KR20150112514A - Hot-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

Disclosed are a hot-rolled steel sheet having excellent low-temperature toughness and corrosion resistance, and a manufacturing method thereof. According to the present invention, the method to manufacture a hot-rolled steel sheet comprises the steps of: reheating, at a temperature of 1200°C or higher for two hours or longer, a slab plate composed of 0.04-0.06 wt% of carbon (C), 0.1-0.2 wt% of silicon (Si), 0.4-0.8 wt% of manganese (Mn), 0.01 wt% or less of phosphorus (P), 0.001 wt% or less of sulfur (S), 0.02-0.05 wt% of aluminum (Al), 0.04-0.08 wt% of vanadium (V), 0.02-0.05 wt% of niobium (Nb), 0.01-0.02 wt% of titanium (Ti), 0.5-0.7 wt% of chromium (Cr), 0.001-0.003 wt% of calcium (Ca), 0.006 wt% or less of nitrogen (N), and the remainder consisting of iron (Fe) and inevitable impurities; rough-rolling the reheated plate in a temperature region of recrystallizing austenite; finish-rolling the plate having completed the rough-rolling in a non-recrystallizing region of the austenite; and cooling the plate having completed the finish-rolling, and winding the plate at a temperature equal to or lower than 600°C.

Description

TECHNICAL FIELD [0001] The present invention relates to a hot-rolled steel sheet,

More particularly, the present invention relates to a hot-rolled steel sheet excellent in low-temperature toughness and corrosion resistance and a method of manufacturing the same.

In recent years, mining and transportation of crude oil and gas have been increasing in harsh environments where steel pipes can corrode due to hydrogen sulfide (H ₂ S) gas as in the deep sea. As a result, large-scale pipeline construction for crude oil and gas mining and transportation is also increasing.

The material used for such pipelines must have high strength, good corrosion resistance to hydrogen sulfide gas, and excellent low temperature toughness.

A background art related to the present invention is Korean Patent Laid-Open Publication No. 10-0770572 (published on October 26, 2007), which discloses a high carbon steel sheet excellent in quenching heat treatment characteristics and a method for manufacturing the same.

An object of the present invention is to provide a hot-rolled steel sheet excellent in corrosion resistance and low-temperature toughness and a method for producing the same.

In order to achieve the above object, the present invention provides a method for manufacturing a hot-rolled steel sheet, comprising the steps of: 0.04 to 0.06% of carbon (C), 0.1 to 0.2% of silicon (Si) (V): 0.04 to 0.08%, niobium (Nb): 0.02 to 0.05%, phosphorus (P): 0.01% or less, sulfur (S) And a slab made of unavoidable impurities and iron (Fe) in an amount of 0.001 to 0.003% of calcium (Ca), 0.006 to less than 0.006% of titanium (Ti), 0.5 to 0.7% of chromium (Cr) Reheating the plate material at a temperature of 1200 캜 or more for 2 hours or more; Subjecting the reheated plate to rough rolling in the austenite recrystallization temperature range; Subjecting the roughly rolled plate to finish rolling in a non-austenite recrystallization zone; And cooling the sheet material after the finish rolling, and winding the sheet material at a temperature of 600 ° C or lower.

At this time, the rough rolling is performed at a reduction ratio of 80% or more, and the finishing rolling is preferably performed at a reduction ratio of 70% or more.

Further, it is preferable that the cooling is performed at an average cooling rate of 20 DEG C / sec or more.

In order to achieve the above object, the hot-rolled steel sheet according to an embodiment of the present invention includes 0.04 to 0.06% of carbon (C), 0.1 to 0.2% of silicon (Si), 0.4 to 0.8% of manganese (Mn) (P): not more than 0.01%, sulfur (S): not more than 0.001%, aluminum (Al): 0.02 to 0.05%, vanadium (V): 0.04 to 0.08%, niobium (Nb) (Fe) and unavoidable impurities, and the surface area of the steel sheet is in the range of 0.01 to 0.02% of Ti, 0.5 to 0.7% of Cr, 0.001 to 0.003% of Ca, 0.006% Shaped ferrite and precipitates at a rate of 90% or more.

The hot-rolled steel sheet has a strength of a yield strength of 380 to 500 MPa, a tensile strength of 510 to 680 MPa, a CLR of 6% or less after HIC test, a CTR of 3% or less and a CSR of 1.5% or less and a low temperature toughness of- .

According to the method for producing a hot-rolled steel sheet according to the present invention, strength, corrosion resistance, and corrosion resistance can be improved through control of alloy components such as manganese (Mn), vanadium (V), and chromium (Cr), and process control such as rough rolling, A hot-rolled steel sheet excellent in low-temperature toughness can be produced.

Accordingly, the hot-rolled steel sheet according to the present invention is suitable for use as a material for an oil pipeline or the like in a corrosive environment due to hydrogen sulfide gas, such as a deep sea floor.

1 shows a method of manufacturing a hot-rolled steel sheet according to the present invention.
Fig. 2 shows the results of C-scan after HIC test for specimen 2 and specimen 3. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with 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 contains 0.04 to 0.06% of carbon (C), 0.1 to 0.2% of silicon (Si), 0.4 to 0.8% of manganese (Mn) (S): 0.001% or less, aluminum (Al): 0.02 to 0.05%, vanadium (V): 0.04 to 0.08%, niobium (Nb): 0.02 to 0.05% (Cr): 0.5 to 0.7%, calcium (Ca): 0.001 to 0.003%, and nitrogen (N): 0.006% or less.

The rest of the above components are composed of iron (Fe) and impurities inevitably incorporated in the steelmaking process.

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 the material strength and for microstructure control, and in order to improve the corrosion resistance while forming the needle-shaped ferrite with the low temperature structure to have sufficient strength even in the after-treatment, 0.04% by weight or more is required. However, when the carbon content is more than 0.06 wt.%, A γ phase which is difficult to diffuse in the liquid phase is formed to cause diffusion inhibition of the alloying element, so there is a great possibility that segregation will occur, and the impact toughness is deteriorated.

Therefore, in the present invention, the content of carbon is limited to 0.04 to 0.06 wt% of the total weight of the steel sheet.

Manganese (Mn)

Manganese (Mn) is a very effective element for solid solution strengthening and improves the hardenability of steel and is an effective element for securing strength. Further, manganese can contribute to grain refinement of ferrite by delaying ferrite and pearlite transformation as an austenite stabilizing element. These effects are remarkable when 0.4 wt% or more of manganese is added. However, when manganese is added in excess of 0.8 wt%, it greatly reduces the weldability, generates MnS inclusions, and induces center segregation, thereby greatly reducing the corrosion resistance of the steel.

Therefore, the content of manganese is preferably 0.4 to 0.8% by weight of the total weight of the steel sheet, and more preferably 0.4 to 0.6% by weight in view of improvement in corrosion resistance.

Silicon (Si), aluminum (Al)

Silicon (Si) acts as a deoxidizer and contributes to strength improvement. The silicon is preferably contained 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 silicon is less than 0.1% by weight, the effect of the addition is insufficient. On the contrary, when the addition amount of silicon exceeds 0.2% by weight, the weldability of the steel is lowered, and the re-heating process during hot rolling and the formation of the rolling scale during hot rolling may cause problems in surface quality, .

Aluminum (Al) together with silicon (Si) acts as a deoxidizer. The aluminum is preferably contained in an amount of 0.02 to 0.05% by weight based on the total weight of the steel sheet. When the content of aluminum is less than 0.02% by weight, the effect of the addition is insufficient. On the contrary, when the content of aluminum exceeds 0.05% by weight, the surface quality of the slab may be deteriorated.

Phosphorus (P), sulfur (S), nitrogen (N)

Phosphorus (P) is deteriorated in weldability and becomes a factor which deteriorates corrosion resistance by slab center segregation. In the present invention, the content of phosphorus is limited to 0.01% or less of the total weight of the steel sheet.

Sulfur (S) inhibits the toughness and weldability of steel and increases MnS nonmetallic inclusions, which causes corrosion resistance of steel to deteriorate. Therefore, in the present invention, the sulfur content is limited to 0.001% or less of the total weight of the steel sheet.

Nitrogen (N) contributes to refining the crystal grains by forming carbonitride by bonding with titanium (Ti), niobium (Nb) and the like. However, when nitrogen is contained in a large amount in the steel, the amount of solute nitrogen is increased, which deteriorates the impact characteristics and elongation of the steel and greatly deteriorates the toughness of the welded part. In the present invention, the content of nitrogen is limited to 0.006% by weight or less based on the total weight of the steel sheet.

Vanadium (V)

Vanadium (V) contributes to the improvement of the strength of the steel sheet by precipitating carbonitride precipitates in the steel or strengthening the solid solution in the steel (Fe). Particularly, in the present invention, the content of manganese is 0.8 wt% or less in order to improve the corrosion resistance, so that the strength can be compensated by adding vanadium.

The vanadium is preferably added in an amount of 0.04 to 0.08% by weight based on the total weight of the steel sheet. When the addition amount of vanadium is less than 0.04% by weight, the effect of compensating the strength is insufficient. On the other hand, when vanadium is added in an amount exceeding 0.08% by weight, the production cost of the steel sheet may be significantly increased without further effect, and the toughness of the steel sheet may be deteriorated.

Niobium (Nb)

Niobium (Nb) contributes to enhance the strength of the steel sheet through precipitation of carbonitride in the steel or solid solution strengthening in Fe. Particularly, niobium increases the recrystallization temperature by raising the recrystallization temperature during rolling, thereby increasing the non-recrystallization reverse pressurization, securing the gamma-alpha transformation temperature due to the reduction of manganese and ensuring sufficient yarn rolling region, And serves to improve the low temperature toughness by refining the crystal grains. Further, in the present invention, since the carbon content is 0.06 wt% or less, the precipitation strengthening effect can be increased by maximizing Nb employment in austenite.

The niobium 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 addition amount of niobium is less than 0.02% by weight, the effect of addition thereof is insufficient. On the contrary, when the addition amount of niobium exceeds 0.05% by weight, the solubility of niobium decreases due to carbon, so that niobium is not completely dissolved, and there is a fear that the material and corrosion resistance are disadvantageous.

Titanium (Ti)

Titanium (Ti) produces Ti (C, N) precipitates with high stability at high temperatures, which hinders the growth of austenite grains during welding and makes the welded structure finer, thereby contributing to improving the toughness and strength of steel.

When the titanium is added in an amount of 0.01% by weight or more based on the total weight of the steel sheet, the above effect is sufficiently exhibited. However, when the addition amount of titanium exceeds 0.02% by weight, coarse precipitates may be formed, which may cause a decrease in corrosion resistance of the steel.

Chromium (Cr)

Chromium (Cr) contributes to suppression of center segregation by increasing diffusion of manganese (Mn) during slab manufacturing and contributes to strength improvement by forming a low temperature transformation phase upon cooling after hot rolling. In addition, chromium, together with molybdenum described above, forms a large amount of carbide during the heat treatment after joining to contribute to improvement of corrosion resistance.

Considering that the content of manganese in the present invention is 0.8 wt% or less, the effect of low temperature toughness effect can be sufficiently exhibited when the chromium is added in an amount of 0.5 wt% or more of the total weight of the steel sheet. However, when the addition amount of chromium exceeds 0.7% by weight, the weldability may be deteriorated.

Calcium (Ca)

Calcium (Ca) functions to prevent formation of MnS inclusions which form CaS inclusions and cause corrosion resistance and deterioration of weldability. When calcium is added in an amount of 0.001% by weight or more based on the total weight of the steel sheet, the above effect is sufficiently exhibited. However, when the addition amount of calcium exceeds 0.003% by weight, the playability is deteriorated.

The hot-rolled steel sheet according to the present invention having the above alloy composition may have a microstructure including an acicular ferrite and a precipitate having an area ratio of 90% or more. In addition, the microstructure such as bainite, pearlite, and polygonal ferrite can be 10% or less in area ratio.

The hot-rolled steel sheet according to the present invention has a strength of a yield strength of 380 to 500 MPa, a tensile strength of 510 to 680 MPa, a crack length ratio (CLR) of 6% or less, a CTR (crack thickness ratio) of 3% And CSR (Crack Sensitivity Ratio) of 1.5% or less and a low temperature toughness of -60 ° C and an average impact toughness of 100 J or more.

Hot-rolled steel sheet manufacturing method

1 shows a method of manufacturing a hot-rolled steel sheet according to the present invention.

Referring to FIG. 1, the illustrated hot-rolled steel sheet manufacturing method includes a slab reheating step S110, a rough rolling step S120, a finishing rolling step S130, and a cooling / winding step S140.

First, in the slab reheating step (S110), the semi-finished slab material having the above-described alloy composition is reheated. Through the reheating of the slab, the segregated components in the casting can be reused and reused such as niobium can be achieved.

The slab reheating is preferably performed at a temperature of 1200 ° C or more for 2 hours or more, and more preferably for 2 to 4 hours at 1200 to 1300 ° C. If the reheating temperature of the slab is less than 1200 占 폚 or the reheating time is less than 2 hours, sufficient reuse of niobium (Nb) or the like may not be achieved.

Next, in the rough rolling step (S120), the reheated plate is rough-rolled in the austenite recrystallization temperature range, approximately 900 to 950 占 폚.

The rough rolling can be performed at a reduction ratio of 80% or more so that sufficient rolling can be performed in the finishing rolling described later.

Next, in the finishing rolling step (S130), the roughly rolled plate is finished in an austenite non-recrystallized region, approximately 820 to 880 캜. In the present invention, it is possible to finish rolling at a high temperature due to an increase in the austenite non-recrystallization temperature. It is preferable that the finishing rolling is performed at a reduction ratio of 70% or more so that a sufficient shear region can be formed for formation of a nucleation site of gamma alpha.

Next, in the cooling / winding step (S140), the plate material on which the finish rolling has been completed is cooled and wound.

At this time, the cooling is preferably performed at an average cooling rate of 20 ° C / sec or more, more preferably 25 to 60 ° C / sec to 600 ° C or less, and more preferably 500 to 600 ° C. In the present invention, the reason why the average cooling rate after hot rolling is 20 ° C / sec or more and the cooling end temperature is limited to 600 ° C or less is to accelerate the transformation of gamma to alpha and to form a fine precipitate at the time of transformation. When the average cooling rate during cooling is less than 20 占 폚 / sec, or when the cooling termination temperature exceeds 600 占 폚, the crystal grains may coarsen and the precipitate formation may be insufficient.

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

1 to 5 were prepared according to the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Property evaluation

The tensile test, the impact test and the HIC (Hydrogen induced cracking) test were performed on the prepared specimens 1 to 5, and the results are shown in Table 3. The results of the C-scan after the HIC test of the specimen 2 and the specimen 3 are shown in Fig.

[Table 3]

Referring to Table 4, in the case of Specimen 1 and Specimen 2 satisfying the alloy composition and manufacturing conditions proposed in the present invention, it was found that the specimens 1 and 2 exhibited high strength of yield strength of 380 to 500 MPa and tensile strength of 510 to 680 MPa, Respectively.

On the other hand, the specimen 3 containing a relatively high content of carbon and manganese and having no titanium had excellent strength characteristics, but was poor in internal characteristics and low temperature impact toughness.

Also, referring to FIG. 2, it can be seen that, in the case of the specimen 3, a large amount of cracks occurred compared to the specimen 2. As a result, it can be seen that the specimen 2 has remarkably excellent anti-corrosion properties as compared with the specimen 3.

On the other hand, in the case of the specimen 4 in which the cooling conditions after hot rolling did not meet the conditions proposed in the present invention and the specimen 5 in which the slab reheating temperature was relatively low, the internal characteristics were good, but the strength characteristics were relatively poor .

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.

Claims (5)

(P): 0.01% or less, sulfur (S): 0.001% or less, carbon (C): 0.04 to 0.06%, silicon (Si): 0.1 to 0.2%, manganese (Ti): 0.01 to 0.02%, and chromium (Cr): 0.5 to 0.7%, and more preferably, the aluminum (Al): 0.02 to 0.05%, the vanadium (V): 0.04 to 0.08%, the niobium Reheating the slab plate composed of 0.001 to 0.003% of calcium (Ca), 0.006% or less of nitrogen (N) and the balance of iron (Fe) and unavoidable impurities at a temperature of 1200 ° C or more for 2 hours or more;
Subjecting the reheated plate to rough rolling in the austenite recrystallization temperature range;
Subjecting the roughly rolled plate to finish rolling in a non-austenite recrystallization zone; And
And cooling the sheet material after the finish rolling and winding the sheet material at a temperature of 600 캜 or less.
The method according to claim 1,
The rough rolling is performed at a reduction ratio of 80% or more,
Wherein the finishing rolling is performed at a reduction ratio of 70% or more.
The method according to claim 1,
Wherein the cooling is performed at an average cooling rate of 20 DEG C / sec or more.
(P): 0.01% or less, sulfur (S): 0.001% or less, carbon (C): 0.04 to 0.06%, silicon (Si): 0.1 to 0.2%, manganese (Ti): 0.01 to 0.02%, and chromium (Cr): 0.5 to 0.7%, and more preferably, the aluminum (Al): 0.02 to 0.05%, the vanadium (V): 0.04 to 0.08%, the niobium 0.001 to 0.003% of calcium (Ca), 0.006% or less of nitrogen (N), and the balance of iron (Fe) and unavoidable impurities,
Characterized in that the steel sheet has a microstructure including acicular type ferrite and precipitate having an area ratio of 90% or more.
5. The method of claim 4,
The hot-rolled steel sheet preferably has a yield strength of 380 to 500 MPa, a tensile strength of 510 to 680 MPa, a crack length ratio (CRR) of 6% or less after HIC test, a CTR (crack thickness ratio) of 3% or less, and a CSR (crack sensitivity ratio) of 1.5% And a low-temperature toughness of -60 deg. C and an impact toughness of 100 J or higher.

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