KR20150076992A - Steel sheet and manufacturing method of the same - Google Patents

Abstract

A steel sheet excellent in corrosion resistance and a method for producing the same are disclosed.
The method for manufacturing a steel sheet according to the present invention comprises the steps of: (a) 0.03 to 0.06% of carbon (C), 0.2 to 0.3% of silicon (Si), 1.1 to 1.3% of manganese (Mn) 0.08% or less of sulfur (S), 0.0008% or less of sulfur, 0.02 to 0.05% of soluble aluminum (S-Al), 0.2 to 0.3% of copper, 0.04 to 0.05% of niobium, : 0.2 to 0.3% of chromium (Cr), 0.1 to 0.25% of molybdenum (Mo), 0.005 to 0.015% of titanium (Ti), 0.045 to 0.055% of vanadium (V) : 0.001 to 0.004% and the balance of iron (Fe) and unavoidable impurities; (b) reheating the slab sheet to a temperature of 1110 to 1150 占 폚; (c) hot rolling the reheated plate to a rolling finish temperature of 840 to 880 占 폚; And (d) cooling the hot-rolled plate to 430 to 510 ° C at an average cooling rate of 20 ° C / sec or more.

Description

STEEL SHEET AND MANUFACTURING METHOD OF THE SAME [0001]

The present invention relates to a steel sheet manufacturing method, and more particularly, to a steel sheet having excellent hydrogen-organic cracking resistance through alloy components and process control, and a manufacturing method thereof.

Line pipes used to transport crude oil or natural gas containing hydrogen sulfide (H ₂ S) are required to have resistance to corrosion caused by hydrogen induced cracking (HIC) in addition to strength, toughness and weldability.

Recently, the crude oil and natural gas sampling environment has expanded to polar regions and deep sea, and crude oil containing a large amount of hydrogen sulfide has been produced. Therefore, the line pipe steel sheet used for collecting or transporting crude oil containing hydrogen sulfide and the like is required to have high strength and good resistance to HIC.

A related art related to the present invention is Korean Patent Laid-Open Publication No. 10-2009-0071224 (published on Jul. 1, 2009), which discloses a hot-rolled steel sheet having excellent hydrogen- .

An object of the present invention is to provide a steel sheet having a tensile strength of 570 MPa or more and excellent resistance to hydrogen-induced organic cracking, and a method of manufacturing the same.

In order to achieve the above object, a steel sheet manufacturing method according to an embodiment of the present invention includes: (a) 0.03 to 0.06% of carbon (C), 0.2 to 0.3% of silicon (Si) (S): 0.0008% or less, S: Al: 0.02 to 0.05%, Cu: 0.2 to 0.3%, niobium (Nb) (Ti): 0.005 to 0.015%, vanadium (V): 0.04 to 0.05%, nickel (Ni): 0.2 to 0.3%, chromium (Cr): 0.15 to 0.25%, molybdenum 0.045 to 0.055%, calcium (Ca): 0.001 to 0.004%, and the balance of iron (Fe) and unavoidable impurities; (b) reheating the slab sheet to a temperature of 1110 to 1150 占 폚; (c) hot rolling the reheated plate to a rolling finish temperature of 840 to 880 占 폚; And (d) cooling the hot-rolled plate to 430 to 510 ° C.

At this time, it is preferable that the step (a) includes a light-hard pressing process in manufacturing the slab plate.

Further, in the step (d), it is preferable that the cooling is performed at an average cooling rate of 20 ° C / sec or more.

In order to achieve the above object, a steel sheet according to an embodiment of the present invention comprises 0.03 to 0.06% of carbon (C), 0.2 to 0.3% of silicon (Si), 1.1 to 1.3% of manganese (Mn) (P) of 0.08% or less, sulfur (S) of 0.0008% or less, soluble aluminum (S-Al) of 0.02 to 0.05%, copper (Cu) of 0.2 to 0.3%, niobium (Nb) , And the amount of nickel (Ni) is 0.2 to 0.3%, the content of chromium (Cr) is 0.15 to 0.25%, the content of molybdenum (Mo) is 0.1 to 0.2%, the content of titanium is 0.005 to 0.015% 0.001 to 0.004% of calcium (Ca), and the balance of iron (Fe) and unavoidable impurities.

At this time, the steel sheet may exhibit a tensile strength of 570 to 760 MPa, a HIC (Crack Length Ratio) of 10% or less, and an average of -10 ° C DWTT ductile fracture rate of 90% or more. In addition, the steel sheet may exhibit a yield strength of 485 to 635 MPa and an elongation of 30% or more.

The steel sheet manufacturing method according to the present invention is characterized in that low sulfur (S) and phosphorus (P) and copper (Cu), nickel (Ni), chromium (Cr) ), Molybdenum (Mo), calcium (Ca), and the like, it is possible to produce a steel sheet capable of exhibiting excellent hydrogen-organic cracking properties with a high strength of 570 MPa or higher in tensile strength.

Accordingly, the steel sheet according to the present invention can be utilized as a material for a line pipe for natural gas, crude oil picking, transportation, and the like.

1 is a flowchart showing a method of manufacturing a steel sheet 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. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Hereinafter, a steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Steel plate

The steel sheet according to the present invention contains 0.03 to 0.06% of carbon (C), 0.2 to 0.3% of silicon (Si), 1.1 to 1.3% of manganese (Mn), 0.08% or less of phosphorus (P) (S): 0.0008% or less, S-Al: 0.02 to 0.05%, Cu: 0.2 to 0.3%, niobium: 0.04 to 0.05%, nickel: 0.2 to 0.3% 0.1 to 0.25% of chromium (Cr), 0.1 to 0.2% of molybdenum (Mo), 0.005 to 0.015% of titanium (Ti), 0.045 to 0.055% of vanadium (V) .

The remainder is composed of iron (Fe) and impurities inevitably included in the steelmaking process.

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

Carbon (C)

In the present invention, carbon (C) is added to secure strength.

Carbon (C) is preferably added in a content ratio of 0.03 to 0.06% by weight based on the total weight of the steel sheet. When the content of carbon (C) is less than 0.03 wt%, it may be difficult to secure strength. On the contrary, when the content of carbon (C) exceeds 0.06% by weight, the strength of the steel increases, but the impact resistance and weldability are deteriorated at low temperatures.

Silicon (Si)

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. Silicon (Si) also has a solid solution strengthening effect.

Silicon (Si) is preferably added in a content ratio of 0.2 to 0.3% by weight of the total weight of the steel sheet. If the content of silicon (Si) is less than 0.2% by weight, the effect of adding silicon can not be exhibited properly. On the contrary, when the content of silicon (Si) exceeds 0.3% by weight, the toughness and weldability are lowered, and the oxide inclusions in the steel are increased, which may lower the low temperature toughness and the hydrogen organic cracking resistance.

Manganese (Mn)

Manganese (Mn) is an element which increases the strength and toughness of steel and increases the ingotability of steel. Addition of manganese (Mn) causes less deterioration of ductility when the strength is higher than that of carbon (C).

Manganese (Mn) is preferably added in a content ratio of 1.1 to 1.3% by weight based on the total weight of the steel sheet. If the content of manganese (Mn) is less than 1.1% by weight, it may be difficult to secure strength even if the content of carbon (C) is high. On the other hand, when the content of manganese (Mn) exceeds 1.3% by weight, the amount of MnS-based nonmetal inclusions increases, which may lead to defects such as cracking during welding.

Phosphorus (P), sulfur (S)

Phosphorus (P) is an impurity which is inevitably contained at the time of production, and is contained in the steel to deteriorate the weldability and toughness, and is segregated at the center of the slab and at the austenite grain boundary during solidification. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.08% by weight or less based on the total weight of the steel sheet.

Sulfur (S), together with phosphorus (P), is an element that is inevitably contained in the production of steel, and forms MnS to lower impact toughness at low temperatures. Therefore, in the present invention, the content of sulfur (S) is limited to 0.0008 wt% or less of the total weight of the steel sheet.

Soluble Al (Sol. Al)

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

The soluble aluminum is preferably added in an amount of 0.02 to 0.05% by weight based on the total weight of the steel sheet. When the content of soluble aluminum is less than 0.02 wt% of the total weight of the steel sheet, the deoxidizing effect described above can not be exhibited properly. On the other hand, when the content of soluble aluminum exceeds 0.05% by weight of the total weight of the steel sheet, the slab surface quality deteriorates.

Copper (Cu)

Copper (Cu) is an element effective for increasing the strength of steel and improving toughness. Copper (Cu), together with silicon (Si) and manganese (Mn), also contributes to the strengthening effect of the steel through controlled amount of content.

Copper (Cu) is preferably added in an amount of 0.2 to 0.3% by weight based on the total weight of the steel sheet. When the content of copper (Cu) is less than 0.2% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) is more than 0.3% by weight, cracks are generated on the surface during hot rolling, thereby deteriorating the surface quality.

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 in a content ratio of 0.04 to 0.05% by weight based on the total weight of the steel sheet. When the content of niobium (Nb) is less than 0.04% by weight, the effect of adding niobium can not be exhibited properly. On the contrary, when the content of niobium (Nb) exceeds 0.05% by weight, coarse secondary phases including niobium are generated and act as a starting point of hydrogen organic cracking.

Nickel (Ni)

Nickel (Ni) fine grains and solidify in the austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving the low-temperature impact toughness.

Nickel (Ni) is preferably added at a content ratio of 0.2 to 0.3% by weight of the total weight of the steel sheet. When the content of nickel (Ni) is less than 0.2% 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.

Chromium (Cr)

Chromium (Cr) is an effective element added to secure strength.

Cr (Cr) is preferably added in a content ratio of 0.15 to 0.25% by weight of the total weight of the steel sheet. If the content of chromium is less than 0.15% by weight, the effect of the addition is insufficient. On the contrary, when the content of chromium exceeds 0.25% by weight, the weldability and the heat-affected zone (HAZ) toughness are lowered.

Molybdenum (Mo)

Molybdenum (Mo) contributes to improving the strength and toughness of steel and securing stable strength at room temperature and high temperature.

Molybdenum (Mo) is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel sheet. When the addition amount of molybdenum is less than 0.1% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of molybdenum exceeds 0.2% by weight, there is a problem that the weldability is lowered.

Titanium (Ti)

Titanium (Ti) has the effect of improving the toughness and strength of steel by refining the texture of the welded part by inhibiting the growth of austenite crystal grains during welding by generating precipitates of Ti (C, N) having high stability at high temperatures.

Titanium (Ti) is preferably added in a content ratio of 0.005-0.015 wt% of the total weight of the steel sheet. When the content of titanium (Ti) is less than 0.005% by weight, there arises a problem that aging hardening occurs due to the remaining solid carbon and nitrogen employed without precipitation. On the contrary, when the content of titanium (Ti) exceeds 0.015 wt%, a large amount of inclusions may exist on the surface of the steel sheet.

Vanadium (V)

Vanadium (V) acts as a pinning to the grain boundaries and contributes to the improvement of strength.

Vanadium (V) is preferably added at a content ratio of 0.045 to 0.055% by weight based on the total weight of the steel sheet. When the content of vanadium (V) is less than 0.045% by weight, it may be difficult to exhibit the above effect properly. On the contrary, when the content of vanadium (V) exceeds 0.055% by weight, coarse vanadium precipitates are formed to deteriorate low-temperature impact toughness.

Calcium (Ca)

Calcium (Ca) is added for the purpose of improving electrical resistance weldability by inhibiting the formation of MnS inclusions by forming CaS inclusions. That is, calcium (Ca) has a higher affinity with sulfur than manganese (Mn), so CaS inclusions are formed and CaS inclusions are reduced when calcium is added. Such MnS is stretched during hot rolling to cause hook defects and the like in electrical resistance welding (ERW), so that electrical resistance weldability can be improved.

The calcium (Ca) is preferably added in an amount of 0.001 to 0.004% by weight based on the total weight of the steel sheet according to the present invention. If the content of calcium is less than 0.001% by weight, the above effects can not be exhibited properly. On the contrary, when the content of calcium (Ca) exceeds 0.004% by weight, generation of CaO inclusions becomes excessive, and performance is deteriorated.

The steel sheet according to the present invention preferably has a tensile strength of 570 to 760 MPa, a hydrogen induced cracking (HIC) -crack length ratio (CLR) of 10% or less and a -10 DEG C DWTT ductile fracture percentage average 90% or more. In addition, the steel sheet according to the present invention can exhibit a yield strength of 485 to 635 MPa and an elongation of 30% or more.

Steel manufacturing method

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

Referring to FIG. 1, the steel sheet manufacturing method includes a slab manufacturing step S110, a slab reheating step S120, a hot rolling step S130, and a cooling step S140.

Slab manufacturing

In the slab manufacturing step (S110), 0.03 to 0.06% of carbon (C), 0.2 to 0.3% of silicon (Si), and 0.03 to 0.06% of a steel material having the above- (P): 0.08% or less, S: not more than 0.0008%, soluble aluminum (S-Al): 0.02 to 0.05%, copper (Cu): 0.2 to 0.3% 0.04 to 0.05% of niobium (Nb), 0.2 to 0.3% of nickel, 0.15 to 0.25% of chromium (Cr), 0.1 to 0.2% of molybdenum (Mo) and 0.005 to 0.015% of titanium (Ti) , 0.045 to 0.055% of vanadium (V), 0.001 to 0.004% of calcium (Ca), and the balance of iron (Fe) and unavoidable impurities.

In the present invention, the aim is to secure hydrogen organic cracking property. In order to minimize the hydrogen content, the reflux time in the RH degassing process is more than 18 minutes. .

Further, in the present invention, it is more preferable to perform the soft-pressing treatment at a reduction rate of about 1 to 3% in continuous casting. In this case, the center segregation can be eliminated and the quality of the slab can be improved.

Reheating slabs

In the slab reheating step S120, the produced slab plate is reheated to 1110 to 1150 ° C. The slab plate having the above composition can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. Manganese (Mn) and phosphorus (P) segregation on the slab are relaxed by diffusion during reheating. Further, by sufficiently employing niobium (Nb), austenite crystal grains can be finely controlled by using a pinning effect.

If the reheating temperature is lower than 1110 ° C, the segregation can not be sufficiently diffused and the low temperature toughness and the hydrogen organic cracking resistance are deteriorated. On the other hand, when the reheating temperature exceeds 1150 占 폚, the crystal grain size of the austenite increases, resulting in a problem of low temperature toughness.

Hot rolling

In the hot rolling step (S130), the reheated slab plate is rolled in the furnace.

The hot rolling is preferably carried out at a rolling end temperature of 840 to 880 캜. If the rolling finish temperature is less than 840 캜, the toughness deterioration and yield ratio due to the abnormal reverse rolling may be increased. On the other hand, when the rolling finish temperature exceeds 880 DEG C, it is difficult to obtain strength and toughness due to recrystallization and crystal grain coarsening.

Cooling

In the cooling step (S140), the hot-rolled steel sheet is cooled to 430 to 510 ° C in order to produce a steel sheet having a uniform and fine structure. When the cooling termination temperature is lower than 430 캜, a large amount of low-temperature transformed structure is formed, which lowers the low-temperature toughness. On the other hand, when the cooling end temperature exceeds 510 ° C, it is difficult to obtain sufficient strength due to the formation of coarse microstructure.

On the other hand, cooling is preferably performed at an average cooling rate of 20 DEG C / sec or more. When the average cooling rate exceeds 20 DEG C / sec, physical properties such as strength may be lowered due to a large amount of ferrite and pearlite transformation.

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. Specimen Manufacturing

The specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the compositions shown in Tables 1-1 and 1-2 and the process conditions shown in Table 2.

[Table 1-1] (Unit:% by weight)

[Table 1-2] (Unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

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

 [Table 3]

Referring to Table 3, the specimens prepared according to Examples 1 to 3 satisfied the target values of strength, elongation, HIC-CLR, and -10 DEG C DWTT characteristics.

On the contrary, in the case of the specimen according to Comparative Example 1 in which the carbon content was high and the cooling rate was relatively slow, the HIC-CLR characteristic was relatively poor. In addition, the specimen according to Comparative Example 2, which did not contain elements such as molybdenum and nickel, and which had relatively high reheating temperature of the slab, had relatively poor strength characteristics.

2 shows the results of HIC evaluation of the specimens according to Comparative Example 1 and Example 1. Fig.

Referring to FIG. 2, it can be seen that, in the case of the specimen according to Example 1, no cracks were generated at all. On the contrary, in the case of the specimen according to Comparative Example 1, it can be seen that a lot of cracks were generated.

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 manufacturing step
S120: Slab reheating step
S120: Hot rolling step
S130: cooling step

Claims (6)

(S): 0.2 to 0.3%, manganese (Mn): 1.1 to 1.3%, phosphorus (P): 0.08% or less, sulfur (S) : 0.0008% or less, S-Al: 0.02 to 0.05%, Cu: 0.2 to 0.3%, Nb: 0.04 to 0.05%, Ni: 0.2 to 0.3% 0.1 to 0.25% of Cr, 0.1 to 0.2% of molybdenum, 0.005 to 0.015% of titanium, 0.045 to 0.055% of vanadium, 0.001 to 0.004% of calcium, (Fe) and inevitable impurities;
(b) reheating the slab sheet to a temperature of 1110 to 1150 占 폚;
(c) hot rolling the reheated plate to a rolling finish temperature of 840 to 880 占 폚; And
(d) cooling the hot-rolled plate to 430 to 510 占 폚.
The method according to claim 1,
The method of claim 1, wherein the step (a) comprises a light-hardening step in the production of the slab sheet.
The method according to claim 1,
In the step (d), the cooling is performed at an average cooling rate of 20 ° C / sec or more.
(P): 0.08% or less, sulfur (S): 0.0008% or less, carbon (C): 0.03 to 0.06%, silicon (Si): 0.2 to 0.3%, manganese (Ni): 0.2 to 0.3%, chromium (Cr): 0.02 to 0.05%, copper (Cu): 0.2 to 0.3%, niobium (Nb) 0.1 to 0.25% of molybdenum, 0.1 to 0.2% of molybdenum (Mo), 0.005 to 0.015% of titanium (Ti), 0.045 to 0.055% of vanadium (V), 0.001 to 0.004% of calcium (Ca) And other unavoidable impurities.
5. The method of claim 4,
Wherein the steel sheet exhibits a tensile strength of 570 to 760 MPa, a HIC (Crack Length Ratio) of 10% or less and a -10 캜 DWTT ductile fracture rate average of 90% or more.
6. The method of claim 5,
Wherein the steel sheet has a yield strength of 485 to 635 MPa and an elongation of 30% or more.

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