 N / mm ² .

In the present invention, C: 0.10
-0.20%, Si: 0.10-0.50%, Mn:
1.5-3.0%, P: 0.02% or less, S: 0.02
% Or less, Cr: 0.5 to 2.0% and Mo: 1.0%
Containing the following, further Nb: 0.015 to 0.05%,
Ti: 0.015 to 0.10%, V: 0.1% or less, N
i: 1.0% or less, B: 0.0005 to 0.0050%
By using a hot-rolled steel sheet containing at least one of the following and hot-rolling a steel consisting of Fe and unavoidable impurities and winding the steel at 500 ° C. or less, the strength and toughness of the hot-rolled steel sheet are reduced. The obtained ERW steel pipe obtained by ERW welding has a maximum hardness of Hv 550 or less including the welded portion, so that it has excellent delayed fracture resistance and a tensile strength of 1180 N / mm.
A high-strength ERW steel pipe that satisfies ² can be obtained.

In the present invention, when the above-mentioned upper limit of the chemical components is used, the case where the maximum hardness of the main body portion and the welded portion does not satisfy Hv550 or less may occur when the electric resistance welding is performed.
On the other hand, the ERW steel pipe after the ERW welding is heat-treated at a temperature of 600 ° C. or less, and the maximum hardness of the steel pipe including the welded portion is set to H.
v550 or less, the tensile strength is 1180 N
/ Mm ² or more, and the delayed fracture resistance can be improved.

In the present invention, the reason why the winding temperature in the hot rolling is set to 500 ° C. or lower is that the chemical composition of the hot-rolled steel sheet is changed from the viewpoint of improving the delayed fracture resistance, especially the C content to 0.10 to 0.10.
Since it is held at a low value of 0.20%, the target tensile strength of 1180 N / mm ² cannot be satisfied in a normal hot-rolling finish. did. Further, the reason why the maximum hardness of the ERW steel pipe obtained by ERW welding is set to Hv550 or less including the welded portion is that delayed fracture has a correlation with strength, and the higher the strength, the higher the delayed fracture sensitivity. Therefore, the base metal part under hydrochloric acid environment,
As a result of the hardness test of the welded portion and the delayed fracture test, it was found that delayed fracture would occur if the hardness of both the welded portion and the main body portion was not less than Hv550, so the maximum hardness of the steel pipe including the welded portion was obtained. Was set to Hv550 or less. Further, when a hot-rolled steel sheet satisfying the above-mentioned component characteristics is subjected to electric resistance welding, the maximum hardness of the base material and the welded portion is Hv5,
Not satisfying 50 or less. The reason for setting the heat treatment temperature to 600 ° C. or lower is that if the temperature exceeds 600 ° C., the target tensile strength of 1180 N / mm ² or more cannot be obtained. In addition, when Hv550 or less is not satisfied only in the welded portion, it is naturally considered that only the welded portion is heat-treated.

[0018]

EXAMPLES Example 1 Inventive steels of the steel types 1 to 13 shown in Table 1 and comparative steels of the steel types 14 to 23 shown in Table 2 were melted in a converter, and as shown in FIG. None, each of these slabs was soaked at 1250 ° C., subjected to hot rolling at 850 ° C., and then wound under the winding conditions shown in Tables 3 and 4 to obtain a hot-rolled coil having a thickness of 2.0 mm. I got These and electric resistance welding after each hot-rolled coil was scan liter coating was outer diameter 31.8mm, thickness 2.0 mm, the electric resistance welded steel pipe of length 10000mm cut. Some of the ERW steel pipes were subjected to tempering heat treatment of the product in a bright annealing furnace. For each of the ERW steel pipes obtained as described above, a tensile test and a Vickers hardness test of the welded portion and the base material portion were performed to determine the maximum hardness of the welded portion and the base material portion. Next, a three-point bending test was carried out to check for breakage. Furthermore, the occurrence of delayed fracture of each ERW pipe was investigated. The results are shown in Tables 3 and 4. The maximum hardness of the welded portion and the base material was determined according to the Vickers hardness test method specified in JIS Z 2244. In the three-point bending test, as shown in FIG. 2, a bending load P was applied with a span L: 800 mm, a stroke S: 150 mm, and a bending radius R: 152 mm, and the presence or absence of breakage was inspected. The presence or absence of the delayed fracture was determined by immersing a steel tube having a length of 500 mm in a 1.0 N HCl solution for 1200 hours, and visually observing the presence and absence of cracks in the penetration test for the entire surface and the entire length.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

As shown in Tables 3 and 4, steel types 1-1
No. 3 of the present invention steel had a target tensile strength of 1180 N / m.
m ² or more, there was no breakage by three-point bending, and no delayed fracture occurred at all. On the other hand, C, Mn, C
As for the comparative steels of steel types 14, 16, and 18 in which r is lower than the lower limit, the target tensile strength of 1180 N / mm ² or more cannot be obtained, the steel type 21 whose winding temperature is outside the upper limit, and the heat treatment temperature Similarly, for the steel type 22 having a value exceeding the upper limit, the target tensile strength of 1180 N / mm ² or more could not be obtained. In addition, steel type 1 in which C, Mn, and Cr exceed the upper limit
5, 17, 19 and 20 have a maximum hardness of Hv 55
0 or more, and delayed fracture occurred in each case. In the case of steel type 23 whose C, Mn, and Cr values are close to the upper limit, the maximum hardness of the welded portion is Hv510, which satisfies the target, but the maximum hardness of the base material portion is Hv580, which is high. Delayed fracture occurred in the material section. These results are summarized and shown in FIG. As shown in FIG. 3, when the maximum hardness of the base material portion and the welded portion is Hv550 or less, it is clear that no delayed fracture occurs in any case.

[0024]

As described above, according to the method of the present invention, a steel pipe having a high tensile strength of 1180 N / mm ² or more, excellent bending properties, and excellent delayed fracture resistance can be easily and easily obtained. It can be manufactured efficiently and at low cost.

[Brief description of the drawings]

FIG. 1 is a system diagram of a manufacturing process of an electric resistance welded steel pipe in an embodiment.

FIG. 2 is an explanatory diagram of a three-point bending test method in an example.

FIG. 3 is a graph showing a relationship among a base material maximum hardness (Hv), a weld maximum hardness (Hv), and the presence or absence of delayed fracture in Examples.

FIG. 4 is a system diagram showing an example of a manufacturing process of a conventional ERW steel pipe.

[Explanation of symbols]

 L span S stroke R bending radius P bending load

Claims (3)

(57) [Claims] 1. C: 0.10 to 0.20%, Si: 0.
10 to 0.50%, Mn: 1.5 to 3.0%, P: 0.
02% or less, S: 0.02% or less, Cr: 0.5-2.
0% and Mo: 1.0% or less, with the balance being Fe and unavoidable impurities, hot rolled to 500 ° C.
A high-strength electrode excellent in delayed fracture resistance characterized in that the maximum hardness of an ERW steel pipe obtained by ERW welding using a hot-rolled steel sheet wound below is Hv550 or less including a welded portion. Sewing pipe manufacturing method. 2. C: 0.10 to 0.20%, Si: 0.
10 to 0.50%, Mn: 1.5 to 3.0%, P: 0.
02% or less, S: 0.02% or less, Cr: 0.5-2.
0% and Mo: contains 1.0% or less, and et al in Nb:
0.015 to 0.05%, Ti: 0.015 to 0.10
%, V: 0.1% or less, Ni: 1.0% or less, B: 0.
A hot-rolled steel sheet containing at least one of 0005 to 0.0050%, the balance being Fe and unavoidable impurities, hot rolled and rolled at 500 ° C. or lower,
A method for producing a high-strength ERW pipe excellent in delayed fracture resistance, characterized in that the maximum hardness of an ERW pipe obtained by ERW welding is Hv550 or less including a welded portion. 3. An ERW steel pipe after ERW welding is heat-treated at a temperature of 600 ° C. or less, and the maximum hardness of the steel pipe including the welded part is Hv.
3. The method for producing a high-strength electric resistance welded steel pipe having excellent delayed fracture resistance according to claim 1 or 2, wherein the resistance is set to 550 or less.