KR101546138B1 - Hot-rolled steel sheet and manufacturing method of the same - Google Patents

Abstract

Hot rolled steel sheets for oil well tubing having excellent impact toughness and satisfying the K55 standard (yield strength: 379 to 552 MPa, tensile strength: 655 MPa or more) by increasing the pearlite lamella spacing by controlling alloy components and hot rolling process conditions Method is disclosed.
The method for manufacturing a hot-rolled steel sheet according to the present invention comprises the steps of: (a) mixing 0.33 to 0.37 weight% of carbon (C), more than 0 to 0.25 weight% of silicon (Si), 1.4 to 1.6 weight% of manganese (Mn) (P): more than 0 wt% to 0.01 wt% or less, S: more than 0 wt% to 0.001 wt%, aluminum (Al): 0.02 to 0.04 wt%, titanium (Ti): 0.01 to 0.02 wt% (Fe) and unavoidable impurities, and the balance of iron (Fe), inevitable impurities, and the balance of iron (Ni): 0.1 to 0.3 wt%, calcium (Ca): 0.001 to 0.003 wt% Reheating the sheet material to a slab reheating temperature (SRT) of 1150 to 1250 占 폚; (b) hot rolling the reheated slab plate to a finishing delivery temperature (FDT) of 780 to 820 占 폚; And (c) cooling and winding the hot-rolled plate to a CT (Coiling Temperature) of 600 to 700 ° C.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-rolled steel sheet,

More particularly, the present invention relates to a hot-rolled steel sheet for oil tubular country goods having a high impact toughness by increasing pearlite lamella spacing and a method for producing the same.

Yujeong Steel pipe is a steel pipe used for drilling oil or gas. The steel pipe for oil wells is usually produced by hot rolling a steel plate.

Recently, as the demand for high-strength steels increased due to the increase of non-traditional mining, the demand for the development of new steel species for reducing the manufacturing cost has increased due to the heat treatment cost and seamless replacement. Among them, high-strength and high toughness are required for a large-diameter oil tube, and therefore, a higher impact toughness is required.

A related prior art document is Korean Patent Laid-Open Publication No. 10-2013-0046920 (published on May 3, 2013), which discloses a hot-rolled steel sheet and a method of manufacturing the same.

It is an object of the present invention to provide a steel sheet having excellent impact toughness satisfying the K55 standard (yield strength: 379 to 552 MPa, tensile strength: 655 MPa or more) by increasing the pearlite lamella spacing by controlling the alloy component and controlling the hot rolling process condition Hot-rolled steel sheet and a method of manufacturing the same.

In order to accomplish the above object, the present invention provides a method of manufacturing a hot-rolled steel sheet, comprising: (a) 0.33 to 0.37 wt% of carbon (C), more than 0 wt% to 0.25 wt% of silicon (Si) (S): more than 0 wt% to 0.001 wt%; aluminum (Al): 0.02 wt% to 0.04 wt%; (Ti): 0.01-0.02 wt%, Ni: 0.1-0.3 wt%, Ca: 0.001-0.003 wt%, N: 0 wt% to 0.006 wt% (Fe) and inevitable impurities to a slab reheating temperature (SRT) of 1150 to 1250 占 폚; (b) hot rolling the reheated slab plate to a finishing delivery temperature (FDT) of 780 to 820 占 폚; And (c) cooling and winding the hot-rolled plate to a CT (Coiling Temperature) of 600 to 700 ° C.

According to another aspect of the present invention, there is provided a hot-rolled steel sheet comprising 0.33 to 0.37 wt% of carbon (C), more than 0 wt% to 0.25 wt% of silicon (Si) (S): more than 0 wt% to 0.001 wt%, aluminum (Al): 0.02 wt% to 0.04 wt%, titanium (Ti) (Fe) in an amount of from 0.01 to 0.02 wt%, nickel (Ni) in an amount of from 0.1 to 0.3 wt%, calcium (Ca) in an amount of from 0.001 to 0.003 wt%, nitrogen (N) And has a tensile strength (TS) of 655 MPa or more and a yield strength (YP) of 379 to 552 MPa.

The hot-rolled steel sheet according to the present invention is advantageous in that nickel is added to increase the lamellar spacing of pearlite by decreasing the temperature of Ae1, thereby having excellent impact toughness.

Accordingly, the hot-rolled steel sheet according to the present invention can provide a hot-rolled steel sheet for a well pipe with excellent impact toughness that can satisfy a tensile strength (TS) of 655 MPa or more, a yield strength (YP) of 379 to 552 MPa, and an impact energy of 50 J or more have.

1 is a flowchart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.
Fig. 2 shows the microstructure of the specimen produced according to Example 1. Fig.
Fig. 3 shows the microstructure of the specimen produced according to Comparative Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Hot-rolled steel sheet

The hot-rolled steel sheet according to the present invention aims to satisfy a tensile strength (TS) of 655 MPa or more, a yield strength (YP) of 379 to 552 MPa, and an impact energy of 50 J or more.

For this, the hot-rolled steel sheet according to the present invention comprises 0.33 to 0.37% by weight of carbon (C), more than 0 to 0.25% by weight of silicon (Si), 1.4 to 1.6% by weight of manganese (Mn) ): More than 0 wt% to 0.01 wt% or less, sulfur (S): 0 wt% to 0.001 wt%, aluminum (Al): 0.02 to 0.04 wt%, titanium (Ti) (Ni): 0.001-0.003 wt%, calcium (Ca): 0.001 wt%, nitrogen (N): 0 wt% to 0.006 wt%, and the balance of iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component included in the hot-rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is added for securing strength and controlling microstructure.

The carbon (C) is preferably added in a content ratio of 0.33 to 0.37% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of carbon (C) is less than 0.33% by weight, the fraction of the second phase structure is lowered and the strength is lowered. On the contrary, when the content of carbon (C) exceeds 0.37% by weight, toughness and weldability are deteriorated.

silicon( Si )

Silicon (Si) is an element stabilizing ferrite, which has the effect of increasing subcooling during ferrite transformation to refine the grain and inhibit carbide formation.

The silicon (Si) is preferably added in a content ratio of more than 0 wt% to 0.25 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention. When the content of silicon (Si) is more than 0.25% by weight, the weldability is lowered and the scale is generated during the reheating step and the hot rolling at the hot rolling step to cause problems in the surface quality, There is a problem.

manganese( Mn )

Manganese (Mn) is an austenite stabilizing element, which is very effective for solid solution strengthening and has a great influence on the hardening ability of steel.

Manganese (Mn) is preferably added in a content ratio of 1.4 to 1.6% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of manganese (Mn) is less than 1.4% by weight, the fraction of the second phase structure is lowered and it may be difficult to secure the strength. On the other hand, when the content of manganese (Mn) exceeds 1.6% by weight, sulfur precipitated in the steel is precipitated as MnS, causing center segregation during casting, thereby greatly reducing the corrosion resistance of the steel.

When the content ratio of manganese (Mn) and silicon (Si) is added according to the following formula (1), the melting temperature is decreased to discharge Mn-Si-O inclusions generated in the welded portion to the outside, .

6? [Mn] / [Si]? 10 (where [] is the weight percentage of each element)

In (P)

When the content of phosphorus (P) exceeds 0.01% by weight, there is a problem that the weldability is deteriorated and corrosion resistance is lowered due to slab center segregation. Therefore, phosphorus (P) is preferably limited to a range of more than 0 wt% to 0.01 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

Sulfur (S)

If the content of sulfur (S) exceeds 0.001% by weight, the toughness and weldability of the steel are impaired and the corrosion resistance of the steel may be lowered by increasing the MnS non-metallic inclusions. Therefore, sulfur (S) is preferably limited to a range of more than 0 wt% to 0.001 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

aluminum( Al )

Aluminum (Al) acts as a deoxidizer to remove oxygen in the steel.

Aluminum (Al) is preferably added in an amount of 0.02 to 0.04% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.02% by weight, the effect of adding silicon can not be exhibited properly. On the contrary, when the content of aluminum (Al) exceeds 0.04% by weight, weldability is deteriorated.

titanium( Ti )

Titanium (Ti) has the effect of improving the toughness and strength of hot-rolled steel sheet by making Ti (C, N) precipitates having high stability at high temperatures, thereby finishing the austenite grain growth and refining the texture of the welded portion.

Titanium (Ti) is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of titanium (Ti) is less than 0.01% by weight, the austenite grain size becomes large. On the other hand, when the content of titanium (Ti) exceeds 0.02% by weight, corrosion resistance of the steel can be lowered by forming coarse TiN precipitates.

nickel( Ni )

Nickel (Ni), together with manganese (Mn), is a typical austenite stabilizing element, which is very effective in strengthening the solid solution and has a great influence on the increase of the hardenability of the steel. Further, by decreasing the temperature of Ae1 as in manganese (Mn), the lamellar spacing of pearlite is increased.

Nickel (Ni) is preferably added at a content ratio of 0.1 to 0.3% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. If the content of nickel (Ni) is less than 0.1% by weight, the effect of adding nickel can not be exhibited properly. On the contrary, when the content of nickel (Ni) is more than 0.3 wt% and added in a large amount, there arises a problem of inducing a hot brittleness.

calcium( Ca )

Calcium (Ca) forms CaS to lower the content of sulfur in steel, and also reduces MnS segregation, thereby reducing steel segregation and sulfur segregation, thereby increasing resistance to reheating cracking.

Calcium (Ca) is preferably added at a content ratio of 0.001 to 0.003% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of calcium (Ca) is less than 0.001% by weight, the effect of addition of calcium (Ca) is difficult to see. On the contrary, when the content of calcium (Ca) exceeds 0.003% by weight, CaS inclusions are formed, thereby inhibiting MnS formation which is effective on corrosion resistance and weldability.

Nitrogen (N)

When the content of nitrogen (N) exceeds 0.006% by weight, the impact characteristics and elongation of the steel are lowered, and the toughness of the welded portion is greatly deteriorated. Therefore, nitrogen (N) is preferably limited to a range of more than 0 wt% to 0.006 wt% or less of the total weight of the hot-rolled steel sheet according to the present invention.

Manufacturing method of hot-rolled steel sheet

1 is a flowchart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing a hot-rolled steel sheet according to the present invention includes a slab reheating step (S110), a hot rolling step (S120), and a cooling and winding step (S130). At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the reheating step to obtain effects such as reuse of precipitates.

In the semi-finished slab plate to be subjected to the hot rolling process in the control rolling method of the present invention, the slab plate has carbon (C) of 0.33 to 0.37 wt%, silicon (Si) of more than 0 wt% to 0.25 wt% (S): more than 0 wt% to 0.001 wt%; aluminum (Al): 0.02 to 0.04 wt%; (Ni): 0.001 to 0.003 wt%; calcium (Ca): 0.001 to 0.003 wt%; nitrogen (N): 0 wt% to 0.006 wt%; and (Fe) and inevitable impurities.

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated at a slab reheating temperature (SRT) of 1150 to 1250 DEG C for 2 hours or more. Here, the slab plate can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.

If the slab reheating temperature (SRT) is less than 1150 ° C, the segregated components are not sufficiently distributed during casting. On the other hand, when the slab reheating temperature exceeds 1250 DEG C, coarse austenite is formed and it is difficult to secure strength.

Hot rolling

In the hot rolling step (S120), the reheated slab plate is subjected to hot rolling at a finishing delivery temperature (FDT) of 780 to 820 占 폚 by a high-grade rolling and a longitudinal low-rolling.

When the finish rolling temperature is less than 780 DEG C, the Ae3 temperature is 760 DEG C, so that the modified organic ferrite transformation occurs during rolling, and the amount of pearlite is reduced to cause a decrease in strength. On the other hand, when the finishing rolling temperature exceeds 820 占 폚, the ductility and toughness are excellent, but the strength is rapidly lowered.

Cooling and Coiling

In the cooling and winding step (S130), the hot-rolled plate is cooled and then wound.

In the cooling and winding step (S130), the hot rolled plate is cooled at a cooling rate of 20 DEG C / sec or more. When the cooling rate is less than 20 ° C / sec, the crystal growth is accelerated and it may be difficult to secure the strength.

The cooled plate is rolled at a temperature of 600 to 700 ° C. (Coil milling temperature). When the coiling temperature is less than 600 ° C, sufficient strength can be secured, but it is difficult to ensure high toughness. On the other hand, when the coiling temperature exceeds 700 캜, the strength is insufficient.

The hot-rolled steel sheet for a sanitary pipe manufactured in the above-described processes (S110 to S130) has an increased pearlite lamella spacing and can have excellent impact toughness.

Further, the hot-rolled steel sheet produced by the above method can satisfy a tensile strength (TS) of 655 MPa or more, a yield strength (YP) of 379 to 552 MPa, and an impact energy of 50 J or more.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

Specimens according to Examples 1 to 2 and Comparative Examples 1 to 2 were prepared with the compositions shown in Table 1 and the process conditions shown in Table 2. In the case of the specimens according to Examples 1 and 2 and Comparative Examples 1 and 2, ingots having respective compositions were prepared and heated using a rolling simulation tester, followed by hot rolling and air cooling.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the evaluation results of the mechanical properties of the specimens prepared according to Examples 1 and 2 and Comparative Examples 1 and 2.

[Table 3]

With reference to Tables 1 to 3, the specimens prepared according to Examples 1 and 2 have a tensile strength (TS) of 655 MPa or more, a yield strength (YP) of 379 to 552 MPa, Impact energy: 50 J or more.

On the other hand, in comparison with Example 1, it was found that, when Ni (Ni) was not added, chromium (Cr) was further added, and FDT was produced at a temperature lower than the range suggested by the present invention The tensile strength (TS) and the yield strength (YP) of the specimen satisfy the target value, but the impact energy value is less than the target value.

The specimen prepared according to Comparative Example 2 in which nickel (Ni) was not added, chromium (Cr) and boron (B) were further added, and cooling rate was lower than the range suggested by the present invention The tensile strength (TS) and the yield strength (YP) satisfied the target value, but the impact energy value fell below the target value.

FIG. 2 shows the microstructure of the specimen produced according to Example 1, and FIG. 3 shows the microstructure of the specimen prepared according to Comparative Example 1. FIG.

2 and 3, it can be seen that the pearlite lamella spacing of the specimen prepared according to Example 1 is wider than the pearlite lamella spacing of the specimen prepared according to Comparative Example 1. [ As a result, the impact energy values of Comparative Example 1 are significantly smaller than those of Example 1, as shown in Table 3.

Accordingly, the hot-rolled steel sheet according to the present invention can be manufactured by increasing the lamellar spacing of pearlite through control of alloy components and process conditions, thereby providing a hot-rolled steel sheet for oil wells satisfying a yield strength of 379 to 552 MPa and a tensile strength of 655 MPa or more, Can be provided.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: Cooling and winding step

Claims (6)

(a) from 0.33 to 0.37 wt% of carbon (C), from more than 0 wt% to 0.25 wt% of silicon (Si), from 1.4 to 1.6 wt% of manganese (Mn) (Al): 0.01 to 0.02% by weight, and the nickel (Ni): 0.1 to 0.3% by weight, (Slab reheating temperature) of 1150 (SRT) is used as a slab plate composed of 0.001 to 0.003% by weight of calcium (Ca), more than 0 to 0.006% by weight of nitrogen (N) Reheating to ~ 1250 ° C;
(b) hot rolling the reheated slab plate to a finishing delivery temperature (FDT) of 780 to 820 占 폚; And
(c) cooling and winding the hot-rolled sheet material under a condition of a CT (Coiling Temperature) of 600 to 700 ° C,
Wherein the plate has a tensile strength (TS) of 655 MPa or more, a yield strength (YP) of 379 to 552 MPa, and an impact energy of 50 J or more after the step (c).
The method according to claim 1,
In the step (c)
The cooling
Cooling rate: 20 캜 / sec or more.
The method according to claim 1,
The slab plate
(Mn) and silicon (Si) within a range satisfying the following formula (1): " (1) "
6? [Mn] / [Si]? 10
(Where [] is the weight percentage of each element)
(P): more than 0% by weight to 0.01% by weight of phosphorus (P), 0.33 to 0.37% by weight of carbon (C), more than 0 to 0.25% (Al), 0.01 to 0.02 wt% of titanium (Ti), 0.1 to 0.3 wt% of nickel (Ni), 0.1 to 0.3 wt% of nickel (Ni) 0.001 to 0.003 wt% of calcium (Ca), more than 0 wt% to 0.006 wt% of nitrogen (N) and the balance of iron (Fe) and unavoidable impurities,
A tensile strength (TS) of 655 MPa or more and a yield strength (YP) of 379 to 552 MPa.
5. The method of claim 4,
The hot-
Impact energy: 50 J or more.
5. The method of claim 4,
The hot-
(Mn) and silicon (Si) within a range satisfying the following expression (1).
6? [Mn] / [Si]? 10
(Where [] is the weight percentage of each element)

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