KR20180073954A - High-strength steel sheet having superior toughness at cryogenic temperatures, and method for manufacturing same - Google Patents

Abstract

The present invention provides a high strength steel sheet with excellent cryogenic toughness and a manufacturing method thereof. According to one embodiment of the present invention, the high strength steel sheet comprises: 0.04 to 0.08 weight percent of C; 0.20 to 0.30 weight percent of Si; 0.50 to 0.70 weight percent of Mn; 0.02 to 0.06 weight percent of soluble Al; greater than 0 to 0.01 weight percent or less of P; greater than 0 to 0.001 weight percent or less of S; greater than 0 to 0.1 weight percent or less of Cr; greater than 0 to 0.08 weight percent or less of Mo; 3.5 to 3.7 weight percent of Ni; greater than 0 to 0.01 weight percent or less of Ti; greater than 0 to 0.01 weight percent or less of Nb; and the remainder being Fe and unavoidable impurities. Accordingly, a high strength steel plate with a yield strength (TS) of 550 to 690 MPa, a yield point of 380 to 620 MPa, and a shock absorption energy of 250 J or more at -130deg;C is able to be provided by quenching and tempering (QT) treatment.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength steel sheet excellent in cryogenic temperature toughness and a method for manufacturing the same,

TECHNICAL FIELD The present invention relates to a cryogenic steel sheet for an ethane carrier and a method for producing the same. More particularly, the present invention relates to a cryogenic high strength steel sheet for an ethane carrier and a method for producing the same.

Ethane is a by-product that accounts for about 13% of the shale gas. Shale gas production in the United States is steadily increasing, and the demand for very large Ethane Carriers (VLECs) for the import and export of ethane, a by-product, is also increasing.

In order to transport ethane having a liquefaction point of -89 ° C, it is necessary to use a steel capable of ensuring low temperature toughness up to -110 ° C. Accordingly, the inventors of the present invention proceeded to develop a steel material for an ethane carrier having excellent low-temperature toughness which can be used for the transportation of ethane.

Related technology is Korean Patent Publication No. 2012-0075274 (published on July 6, 2012, a high-strength steel sheet excellent in cryogenic toughness and a manufacturing method thereof).

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a high strength steel sheet excellent in cryogenic toughness which can be used for an ethane carrier and a method for producing the same.

According to an aspect of the present invention, there is provided a high strength steel sheet comprising 0.04% to 0.08% carbon (C), 0.20% to 0.30% silicon (Si), 0.50% manganese (Mn) (S): more than 0% to 0.001%, chromium (Cr): more than 0% to 0.1% or less, (Mo): more than 0 to 0.08%, titanium (Ti): more than 0 to 0.01%, niobium (Nb): more than 0 to 0.01%, and the balance of iron (Fe) and other inevitably contained impurities , And nickel (Ni) in an amount of 3.5% by weight to 3.7% by weight of the entire steel sheet.

(B): more than 0 and not more than 5 ppm, tin (Sn): more than 0 and not more than 0.015% by weight , And nitrogen (N): 0.005 wt% or less.

In the present invention, the steel sheet has a tensile strength (TS) of 550 to 690 MPa, a yield point (YP) of 380 to 620 MPa or more, and an impact absorption energy of 250 J or more at -130 캜.

According to an aspect of the present invention, there is provided a method of manufacturing a high strength steel sheet, comprising: 0.04% to 0.08% carbon (C), 0.20% to 0.30% silicon (Si) (S): more than 0 and not more than 0.001%, chromium (Cr): more than 0 and not more than 0.1% (Ti): more than 0 and not more than 0.01%, niobium (Nb): more than 0 and not more than 0.01%, and the remainder is molybdenum (Mo) Producing a sheet comprising iron (Fe) and other inevitably contained impurities; And (b) normalizing and quenching the sheet material at a temperature of 890 ° C to 930 ° C, followed by tempering at a temperature of 500 ° C to 700 ° C.

(B): more than 0 and not more than 5 ppm, tin (Sn): more than 0 and not more than 0.015 (V) By weight or less, and nitrogen (N): 0.005% by weight or less.

In the present invention, the step of manufacturing the sheet material may include a step of reheating the slab sheet material, a step of finishing hot-rolling the reheated sheet material at a finish rolling temperature of 800 ° C or higher; And cooling the hot rolled plate material.

In the present invention, the quenching can be carried out at a rate of 10 to 30 DEG C / sec.

According to the present invention, a steel sheet according to an embodiment of the present invention comprises iron (Fe) as austenite in a high chromium (Cr) alloy, finely graining the crystal, solidifying it in austenite and ferrite, It is possible to obtain a high strength steel sheet containing 3.5 wt% to 3.7 wt% of nickel (Ni), which is an element effective for improving impact resistance at low temperatures, and having increased impact resistance at low temperatures.

Further, by conducting the QT heat treatment on the plate material after the hot-rolling, predetermined impact toughness can be ensured even at a cryogenic temperature of about -130 ° C where an ethane carrier is used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow chart schematically showing a method of manufacturing a steel sheet according to an embodiment of the present invention. FIG.
2 is a graph showing changes in shock absorption energy according to temperatures of a comparative steel and an inventive steel.
3 is a graph showing changes in shock absorption energy depending on the temperature after the rolling, quenching, and quenching steps of the steel of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may be embodied in many different forms and is not limited to the embodiments described herein. Throughout this specification, the same or similar components are denoted by the same reference numerals. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The embodiment of the present invention described below is a high strength steel sheet capable of securing low temperature toughness to a temperature of about -130 DEG C that can be used at a cryogenic temperature, for example, an ethane carrier through the development of a proper alloy composition designing and manufacturing process, .

Steel plate

The steel sheet according to the embodiment of the present invention comprises 0.04% to 0.08% of carbon (C), 0.20% to 0.30% of silicon (Si), 0.50% to 0.70% of manganese (Mn) (S): more than 0 and not more than 0.001%, chromium (Cr): more than 0 and not more than 0.1%, molybdenum (Mo): more than 0 and not more than 0.08 (Ni): 3.5 to 3.7%, Ti: more than 0 to 0.01%, niobium (Nb): more than 0 to 0.01%, and the balance of iron (Fe) and other inevitably contained impurities .

The steel sheet of the present invention is characterized in that the steel sheet comprises at least one of copper (Cu): not less than 0.1 wt%, vanadium (V): not less than 0.01 wt%, boron (B): more than 0 and not more than 5ppm, tin (Sn) % Or less, and nitrogen (N): 0.005 wt% or less.

The steel sheet according to the embodiment of the present invention is made of austenite of iron (Fe) in a high chromium (Cr) alloy to refine the crystal grains and strengthen the matrix by being dissolved in austenite and ferrite, A high-strength steel sheet containing 3.5 to 3.7% by weight of nickel (Ni), which is an effective element for increasing the impact resistance at low temperatures, can be obtained.

Hereinafter, the role and content of each component included in the alloy composition of the high-strength steel sheet excellent in cryogenic temperability according to the present invention will be described in more detail.

Carbon (C)

Carbon (C) is an indispensable element for imparting high strength to a steel sheet. It is a main element for enhancing the hardenability of the steel sheet and determining the strength after quenching. Depending on the content of carbon (C) and the method of production, it may be used as a solid carbon in the material structure and may be combined with elements having a very high property to bond with carbon (C) to form a carbide. In order to obtain such an effect, it is preferable that the steel material contains 0.04% or more of the total steel material weight. However, if the content of carbon (C) is excessive, the weldability, the elongation and the corrosion resistance are lowered, and therefore, the content thereof is preferably controlled to 0.08%.

Silicon (Si)

Silicon (Si) is a solid solution strengthening element, which contributes to strengthening of a steel sheet and is a necessary element for improving ductility. It also acts to inhibit the formation of cementite, which is the starting point of cracking due to hydrogen embrittlement. Silicon (Si) is preferably added in an amount of 0.20% to 0.30% of the total steel weight. When the content of silicon (Si) is less than 0.20% by weight, the effect of adding silicon (Si) described above can not be exhibited properly. On the other hand, when the content of silicon (Si) exceeds 0.30% by weight, an oxide is formed on the surface of the steel to lower the weldability of the steel.

Manganese (Mn)

Manganese (Mn) is an element which increases the strength and toughness of steel and increases the ingotability of steel. In order to effectively exert such action, manganese (Mn) is added in an amount of 0.3% to 0.5% of the total steel weight. When the content of manganese (Mn) is less than 0.5% by weight, it may be difficult to secure strength. On the other hand, when the content of manganese (Mn) exceeds 0.7% by weight, the strength is increased but segregation may occur, resulting in irregularity of the structure.

Soluble Aluminum (S_Al)

Soluble aluminum (S_Al) is used as a deacidification material and at the same time it plays a role of suppressing precipitation of cementite and stabilizing austenite like silicon (Si) and enhancing strength.

The soluble aluminum (S_Al) is preferably added in a content ratio of 0.02% to 0.06% of the total weight of the steel according to the present invention. When the content of soluble aluminum (S_Al) is less than 0.02 wt%, it is difficult to expect an austenite stabilizing effect. On the contrary, when the content of soluble aluminum (S_Al) exceeds 0.06% by weight, there is a problem that the nozzle clogging problem may occur at the time of steel making, and hot brittleness occurs due to Al oxide or the like during casting, and cracking and ductility are deteriorated.

In (P)

Phosphorus (P) is added to deteriorate cementite formation and increase strength. However, when the content of phosphorus (P) exceeds 0.01% of the total weight of the steel material according to the present invention, the weldability is deteriorated, which may cause the final material deviation due to the slab center segregation. Therefore, in the embodiment of the present invention, phosphorus (P) controls the content to 0.01% or less of the total steel weight.

Sulfur (S)

Sulfur (S) has the effect of enhancing the hardenability of the steel sheet to enhance the stabilization of strength after quenching, but it promotes the absorption of hydrogen into the steel material in a corrosive environment and forms a sulfide such as MnS which is a starting point of cracking due to hydrogen embrittlement Therefore, it is desirable to minimize. In the embodiment of the present invention, sulfur (S) is preferably controlled to 0.001% or less of the total steel mass.

Chromium (Cr)

Chromium (Cr) is an element added to secure strength and improve corrosion resistance. In addition, chromium (Cr) plays a role to increase the hardenability, but as the content increases, local corrosion occurs due to Cr-O ₂ produced by the combination of Cr and oxygen, and toughness is lowered. Therefore, the impression of chromium (Cr) is controlled to 0.1 wt% or less.

Molybdenum (Mo)

Molybdenum (Mo) is a substitutional element and improves the strength of steel by solid solution strengthening effect. In addition, molybdenum (Mo) improves corrosion resistance by forming a MnO ^{2 -} selective layer in an aqueous solution to inhibit the permeation of chloride ions. However, as the content increases, the workability and toughness of the steel decrease, so the content is controlled to 0.08 wt% or less.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability of the steel and the low temperature and impact toughness. However, when the content of copper (Cu) exceeds 0.1 wt%, copper (Cu) having a low melting point at the time of reheating for resistance to stress corrosion cracking and hot rolling is deposited on the grain boundaries of the steel and cracks are generated during hot working. It is preferable to control the content to 0.1% by weight or less.

Nickel (Ni)

In the present invention, nickel (Ni) is refined in crystal grains and solidified in austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving the low temperature impact toughness. However, when a large amount of nickel (Ni) is added in an amount exceeding 3.7% by weight, the weldability is deteriorated and a problem arises that it causes fuming in heat. Therefore, it is preferable that nickel (Ni) is controlled to 3.5 wt% to 3.7 wt% of the total weight of the steel material according to the present invention.

titanium( Ti )

Titanium (Ti) has the effect of improving the toughness and strength of steel by reducing the austenite grain growth by refining the structure of the welded part by generating Ti (C, N) high-temperature high-temperature exotherm. When titanium (Ti) is added in an amount exceeding 0.01% by weight, coarse precipitates are formed, which lowers the low-temperature impact properties of the steel and raises manufacturing costs without further effect of addition. It is preferable to control titanium (Ti) to 0.01% or less of the total weight of the steel material according to the present invention.

Niobium ( Nb )

Niobium (Nb) is one of the elements that greatly affects the strength of steel, and it plays a role of precipitating Nb (C, N), which is carbonitride precipitate in steel, or enhancing strength through solid solution strengthening in Fe. However, when the content of niobium (Nb) is added in excess of 0.01% by weight, the solubility of niobium (Nb) decreases with an increase in carbon (C) content, , It is preferable to control it to 0.01% or less of the total weight of the steel material according to the present invention.

Vanadium (V)

Vanadium (V) is an alloy element used for precipitation strengthening purpose, and is preferably controlled to 0.01% or less of the total weight of the steel according to the present invention.

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.

When boron (B) is added in an amount exceeding 5 ppm, formation of boron oxide may cause a problem of inhibiting the surface quality of the steel. Therefore, it is preferable to control the amount of boron (B) to 5 ppm or less in the steel according to the present invention.

Remark( Sn )

Tin (Sn) is an element having an effect of suppressing sedimentation from the surface of a steel sheet, and when the content of tin (Sn) exceeds 0.015 wt%, saturation suppression effect is saturated and segregation occurs in grain boundaries, . Therefore, tin (Sn) is preferably limited to 0.015 wt% or less of the entire steel sheet.

Nitrogen (N)

Nitrogen (N) combines with niobium (Nb) to form carbonitride, which causes fine grains to be refined. However, when added in large amounts, the amount of dissolved nitrogen is increased to lower the impact characteristics and elongation of the steel. Therefore, nitrogen is limited to 0.005% by weight or less of the total weight of the steel material according to the present invention.

Method of manufacturing steel sheet

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow chart schematically showing a method of manufacturing a steel sheet according to an embodiment of the present invention. FIG.

Referring to FIG. 1, a method of manufacturing a steel sheet according to an embodiment of the present invention includes a reheating step S110 for reusing a steel strip, a hot rolling step S120 for rolling to a final thickness, and a QT heat treatment for heat- 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 steel sheet manufacturing method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process can be obtained through a continuous casting process after obtaining a molten steel having a predetermined composition through a steelmaking process. Wherein the slab plate comprises 0.04 to 0.08% carbon (C), 0.20 to 0.30% silicon (Si), 0.50 to 0.70% manganese (Mn), 0.02 to 0.06% soluble aluminum (S_Al) (S): more than 0 and not more than 0.001%, chromium (Cr): more than 0 and not more than 0.1%, molybdenum (Mo): more than 0 and not more than 0.08%, copper (Cu): 0 (Ti): more than 0 and not more than 0.01%, niobium (Nb): more than 0 and not more than 0.01%, vanadium (V): more than 0 and not more than 0.01%, boron (B): more than 0 and not more than 5 ppm, tin (Sn): more than 0 and not more than 0.015%, nitrogen (N): not more than 0.005%, and the balance of iron (Fe) and other inevitably contained impurities. Here, the% means% by weight.

Slab reheat step

In the slab reheating step S110, the slab plate having the above composition is reheated at a slab reheating temperature (SRT) of 1,100 ° C to 1,200 ° C. This reheating can result in re-use of the segregated components and re-use of precipitates during casting. At this time, the slab may be a slab manufactured by a continuous casting process performed before the slab reheating step (S110).

When the reheating temperature of the slab is less than 1,100 DEG C, there is a problem that the heating load is insufficient and the rolling load becomes large. In addition, since the Nb-based precipitates do not reach the solid solution temperature, they can not be precipitated as fine precipitates upon hot rolling, and the austenite grain growth can not be suppressed, and the austenite grains are rapidly coarsened. On the contrary, when the reheating temperature exceeds 1,200 占 폚, the austenite grains are rapidly coarsened or the decarburization phenomenon occurs, which makes it difficult to secure the strength and low temperature toughness of a steel to be produced.

Hot rolling step

In the hot rolling step (S120), the reheated plate is subjected to finish hot rolling in the form of a steel sheet having a final thickness. The hot rolling may be performed at a finishing rolling temperature (FDT). Specifically, the finishing rolling temperature may be 900 ° C to 1,100 ° C. If the finish rolling temperature exceeds 1,100 DEG C, the austenite grains are coarsened and the ferrite grains are not sufficiently refined after the transformation, which may make it difficult to secure strength. On the contrary, when the finish rolling temperature is lower than 900 캜, the rolling load is induced to lower the productivity and reduce the heat treatment effect.

QT (Quenching + Tempering) heat treatment step

In the QT heat treatment step (S130), the hot-rolled plate is normalized at a temperature of 890 ° C to 930 ° C to form austenite, followed by quenching to transform it into martensite. Thereafter, for the purpose of improving toughness, a tempering process is performed at a temperature of 500 ° C to 700 ° C for 1 to 30 minutes.

At this stage, when the normalizing temperature is lower than 890 DEG C, the formation of austenite is insufficient and the steel can not be sufficiently transformed into martensite, so that the strength of the steel may be lowered. On the other hand, when the normalizing temperature exceeds 930 DEG C, the desired strength can not be secured due to the increase of the production cost and the austenite coarsening.

In particular, quenching can be carried out at an average cooling rate of 10 to 30 DEG C / sec. When the quenching rate is less than 10 ° C / sec, it may be difficult to secure sufficient strength. On the other hand, when the quenching rate exceeds 30 DEG C / sec, there is a problem that the strength is secured but the quenching toughness is lowered.

On the other hand, if the tempering temperature is less than 500 ° C, the effect is insignificant. On the contrary, if the tempering temperature exceeds 700 ° C, the excessive chemfiring process may be performed to deteriorate the mechanical properties of the heat treated steel sheet.

The steel sheet produced in the above steps S110 to S130 is subjected to a quenching and tempering (Q + T) heat treatment after rolling to have a tensile strength TS of 550 to 690 MPa, a yield strength YS of 380 to 620 MPa, Toughness: -130 占 폚, and low-temperature fatigue characteristic: -100 占 폚.

Example

Hereinafter, the structure and function of the present invention will be described in more detail with reference to preferred embodiments and comparative examples of the present invention. It should be understood, however, that this is provided as illustrative of the present invention and is not to be construed as limiting the invention in any way.

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 and 1 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 produced according to Examples 1 and 2 and Comparative Examples 1 and 2, ingots having respective compositions were prepared and subjected to a hot rolling process such as heating, hot rolling and cooling. Thereafter, the specimens prepared according to Examples 1 and 2 were subjected to normalizing, quenching and tempering. On the other hand, a TMCP (Thermo Mechcanical Controlling Process) process was performed on the specimens prepared according to Comparative Examples 1 and 2.

(Unit: wt%) division C Si Mn S_Al P S Cr Comparative Example 1 0.07 0.3 1.5 0.03 0.001 0.002 - Comparative Example 2 0.07 0.3 1.45 0.03 0.001 0.002 Example 1 0.061 0.25 0.6 0.03 0.0013 0.002 0.06 Example 2 0.056 0.25 0.6 0.03 0.0007 0.002 0.2

(Unit: wt%) division Mo Ni Ti Nb Comparative Example 1 - 0.2 0.012 0.035 Comparative Example 2 - 0.2 0.012 0.035 Example 1 - 3.55 - - Example 2 0.15 3.49 - -

division Cooling rate
(° C / sec) Quenching
(° C) Tempering
(° C) Remarks Comparative Example 1 8 (TMCP) - - FRT: 830 DEG C, FCT: 510 DEG C Comparative Example 2 8 (TMCP) - - FRT: 830 DEG C, FCT: 510 DEG C Example 1 30 (QT) 910 620 Rolling: AR, quenching termination: room temperature Example 2 3 (QT) 910 620 Rolling: AR, quenching termination: room temperature

2. Evaluation of mechanical properties

Table 4 shows the evaluation results of the mechanical properties of the specimens prepared according to Examples 1 and 2 and Comparative Examples 1 and 2. The tensile strength (TS), the yield strength (YS), the tensile strength 130 ° C and -80 ° C, and the results are shown.

division TS
(MPa) YS
(MPa) Impact 1
(-130 < 0 > C, J) Shock 2
(-130 < 0 > C, J) Shock 3
(-130 < 0 > C, J) Impact 1
(-80 < 0 > C, J) Shock 2
(-80 < 0 > C, J) Shock 3
(-80 < 0 > C, J) Comparative Example 1 540 398 - - - 351 38 27 Comparative Example 2 571 420 - - - 21 17 34 Example 1 679 563 250 257 253 Example 2 721 628 300 254 21

With reference to Tables 1 to 4, the specimens prepared according to Examples 1 and 2 exhibited the desired mechanical properties at a cryogenic temperature of -130 ° C, that is, a tensile strength (TS) of 550 to 690 MPa, a yield point YP ): 380 MPa or more, and good results were obtained in the impact toughness test conducted at -130 ° C.

On the other hand, the alloying elements of Example 1 and most of the alloying elements were added in a similar amount, but showed a large difference in the content of nickel (Ni), and Comparative Examples 1 and 2 in which the TMCP process was performed in place of the quenching (Q) The yield strength (YS) of the specimens produced according to It can also be seen that the impact toughness at -80 占 폚 shows a large difference from the specimens prepared at -130 占 폚 according to Examples 1 and 2.

FIG. 2 is a graph showing changes in shock absorption energy according to temperatures of a comparative steel and an inventive steel.

Referring to FIG. 2, A is a graph showing a change in shock absorption energy according to the steel temperature of the present invention, and B is a graph showing a change in impact absorption energy according to a temperature of a comparative steel. As shown in the drawing, the steel according to the present invention exhibits a high energy value of 250 J or more even at a cryogenic temperature of -130 ° C, which indicates excellent cryogenic toughness. On the other hand, in the case of the comparative steel (B), it can be seen that the energy value is appropriate at a temperature of about -60 ° C or more.

FIG. 3 is a graph showing changes in shock absorption energy depending on temperatures after the rolling, quenching, quenching and tempering steps of the steel of the present invention, and the results are shown at -110 ° C.

Referring to FIG. 3, it can be seen that both inventive steels exhibit higher energy values when the quenching and tempering steps are performed than when rolling and quenching steps are performed.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the appended claims. It will be understood that various modifications and changes may be made by those skilled in the art within the scope of the present invention.

S110: Reheating step
S120: Hot rolling step
S130: cooling step
S140: QT heat treatment step

Claims (7)

(C): 0.04% to 0.08%, silicon (Si): 0.20% to 0.30%, manganese (Mn): 0.50% to 0.70%, soluble aluminum (S_Al): 0.02% to 0.06% (S): more than 0 and not more than 0.001%, chromium (Cr): more than 0 and not more than 0.1%, molybdenum (Mo): more than 0 and not more than 0.08%, titanium (Ti): more than 0 0.01% or less, niobium (Nb): more than 0 and 0.01% or less, and the balance of iron (Fe) and other inevitably contained impurities,
And the content of nickel (Ni) is 3.5% by weight to 3.7% by weight of the entire steel sheet. The method according to claim 1,
(B): more than 0 and not more than 5 ppm, tin (Sn): more than 0 and not more than 0.015 wt%, and nitrogen (N) ): 0.005% by weight or less. The method according to claim 1,
The steel sheet has a tensile strength (TS) of 550 to 690 MPa, a yield point (YP) of 380 to 620 MPa or more, and a shock absorption energy of 250 J or more at -130 ° C. (Si): 0.20% to 0.30%, manganese (Mn): 0.50% to 0.70%, soluble aluminum (S_Al): 0.02% to 0.06% (S): more than 0 and not more than 0.001%, chromium (Cr): more than 0 and not more than 0.1%, molybdenum (Mo): more than 0 and not more than 0.08%, nickel (Ni) : 3.5 to 3.7%, titanium (Ti): more than 0 to 0.01%, niobium (Nb): more than 0 to 0.01%, and the balance of iron (Fe) and other inevitably contained impurities step; And
(b) normalizing the plate material at a temperature of 890 ° C to 930 ° C, quenching it, and then tempering the plate material at a temperature of 500 ° C to 700 ° C. 5. The method of claim 4,
(B): more than 0 and not more than 5 ppm, tin (Sn): not less than 0 and not more than 0.015% by weight, and copper (Cu) And nitrogen (N): 0.005 wt% or less. 5. The method of claim 4,
The step of producing the sheet material may include:
Reheating the slab plate;
Subjecting the reheated plate to finish hot rolling at a finish rolling temperature of 800 ° C or higher; And
And cooling the hot-rolled plate material. 5. The method of claim 4,
Wherein said quenching is performed at a rate of 10 to 30 DEG C / sec.

Images