JP3383156B2 - Manufacturing method of steel sheet for high strength electric resistance welded steel pipe - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In recent years, stricter regulations on overloading of truck transportation have made it possible to hollow out automobile (truck) parts for the purpose of weight reduction, and to further reduce the weight of parts materials. It has become possible to achieve high strength. Therefore, in Japanese Unexamined Patent Publication No. 2-156021, C,
Disclosed is a method for producing a steel sheet having high tensile strength and excellent low temperature toughness by rolling steel containing Si, Mn, Ti, B and the like under specific conditions.

[0003]

As methods for obtaining high strength, methods such as precipitation strengthening and transformation strengthening have been used, but each has the following drawbacks. The properties (strength, toughness) of TiC steel vary greatly depending on the chemical composition of the base metal and the manufacturing process conditions in hot rolling. TiC steel is α
The strength is secured by maximally utilizing the precipitation of fine Ti carbonitrides in the region. Therefore, if the rolling end temperature and the winding temperature in hot rolling vary greatly, the strength also varies greatly and it is difficult to obtain a desired steel sheet. Transformation strengthening is to obtain a quenched structure by quenching from the γ region or γ + α region, or to cool a high Mn steel to obtain a quenched structure to obtain high strength. Is extremely poor, and since a large amount of alloying elements are added, the manufacturing cost is high and the economy is poor. The present invention is an inexpensive solution to these problems in the past.
The present invention provides a steel plate for ERW steel pipe having a high strength of 80 N / mm ² .

[0004]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
, C: 0.12-0.25, Si: 0.6 or less, Mn: 1.0-2.0, P: 0.03 or less, S: 0.01 or less, Cr: 0.1-1 0.0 , Ti: 0.04 to 0.12, Al: 0.05 or less, N: 0.006 or less, O: 0.005 or less, and if necessary, Ni: 0.1 to 1.0, Cu. : 0.1~1.0, Mo: 0.1~1.0, V : 0.01~0.10, Nb: 0.07 ~0.1, Ca: of the 0.001 to 0.005 After heating a steel slab containing one kind or two or more kinds and the balance consisting of iron and unavoidable impurities to 1100 to 1250 ° C, 850 to 950 ° C
After finishing the rolling in the temperature range of, the material is cooled at a cooling rate of 5 to 30 ° C./sec and wound into a coil at 550 to 650 ° C.

The method for producing an electric resistance welded steel pipe having strength and high toughness of 780 N / mm ² or more according to the present invention will be described in detail below. The characteristics of the steel of the present invention are (1) reduction of alloying elements,
i Strengthening at low cost by positively utilizing precipitation strengthening by precipitation of carbonitride, and (2) Higher C to secure fatigue strength of HAZ softened part of friction welding part. In order to manufacture high-strength steel at low cost, it is necessary to minimize addition of other expensive alloy elements, and therefore it is necessary to finely precipitate Ti carbonitride to secure strength. For this reason, the precipitation behavior of Ti carbonitrides is sensitive to the chemical composition of steel (balance between C and Ti) and the manufacturing process conditions, so optimization of these is important. Generally, fatigue strength has a good correlation with hardness, and fatigue strength improves as hardness increases. From this, in order to increase the fatigue strength of the HAZ softened part of the friction welding part, it is effective to increase C effective for increasing the hardness.

The reasons for limiting the chemical components and manufacturing 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 strength,
In the TiC-based precipitation strengthening type, when C is high, fine Ti carbonitrides acting as precipitation strengthening are coarsened and the fine Ti carbonitrides contributing to the increase in strength are reduced and the strength is lowered. 780N
The upper limit of C is 0.25 to secure the strength of 1 / mm ² class.
%. The reduction of C makes it difficult to strengthen the precipitation of Ti carbonitride and to secure the strength of the base material. Also, the HAZ of friction welding parts
The lower limit is 0.12% to secure the fatigue strength of the softened part.
And

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

The lower limit of the heating temperature is 1100 in order to sufficiently dissolve the carbonitride of Ti and to secure the rolling end temperature.
℃ was made. However, if the heating temperature exceeds 1250 ° C, γ
The upper limit of the heating temperature was set to 1250 ° C because excellent low temperature toughness cannot be obtained because grains are significantly coarsened and cannot be completely refined by rolling. The rolling end temperature is set to 8 in order to promote fine precipitation of Ti carbonitride after rolling and refinement of crystal grains.
It 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 set to 950 ° C.

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

Next, the reasons for limiting the other constituent elements of the present invention will be explained. Si is an element contained in the deoxidized upper steel, and the toughness of the steel sheet deteriorates when the Si content increases, so the upper limit was made 0.6%. Mn is an essential element for ensuring the strength and toughness of steel, and its lower limit is 1.0%. However, if Mn is too much, not only the cost becomes high, but also the hardenability increases and the toughness deteriorates. Therefore, the upper limit of Mn is set to 2.0%. From the viewpoint of improving weldability and workability, P must be thoroughly reduced, and the upper limit is 0.03.
%.

It is necessary to thoroughly reduce S. In the component system of the present invention, since Mn is added and it is necessary to prevent MnS from being generated in order to effectively use Mn, the upper limit was made 0.01%. Al is necessary for deoxidation, but if added in excess, the amount of deoxidation products centering on Al ₂ O ₃ remains in the steel increases. In particular, when used in an electric resistance welded steel pipe as in the case of the present invention, since a huge inclusion in the welded portion becomes a fatal defect, its upper limit is set to 0.
It was set to 05%.

N forms TiN and improves the base metal 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 large, it causes deterioration of the HAZ toughness due to solid solution N, so the upper limit is 0.
It is necessary to suppress it to 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 composition is to increase the manufacturable plate thickness and improve the properties such as the strength of the base material without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself.

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 field weldability. Compared to the addition of Mn, Cr, or Mo, the addition of Ni rarely forms a hardened structure detrimental to the low temperature toughness in the rolled structure (especially the central segregation zone of the slab) and increases the strength. To exert this effect, 0.1%
The above additions are necessary. However, if the addition amount is too large, the economical efficiency is deteriorated, so the upper limit was made 1.0%.

Cu has substantially the same effect as Ni. In order to exert this effect, it is necessary to add 0.1% or more. However, if added excessively, toughness decreases and Cu cracks occur during hot rolling, so the upper limit was made 1.0%. Cr has the effect of increasing the strength of the base material and the welded portion, and in order to exert this effect, addition of 0.1% or more is necessary. However, if it is too large, the toughness is significantly deteriorated. Therefore, 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 welded portion. To obtain this effect, M
o must be at least 0.1%. However, excessive addition of Mo 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 0.07%
Is. If the amount of Nb 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 exert this effect, addition of 0.01% or more is necessary. The upper limit is 0.10% in terms of weldability and toughness. Ca controls the morphology of sulfide (MnS) and improves the characteristics of welds and low temperature toughness. But C
If the content of a is 0.001% or less, there is no practical effect, and if it is added in excess of 0.005%, a large amount of CaO-CaS is formed to form clusters and large inclusions, which not only impairs the cleanliness of steel. It also adversely affects the weldability. Therefore, the amount of Ca added is 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 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 . On the other hand, the comparative steels do not have the appropriate chemical composition or hot rolling conditions and cannot obtain the desired strength. Steel L is too high in strength because the amount of C is too small. Steel M has a low strength because it contains too much C. Steel N has a low strength because the Ti content is too small. Steel O has too high a Ti content, so its strength is too high. Steel P
The chemical composition is appropriate, but the rolling finish temperature during the manufacturing process conditions in hot rolling is too high, resulting in coarsening of crystal grains and low strength. Steel Q has a too low rolling end temperature, so that the crystal grains are made fine, but the strength is low because the fine Ti carbonitride effective for strength is reduced. Steel R has a too high coiling temperature, so Ti
Carbonitride is coarsened and its strength is low. Steel S has a low winding temperature and thus has low strength.

[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.

Continuation of the front page (56) Reference JP-A-6-10046 (JP, A) JP-A-4-202711 (JP, A) JP-A-62-89813 (JP, A) JP-A-53-88620 (JP , A) JP-A-59-1632 (JP, A) JP-B-55-45614 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/46-9/48 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. 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, Cr in weight%. : 0.1 to 1.0, Ti: 0.04 to 0.12, Al: 0.05 or less, N: 0.006 or less, O: 0.005 or less, and the balance iron and inevitable impurities. After heating the steel slab consisting of 1 to 1250 to 1250 ° C, after finishing rolling in the temperature range of 850 to 950 ° C, it is cooled at a cooling rate of 5 to 30 ° C / sec and wound into a coil at 550 to 650 ° C. A method for producing a steel plate for high strength ERW steel pipe, which is characterized in that 2. 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, Cr in weight%. : 0.1 to 1.0, Ti: 0.04 to 0.12, Al: 0.05 or less, N: 0.006 or less, O: 0.005 or less, and further Ni: 0.1 to 1. 0, Cu: 0.1 to 1.0, Mo: 0.1 to 1.0, V: 0.01 to 0.10, Nb: 0.07 to 0.1, Ca: 0.001 to 0. After heating a steel slab containing one or more of 005 and the balance iron and unavoidable impurities to 1100 to 1250 ° C., then 850 to 950 ° C.
The method for producing a steel sheet for high-strength electric resistance welded steel pipe, which comprises cooling at a cooling rate of 5 to 30 ° C./sec and winding the material into a coil at 550 to 650 ° C.