KR20220028375A - High strength Cold-rolled steel sheet having excellent hole expansion rate and bending properties and method for manufacturing the same - Google Patents

Abstract

The present invention provides a high-strength cold-rolled steel material having an excellent hole expansion rate and bending property. According to an embodiment of the present invention, the high-strength cold-rolled steel material comprises: 0.04 to 0.10 wt% of carbon (C); 0.3 to 0.7 wt% of silicon (Si); 1.5 to 3.5 wt% of manganese (Mn); above 0 to 0.02 wt% of phosphorus (P); above 0 to 0.01 wt% of sulfur (S); 0.1 to 0.5 wt% of aluminum (Al); 0.2 to 0.5 wt% of chrome (Cr); 0.01 to less than 0.05 wt% of titanium (Ti); and a remainder of iron (Fe) and inevitable impurities. In addition, the high-strength cold-rolled steel material satisfies 350 MPa or more of a yield strength (YS), 780 MPa or more of a tensile strength (TS), 20% or more of an elongation percentage (EL), 1.0 or less of a bending property (R/t), and 26% or more of a hole expansion rate (HER).

Description

High strength cold-rolled steel sheet having excellent hole expansion rate and bending properties and method for manufacturing the same

The technical idea of the present invention relates to a steel material, and more particularly, to a high-strength cold-rolled steel material having excellent hole expandability and bendability, and a method for manufacturing the same.

In recent years, improvement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. For this reason, the movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself is increasing. In addition, even if the vehicle body is lightened in this way, it is essential to secure passenger stability in a collision.

High-strength cold-rolled steel has an ideal composite structure and exhibits high strength and high elongation characteristics. The use of high-strength cold-rolled steel sheets of 780 MPa or higher for automobiles is increasing as the demand for improvement of crash performance and weight reduction of car bodies increases due to the recent strengthening of collision laws and environmental regulations. However, in order to apply the high-strength cold-rolled steel to a vehicle having a complex shape, it is important to improve the formability as well as to improve the strength of the steel. In order to improve the strength of steel, a method of increasing the content of manganese (Mn) is mainly used, which facilitates the formation of manganese bands and manganese precipitates, MnS, which are known to degrade the formability of steel. Therefore, in order to increase the formability of steel, it is important to refine the grains and reduce the difference in hardness between phases, as well as suppress the formation of manganese bands and MnS.

Korean Patent Application No. 10-2018-0166158

The technical problem to be achieved by the technical idea of the present invention is to provide a high-strength cold-rolled steel material having excellent hole expandability and bendability and a method for manufacturing the same.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, a high-strength cold-rolled steel material having excellent hole expandability and bendability and a manufacturing method thereof are provided.

According to an embodiment of the present invention, the cold-rolled steel material is, by weight, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn): 1.5% to 3.5% , Phosphorus (P): >0% to 0.02%, Sulfur (S): >0% to 0.01%, Aluminum (Al): 0.1% to 0.5%, Chromium (Cr): 0.2% to 0.5%, Titanium (Ti) ): 0.01% to less than 0.05%, and the balance contains iron (Fe) and unavoidable impurities, yield strength (YS): 350 MPa or more, tensile strength (TS): 780 MPa or more, elongation (EL): 20% Above, bendability (R/t): 1.0 or less, and hole expandability (HER): 26% or more may be satisfied.

According to an embodiment of the present invention, the cold rolled steel includes a mixed structure of ferrite and martensite, the fraction of martensite is in the range of 10% to 20%, and the fraction of ferrite is 80% to 90% It can be a range.

According to an embodiment of the present invention, the ferrite may include titanium carbide (TiC).

According to an embodiment of the present invention, the cold-rolled steel material may include crystal grains having a size in the range of 3 μm to 7 μm.

According to an embodiment of the present invention, the cold-rolled steel material may exhibit a manganese band interval in the range of 40 μm to 50 μm.

According to an embodiment of the present invention, the cold-rolled steel material may satisfy a hardness difference between phases of 40 Hv or less.

According to an embodiment of the present invention, in the cold rolled steel, a value obtained by dividing the hardness difference between phases by the total hardness may be 20% or less.

According to an embodiment of the present invention, the manufacturing method of the cold-rolled steel material is (a) by weight, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn) : 1.5% to 3.5%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, aluminum (Al): 0.1% to 0.5%, chromium (Cr): 0.2% to 0.5%, titanium (Ti): 0.01% to less than 0.05%, and the remainder of preparing a hot-rolled steel sheet containing iron (Fe) and unavoidable impurities; (b) cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel material; (c) heat-treating the cold-rolled steel material at a temperature in the range of 780°C to 860°C; and (d) cooling the cold-rolled steel to a temperature in the range of 400°C to 600°C.

According to an embodiment of the present invention, the step (a) comprises the steps of: (a-1) preparing a steel slab having the alloy composition; (a-2) reheating the steel slab in the range of 1,000 to 1,300°C; (a-3) preparing a hot-rolled steel sheet by hot finish rolling the reheated steel slab in a range of 800°C to 1,000°C; and (a-4) winding the hot-rolled steel sheet in the range of 500°C to 650°C.

According to an embodiment of the present invention, in step (c), the cold-rolled steel is heated at a temperature increase rate in the range of 1° C./sec to 10° C./sec and maintained at a temperature in the range of 780° C. to 860° C., In step (d), the heat-treated cold-rolled steel may be cooled to a temperature in the range of 400 to 600° C. at a cooling rate of 3° C./sec to 20° C./sec.

According to an embodiment of the present invention, after performing the step (d), (e) immersing the cold-rolled steel material in a hot-dip galvanizing bath to form a hot-dip galvanizing layer at a temperature of 400 ℃ ~ 520 ℃; and (f) cooling the cold-rolled steel material having the hot-dip galvanized layer formed thereon at a cooling rate of 1° C./sec to 100° C./sec.

According to an embodiment of the present invention, the cold rolled steel manufactured by the method for manufacturing the cold rolled steel has a yield strength (YS): 350 MPa or more, a tensile strength (TS): 780 MPa or more, and an elongation (EL): 20% Above, bendability (R/t): 1.0 or less, and hole expandability (HER): 26% or more may be satisfied.

According to the technical idea of the present invention, by controlling the components of the additive element to increase the formability and hole expandability, in particular, by increasing the content of titanium to suppress the formation of manganese bands and MnS, and to form a fine and uniform structure. To provide high-strength cold-rolled steel with excellent hole expandability and bendability.

A large amount of manganese (Mn) is added to the 780 MPa grade cold-rolled steel sheet manufactured through the conventional continuous annealing process to secure strength, thereby forming a manganese band, making it difficult to obtain a uniform structure and excellent formability. In the present invention, it is possible to provide a cold-rolled hot-dip galvanized steel sheet having a uniform structure and excellent bending properties by controlling grain refinement, reduction of interphase hardness difference, and manganese band layer through a lower winding temperature and a change in chemical composition compared to the prior art.

When such a steel plate is applied to automobiles, a high-strength steel plate can be used even for parts having a complex shape, thereby improving the crash performance of the car body and contributing to fuel economy improvement by reducing the weight of the car body.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method of manufacturing a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.
2 is a scanning electron microscope photograph showing the microstructure of a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.
3 is a scanning electron microscope photograph showing titanium carbide in the microstructure of a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.
4 is an electron probe microanalysis photograph showing a manganese band in a high-strength cold-rolled steel having excellent hole expandability and bendability according to an embodiment of the present invention.
5 is an optical micrograph showing a manganese band in a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.
6 is a scanning electron microscope photograph showing MnS segregation that can be suppressed in a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

Note that the cold-rolled steel material described herein may refer to a cold-rolled steel sheet, but this is exemplary, and the technical idea of the present invention is not limited thereto, and is used to refer to cold-rolled steel material of various shapes.

The present invention provides a high-strength cold-rolled steel material having excellent hole expandability and bendability through control of the alloy composition and heat treatment conditions of the steel sheet, and a method for manufacturing the same.

As a high-strength cold-rolled steel is applied to a difficult-to-form car body, if the formability and elongation are low, there is a possibility that cracks may occur during steel forming. The formability of cold rolled steel can be improved by minimizing the difference in hardness between phases, generating fine and uniform crystal grains, and minimizing inclusions. In the present invention, titanium (Ti) is added as a precipitation element to induce the formation of titanium carbide (TiC) precipitates, thereby suppressing the formation of manganese bands and manganese sulfide (MnS), and obtaining a fine and uniform structure. In addition, by reducing the hardness of the hard phase martensite and increasing the hardness of the soft phase ferrite through the titanium carbide (TiC) precipitation, to reduce the hardness difference between the phases.

Hereinafter, a high-strength cold-rolled steel material having excellent hole expandability and bendability, which is an aspect of the present invention, will be described.

High-strength cold-rolled steel with excellent hole expandability and bendability

High-strength cold-rolled steel having excellent hole expandability and bendability, which is an aspect of the present invention, in weight %, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn): 1.5 % to 3.5%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, aluminum (Al): 0.1% to 0.5%, chromium (Cr): 0.2% to 0.5% , titanium (Ti): 0.01% to less than 0.05%, and the balance includes iron (Fe) and unavoidable impurities.

Hereinafter, the role and content of each component included in the high-strength cold-rolled steel material having excellent hole expandability and bendability according to the present invention will be described. At this time, the content of the component elements all mean weight % with respect to the entire steel material.

Carbon (C): 0.04% to 0.10%

Carbon (C) is an element that affects the strength and toughness of steel, and is an element that increases the strength of martensite and forms carbides. When the content of carbon (C) is less than 0.04%, it is difficult to secure the desired strength. When the content of carbon (C) exceeds 0.10%, it causes deterioration of weldability and deterioration of formability. Therefore, it is preferable to add the content of carbon (C) to 0.04% to 0.10% of the total steel.

Silicon (Si): 0.3% to 0.7%

Silicon (Si) acts as a deoxidizer and is an element that works effectively for solid solution strengthening. In addition, silicon (Si) is added to strengthen ferrite solid solution and secure elongation. Specifically, silicon (Si) is effective in improving toughness and ductility of steel by inducing formation of ferrite as a ferrite stabilizing element. When the content of silicon (Si) is less than 0.3%, the effect of adding silicon is insufficient, and in particular, it is difficult to ensure ductility. When the content of silicon (Si) exceeds 0.7%, it may give a problem of deteriorating the surface of the steel by generating red scale in the heating furnace, and also has a problem of lowering the plating property due to the formation of surface oxides. Therefore, it is preferable to add the content of silicon (Si) to 0.3% to 0.7% of the total steel material.

Manganese (Mn): 1.5% to 3.5%

Manganese (Mn) is a substitution element having an atomic diameter similar to that of iron (Fe), and may facilitate the formation of a low-temperature transformation phase and provide an effect of increasing strength. When the content of manganese (Mn) is less than 1.5%, it is difficult to secure strength. When the content of manganese (Mn) exceeds 3.5%, the carbon equivalent is increased to generate MnS inclusions, and segregation zones are formed inside and outside the slab and steel plate, thereby greatly reducing the ductility and impact properties of the steel. Therefore, it is preferable to add the content of manganese (Mn) to 1.5% to 3.5% of the total steel.

Aluminum (Al): 0.1% to 0.5%

Aluminum (Al) acts as a deoxidizer and promotes ferrite formation, and exhibits an effect similar to that of silicon (Si). When the content of aluminum (Al) is less than 0.1%, the deoxidation function may not be properly performed. When the content of aluminum (Al) exceeds 0.5%, aluminum inclusions may increase, and plating properties may decrease. Therefore, it is preferable to add the content of aluminum (Al) to 0.1% to 0.5% of the total steel material.

Chromium (Cr): 0.2% to 0.5%

Chromium (Cr) is an element that improves strength by effectively acting on solid solution strengthening. When the content of chromium (Cr) is less than 0.2%, it is difficult to secure strength. When the content of chromium (Cr) exceeds 0.5%, elongation may decrease. Therefore, it is preferable to add the content of chromium (Cr) to 0.2% to 0.5% of the total steel.

Titanium (Ti): 0.01% to less than 0.05%

Titanium (Ti) is added for high yield and high bendability through grain refinement and homogenization. Titanium (Ti) suppresses the formation of BN and forms TiC to reduce the hardness of martensite and increase the hardness of ferrite. When the content of titanium (Ti) is less than 0.01%, AIN or BN precipitates may be excessively precipitated. When the content of titanium (Ti) is 0.05% or more, the recrystallization temperature may be excessively increased to cause a non-uniform structure. Therefore, it is preferable to add the content of titanium (Ti) to less than 0.01% to less than 0.05% of the total steel. Alternatively, it is preferable to add the content of titanium (Ti) to 0.01% to 0.04% of the entire steel.

Phosphorus (P): >0% to 0.02%

Phosphorus (P) is an element that is unavoidably contained in the manufacture of steel, and has a high probability of segregation during the steel manufacturing process, and segregation of phosphorus reduces toughness and causes delayed fracture, which is broken after a certain period of time after forming. Phosphorus (P) needs to be minimized to reduce grain boundary embrittlement, inclusions, and secure weldability. Therefore, it is preferable to limit the content of phosphorus (P) to more than 0% to 0.02% of the entire steel.

Sulfur (S): >0% to 0.01%

Sulfur (S) is an element that is unavoidably contained in the manufacture of steel, and inhibits the toughness and weldability of the steel and combines with manganese (Mn) to form MnS, thereby reducing the corrosion resistance and impact properties of the steel. Sulfur (S) needs to be minimized to reduce grain boundary embrittlement, inclusions, and secure weldability. Therefore, it is preferable to limit the content of sulfur (S) to more than 0% ~ 0.01% of the entire steel.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steelmaking process, it cannot be excluded. Since these impurities are known to any person skilled in the art in the manufacturing process, all details thereof are not specifically mentioned in the present specification.

The high-strength cold-rolled steel material having excellent hole expandability and bendability manufactured by controlling the specific components of the above alloy composition and their content ranges, and by the manufacturing method to be described later, has a yield strength (YS): 350 MPa or more, tensile strength ( TS): 780 MPa or more, elongation (EL): 20% or more, bendability (R/t): 1.0 or less, and hole expandability (HER): 26% or more may be satisfied. The high-strength cold-rolled steel is, yield strength (YS): 350 MPa ~ 500 MPa, tensile strength (TS): 780 MPa ~ 1000 MPa, elongation (EL): 20% ~ 30%, bendability (R/t): 0 ~ 1.0, and hole expandability (HER): 25 to 30 may be satisfied.

The high-strength cold-rolled steel may have a mixed structure in which ferrite and martensite are mixed. The fraction of martensite may be, for example, in the range of 10% to 20%. The fraction of ferrite may be included as the remaining fraction, for example, may be in the range of 80% to 90%. When the fraction of the ferrite is 80% or less, the target elongation and formability cannot be obtained. When the fraction of ferrite is more than 90%, the martensite fraction cannot be secured. When the fraction of martensite is 10% or less, the target strength cannot be obtained. When the fraction of martensite is more than 20%, elongation may be reduced. The fraction means an area ratio derived from a microstructure photograph through an image analyzer.

The cold-rolled steel material can control the grain size by forming precipitates by adding titanium. The cold-rolled steel material may include crystal grains having a size in the range of 3 μm to 7 μm. The size of these grains may be based on a 1/4 point in the thickness direction when the cold-rolled steel is a steel sheet. When the crystal grains have a size exceeding 7 μm, a manganese band layer may be easily formed, and thus bending properties may be deteriorated and formability may be deteriorated.

In this way, by refining the crystal grains, the manganese band interval can be controlled to be 40 μm to 50 μm. The manganese band interval may be based on a 1/2 point in the thickness direction when the cold-rolled steel material is a steel sheet. When the manganese band interval is less than 40 μm, stress concentration may be facilitated, and bending properties may be deteriorated.

The cold-rolled steel material may satisfy a hardness difference between phases, for example, less than or equal to 40 Hv, for example, may satisfy a range of 30 Hv to 40 Hv. Alternatively, a value obtained by dividing the interphase hardness difference by the total hardness may satisfy 20% or less, for example, it may satisfy a range of 10% to 20%. The interphase hardness difference is a value obtained by subtracting the ferrite hardness from the martensite hardness. For reference, the hardness of ferrite and martensite is measured using a nano indenter, and the overall hardness is measured using a macro indenter including both phases of ferrite and martensite.

The high-strength cold-rolled steel material having excellent hole expandability and bendability of the present invention can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an embodiment thereof, it may be prepared by the following method.

Hereinafter, a method for manufacturing a high-strength cold-rolled steel material having excellent hole expandability and bendability according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of high-strength cold-rolled steel with excellent hole expandability and bendability

1 is a process flowchart schematically illustrating a method of manufacturing a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling and cold rolling processes may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process. also,

The cold-rolled steel material is, by weight, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn): 1.5% to 3.5%, phosphorus (P): more than 0% ~ 0.02%, sulfur (S): more than 0% ~ 0.01%, aluminum (Al): 0.1% ~ 0.5%, chromium (Cr): 0.2% ~ 0.5%, titanium (Ti): 0.01% ~ less than 0.05%, and the balance includes iron (Fe) and unavoidable impurities.

Referring to FIG. 1, the method for manufacturing a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention includes the steps of manufacturing a hot-rolled steel material using the steel of the composition (S110), the hot-rolled steel material Cold rolling to manufacture a cold rolled steel (S120); heat-treating the cold-rolled steel material (S130); and cooling the heat-treated cold-rolled steel (S140).

In addition, the manufacturing method of the high-strength cold-rolled steel material may further include the step of immersing the cold-rolled steel material in a hot-dip galvanizing bath to form a hot-dip galvanizing layer (S150).

Hot-rolled steel manufacturing step (S110)

Prepare a steel slab having the above composition, and reheat the steel slab at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1,000 ° C to 1300 ° C for about 40 minutes to 60 minutes. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur. If the reheating temperature is less than 1,000 ℃, there may be a problem that the hot rolling load rapidly increases. When the reheating temperature exceeds 1,300° C., the amount of surface scale increases, which may lead to material loss.

After the reheating, hot rolling may be performed in a conventional manner, and finish rolling may be performed at a finish delivery temperature (FDT) in the range of 800°C to 1,000°C. If the finish rolling end temperature exceeds 1,000 ℃, there is a fear that the quality of the steel material due to the generation of scale on the surface of the steel material. In addition, when the finish rolling end temperature is less than 800 ℃, the crystal grains are refined to increase the strength, but may cause an increase in the rolling load and a decrease in productivity. The hot rolling, for example, may be performed at a reduction ratio of 90% or more, for example, a reduction ratio of 90% to 95%.

Then, the hot-rolled steel material is cooled to a predetermined coiling temperature. The cooling may be either air cooling or water cooling, and cooling may be performed at a cooling rate of 1° C./sec to 100° C./sec. A faster cooling rate is advantageous in reducing the average grain size. Preferably, the cooling is performed to a coiling temperature.

Then, the hot-rolled steel is wound at a coiling temperature (CT) in the range of 500°C to 650°C. If the winding temperature is less than 500 ℃, it may cause a shape non-uniformity of the hot-rolled coil, it may cause a load during cold rolling. When the coiling temperature exceeds 640 ℃, it may cause a non-uniform microstructure due to a decrease in elongation due to grain coarsening and a difference in cooling rate during winding.

Cold rolling step (S120)

A pickling treatment of washing the formed hot-rolled steel with acid is performed. Then, cold rolling is performed on the pickling-treated hot-rolled steel material at an average reduction ratio of 40% to 60% to form a cold-rolled steel material. As the average reduction ratio is higher, there is an effect of increasing the formability due to the effect of refining the tissue. When the average reduction ratio is less than 40%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 60%, the roll force is increased to increase the process load.

Heat treatment step (S130)

The cold-rolled steel is heat-treated in a continuous annealing furnace with a normal slow cooling section. The heat treatment may be referred to as annealing heat treatment. The heat treatment is performed by heating at a temperature increase rate in the range of 1° C./sec to 10° C./sec, and at a temperature of Ac1 or more and Ac3 or less, for example, a temperature in the range of 700° C. to 900° C., for example, in the range of 780° C. to 860° C. The heat treatment is carried out at a temperature of 60 to 600 seconds.

Cooling step (S140)

The heat-treated cold-rolled steel is cooled at a cooling rate of 3° C./sec to 20° C./sec, for example, to a temperature in the range of 400 to 600° C., for example, to a temperature in the range of 420 to 520° C.

Hot-dip galvanizing layer forming step (S150)

The step of forming a hot-dip galvanizing layer by immersing the cold-rolled steel material in a hot-dip galvanizing bath is performed. The temperature of the plating bath may be in the range of 400°C to 520°C depending on the type and ratio of alloying elements constituting the plating layer, and the composition of the cold-rolled steel material. While the hot-dip galvanizing layer is easily formed on the surface of the cold-rolled sheet under the plating bath conditions, the adhesion of the plating layer may be excellent. Then, it is cooled to room temperature at a cooling rate of 1° C./sec to 100° C./sec.

If necessary, the cold-rolled steel material on which the hot-dip galvanized layer is formed may be subjected to alloying heat treatment. The alloying heat treatment may be carried out for 10 seconds to 60 seconds at a temperature in the range of 500 ℃ ~ 620 ℃. Under the above conditions, while the hot-dip galvanized layer is stably grown during the alloying heat treatment, the adhesion of the plating layer may be excellent. If the alloying heat treatment temperature is less than 500 ℃, alloying may not proceed sufficiently, the soundness of the hot-dip galvanizing layer may be reduced. When the alloying heat treatment temperature exceeds 620° C., a change in material may occur while passing to an abnormal temperature range.

The cold-rolled steel material manufactured by the method of the present invention described above may have a mixed structure in which ferrite and martensite are mixed.

The fraction of martensite may be, for example, 50% or more, for example, in the range of 10% to 20%. The fraction of ferrite may be included as the remaining fraction, for example, may be in the range of 80% to 90%.

The cold-rolled steel material may include crystal grains having a size in the range of 3 μm to 7 μm. The cold-rolled steel may have a manganese band interval in the range of 40 μm to 50 μm. The cold-rolled steel material may satisfy a hardness difference between phases of 40 Hv or less.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Steel having the composition (unit: weight %) shown in Table 1 below was prepared, and cold rolled steel sheets according to Examples and Comparative Examples were prepared through predetermined hot rolling and cold rolling processes.

(weight%) C Si Mn Cr Al Ti Fe Example 1 0.07 0.5 2.2 0.4 0.3 0.01 Bal. Example 2 0.07 0.5 2.2 0.4 0.3 0.03 Bal. Example 3 0.07 0.5 2.2 0.4 0.3 0.04 Bal. Comparative Example 1 0.07 0.5 2.2 0.4 0.3 - Bal. Comparative Example 2 0.07 0.5 2.2 0.4 0.3 0.05 Bal. Comparative Example 3 0.07 0.5 2.1 - 0.3 - Bal. Comparative Example 4 0.07 0.5 2.4 - 0.3 - Bal.

Referring to Table 1, comparing the compositions of Examples and Comparative Examples of the present invention, Comparative Example 1 is a case that does not include titanium (Ti), Comparative Example 2 is titanium (Ti) exceeding the scope of the present invention In the case of including, Comparative Examples 3 and 4 are cases in which chromium (Cr) and titanium (Ti) are not included, and in Comparative Example 3 and Comparative Example 4, the manganese content is large.

The microstructure characteristics and mechanical properties of the cold-rolled steel sheet were measured, and the results are shown in Tables 2 and 3.

division grain
size
(μm) manganese band
interval
(μm) all
Hardness
(GPa) incest
hardness difference
(Hv) Hardness difference between phases/Total hardness
(%) Example 1 4.8 41.0 189.7 37.2 19.6 Example 2 4.0 43.2 191.0 36.4 19.1 Example 3 3.9 43.5 221.7 34.7 15.7 Comparative Example 1 9.3 12.8 179.4 166.5 92.8 Comparative Example 2 3.7 45.2 225.1 32.2 14.3 Comparative Example 3 8.7 10.5 220.0 157.3 71.5 Comparative Example 4 8.9 9.2 243.0 160.3 66.7

division yield strength
(MPa) tensile strength
(MPa) elongation
(%) n bendability
(0 degree direction) bendability
(90 degree direction) Hall expandability
(%) Example 1 362 812 22.5 0.184 One One 26.5 Example 2 368 815 22.3 0.186 0 0 28.7 Example 3 380 825 21.8 0.185 0 0 28.9 Comparative Example 1 365 816 19.9 0.186 3 3 23.7 Comparative Example 2 389 840 20.7 0.191 One One 27.5 Comparative Example 3 382 820 18.1 0.179 3 3 21.5 Comparative Example 4 392 825 17.8 0.178 3 3 20.7

Referring to Tables 2 and 3, it can be seen that the examples have a target grain size, manganese band spacing, interphase hardness difference, yield strength, tensile strength, elongation, bendability, and hole expandability.

Comparative Example 1 had a large grain size, a narrow gap between manganese bands, a large difference in hardness between phases, and a low elongation and low hole expansion compared to the Example. Also, the bendability value was higher than 1.

Comparative Example 2, compared to the Example, the grain size, the manganese band interval, and the interphase hardness difference appeared similar to the Example, but the hole expandability was reduced, which is because titanium was excessively added to increase the carbide formation is analyzed as

Comparative Example 3 and Comparative Example 4 showed a large crystal grain size, a narrow manganese band interval, a large interphase hardness difference, and low elongation and low hole expansion compared to Examples. Also, the bendability value was higher than 1.

Although the Examples and Comparative Examples were manufactured under the same manufacturing process conditions, crystal grain refinement, manganese band spacing increase, bending characteristics improvement, and hole expansion improvement were confirmed in the titanium-added steel material. In addition, it can be seen that the Example shows a similar range in strength compared to the comparative example, but the elongation is more excellent. However, it can be seen that when the titanium of Comparative Example 2 is 0.05 wt%, hole expandability, bendability, and elongation are reduced.

In addition, even when manufactured under the same process conditions, when the manganese content is increased, the manganese band spacing is decreased and the yield strength and tensile strength are increased, but the elongation and hole expandability are decreased.

For reference, MnS inclusions can be controlled to 5 μm or less in the longitudinal direction, for example, in the range of 1 μm to 5 μm.

2 is a scanning electron microscope photograph showing the microstructure of a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

Referring to FIG. 2 , it can be seen that the grains of Example 2 are refined compared to Comparative Example 1. This is consistent with the results in Table 2.

3 is a scanning electron microscope photograph showing titanium carbide in the microstructure of a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

Referring to FIG. 3 , in Example 2, fine titanium carbide (TiC) was uniformly distributed, whereas in Comparative Example 1, such titanium carbide was not observed. Such titanium carbide may provide an effect of refining crystal grains. The titanium carbide (TiC) may be precipitated in ferrite to increase hardness of the ferrite. In addition, since carbon and titanium react to form the titanium carbide (TiC), the carbon content dissolved in martensite may be reduced, thereby reducing the hardness of the martensite. Therefore, the hardness difference between the phases can be reduced.

4 is an electron probe microanalysis (EPMA) photograph showing a manganese band in a high-strength cold-rolled steel having excellent hole expandability and bendability according to an embodiment of the present invention.

Referring to FIG. 4 , it is measured at the center of the cold rolled steel sheet, and manganese bands are shown in Example 2 and Comparative Example 1. The higher the manganese content, the brighter the color. It can be seen that the width of the manganese band in Example 2 is larger than in Comparative Example 1.

5 is an optical micrograph showing a manganese band in a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

Referring to FIG. 5 , it is measured at the center of the cold rolled steel sheet, and manganese bands are shown in Example 2 and Comparative Example 1. Example 2 has an average manganese band interval of 43.2 μm. In Comparative Example 1, the manganese band interval was 12.8 μm on average. In Example 2, it can be seen that the crystal grains are refined and thus the manganese band interval is increased.

6 is a scanning electron microscope photograph showing MnS segregation that can be suppressed in a high-strength cold-rolled steel material having excellent hole expandability and bendability according to an embodiment of the present invention.

Referring to FIG. 6 , in Comparative Example 1, MnS segregation appears in the central portion. In the case of the embodiment, as described above, MnS segregation can be suppressed by controlling the manganese band, that is, increasing the manganese band interval.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (12)

By weight, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn): 1.5% to 3.5%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, aluminum (Al): 0.1% to 0.5%, chromium (Cr): 0.2% to 0.5%, titanium (Ti): 0.01% to less than 0.05%, and the balance is iron ( Fe) and unavoidable impurities,
Yield strength (YS): 350 MPa or more, tensile strength (TS): 780 MPa or more, elongation (EL): 20% or more, bendability (R/t): 1.0 or less, and hole expansion (HER): 26% Satisfying more than
cold rolled steel. The method of claim 1,
The cold-rolled steel includes a mixed structure of ferrite and martensite,
The fraction of martensite is in the range of 10% to 20%, and the fraction of ferrite is in the range of 80% to 90%,
cold rolled steel. 3. The method of claim 2,
The ferrite comprises titanium carbide (TiC),
cold rolled steel. The method of claim 1,
The cold-rolled steel includes crystal grains having a size in the range of 3 μm to 7 μm,
cold rolled steel. The method of claim 1,
The cold-rolled steel material represents a manganese band interval in the range of 40 μm to 50 μm,
cold rolled steel. The method of claim 1,
The cold-rolled steel material, the interphase hardness difference satisfies 40 Hv or less,
cold rolled steel. The method of claim 1,
In the cold-rolled steel material, the value obtained by dividing the hardness difference between phases by the total hardness is 20% or less,
cold rolled steel. (a) by weight, carbon (C): 0.04% to 0.10%, silicon (Si): 0.3% to 0.7%, manganese (Mn): 1.5% to 3.5%, phosphorus (P): greater than 0% to 0.02 %, sulfur (S): greater than 0% to 0.01%, aluminum (Al): 0.1% to 0.5%, chromium (Cr): 0.2% to 0.5%, titanium (Ti): 0.01% to less than 0.05%, and glass manufacturing a hot-rolled steel sheet containing iron (Fe) and unavoidable impurities;
(b) cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel material;
(c) heat-treating the cold-rolled steel material at a temperature in the range of 780°C to 860°C; and
(d) comprising the step of cooling the cold-rolled steel to a temperature in the range of 400 ℃ ~ 600 ℃,
Method for manufacturing cold rolled steel. 9. The method of claim 8,
The step (a) is,
(a-1) preparing a steel slab having the alloy composition;
(a-2) reheating the steel slab in the range of 1,000 to 1,300°C;
(a-3) preparing a hot-rolled steel sheet by hot finish rolling the reheated steel slab in a range of 800°C to 1,000°C; and
(a-4) comprising the step of winding the hot-rolled steel sheet in the range of 500 ℃ ~ 650 ℃,
Method for manufacturing cold rolled steel. 9. The method of claim 8,
In the step (c), the cold-rolled steel is heated at a temperature increase rate in the range of 1° C./sec to 10° C./sec and maintained at a temperature in the range of 780° C. to 860° C.,
In the step (d), cooling the heat-treated cold-rolled steel to a temperature in the range of 400 to 600 °C at a cooling rate of 3 °C / sec to 20 °C / sec,
Method for manufacturing cold rolled steel. 9. The method of claim 8,
After performing step (d),
(e) immersing the cold-rolled steel material in a hot-dip galvanizing bath to form a hot-dip galvanizing layer at a temperature of 400°C to 520°C; and
(f) cooling the cold-rolled steel material having the hot-dip galvanized layer formed thereon at a cooling rate of 1°C/sec to 100°C/sec; further comprising,
Method for manufacturing cold rolled steel. 9. The method of claim 8,
The cold-rolled steel material manufactured by the manufacturing method of the cold-rolled steel material,
Yield strength (YS): 350 MPa or more, tensile strength (TS): 780 MPa or more, elongation (EL): 20% or more, bendability (R/t): 1.0 or less, and hole expansion (HER): 26% Satisfying more than
Method for manufacturing cold rolled steel.

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