KR102109269B1 - Hot-rolled steel sheet for steel pipe and manufacturing method for the same - Google Patents

Abstract

The hot-rolled steel sheet for a steel pipe according to one preferred aspect of the present invention is in weight%, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% or less, remainder It contains Fe and unavoidable impurities, contains at least 92 area% of polygonal ferrite as a microstructure, and the fraction of mixed tissue at the point t / 4 (t: steel sheet thickness) along the thickness direction from the upper and lower surfaces is 1 area%. It may be:

Description

Hot-rolled steel sheet for steel pipe and manufacturing method for the same

The present invention relates to a hot-rolled steel sheet for a steel pipe and a method for manufacturing the same, in detail, it is possible to effectively prevent processing defects due to non-uniform stretching, and to a hot-rolled steel sheet for a steel pipe having properties suitable for manufacturing steel pipes for various uses and a method for manufacturing the same It is about.

The steel pipe is provided in a rod shape and means a steel product having a space formed therein along the longitudinal direction, and can be divided into structural, piping, oil well, oil pipeline, and steel pipes according to the purpose of the steel pipe. The physical properties required for steel pipes are different for each application, and industrial standards (KS, JIS, ASTM, etc.) of each country are also defined by classifying the component system and physical properties of the steel pipes according to the application of the steel pipes. Therefore, a steel pipe manufacturer is required to manufacture a steel pipe provided with a component system and physical properties suitable for various applications and supply it to a customer.

Generally, a steel pipe manufacturer manufactures a steel pipe by supplying a steel plate from a steel plate supplier, but manufactures a steel pipe by processing steel plates having different component systems and properties according to the use of the steel pipe. That is, since the types of steel sheets used for the production of steel pipes differ depending on the use of the steel pipes, there may be a problem of insufficient or excessive supply of steel plates for manufacturing steel pipes for a specific use depending on the market conditions of steel pipes and the supply and demand conditions of steel plates. .

Republic of Korea Patent Publication No. 10-2017-0074285 (2017.06.30. Public)

According to one aspect of the present invention, a hot rolled steel sheet for a steel pipe and a manufacturing method thereof can be provided.

The subject of this invention is not limited to the above-mentioned content. Those skilled in the art will have no difficulty in understanding the additional subject matter of the present invention from the general contents of this specification.

The hot-rolled steel sheet for a steel pipe according to one preferred aspect of the present invention is in weight%, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% or less, remainder It contains Fe and unavoidable impurities, contains at least 92 area% of polygonal ferrite as a microstructure, and the fraction of mixed tissue at the point t / 4 (t: steel sheet thickness) along the thickness direction from the upper and lower surfaces is 1 area%. It may be:

The mixed structure may be a structure in which polygonal ferrite and austenite are mixed.

The hot rolled steel sheet may be provided with a tensile strength of 340 ~ 490MPa and a yield strength of 210 ~ 370MPa.

Method for manufacturing a hot-rolled steel sheet for a steel pipe according to a preferred aspect of the present invention, in weight percent, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% Hereinafter, the slab containing the residual Fe and unavoidable impurities is heated to extract in a temperature range of 1140 to 1180 ° C, and the extracted slab is roughly rolled in a temperature range of 980 to 1000 ° C to provide a rough rolling bar, and the rough rolling The bar can be hot rolled and cooled in a temperature range of 840 to 880 ° C based on the exit temperature to provide a hot rolled steel sheet.

The hot-rolled hot-rolled steel sheet may be cooled to a temperature of 600 ° C. or less at a cooling rate of 10 to 15 ° C./s.

The solving means of the above problems does not list all the features of the present invention, and various features of the present invention and the advantages and effects thereof may be understood in more detail with reference to specific embodiments below.

The hot-rolled steel sheet for a steel pipe according to a preferred aspect of the present invention actively suppresses the formation of mixed structures in the surface layer portion, and thus can effectively prevent processing defects due to non-uniform stretching during steel pipe processing in a steel pipe processing company.

The hot-rolled steel sheet for a steel pipe according to one preferred aspect of the present invention has a component system and physical properties applicable to the production of steel pipes for various uses, and thus, it is possible to effectively solve the problem of lack of a specific steel plate or a specific steel plate inventory in a steel pipe manufacturer.

1 is a microstructure photograph of a cross section of the upper surface layer of Inventive Example 1.
2 is a microstructure photograph of a cross section of the upper surface layer of Comparative Example 1.

The present invention relates to a hot-rolled steel sheet for a steel pipe and a method of manufacturing the same, hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to those skilled in the art to further detail the present invention.

The hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention, by weight, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% or less, It contains the remaining Fe and unavoidable impurities, contains at least 92 area% of polygonal ferrite as a microstructure, and the mixed tissue fraction at the 4 / t (t: steel sheet thickness) point along the thickness direction from the upper and lower surfaces is 1 area. %.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% representing the content of each element is based on weight.

C: 0.05 to 0.12%

Carbon (C) is an element that affects the strength and toughness of the steel, and the present invention can include 0.05% or more of C to secure the desired strength. However, when the content of C is excessive, the weldability and the toughness of the weld zone may be degraded, so the present invention may limit the upper limit of the C content to 0.12%. Therefore, the C content of the present invention may be 0.05 to 0.12%.

Si: 0.03% or less

Silicon (Si) is an element necessary for deoxidation, and is an effective element for improving the strength of steel materials by strengthening solid solution. However, when the content of Si is excessive, hot rolling and weldability may be deteriorated, so the upper limit of the Si content of the present invention may be limited to 0.03%.

Mn: 0.3-0.6%

Manganese (Mn) is an element that contributes to increasing the strength of steel as a solid solution strengthening element, and the present invention may include 0.3% or more Mn to secure a desired strength. However, if the Mn content is excessive, there is a high possibility of segregation zone, so the present invention can limit the upper limit of the Mn content to 0.6%. Therefore, the Mn content of the present invention may be 0.3 to 0.6%.

P: 0.02% or less

Phosphorus (P) is a solid solution strengthening element, but when a large amount of P is added, weldability may decrease, and the risk of brittleness of steel may increase. Therefore, the present invention can limit the upper limit of the P content to 0.02%.

S: 0.015% or less,

Sulfur (S) is an impurity element inevitably contained in steel and is an element that inhibits the ductility and weldability of steel. Therefore, the present invention can limit the upper limit of the S content to 0.015%.

The remaining component of the invention is Fe. However, unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, and thus cannot be entirely excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

Hereinafter, the microstructure of the present invention will be described in more detail.

The hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention may include polygonal ferrite as a microstructure. The fraction of polygonal ferrite among the microstructures may be 92% by area or more, and the residual structure may include ecicular ferrite.

In addition, the hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention can be actively suppressed the formation of mixed tissue in the surface layer portion. That is, when the total thickness of the hot-rolled steel sheet is t, the fraction of the mixed tissue from the upper and lower surfaces of the hot-rolled steel sheet to the point t / 4 along the thickness direction can be actively suppressed to 1 area% or less. It may be more preferable that no mixed tissue exists from the upper and lower surfaces of the hot rolled steel sheet to the point t / 4 along the thickness direction. This is because if a mixed structure exceeding a certain fraction exists in the surface layer portion of the hot-rolled steel sheet, non-uniform stretching may occur during steel pipe processing, resulting in defective processing.

The mixed structure may be a structure in which a polygonal ferrite structure and an austenite structure are mixed, and may mean a structure in which crystal grains having a particle size difference of 3 or more compared to an average particle size per unit area exist in a proportion of 20 area% or more.

The hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention actively inhibits formation of mixed structures in the surface layer portion, and thus can effectively prevent processing defects due to non-uniform stretching during steel pipe processing at a steel pipe processing company.

The hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention satisfies a tensile strength of 340 to 490 MPa and a yield strength of 210 to 370 MPa, and thus can be comprehensively applied to the production of steel pipes applied to various applications.

Hereinafter, the manufacturing method of the present invention will be described in more detail.

Method of manufacturing a hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention, by weight, C: 0.05 ~ 0.12%, Si: 0.03% or less, Mn: 0.3 ~ 0.6%, P: 0.02% or less, S: 0.015 % Or less, heating the slab containing the residual Fe and unavoidable impurities in the temperature range of 1140 ~ 1180 ° C, and roughly rolling the extracted slab in the temperature range of 980 ~ 1000 ° C to provide a crude rolling bar. The hot rolled steel sheet can be provided by hot rolling and cooling the rolling bar in a temperature range of 840 to 880 ° C based on the exit temperature.

Slab heating

Since the slab composition of the present invention corresponds to the composition of the hot-rolled steel sheet for the steel pipe described above, the description of the slab composition is replaced by the description of the composition of the hot-rolled steel sheet for the steel pipe described above.

Slab heating is a process of heating the steel so that the subsequent rolling process can be performed smoothly and the physical properties of the target steel sheet can be sufficiently secured. Therefore, the heating process should be performed within a suitable temperature range according to the purpose. However, when the slab extraction temperature exceeds 1180 ° C, the initial grains are excessively grown, which makes it difficult to refine the final grain size, and when the slab extraction temperature is less than 1140 ° C, excessive rolling load in a subsequent rolling process is caused. The invention can limit the slab extraction temperature to 1140 ~ 1180 ℃.

Rough rolling

The extracted slab may be provided as a rough rolling bar through a rough rolling process. If the temperature range of rough rolling is less than 980 ℃, excessive rolling load may be a problem during rough rolling, and if the temperature range of rough rolling exceeds 1000 ℃, oxidation and damage of the rough rolling equipment due to high temperature exposure may be a problem. Bar, the rough rolling temperature range of the present invention may be 980 ~ 1000 ℃. At this time, rough rolling may be performed by multi-pass rolling, and the cumulative rolling reduction of rough rolling may be 60 to 85%.

Hot rolled

The rough rolling bar may be hot rolled in the range of 840 to 880 ° C based on the exit temperature. When the temperature range of hot rolling is less than 840 ° C, roll fatigue scale may occur depending on the rolling load, and when the temperature range of hot rolling exceeds 880 ° C, surface properties may be deteriorated by excessive surface scale formation. The hot rolling temperature of the invention may be limited to 840 ~ 880 ℃.

Cooling

After hot rolling, the hot rolled steel sheet can be provided by cooling. When cooling the steel sheet, slow cooling conditions may be applied to prevent shape defects of the steel sheet, and cooling may be performed to a temperature of 600 ° C. or less at a cooling rate of 10 ° C./s to 15 ° C./s. In addition, the present invention may apply cooling conditions in which the cooling rate is gradually increased.

Hereinafter, the present invention will be described in more detail through examples.

( Example )

For the slab having the component composition of Table 1, a hot-rolled steel sheet specimen was prepared under the conditions of Table 2. For each specimen, microstructure, physical properties, and whether or not non-uniform stretching occurred during steel pipe processing were evaluated, and the results are shown in Table 3. At this time, the microstructure was evaluated by observing the etched specimens after the Nital corrosion with an optical microscope, and the surface layer part mixed tissue fraction was evaluated by measuring the area fraction of the mixed tissue from the cross section of each specimen up to t / 4 point in the thickness direction. The tensile strength and yield strength of each specimen were measured using a tensile tester. In addition, whether or not non-uniform elongation occurred during steel pipe processing was evaluated through simulation of each specimen, and it was evaluated that non-uniform elongation occurs when the surface defects on the surface of the processed specimen are 100 μm or more.

Steel Weight (wt%) C Si Mn P S Invention Steel 1 0.10 0.03 0.45 0.02 0.01 Invention Steel 2 0.05 0.03 0.40 0.01 0.002 Comparative steel 1 0.04 0.3 0.20 0.03 0.02 Comparative steel 2 0.04 0.03 0.20 0.03 0.02

Steel Condition Slavic
Extraction temperature
(℃) Rough rolling Hot rolled division Temperature
(℃) Rolling reduction
(%) Temperature
(℃) Rolling reduction
(%) Invention Steel 1 A 1160 980 85 850 92 Inventive Example 1 Invention Steel 1 B 1050 920 85 810 90 Comparative Example 1 Invention Steel 1 C 1160 980 65 830 80 Comparative Example 2 Invention Steel 2 A 1160 980 85 850 92 Inventive Example 2 Invention Steel 2 B 1050 920 85 810 90 Comparative Example 3 Invention Steel 2 C 1160 980 65 830 80 Comparative Example 4 Comparative steel 1 A 1160 980 85 850 92 Comparative Example 5 Comparative steel 2 B 1050 920 85 810 90 Comparative Example 6

division Microstructure TS (MPa) YP (MPa) Whether non-uniform stretching occurs Polygonal
ferrite
Fraction (area%) Surface layer
Organization
Fraction (area%) Inventive Example 1 99% 0 375 245 Not occurring Comparative Example 1 95% 10 366 223 Occur Comparative Example 2 95% 5 368 232 Occur Inventive Example 2 92% One 342 210 Not occurring Comparative Example 3 90% 20 331 205 Occur Comparative Example 4 85% 15 311 201 Occur Comparative Example 5 85% 30 298 199 Occur Comparative Example 6 60% 40 292 195 Occur

As shown in Tables 1 to 3, in the case of Inventive Examples 1 and 2 satisfying the alloy composition and manufacturing conditions of the present invention, the present invention satisfies the desired tissue fraction and strength, particularly, the surface layer mixed tissue fraction 1 It can be confirmed that non-uniform stretching does not occur when the steel pipe is processed because it is actively suppressed to an area of less than or equal to an area. On the other hand, in Comparative Examples 1 to 6, which do not satisfy any one or more of the alloy composition and manufacturing conditions of the present invention, the present invention does not satisfy the desired tissue fraction or strength, and in particular, the surface layer mixed tissue fraction is 1 area It can be confirmed that uneven stretching occurs when the steel pipe is processed in excess of%.

1 is a microstructure photograph of a cross section of the upper surface layer of Inventive Example 1, and FIG. 2 is a microstructure photograph of a cross section of the upper surface layer of Comparative Example 1. As shown in Figure 1 and 2, Inventive Example 1, the grain size of the surface layer portion is formed uniformly, there is no mixed tissue, while Comparative Example 1 can be confirmed that the grain size of the mixed layer is formed on the surface layer is non-uniform. .

Therefore, the hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention actively suppresses the formation of mixed structures in the surface layer portion, and thus it can be confirmed that a processing defect due to non-uniform stretching during steel pipe processing by a steel pipe processing company can be effectively prevented. .

Although the present invention has been described in detail through the above embodiments, other types of embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (5)

In weight percent, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% or less, balance Fe and unavoidable impurities,
Polygonal ferrite is included as the base tissue, and cyclic circular ferrite is included as the remainder.
The fraction of the polygonal ferrite is 92 area% or more,
A hot-rolled steel sheet for steel pipes, wherein the fraction of mixed structure at the point t / 4 (t: steel sheet thickness) along the thickness direction from the upper and lower surfaces is 1 area% or less. According to claim 1,
The mixed structure is a structure in which polygonal ferrite and austenite are mixed, a hot rolled steel sheet for steel pipes. According to claim 1,
The hot rolled steel sheet is provided with a tensile strength of 340 ~ 490MPa and a yield strength of 210 ~ 370MPa, hot rolled steel sheet for steel pipes. In weight percent, C: 0.05 to 0.12%, Si: 0.03% or less, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.015% or less, residual slab and slabs containing unavoidable impurities are heated to 1140 to Extracted in the temperature range of 1180 ℃,
The extracted slab is roughly rolled in a temperature range of 980 to 1000 ° C to provide a rough rolled bar,
A method of manufacturing a hot rolled steel sheet for a steel pipe to provide a hot rolled steel sheet by hot rolling and cooling the crude rolled bar in a temperature range of 840 to 880 ° C based on the exit temperature. The method of claim 4,
The hot-rolled hot-rolled steel sheet is cooled to a temperature of 600 ° C. or less at a cooling rate of 10 to 15 ° C./s, and a method for manufacturing a hot rolled steel sheet for steel pipes.

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