WO2023096453A1 - Ultra-high strength cold-rolled steel sheet having excellent elongation and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an ultra-high strength cold-rolled steel sheet and a manufacturing method thereof and, more specifically, to a cold-rolled steel sheet and a manufacturing method thereof, wherein the cold-rolled steel sheet has a tensile strength of 1.5 GPa and exhibits an excellent elongation rate, and as such, can be suitably used for cold stamping.

Description

Ultra-high strength cold-rolled steel sheet with excellent elongation and its manufacturing method

The present invention relates to an ultra-high-strength cold-rolled steel sheet having excellent elongation and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet having a tensile strength of 1.5 GPa and having an excellent elongation rate, which can be suitably used for cold stamping, and a cold-rolled steel sheet thereof It's about manufacturing methods.

In the domestic and foreign automobile industries, along with the gradual increase in carbon dioxide regulations, the purpose of vehicle body stability and weight reduction for improving fuel efficiency and protecting passengers is emerging. To achieve this purpose, the use and development of ultra-high-strength cold stamping steel sheets having a tensile strength of 1.3 GPa or more compared to high-strength steel used in existing automobile parts are increasing. However, since the cold-rolled sheet material has an inversely proportional relationship in which elongation decreases as strength increases, application of cold stamping materials is generally very limited due to inferior formability.

In order to solve the above problems, in order to secure excellent strength and elongation, dual steel (pre-steel) and transformation induced plasticity steel (Transformation Induced Plasticity Steep; TRIP) are composite structure steels that introduce a low-temperature transformation phase in a soft matrix. Steel), such as advanced high strength steel, is employed as a vehicle body member. Gradually, with the development of the times, demand for cold-rolled steel sheets with relatively excellent elongation and formability is increasing for application to corresponding vehicle members, while requiring strength improvement against thickness reduction in terms of light weight for fuel efficiency improvement. are doing However, in the case of high-strength steel having a strength of 1 GPa or more, there is a disadvantage in that elongation and hole expansion ratio (HER) are inferior due to the occurrence of a local difference in hardness between phases.

In order to overcome such disadvantages, ultra-high strength steel (UHSS) composed of bainite, particularly martensite, which is a low-temperature transformation phase, has been manufactured, and it is mainly formed by roll forming even though its bendability is low. It is used as a possible part.

On the other hand, since the use of high-strength steel parts is gradually increasing, the need for ultra-high strength steel having high ductility and a manufacturing method thereof is increasing in order to satisfy the characteristics of various parts. However, a level of technology and a manufacturing method thereof capable of meeting the high-grade demand having high ductility while having a tensile strength of 1.5 GPa or more have not been developed.

(Patent Document 1) Korean Publication No. 2017-7022118

(Patent Document 2) Japanese Unexamined Publication No. 2016-28760

According to one aspect of the present invention, it is intended to provide an ultra-high-strength cold-rolled steel sheet having excellent elongation and a manufacturing method thereof.

Alternatively, according to one aspect of the present invention, it is intended to provide a cold-rolled steel sheet that can be suitably used for cold stamping and a manufacturing method thereof, having a total elongation of 10% or more while having an ultra-high strength of 1.5 GPa or more.

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%, C: 0.15 to 0.3%, Si: 0.1 to 1.5%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 ~0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the balance including Fe and other unavoidable impurities,

Microstructure, by area%, including retained austenite: 0.5-20% and martensite: 80-99.5%,

Provided is a cold-rolled steel sheet that satisfies the following relational expression 1-1.

[Relationship 1-1]

(In the relational expression 1-1, the

Represents the area fraction of low Mn crystal grains in which the Mn content in the martensite is greater than 2% and less than 5%, and its unit is area%. Also, the above

Represents the area fraction of high Mn crystal grains having a Mn content of 5% or more in the martensite, and the unit is area%.)

Another aspect of the present invention is,

In weight%, C: 0.15 to 0.3%, Si: 0.1 to 1.5%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 reheating to 1100-1300° C. a steel slab having a composition comprising ~0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the balance being Fe and other unavoidable impurities;

Obtaining a hot-rolled steel sheet by hot-rolling the reheated slab at 800 to 1000 ° C;

winding the hot-rolled steel sheet at 400 to 700° C.;

Obtaining a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 20 to 75%;

Primary annealing step of heating the cold-rolled steel sheet in the range of 600 ~ 700 ℃, and maintaining for 2 ~ 24 hours;

Average of 30 ℃ / s or more of the primary annealed cold-rolled steel sheet A first cooling step of cooling at a cooling rate;

A secondary annealing step of heating the primary cooled cold-rolled steel sheet to a temperature higher than the austenite single-phase region at an average temperature increase rate of 30° C./s or higher; and

The secondary annealed cold-rolled steel sheet averaged over 30 ℃ / s It provides a method for manufacturing a cold-rolled steel sheet comprising a; secondary cooling step of cooling at a cooling rate.

According to one aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet having excellent elongation and a manufacturing method thereof.

Alternatively, according to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet that has a total elongation of 10% or more while having an ultra-high strength of 1.5 GPa or more, and can be suitably used for cold stamping and a manufacturing method thereof.

According to one aspect of the present invention, the main structure is the same from the martensite main structure obtained in the annealing region and the Mn content gradient of the retained austenite, rather than controlling the microstructure having a uniform chemical composition by utilizing the C and Mn contents of the entire steel sheet. However, a non-uniform gradient in chemical composition occurs, microstructure yielding with a relatively low Mn content occurs during processing, and local stress and deformation occur within the boundary as hard martensitic transformation progresses, resulting in a microstructure with a low Mn content. The elongation rate can provide an additional strength increase effect, thereby providing a 1.5 GPa class ultra-high strength steel sheet.

In addition, from the Mn distribution, C and Mn in martensite are additionally distributed to the remaining austenite, thereby finally securing more stable austenite than before, minimizing martensitic transformation in the uniform elongation section during plastic deformation, and high strength / high resistance It is possible to provide a method for manufacturing a cold-rolled steel sheet having a compound ratio and excellent workability.

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 is a schematic diagram schematically showing a manufacturing process of a cold-rolled steel sheet according to an aspect of the present invention.

Figure 2 is a photograph of the final microstructure after secondary annealing-secondary cooling for Example 8 of Table 2 measured at high magnification by electron backscattering diffraction (EBSD), Figure 2 (a) is a crystallographic orientation map (Inverse Pole Figure, IPF), Figure 2(b) shows the phase distribution map for inventive steel, Figure 2(c) is a phase distribution map that enlarges a specific area where retained austenite is distributed in the final microstructure indicates

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.

Hereinafter, an ultra-high strength cold-rolled steel sheet having excellent elongation according to an aspect of the present invention will be described in detail.

According to one aspect of the present invention, the cold-rolled steel sheet contains, by weight, C: 0.15-0.3%, Si: 0.1-1.5%, Mn: 2.5-5.0%, P: 0.1% or less (excluding 0%), S: 0.03 % or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), 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.

Carbon (C): 0.15 to 0.3%

Carbon (C) is an essential element for securing the strength and hardenability of martensitic steel, and it is preferable to add 0.15% or more in order to have a tensile strength of 1.5 GPa or more. However, if the C content exceeds 0.3%, the elongation rate decreases due to an excessive increase in strength compared to the target, the potential for causing brittle fracture is high, and the inferiority of spot weldability occurs, so the upper limit is 0.3%. It is preferable to control below. On the other hand, since the degree of generation of carbides increases when martensite is formed as the carbon content increases, more preferably, the upper limit of the C content can be controlled to 0.27%.

Silicon (Si): 0.1 to 1.5%

Silicon (Si) is added as a deoxidizer in the steelmaking process, and is an element that suppresses the formation of carbides together with a solid solution strengthening element. In addition, the addition of Si serves to uniformly disperse the structure during annealing heat treatment, increases the stability of austenite during cooling, and makes it possible to secure retained austenite at room temperature. Therefore, in order to secure the above effect, it is preferable to add 0.15% or more of Si. However, if the Si content exceeds 1.5%, excessive Si-based oxides are generated on the surface of the steel sheet during hot rolling, which may cause surface defects during cold rolling. In addition, the Si content is controlled to 1.5% or less because the resistivity of the final cold-rolled steel sheet increases and spot weldability deteriorates. On the other hand, more preferably in order to improve the above effect, the upper limit of the Si content may be 1.25%.

Manganese (Mn): 2.5 to 5.0%

Manganese (Mn) is an austenite stable element, which is added to secure martensite hardenability, and is easy to suppress ferrite generation during annealing heat treatment after cold rolling. When the Mn content is less than 2.5%, it is possible to secure martensitic hardenability, but it is difficult to generate a gradient of Mn concentration in the cold-rolled steel sheet, making it difficult to secure a non-uniform microstructure according to the Mn concentration difference sought in the present invention. In addition, if the Mn content exceeds 5.0%, excessive strength and Mn bands may occur in the base iron in the thickness direction from the steelmaking and casting stages, and thus the crash resistance deteriorates, so the upper limit of the Mn content is set to 5.0% or less. It is desirable to control However, more preferably, the lower limit of the Mn content may be controlled to 3.0% and the upper limit of the Mn content may be controlled to 4.12% in order to achieve the object of the present invention.

Phosphorus (P): 0.1% or less (excluding 0%)

Phosphorus (P) is an impurity element in steel, and when it exceeds 0.1%, weldability deteriorates due to P segregation, and the upper limit of the P content is controlled to 0.1% because the potential to cause brittleness of the steel is high. On the other hand, the lower limit of the P content may exclude 0% (ie, greater than 0%) in consideration of the case where it is inevitably included. However, more preferably in order to achieve the object of the present invention, the lower limit of the P content may be controlled to 0.005. Alternatively, the upper limit of the P content may be controlled to 0.03%, most preferably 0.02%.

Sulfur (S): 0.03% or less (excluding 0%)

Sulfur (S), like P, is an impurity element that is inevitably added to steel, and has characteristics that impair ductility and weldability of the final cold-rolled steel sheet. In addition, when the S content exceeds 0.03%, precipitates generated in the hot rolling process due to the precipitation of MnS are not completely decomposed during the annealing heat treatment, deteriorating the ductility of the steel sheet. In addition, since it may cause problems in weldability, the upper limit of the S content is controlled to 0.03% or less. On the other hand, the lower limit of the S content may exclude 0% (ie, greater than 0%) in consideration of the case where it is inevitably included. However, more preferably in order to achieve the object of the present invention, the lower limit of the S content may be controlled to 0.002%, and the upper limit of the S content may be controlled to 0.005%.

Aluminum (Al): 0.01 to 0.1%

Aluminum (Al), like Si, is added to remove oxygen from molten steel during the steelmaking process, and is an element that is advantageous for removing impurity elements from steel. In addition, although it is relatively weaker than Si, it helps to suppress the formation of carbides and contributes to the stabilization of austenite C, so it is possible to manufacture a steel sheet with excellent elongation. Therefore, in order to secure the above effect, the Al content is controlled to 0.01% or more. However, if excessive Al is added, cracks may occur in the slab due to excessive precipitation of AlN, which may cause defects in the process product, so the upper limit of the Al content is limited to 0.1%. Meanwhile, more preferably, the lower limit of the Al content may be 0.02% and the upper limit of the Al content may be 0.06% in order to secure the desired effect of the present invention.

Nitrogen (N): 0.01% or less (excluding 0%)

Nitrogen (N) is an impurity element in steel, and if its content exceeds 0.01%, the risk of cracking during playing due to AlN formation greatly increases, so the upper limit of the N content is limited to 0.01%. On the other hand, the lower limit of the N content may exclude 0% (ie, greater than 0%) in consideration of the case where it is unavoidably included. However, more preferably in order to achieve the object of the present invention, the lower limit of the N content may be controlled to 0.007%, and the upper limit of the N content may be controlled to 0.03%.

Boron (B): 0.005% or less (excluding 0%)

Boron (B) is an element that is advantageous for inhibiting ferrite phase transformation during annealing heat treatment, and can improve the hardenability of martensite through grain boundary reinforcement and solid solution strengthening. However, if the B content exceeds 0.005%, as Fe ₂₃ (B, C) ₆ , a B-based precipitate phase, is formed at the austenite grain boundary, it may cause brittle fracture in the hot rolling state. Therefore, the upper limit of the B content is 0.005%. limited to less than %. On the other hand, the lower limit of the B content may exclude 0% (ie, more than 0%) in consideration of the case where it is inevitably included. However, more preferably in order to achieve the object of the present invention, the lower limit of the B content may be controlled to 0.001%, and the upper limit of the B content may be controlled to 0.003%.

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.

Meanwhile, according to one aspect of the present invention, the cold-rolled steel sheet may optionally further include one or more selected from Cr: 0.1% or less and Mo: 0.1% or less. Hereinafter, the reason for adding each element and the reason for limiting the content will be described.

Chromium (Cr): 0.1% or less (including 0%)

Chromium (Cr), like Mn, is an element that increases the hardenability of martensite and suppresses ferrite transformation, thereby finally enabling martensite having appropriate strength to be produced. Therefore, when the Mn content is designed within a certain range, since the hardenability effect according to the Cr content is lowered, martensitic strength can be secured without adding it, so that a small amount of Cr can be added. On the other hand, if the Cr content exceeds 0.1%, there is a risk of causing cracks due to local deformation and stress generation at the boundary between the carbide and the steel structure during part molding due to the formation of coarse Cr-based carbides, so the upper limit of the Cr content is 0.1%. can be controlled with However, since the case in which Cr is not added may also be included, the lower limit of the Cr content may be 0%. On the other hand, in the case of adding Cr, more preferably to achieve the object of the present invention, the lower limit of the Cr content may be controlled to 0.005%, and the upper limit of the Cr content may be controlled to 0.05%.

Molybdenum (Mo): 0.1% or less (including 0%)

Molybdenum (Mo) is an element effective in increasing martensitic hardenability and suppressing ferrite generation in the cooling section during annealing heat treatment in the same way as Cr. However, if Mo is added excessively, since it is expensive compared to other elements, the problem of increasing the cost of ferroalloy due to the excessive amount of alloy input is caused. In the present invention, the upper limit of the Mo content is limited to 0.1% or less. However, since Mo is expensive and can be included in the scope of the present invention if it is possible to achieve the purpose of the present invention even without adding it, the lower limit of the Mo content may be 0% and the upper limit of the Mo content may be 0.05%. there is.

According to one aspect of the present invention, the microstructure of the cold-rolled steel sheet includes, in area fraction, retained austenite: 0.5 to 20% and martensite: 80 to 99.5%.

Among the microstructures of the cold-rolled steel sheet, if the retained austenite is less than 0.5% or the martensite exceeds 99.5%, the Mn content distribution does not occur properly, and a cold-rolled steel sheet having a martensite matrix is manufactured, resulting in insufficient elongation. can occur On the other hand, in the microstructure of the cold-rolled steel sheet, if the retained austenite is more than 20% or the martensite is less than 80%, carbon stability in the retained austenite is poor, resulting in martensite transformation due to strain-induced transformation during processing, resulting in formability. This can lead to exacerbation of the problem. On the other hand, according to one aspect of the present invention, in terms of further improving the above-mentioned effect, the lower limit of the area fraction of retained austenite may be 1.2%, or the upper limit of the area fraction of retained austenite may be 10%. can On the other hand, according to another aspect of the present invention, in terms of further improving the above-described effect, the lower limit of the area fraction of martensite may be 90%, or the upper limit of the area fraction of martensite may be 98.5%. can

In addition, according to one aspect of the present invention, the cold-rolled steel sheet may satisfy the following relational expression 1-1.

[Relationship 1-1]

(In the relational expression 1-1, the

Represents the area fraction of low Mn crystal grains in which the Mn content in the martensite is greater than 2% and less than 5%, and its unit is area%. Also, the above

Represents the area fraction of high Mn crystal grains having a Mn content of 5% or more in the martensite, and the unit is area%.)

According to the present invention, the

If the value of is less than 5, it is difficult to develop an additional strength effect resulting from a difference in Mn content, and it is difficult to secure retained austenite, which may limit the production of a cold-rolled steel sheet having a total elongation of 10% or more. On the other hand, the above

When the value of is greater than 25, the corresponding C and Mn contents are high, resulting in deterioration of workability due to the formation of Mn bands and carbides in the cold-rolled steel sheet. On the other hand, according to one aspect of the present invention, in terms of further improving the above-mentioned effect,

The lower limit of the value of may be 9, or

The upper limit of the value of may be 19.

remind

and

The measurement method of is not particularly limited. However, as a representative example, after mechanical polishing is performed from the steel sheet surface to the 1/4t position based on the total thickness t of the steel sheet, the EPMA surface analysis is performed quantitatively for the Mn content at an area magnification of 30 μm to 30 μm. Subsequently, it can be measured by analyzing a region having a martensitic structure having a locally high Mn content and a region having a martensitic structure having a locally low Mn content, respectively.

Meanwhile, although not particularly limited, according to one aspect of the present invention, the cold-rolled steel sheet may satisfy the following relational expression 1-2. At this time, by satisfying the following relational expression 1-2, additional strength is realized in the region where the Mn content is relatively low, making it possible to manufacture a 1.5 GPa class cold-rolled steel sheet, thereby reducing the difference in hardness within the structure and providing excellent elongation compared to conventional martensitic steel. it is possible to secure On the other hand, according to one aspect of the present invention, the

The lower limit of the value of may be 829 MPa, or the above

The upper limit of the value of may be 1844 MPa.

[Relationship 1-2]

(In the above relational expression 1-2, the above

Is the average value of tensile strength for high Mn grains having a Mn content of 5% or more in the martensite (

) and the average value of tensile strength for low Mn crystal grains having a Mn content of more than 2% and less than 5% in the martensite (

), and its unit is MPa.)

On the other hand, the above

and

Is defined by the following relational expression 1-3 for each region

can be obtained based on Therefore, the above

can be obtained as the content of each component obtained through EPMA analysis in a region having a martensitic structure with a high Mn content locally. Also, the above

can be obtained by the content of each component obtained through EPMA analysis in a region having a martensitic structure with a low Mn content locally.

[Relationship 1-3]

(In the above relational expression 1-3, [P], [Si], [Cu], [Ni], [Cr], [Mn] and [Mo] represent the weight % content for each element in parentheses in the steel sheet , [Nss] represents the weight percent content of nitrogen in the solid solution of the steel sheet.)

In order to achieve the desired effect of the present invention, by distributing the Mn concentration in martensite by controlling the annealing heat treatment process, a hard/soft phase is obtained according to the Mn concentration, and additional reinforcement is induced in the hard phase from the resulting strain distribution. Should be. By securing austenite stability in the hard Mn phase due to the Mn concentration gradient, retained austenite can be secured at room temperature to provide ultra-high strength steel having excellent elongation.

The cold-rolled steel sheet according to the present invention has a tensile strength of 1500 MPa or more (or 1500 MPa or more and 1700 MPa or less, or 1554 MPa or more and 1660 MPa or less), and a total elongation of 10% or more (or 10% or more and 12% or less, or 10.2% or more and 11.2% or more). below) can be satisfied, and by satisfying this, it can be suitably used for cold stamping as an ultra-high-strength cold-rolled steel sheet having excellent elongation. On the other hand, although not particularly limited, more preferably for achieving the object of the present invention, the yield strength of the cold-rolled steel sheet is 940MPa or more (or, 940MPa or more and 1200MPa or less, 1000MPa or more, or 1000MPa or more and 1200MPa or less) can be Further, according to one aspect of the present invention, the uniform elongation of the cold-rolled steel sheet may be 5.0% or more, or may be 5.0% or more and 7.0% or less.

Hereinafter, a method for manufacturing a 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.

Slab reheating step

A method for manufacturing a cold-rolled steel sheet according to an aspect of the present invention includes reheating a steel slab having the above composition at 1,100 to 1,300 °C. At this time, the composition of the steel slab is the same as the composition of the above-mentioned cold-rolled steel sheet, and the description of the above-described cold-rolled steel sheet can be equally applied to the reason for adding each component and the reason for limiting the content in the slab.

On the other hand, in the present invention, it is preferable to go through a process of reheating and homogenizing the steel slab prior to performing hot rolling, and at this time, it is preferable to perform the temperature at 1,100 ~ 1,300 ℃ during reheating. If the reheating temperature is less than 1100° C., a problem in that the load rapidly increases during subsequent hot rolling may occur. In addition, when the reheating temperature exceeds 1,300° C., the amount of surface scale increases, which may lead to material loss.

hot rolling step

It is preferable to manufacture a hot-rolled steel sheet by hot-rolling the above-described reheated slab at 800 to 1,000 ° C. If the temperature of the hot rolling is less than 800 ° C., it is not preferable because there is a possibility that the rolling load may increase due to the introduction of non-recrystallized ferrite. On the other hand, if the temperature of the hot rolling exceeds 1,000 ° C., it is not preferable because the possibility of increasing surface defects and wear of the rolling roll due to scale increases.

winding step

It is preferable to wind the hot-rolled steel sheet manufactured according to the above-described hot rolling at 400 to 700 ° C. When the coiling temperature exceeds 700° C., an excessive oxide film is formed on the surface of the steel sheet to cause defects, so it is preferable to limit this. On the other hand, if the coiling temperature is lower than 400 ° C, the strength of the hot-rolled steel sheet is excessively high, so that the rolling load in the cold-rolling process is increased, and productivity is deteriorated because there are many control variables during the cold-rolling process to control it. .

cold rolling step

After removing the oxide layer formed on the surface of the hot-rolled steel sheet wound after the hot rolling by an pickling process, cold rolling is performed at a reduction ratio of 20 to 75% to manufacture a cold-rolled steel sheet. If the reduction ratio is less than 20% during the cold rolling, it is difficult to secure the target thickness, and the remaining hot-rolled crystal grains affect the generation of austenite and final physical properties during annealing heat treatment. Therefore, the reduction ratio during cold rolling is preferably performed in the range of 20 to 75%.

The present invention is to manufacture a highly-stretched cold-rolled steel sheet having mechanical properties of 10% or more in elongation while securing a tensile strength of 1.5 GPa. To this end, as shown in FIG. 1, primary annealing - primary cooling - secondary A two-stage annealing process including an annealing-secondary cooling process is performed. It is explained in detail below.

1st annealing step

The cold-rolled steel sheet obtained from the aforementioned cold rolling is heated to a range of 600 to 700° C. (or 620 to 700° C.) and maintained for 2 to 24 hours. In the present invention, in order to reduce the reverse transformation effect due to the average temperature increase rate during the temperature increase in the primary annealing step, the temperature increase rate of 30 ℃ / s or more (more preferably 30 ~ 50 ℃ / s range) is longer than the cementite is decomposed The holding time is controlled to 2 hours or more so that Mn decomposition can occur at an annealing temperature with an inverse temperature (T _{I A} ) range of 600 ° C to 700 ° C.

Although not particularly limited, if the temperature increase rate during the primary annealing is less than 30° C./s, a problem of insignificant C and Mn inhomogeneity may occur during the primary annealing. On the other hand, if the temperature increase rate during the primary annealing exceeds 50° C./s, the accuracy of the primary annealing target temperature may be lowered, resulting in material problems of the final steel type due to temperature deviations in the annealing station.

At the same time as controlling to satisfy the heat treatment conditions in the first annealing step described above, cooling proceeds at an average cooling rate of 30°C/s or more in the first cooling step described later. This is to generate a difference in the Mn concentration of the nitrite and to obtain ferrite with a low Mn concentration and martensite with a high Mn content during cooling. In order to maximize the Mn concentration more preferably in the primary annealing, it is carried out in a holding time range of up to 24 hours.

On the other hand, it is not particularly limited, but preferably in order to further improve the desired effect of the present invention, at this time, the primary annealing step has a maximum temperature in the temperature range of 600 ~ 700 ℃ based on the surface temperature of the steel sheet. It is heated to be, and the time to maintain in the temperature range of 600 ~ 700 ℃ from the time of reaching the maximum temperature may be 2 to 24 hours.

In addition, if the holding time during the primary annealing is less than 2 hours, the distribution effect of C and Mn in the ideal range temperature range is insignificant, so that the C and Mn concentrations in the ferrite martensite may be uniform during cooling. On the other hand, if the holding time during the primary annealing exceeds 24 hours, material deterioration may occur due to coarsening of grains, and a problem in that heterogeneity of C and Mn concentrations in ferrite and austenite may be weakened during annealing may occur.

1st cooling step

Average of 30 ℃ / s or more of the primary annealed cold-rolled steel sheet The primary cooling to cool at the cooling rate is performed. More specifically, the first cooling step may be cooled at an average cooling rate in the range of 30 ~ 50 ℃ / s to 25 ℃ or less.

If the average cooling rate in the primary cooling step is less than 30° C./s, redistribution of C and Mn may occur during cooling after primary annealing, which may cause problems in securing chemical heterogeneity. In addition, when the temperature range in the primary cooling step exceeds 25 ° C, due to the extreme distribution of Mn and C during primary annealing, the formation of martensite and the end point temperature between each grain are different, resulting in untransformed austenite fraction even in the low temperature range. This may cause problems with the introduction of phase 2.

2nd annealing step

Finally, in order to secure an annealed cold-rolled steel sheet having an area fraction of martensite of 80% or more (and retained austenite of 20% or less), the primary cooled cold-rolled steel sheet is heated at 30 ° C / s or more (more preferably 30 to 50%). Secondary annealing is performed by heating to a temperature higher than the austenite single phase region at an average temperature increase rate of ℃/s range).

At this time, in the second annealing step, controlling the average temperature increase rate to 30 ° C / s or more is to maintain the Mn distribution obtained from the first annealing, and the average temperature increase rate in the second annealing step is 30 ° C / s If it is less than s, there may be a problem that the Mn concentration difference becomes uniform due to Mn redistribution.

In addition, in the secondary annealing step, the temperature above the austenite single phase region may be 820 ° C. or higher (more preferably, 850 ° C. or higher and 900 ° C. or lower), and the reason for controlling the temperature range is Mn distributed austenite. This is to produce martensite of 80% or more, which is the final microstructure.

On the other hand, in order to further improve the desired effect of the present invention, preferably, the average temperature increase rate in the primary annealing step and the secondary annealing step may be controlled to satisfy the following relational expression 2.

[Relationship 2]

1.0 ≤ TH1/TH2 ≤ 1.5

(In the relational expression 2-1, TH1 represents the highest temperature of the steel sheet surface in the first annealing step, and TH2 represents the highest temperature of the steel sheet surface in the second annealing step.)

2nd cooling step

Then, the secondary annealed cold-rolled steel sheet is averaged at 30 ° C / s or more (more preferably, in the range of 30 to 50 ° C / s). Secondary cooling to cool at the cooling rate is performed. At this time, if the average cooling rate in the secondary cooling step is less than 30° C./s, problems may arise in securing elongation due to redistribution of C and Mn during cooling.

According to the present invention, in order to secure a cold-rolled steel sheet having mechanical properties of 1.5 grade strength and total elongation of 10% or more, in the two-stage annealing process, the critical heating rate and heat treatment temperature, and the critical cooling rate and cooling in the cooling process Precise control of temperature is required. If it is out of this range, it may be difficult to secure the tensile strength and elongation within the desired range in the present invention.

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)

After vacuum melting the steel having the composition of Table 1 below into an ingot, maintaining it at a temperature of 1200 ° C. for 1 hour, finishing rolling at 900 ° C., charging into a furnace preheated to 600 ° C., maintaining for 1 hour, and then hot rolling by furnace cooling Winding was simulated. After pickling, cold rolling was performed at a reduction rate of 50% to prepare a cold rolled steel sheet.

Under the conditions shown in Table 2 below, the cold-rolled steel sheet thus prepared was heated at an average temperature increase rate of 30° C./s in various ideal temperature ranges, and then primary annealing was performed by varying the holding time. Subsequently, after performing primary cooling to 25 ° C at an average cooling rate of 30 ° C / s, secondary annealing was performed by heating at an average heating rate of 30 ° C / s in the austenite single-phase region of 870 ° C, and 30 ° C Secondary cooling was performed at an average cooling rate of /s to simulate continuous annealing heat treatment.

For each of the specimens subjected to this heat treatment, the fraction of retained austenite in the specimen including minute austenite was measured using a magnetic induction method (Metis), and the other fractions were calculated as martensite. In order to analyze the difference in Mn concentration in the same martensite, EPMA surface analysis was performed at 1,000 times or more and shown in Table 2 below, and the results of measuring mechanical properties are shown in Table 3 below. At this time, in order to measure the mechanical properties, the yield strength, tensile strength, and elongation were measured by processing perpendicular to the rolling direction according to the JIS standard and attaching a tensile tester and an extensometer. also,

and

was measured in the same way as the method and method described herein.

steel grade	Ingredient Composition (% by weight)
	C	Si	Mn	P	S	Al	Cr	Ti	Nb	B	N	Mo
A	0.18	0.1	3.5	0.01	0.002	0.025	0	0.02	0	0.002	40	0	invention steel
B	0.18	0.1	4.0	0.01	0.002	0.025	0.05	0.02	0	0.002	40	0	invention steel
C	0.15	1.4	3.5	0.01	0.002	0.025	0.05	0.02	0	0.002	40	0	invention steel
D	0.24	0.1	2.0	0.01	0.002	0.025	0.1	0.025	0.04	0.002	40	0.05	comparative steel
E	0.26	0.2	1.5	0.01	0.001	0.025	0.2	0.025	0.04	0.002	40	0.1	comparative steel
F	0.15	0.1	1.7	0.01	0.001	0.025	0.3	0.025	0	0.002	40	0.2	comparative steel
G	0.18	0.6	2.0	0.01	0.001	0.025	0.25	0.025	0.04	0.004	40	0.15	comparative steel

note	steel grade	Annealing process	Annealing heat treatment conditions			Microstructure characteristics (area fraction %)
			1st annealing temperature (IA)	maintain hour	Secondary Annealing temperature	AF Mn_s /AF Mn_h	F	M	P	γ
			(℃)	hr	(℃)		(%)	(%)	(%)	(%)
Example 1	A	CAL	500	2	870	19	0	99.7	0	0.3
Example 2	A	CAL	600	12	870	19	0	98	0	2
example 3	A	CAL	660	24	870	9	0	98.5	0	1.5
example 4	A	CAL	720	2	870	One	0	100	0	0
Example 5	B	CAL	660	2	870	9	0	98.8	0	1.2
Example 6	B	CAL	720	24	870	19	0	99.45	0	0.55
yes 7	B	CAL	660	24	870	5.7	0	99.2	0	0.8
yes 8	C	CAL	660	2	870	9	0	97	0	3
yes 9	D	CAL	660	24	870	3	0	100	0	0
Example 10	D	CAL	720	24	870	One	0	100	0	0
Example 11	E	CAL	600	12	870	One	0	100	0	0
Example 12	E	CAL	660	12	870	One	0	100	0	0
Example 13	E	CAL	720	12	870	One	0	100	0	0
Example 14	F	CAL	660	2	870	One	0	100	0	0
yes15	G	CAL	660	2	870	One	0	100	0	0

(In Table 2, F: ferrite, M: martensite, P: pearlite, and γ: retained austenite.)

note	yield strength (MPa)	Tensile Strength (MPa)	Total Elongation (%)	(MPa)	Uniform elongation (%)
Example 1	1095	1643	9.1	1390	5.2
Example 2	943	1554	11.2	1844	6.7
Example 3	1021	1643	10.2	829	5.0
example 4	1020	1561	8.9	277	4.6
Example 5	1200	1660	10.5	922	6.2
Example 6	1050	1580	9.8	720	4.7
yes 7	1158	1645	10.1	645	5.3
yes 8	1011	1632	10.5	829	6.7
yes 9	1020	1520	8.9	450	4.2
Example 10	975	1508	7.8	225	3.8
Example 11	1011	1570	8.3	425	3.5
Example 12	987	1562	7.8	389	3.9
Example 13	950	1523	6	150	2.8
Example 14	980	1565	9.2	438	4.6
yes15	1080	1589	8.9	573	3.7

In the case of examples 2, 3, 5, 7, and 8 that satisfy the alloy composition and manufacturing conditions of the present invention, the relational expression 1-1 was satisfied, whereby the tensile strength was 1500 MPa or more, the yield strength was 1000 MPa or more, the total elongation was 10% or more, and In addition to satisfying the uniform elongation of 5.0% or more, uniformity was also secured by satisfying the relational expression 1-2.

In particular, a photograph of the final microstructure after secondary annealing and secondary cooling for Example 8 measured at high magnification by electron backscattering diffraction (EBSD) is shown in FIG. 2 . Figure 2 (a) shows the crystallographic orientation map (Inverse Pole Figure, IPF), Figure 2 (b) shows the phase distribution map for the inventive steel, Figure 2 (c) shows the retained austenite in the final microstructure It shows a phase distribution map in which a specific area of distribution is enlarged.

On the other hand, it was confirmed that Examples 9 to 15, which do not satisfy the alloy composition of the present invention, do not satisfy one or more of tensile strength of 1500 MPa or more, yield strength of 1000 MPa or more, total elongation of 10% or more, and uniform elongation of 5.0% or more.

In addition, in the case of Example 1, which meets the alloy composition of the present invention but does not meet the manufacturing conditions because the primary annealing temperature is too low, the cementite generated in the ferrite phase is not completely dissolved, and there is a problem remaining after secondary annealing. , which resulted in a total elongation of less than 10%.

In addition, in the case of Examples 4 and 6, which satisfy the alloy composition of the present invention but do not meet the manufacturing conditions because the primary annealing temperature is too high, the primary annealing temperature is 720 ° C. , the difference in Mn concentration was small, and due to this, it was difficult to secure the retained austenite fraction in the final microstructure, and the elongation was less than 10%.

Claims (8)

1. In weight%, C: 0.15 to 0.3%, Si: 0.1 to 1.5%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 ~0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the balance including Fe and other unavoidable impurities,

   Microstructure, by area%, including retained austenite: 0.5-20% and martensite: 80-99.5%,

   A cold-rolled steel sheet that satisfies the following relational expression 1-1.

   [Relationship 1-1]

   

   (In the relational expression 1-1, the

   

   Represents the area fraction of low Mn crystal grains in which the Mn content in the martensite is greater than 2% and less than 5%, and its unit is area%. Also, the above

   

   Represents the area fraction of high Mn crystal grains having a Mn content of 5% or more in the martensite, and the unit is area%.)
2. The method of claim 1,

   A cold-rolled steel sheet that satisfies the following relational expression 1-2.

   [Relationship 1-2]

   

   (In the above relational expression 1-2, the above

   

   Is the average value of tensile strength for high Mn grains having a Mn content of 5% or more in the martensite (

   

   ) and the average value of tensile strength for low Mn crystal grains having a Mn content of more than 2% and less than 5% in the martensite (

   

   ), and its unit is MPa.)
3. The method of claim 1,

   Cr: 0.1% or less (including 0%) and Mo: 0.1% or less (including 0%), further comprising at least one selected from among, cold-rolled steel sheet.
4. The method of claim 1,

   A cold-rolled steel sheet having a tensile strength of 1500 MPa or more and a total elongation of 10% or more.
5. The method of claim 1,

   Cold-rolled steel sheet with a yield strength of 1000 MPa or more.
6. In weight%, C: 0.15 to 0.3%, Si: 0.1 to 1.5%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 reheating to 1100-1300° C. a steel slab having a composition comprising ~0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), the balance being Fe and other unavoidable impurities;

   Obtaining a hot-rolled steel sheet by hot-rolling the reheated slab at 800 to 1000 ° C;

   winding the hot-rolled steel sheet at 400 to 700° C.;

   Obtaining a cold-rolled steel sheet by cold-rolling the rolled hot-rolled steel sheet at a reduction ratio of 20 to 75%;

   Primary annealing step of heating the cold-rolled steel sheet in the range of 600 ~ 700 ℃, and maintaining for 2 ~ 24 hours;

   Average of 30 ℃ / s or more of the primary annealed cold-rolled steel sheet A first cooling step of cooling at a cooling rate;

   A secondary annealing step of heating the primary cooled cold-rolled steel sheet to a temperature higher than the austenite single-phase region at an average temperature increase rate of 30° C./s or higher; and

   The secondary annealed cold-rolled steel sheet averaged over 30 ℃ / s A method of manufacturing a cold-rolled steel sheet comprising a; secondary cooling step of cooling at a cooling rate.
7. The method of claim 6,

   The primary annealing step has an average temperature increase rate of 30 ° C. / s or more, a method for producing a cold-rolled steel sheet.
8. The method of claim 6,

   A method for manufacturing a cold-rolled steel sheet that satisfies the following relational expression 2.

   [Relationship 2]

   1.0 ≤ TH1/TH2 ≤ 1.5

   (In the relational expression 2-1, TH1 represents the highest temperature of the steel sheet surface in the first annealing step, and TH2 represents the highest temperature of the steel sheet surface in the second annealing step.)

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