KR20230043352A - High strength cold rolled steel sheet having excellent surface quality and low mechanical property deviation and manufacturing method of the same - Google Patents

Abstract

The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and low physical property variation and a manufacturing method thereof, and more specifically, to a high-strength cold-rolled steel sheet which has less surface defects and physical property variation while ensuring high strength and elongation, thereby being suitable for vehicle parts, and a manufacturing method thereof. To this end, the high-strength cold-rolled steel sheet comprises 0.05 to 0.3 wt% of C, 0.01 to 2.0 wt% of Si, 1.5 to 3.0 wt% of Mn, 0.01 to 0.1 wt% of Al, 0.001 to 0.015 wt% of P, 0.001 to 0.01 wt% of S, 0.001 to 0.01% of N, the balance Fe and other unavoidable impurities.

Description

High-strength cold-rolled steel sheet with excellent surface quality and low material variation and manufacturing method thereof

The present invention relates to a high-strength cold-rolled steel sheet used for structural members having a large amount of molding, such as pillars, seat rails, and members of automobile bodies, and a method for manufacturing the same, and more particularly, automotive parts having excellent surface quality and low material variation. It relates to a high-strength cold-rolled steel sheet that can be suitably used for and a manufacturing method thereof.

In recent years, as safety and environmental regulations have been strengthened in the automobile industry, the use of high-strength steel with a tensile strength of 780 MPa or more is increasing when manufacturing a vehicle body to improve vehicle fuel efficiency and protect passengers.

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

However, in high-strength steel, when a large amount of Si, Al, Mn, etc. is added, there is a problem in that weldability is poor and surface defects of the steel sheet due to furnace dents occur during annealing. In addition, when a large amount of hardenable elements such as Mn, Cr, and Mo are added, there is a problem in that the quality of the thickness deteriorates during cold rolling due to material deviation of the hot-rolled coil. At this time, the surface defect due to the dent in the furnace refers to a surface defect of the steel sheet formed by contact between the steel sheet and the roll during sheet passing, in which metal-based oxides on the surface of the steel sheet are adsorbed and accumulated on the annealing furnace rolls.

The prior art related to the manufacturing technology of high-strength cold-rolled steel sheets and hot-dip galvanized steel sheets to solve the above problems is briefly described as follows.

Among the prior art, Patent Document 1 discloses a process of cold rolling a hot-rolled steel sheet containing a low-temperature transformation phase of 60% or more by volume at a cold reduction ratio of more than 60% and less than 80%, and ferrite and austenite steel sheets after cold rolling. A high-strength cold-rolled steel sheet and its manufacturing method are presented through a continuous annealing process in the second phase. However, the cold-rolled steel sheet obtained from Patent Document 1 has a low strength of 370 to 590 MPa, so it is difficult to apply to an automobile shock-resistant member and has a problem that it is limited only to the use of inner and outer panels.

In addition, Patent Document 2 discloses a method of manufacturing a cold-rolled steel sheet having high strength and high ductility at the same time by utilizing a tempered martensite phase and having excellent sheet shape after continuous annealing. However, the technology of Patent Document 2 has a problem of poor weldability due to a high carbon content of 0.2% or more in the steel, and a problem of surface defects due to dents in the furnace due to a large amount of Si.

Korean Patent Publication No. 2004-0066935 Japanese Unexamined Patent Publication No. 2010-090432

According to one aspect of the present invention, it is intended to provide a high-strength cold-rolled steel sheet with excellent surface quality and low material variation and a manufacturing method thereof.

The object of the present invention is not limited to the foregoing. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the additional objects of the present invention from the contents throughout the present specification.

One aspect of the present invention,

In weight percent, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.001% 0.01%, the balance including Fe and other unavoidable impurities,

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

As a microstructure, by area%, ferrite: 50% or more, balance: including bainite and martensite,

A high-strength cold-rolled steel sheet having an average number of surface defects satisfying at least one condition of a depth of 100 µm or more and a short side length of 1 mm or more is less than 10/m ² .

[Relationship 1]

C + (1.3×Si+Mn)/6 + (Cr+1.2×Mo)/5 + 100×B

(In the relational expression 1, the C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. In addition, when each element is not added, 0 is substituted.)

Another aspect of the present invention is,

In weight percent, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.001% Reheating a steel slab containing 0.01%, the balance Fe and other unavoidable impurities, and satisfying a value defined by the following relational expression 1 of 0.6 or more and less than 0.9 to 1100 to 1350 ° C;

hot-rolling the reheated steel slab at 850 to 1150° C.;

cooling the hot-rolled steel sheet to 450-700°C at an average cooling rate of 10-70°C/s;

Winding at 450 ~ 700 ℃ of the cooled steel sheet;

Cold rolling the rolled steel sheet at a reduction ratio of 40 to 70%; and

continuously annealing the cold-rolled steel sheet at 740 to 900° C.;

including,

The winding step is controlled so that the surface temperature (Te) of both ends in the width direction meets 601 to 700 ° C and the surface temperature (Tc) of the central portion meets 450 to 600 ° C, based on the entire width of the steel sheet. Provided is a method for manufacturing a high-strength cold-rolled steel sheet.

[Relationship 1]

C + (1.3×Si+Mn)/6 + (Cr+1.2×Mo)/5 + 100×B

(In the above relational expression 1, the C, Si, Mn, Cr, Mo and B represent the weight percent average content of each element. At this time, when each element is not added, 0 is substituted.)

According to one aspect of the present invention, it is possible to provide a high-strength cold-rolled steel sheet with excellent surface quality and low material variation and a manufacturing method thereof.

Various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1 shows pictures taken with a general low-magnification camera of surface defects of each cold-rolled steel sheet obtained from Inventive Example 1 and Comparative Example 1 of the present invention.
Figure 2 shows a photograph taken with a high-magnification scanning cell microscope (SEM) of the surface defect defined in the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in many different forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Meanwhile, terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, the singular forms used herein include the plural forms unless the relevant definition clearly dictates the contrary. Also, the meaning of "comprising" used in the specification specifies a component, and does not exclude the presence or addition of other components.

In the prior art, technology that satisfies the high-end demand for cold-rolled steel sheets with excellent surface quality and low material variation, while being applicable to structural members with a high tensile strength (TS) of 780 MPa or more and excellent formability, has not been developed. situation is not.

Accordingly, the inventors of the present invention conducted intensive studies to provide a cold-rolled steel sheet that satisfies all of the above-described characteristics while solving the problems of the prior art. As a result, the composition and manufacturing conditions of the steel sheet were optimized, and the microstructure and surface defects were controlled By doing so, it was found that the above object can be achieved and the present invention has been completed.

That is, according to the present invention, the product of tensile strength and elongation satisfies 12,000 MPa% or more even though it has a high strength of 780 MPa or more, so that a structure such as a filler that requires a stable strength-elongation balance and shock absorption among parts constituting an automobile body A high-strength cold-rolled steel sheet that can be suitably applied to member parts can be effectively provided.

Hereinafter, a high-strength steel sheet having excellent surface quality and small material variation according to an aspect of the present invention will be described in detail.

In the high-strength cold-rolled steel sheet according to one aspect of the present invention, by weight, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015% , S: 0.001 to 0.01%, N: 0.001 to 0.01%, the balance including Fe and other unavoidable impurities.

Hereinafter, the reason for adding components and the reason for limiting the content of the cold-rolled steel sheet according to the present invention will be described in detail. At this time, in the present specification, when indicating the content of each element, unless otherwise specified, weight % is indicated.

C: 0.05 to 0.3%

The carbon (C) is a very important component in securing a martensitic structure effective in strengthening steel. When the addition amount of C increases, the martensite phase and bainite phase fractions increase, resulting in an increase in tensile strength. 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 fraction of the martensite phase and bainite phase, which are hard phases, increases by expanding the austenite region during the two-phase annealing, and the fraction of the ferrite phase, which is a soft phase, decreases, resulting in poor formability and poor weldability. It happens. Therefore, the upper limit of the C content is controlled to 0.3%. On the other hand, in terms of further improving the above-mentioned effect, 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 to 2.0%

The silicon (Si) is an element advantageous for deoxidizing molten steel, having a solid solution strengthening effect, and delaying the formation of coarse carbides to improve formability. However, when the Si content is less than 0.01%, it is difficult to improve formability due to the small effect described above. On the other hand, when the Si content exceeds 2.0%, red scale due to Si is severely formed on the surface of the steel sheet during hot rolling. Accordingly, surface defects are generated, or non-plating occurs due to concentration on the surface during the annealing process. In addition, there is a problem in that the plating adhesion is inferior due to the formation of surface oxide, and the surface quality is very poor. Therefore, in the present invention, the Si content is controlled to 0.01 to 2.0%. On the other hand, in terms of further improving the above-mentioned effect, 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 to 3.0%

Manganese (Mn), like Si, is an element effective in solid solution strengthening of steel and greatly increases hardenability. However, if the Mn content is less than 1.5%, the aforementioned effects cannot be obtained, and if the Mn content exceeds 3.0%, the strengthening effect is greatly increased and the ductility is reduced. In addition, the segregation part is greatly developed in the thickness center during slab casting in the casting process, and the microstructure in the thickness direction is non-uniform during cooling after hot rolling, and MnS is formed, resulting in poor formability such as stretch flangeability. Therefore, in the present invention, the content of Mn is controlled to 1.5 to 3.0%. On the other hand, in terms of further improving the above-mentioned effect, 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 to 0.1%

The aluminum (Al) is a component mainly added for deoxidation. If the Al content is less than 0.01%, the addition effect is insufficient. On the other hand, when the Al content exceeds 0.1%, AlN is formed in combination with nitrogen, so that corner cracks are likely to occur in the slab during cast casting, and defects due to inclusion formation are likely to occur. Therefore, in the present invention, the Al content is controlled to 0.01 to 0.1%. On the other hand, in terms of further improving the above 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 to 0.015%

The phosphorus (P) is an alloying element having a very large solid solution strengthening effect, and is characterized in that a large strengthening effect can be obtained even with a small content. However, when P is excessively added, brittleness occurs due to grain boundary segregation, and fine cracks easily occur during molding, greatly deteriorating ductility and impact resistance. In addition, there is also a problem of causing defects on the surface during plating. Therefore, the upper limit of the P content is controlled to 0.015%. On the other hand, if the P content is less than 0.001%, the manufacturing cost is excessively required to meet this, which is not only economically disadvantageous, but also the strength obtained is insufficient, so the lower limit of the P content is controlled to 0.001% or more. Therefore, in the present invention, it is preferable to control the P content to 0.001 to 0.015%. On the other hand, in terms of further improving the above-mentioned effect, 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 to 0.01%

Sulfur (S) is an impurity present in steel, and when the S content exceeds 0.01%, it combines with Mn to form non-metallic inclusions. Accordingly, it is easy to generate fine cracks during cutting and processing of steel, and the elongation flange property and impact resistance are improved. There is a problem that is greatly aggravating. In addition, in order to manufacture the S content to be less than 0.001%, there is a problem in that productivity is reduced due to the long time required during steelmaking operation. Therefore, in the present invention, it is preferable to control the S content to 0.001 to 0.01%. On the other hand, in terms of further improving the above-mentioned effect, 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 to 0.01%

Nitrogen (N) is a representative solid solution strengthening element together with C, and contributes to forming coarse precipitates together with Ti and Al. In general, the solid solution strengthening effect of N is superior to that of carbon, but as the amount of N in steel increases, there is a problem in that toughness decreases significantly. In addition, in order to manufacture the N content to be less than 0.001%, there is a problem of low productivity due to the long time required during steelmaking operation. Therefore, in the present invention, it is preferable to control the N content to 0.001 to 0.01%. On the other hand, in terms of further improving the above-mentioned effect, 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%.

On the other hand, according to one aspect of the present invention, although not particularly limited, optionally, the cold-rolled steel sheet, by weight%, Cr: 1.0% or less (including 0%), Mo: 0.2% or less (including 0%) and B : 0.005% or less (including 0%) may further include one or more selected from among. Hereinafter, the reason for adding the optional additional element and the reason for limiting the content will be described.

Cr: 1.0% or less (including 0%)

Chromium (Cr) is a component added to improve the hardenability of steel and secure high strength, and is an element that plays a very important role in the formation of martensite. It is advantageous. Therefore, the Cr may be selectively added for the above effect. However, when the Cr content exceeds 1.0%, the aforementioned effect is saturated, and there is a problem in that cold rolling property deteriorates due to an excessive increase in hot rolling strength. In addition, since the martensite fraction greatly increases after annealing, there is a problem of reducing the elongation, so the upper limit of the Cr content is controlled to 1.0% or less. On the other hand, in terms of further improving the above-mentioned effect, 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: 0.2% or less (including 0%)

Molybdenum (Mo) is an element that suppresses pearlite formation and increases hardenability. Therefore, in order to secure the above effect, Mo may be selectively added in the present invention. However, when the Mo content exceeds 0.2%, the strength improvement effect is not greatly increased, but the ductility is deteriorated, which may be economically disadvantageous. Therefore, the Mo content is preferably controlled to 0.2% or less. On the other hand, in terms of further improving the above-mentioned effect, 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: 0.005% or less (including 0%)

Boron (B), when present in a solid solution state in steel, has an effect of improving brittleness of steel in a low temperature range by stabilizing grain boundaries, and greatly increases hardenability of steel. Therefore, the B may be selectively added for the above effect. However, if the upper limit of B exceeds 0.005%, recrystallization is delayed during annealing, and oxide is formed on the surface, resulting in poor plating properties. Therefore, it is preferable to control the content of B to 0.005% or less. On the other hand, in terms of further improving the above-mentioned effect, 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 remaining component of the present invention is iron (Fe). However, since unintended impurities may inevitably be mixed due to raw materials or surrounding environmental variables in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary steel manufacturing process, not all of them are specifically mentioned in this specification.

According to one aspect of the present invention, the high-strength cold-rolled steel sheet may have a value defined by the following relational expression 1 of 0.6 or more and less than 0.9, thereby minimizing the material deviation of the cold-rolled steel sheet and suppressing the occurrence of surface defects to achieve the desired result. material can be obtained.

[Relationship 1]

C + (1.3×Si+Mn)/6 + (Cr+1.2×Mo)/5 + 100×B

(In the above relational expression 1, the C, Si, Mn, Cr, Mo and B represent the weight percent average content of each element. At this time, when each element is not added, 0 is substituted.)

In the present invention, the relational expression 1 is an expression representing the hardenability of the steel material according to the composition of the present invention, and the coefficient in front of each element quantitatively represents the scale that the corresponding element contributes to the hardenability. When the hardenability of the steel is high, it is advantageous to secure hard low-temperature transformation phases such as bainite and martensite phases, contributing to strength improvement, and when the hardenability is low, ferrite transformation is promoted, which is disadvantageous in securing strength.

In particular, in order to secure high strength of 780 MPa or more of tensile strength (TS), which is the target in the present invention, the value defined by the relational expression 1 must satisfy 0.6 or more. On the other hand, when the value defined by the relational expression 1 is 0.9 or more, the strength is excessively high, resulting in deterioration of elongation. In addition, when the value defined by the relational expression 1 is 0.9 or more, the phase transformation of ferrite is greatly delayed in the step of cooling the hot-rolled steel sheet to 450 to 700 ° C. at an average cooling rate of 10 to 70 ° C. / s immediately after hot rolling. Accordingly, in the subsequent winding step, too many lower bainite phases and martensite phases having high hardness among the bainite phases in the hot-rolled steel sheet are formed, resulting in severe material deviation depending on the position in the width direction and deterioration of the shape. Therefore, in the present invention, it is preferable to control the value defined by the relational expression 1 to satisfy 0.6 or more and less than 0.9. On the other hand, in terms of maximizing the above-mentioned effect more, the lower limit of the value defined by the relational expression 1 may be 0.62, or the upper limit of the value defined by the relational expression 1 may be 0.84.

On the other hand, according to one aspect of the present invention, the high-strength cold-rolled steel sheet, as a microstructure, includes ferrite: 50% or more, balance: bainite and martensite, in area%. Among the microstructures, if ferrite is less than 50%, there is a problem of poor formability due to insufficient elongation. Also, the remainder may be 50% or less as bainite and martensite. If the sum of the bainite and martensite exceeds 50%, there is a problem in that the strength is too high and the elongation is insufficient.

Alternatively, according to one aspect of the present invention, although not particularly limited, in terms of elongation and formability improvement, the microstructure of the high-strength cold-rolled steel sheet is, in area %, ferrite: 50 to 85% and bainite and martensite Total: May contain 15-50%.

Among the high-strength cold-rolled steel sheets, if the ferrite content exceeds 85%, the target strength may be insufficient, and if the total content of bainite and martensite is less than 15%, the target strength may be insufficient. On the other hand, in terms of further improving the above-mentioned effect, the microstructure of the high-strength cold-rolled steel sheet may include, more preferably, ferrite: 66 to 75% in area %.

In addition, according to one aspect of the present invention, although not particularly limited, the microstructure of the high-strength cold-rolled steel sheet may include bainite: 3-7%, and/or martensite: 19- may contain 31%. In the high-strength cold-rolled steel sheet, if the bainite is less than 3%, a problem may occur that the target strength is not met, and if it exceeds 7%, the strength is high but the elongation is low. Alternatively, in the high-strength cold-rolled steel sheet, if the martensite is less than 19%, a problem may occur that the target strength is not reached, and if it exceeds 31%, the strength is high but the elongation is low.

According to one aspect of the present invention, in the high-strength cold-rolled steel sheet, the average number of surface defects that satisfy at least one condition of a depth of 100 μm or more and a short side length of 1 mm or more is less than 10 / m ² (0 / m ² included). In the present invention, a surface defect means a defect having a groove shape, and specifically, a defect in the form of a depression in the thickness direction, and refers to a defect that can be confirmed when the surface of the steel sheet is observed with the naked eye. In addition, the depth of the surface defect is the 'maximum depth' in the thickness direction of the groove-shaped defect, based on the cross section in the thickness direction of the cold-rolled steel sheet (ie, on a cross-sectional basis, meaning a direction perpendicular to the rolling direction). can mean In addition, the short side length of the surface defect may mean the shortest length passing through the point where the maximum depth is obtained based on the surface of the cold-rolled steel sheet. On the other hand, in order to observe groove-shaped surface defects on the surface of the above-described steel sheet and to confirm the depth and short side length of each surface defect, a photo taken using a high-magnification scanning electron microscope (SEM) is shown in FIG. shown in

The inventors of the present invention have been intensively researched to provide a cold-rolled steel sheet capable of solving the problems of the prior art and minimizing surface defects and material variations while securing desired levels of strength and formability.

As a result, it was found that the above effect can be secured by controlling the average number of surface defects satisfying at least one of the aforementioned depth of 100 µm or more and short side length of 1 mm or more to less than 10/m ² . That is, in the present invention, if the average number of surface defects is 10/m ² or more, surface dents may occur. On the other hand, in terms of further improving the above-described effect, preferably, the average number of the above-described surface defects may be 8/m ² or less.

On the other hand, according to one aspect of the present invention, the present inventors are conducting additional research to provide a cold-rolled steel sheet capable of securing desired levels of strength and formability at the same time without affecting material variation, even if surface defects exist on the surface of the steel sheet. repeated. As a result, even if surface defects exist in the present invention, surface defect characteristics of a level that does not affect material variation or the like were additionally discovered. Specifically, although not particularly limited in the present invention, the maximum depth of the surface defect may satisfy 500 μm or less. In this case, the maximum depth of the surface defect may mean the maximum value of the depth of each surface defect existing on the surface of the steel sheet.

On the other hand, according to one aspect of the present invention, in the width direction of the cold-rolled steel sheet, the difference between the yield strength (YS) of both ends and the central portion may be 100 MPa or less. By satisfying the difference between the yield strength of the both ends and the central portion of 100 MPa or less, it is possible to provide a steel sheet in which material variation in the width direction is reduced, and the material is uniform in the width direction. At this time, the 'both ends' refers to a 30% section (total: corresponding to 60%) from both ends based on the total width (referred to as 100%) in the width direction of the cold-rolled steel sheet, and the 'central part' refers to the cold-rolled steel sheet Based on the entire width in the width direction of the steel sheet, it may mean the remaining 40% section excluding the both ends.

Meanwhile, according to one aspect of the present invention, the cold-rolled steel sheet may have a tensile strength (TS) of 780 MPa or more, preferably 780 MPa or more and less than 1180 MPa, and more preferably 800 MPa or more and 1100 MPa or less. If the tensile strength of the cold-rolled steel sheet is less than 780 MPa, there may be a problem of not satisfying the target strength required for the applied part, and if it exceeds 1100 MPa, cracks may occur during molding of the part or the impact resistance of the part may be significantly reduced. .

In addition, according to one aspect of the present invention, the yield strength (YS) of the cold-rolled steel sheet may be 380 MPa or more, more preferably 390 MPa or more and 650 MPa or less. If the yield strength of the cold-rolled steel sheet is less than 380 MPa, a problem of deteriorating collision resistance of the part may occur, and if it exceeds 650 MPa, a problem of deterioration in formability may occur.

In addition, according to one aspect of the present invention, the cold-rolled steel sheet has a product of tensile strength and elongation of 12,000 MPa% or more (more preferably, 12,000 MPa% or more and 16,500 MPa% or less, most preferably 12,000 MPa% or more and 16,200 MPa%). below). By satisfying the above-described physical properties, it is possible to secure effects suitable for structural member parts such as pillars that require a stable strength-elongation balance and shock absorption among parts constituting an automobile body.

Although not particularly limited, optionally, the cold-rolled steel sheet may further include a plating layer formed on a surface thereof. At this time, the plating layer may be formed by a plating process to be described later. In addition, since the composition of the plating layer can be applied differently depending on the purpose, it is not particularly limited in the present specification, and a zinc-based plating layer and the like can be cited as an example.

Hereinafter, a method for manufacturing a high-strength cold-rolled steel sheet according to an aspect of the present invention will be described in detail. However, the manufacturing method of the cold-rolled steel sheet according to the present invention does not necessarily mean that it must be manufactured by the following manufacturing method.

Steel slab reheating stage

A steel slab meeting the above composition is reheated to 1100-1350°C. The composition of the steel slab is the same as that of the above-mentioned cold-rolled steel sheet, and at this time, the description of the cold-rolled steel sheet described above is equally applied to the reason for adding each component and the reason for limiting the content of each component in the steel slab. On the other hand, if the reheating temperature of the steel slab is less than 1100 ° C., alloy elements segregated in the center of the slab remain, and the starting temperature of hot rolling is too low, causing a problem that the rolling load becomes severe. On the other hand, when the reheating temperature of the steel slab exceeds 1350° C., a problem in that strength is lowered due to coarsening of austenite crystal grains occurs. Therefore, in the present invention, it is preferable to control the reheating temperature of the steel slab to 1100 to 1350 ° C.

hot rolling step

The reheated steel slab is hot rolled at 850 to 1150 ° C. When the temperature of the hot rolling exceeds 1150° C., the temperature of the hot-rolled steel sheet increases, resulting in a coarse grain size and deterioration in surface quality of the hot-rolled steel sheet. If the temperature of the hot rolling is less than 850 ° C., due to the development of elongated grains due to excessive recrystallization delay, the load during rolling increases and the temperature at both ends decreases significantly, resulting in an uneven microstructure during cooling, resulting in increased material deviation. and formability deteriorates.

After hot rolling, cooling step

The hot-rolled steel sheet is cooled to 450 to 700°C at an average cooling rate of 10 to 70°C/s (more preferably, 20 to 50°C/s). When the cooling temperature of the hot-rolled steel sheet is less than 450° C., a problem of deterioration in material variation occurs, and when it exceeds 700° C., material variation occurs, as well as internal oxidation of the hot-rolled steel sheet, resulting in surface defects. In addition, if the average cooling rate is less than 10 °C / s, the crystal grains of the base structure become coarse and the microstructure becomes non-uniform. In addition, when the average cooling rate exceeds 70 °C / s, there is a problem that the load increases during cold rolling because bainite phase and martensite phase are easily formed.

winding step

It is wound at 450 ~ 700 ℃ of the cooled steel sheet. When the coiling temperature is cooled to less than 450° C. and coiled, the bainite phase and the martensite phase of the steel are unnecessarily formed, resulting in non-uniform shape and greatly increasing the rolling load during cold rolling. When coiled at a temperature exceeding 700° C., ferrite crystal grains become large and coarse pearlite phases are easily formed, resulting in a non-uniform microstructure during annealing, resulting in poor formability of the steel. In addition, hot-rolled oxides increase and are adsorbed to the rolls during annealing, resulting in accumulation of oxides on the rolls, and friction between the steel sheet and the rolls during sheet passing, causing surface defects such as dent defects on the surface of the steel sheet. do. In addition, when hot-rolled oxides remain on the steel sheet, plating quality and coating adhesion are deteriorated during plating of the steel sheet.

Usually, after winding, cooling proceeds rapidly at both ends in the width direction of the coiled steel sheet (coil) due to exposure to the surrounding atmosphere, and cooling proceeds slowly at the central portion in the width direction. Due to this, a cooling deviation occurs in the width direction of the steel sheet from the winding step, and as a result, a difference occurs in the microstructure of each position of the coiled steel sheet, resulting in material variation with respect to the hot-rolled steel sheet. In the process of performing cold rolling, the hot-rolled steel sheet with such large material variation not only intensifies material variation, but also groove-shaped surface defects that were not observed with the naked eye in the hot-rolled steel sheet become more severe after cold rolling, resulting in surface defects. A big problem arises. That is, hot-rolled steel sheet with large material variation not only deteriorates in shape during cold rolling, but also causes material variation by position in the width direction in the final annealed material. As a result, a manufacturing method of differently controlling the temperature of both ends and the central part in the winding step has been sought.

Specifically, in the present invention, as a method for reducing repainting variation in the width direction of the steel sheet and suppressing surface defects, the surface temperature (Te) of both ends in the width direction based on the entire width of the steel sheet during the winding is controlled to satisfy 601 to 700 ° C, and the surface temperature (Tc) of the central portion to meet 450 to 600 ° C. At this time, the 'width direction of the steel sheet' means a direction perpendicular to the transport direction of the steel sheet based on the surface of the steel sheet. In addition, the above description is equally applied to the both ends and the central portion.

At this time, if the Te is less than 601 ° C, there is a problem that material deviation due to overcooling of both ends is intensified, and if the Te exceeds 700 ° C, there is a problem that material deviation and surface defects are intensified due to deterioration of the central part. In addition, if the Tc is less than 450 ° C, the temperature difference between the central part and both ends becomes severe, resulting in a problem of material deviation, and if the Tc exceeds 600 ° C, the temperature of the central part is too high, resulting in material deviation and surface defects. there is

As such, in the above-described winding step, in order to control the surface temperature of both ends and the center of the steel sheet differently in the width direction of the steel sheet, various methods can be applied, and thus are not particularly limited. For example, in order to control the temperatures of both ends and the center of the steel sheet differently during the winding, in the cooling step before winding, the cooling water injected at both ends can be blocked before reaching the steel sheet, or the amount of cooling water injected can be controlled differently , or both methods may be combined. As an example, according to one aspect of the present invention, in the step of cooling before winding, based on the entire width of the steel sheet, the amount of cooling water injected onto both ends in the width direction is higher than the injection amount of the cooling water injected into the central portion excluding the both ends. The amount of cooling water can be controlled to be larger.

In addition, according to one aspect of the present invention, although it is not particularly limited, in terms of further improving the effect of reducing material deviation and suppressing surface defects, the winding step is the surface temperature of the both ends and the surface temperature of the central part The difference (Te-Tc) can be 150 ° C or less. At this time, when the value of Te-Tc exceeds 150 ° C., a problem of deterioration of material variation in the width direction may occur. However, the smaller the temperature deviation calculated from the Te-Tc is, the better it is, so the lower limit may not be separately limited, and may be preferably 0°C. Meanwhile, more preferably, the lower limit of the Te-Tc value may be 50°C, and the upper limit of the Te-Tc value may be 90°C.

Maintenance step in the heat insulation cover

After the above-described winding step, it may optionally be moved into a heat-retaining cover and maintained at a temperature of 400 to 500° C. for 6 hours or more. After the winding step, by maintaining in the heat-retaining cover for a long time, when the steel sheet is maintained at a temperature in the range of 601 to 700 ° C and 450 to 600 ° C, respectively, both ends and the center in the width direction of the steel plate for a long time, both ends and the center of the coil lengthwise. A large amount of uniformly formed bainite structure is excellent in shape quality, and it is possible to manufacture a cold-rolled steel sheet having a uniform thickness with a small rolling load during cold rolling.

In the holding step in the heat-retaining cover, the surface temperature of the steel sheet can be adjusted to 400 to 500°C. At this time, in the holding step in the heat-retaining cover, if the surface temperature of the steel sheet is less than 400 ° C, the above-mentioned effect cannot be secured, and if it exceeds 500 ° C, coarse carbides are formed locally and hot-rolled oxides increase, thereby increasing the formability and surface of the steel. Quality may deteriorate.

In addition, if the holding time in the heat insulating cover is less than 6 hours, 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.

Additionally, in terms of further improving the above-mentioned effect, the rolled steel sheet can be stored in the heat-retaining cover within 90 minutes immediately after winding, and if the time before being accommodated in the heat-retaining cover exceeds 90 minutes, due to excessive air cooling The range of 450 to 600 ° C. may not be satisfied due to supercooling occurring in the central portion in the width direction. Alternatively, air cooling or water cooling may be additionally performed to room temperature after the step of maintaining the heat insulating cover.

cold rolling step

The coiled steel sheet is subjected to cold rolling at a cold rolling reduction of 40 to 70%. If the cold reduction ratio is less than 40%, it is difficult to secure the target thickness and it is difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, cracks at the edge of the steel sheet are likely to occur and the cold rolling load is There is a problem to come. Therefore, in the present invention, it is preferable to limit the cold rolling reduction to 40 to 70%.

annealing step

The cold-rolled steel sheet is continuously annealed at 740 to 900 ° C. If the annealing temperature is less than 740 ° C, non-recrystallization may occur, resulting in insufficient strength and elongation, and if the annealing temperature exceeds 900 ° C, surface oxide may occur. On the other hand, in terms of further improving the above-mentioned effect, more preferably the annealing temperature may be 750 ~ 850 ℃.

In addition, although not particularly limited, according to one aspect of the present invention, after the continuous annealing step, optionally primary cooling to 650 ~ 700 ℃ at a cooling rate of 1 ~ 10 ℃ / sec; After the primary cooling step, secondary cooling at a cooling rate of 11 to 20 °C/sec from Ms-100 °C to Ms+100 °C; may be further included. In addition, after the secondary cooling step, an overaging step may be further included while selectively maintaining a constant temperature. the primary cooling step; Strength and elongation can be further improved by satisfying the conditions of the secondary cooling step and overaging step. At this time, the Ms means the starting temperature at which martensite is generated when the steel sheet is cooled after annealing, and from the following relational expression 2 can be saved

[Relationship 2]

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

(In the relational expression 2, the C, Mn, Cr, Ni, and Mo represent the weight percent average content of each element. In addition, when each element is not added, 0 is substituted.)

Further, according to one aspect of the present invention, optionally, a step of plating (preferably, hot-dip galvanizing) the cold-rolled steel sheet may be further included, and a coated steel sheet may be obtained by performing the plating.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for explaining the present invention through examples, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A steel slab satisfying the composition of Table 1 below was reheated at 1200 ° C, hot rolled at 900 ° C, cooled to 450 to 700 ° C at a cooling rate of 20 to 50 ° C / s, and then wound. At this time, at the time of winding, the surface temperature (Te) of the steel sheet at both ends of the 30% section from both ends and the surface temperature (Tc) of the remaining 40% of the central portion of the steel sheet are as follows, based on the total width of the steel sheet in the width direction In order to satisfy the hot rolling conditions described in Table 2, the amount of cooling water injected into the central portion excluding the both ends was controlled to be greater than the amount of cooling water injected onto both end portions in the width direction of the steel sheet. In addition, the rolled hot-rolled steel sheet was moved into the heat-retaining cover and controlled to satisfy the average temperature and holding time before and after charging into the cover as conditions for the heat-retaining cover described in Table 2 below. Subsequently, the hot-rolled steel sheet is cold-rolled at a cold rolling reduction of 50%, continuously annealed at 800°C, followed by primary cooling at an average cooling rate of 8°C/s to 670°C, and then average cooling rate up to Ms+100°C. A cold-rolled steel sheet was obtained by secondary cooling at 12°C/s.

For each cold-rolled steel sheet obtained in this way, the average number of surface defects per unit area (pcs/m ² ) observed on the microstructure, mechanical properties, and surface of Inventive Examples and Comparative Examples was measured and shown in Tables 3 to 5 below. At this time, YS, TS, and El mean 0.2% off-set yield strength, tensile strength, and breaking elongation, respectively, and the results of the test by taking samples from the center and both ends of a JIS No. 5 standard test piece in a direction perpendicular to the rolling direction it is shown In addition, the above-described microstructure was obtained by using a scanning electron microscope (FE-SEM) and measuring the area% with a photograph observed at a magnification of 3,000 to 5,000. In addition, the average number of surface defects is obtained by visually observing the surface of the manufactured steel sheet and measuring the average number of surface defects that satisfy at least one condition of a depth of 100 μm or more and a short side length of 1 mm or more. In particular, the maximum depth of the surface defect was measured in the same manner as described herein. In addition, with respect to the specimens taken from the center and both ends of the cold-rolled steel sheet in the width direction, the yield strength was measured in the same manner as described above, and the material deviation in the width direction was measured and shown in Tables 4 and 5 below. was

division Composition [wt%] (balance Fe and impurities) relational expression 1 C Si Mn Cr Mo B Al P S N Invention Steel 1 0.1 0.4 2.5 0.1 0 0 0.03 0.010 0.002 0.004 0.62 Invention Steel 2 0.06 0.9 2.4 0.1 0.02 0.0007 0.025 0.008 0.003 0.005 0.75 Invention Steel 3 0.12 1.2 2.6 0 0.1 0 0.035 0.009 0.004 0.002 0.84 Comparative Lecture 1 0.18 2.1 2.2 0 0 0 0.04 0.009 0.005 0.004 1.00 comparative steel 2 0.09 0.9 1.7 0 0 0 0.1 0.0011 0.007 0.003 0.57

division winding conditions Insulating cover conditions Te
[℃] Tc
[℃] Te-Tc
[℃] temperature
[℃] hour
[hr] invention steel 1 Invention example 1 650 580 70 480 8 Invention example 2 680 590 90 450 7 Comparative Example 1 720 650 70 490 7 Comparative Example 2 580 400 180 420 8 Comparative Example 3 610 490 120 Unapplied Comparative Example 4 650 590 60 570 10 invention steel 2 Inventive example 3 640 570 75 490 8 Inventive example 4 680 590 90 450 7 Comparative Example 5 720 650 70 490 7 Comparative Example 6 580 400 180 420 8 Comparative Example 7 610 490 120 Unapplied Comparative Example 8 650 590 60 570 10 invention steel 3 Inventive Example 5 640 570 75 490 8 Inventive Example 6 680 590 90 450 7 Comparative Example 9 720 650 70 490 7 Comparative Example 10 580 400 180 420 8 Comparative Example 11 610 490 120 Unapplied Comparative Example 12 650 590 60 570 10 comparative steel 1 Comparative Example 13 650 580 70 480 8 Comparative Example 14 680 590 90 450 7 comparative steel 2 Comparative Example 15 650 580 70 480 8 Comparative Example 16 680 590 90 450 7

division microstructure [area%] ferrite bainite martensite Invention Steel 1 Invention Example 1 75 5 20 Invention example 2 74 7 19 Comparative Example 1 73 6 21 Comparative Example 2 77 4 19 Comparative Example 3 79 2 19 Comparative Example 4 77 6 17 Invention Steel 2 Inventive example 3 75 6 19 Inventive example 4 74 7 19 Comparative Example 5 73 6 21 Comparative Example 6 77 4 19 Comparative Example 7 79 2 19 Comparative Example 8 77 6 17 Invention Steel 3 Inventive Example 5 66 3 31 Inventive example 6 66 4 30 Comparative Example 9 66 5 29 Comparative Example 10 64 3 33 Comparative Example 11 67 6 27 Comparative Example 12 64 6 30 Comparative Lecture 1 Comparative Example 13 59 5 36 Comparative Example 14 56 7 37 comparative steel 2 Comparative Example 15 85 2 13 Comparative Example 16 83 One 16

division width center surface defects YS
[MPa] TS
[MPa] El
[%] TS*El
[MPa%] average count
[pcs/m ² ] Invention Steel 1 Invention example 1 440 812 19 15428 One Invention example 2 465 807 20 16140 One Comparative Example 1 423 819 19 15561 15 Comparative Example 2 415 829 19 15751 3 Comparative Example 3 390 831 20 16620 2 Comparative Example 4 397 817 21 17157 18 Invention Steel 2 Inventive example 3 412 825 19 15675 0 Inventive example 4 399 844 19 16036 0 Comparative Example 5 445 835 20 16700 18 Comparative Example 6 427 799 21 16779 One Comparative Example 7 399 824 21 17304 2 Comparative Example 8 408 809 21 16989 22 Invention Steel 3 Inventive Example 5 605 1028 15 15420 One Inventive Example 6 635 1054 15 15810 One Comparative Example 9 645 1038 16 16608 14 Comparative Example 10 607 1019 16 16304 One Comparative Example 11 616 1038 14 14532 One Comparative Example 12 635 1041 15 15615 22 Comparative Lecture 1 Comparative Example 13 689 1097 17 18649 16 Comparative Example 14 678 1087 18 19566 22 comparative steel 2 Comparative Example 15 345 658 26 17108 One Comparative Example 16 332 628 27 16956 One

division Material of Both Ends YS deviation in the width direction
[MPa] YS [MPa] Invention Steel 1 Invention example 1 480 40 Invention example 2 475 10 Comparative Example 1 441 18 Comparative Example 2 523 108 Comparative Example 3 502 112 Comparative Example 4 412 15 Invention Steel 2 Inventive example 3 479 67 Inventive example 4 465 66 Comparative Example 5 448 3 Comparative Example 6 533 106 Comparative Example 7 517 118 Comparative Example 8 432 24 Invention Steel 3 Inventive Example 5 623 18 Inventive example 6 669 34 Comparative Example 9 650 5 Comparative Example 10 726 119 Comparative Example 11 729 113 Comparative Example 12 669 34 Comparative Lecture 1 Comparative Example 13 725 36 Comparative Example 14 700 22 comparative steel 2 Comparative Example 15 355 10 Comparative Example 16 339 7

As can be seen from the experimental results of Tables 1 to 5, in the case of Inventive Examples 1 to 6 satisfying the composition and manufacturing conditions of the present invention, it is possible to secure a tensile strength (TS) of 780 MPa or more, while suppressing material deviation and surface defects. A cold-rolled steel sheet could be obtained. At this time, it was confirmed that the maximum depth of surface defects measured in the cold-rolled steel sheets obtained from Examples 1 to 6 of the present invention satisfies 500 μm or less.

On the other hand, in the case of Comparative Examples 1 to 16 that do not satisfy at least one of the composition and manufacturing conditions of the present invention, material variation is poor, surface defects occur, and / or it is difficult to secure desired physical properties in the present invention.

In particular, Comparative Steel 1 did not satisfy Relational Expression 1 because the amount of Si added exceeded 2.0%. Therefore, in the case of Comparative Examples 13 and 14 using Comparative Steel 1, even if the material deviation is good by satisfying the manufacturing conditions presented in the present invention, a dent problem occurs due to the accumulation of Si oxide in the annealing furnace, resulting in product surface defects. There was a problem that the average number exceeded the target value.

In addition, Comparative Steel 2 did not satisfy Relational Equation 1 because the amount of added alloy was small. Therefore, in the case of Comparative Examples 15 and 16 using Comparative Steel 2, even though surface defects and material variation were good by satisfying the manufacturing conditions presented in the present invention, the tensile strength was less than 780 MPa and did not satisfy the target material.

In addition, in the case of Comparative Examples 1, 5, and 9, the temperatures of both ends and the center in the width direction are higher than the temperature suggested in the present invention, and in the case of Comparative Examples 4, 8, and 12, the heat insulating cover An example in which the temperature exceeds the standard temperature is shown. Accordingly, in the Comparative Examples, hot-rolled oxide was excessively generated, and a large number of surface defects occurred in the final steel sheet due to the oxide.

In addition, in the case of Comparative Examples 2, 6, and 10, the temperature of both ends and the center in the width direction is lower than the temperature suggested in the present invention, and the difference (Te-Tc) between the surface temperature of the both ends and the surface temperature of the center is 150 Examples exceeding ° C are shown, and in the case of Comparative Examples 3, 7, and 11, examples in which a heat insulating cover is not applied are shown. Accordingly, in the comparative examples, it was possible to secure the target material of the annealed steel sheet, and the average number of surface defects was good, but there was a problem in that the deviation of the yield strength of the annealed steel sheet in the width direction exceeded the target value of 100 MPa. .

Claims (13)

In weight percent, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.001% 0.01%, the balance including Fe and other unavoidable impurities,
The value defined by the following relational expression 1 satisfies 0.6 or more and less than 0.9,
As a microstructure, by area%, ferrite: 50% or more, balance: including bainite and martensite,
The average number of surface defects satisfying at least one condition of a depth of 100 μm or more and a short side length of 1 mm or more is less than 10 / m ² , high-strength cold-rolled steel sheet.
[Relationship 1]
C + (1.3×Si+Mn)/6 + (Cr+1.2×Mo)/5 + 100×B
(In the relational expression 1, the C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. In addition, when each element is not added, 0 is substituted.)
The method of claim 1,
The microstructure, in area%, ferrite: 50 to 85% and the sum of bainite and martensite: 15 to 50%, high-strength cold-rolled steel sheet.
The method of claim 1,
The microstructure, in area%, ferrite: 66 to 75%, including, high-strength cold-rolled steel sheet.
The method of claim 3,
The microstructure, in area%, bainite: 3 to 7%, high-strength cold-rolled steel sheet.
The method of claim 3,
The microstructure, in area %, martensite: 19 to 31%, including, high-strength cold-rolled steel sheet.
The method of claim 1,
In weight percent, Cr: 1.0% or less (including 0%), Mo: 0.2% or less (including 0%), and B: 0.005% or less (including 0%), further comprising at least one member selected from the group consisting of, high-strength cold-rolled steel sheet.
The method of claim 1,
A high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and a yield strength of 380 MPa or more.
The method of claim 1,
A high-strength cold-rolled steel sheet in which the product of tensile strength and elongation is 12,000 MPa% or more.
The method of claim 1,
In the width direction of the cold-rolled steel sheet, the difference between the yield strength of both ends and the central portion is 100 MPa or less, high-strength cold-rolled steel sheet.
In weight percent, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.001% Reheating a steel slab containing 0.01%, the balance Fe and other unavoidable impurities, and satisfying a value defined by the following relational expression 1 of 0.6 or more and less than 0.9 to 1100 to 1350 ° C;
hot-rolling the reheated steel slab at 850 to 1150° C.;
cooling the hot-rolled steel sheet to 450-700°C at an average cooling rate of 10-70°C/s;
Winding at 450 ~ 700 ℃ of the cooled steel sheet;
Cold rolling the rolled steel sheet at a reduction ratio of 40 to 70%; and
continuously annealing the cold-rolled steel sheet at 740 to 900° C.;
including,
The winding step is controlled so that the surface temperature (Te) of both ends in the width direction meets 601 to 700 ° C and the surface temperature (Tc) of the central portion meets 450 to 600 ° C, based on the entire width of the steel sheet. Manufacturing method of high-strength cold-rolled steel sheet.
[Relationship 1]
C + (1.3×Si+Mn)/6 + (Cr+1.2×Mo)/5 + 100×B
(In the relational expression 1, the C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. In addition, when each element is not added, 0 is substituted.)
The method of claim 10,
After the winding step, the step of moving the rolled steel sheet into a heat-retaining cover and maintaining it in the range of 400 ~ 500 ℃ for 6 hours or more; further comprising a method of manufacturing a high-strength cold-rolled steel sheet.
The method of claim 10,
The winding step is a method of manufacturing a high-strength cold-rolled steel sheet, which is controlled so that the difference (Te-Tc) between the surface temperature of the both ends and the surface temperature of the central portion meets 150 ° C. or less.
The method of claim 10,
In the cooling step, based on the entire width of the steel sheet, high-strength cold-rolled steel is controlled so that the injection amount of cooling water injected onto the central portion excluding the both ends is greater than the injection amount of cooling water injected onto both end portions in the width direction. Manufacturing method of steel plate.

Images