KR20140098900A

KR20140098900A - High strength thick steel plate and method for manufacturing the same

Info

Publication number: KR20140098900A
Application number: KR1020130010891A
Authority: KR
Inventors: 이동진; 고상기; 권승오; 황성두
Original assignee: 현대제철 주식회사
Priority date: 2013-01-31
Filing date: 2013-01-31
Publication date: 2014-08-11

Abstract

Disclosed are a high strength thick steel plate having excellent strain aging impact toughness, and a method to manufacture the same. According to the present invention, the method to manufacture the high strength thick steel plate comprises: a step of re-heating a slab plate, by weight%, including 0.07-0.12% of carbon (C), 0.05-0.3% of silicon (Si), 1.2-2.0% of manganese (Mn), 0.015% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.01-0.04% of niobium (Nb), 0.003-0.02% of titanium (Ti), 0.005% or less of nitrogen (N), 0.4% or less of copper (Cu), 1.0% or less of nickel (Ni), 0.5% or less of molybdenum (Mo), 0.05% or less of calcium (Ca), and the remainder consisting of Fe and inevitable impurities at 1000-1200°C; a step of primary rolling the slab plate on the condition of the reduction rate of 40% or more in an austenite recrystallization area; a step of forming a plate whose thickness is 70-120 mm by secondary rolling the primary rolled plate at 650-900°C; and a step of cooling the secondary rolled plate to 200-500°C in an accelerated cooling method.

Description

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

More particularly, the present invention relates to a high strength steel sheet having a high strength and impact toughness with high strength through an alloy component and a process control, and a method for manufacturing the same.

Recently, the demand for offshore structures has increased sharply with rising oil prices. In addition, the use of ultra-fine steel plates with a thickness of several tens of mm or more is also increasing for the production of offshore structures.

In the case of such a superfine steel sheet, the variations in the microstructure and material properties of the steel sheet are very large depending on the alloy composition and the manufacturing process. Particularly, in the case of a very thin steel sheet, deformation aging causes deformation of the base material, which results in poor impact toughness.

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 strength and impact toughness with high strength through an alloy component and process control.

Another object of the present invention is to provide a high-strength steel sheet having excellent strength and strain-age impact toughness, which can be utilized for marine structure.

In order to accomplish the above object, there is provided a method for manufacturing a high strength steel sheet having a high strength, comprising the steps of: 0.07 to 0.12% of carbon; 0.05 to 0.3% of silicon; (S): 0.005% or less, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.003 to 0.02%, nitrogen (N): 0.005 (Ni): not more than 1.0%, molybdenum (Mo): not more than 0.5% and calcium (Ca): not more than 0.05%, and the balance of iron (Fe) and inevitable impurities To 1000 < 0 > C to 1200 < 0 >C; Subjecting the slab plate to primary rolling at a reduction ratio of at least 40% in austenite recrystallization region; Secondarily rolling the primary rolled plate at a temperature of 650 to 900 캜 to form a plate having a thickness of 70 to 120 mm; And cooling the secondary rolled plate by an accelerated cooling method to 200 to 500 ° C.

[S], [Ti], and [N] are selected from the group consisting of Ca [Ca] / [S] , S, Ti, and N by weight).

Also, it is preferable that the secondary rolling is performed under the condition of a reduction in the rolling reduction of 30 to 60% and a shape factor of 0.5 to 0.7.

Also, it is preferable that the cooling is performed at an average cooling rate of 2 to 8 占 폚 / sec.

According to another aspect of the present invention, there is provided a high strength steel sheet having a high strength steel sheet comprising 0.07 to 0.12% of carbon, 0.05 to 0.3% of silicon, 1.2 to 1.2% of manganese, (S): 0.005% or less, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.002 to 0.03%, nitrogen (N): 0.005% or less, (Fe) and unavoidable impurities, and contains at most 0.4% of copper (Cu), at most 1.0% of nickel (Ni), at most 0.5% of molybdenum (Mo) , A thickness of 70 to 120 mm, a volume fraction of crystal grains having a degree of misorientation of 15 degrees or more of 60% or more, and a volume fraction of crystal grains having a bearing difference of less than 15 degrees of less than 40%.

In this case, the high-strength steel sheet steel sheet is characterized in that 1.5 [Ca] / [S]? 2.5 and 1.5? [Ti] / [N]? 4.5 where [Ca], [S], [Ti] Preferably contains Ca, S, Ti and N in a range that satisfies the following expression (% by weight of Ca, S, Ti and N).

The high strength steel sheet may have a microstructure containing at least 50% by volume of needle-shaped ferrite and lower bainite having an average grain size of 10 탆 or less.

In addition, the high-strength steel backing sheet may have a tensile strength at impact of 590 MPa or more and a fracture transition temperature (vTrs) at 5% deformation of -40 캜 or lower.

According to the method for manufacturing a high strength steel sheet having a high strength according to the present invention, it is possible to control the alloy component such as carbon, niobium, titanium, copper, nickel, molybdenum and calcium, Lt; RTI ID = 0.0 > C, < / RTI >

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 center portion of the steel sheet in the thickness direction prepared according to Example 1. Fig.
Figs. 3 to 5 show the azimuth difference (hardness angle) at each point in the thickness direction of the extreme-behind-grain steel sheet produced according to Example 1. 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a high strength steel sheet according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

High strength steel sheet

The high strength steel sheet according to the present invention comprises 0.07 to 0.12% of carbon (C), 0.05 to 0.3% of silicon (Si), 1.2 to 2.0% of manganese (Mn) (N): not more than 0.005%, sulfur (S): not more than 0.005%, niobium (Nb): 0.01 to 0.04%, titanium (Ti): 0.003 to 0.02% (Ni): not more than 1.0%, molybdenum (Mo): not more than 0.5%, and calcium (Ca): not more than 0.05%.

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 included 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 0.07 to 0.12% by weight based on the total weight of the steel sheet. When the amount of carbon added is less than 0.07% by weight, the strength of the steel sheet is insufficient. On the other hand, when the amount of carbon added exceeds 0.12% by weight, there is a problem that the ductility and weldability of the steel sheet are deteriorated due to aging impact.

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 1.2 to 2.0% by weight based on the total weight of the steel sheet. When the addition amount of manganese is less than 1.2% 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 of lowering the tensional impact toughness.

In (P)

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

In the present invention, the content of phosphorus is limited to 0.015 wt% or less of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) together with phosphorus (P) is an element which is inevitably contained in the production of steel, and forms an emulsion inclusion (MnS) to lower the strain aging impact toughness.

Therefore, in the present invention, the sulfur content is limited to 0.005% by weight or less based on 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, which improves strength and strain aging impact 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 other hand, when the addition amount of niobium exceeds 0.04% by weight, the performances and the rolling properties of the steel sheet may be lowered.

Titanium (Ti)

Titanium (Ti) finely grains the crystal grains of the steel sheet and contributes to the improvement of toughness and impact toughness.

The titanium is preferably added in an amount of 0.003 to 0.02% by weight based on the total weight of the steel sheet. When the addition amount of titanium is less than 0.003% by weight, the effects such as improvement in strain age toughness and the like are insufficient. On the other hand, if the added amount of titanium exceeds 0.02 wt%, the solid solution titanium reacts with the carbon (C) to form a carbide, which may result in a problem of deteriorating the toughness and impact toughness.

In particular, the titanium is more preferably contained in an amount satisfying 1.5? [Ti] / [N]? 4.5 wherein [Ti] and [N] are Ca, S, Ti and N weight%. 1.5> [Ti] / [N], the steel sheet quality may be deteriorated by the influence of free nitrogen (Free N). Conversely, when [Ti] / [N] > 4.5, the strain impact toughness may be lowered.

Nitrogen (N)

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

In the present invention, the content of nitrogen is limited to 0.005 wt% or less of the total weight of the steel sheet.

Copper (Cu)

Copper (Cu) contributes to an increase in the strength in the center in the thickness direction and an improvement in the strain aging impact toughness.

The amount of copper added is preferably 0.4% by weight or less based on 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 the improvement of the strength and strain aging impact toughness of the steel sheet.

The amount of nickel added is preferably 1.0% by weight or less based on the total weight of the steel sheet. If the addition amount of nickel exceeds 1.0% by weight, there may arise a problem of inducing a hot 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 0.5% by weight or less based on the total weight of the steel sheet. When the addition amount of molybdenum exceeds 0.5% by weight, there is a problem that the toughness toughness of the strain is deteriorated.

Calcium (Ca)

Calcium (Ca) forms CaS to lower the content of sulfur in the steel and prevents formation of MnS inclusions, thereby preventing impact toughness deterioration at the steel, especially at the center in the thickness direction.

The calcium is preferably added in an amount of 0.05% by weight or less based on the total weight of the steel sheet according to the present invention. When the addition amount of calcium exceeds 0.05% by weight, undesired CaO is generated.

Particularly, calcium is more preferably added within a range satisfying 1.5? [Ca] / [S]? 2.5 where [Ca] and [S] are Ca and S weight percent. The low-temperature impact toughness due to the formation of CaS can be sufficiently exerted in the addition range of calcium. When [Ca] / [S] < 1.5, the amount of CaS produced is insufficient and the effect of adding calcium is insufficient. Further, when [Ca] / [S] > 2.5, the effect is no longer improved with respect to the calcium input amount.

The high strength steel sheet according to the present invention has a thickness of 70 to 120 mm according to the above-described components and process control described below, and has a tensile strength of 590 MPa or more and a fracture transition temperature (vTrs) Lt; RTI ID = 0.0 > 40 C < / RTI >

In one aspect of the microstructure of the high-strength steel sheet according to the present invention, the volume fraction of crystal grains having a crystal grain size of not less than 15 degrees and a bearing difference of less than 15 degrees is not less than 60% Less than 40%. The azimuth difference means the degree of deviation of the orientation of two crystal grains having a grain boundary therebetween. Since the volume fraction of crystal grains having an azimuth difference of 15 degrees or more is 60% or more, it is possible to improve the toughness by making crack propagation difficult in a specific direction at the time of impact.

Further, the high-strength steel sheet according to the present invention is characterized in that on the other side of the microstructure, the steel sheet contains needle-shaped ferrite and lower bainite having an average grain size of 10 탆 or less, more specifically 1 to 10 탆 in a volume fraction of 50% It can have a microstructure. The remaining structure may be polygonal ferrite, upper bainite, retained austenite, martensite, and the like. High strength of 590 MPa or more in tensile strength can be obtained through the needle-like ferrite and the lower bainite microstructure with such a high fraction, and the strain age impact toughness can be improved.

Method of manufacturing high strength steel sheet

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

Referring to FIG. 1, the method for manufacturing high strength steel sheet steel 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 having the above-described composition is reheated.

At this time, the slab reheating is preferably performed at 1000 to 1200 ° C for about 1 to 3 hours. If the slab reheating temperature is higher than 1200 ° C, the initial austenite growth may increase the structure and material deviation particularly in the thickness direction. On the other hand, when the slab reheating temperature is lower than 1000 占 폚, the rolling property is lowered and the material variation 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 at a temperature of 1000 to 1200 DEG C in the austenite recrystallization region.

At this time, it is preferable that the primary rolling is performed at a reduction rate condition of 40% or more, that is, under a reduced pressure condition. Through the primary rolling under the downward pressing condition, the deformation of the central portion in the thickness direction of the steel sheet to be produced is maximized, and hydrogen in the MnS segregation zone can be removed, thereby improving the strength and toughness at the center in the thickness direction. .

Secondary rolling

Next, in the secondary rolling step (S130), the primary rolled plate is secondarily rolled at 650 to 900 占 폚, which is an austenite non-recrystallized region, to form a plate having a thickness of 70 to 120 mm.

The finish temperature of the secondary rolling is preferably 650 ° C or higher. If the end temperature of the secondary rolling is less than 650 ° C, blast texturing may occur due to abnormal reverse rolling, thereby deteriorating the physical properties of the steel sheet.

Further, the secondary rolling is performed under conditions of a residual reduction ratio of 30 to 60% ((AB) / AX 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) desirable.

If the remainder reduction ratio of the secondary rolling is less than 30%, it is difficult to obtain a uniform but fine structure, and the structure of the center portion in the thickness direction may be coarsened, so that the torsional 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.

Further, it is preferable that the secondary rolling is performed such that the shape factor determined by the following formula 1 is 0.5 to 0.7.

[Formula 1]

Shape factor =

(Wherein, R denotes the radius of the rolling rolls and, t ₀ denotes the thickness of the plate material, and the rolling roll inlet, t _i is the rolling roll exit side sheet thickness)

If the shape factor is less than 0.5 in the secondary rolling, the strength and strain-age impact toughness may be reduced. On the contrary, when 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 200 to 500 占 폚 by an accelerated cooling method.

When the cooling-cooling termination temperature is less than 200 ° C, a large amount of low-temperature transformed structure such as martensite is formed and the impact toughness is deteriorated. On the other hand, when the cooling end temperature exceeds 500 ° C, there is a problem that the strength becomes insufficient due to formation of coarse microstructure or the like.

The cooling is preferably carried out at an average cooling rate of 2 to 8 占 폚 / sec. When the cooling rate is less than 2 캜 / sec, the grain growth is promoted by the double heat due to the characteristics of the ultra-fine steel sheet, and the strength may be lowered. On the other hand, when the cooling rate exceeds 8 DEG C / sec, there is a problem that the difference in cooling rate between the surface portion in the thickness direction and the center portion is excessively increased.

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

A slab plate of 300 mm in thickness comprising the components listed in Table 1 and composed of the remaining iron and impurities was prepared and then extruded steel sheets according to Examples 1 to 3 and Comparative Examples 1 to 3 were produced under the process conditions shown in Table 2 .

[Table 1] (unit:% by weight)

[Table 2]

2. Results of physical property evaluation

Table 3 shows the evaluation results of the physical properties of the produced steel sheet.

The tensile strength was measured according to the JIS No. 4 test piece.

The deformation aged impact toughness was evaluated by applying a deformation of 5% to the steel sheet produced according to Examples 1 to 3 and Comparative Examples 1 to 3, aging treatment at 250 ° C for 1 hour, and then performing a Charpy impact test at various temperatures, The wave front transition temperature (vTrs) was measured. The lower the fracture transition temperature, the better the strain aging impact toughness.

[Table 3]

Referring to Tables 2 and 3, both of the ultrafine wall steel sheets prepared according to Examples 1 to 3 and the ultrafine wall steel sheets according to Comparative Examples 1 to 3 had a thickness of 70 to 120 mm and a tensile strength of 590 MPa or more.

However, in the case of the ultrafine metal sheet according to Examples 1 to 3, the fracture transition temperature for the 5% deformation was -40 ° C or less, but not for the ultrafine metal sheet according to Comparative Examples 1 to 3. It can be considered that the content of titanium, calcium, and the like in the case of the extreme-rolled steel sheet according to Comparative Example 1 is insufficient. In the case of the extreme-rolled steel sheet according to Comparative Example 2, . Further, in the case of the ultrafine-rolled steel sheet according to Comparative Example 3, it can be considered that the Ti / N ratio exceeded 4.5.

3. Microstructure

Fig. 2 is a cross-sectional photograph of a center portion of the steel sheet in the thickness direction prepared according to Example 1. Fig.

Referring to FIG. 2, it can be seen that, in the case of the ultra-fine steel sheet produced according to Example 1, fine ferrite and lower bainite of less than 10 μm are formed in the main structure.

Figs. 3 to 5 show the azimuth difference (hardness angle) at each point in the thickness direction of the extreme-behind-grain steel sheet produced according to Example 1. Fig.

Referring to FIGS. 3 to 5, it can be seen that the portion of the pole headlining steel sheet manufactured according to the first embodiment has an azimuth difference of 15 degrees or more at all points in the thickness direction of 60% or more.

The high strength steel sheet according to the present invention has excellent strength and strain impact toughness 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.

Claims

(P): 0.015% or less, sulfur (S): 0.005% or less, carbon (C): 0.07 to 0.12%, silicon (Si): 0.05 to 0.3%, manganese (N): not more than 0.4%, nickel (Ni): not more than 1.0%, molybdenum (Nb) Reheating the slab plate including the remaining iron (Fe) and unavoidable impurities at a temperature of 1000 to 1200 占 폚, containing not more than 0.5% of molybdenum (Mo) and not more than 0.05% of calcium (Ca)
Subjecting the slab plate to primary rolling at a reduction ratio of at least 40% in austenite recrystallization region;
Secondarily rolling the primary rolled plate at a temperature of 650 to 900 캜 to form a plate having a thickness of 70 to 120 mm; And
And cooling the secondary rolled plate by an accelerated cooling method to 200 to 500 ° C.

The method according to claim 1,
[S], [Ti], and [N] are Ca, S, and N, where 1.5 ≤ [Ca] / [S] ≤ 2.5 and 1.5 ≤ [ , Ti, and N by weight), wherein the content of Ca, S, Ti, and N is in the range satisfying the following formula (1).

3. The method according to claim 1 or 2,
Wherein the secondary rolling is performed under a condition of a reduction in the yield of 30 to 60% and a shape factor of 0.5 to 0.7.

3. The method according to claim 1 or 2,
Wherein the cooling is performed at an average cooling rate of 2 to 8 占 폚 / sec.

(P): 0.015% or less, sulfur (S): 0.005% or less, carbon (C): 0.07 to 0.12%, silicon (Si): 0.05 to 0.3%, manganese (N): not more than 0.4%, nickel (Ni): not more than 1.0%, molybdenum (Nb) 0.5% or less of molybdenum (Mo) and 0.05% or less of calcium (Ca), the balance being Fe and unavoidable impurities,
It has a thickness of 70 ~ 120mm,
Wherein a volume fraction of the crystal grains having a degree of misorientation of 15 degrees or more is 60% or more and a volume fraction of crystal grains having a bearing difference of less than 15 degrees is less than 40%.

6. The method of claim 5,
Wherein the high strength steel sheet has a composition of 1.5 ≤ [Ca] / [S] ≤ 2.5 and 1.5 ≤ [Ti] / [N] ≤ 4.5 where [Ca], [S], [Ti] S, Ti, and N in the range satisfying the weight percentages of S, Ti, and N. The high strength steel sheet according to claim 1,

The method according to claim 5 or 6,
Characterized in that the high strength steel sheet has a microstructure containing at least 50% by volume of an acicular ferrite having an average grain size of 10 탆 or less and a lower bainite.

The method according to claim 5 or 6,
In the high-strength steel sheet,
A tensile strength of 590 MPa or more,
And a fracture transition temperature (vTrs) for a 5% deformation of less than -40 占 폚.