JPH1017930A - Production of high strength electric resistance welded tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet for an electric resistance welded tube having high strength and high toughness by subjecting a slab obtd. by adding Ti to a C-Si-Mn low alloy steel to rolling, cooling and coiling at specified temps. SOLUTION: The compsn. of a steel is composed of, by weight, 0.12 to 0.25% C, <=0.6% Si, 1.0 to 2.0% Mn, <=0.03% P, <=0.01% S, 0.04 to 0.12% Ti, <=0.05% Al, <=0.006% N, <=0.005% O, and the balance Fe with inevitable impurities. This slab is heated at 1100 to 1250 deg.C, and after that, rolling is finished in the temp. range of 850 to 950 deg.C. Next, it is cooled at a cooling rate ofo 5 to 30 deg.C/sec and is thereafter coiled at 550 to 650 deg.C into a coiled shape. Ti in the steel sheet compsn. finely disperses into the steel as carbon nitride and is effective for increasing its strength as a precipitation strengthening type. Furthermore, one or >= two kinds among Ni, Cu, Cr, Mo, V, Nb and Ca may be incorporated into the steel compsn. according to necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a 780 N / mm ²
The present invention relates to a method for producing an electric resistance welded steel pipe having the above strength and high toughness.

[0002]

2. Description of the Related Art In recent years, overloading regulations for trucking have been strengthened, so that automobile (truck) parts can be made hollow for the purpose of weight reduction, and in order to further reduce the weight of parts materials. Higher strength can be achieved. Therefore, Japanese Patent Application Laid-Open No. 2-15621 discloses C,
A method for producing a steel sheet having high tensile strength and excellent low-temperature toughness by rolling a steel containing Si, Mn, Ti, B, and the like under specific conditions is disclosed.

[0003]

As methods for obtaining high strength, methods such as precipitation strengthening and transformation strengthening have been used, but each has the following disadvantages. The properties (strength and toughness) of the TiC-based steel significantly change depending on the chemical composition of the base material and the manufacturing process conditions in hot rolling. TiC steel is α
The strength is ensured by making the most of the precipitation of fine Ti carbonitride in the region. Therefore, if the temperature at the end of rolling in hot rolling and the temperature at the coiling temperature fluctuate greatly, the fluctuation in strength is also large, making it difficult to obtain a desired steel sheet. Transformation strengthening is to obtain a quenched structure by quenching from the γ range or γ + α range or to obtain a quenched structure by cooling high Mn steel to obtain high strength. And the production cost is high due to the addition of a large amount of alloying elements, resulting in poor economic efficiency. The present invention is an inexpensive 7 that solves the conventional problems.
An object of the present invention is to provide a steel sheet for an electric resistance welded steel pipe having a high strength of 80 N / mm ² class.

[0004]

Means for Solving the Problems The gist of the present invention is that the weight%
And C: 0.12-0.25, Si: 0.6 or less, M
n: 1.0 to 2.0, P: 0.03 or less, S: 0.
01 or less, Ti: 0.04 to 0.12, Al: 0.05
Hereinafter, N: 0.006 or less, O: 0.005 or less, Ni: 0.1 to 1.0, and Cu: 0.
1 to 1.0, Cr: 0.1 to 1.0, Mo: 0.1 to
1.0, V: 0.01 to 0.10, Nb: 0.003
０．１0.1, Ca: One or more of 0.001 to 0.005, and the balance is heated to 1100 to 1250 ° C. after heating a steel slab consisting of iron and unavoidable impurities to 850
After finishing the rolling in the temperature range of ~ 950 ° C, 5 ~ 30 ° C
Cooling at a cooling rate of / sec and winding up at 550 to 650 ° C in a coil shape.

[0005] Hereinafter, a method of manufacturing an electric resistance welded steel pipe having a strength of 780 N / mm ² or more and a high toughness of the present invention will be described in detail. The features of the steel according to the present invention are as follows:
(i) Higher C in order to secure the fatigue strength of the HAZ softened portion of the friction welded portion by actively utilizing precipitation strengthening by i carbonitride precipitation. In order to manufacture high-strength steel at low cost, it is necessary to minimize the addition of other expensive alloying elements as much as possible. Therefore, it is necessary to secure the strength by finely precipitating Ti carbonitride. For this reason, the precipitation behavior of Ti carbonitride is sensitive to the chemical composition of steel (the balance between C and Ti) and the manufacturing process conditions, so that their optimization is important. Generally, the fatigue strength has a good correlation with the hardness, and the fatigue strength improves as the hardness increases. From this, it is effective to increase the amount of C effective for increasing the hardness in order to increase the fatigue strength of the HAZ softened portion of the friction welding portion.

The reasons for limiting the chemical components and production conditions in the present invention will be described. First, the reasons for limiting the chemical components will be described. C is an element effective for increasing the strength,
In a TiC-based precipitation-strengthening type, when C is high, fine Ti carbonitrides acting as precipitation strengthening are coarsened, and fine Ti carbonitrides contributing to an increase in strength are reduced and strength is reduced. 780N
/ Mm upper limit of 0.25 in order to secure class ² strength
%. Reduction of C makes it difficult to strengthen precipitation by Ti carbonitride and to secure the strength of the base material. Also, the HAZ of the friction welding part
The lower limit is 0.12% to secure the fatigue strength of the softened part
And

Ti is finely dispersed in steel as a carbonitride and is effective in increasing the strength as a precipitation strengthened type. To obtain precipitation strengthening, 0.04% or more of Ti is required.
However, excessive addition leads to deterioration of low-temperature toughness and weldability due to coarsening of carbonitride. Therefore, the upper limit is set to 0.12%. In the above TiC steel,
In order to ensure stable strength, further manufacturing conditions must be appropriate. Next, the reasons for limiting the manufacturing conditions of the heating temperature, the rolling end temperature, and the winding temperature will be described.

The lower limit of the heating temperature is 1100 in order to sufficiently dissolve the carbonitride of Ti and secure the rolling end temperature.
° C. However, when the heating temperature exceeds 1250 ° C., γ
The upper limit of the heating temperature was set to 1250 ° C., since the grains were remarkably coarsened and could not be completely refined even by rolling, so that excellent low-temperature toughness could not be obtained. The rolling end temperature is set at 8 in order to promote fine precipitation of Ti carbonitride and refinement of crystal grains after rolling.
The temperature was set to 50 ° C. or higher. When the rolling end temperature exceeds 950 ° C., the crystal grains become coarse and the strength of the steel sheet decreases, so the upper limit of the finishing temperature was 950 ° C.

[0009] The cooling from the end of rolling to the winding is 5 to 3 in order to suppress precipitation of Ti carbonitride during this time.
The cooling rate was 0 ° C./sec. If the winding temperature is less than 550 ° C, precipitation of fine carbonitrides effective for strength is not sufficient, and if it exceeds 650 ° C, Ti carbonitrides are coarsened and fine carbonitrides effective for direct strength are reduced. Therefore, a desired steel plate cannot be obtained. Therefore, the winding temperature is set to 550
650 ° C. With the above components and hot rolling conditions, it is possible to stably produce a steel sheet for electric resistance welded steel pipe having a tensile strength of 780 N / mm ² class.

Next, the reasons for limiting the other component elements of the present invention will be described. Si is an element contained in the deoxidized upper steel, and if the amount of Si increases, the toughness of the steel sheet deteriorates. Therefore, the upper limit is set to 0.6%. Mn is an indispensable element for securing the strength and toughness of steel, and the lower limit is 1.0%. However, if the amount of Mn is too large, not only becomes expensive, but also the hardenability increases and the toughness deteriorates. Therefore, the upper limit of Mn is set to 2.0%. P must be reduced thoroughly from the viewpoint of improving weldability and workability, and the upper limit is 0.03.
%.

S needs to be reduced thoroughly. In the component system of the present invention, Mn is added, and it is necessary to prevent MnS from being generated in order to effectively use Mn. Therefore, the upper limit is set to 0.01%. Al is necessary for deoxidation, but when added in excess, the amount of deoxidized products centering on Al ₂ O ₃ remaining in the steel increases. In particular, when used in an electric resistance welded steel pipe as in the case of the present invention, a large inclusion at the welded portion becomes a fatal defect, so the upper limit is set to 0.
05%.

N forms TiN and improves the base material and HAZ toughness through the effect of suppressing the coarsening of γ grains. The minute amount for this is 0.002%. However, if it is too much, it causes the HAZ toughness to deteriorate due to solid solution N.
006% or less. Next, Ni, Cu,
The reason for adding Cr, Mo, V, and Ca will be described. The main purpose of adding these elements to the basic components is to increase the plate thickness that can be produced and to improve properties such as the strength of the base material without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is of a nature that should be restricted.

The purpose of adding Ni is to improve the strength of the low carbon steel of the present invention without deteriorating the low-temperature toughness and the on-site weldability. The addition of Ni is less likely to form a hardened structure that is harmful to low-temperature toughness in the rolled structure (especially the central segregation zone of the slab) than the addition of Mn, Cr, and Mo, and increases the strength. To achieve this effect, 0.1%
The above addition is necessary. However, if the added amount is too large, the economic efficiency is degraded, and the upper limit is set to 1.0%.

Cu has almost the same effect as Ni. In order to exhibit this effect, it is necessary to add 0.1% or more. However, if added excessively, the toughness is reduced and Cu cracks occur during hot rolling, so the upper limit was set to 1.0%. Cr has the effect of increasing the strength of the base material and the welded portion, and in order to exhibit this effect, it is necessary to add 0.1% or more. However, if it is too much, the toughness is significantly deteriorated. For this reason, the upper limit of the amount of Cr is 1.0%.

The reason for adding Mo is that it has the effect of increasing the strength of the base material and the weld. To achieve this effect, M
o must be at least 0.1%. However, excessive Mo addition deteriorates toughness and weldability, so the upper limit was made 1.0%. Nb is an element that forms fine precipitates and increases strength. To obtain this effect, the lower limit is 0.003
%. If the Nb content is too large, the weldability deteriorates and the toughness further deteriorates, so the upper limit was made 0.1%.

V has almost the same effect as Nb. In order to exhibit this effect, 0.01% or more must be added. The upper limit is allowable up to 0.10% from the viewpoint of weldability and toughness. Ca controls the form of sulfide (MnS) and improves the properties and low-temperature toughness of the weld. But C
If the a content is 0.001% or less, there is no practical effect, and if it exceeds 0.005%, CaO—CaS is generated in large amounts to form clusters and large inclusions, which not only impairs the cleanliness of the steel but also , Also has an adverse effect on weldability. For this reason, the amount of Ca added was limited to 0.001 to 0.005%.

[0017]

EXAMPLES Table 1 shows the chemical composition, hot rolling conditions and mechanical properties of the steel of the present invention. Table 2 shows the chemical composition, hot rolling conditions, and mechanical properties of the conventional steel. As is clear from Table 1, the steel sheet manufactured according to the steel of the present invention has a strength of 780 N / mm ² class. On the other hand, the comparative steel is not suitable for the chemical composition or the hot rolling conditions, and cannot obtain the desired strength. Steel L has too high a C content, so that the strength is too high. Steel M has a low strength because the amount of C is too large. Steel N has low strength because the amount of Ti is too small. Steel O has an excessively high Ti content, so that its strength is too high. Steel P
Although the chemical components are appropriate, the rolling end temperature during the manufacturing process conditions in hot rolling is too high, so that the crystal grains become coarse and the strength is low. Since the rolling end temperature of the steel Q is too low, the crystal grains are refined, but the strength is low due to a decrease in fine Ti carbonitride, which is effective for strength. Steel R has too high winding temperature,
Carbonitride coarsens and has low strength. Steel S has a low strength because the winding temperature is too low.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

According to the present invention, an inexpensive electric resistance welded steel pipe having a high strength of 780 N / mm ² class can be stably manufactured.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location C22C 38/00 301 C22C 38/00 301A 301B 301Z 38/14 38/14

Claims (2)

[Claims] C .: 0.12 to 0.25, Si: 0.6 or less, Mn: 1.0 to 2.0, P: 0.03 or less, S: 0.01 or less, Ti by weight% : 0.04 to 0.12, Al: 0.05 or less, N: 0.006 or less, O: 0.005 or less, the balance being iron and unavoidable impurities. After heating, after finishing rolling in the temperature range of 850 to 950 ° C,
Cool at a cooling rate of 5-30 ° C./sec.
A method for producing a steel plate for a high-strength ERW steel pipe, characterized in that the steel sheet is wound up in a coil shape at a temperature. 2. In% by weight, C: 0.12 to 0.25, Si: 0.6 or less, Mn: 1.0 to 2.0, P: 0.03 or less, S: 0.01 or less, Ti: : 0.04 to 0.12, Al: 0.05 or less, N: 0.006 or less, O: 0.005 or less, and Ni: 0.1 to 1.
0, Cu: 0.1 to 1.0, Cr: 0.1 to 1.0, Mo: 0.1 to 1.0, V: 0.01 to 0.10, Nb: 0.003 to 0. 1, Ca: One or two or more of 0.001 to 0.005, the balance being iron and slabs composed of unavoidable impurities are heated to 1100 to 1250 ° C, and then 850 to 950 ° C.
A method for producing a steel plate for a high-strength electric resistance welded steel pipe, wherein the steel is cooled at a cooling rate of 5 to 30 ° C./sec and rolled up at 550 to 650 ° C.