KR20150025950A - Steel plate and manufacturing method of the same - Google Patents

Abstract

Disclosed are a steel material with excellent flatness capable of increasing productivity by simplifying a reheating step once by direct quenching and tempering (DQT) an alloy composition adjusting and manufacturing step; and a method to manufacture the same. According to the present invention, the method to manufacture a steel material comprises: (a) step of reheating a steel slab comprising 0.08-0.16 wt% of carbon (C), 0.1-0.5 wt% or less of silicon (Si), 1.0-1.7 wt% of manganese (Mn), 120 ppm or less of phosphorus (P), 30 ppm or less of sulfur (S), 0.01-0.05 wt% of soluble aluminum (S-Al), 0.01-0.20 wt% of copper (Cu), 0.01 wt% or less of niobium (Nb), 0.0005 wt% or less of boron (B), 5-40 ppm of calcium (Ca), and the remainder consisting of iron (Fe) and inevitable impurities at a slab reheating temperature (SRT) of 1150-1250°C; (b) step of hot rolling the reheated steel material at a finish rolling temperature (FRT) of 850-950°C; (c) step of quenching the hot rolled steel material; and (d) step of tempering the cooled steel material at 620-700°C.

Description

STEEL PLATE AND MANUFACTURING METHOD OF THE SAME [0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material manufacturing technique, and more particularly, to a steel material for a pressure vessel capable of managing flatness through DQT (Direct Quenching & Tempering) and alloy composition adjustment, and a manufacturing method thereof.

A537-C2 steels used for pressure vessels are mainly produced through RQT (Reheat Quenching & Tempering). It is subjected to a process of reheating and quenching at a temperature higher than the Ar3 temperature and then tempering again. This process has a problem that the reheating process is performed twice, and thus the manufacturing cost is large.

In addition, since the RQT process is performed nonuniformly at the time of quenching, the thermal stress and the transformation stress are different locally, so that warping or cracking may occur after quenching.

As a related prior art, Korean Patent Laid-Open Publication No. 10-2012-0074638 (published on Jul. 6, 2012) discloses a pole steel sheet for pressure vessel excellent in core property and hydrogen organic cracking resistance and a method for producing the same. .

It is an object of the present invention to provide a steel material which can increase productivity and simplify the reheating step by performing DQT (Direct Quenching & Tempering) of the alloy component adjustment and manufacturing process, and a method of manufacturing the same.

In order to accomplish the above object, the present invention provides a method of manufacturing a steel material, comprising the steps of: (a) providing 0.08 to 0.16 wt% carbon, 0.1 to 0.5 wt% silicon, manganese (S): not more than 30 ppm, soluble aluminum (S-Al): 0.01 to 0.05 wt%, copper (Cu): 0.01 to 0.20 wt%, niobium A steel slab composed of Nb: 0.01 wt% or less, B: 0.0005 wt% or less, Ca: 5 to 40 ppm, and the balance iron (Fe) and unavoidable impurities is slab reheating temperature (SRT) Reheating to 1250 占 폚; (b) hot rolling the reheated steel material to a finishing rolling temperature (FRT) of 850 to 950 占 폚; (c) quenching the hot-rolled steel material; And (d) tempering the cooled steel at 620 to 700 占 폚.

According to another aspect of the present invention, there is provided a steel material comprising 0.08 to 0.16% by weight of carbon (C), 0.1 to 0.5% by weight of silicon (Si), 1.0 to 1.7% by weight of manganese (Mn) (P): 120 ppm or less, S: 30 ppm or less, S-Al: 0.01 to 0.05 wt%, Cu: 0.01 to 0.20 wt%, Niobium (Nb) (Y) and a tensile strength (TS) of 500 to 600 MPa and a yield strength (YP) of at least 0.5% by weight, boron (B) at 0.0005% : 400 to 480 MPa.

In order to improve the productivity of the steel material according to the present invention, it is possible to provide a well steel pipe having excellent flatness by controlling the alloy component and the manufacturing process, which is problematic in that the flatness is lowered by the DQT process.

1 is a flowchart showing a method of manufacturing a steel material according to an embodiment of the present invention.

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 steel material according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Steel

The steel material according to the present invention aims to satisfy a tensile strength (TS) of 500 to 600 MPa and a yield strength (YP) of 400 to 480 MPa.

For this, the steel material according to the present invention contains 0.08 to 0.16% by weight of carbon (C), 0.1 to 0.5% by weight of silicon (Si), 1.0 to 1.7% by weight of manganese (Mn) (B) 0.01 to 0.20% by weight of niobium (Nb), 0.01 to 0.5% by weight of aluminum (S-Al) : 0.0005 wt% or less, calcium (Ca): 5 to 40 ppm, and the balance of iron (Fe) and unavoidable impurities.

The steel material according to the present invention may further contain 0.1 to 0.4 wt% of nickel (Ni), 0.1 to 0.3 wt% of chromium (Cr), 0.02 to 0.12 wt% of molybdenum (Mo), 0.005 to 0.030 wt% of vanadium % And nitrogen (N): 60 ppm or less.

Hereinafter, the role and content of each component contained in the steel 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 0.08 to 0.16% by weight of the total weight of the steel material according to the present invention. When the content of carbon (C) is less than 0.08% 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 0.16% 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.

Silicon (Si) is preferably added in an amount of 0.1 to 0.5% by weight based on the total weight of the steel according to the present invention. When the content of silicon (Si) is less than 0.1% by weight, the effect of the addition is insufficient. On the other hand, when the content of silicon (Si) exceeds 0.5% by weight, the weldability is lowered, and the scale is generated at the reheating step and the hot rolling at the hot rolling step to cause problems in surface quality. There is a problem that it inhibits.

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 an amount of 1.0 to 1.7% by weight based on the total weight of the steel according to the present invention. When the content of manganese (Mn) is less than 1.0% 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 1.7% by weight, sulfur precipitated in the steel is precipitated as MnS, which causes center segregation during casting, thereby greatly reducing the corrosion resistance of the steel.

In (P)

When the content of phosphorus (P) exceeds 120 ppm, there is a problem that the weldability is deteriorated and the corrosion resistance is lowered due to slab center segregation. Therefore, phosphorus (P) is preferably limited to a range of 120 ppm or less based on the total weight of the steel material according to the present invention.

Sulfur (S)

When the content of sulfur (S) exceeds 30 ppm, the toughness and weldability of the steel are impaired and the corrosion resistance of the steel may be lowered by increasing the MnS nonmetallic inclusions. Therefore, it is preferable to limit the sulfur (S) to a range of 30 ppm or less of the total weight of the steel material according to the present invention.

The soluble aluminum (S- Al )

Soluble aluminum (S-Al) acts as a deoxidizer to remove oxygen in the steel.

The soluble aluminum (S-Al) is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the steel according to the present invention. If the content of soluble aluminum (S-Al) is less than 0.01% by weight, the effect of adding silicon can not be exhibited properly. On the contrary, when the content of soluble aluminum (S-Al) exceeds 0.05% by weight, there is a problem that the weldability is deteriorated.

Copper( Cu )

Copper (Cu) contributes to solid solution strengthening and enhances strength.

Copper (Cu) is preferably added in an amount of 0.01 to 0.20% by weight based on the total weight of the steel according to the present invention. When the content of copper (Cu) is 0.01% by weight or less, the addition effect is insignificant. On the other hand, when the content of copper (Cu) exceeds 0.20% by weight, the hot workability of the steel is lowered and the susceptibility to reheat cracking after welding is increased.

Niobium ( Nb )

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium-based carbides or nitrides improve grain strength and low-temperature toughness by suppressing grain growth during rolling and making crystal grains finer.

Niobium (Nb) is preferably added at a content ratio of 0.01% by weight or less based on the total weight of the steel sheet according to the present invention. When the content of niobium (Nb) exceeds 0.01% by weight, coarse secondary phases including niobium are generated and cracks are generated.

Boron (B)

Boron (B) is a strong incipient element, which plays a role in blocking segregation of phosphorus (P) and improving strength. If segregation of phosphorus (P) occurs, secondary processing brittleness may occur, so boron (B) is added to block segregation of phosphorus (P) to increase resistance to process embrittlement.

Boron (B) is preferably added in a content ratio of 0.0005% by weight or less based on the total weight of the steel sheet according to the present invention. If the boron (B) content exceeds 0.005% by weight, excess boron oxide may cause a problem of inhibiting the surface quality of the steel due to the formation of boron oxide.

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.

Ca is preferably added at a content ratio of 5 to 40 ppm based on the total weight of the steel according to the present invention. When the content of calcium (Ca) is less than 5 ppm, the addition of calcium (Ca) is hard to see. On the contrary, when the content of calcium (Ca) exceeds 40 ppm, formation of CaS inclusions hinders the production of MnS which is effective on corrosion resistance and weldability.

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 0.4% by weight based on the total weight of the steel material 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 other hand, when the content of nickel (Ni) exceeds 0.4% by weight and is added in a large amount, there arises a problem of causing redispersible brittleness.

chrome( Cr )

Chromium (Cr) is a ferrite stabilizing element and contributes to strength improvement. In addition, chromium expands the delta ferrite region and transitions the hypo-peritectic region to the high carbon side to improve the slab surface quality.

Cr (Cr) is preferably added in an amount of 0.1 to 0.3% by weight based on the total weight of the steel according to the present invention. When the content of chromium (Cr) is less than 0.1% by weight, the addition effect is insignificant. On the other hand, when the addition amount of chromium (Cr) exceeds 0.3% by weight, the weld heat affected zone (HAZ) tends to deteriorate toughness.

molybdenum( Mo )

Molybdenum (Mo) stably produces carbides, which together with chromium contribute to the improvement of high temperature strength.

Molybdenum (Mo) is preferably added in an amount of 0.02 to 0.12% by weight based on the total weight of the steel material according to the present invention. If the addition amount of molybdenum is less than 0.02 wt%, the effect of increasing the strength by adding molybdenum (Mo) is insufficient. On the contrary, if the amount of molybdenum (Mo) added exceeds 0.12 wt%, there is a problem that weldability such as low temperature cracking and reheat cracking is deteriorated.

Vanadium (V)

Vanadium (V) is a carbide-generating element, and is particularly effective in increasing the high-temperature strength.

Vanadium (V) is preferably added in an amount of 0.005 to 0.030% by weight based on the total weight of the steel material according to the present invention. When the addition amount of vanadium (V) is less than 0.005% by weight, the effect of improving the strength is insufficient. On the contrary, when the amount of vanadium (V) added exceeds 0.030 wt%, there is a problem of increasing the susceptibility to reheat cracking.

Nitrogen (N)

If the content of nitrogen (N) exceeds 60 ppm, the impact characteristics and elongation of the steel are lowered and the toughness of the welded portion is significantly deteriorated. Therefore, nitrogen (N) is preferably limited to a range of 60 ppm or less of the total weight of the steel material according to the present invention.

In the steel material for a pressure vessel according to the present invention, the carbon equivalent (Ceq) determined by the following formula 1 preferably satisfies the following formula (2).

Equation 1: 0.43? Ceq

[Formula 1] Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Cr] / 5 + [Mo] / 4 + [V] / 14 [

When the carbon equivalent (Ceq) was less than 0.43 regardless of the thickness of the steel, the high temperature strength was insufficient. Therefore, the carbon equivalent is preferably 0.43 or less, and the addition amount of carbon, silicon, manganese, chromium, molybdenum, and vanadium can be adjusted to satisfy the requirement.

Manufacturing method of steel

1 is a flowchart showing a method of manufacturing a steel material according to an embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing a steel material according to the present invention includes a slab reheating step S110, a hot rolling step S120, a quenching step S130, and a tempering step S140. 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.

The steel slab in the semi-finished product state to be the subject of the steel according to the present invention is composed of 0.08 to 0.16 wt% of carbon (C), 0.1 to 0.5 wt% or less of silicon (Si), 1.0 to 1.7 wt% of manganese (Mn) (P): 120 ppm or less, S: 30 ppm or less, S-Al: 0.01 to 0.05 wt%, Cu: 0.01 to 0.20 wt%, Niobium: , Boron (B): 0.0005 wt% or less, calcium (Ca): 5 to 40 ppm, and the balance of iron (Fe) and unavoidable impurities.

The steel material according to the present invention may further contain 0.1 to 0.4 wt% of nickel (Ni), 0.1 to 0.3 wt% of chromium (Cr), 0.02 to 0.12 wt% of molybdenum (Mo), 0.005 to 0.030 wt% of vanadium % And nitrogen (N): 60 ppm or less.

Reheating slabs

In the slab reheating step S110, the steel slab having the above composition is reheated at a slab reheating temperature (SRT) of 1150 to 1250 占 폚 for 300 to 400 minutes. Here, the steel slab 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 steel slabs obtained through the continuous casting process are 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 steel is subjected to hot rolling at a finishing rolling temperature FRT (Finish Rolling Temperature) of 850 to 950 DEG C by means of rough rolling and longitudinal cold rolling.

If the finish rolling temperature is less than 850 ° C, the modified organic ferrite transformation occurs during rolling, and the amount of pearlite is reduced, which causes a decrease in strength. On the other hand, when the finish rolling temperature exceeds 950 DEG C, ductility and toughness are excellent, but the strength is rapidly lowered.

Quenched

The quenching step (S130) is a step for imparting strength and hardness to the hot-rolled steel material. The quenching is started at a temperature of 910 to 950 占 폚, followed by quenching to room temperature.

In the quenching step S130, the hot-rolled steel material may be directly quenched (DT) while passing through a cooling zone provided in the rolling line. Therefore, the manufacturing cost can be reduced by omitting the austenitizing heat treatment step by reheating in the conventional reheating quenching method.

When the quenching start temperature is less than 910 占 폚, coarse ferrite is formed in the surface layer portion and the toughness is greatly deteriorated. On the other hand, when the quenching start temperature exceeds 950 占 폚, there is a problem that a large amount of low-temperature transformation structures are formed.

Tempering

The tempering step S140 is a step for removing the internal stress of the quenched steel, and is preferably carried out at a temperature of 620 to 700 占 폚. When the tempering temperature is less than 620 占 폚, the effect of tempering is low and it is difficult to secure toughness. On the other hand, when the tempering temperature exceeds 700 캜, it is difficult to secure the strength.

At this time, it is preferable to perform the tempering holding time for 2.7 * t + 10 min to 2.7 * t + 30 min (t: thickness of the steel). When the tempering holding time is carried out under the above-mentioned conditions, sufficient toughness can be ensured and a steel material having excellent flatness can be produced. The steel material after the tempering process can be cooled to room temperature through air cooling.

The steel produced in the above steps S110 to S140 is subjected to DQT (Direct Quenching & Tempering), so that the reheating process is simplified from one to two times, and productivity is excellent.

Also, the steel material produced by the above method can satisfy a tensile strength (TS) of 500 to 600 MPa and a yield strength (YP) of 400 to 480 MPa.

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

The specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the compositions shown in Tables 1 and 2 and the process conditions shown in Table 3. At this time, in the case of the specimens according to Examples 1 to 3 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] (unit:% by weight)

[Table 3]

2. Evaluation of mechanical properties

Table 4 shows the evaluation results of the mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2.

[Table 4]

Referring to Tables 1 to 4, the specimens prepared according to Examples 1 to 3 satisfy a tensile strength (TS) of 400 to 480 MPa or more and a yield strength (YP) of 500 to 600 MPa Able to know.

On the other hand, in comparison with Example 1, the specimens prepared according to Comparative Example 1 and Comparative Example 2 in which RQT (Reheat Quenching & Tempering) was applied were different in alloy component and composition range, and yield strength (YP) However, it can be seen that the tensile strength (TS) and the carbon equivalent (Ceq) do not satisfy the target value.

Therefore, the steel according to the present invention can reduce the production cost by simplifying the reheating process one time by controlling the alloying components and the DQT process condition, and it is possible to reduce the production cost by satisfying the yield strength: 400 to 480 MPa and the tensile strength: 500 to 600 MPa, It is possible to provide a steel material having a high degree of corrosion resistance.

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: Quenching step
S140: Tempering step

Claims (6)

(S): not more than 0.1 to 0.5 wt%, manganese (Mn): 1.0 to 1.7 wt%, phosphorus (P): not more than 120 ppm, sulfur (S) : Not more than 30 ppm, soluble aluminum (S-Al): 0.01 to 0.05 wt%, copper: 0.01 to 0.20 wt%, niobium (Nb): not more than 0.01 wt%, boron (B) Reheating a steel slab composed of calcium (Ca): 5 to 40 ppm and the balance of iron (Fe) and unavoidable impurities to a slab reheating temperature (SRT) of 1150 to 1250 占 폚;
(b) hot rolling the reheated steel material to a finishing rolling temperature (FRT) of 850 to 950 占 폚;
(c) quenching the hot-rolled steel material; And
(d) tempering the cooled steel at 620 to 700 占 폚.
The method according to claim 1,
In the step (a)
The steel slab
(Ni): 0.1 to 0.4 wt%, chromium (Cr): 0.1 to 0.3 wt%, molybdenum (Mo): 0.02 to 0.12 wt%, vanadium (V): 0.005 to 0.030 wt% ppm or less, based on the total weight of the steel material.
The method according to claim 1,
In the step (a)
The steel slab
Has a carbon equivalent (Ceq) satisfying the following formula (1)
Silicon, manganese, chromium, molybdenum and vanadium in a content ratio satisfying the following formula (2).
Equation 1: 0.43? Ceq
[Formula 1] Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Cr] / 5 + [Mo] / 4 + [V] / 14 [
(Where [] is the weight percentage of each element)
The method according to claim 1,
In the step (d)
Wherein said tempering holding time is performed for 2.7 * t + 10 min to 2.7 * t + 30 min (t: thickness of steel material).
(Si): 0.1 to 0.5 wt% or less, manganese (Mn): 1.0 to 1.7 wt%, phosphorus (P): 120 ppm or less, sulfur (S): 30 ppm 0.01 to 0.20 wt% of soluble aluminum (S-Al), 0.01 to 0.20 wt% of copper (Cu), 0.01 wt% or less of niobium (Nb), 0.0005 wt% or less of boron (B) ): 5 to 40 ppm and the balance of iron (Fe) and unavoidable impurities,
A tensile strength (TS) of 500 to 600 MPa and a yield strength (YP) of 400 to 480 MPa.
6. The method of claim 5,
The steel
(Ni): 0.1 to 0.4 wt%, chromium (Cr): 0.1 to 0.3 wt%, molybdenum (Mo): 0.02 to 0.12 wt%, vanadium (V): 0.005 to 0.030 wt% ppm. < RTI ID = 0.0 > 11. < / RTI >

Images