KR101412427B1 - High strength steel plate and method for manufacturing the steel plate - Google Patents

Abstract

A high-strength steel sheet excellent in thickness direction characteristics and low-temperature impact toughness at a central portion, and a method for producing the same.
The method for manufacturing a high strength steel sheet according to the present invention comprises 0.05 to 0.12% of carbon (C), 0.05 to 0.3% of silicon (Si), 1.4 to 2.0% of manganese (Mn) , 0.005% or less of sulfur (S), 0.01 to 0.04% of niobium (Nb), 0.02 to 0.05% of titanium (Ti), 0.05 to 0.2% of molybdenum (Mo) (B): not more than 0.0005 to 0.002% and not more than 0.02% of calcium (Ca), not more than 0.4% of copper (Cu) and not more than 0.4% of nickel (Fe) and inevitable impurities at 900 to 1150 캜; Subjecting the reheated plate to primary rolling in an austenite recrystallization zone; Secondarily rolling the primary rolled plate at a finish rolling temperature of 650 to 850 캜; And cooling the secondary rolled plate to 300 to 600 ° C.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet,

The present invention relates to a high-strength steel sheet and a method of manufacturing the same, and more particularly, to a high-strength steel sheet excellent in thickness direction characteristics and excellent in impact toughness at a low temperature region through alloy components and process control, and a method for manufacturing the same.

In recent years, the application of high-strength extreme-strength steel plates with a thickness of 70 mm or more has been gradually increasing in upper decks and hatch coaming portions of super-large container ships of 12,000 TEU (Twenty-foot Equivalent Unit)

In the case of a superficial steel sheet having a thickness of 70 mm or more, the variations in the microstructure and material properties of the steel sheet are very large depending on the alloy components and the manufacturing process. Particularly, since the steel sheet has a large material deviation in the thickness direction and the core low temperature impact toughness is poor, it is very difficult to satisfy the characteristics required by the consumer.

Background art related to the present invention is a high strength steel material excellent in low temperature impact toughness disclosed in Korean Patent Laid-Open Publication No. 10-2010-0062693 (published on Jun. 10, 2010) and a manufacturing method thereof.

It is an object of the present invention to provide a method of manufacturing a high strength steel sheet having excellent thickness direction properties through an alloy component and process control, and also having excellent low temperature impact toughness at the center.

Another object of the present invention is to provide a high strength steel sheet which is manufactured by the above method and has a small variation in material in the thickness direction and is excellent in low temperature impact toughness at the center portion so that it can be utilized as a material for shipbuilding.

In order to accomplish the above object, according to an embodiment of the present invention, there is provided a method of manufacturing a high strength steel sheet, comprising: 0.05 to 0.12% carbon, 0.05 to 0.3% silicon, manganese (Mn) (S): more than 0% to 0.005%, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.02 to 0.05% (Cu): not less than 0% to not more than 0.4%, and nickel (B): 0.05 to 0.2% of molybdenum (Mo), more than 0 to 0.005% (Ni): more than 0% to 0.4%; and reheating the slab plate made of the remaining iron (Fe) and unavoidable impurities at 900 to 1150 캜; Subjecting the reheated plate to primary rolling in an austenite recrystallization zone; Secondarily rolling the primary rolled plate at a finish rolling temperature of 650 to 850 캜; And cooling the secondary rolled plate to 300 to 600 ° C.

At this time, the slab plate material may further include 0% to 0.02% by weight of calcium (Ca).

The high strength steel sheet according to an embodiment of the present invention includes 0.05 to 0.12% carbon, 0.05 to 0.3% silicon, and manganese (Mn) in weight percent, (S): more than 0% to 0.005%, niobium (Nb): 0.01 to 0.04%, and titanium (Ti): 0.02 to 0.05% (Cu): not less than 0% to not more than 0.4%, and (B) a boron (B): 0.05 to 0.2% (Ni): at least one of more than 0% and 0.4% or less of at least one of iron (Fe) and unavoidable impurities and having a thickness of 70 to 100 mm, a tensile strength at the center in the thickness direction of 590 MPa or more And has an average impact absorption energy of not less than 290 J at -40 캜 in the center in the thickness direction.

At this time, the steel sheet may further contain calcium (Ca) in an amount of more than 0% to 0.02% by weight.

The steel sheet may have a core microstructure including ferrite and bainite having an average grain size of 10 mu m or less.

The method for manufacturing a high strength steel sheet according to the present invention ensures fineness of the initial austenite grains through controlling the process conditions such as the slab reheating temperature while securing strength through adjustment of alloy components such as niobium, titanium, copper, nickel, molybdenum and calcium By applying accelerated cooling during controlled rolling, a high strength steel sheet having a small material deviation in the thickness direction and excellent in impact toughness at low temperature at the center can be manufactured.

1 is a flowchart schematically showing a method of manufacturing a high-strength steel sheet according to an embodiment of the present invention.
Fig. 2 is a cross-sectional photograph of a central portion in the thickness direction of the steel sheet produced according to Example 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. 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 high strength steel sheet according to an embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

High strength steel plate

The high strength steel sheet according to the present invention comprises 0.05 to 0.12% of carbon (C), 0.05 to 0.3% of silicon (Si), 1.4 to 2.0% of manganese (Mn), more than 0% of phosphorus (P) (S): not less than 0% to not more than 0.005%, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.02 to 0.05%, molybdenum (Mo): 0.05 to 0.2% N): more than 0% to 0.005% and boron (B): 0.0005 to 0.002%.

The high-strength steel sheet according to the present invention further contains at least one of copper (Cu) in an amount of more than 0% to 0.4% and nickel (Ni) in an amount of more than 0% to 0.4% by weight.

The high-strength steel sheet according to the present invention may further contain, by weight, more than 0% to 0.02% of calcium (Ca).

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

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

Carbon (C)

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

The carbon is preferably added in an amount of 0.05 to 0.12% by weight based on the total weight of the steel sheet. When the addition amount of carbon is less than 0.05% by weight, the strength of the steel sheet is insufficient. On the other hand, if the amount of carbon added exceeds 0.12 wt%, there is a problem that the impact resistance and weldability of the steel sheet at low temperatures are deteriorated.

silicon( Si )

Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process. Silicon also contributes to the strength improvement of the steel sheet through solid solution strengthening.

The silicon is preferably added in an amount of 0.05 to 0.3% by weight based on the total weight of the steel sheet. When the addition amount of silicon is less than 0.05% by weight, the effect of deoxidation by adding silicon is insufficient. On the contrary, when the addition amount of silicon exceeds 0.3% by weight, a large amount of oxides are formed on the surface of the steel sheet, which deteriorates the plating characteristics of the steel sheet and deteriorates the weldability.

manganese( Mn )

Manganese (Mn) is an austenite stabilizing element and serves to improve the strength and impact resistance at low temperatures by making the grain finer.

The manganese is preferably added in an amount of 1.4 to 2.0% by weight based on the total weight of the steel sheet. When the addition amount of manganese is less than 1.4% by weight, the effect of addition thereof is insufficient. On the contrary, when the addition amount of manganese exceeds 2.0% by weight, there is a problem that the low-temperature impact toughness is lowered.

In (P)

Phosphorus (P) contributes partly to the strength improvement, but it is a representative element that lowers impact toughness at low temperatures. The lower the content is, the better.

Therefore, in the present invention, the content of phosphorus is limited to more than 0 weight% and not more than 0.015 weight% of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) is an element which is inevitably contained in the production of steel together with phosphorus (P), and emulsifier inclusions (MnS) are formed to lower the impact resistance at low temperatures.

Therefore, in the present invention, the content of sulfur is limited to more than 0 wt% and 0.005 wt% or less of the total weight of the steel sheet.

Niobium ( Nb )

Niobium (Nb) combines with carbon (C) and nitrogen (N) to form carbides or nitrides. This suppresses crystal grain growth during rolling and makes crystal grains finer, thereby improving strength and low temperature toughness.

The niobium is preferably added in an amount of 0.01 to 0.04% by weight based on the total weight of the steel sheet. When the addition amount of niobium is less than 0.01% by weight, the effect of adding niobium can not be sufficiently exhibited. On the contrary, when the addition amount of niobium exceeds 0.04% by weight, the weldability of the steel sheet is lowered and there is a risk of lowering the impact resistance at low temperatures.

titanium( Ti )

Titanium (Ti) contributes to finer crystal grains in the steel sheet and to improvement in impact resistance at low temperatures.

The titanium 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 titanium is less than 0.02% by weight, the effect of improving the impact resistance at low temperatures is insufficient. On the other hand, if the amount of titanium added exceeds 0.05% by weight, the solid titanium may bond with carbon (C) to form a carbide, which may cause deterioration of toughness.

Nitrogen (N)

Nitrogen (N) generates inclusions in the steel to deteriorate the inner quality of the steel sheet.

Therefore, in the present invention, the content of nitrogen is limited to more than 0 wt% and 0.005 wt% or less of the total weight of the steel sheet.

Copper( Cu )

Copper (Cu) contributes to an increase in strength in the center in the thickness direction and an improvement in low temperature toughness.

When the copper is included, the content of the copper is preferably more than 0 wt% to 0.4 wt% or less of the total weight of the steel sheet. When the addition amount of copper exceeds 0.4% by weight, surface defects can be caused.

nickel( Ni )

Nickel (Ni) contributes to improvement of the strength in the center in the thickness direction and improvement of the low temperature toughness, like copper.

When the steel sheet contains the nickel, the content thereof is preferably more than 0 wt% to 0.4 wt% or less of the total weight of the steel sheet. If the addition amount of nickel is more than 0.4% by weight, there may arise a problem of inducing heat-induced brittleness.

molybdenum( Mo )

Molybdenum (Mo) contributes to the strength improvement through the solid solution strengthening effect.

The molybdenum is preferably added in an amount of 0.05 to 0.2 wt% based on the total weight of the steel sheet. When the addition amount of molybdenum is less than 0.05 wt%, the addition effect is insignificant. On the contrary, when the addition amount of molybdenum exceeds 0.2% by weight, the low-temperature impact toughness is lowered.

calcium( Ca )

Calcium (Ca) forms CaS to lower the content of sulfur in the steel, and stretches during rolling to inhibit the formation of MnS inclusions that cause defects such as hook cracks during electric resistance welding. This is because calcium has a higher affinity with sulfur than manganese. Therefore, the addition of calcium contributes to the improvement of electric resistance welding characteristics and impact properties of the produced steel.

However, when calcium is added in an amount exceeding 0.02% by weight, excessive CaS is formed or undesired CaO is produced.

Therefore, when calcium is added, the content thereof is preferably added in an amount of more than 0 wt% to 0.02 wt% of the total weight of the steel sheet according to the present invention.

Boron (B)

Boron (B) is an element which is useful for improving the ingotability of steel and greatly increases the fraction of bainite structure which greatly contributes to high-temperature strength, and plays a role of preventing the segregation of phosphorus and improving the strength.

The boron (B) is preferably added in a content ratio of 0.0005 to 0.002 wt% or less of the total weight of the steel material. When the content of boron (B) is over 0.002 wt%, there is a problem that the surface quality of the steel is deteriorated by the formation of boron oxide. On the other hand, when the content of boron (B) is less than 0.0005% by weight, the amount of boron (B) is insignificant and the above effect can not be exhibited properly.

The high strength steel sheet according to the present invention has a thickness of 70 to 100 mm by controlling the above components and process conditions to be described below and has a tensile strength at the center in the thickness direction of 590 MPa or more in the thickness direction and an average impact absorption energy It can display more than 290J.

Further, the high-strength steel sheet according to the present invention has a composite structure including ferrite and bainite, and each average grain size can be 10 탆 or less, more specifically 1 to 10 탆.

Method of manufacturing high strength steel sheet

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

Referring to FIG. 1, a method of manufacturing a high strength steel sheet according to the present invention includes a slab reheating step (S110), a primary rolling step (S120), a secondary rolling step (S130), and a cooling step (S140).

Reheating slabs

First, in the slab reheating step (S110), the slab plate material having the above-described composition is reheated.

At this time, the slab reheating is preferably performed at 900 to 1150 ° C for about 1 to 3 hours. If the slab reheating temperature exceeds 1150 DEG C, the initial austenite growth may increase the texture and material deviation in the thickness direction. On the other hand, when the slab reheating temperature is lower than 900 占 폚, the material deviation in the length and width direction of the steel sheet may become large.

Primary rolling

Next, in the primary rolling step (S120), the reheated plate is primarily rolled in the austenite recrystallization region.

In the primary rolling, rolling is performed before the secondary rolling to control the reduction rate of the secondary rolling. The primary rolling is carried out in the austenite recrystallization region and can be carried out at a temperature of about 850 to 1000 占 폚.

Secondary rolling

Next, in the secondary rolling step (S130), the primary rolled plate is secondarily rolled in the austenite non-recrystallized region.

At this time, it is preferable that the secondary rolling is performed at a finish rolling temperature of 650 to 850 캜. When the finish temperature of the secondary rolling exceeds 850 DEG C, it may become difficult to secure sufficient strength. On the contrary, when the finish temperature of the secondary rolling is less than 650 占 폚, blended structure may occur due to abnormal reverse rolling, thereby deteriorating the physical properties of the steel sheet.

Further, the secondary rolling is carried out such that the residual reduction ratio ((AB) / AX 100, where A is the plate thickness at the start of secondary rolling and B is the plate thickness at the end of secondary rolling) is 40 to 60% desirable. If the residual reduction ratio of the secondary rolling is less than 40%, it is difficult to obtain a uniform but fine structure, and the core structure in the thickness direction may be coarsened and the low temperature impact toughness may be lowered. On the contrary, when the secondary rolling exceeds 60%, the seismic resistance and the like may be lowered due to an increase in the yield strength.

The secondary rolling is preferably carried out so that the shape factor is 0.5 to 0.7.

If the shape factor is less than 0.5 in the secondary rolling, the strength and low temperature impact toughness may be reduced. On the other hand, if the shape factor exceeds 0.7 in the secondary rolling, the seismic resistance and the like may be lowered by increasing the yield strength.

Cooling

Next, in the cooling step (S140), the secondary rolled plate is cooled to 300 to 600 占 폚.

The cooling is performed by an accelerated cooling method using a squeeze water. At this time, the cooling end temperature is preferably 300 to 600 ° C. When the cooling and cooling end temperature is less than 300 ° C, a large amount of low-temperature transformed structure such as martensite is formed and low-temperature impact toughness is deteriorated. On the other hand, when the cooling end temperature exceeds 600 ° C, there is a problem that strength becomes insufficient due to formation of coarse microstructure and the like.

The cooling is preferably carried out at a cooling rate of 2 to 7 DEG C / sec. When the cooling rate is less than 2 캜 / sec, grain growth is promoted and it is difficult to secure strength. Conversely, when the cooling rate exceeds 7 DEG C / sec, the low-temperature impact toughness may be lowered.

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. Manufacture of steel sheet

The steel sheets according to Examples 1 to 3 and Comparative Examples 1 to 2 having a thickness of 78.3 mm were produced in accordance with the process conditions shown in Table 2 after producing the slab plate comprising the components listed in Table 1 and the balance of iron and impurities Respectively.

[Table 1] (unit:% by weight)

[Table 2]

2. Results of physical property evaluation

Table 3 shows the results of tensile test at 1/2 point in the thickness direction and 1/4 point in the thickness direction of the produced steel sheet.

[Table 3]

Referring to Table 3, the steel sheet produced according to Examples 1 to 3 satisfying the composition and process conditions proposed in the present invention exhibited a tensile strength of 590 MPa or more at the central portion (1/2 point in the thickness direction) The intensity deviation at the point of 1/2 point in the thickness direction and 1/4 point in the thickness direction was very low, 20Mpa or less. In addition, the steel sheet produced according to Examples 1 to 3 exhibited a Charpy average impact energy of -40 DEG C at the central portion in the thickness direction of 290 J or more, which was excellent in impact resistance at low temperatures.

On the other hand, in the case of the steel sheet prepared according to Comparative Example 1 in which the process conditions proposed in the present invention were satisfied but the composition was unsatisfactory, the tensile strength at the center in the thickness direction was less than 590 MPa, ℃ Charpy average impact energy showed very low value.

The steel sheet prepared according to Comparative Example 2 satisfying the composition of the present invention but satisfying the process conditions did not have a tensile strength of 590 MPa at the central portion in the thickness direction, The intensity deviation at the 1/4 direction was relatively large. This is because, in the case of the steel sheet prepared in accordance with Comparative Example 2, the slab reheating temperature was 1250 ° C, indicating that the initial austenite growth was not suppressed.

3. Microstructure

Fig. 2 is a cross-sectional photograph of a central portion in the thickness direction of the steel sheet produced according to Example 3. Fig.

Referring to FIG. 2, in the case of the steel sheet produced according to Example 3, it can be seen that the thickness central portion structure includes fine ferrite and bainite. The high-strength steel sheet according to the present invention is excellent in strength at the center portion and impact resistance at low temperature due to such microstructure.

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: Primary rolling step
S130: Secondary rolling step
S140: cooling step

Claims (8)

(P): more than 0% to 0.015%, sulfur (S): 0.05 to 0.12%, silicon (Si): 0.05 to 0.3%, manganese ): More than 0% to less than 0.005%, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.02 to 0.05%, molybdenum (Mo): 0.05 to 0.2% (Fe): 0.4% or less; and Ni: 0.4% or less, wherein the balance of iron (Fe) and inevitable impurities (B) is 0.0005 to 0.002% Reheating the slab plate at 900 to 1150 占 폚;
Subjecting the reheated plate to primary rolling in an austenite recrystallization zone;
Secondarily rolling the primary rolled plate at a finish rolling temperature of 650 to 850 캜; And
And cooling the secondary rolled plate to 300 to 600 占 폚.
The method according to claim 1,
The slab plate
By weight, and calcium (Ca): not less than 0% and not more than 0.02%.
The method according to claim 1,
In the secondary rolling,
(A / 100), where A is the plate thickness at the start of the secondary rolling and B is the plate thickness at the end of the secondary rolling) is 50 to 70%. Way.
The method according to claim 1,
In the secondary rolling,
And the following shape factor is 0.5 to 0.7.
The method according to claim 1,
The cooling
At a cooling rate of 2 to 7 占 폚 / sec.
(P): more than 0% to 0.015%, sulfur (S): 0.05 to 0.12%, silicon (Si): 0.05 to 0.3%, manganese ): More than 0% to less than 0.005%, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.02 to 0.05%, molybdenum (Mo): 0.05 to 0.2% (Fe): 0.4% or less; and Ni: 0.4% or less, wherein the balance of iron (Fe) and inevitable impurities (B) is 0.0005 to 0.002% Lt; / RTI >
A tensile strength at the central portion in the thickness direction of 590 MPa or more and an average impact absorbing energy at 0 DEG C in the thickness direction central portion of 290 J or more.
The method according to claim 6,
The steel sheet
By weight, and calcium (Ca): not less than 0% and not more than 0.02%.
The method according to claim 6,
The steel sheet
Wherein the bainite has an area ratio of 50 to 70% and an area ratio of the ferrite is 30 to 50%, wherein the area ratio of the bainite is 50 to 70% and the area ratio of the ferrite is 30 to 50%. .

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