CN118019871A

CN118019871A - High-strength cold-rolled steel sheet having excellent surface quality and little material deviation, and method for producing same

Info

Publication number: CN118019871A
Application number: CN202280063315.3A
Authority: CN
Inventors: 柳朱炫; 金成一; 徐昌孝
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-09-24
Filing date: 2022-09-20
Publication date: 2024-05-10

Abstract

The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and less material variation and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having less surface defects and less material variation and ensuring high strength and elongation, which can be suitably used for automobile parts, and a method for manufacturing the same.

Description

High-strength cold-rolled steel sheet having excellent surface quality and little material deviation, and method for producing same

Technical Field

The present invention relates to a high-strength cold-rolled steel sheet used for structural members having a large molding amount such as pillars, seat rails, and modules of an automobile body, and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet excellent in surface quality and having little material deviation, and suitable for use as an automobile member, and a method for manufacturing the same.

Background

In recent years, with the enhancement of safety and environmental regulations in the automotive industry, the use of high-strength steel having a tensile strength of 780MPa class or more is increasing in the manufacture of vehicle bodies for the purpose of improving fuel efficiency of vehicles and protecting passengers.

High-strength steels used for conventional automobile bodies include Dual Phase (DP) steels composed of two phases of soft ferrite matrix and hard martensite, transformation induced plasticity (Transformation Induced Plasticity, TRIP) steels using transformation induced plasticity of retained austenite, and complex Phase (Complexed Phase, CP) steels composed of a complex structure of ferrite and hard bainite or martensite.

However, high strength steel has problems in that it is poor in weldability when a large amount of Si, al, mn, etc. is added, and in that defects occur in the surface of the steel sheet due to dents in the furnace during annealing. In addition, when a large amount of hardenability elements such as Mn, cr, mo, etc. are added, there is a problem in that the quality of the thickness is deteriorated during cold rolling due to variation in the quality of the hot rolled sheet. At this time, the surface defect caused by the dent in the furnace means a surface defect of the steel sheet, which is formed by contact of the steel sheet and the roller when the sheet passes through, in which the metal-based oxide on the surface of the steel sheet is adsorbed and accumulated on the roller of the annealing furnace.

In order to solve the above problems, the prior art related to the manufacturing technology of the high strength cold rolled steel sheet and the hot dip galvanized steel sheet is briefly described as follows.

In the prior art, patent document 1 proposes a high-strength cold-rolled steel sheet and a method of manufacturing the same, the method including the following processes: cold rolling a hot rolled steel sheet at a cold rolling reduction of more than 60% and less than 80%, the steel sheet comprising 60% or more of a low temperature transformation phase by volume; the cold-rolled steel sheet is continuously annealed in a two-phase region of ferrite and austenite. However, the cold-rolled steel sheet obtained from patent document 1 has a strength as low as 370 to 590MPa, and thus has a problem that it is difficult to apply it to an automobile impact-resistant member and is limited to only the use of inner and outer panels (panels).

In addition, patent document 2 discloses a method of manufacturing a cold-rolled steel sheet which is identical by using tempered martensite (TEMPERED MARTENSITE), while attaining high strength and high ductility, and which is excellent in shape of a sheet after continuous annealing. However, the steel of the technique of patent document 2 has a problem of poor weldability because the carbon content is as high as 0.2% or more, and has a problem of surface defects due to dents in the furnace because it contains a large amount of Si.

(Patent document 1) Korean laid-open patent publication No. 2004-0066935

(Patent document 2) Japanese laid-open patent publication No. 2010-090432

Disclosure of Invention

Technical problem to be solved

According to one aspect of the present invention, there is provided a high-strength cold-rolled steel sheet having excellent surface quality and less material variation, and a method for manufacturing the same.

The technical problem of the present invention is not limited to the above. Additional technical problems of the invention will be readily apparent to one skilled in the art from the present description.

Technical proposal

One aspect of the present invention provides a high-strength cold-rolled steel sheet comprising, in weight%: c:0.05-0.3%, si:0.01-2.0%, mn:1.5-3.0%, al:0.01-0.1%, P:0.001-0.015%, S:0.001-0.01%, N:0.001 to 0.01%, the balance being Fe and other unavoidable impurities, the value defined by the following relation 1 satisfying 0.6 or more and less than 0.9, and containing, in area%, as a microstructure: ferrite: more than 50 percent, the balance: bainite and martensite, and the average number of surface defects satisfying one or more of the conditions of a depth of 100 μm or more and a short side length of 1mm or more is less than 10/m ².

[ Relation 1]

C+(1.3×Si+Mn)/6+(Cr+1.2×Mo)/5+100×B

( In the relational expression 1, C, si, mn, cr, mo and B represent average contents of weight% of each element. And substituting 0 in the case where the elements are not added. )

In addition, another aspect of the present invention provides a method of manufacturing a high-strength cold-rolled steel sheet, comprising the steps of: reheating a steel billet to 1100-1350 ℃, said steel billet comprising, in weight-%: c:0.05-0.3%, si:0.01-2.0%, mn:1.5-3.0%, al:0.01-0.1%, P:0.001-0.015%, S:0.001-0.01%, N:0.001 to 0.01%, the balance being Fe and other unavoidable impurities, the value defined by the following relation 1 satisfying 0.6 or more and less than 0.9; hot rolling the reheated steel billet at 850-1150 ℃; cooling the hot rolled steel plate to 450-700 ℃ at an average cooling rate of 10-70 ℃/sec; rolling the cooled steel plate at 450-700 ℃; cold rolling the rolled steel plate at a rolling reduction of 40-70%; and continuously annealing the cold-rolled steel sheet at 740-900 ℃, wherein the rolling step is controlled such that the surface temperature (Te) of both end portions in the width direction satisfies 601-700 ℃ and the surface temperature (Tc) of the center portion satisfies 450-600 ℃ based on the total width of the steel sheet.

[ Relation 1]

C+(1.3×Si+Mn)/6+(Cr+1.2×Mo)/5+100×B

( In the relational expression 1, C, si, mn, cr, mo and B represent average contents of weight% of each element. At this time, when the elements are not added, 0 is substituted. )

Effects of the invention

According to one aspect of the present invention, a high-strength cold-rolled steel sheet having excellent surface quality and less material variation and a method for manufacturing the same can be provided.

The various advantageous advantages and effects of the present invention are not limited to the above, and will be more readily understood in describing particular embodiments of the present invention.

Drawings

Fig. 1 is a view showing photographs of surface defects of respective cold rolled steel sheets obtained in inventive example 1 and comparative example 1 of the present invention taken with a conventional low magnification camera.

Fig. 2 is a view showing a photograph of a surface defect defined in the present invention taken with a Scanning Electron Microscope (SEM) at a high magnification.

Best mode for carrying out the invention

Preferred embodiments of the present invention will be described below. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully illustrate the invention to those skilled in the art.

In addition, the terminology used in the description presented herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. For example, as used in this specification, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. In addition, the meaning of "comprising" or "including" as used in the specification is used to embody the constitution and does not exclude the presence or addition of other constitution.

In the prior art, a technology has not been developed which satisfies the high-grade requirements of a cold-rolled steel sheet having high strength of 780MPa or more in Tensile Strength (TS) and excellent formability, and which can be applied to a structural member having a large forming amount, and also has excellent surface quality and less material variation.

In this regard, the present inventors have conducted intensive studies to provide a result of a cold rolled steel sheet satisfying all of the above characteristics while solving the problems of the prior art, and have found that the above object can be achieved by optimizing the composition and manufacturing conditions of the steel sheet and controlling the characteristics of the microstructure and surface defects, thereby completing the present invention.

That is, according to the present invention, it is possible to effectively provide a high-strength cold-rolled steel sheet having a high strength of 780MPa or more and a product of tensile strength and elongation satisfying 12000MPa% or more, so that it can be suitably applied to structural members such as a column to be stabilized in strength-elongation balance and impact absorbability among members constituting an automobile body.

A high strength cold rolled steel sheet having excellent surface quality and less material deviation according to an aspect of the present invention will be described in detail.

According to one aspect of the present invention, a high strength cold rolled steel sheet comprises, in weight%: c:0.05-0.3%, si:0.01-2.0%, mn:1.5-3.0%, al:0.01-0.1%, P:0.001-0.015%, S:0.001-0.01%, N:0.001-0.01%, and the balance of Fe and other unavoidable impurities.

The reasons for adding components and for limiting the content of the cold-rolled steel sheet according to the present invention will be specifically described below. In this case, in the present specification, unless otherwise defined, the content of each element is expressed as weight%.

C：0.05-0.3％

The carbon (C) is a very important component in ensuring a martensitic structure effective for strengthening the steel. As the amount of C added increases, the fraction of the martensite phase and the bainite phase increases, and thus the tensile strength increases. Therefore, in order to secure high strength, the lower limit of the C content is controlled to 0.05%. However, when the C content increases, the austenite region expands upon annealing of the two-phase region, so that the fraction of the martensite and bainite phases as the hard phase increases, and the fraction of the ferrite phase as the soft phase decreases, and thus the formability and weldability deteriorate. Therefore, the upper limit of the C content is controlled to 0.3%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the C content may be 0.06%, or the upper limit of the C content may be 0.12%.

Si：0.01-2.0％

The silicon (Si) is an element that deoxidizes molten steel and has a solid solution strengthening effect, and is advantageous in improving formability by delaying the formation of coarse carbides. However, when the Si content is less than 0.01%, the above effect is small and it is difficult to improve the moldability. On the other hand, when the Si content exceeds 2.0%, a red oxide scale due to Si is seriously formed on the surface of the steel sheet at the time of hot rolling. Thus, surface defects are generated or concentrated on the surface during the annealing process and non-plating occurs. Further, the plating adhesion also becomes poor due to the formation of surface oxides, and thus there is a problem in that the surface quality becomes very poor. Therefore, in the present invention, the Si content is controlled to be 0.01 to 2.0%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the Si content may be 0.4%, or the upper limit of the Si content may be 1.2%.

Mn：1.5-3.0％

Like Si, the manganese (Mn) is an element effective for solid solution strengthening of steel, and is an element that greatly increases hardenability. However, when the Mn content is less than 1.5%, the above-mentioned effect by adding Mn cannot be obtained, and when the Mn content exceeds 3.0%, the strengthening effect is greatly increased and the ductility is lowered. In addition, when a slab is cast in a continuous casting process, segregation is greatly developed in the thickness center portion, so that fine structure in the thickness direction becomes uneven and MnS is formed when cooling after hot rolling, and thus formability such as stretch flangeability is deteriorated. Therefore, in the present invention, the Mn content is controlled to 1.5-3.0%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the Mn content may be 1.8%, or the upper limit of the Mn content may be 2.6%.

Al：0.01-0.1％

The aluminum (Al) is mainly a component added for deoxidization. When the Al content is less than 0.01%, the effect of addition is insufficient. On the other hand, when the Al content exceeds 0.1%, alN is formed in combination with nitrogen, so that the slab is liable to suffer from corner cracks at the time of continuous casting and defects caused by the formation of inclusions. Therefore, in the present invention, the content of Al is controlled to 0.01 to 0.1%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the Al content may be 0.015%, or the upper limit of the Al content may be 0.06%.

P：0.001-0.015％

The phosphorus (P) is an alloy element having a very large solid solution strengthening effect, and is characterized in that a large strengthening effect can be obtained even when used in a small amount. However, when an excessive amount of P is added, brittleness due to grain boundary segregation occurs, microcracks are easily generated during molding, and ductility and impact resistance are greatly reduced. In addition, there is a problem that defects are induced on the surface at the time of plating. Therefore, the upper limit of the P content is controlled to 0.015%. When the P content is less than 0.001%, excessive manufacturing cost is required to meet the requirement, which is economically disadvantageous, and the ensured strength is insufficient, so that the lower limit of the P content is controlled to 0.001% or more. Therefore, in the present invention, the P content is preferably controlled to 0.001 to 0.015%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the P content may be 0.003%, or the upper limit of the P content may be 0.012%.

S：0.001-0.01％

The sulfur (S) is an impurity present in steel, and when the S content exceeds 0.01%, it combines with Mn or the like to form nonmetallic inclusions, so that fine cracks are easily generated at the time of cutting processing of steel, and there is a problem that stretch flangeability and impact resistance are greatly deteriorated. In addition, in order to manufacture the steel-making alloy having the S content of less than 0.001%, a lot of time is required for the steel-making operation, and thus there is a problem in that productivity is lowered. Therefore, in the present invention, the S content is preferably controlled to 0.001 to 0.01%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the S content may be 0.002%, or the upper limit of the S content may be 0.007%.

N：0.001-0.01％

Together with C, the nitrogen (N) is a representative solid solution strengthening element, and together with Ti, al, and the like, contributes to the formation of coarse precipitates. In general, N has a solution strengthening effect superior to that of carbon, but as the amount of N in steel increases, there is a problem in that toughness is greatly reduced. In addition, in order to produce a steel having an N content of less than 0.001%, a lot of time is required for the steel-making operation, and thus there is a problem in that productivity is lowered. Therefore, in the present invention, the N content is preferably controlled to 0.001 to 0.01%. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the N content may be 0.002%, and the upper limit of the N content may be 0.006%.

In addition, according to an aspect of the present invention, although not particularly limited, optionally, the cold rolled steel sheet may further comprise, in weight%, a metal selected from the group consisting of: cr: less than 1.0% (including 0%), mo: below 0.2% (including 0%) and B: less than 0.005% (including 0%) of one or more of the following. The reason for adding the optional additive element and the reason for limiting the content will be described below.

Cr: less than 1.0% (including 0%)

Chromium (Cr) is a component added to improve hardenability of steel and secure high strength, and chromium is an element that plays a very important role in the formation of martensite, and minimizes a decrease in elongation compared to an increase in strength, thereby also facilitating the manufacture of complex phase steel having high ductility. Therefore, the Cr may be selectively added for the above effect. However, when the Cr content exceeds 1.0%, not only the above effect is saturated, but also cold-rolling property is deteriorated due to an excessive increase in hot-rolling strength. Further, since the martensite fraction after annealing is greatly increased and the elongation is reduced, the upper limit of the Cr content is controlled to 1.0% or less. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content may be 0.8%.

Mo: less than 0.2% (including 0%)

Molybdenum (Mo) is an element that suppresses the formation of pearlite and increases hardenability. Therefore, mo may be selectively added in the present invention in order to secure the above-described effects. However, when the Mo content exceeds 0.2%, the effect of improving strength is not greatly increased, but on the other hand, ductility becomes poor, and thus it may be economically disadvantageous. Therefore, the Mo content is preferably controlled to 0.2% or less. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the Mo content may be 0.01%, or the upper limit of the Mo content may be 0.1%.

B: less than 0.005% (including 0%)

When boron (B) exists in a solid solution state in steel, it has an effect of stabilizing grain boundaries to improve brittleness of steel in a low temperature region, and greatly increases hardenability of steel. Therefore, the B may be selectively added for the above effect. However, when the upper limit of B exceeds 0.005%, recrystallization at the time of annealing is delayed, and oxide is formed on the surface to deteriorate the plating property. Therefore, the content of B is preferably controlled to 0.005% or less. In addition, in terms of further improving the above-described effects, more preferably, the lower limit of the B content may be 0.0003%, or the upper limit of the B content may be 0.0025%.

The remainder of the invention is iron (Fe). However, in the conventional manufacturing process, since raw materials or surrounding environmental factors may inevitably mix in unnecessary impurities, these impurities cannot be excluded. These impurities are well known to those skilled in the art of general steel manufacturing processes, and therefore, not specifically described in the present specification in their entirety.

According to an aspect of the present invention, the high-strength cold-rolled steel sheet may have a value defined by the following relation 1 of 0.6 or more and less than 0.9, and by satisfying this condition, deviation in material quality of the cold-rolled steel sheet is minimized and occurrence of surface defects is suppressed, so that a desired material quality may be ensured.

[ Relation 1]

C+(1.3×Si+Mn)/6+(Cr+1.2×Mo)/5+100×B

In the present invention, the relation 1 is a formula showing the hardenability (Hardenability) of the steel material having the composition according to the present invention, and the coefficient before each element is a coefficient quantitatively showing the degree of contribution of the element to the hardenability. When the hardenability of the steel is high, it is advantageous to ensure a low-temperature transformation phase such as a bainite phase and martensite phase, and therefore it is advantageous to improve the strength, and as the hardenability is low, it is advantageous to promote ferrite transformation, and it is disadvantageous to ensure the strength.

In particular, in the present invention, in order to secure a high strength where the desired Tensile Strength (TS) is 780MPa or more, the value defined by the above-mentioned relational expression 1 needs to satisfy 0.6 or more. On the other hand, when the value defined by the above-mentioned relation 1 is 0.9 or more, the strength becomes excessively high, and there is a problem that the elongation becomes poor. Further, when the value defined by the above-mentioned relation 1 is 0.9 or more, in the step of cooling the hot-rolled steel sheet to 450-700 ℃ at an average cooling rate of 10-70 ℃/sec immediately after hot rolling, the transformation of ferrite is greatly delayed. Therefore, in the subsequent winding step, excessive lower bainite phase and martensite phase having high hardness are formed in the bainite phase in the hot rolled steel sheet, and there is a problem that the material deviation according to the position in the width direction is increased and the shape is deteriorated. Therefore, in the present invention, the value defined by the above-mentioned relation 1 is preferably controlled to satisfy 0.6 or more and less than 0.9. In addition, in terms of further maximizing the above-described effect, the lower limit of the value defined by the relation 1 may be 0.62, or the upper limit of the value defined by the relation 1 may be 0.84.

In addition, according to an aspect of the present invention, the high-strength hot-rolled steel sheet includes, in area%, as a microstructure: ferrite: more than 50 percent, the balance: bainite and ferrite. When the ferrite content in the microstructure is less than 50%, there is a problem that the elongation is insufficient and the formability is deteriorated. The balance of bainite and ferrite may be 50% or less. When the total of the bainite and ferrite exceeds 50%, there are problems in that the strength is too high and the elongation is insufficient.

Or according to an aspect of the present invention, although not particularly limited, in terms of improving elongation and formability, the microstructure of the high-strength cold-rolled steel sheet may include, in area%: ferrite: 50-85%, total of bainite and ferrite: 15-50%.

In the high-strength cold-rolled steel sheet, when ferrite exceeds 85%, a problem may occur in that the desired strength may not be achieved, and when the total of bainite and martensite is less than 15%, a problem may occur in that the desired strength may not be achieved. In addition, in terms of further improving the above-described effects, more preferably, the microstructure of the high-strength cold-rolled steel sheet may include, in area%: ferrite: 66-75%.

In addition, according to an aspect of the present invention, although not particularly limited, the microstructure of the high-strength cold-rolled steel sheet may include, in area%: bainite: 3-7%, and/or martensite: 19-31%. In the high-strength cold-rolled steel sheet, when the bainite is less than 3%, a problem may occur in that the desired strength cannot be achieved, and when the bainite exceeds 7%, although the strength is high, a problem may occur in that the elongation is low. Or in the high-strength cold-rolled steel sheet, when the martensite is less than 19%, a problem may occur in that the desired strength cannot be achieved, and when the martensite exceeds 31%, although the strength is high, a problem may occur in that the elongation is low.

According to an aspect of the present invention, in the high-strength cold-rolled steel sheet, an average number of surface defects satisfying one or more of a depth of 100 μm or more and a short side length of 1mm or more is less than 10 pieces/m ² (including 0 pieces/m ²). In measuring the average number of the surface defects, the condition that "the depth is 100 μm or more" or "the short side length is 1mm or more" is merely a judgment criterion for measuring the surface defects, as long as it is satisfied. Therefore, in the present specification, the respective upper limit values of the depth and the short side length are not particularly limited.

In the present invention, the surface defect means a defect having a groove shape, specifically a defect in the form of a dent in the thickness direction, and means a defect identifiable when the surface of the steel sheet is visually observed. The depth of the surface defect may be the "highest depth" in the thickness direction of the groove shape defect with respect to the cross section in the thickness direction of the cold-rolled steel sheet (i.e., the direction perpendicular to the rolling direction with respect to the cross section). The short side length of the surface defect may be the shortest length passing through the position of the highest depth with reference to the surface of the cold-rolled steel sheet. Fig. 2 is a photograph showing a surface defect of a groove shape formed on the surface of the steel sheet, and a depth and a short side length of each surface defect are confirmed, which is photographed by a Scanning Electron Microscope (SEM) at a high magnification.

The present inventors have conducted intensive studies to provide a cold rolled steel sheet that can solve the problems of the prior art and ensure a target level of strength and formability while minimizing surface defects and material deviation.

As a result, it was found that the above-described effects can be ensured by controlling the average number of surface defects satisfying one or more of the above-described conditions of a depth of 100 μm or more and a short side length of 1mm or more to be less than 10 pieces/m ². That is, in the present invention, when the average number of the surface defects is 10 pieces/m ² or more, a problem of occurrence of surface dents may occur. In order to further improve the effect, the average number of the surface defects may be preferably 8 or less per m ².

Further, according to an aspect of the present invention, the present inventors have made further repeated studies to provide a cold-rolled steel sheet which does not affect material variations and the like even if there is a defect on the surface of the steel sheet, and which can simultaneously ensure a target level of strength and formability. As a result, the present invention has further found out the surface defect characteristics at a level that does not affect the material deviation or the like even if there is a surface defect. Specifically, although not particularly limited in the present invention, the maximum depth of the surface defect may be 500 μm or less. In this case, the maximum depth of the surface defect may be a maximum value of depths of the surface defects existing on the surface of the steel sheet.

In addition, according to an aspect of the present invention, a difference between Yield Strengths (YS) of both end portions and a center portion in a width direction of the cold-rolled steel sheet may be 100MPa or less. The difference between the yield strengths of the two end portions and the center portion is 100MPa or less, and a steel sheet having reduced material variation in the width direction and having an effect of uniform material in the width direction can be provided. In this case, the "both end portions" may refer to a section from both ends to 30% (corresponding to the sum: 60%) based on the total width (referred to as 100%) of the cold-rolled steel sheet in the width direction, and the "center portion" may refer to a section of 40% excluding the both end portions based on the total width of the cold-rolled steel sheet in the width direction.

Further, according to an aspect of the present invention, the Tensile Strength (TS) of the cold-rolled steel sheet may be 780MPa or more, preferably 780MPa or more and less than 1180MPa, and more preferably 800MPa or more and 1100MPa or less. When the tensile strength of the cold-rolled steel sheet is less than 780MPa, a problem may occur in that the target strength required for the application part cannot be satisfied, and when the tensile strength of the cold-rolled steel sheet exceeds 1100MPa, a problem may occur in that cracks occur in the part formation or the impact absorption resistance of the part is significantly reduced.

In addition, according to an aspect of the present invention, the Yield Strength (YS) of the cold-rolled steel sheet may be 380MPa or more, and more preferably may be 390MPa or more and 650MPa or less. When the yield strength of the cold-rolled steel sheet is less than 380MPa, a problem of deterioration of collision resistance of the component may occur, and when the yield strength of the cold-rolled steel sheet exceeds 650MPa, a problem of deterioration of formability may occur.

In addition, according to an aspect of the present invention, the product of the tensile strength and the elongation of the cold-rolled steel sheet may be 12000MPa% or more (more preferably 12000MPa% or more and 16500MPa% or less, most preferably 12000MPa% or more and 16200MPa% or less). By satisfying the above physical properties, it is possible to ensure the effect that the composition can be suitably applied to structural members such as pillars, which require stable strength-elongation balance and impact absorbability, among members constituting an automobile body.

Although not particularly limited, the cold-rolled steel sheet may optionally further include a plating layer formed on the surface. At this time, the plating layer may be formed by a plating process described later. Further, the composition of the plating layer may be different depending on the purpose, and therefore is not particularly limited in this specification, and as an example, a zinc-based plating layer and the like may be exemplified.

A method of manufacturing a high-strength cold-rolled steel sheet according to an aspect of the present invention is described in detail as follows. But does not mean that the method of manufacturing a cold rolled steel sheet according to the present invention must be manufactured by the following manufacturing method.

Reheating step of billet

The billet satisfying the above composition is reheated to 1100-1350 ℃. The composition of the steel slab is the same as that of the cold-rolled steel sheet, and in this case, the explanation of the cold-rolled steel sheet is similarly applied to the reasons for adding each component and the reasons for limiting the content in the steel slab. In addition, when the reheating temperature of the slab is lower than 1100 ℃, alloy elements segregated in the center portion of the slab remain, and the initial temperature of hot rolling is too low, so that a problem of serious rolling load occurs. On the other hand, when the reheating temperature of the slab exceeds 1350 ℃, the strength is lowered due to coarsening of austenite grains. Therefore, in the present invention, the reheating temperature of the billet is preferably controlled to 1100 to 1350 ℃.

Hot rolling step

And hot rolling the reheated steel billet at 850-1150 ℃. When the hot rolling temperature exceeds 1150 ℃, the temperature of the hot rolled steel sheet increases, the grain size becomes coarse, and the surface quality of the hot rolled steel sheet deteriorates. When the hot rolling temperature is lower than 850 ℃, the extended crystal grains develop due to excessive delay of recrystallization, the load increases during rolling, the temperature of both ends is greatly reduced, and uneven microstructure is formed during cooling, so that the material deviation increases and the formability also deteriorates.

Cooling step after hot rolling

The hot rolled steel sheet is cooled to 450-700 c at an average cooling rate of 10-70 c/sec, more preferably 20-50 c/sec. When the cooling temperature of the hot rolled steel sheet is lower than 450 ℃, there occurs a problem that the deviation of the quality becomes poor, and when the cooling temperature of the hot rolled steel sheet exceeds 700 ℃, not only the deviation of the quality but also the internal oxidation of the hot rolling occurs, so that there occurs a problem that the surface defect occurs. In addition, when the average cooling rate is less than 10 ℃/sec, there is a problem that crystal grains of the matrix structure become coarse and the microstructure becomes uneven. In addition, when the average cooling rate exceeds 70 ℃/sec, a bainite phase and a martensite phase are easily formed, and thus there is a problem in that the load at the time of cold rolling increases.

Winding step

And rolling the cooled steel plate at 450-700 ℃. When the winding temperature is cooled to below 450 ℃ and winding is performed, a bainite phase and a martensite phase are unnecessarily formed in the steel, resulting in uneven shape, and a rolling load at the time of cold rolling is greatly increased. When the winding temperature exceeds 700 ℃ and winding is performed, ferrite grains become large, coarse pearlite is easily formed, and uneven microstructure is formed at the time of annealing, so that there is a problem that formability of steel is deteriorated. Further, hot rolled oxides are increased, adsorbed on the rolls at the time of annealing, so that oxides accumulate on the rolls, and when a sheet of a steel sheet passes through, problems of surface defects such as dent defects and the like of the surface of the steel sheet occur due to friction of the steel sheet with the rolls. In addition, when hot rolled oxides remain in the steel sheet, the plating quality and plating adhesion are deteriorated when the steel sheet is plated.

Generally, after rolling, both widthwise end portions of a rolled steel sheet (rolled sheet) are rapidly cooled by exposure to the surrounding atmosphere, and widthwise center portions are slowly cooled. Therefore, since the rolling step starts, a cooling deviation occurs in the width direction of the steel sheet, and the microstructure of the rolled steel sheet varies from position to position, and finally, a material deviation occurs in the hot-rolled steel sheet. In the hot-rolled steel sheet having such a large material deviation, not only the material deviation is increased during the cold rolling, but also the surface defects of the groove shape, which are not observed visually in the hot-rolled steel sheet, are further increased after the cold rolling, and thus the problem of the occurrence of large surface defects occurs. That is, since the hot-rolled steel sheet having a large material deviation causes a material deviation at different positions in the width direction as well as a shape deterioration at the time of cold rolling, the present inventors have studied intensively to solve the above-mentioned problems, and have studied a manufacturing method in which the temperatures of both end portions and the center portion are controlled to be different in the rolling step.

Specifically, in the present invention, as a method for reducing the material deviation in the width direction of the steel sheet and suppressing the surface defects, at the time of the rolling, the surface temperature (Te) of both end portions in the width direction is controlled to satisfy 601 to 700 ℃ and the surface temperature (Tc) of the center portion is controlled to satisfy 450 to 600 ℃ based on the total width of the steel sheet. In this case, the "width direction of the steel sheet" refers to a direction perpendicular to the conveying direction of the steel sheet with reference to the surface of the steel sheet. The above description applies equally to the both end portions and the center portion.

In this case, if Te is lower than 601 ℃, there is a problem that the supercooling of both ends increases the material deviation, and if Te exceeds 700 ℃, there is a problem that the deterioration of the center portion increases the material deviation and surface defects. When the Tc is lower than 450 ℃, the temperature difference between the center portion and both end portions becomes large, and the material deviation becomes poor, and when the Tc exceeds 600 ℃, the temperature of the center portion becomes too high, and there are problems that the material deviation and surface defects occur.

As described above, in the winding step, various methods may be applied in order to control the surface temperatures of both end portions in the width direction of the steel sheet to be different from the surface temperature of the center portion, and thus, the method is not particularly limited. For example, in the above-described rolling, in order to control the temperatures of both end portions and the center portion of the steel sheet to be different, in the cooling step before rolling, the cooling water injected into both end portions may be blocked before reaching the steel sheet, or the amount of the cooling water injected may be controlled to be different, or both methods may be simultaneously performed. As an example, according to an aspect of the present invention, in the cooling step before winding, the water injection amount of the cooling water injected to the center portion except the both end portions is controlled to be larger than the water injection amount of the cooling water injected to the both end portions in the width direction based on the total width of the steel sheet.

In addition, according to an aspect of the present invention, although not particularly limited, in the winding step, a difference (Te-Tc) between the surface temperatures of the both end portions and the surface temperature of the center portion may be set to 150 ℃ or less in terms of further reducing material deviation and improving an effect of suppressing surface defects. At this time, when the Te-Tc value exceeds 150 ℃, there may occur a problem that the widthwise material variation becomes worse. However, the smaller the temperature deviation calculated from the Te-Tc is, the more preferable, and thus the lower limit thereof may not be limited alone, and may be preferably 0 ℃. Further, more preferably, the lower limit of the Te-Tc value may be 50℃and the upper limit of the Te-Tc value may be 90 ℃.

Holding step in heat-insulating cover

After the above-described winding step, optionally, it may be moved into a heat-retaining cover and held at 400-500 ℃ for more than 6 hours. After the rolling step, by holding in the heat-retaining cover for a long period of time so that the steel sheet is held at temperatures in the range of 601 to 700 ℃ and 450 to 600 ℃ at both end portions and the center portion in the width direction of the steel sheet, respectively, a large amount of bainite structure is uniformly formed at both end portions and the center portion of the entire length of the rolled sheet, whereby a cold-rolled steel sheet excellent in shape quality and having a small rolling load at the time of cold rolling and a uniform thickness can be manufactured.

In the step of holding in the heat-retaining cover, the surface temperature of the steel sheet may be adjusted to 400-500 ℃. At this time, in the step of holding in the heat insulating cover, when the surface temperature of the steel sheet is lower than 400 ℃, the above effect cannot be ensured, and when the surface temperature of the steel sheet exceeds 500 ℃, coarse carbides are locally formed, and hot rolling oxides increase, so that the formability and surface quality of the steel may be deteriorated.

When the holding time in the heat-insulating cover is less than 6 hours, there is a possibility that a problem of material deviation may occur, and the upper limit of the holding time in the heat-insulating cover is not particularly limited, but may be 8 hours or less as an example.

In addition, in terms of further improving the above effect, the rolled steel sheet may be accommodated in the heat insulation cover within 90 minutes immediately after rolling, and when the time before being accommodated in the heat insulation cover exceeds 90 minutes, supercooling may occur in the center portion in the width direction due to excessive air cooling, so that the range of 450-600 ℃ may not be satisfied. Or after the step of holding in the heat-retaining cover, air cooling or water cooling may be further performed to room temperature.

Cold rolling step

And (3) cold rolling the rolled steel plate at a cold rolling reduction of 40-70%. When the cold rolling reduction is less than 40%, it is difficult to ensure not only a desired thickness but also the shape of the steel sheet, whereas when the cold rolling reduction exceeds 70%, there is a high possibility of occurrence of cracks in the edge (edge) portion of the steel sheet, and there is a problem of bringing about a cold rolling load. Therefore, in the present invention, the cold rolling reduction is preferably limited to 40 to 70%.

Annealing step

And continuously annealing the cold-rolled steel plate at 740-900 ℃. When the annealing temperature is lower than 740 ℃, problems may occur in that the strength and elongation do not reach standards without recrystallization, and when the annealing temperature exceeds 900 ℃, problems may occur in that surface oxides occur. In addition, in terms of further improving the above effect, more preferably, the annealing temperature may be set to 750 to 850 ℃.

Further, although not particularly limited, according to an aspect of the present invention, after the continuous annealing step, the following steps may be optionally further included: primary cooling is carried out at a cooling rate of 1-10 ℃/s, and cooling is carried out to 650-700 ℃; after the primary cooling step, secondary cooling is performed at a cooling rate of 11-20 ℃/sec to Ms-100 ℃ to ms+100 ℃. In addition, after the secondary cooling step, optionally, a step of overaging while maintaining a constant temperature may be further included. By satisfying the conditions of the primary cooling step, the secondary cooling step, and the overaging step, the strength and elongation can be further improved. In this case, ms is the initial temperature at which martensite is formed when the annealed steel sheet is cooled, and can be obtained from the following relational expression 2.

[ Relation 2]

Ms＝539-423×C-30.4×Mn-12.1×Cr-17.7×Ni-7.5×Mo

( In the relational expression 2, C, mn, cr, ni and Mo represent the average content of the weight% of each element. And substituting 0 in the case where the elements are not added. )

In addition, according to an aspect of the present invention, optionally, a step of plating (preferably hot dip galvanization) the cold rolled steel sheet may be further included, by which a plated steel sheet may be obtained.

Detailed Description

The present invention will be described more specifically with reference to examples. It should be noted, however, that the following examples are illustrative of the present invention and are not intended to limit the scope of the claims. This is because the scope of the invention is determined by what is recited in the claims and what is reasonably derived therefrom.

Example (example)

The billets satisfying the composition of table 1 below were reheated at 1200 ℃, hot rolled at 900 ℃, then cooled to 450-700 ℃ at a cooling rate of 20-50 ℃/sec, and then wound. At this time, the water injection amount of the cooling water injected into the center portion except the both end portions was controlled to be larger than the water injection amount of the cooling water injected into the both end portions of the steel sheet in the width direction based on the total width of the steel sheet in the width direction at the time of winding so that the steel sheet surface temperature (Te) from both end portions to both end portions of the 30% section and the steel sheet surface temperature (Tc) of the remaining 40% center portion satisfy the hot rolling conditions described in table 2 below. The rolled hot rolled steel sheet was transferred to a heat-retaining jacket, and the heat-retaining jacket conditions described in table 2 below were controlled so as to satisfy the average temperature and the retention time before and after the heat-retaining jacket was installed. Subsequently, the hot-rolled steel sheet was cold-rolled at a cold rolling reduction of 50%, continuously annealed at 800 ℃, then primary-cooled at an average cooling rate of 8 ℃/sec to 670 ℃, then secondary-cooled at an average cooling rate of 12 ℃/sec to ms+100 ℃, thereby obtaining a cold-rolled steel sheet.

For each of the cold-rolled steel sheets thus obtained, the microstructure, mechanical properties, and average number of surface defects per unit area (number/m ²) observed on the surface of the invention examples and comparative examples were measured and shown in tables 3 to 5 below. At this time, YS, TS, and El represent the yield strength, tensile strength, and elongation at break of 0.2% offset (off-set), respectively, and represent the results of collecting JIS No. 5 standard test pieces at the center and both ends, respectively, in the direction perpendicular to the rolling direction, and performing the test. The microstructure was the result of measuring the area% of a photograph observed at a magnification of 3000 to 5000 using a scanning electron microscope (field emission scanning electron microscope (FE-SEM)). The average number of surface defects is obtained by visually observing the surface of a steel sheet manufactured, and measuring the average number of surface defects satisfying one or more conditions of a depth of 100 μm or more and a short side length of 1mm or more. In particular, the maximum depth of the surface defect is measured by the same method as described in the present specification. The yield strength of the test pieces taken from the center and both ends in the width direction of the cold-rolled steel sheet was measured by the same method as described above, and the material deviation in the width direction of these test pieces was measured and shown in tables 4 and 5 below.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

From the experimental results shown in tables 1 to 5, it was confirmed that cold-rolled steel sheets capable of suppressing material deviation and surface defects while securing Tensile Strength (TS) of 780MPa or more were obtained in the cases of invention examples 1 to 6 satisfying the composition and manufacturing conditions of the present invention. At this time, it was confirmed that the maximum depth of the surface defects measured in the cold-rolled steel sheets obtained in invention examples 1 to 6 of the present invention satisfied 500 μm or less.

In addition, in the case of comparative examples 1 to 16, which do not satisfy one or more of the composition and the manufacturing conditions of the present invention, material deviation is poor, or surface defects occur, and/or it is difficult to ensure the desired physical properties of the present invention.

In particular, the Si addition amount of the comparative steel 1 exceeds 2.0% and does not satisfy the relation 1. Therefore, in the case of using comparative examples 13 and 14 of the comparative steel 1, even if the manufacturing conditions proposed by the present invention are satisfied and the material deviation is good, the problem of dents is caused by the accumulation of Si oxide in the annealing furnace, and thus there is a problem that the average number of surface defects of the product exceeds the target value.

In addition, the alloy addition amount of the comparative steel 2 is small, and thus the relation 1 is not satisfied. Therefore, in the case of comparative examples 15 and 16 using the comparative steel 2, although the production conditions proposed by the present invention are satisfied and the surface defects and the material deviation are good, the tensile strength is less than 780MPa, and the target material cannot be satisfied.

In comparative examples 1, 5 and 9, temperatures of both end portions and the center portion in the width direction are higher than the temperatures proposed in the present invention, and in comparative examples 4, 8 and 12, temperatures of the heat insulating covers are higher than the standard temperatures. Therefore, in the comparative example, too much hot rolled oxide was generated, and based on this oxide, the final steel sheet produced a large number of surface defects.

In addition, comparative examples 2, 6 and 10 are examples showing that the temperatures of both end portions and the center portion in the width direction are lower than the temperatures proposed in the present invention and that the difference (Te-Tc) between the surface temperatures of both end portions and the surface temperature of the center portion exceeds 150 ℃, and comparative examples 3, 7 and 11 are examples showing that no heat insulating cover is applied. Therefore, in the comparative example, although the desired material quality of the annealed steel sheet can be ensured and the average number of surface defects is good, there is a problem in that the yield strength deviation in the width direction of the annealed steel sheet exceeds the target value of 100 MPa.

Claims

1. A high-strength cold-rolled steel sheet comprising, in weight-%: c:0.05-0.3%, si:0.01-2.0%, mn:1.5-3.0%, al:0.01-0.1%, P:0.001-0.015%, S:0.001-0.01%, N:0.001-0.01%, and the balance of Fe and other unavoidable impurities,

The value defined by the following relation 1 satisfies 0.6 or more and less than 0.9,

The microstructure comprises, in area percent: ferrite: more than 50 percent, the balance: the bainite and martensite are present in the composition,

The average number of surface defects satisfying one or more of the conditions of a depth of 100 [ mu ] m or more and a short side length of 1mm or more is less than 10 pieces/m ²,

[ Relation 1]

C+(1.3×Si+Mn)/6+(Cr+1.2×Mo)/5+100×B

In the relational expression 1, C, si, mn, cr, mo and B represent average contents of weight% for each element, and are substituted with 0 in the case where the elements are not added.

2. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the microstructure comprises, in area%: ferrite: 50-85%, total of bainite and ferrite: 15-50%.

3. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the microstructure comprises, in area%: ferrite: 66-75%.

4. The high-strength cold-rolled steel sheet as claimed in claim 3, wherein the microstructure comprises, in area%: bainite: 3-7%.

5. The high-strength cold-rolled steel sheet as claimed in claim 3, wherein the microstructure comprises, in area%: martensite: 19-31%.

6. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet further comprises, in weight%, a composition selected from the group consisting of Cr: less than 1.0% and including 0%, mo:0.2% or less and comprising 0% and B: less than 0.005% and including more than one of 0%.

7. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet has a tensile strength of 780MPa or more and a yield strength of 380MPa or more.

8. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the product of the tensile strength and elongation of the high-strength cold-rolled steel sheet is 12000mpa% or more.

9. The high-strength cold-rolled steel sheet according to claim 1, wherein a difference between yield strengths of both end portions and a center portion in a width direction of the cold-rolled steel sheet is 100MPa or less.

10. A method of manufacturing a high-strength cold-rolled steel sheet, comprising the steps of:

Reheating a steel billet to 1100-1350 ℃, said steel billet comprising, in weight-%: c:0.05-0.3%, si:0.01-2.0%, mn:1.5-3.0%, al:0.01-0.1%, P:0.001-0.015%, S:0.001-0.01%, N:0.001 to 0.01%, the balance being Fe and other unavoidable impurities, the value defined by the following relation 1 satisfying 0.6 or more and less than 0.9;

Hot rolling the reheated steel billet at 850-1150 ℃;

Cooling the hot rolled steel plate to 450-700 ℃ at an average cooling rate of 10-70 ℃/sec;

Rolling the cooled steel plate at 450-700 ℃;

Cold rolling the rolled steel plate at a rolling reduction of 40-70%; and

Continuously annealing the cold-rolled steel plate at 740-900 ℃,

Wherein, in the winding step, the surface temperature Te of the two end parts in the width direction based on the total width of the steel plate is controlled to be 601-700 ℃, the surface temperature Tc of the central part is controlled to be 450-600 ℃,

[ Relation 1]

C+(1.3×Si+Mn)/6+(Cr+1.2×Mo)/5+100×B

11. The method of manufacturing a high-strength cold-rolled steel sheet according to claim 10, wherein after the rolling step, further comprising the step of moving the rolled steel sheet into a heat-insulating cover and maintaining the temperature in the range of 400-500 ℃ for more than 6 hours.

12. The method of manufacturing a high-strength cold-rolled steel sheet according to claim 10, wherein the rolling step is controlled such that a difference between the surface temperatures of the both end portions and the surface temperature of the center portion satisfies 150 ℃ or less.

13. The method of manufacturing a high-strength cold-rolled steel sheet according to claim 10, wherein the cooling step is controlled such that the amount of water injected into the central portion except the both end portions is greater than the amount of water injected into the both end portions in the width direction, based on the total width of the steel sheet.