KR20210037110A - Steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention provides a steel sheet with excellent forming, plating, and welding properties. The steel sheet comprises 0.05-0.09 wt.% of carbon (C), 0.3-0.7 wt.% of silicon (Si), 2-2.8 wt.% of manganese (Mn), 0-150 ppm (excluding 0 ppm) of phosphorus (P), 0-30 ppm (excluding 0 ppm) of sulfur (S), 0.2-0.5 wt.% of aluminum (Al), 0.8-1.2 wt.% of chromium (Cr), 0.05-0.1 wt.% of molybdenum (Mo), 0.03-0.06 wt.% of titanium (Ti), 10-30 ppm of boron (B), 0.02-0.05 wt% of antimony (Sb), and the remainder consisting of Fe and inevitable impurities, and has a yield strength (YP) of 600MPa or higher, a tensile strength (TS) of 1,000-1,180MPa, an elongation (EL) of 11% or higher, and a bendability (R/t) of 0.6 or lower.

Description

Steel sheet and its manufacturing method {STEEL SHEET AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel sheet and a method of manufacturing the same, and more particularly, to a steel sheet having excellent formability, plating properties, and weldability, and a method of manufacturing the same.

In recent years, from the standpoint of preserving the global environment, improvement of the fuel economy of automobiles has become an important issue. For this reason, a movement to reduce the thickness of the vehicle body itself by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself is becoming active. However, in general, increasing the strength of a steel sheet causes a decrease in formability, causing problems such as cracks during molding. For this reason, simple thickness reduction of the steel sheet is not achieved. Therefore, development of a material having both high strength and high formability is desired. Further, a steel sheet having a tensile strength (TS) of 980 MPa or more is particularly required to have a high impact absorption energy in addition to this high formability. In order to improve the collision absorption energy characteristics, it is effective to increase the yield ratio (YR), because if the yield ratio is high, the collision energy can be efficiently absorbed by the steel sheet with a low deformation amount.

As a related technology, there is Korean Patent Application Publication No. 2018-0095529 (published on August 27, 2018, a method of manufacturing a high-strength steel sheet with improved strength and formability, and a high-strength steel sheet obtained).

The problem to be solved by the present invention is to provide a steel sheet having excellent formability, plating properties, and weldability, and a method of manufacturing the same.

The steel sheet according to an aspect of the present invention is in wt%, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus (P): greater than 0 150ppm or less, sulfur (S): greater than 0 and 30ppm or less, aluminum (Al): 0.2 to 0.5%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.05 to 0.1%, titanium (Ti): 0.03 to 0.06%, boron (B): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05%, the balance consists of Fe and inevitable impurities, yield strength (YP) of 600MPa or more, tensile strength (TS) of 1000 ~ 1180MPa , The elongation (EL) is 11% or more, and the bendability (R/t) is 0.6 or less.

In the steel sheet, a cross tension strength (CTS) as a strength of a weld may be 1000 kgf/spot or more.

In the steel sheet, carbon equivalents defined as [C]+[Mn]/20+[Si]/30+2[P]+4[S] (the above [C], [Mn], [Si], [P] ] And [S] are the weight ratios of carbon, manganese, silicon, phosphorus and sulfur, respectively) may be 0.25 or less.

In the steel sheet, the microstructure may include ferrite, martensite, and bainite.

In the steel sheet, a fraction of ferrite may be less than 60%, a fraction of martensite may be 40% or more, and a fraction of bainite may be 5% or less.

In the steel sheet, the average size of ferrite may be 10 μm or less, the average size of martensite may be 10 μm or less, and the average size of bainite may be 10 μm or less.

In the steel sheet, a size ratio (SR), which is a ratio of a minimum size and a maximum size of crystal grains, may be 0.5 or more.

The manufacturing method of the steel sheet according to an aspect of the present invention is (a) in weight %, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus (P): greater than 0 and less than 150ppm, sulfur (S): greater than 0 and less than 30ppm, aluminum (Al): 0.2 to 0.5%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.05 to 0.1%, titanium (Ti): 0.03 ~ 0.06%, boron (B): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05%, the balance is preparing a hot-rolled steel sheet having an alloy composition consisting of Fe and inevitable impurities; (b) cold-rolling the hot-rolled steel sheet to prepare a cold-rolled steel sheet; And (c) heat-treating the cold-rolled steel sheet by heating and maintaining the cold-rolled steel sheet at a temperature of 790 to 860°C. Includes.

In the method of manufacturing the steel sheet, the step (a) includes: (a-1) preparing a steel slab having the alloy composition; (a-2) reheating the steel slab at 1180°C or higher; (a-3) manufacturing a hot-rolled steel sheet by hot finish rolling the reheated steel slab at 880 to 920°C; And (a-4) winding the hot-rolled steel sheet at 560 to 640°C.

In the method of manufacturing the steel sheet, step (b) may be characterized in that the cold rolling reduction ratio is 40 to 80%.

In the method of manufacturing the steel sheet, in step (c), the cold rolled steel sheet is heated at a rate of 2 to 8°C/s, heated and maintained at a temperature of 790 to 860°C, and then at a rate of 3 to 30°C/s. It may include the step of heat treatment by rapid cooling to a temperature of 240 ~ 360 ℃.

In the method of manufacturing the steel sheet, in step (c), the cold rolled steel sheet is heated at a rate of 2 to 8°C/s, heated and maintained at a temperature of 790 to 860°C, and then at a rate of 3 to 30°C/s. It includes the step of heat-treating by rapid cooling to a temperature of 460 ~ 520 ℃, and after the step (c), dipping the steel plate in a plating bath of 450 ~ 470 ℃ to form a plating layer on the steel plate; further comprising I can. In this case, the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the steel sheet and the plating layer may be 90% or more.

In the method of manufacturing the steel sheet, in step (c), the cold rolled steel sheet is heated at a rate of 2 to 8°C/s, heated and maintained at a temperature of 790 to 860°C, and then at a rate of 3 to 30°C/s. Including the step of heat treatment by rapid cooling to a temperature of 460 ~ 520 ℃, and after step (c), after the step (c), after forming a plating layer on the steel plate by dipping the steel plate in a plating bath of 450 ~ 470 ℃ to 500 ~ 540 ℃ Performing and maintaining alloying heating; may further include. In this case, the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer may be 95% or more.

According to the present invention, by designing a component to have a low carbon equivalent (Ceq) of 0.25 or less and controlling the process conditions, a steel sheet having excellent formability, plating properties, and weldability, and a manufacturing method thereof can be implemented. However, the scope of the present invention is not limited by these effects.

1 is a graph showing the relationship between the carbon equivalent (Ceq) and the cross tensile strength (CTS) of a weld.
2 is a flowchart illustrating a method of manufacturing a high-strength steel sheet having excellent weld strength and formability according to an embodiment of the present invention.
3 is a graph showing the relationship between the fraction of the Fe-Al compound (inhibition layer coverage) present in the interface region between the steel sheet and the plating layer according to the content of antimony in the experimental example of the present invention.
4 is a photograph showing the plating peeling characteristics according to the content of antimony in the experimental example of the present invention.
5 are photographs illustrating an area fraction of a Zn-Fe alloyed portion on the surface of a plating layer in an experimental example of the present invention.
6 and 7 are diagrams illustrating a comparison of a size ratio (SR), which is a ratio between a minimum size and a maximum size of a steel plate.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. The present invention may be implemented in various different forms, and is not limited to the embodiments described herein. The same reference numerals are assigned to the same or similar components throughout the present specification. In addition, detailed descriptions of known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

Recently, as steel sheets for automobiles, steel sheets with higher strength are required to improve fuel efficiency or durability, and ultra-high strength steel sheets having a tensile strength of 1000 MPa or more are used for vehicle body structures or as reinforcing materials for collision safety and passenger protection. However, as the strength of the steel plate increases, the formability deteriorates, making it difficult to apply the steel plate, and the strength of the welding portion also decreases, so there is a limit to maximizing the collision safety of a vehicle.

When the base material is heated during welding, the Heat Affected Zone (HAZ) is easy to harden, and the element that has the greatest influence on the degree of hardening is carbon (C). In addition to carbon, there are elements that affect the degree of hardening, and the carbon equivalent is a combination of those elements and expressing the ratio to the effect of carbon. Therefore, the cross tension strength (CTS) representing the strength of the weld is a carbon equivalent defined as [C]+[Mn]/20+[Si]/30+2[P]+4[S] (above [C], [Mn], [Si], [P] and [S] are the weight ratios of carbon, manganese, silicon, phosphorus and sulfur, respectively).

1 is a graph showing the relationship between the carbon equivalent (Ceq) and the cross tensile strength (CTS) of a weld.

Referring to FIG. 1, as the carbon equivalent (Ceq) increases, the cross tensile strength (CTS) tends to decrease. That is, the lower the carbon equivalent, the better the weldability and the better the strength of the weld.

The present invention was designed to have a low carbon equivalent (Ceq) of 0.25 or less in order to be applicable to complex automobile parts and members, and to secure weld strength and formability. In order to secure the strength and material of the welded part, boron (B), an element that is advantageous for increasing the strength of the steel by increasing the hardenability, was added, and for the purpose of refining the grains, precipitation hardening, and uniformity of the structure to compensate for the strength due to the decrease in the content of hardenable elements. Antimony (Sb) and titanium (Ti) were added, and manganese (Mn), molybdenum (Mo), and chromium (Cr) were added to secure strength. In this case, the cross tension strength (CTS) as the strength of the weld may be 1000 kgf/spot or more.

Hereinafter, a high-strength steel sheet having excellent weld strength and formability of the present invention will be described in detail.

High strength steel plate with excellent weld strength and formability

The high-strength steel sheet having excellent weld strength and formability of the present invention, by weight, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus ( P): greater than 0 and less than 150 ppm, sulfur (S): greater than 0 and less than 30 ppm, aluminum (Al): 0.2 to 0.5%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.05 to 0.1%, titanium ( Ti): 0.03 to 0.06%, boron (B): 10 to 30 ppm, and antimony (Sb): 0.02 to 0.05%. In addition to the above-described components, the steel sheet is made of the remainder of iron (Fe) and impurities that are inevitably contained in the steelmaking process.

Hereinafter, the role and content of each alloy component included in the high-strength steel sheet having excellent weld strength and formability according to the present invention will be described as follows.

Carbon (C): 0.05 ~ 0.09% by weight

Carbon (C) is an element that affects the strength, toughness, and toughness of welds of steel. In addition, as an element that increases the hardenability of steel, it increases the fraction of pearlite by delaying ferrite transformation during cooling after hot finish rolling, thereby increasing not only yield strength but also tensile strength. However, if the content is less than 0.05% by weight of the entire steel sheet, it is possible to secure sufficient tensile strength through the addition of alloying elements, but it is difficult to secure the desired yield strength and elongation. On the contrary, when the amount of carbon (C) exceeds 0.09% by weight, the possibility of cracking the slab due to the formation of primary ferrite is high, resulting in a decrease in toughness and weldability, and a higher percentage of pearlite phase, resulting in a desired microstructure. It becomes difficult to control. In the present invention, in order to reduce martensite strength and secure weldability, a low carbon design is promoted, and in order to realize this, it is preferable to add the content of carbon (C) in an amount of 0.05 to 0.09% by weight of the entire steel sheet.

Silicon (Si): 0.3 to 0.7% by weight

Silicon (Si) acts as a deoxidizing agent and is an element that works effectively for solid solution strengthening. In addition, it is effective in improving the toughness and ductility of steel by inducing ferrite formation as a ferrite stabilizing element. However, by generating a red scale in the heating furnace, when a large amount is added, it may give a problem of deteriorating the surface of the steel, and also has a problem of deteriorating weldability due to the generation of oxides. In the present invention, it is preferable to limit the addition amount of silicon (Si) to 0.3 to 0.7% by weight of the entire steel sheet in order to achieve solid solution strengthening of ferrite and securing elongation.

Manganese (Mn): 2.0 ~ 2.8% by weight

Manganese (Mn) is a substitutional element having an atomic diameter similar to that of iron (Fe). It is very effective in solid solution strengthening and is an element effective in securing strength after heat treatment by improving the hardenability of steel. In addition, as an austenite stabilizing element, it can contribute to grain refinement of ferrite by delaying the transformation of ferrite and pearlite. If the amount of manganese (Mn) is less than 2.0% by weight, it is difficult to secure strength.If the amount of manganese (Mn) is less than 2.0% by weight, it is difficult to secure the strength, and when it is added exceeding 2.8% by weight, the weldability is greatly reduced by increasing the carbon equivalent, and by creating MnS inclusions and generating central segregation in the slab/coil, lectures It greatly reduces ductility and impact characteristics. Therefore, the content of manganese (Mn) is preferably limited to 2.0 to 2.8% by weight of the entire steel sheet.

Phosphorus (P), sulfur (S)

In the case of phosphorus (P), there is a high probability of segregation during the manufacturing process of steel, and the segregation of phosphorus decreases toughness and causes delayed fracture that is destroyed after a certain period of time after forming. Therefore, it is preferable to limit the content of phosphorus (P) in the steel sheet to 150 ppm or less. Sulfur (S) impairs the toughness and weldability of the steel and combines with manganese (Mn) to form MnS, thereby lowering the corrosion resistance and impact properties of the steel. Accordingly, in the present invention, the content of sulfur (S) is limited to 30 ppm or less. That is, in the present invention, in order to implement grain boundary embrittlement, inclusion reduction, and securing weldability, phosphorus (P): greater than 0 and less than 150 ppm, sulfur (S): greater than 0 and less than 30 ppm may have a composition range.

Aluminum (Al): 0.2 to 0.5% by weight

Aluminum (Al) acts as a deoxidizing agent, promotes ferrite formation, and exhibits invaluable effects with silicon (Si). It is preferable to add such aluminum (Al) in an amount of 0.2 to 0.5% by weight of the entire steel sheet. When aluminum (Al) is added in an amount of less than 0.2% by weight, the effect is insignificant, and when it is added in an amount exceeding 0.5% by weight, it combines with nitrogen (N) present in the steel to form coarse AlN-based nitride. In the present invention, it is preferable to limit the amount of aluminum (Al) added to 0.2 to 0.5% by weight of the entire steel sheet in order to secure plating properties and elongation.

Chrome (Cr): 0.8 ~ 1.2% by weight

Chromium (Cr) is an element that effectively acts on solid solution strengthening to improve strength, and improves the hardenability of steel to ensure high strength. If such chromium (Cr) is added in an amount of less than 0.8% by weight, it is difficult to secure high strength, and if it is added in excess of 1.2% by weight, the elongation may be greatly reduced. Therefore, in the present invention, the content of chromium (Cr) is limited to 0.8 to 1.2% by weight.

Molybdenum (Mo): 0.05 to 0.1% by weight

Molybdenum (Mo) is effective in refining ferrite and improving strength. When molybdenum (Mo) is added in an amount of less than 0.05% by weight, the effect of refining ferrite and improving strength is insignificant, and when it is added in excess of 0.1% by weight, the elongation may be greatly reduced.In the present invention, 0.05 ~ 0.1 of molybdenum (Mo) It is limited to weight percent.

Titanium (Ti): 0.03 ~ 0.06% by weight

Titanium (Ti) precipitates to form carbonitrides and contributes to hardening. Titanium (Ti) is also involved in the formation of coarse TiN that appears during solidification of the cast product. Therefore, the content of titanium (Ti) is limited to 0.06% by weight or less in order to avoid coarse TiN which is detrimental to hole expansion. When the content of titanium (Ti) is added in an amount of less than 0.03% by weight, it does not have any effect on the steel of the present invention. In the present invention, it is preferable to limit the amount of titanium (Ti) added to 0.03 to 0.06% by weight of the entire steel sheet in order to achieve high yield and excellent bendability through grain refinement and homogenization.

Boron (B): 10 ~ 30ppm

Boron (B) is an element that is advantageous for enhancing the strength by securing the strength of the weld and increasing the hardenability of the steel. When the addition amount of boron (B) is less than 10 ppm, it is difficult to secure the martensite fraction, and it is difficult to secure strength.When a large amount is added, the impact property is rapidly deteriorated.Therefore, the content of boron (B) is preferably controlled to 10 to 30 ppm. Do. That is, in the present invention, it is preferable to limit the addition amount of boron (B) to 10 to 30 ppm of the entire steel sheet in order to achieve strength through suppression of ferrite transformation during cooling.

Antimony (Sb): 0.02 ~ 0.05% by weight

Antimony (Sb) is an element added to improve the surface properties of steel and to secure plating properties. When the content of antimony (Sb) is less than 0.02% by weight, the effect of suppressing the formation of surface oxides and nitrides is insignificant. However, when the content of antimony (Sb) exceeds 0.05% by weight, it may cause cracks and secondary processing brittleness. In the present invention, it is preferable to limit the amount of antimony (Sb) added to 0.02 to 0.05% by weight of the entire steel sheet in order to suppress the generation of surface oxides and nitrides to secure plating properties.

The remaining component of the present invention is iron (Fe). However, in a typical manufacturing process, since unintended impurities may inevitably be mixed from the raw material or the surrounding environment, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

The steel sheet according to the present invention having the above alloy composition has a yield strength (YP) of 600 MPa or more, a tensile strength (TS) of 1000 to 1180 MPa, an elongation (EL) of 11% or more, and a bendability (R/t) of 0.6 or less. I can. In addition, the steel sheet of the present invention has a carbon equivalent defined by [C]+[Mn]/20+[Si]/30+2[P]+4[S] (the above [C], [Mn], [Si ], [P] and [S] are the weight ratios of carbon, manganese, silicon, phosphorus and sulfur, respectively) may be 0.25 or less. In addition, the steel sheet according to the present invention has a cross tensile strength (CTS) of 1000 kgf/spot or more as a weld strength.

In the steel sheet, the microstructure may include ferrite, martensite, and bainite. In this case, the fraction of ferrite may be less than 60%, the fraction of martensite may be 40% or more, and the fraction of bainite may be 5% or less. Further, the average size of ferrite may be 10 μm or less, the average size of martensite may be 10 μm or less, and the average size of bainite may be 10 μm or less.

In the steel sheet, a size ratio (SR), which is a ratio of a minimum size and a maximum size of crystal grains, may be 0.5 or more.

The high-strength steel sheet having excellent weld strength and formability of the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an embodiment, it may be manufactured by the following method. Hereinafter, a method of manufacturing a high-strength steel sheet having excellent weld strength and formability, which is another aspect of the present invention, will be described in detail.

Manufacturing method of high-strength steel sheet with excellent weld strength and formability

2 is a process flow chart schematically showing a method of manufacturing a high-strength steel sheet having excellent weld strength and formability according to an embodiment of the present invention.

The method of manufacturing a high-strength steel sheet having excellent weld strength and formability of the present invention, as a weight%, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8% , Phosphorus (P): greater than 0 and less than 150ppm, sulfur (S): greater than 0 and less than 30ppm, aluminum (Al): 0.2 to 0.5%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.05 to 0.1% , Titanium (Ti): 0.03 ~ 0.06%, boron (B): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05% and the remaining iron (Fe) and preparing a hot-rolled steel sheet containing other inevitable impurities (S210 ); Cold rolling the hot-rolled steel sheet (S220); And annealing heat treatment of the cold-rolled cold-rolled steel sheet (S230).

First, as a step (S210) of preparing a hot-rolled steel sheet, a steel ingot or slab satisfying the above composition (hereinafter, collectively referred to as slab) is prepared, and the slab is heated to 1,180°C or higher to re-use and precipitate segregated components during casting. Re-employment. The reheating temperature may be preferably 1,180 to 1,220°C so as to secure a normal hot rolling temperature, for example. If the reheating temperature is less than 1,180°C, the hot rolling load may rapidly increase, and if it exceeds 1,220°C, the amount of surface scale may increase, leading to material loss.

After the reheating, it is preferable to perform hot rolling by a conventional method, and finish rolling at a temperature range of 880 to 920°C. When the finish rolling temperature exceeds 920°C, there is a concern that the quality of the steel sheet may deteriorate due to the occurrence of surface scale of the steel sheet. In addition, at a rolling temperature of less than 880°C, grains are refined to increase strength, but may cause an increase in rolling load and a decrease in productivity.

Next, the hot-rolled sheet is cooled to a predetermined coiling temperature. The cooling may be air-cooled or water-cooled, and may be cooled at a cooling rate of 30 to 150°C/s. The cooling is preferably cooled to the coiling temperature. In the present invention, it is preferable that the coiling temperature satisfies 560 to 640°C. The coiling temperature is to secure an appropriate amount of ferrite and pearlite, and when the coiling temperature is too high, coarse ferrite and pearlite are produced, making it difficult to secure strength. If the coiling temperature exceeds 640℃, the yield ratio decreases due to the formation of coarse grains, but the toughness decreases and the target strength may be lowered. As it becomes fine, the strength and toughness may increase, but it is difficult to meet the elongation.

After preparing the hot-rolled steel sheet, as a step (S220) of cold-rolling the hot-rolled steel sheet, cold rolling is performed at an average reduction ratio of 40 to 80% after pickling treatment using the hot-rolled steel sheet prepared as above.

After the cold rolling is performed, as an annealing heat treatment step (S230), the cold-rolled cold-rolled steel sheet is annealed in a temperature range of 790 to 860°C in a continuous annealing furnace having a normal slow cooling section. If the annealing temperature is less than 790°C, the risk of generating non-recrystallized grains may increase, and it may be difficult to form sufficient austenite, so that it may be difficult to secure the target strength in the present invention. In addition, when the annealing temperature exceeds 860°C, the amount of bainite increases rapidly due to the formation of excessive austenite, which may lead to excessive increase in yield strength and deterioration of ductility.

The steel sheet manufactured by the method of the present invention described above may have a final structure including ferrite and martensite, and has a characteristic of having a size ratio (SR) of 0.5 or more, which is a ratio of the maximum size to the minimum size of crystal grains. SR represents the ratio of the maximum size to the minimum size when the size of the crystal grains are measured. The smaller this value is, the larger the difference in grain size is and the texture is uneven. It means that the difference is small and the organization is uniform. In the case of the present invention, SR represents 0.5 or more.

On the other hand, in the case of implementing the cold-rolled steel sheet (CR) in this embodiment, the annealing heat treatment step (S230) is performed by heating the cold-rolled steel sheet at a rate of 2 to 8°C/s, heating and maintaining it at a temperature of 790 to 860°C. , It may include the step of heat treatment by rapid cooling to a temperature of 240 ~ 360 ℃ at a rate of 3 ~ 30 ℃ / s.

On the other hand, in the case of implementing the hot-dip galvanized steel sheet (GI) in this embodiment, the annealing heat treatment step (S230) is performed by heating the cold-rolled steel sheet at a rate of 2 to 8°C/s and heating at a temperature of 790 to 860°C. After maintaining, including the step of heat treatment by rapid cooling to a temperature of 460 ~ 520 ℃ at a rate of 3 ~ 30 ℃ / s, and after step (c), dipping the steel sheet in a plating bath of 450 ~ 470 ℃, the Forming a plating layer on the steel plate; may further include. In this case, the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the steel sheet and the plating layer may be 90% or more.

Meanwhile, in the case of implementing the alloyed hot-dip galvanized steel sheet (GA) in this embodiment, the annealing heat treatment step (S230) is performed by heating the cold-rolled steel sheet at a rate of 2 to 8°C/s and heating at a temperature of 790 to 860°C. And then heat-treating by rapid cooling to a temperature of 460 to 520°C at a rate of 3 to 30°C/s, and after step (c), dipping the steel sheet in a plating bath of 450 to 470°C After forming a plating layer on the steel sheet, the step of performing and maintaining alloying heating to 500 ~ 540 ℃; may further include. In this case, the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer may be 95% or more.

Hereinafter, the present invention will be described in detail through experimental examples. However, this is only a preferred experimental example of the present invention, and the scope of the present invention is not limited by the description range of these experimental examples. Contents not described herein can be sufficiently technically inferred by those skilled in the art, and thus the description thereof will be omitted.

Experimental example

Table 1 shows the composition of the steel sheet in the experimental example of the present invention, and Table 2 shows the process conditions in the experimental example of the present invention.

In this experimental example, a steel sheet having the composition (unit: wt% or ppm) shown in Table 1 and subjected to the process conditions shown in Table 2 is prepared. In the categories in Table 2, CR means cold-rolled steel sheet, GI means hot-dip galvanized steel sheet, and GA means alloyed hot-dip galvanized steel sheet. The plating bath for implementing the plated steel sheet has a composition consisting of aluminum (Al): 0.1 to 0.13% by weight and the remainder of zinc (Zn), and the alloying heat treatment temperature for realizing the alloyed hot-dip galvanized steel sheet was applied at 520°C.

Referring to Table 1, the composition of Example 1 is in wt%, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus (P) : More than 0 and less than 150ppm, Sulfur (S): More than 0 and less than 30ppm, Aluminum (Al): 0.2 to 0.5%, Chromium (Cr): 0.8 to 1.2%, Molybdenum (Mo): 0.05 to 0.1%, Titanium (Ti) : 0.03 ~ 0.06%, boron (B): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05%, the balance is Fe satisfies all the composition of the steel sheet according to the technical idea of the present invention.

On the contrary, the composition of the comparative examples in Table 1 does not satisfy all of the composition of the steel sheet according to the technical idea of the present invention. Specifically, the composition of Comparative Example 1 and Comparative Example 2 is antimony (Sb): 0.02 ~ 0.05% The range was not satisfied, and the composition of Comparative Example 3 did not satisfy the range of silicon (Si): 0.3 to 0.7%, aluminum (Al): 0.2 to 0.5%, and antimony (Sb): 0.02 to 0.05%, Comparative Example The composition of 4 does not satisfy the range of aluminum (Al): 0.2 to 0.5% and antimony (Sb): 0.02 to 0.05%, and the composition of Comparative Example 5 does not satisfy the range of titanium (Ti): 0.03 to 0.06%. The composition of Comparative Example 6 is carbon (C): 0.05 ~ 0.09%, sulfur (S): more than 0 30ppm or less, aluminum (Al): 0.2 ~ 0.5%, chromium (Cr): 0.8 ~ 1.2%, titanium ( Ti): 0.03 to 0.06%, boron (B): 10 to 30 ppm and antimony (Sb): do not satisfy the range of 0.02 to 0.05%, and the composition of Comparative Example 7 is titanium (Ti): 0.03 to 0.06% The composition of Comparative Examples 8 and 9 does not satisfy the range of boron (B): 10 ~ 30ppm, and the composition of Comparative Example 10 does not satisfy the range of antimony (Sb): 0.02 ~ 0.05%.

Furtherance C
(wt.%) Si
(wt.%) Mn
(wt.%) P
(wt.%) S
(wt.%) Al
(wt.%) Cr
(wt.%) Mo
(wt.%) Ti
(wt.%) B
(ppm) Sb
(wt.%) Example 1 0.073 0.49 2.45 0.012 0.0020 0.29 0.97 0.06 0.042 23 0.028 Comparative Example 1 0.071 0.47 2.39 0.011 0.0019 0.31 1.01 0.07 0.043 24 0.013 Comparative Example 2 0.075 0.51 2.39 0.013 0.0019 0.31 1.01 0.07 0.039 21 - Comparative Example 3 0.069 1.00 2.41 0.013 0.0021 0.04 0.99 0.06 0.044 23 - Comparative Example 4 0.072 0.52 2.39 0.015 0.0022 0.03 1.03 0.07 0.042 25 - Comparative Example 5 0.072 0.48 2.42 0.014 0.0021 0.32 0.97 0.07 0.023 24 0.032 Comparative Example 6 0.123 0.31 2.38 0.015 0.0034 0.03 0.4 0.07 0.003 5 - Comparative Example 7 0.069 0.53 2.41 0.013 0.0030 0.29 1.01 0.06 0.065 23 0.029 Comparative Example 8 0.074 0.48 2.37 0.012 0.0021 0.31 0.98 0.07 0.038 4 0.028 Comparative Example 9 0.071 0.52 2.42 0.011 0.0019 0.28 1.02 0.06 0.042 46 0.032 Comparative Example 10 0.068 0.47 2.45 0.018 0.0015 0.32 0.99 0.07 0.043 22 0.063

Experimental example division Winding temperature
(℃) Internal oxidation of hot rolled material
Depth (㎛) SS temperature
(℃) RCS temperature
(℃) Example 1-1 CR 600 2.2 830 300 Example 1-2 GI 600 2.5 830 480 Example 1-3 GA 600 2.3 810 480 Comparative Example 1-1 GI 600 3.9 830 480 Comparative Example 2-1 GI 600 6.9 830 480 Comparative Example 2-2 GI 560 5.1 830 480 Comparative Example 2-3 GI 520 2.5 830 480 Comparative Example 3-1 GA 600 7.6 810 480 Comparative Example 4-1 CR 600 3.2 830 300 Comparative Example 4-2 GI 600 3.1 830 480 Comparative Example 4-3 GA 600 2.9 810 480 Comparative Example 5-1 CR 600 2.1 830 300 Comparative Example 5-2 GI 600 2.7 830 480 Comparative Example 5-3 GA 600 2.4 810 480 Comparative Example 6-1 GA 600 3.5 810 480 Comparative Example 7-1 CR 600 2.2 830 300 Comparative Example 7-2 GI 600 2.1 830 480 Comparative Example 7-3 GA 600 1.8 810 480 Comparative Example 8-1 CR 600 2.6 830 300 Comparative Example 8-2 GI 600 2.8 830 480 Comparative Example 8-3 GA 600 2.2 810 480 Comparative Example 9-1 CR 600 2.1 830 300 Comparative Example 9-2 GI 600 2.6 830 480 Comparative Example 9-3 GA 600 2.3 810 480 Comparative Example 10-1 GI 600 2.4 830 480

Referring to Table 2, Example 1-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Example 1 of Table 1, and Example 1-2 is provided with a holding steel sheet that satisfies the composition of Example 1 of Table 1. It is a hot-dip galvanized steel sheet (GI), and Example 1-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Example 1 in Table 1. Comparative Example 1-1 is a hot-dip galvanized steel sheet (GI) provided with a base steel sheet that satisfies the composition of Comparative Example 1 in Table 1. Comparative Example 2-1, Comparative Example 2-2, and Comparative Example 2-3 are hot-dip galvanized steel sheets (GI) having a base steel sheet satisfying the composition of Comparative Example 2 in Table 1. Comparative Example 3-1 is an alloyed hot-dip galvanized steel sheet (GA) provided with a base steel sheet that satisfies the composition of Comparative Example 3 in Table 1. Comparative Example 4-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Comparative Example 4 in Table 1, and Comparative Example 4-2 is a hot-dip galvanized steel sheet (GI ), and Comparative Example 4-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 4 in Table 1. Comparative Example 5-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Comparative Example 5 in Table 1, and Comparative Example 5-2 is a hot-dip galvanized steel sheet (GI ), and Comparative Example 5-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 5 in Table 1. Comparative Example 6-1 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 6 in Table 1. Comparative Example 7-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Comparative Example 7 in Table 1, and Comparative Example 7-2 is a hot-dip galvanized steel sheet (GI ), and Comparative Example 7-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 7 in Table 1. Comparative Example 8-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Comparative Example 8 in Table 1, and Comparative Example 8-2 is a hot-dip galvanized steel sheet (GI ), and Comparative Example 8-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 8 in Table 1. Comparative Example 9-1 is a cold-rolled steel sheet (CR) that satisfies the composition of Comparative Example 9 in Table 1, and Comparative Example 9-2 is a hot-dip galvanized steel sheet (GI ), and Comparative Example 9-3 is an alloyed hot-dip galvanized steel sheet (GA) having a base steel sheet that satisfies the composition of Comparative Example 9 in Table 1. Comparative Example 10-1 is a hot-dip galvanized steel sheet (GI) having a base steel sheet that satisfies the composition of Comparative Example 10 in Table 1.

Referring to Table 2, in Example 1-1, the coiling temperature of the hot-rolled steel sheet: 560 to 640°C, the internal oxidation depth of the hot-rolled material: 5 μm or less, and the heating and holding temperature (SS) in the heat treatment of the cold-rolled steel sheet: 790 to 860°C, In the heat treatment of the cold-rolled steel sheet, the quenching end temperature (RCS): satisfies the range of 240 to 360°C. Example 1-2 is the coiling temperature of the hot-rolled steel sheet: 560 ~ 640 ℃, the internal oxidation depth of the hot-rolled material: 5㎛ or less, the heating holding temperature (SS) in the heat treatment of the hot-dip galvanized steel sheet (GI): 790 ~ 860 ℃, melting In the heat treatment of the galvanized steel sheet (GI), the quenching end temperature (RCS): satisfies the range of 460 ~ 520 ℃. Example 1-3 is the coiling temperature of the hot-rolled steel sheet: 560 to 640 °C, the internal oxidation depth of the hot-rolled material: 5 µm or less, the heating and holding temperature (SS) in the heat treatment of the alloyed hot-dip galvanized steel sheet (GA): 790 to 860 °C, In the heat treatment of the hot-dip galvanized steel sheet (GA), the quenching end temperature (RCS): satisfies the range of 460 ~ 520 ℃.

Meanwhile, most of the comparative examples in Table 2 also include coiling temperature of hot-rolled steel sheet: 560 to 640°C, internal oxidation depth of hot-rolled material: 5µm or less, cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), or alloyed hot-dip galvanized Heat-holding temperature (SS) in heat treatment of steel sheet (GA): 790 to 860°C, quenching end temperature (RCS) in heat treatment of cold-rolled steel sheet (CR): 240 to 360°C, hot-dip galvanized steel sheet (GI) or molten zinc alloy In the heat treatment of the plated steel sheet (GA), the quenching end temperature (RCS): satisfies the range of 460 ~ 520 ℃. However, the coiling temperature of the hot-rolled steel sheet in Comparative Example 2-3: does not satisfy the range of 560 to 640°C, and the internal oxidation depth of the hot-rolled material in Comparative Examples 2-1, 2-2, and 3-1: 5㎛ or less Does not satisfy the range of.

Table 3 shows the results of physical properties and plating peeling according to the experimental example of the present invention. In the section of plating peeling,'OK' means that plating peeling does not occur, and the'NG' item means that plating peeling occurs.

Referring to Table 3, the cold-rolled steel sheet (CR) according to Example 1-1 has yield strength (YP): 600 MPa or more, tensile strength (TS): 1000 to 1180 MPa, elongation (EL): 11% or more, and bendability ( R/t): 0.6 or less, and the cross tension strength (CTS) as the weld strength: satisfies the range of 1000 kgf/spot or more.

The hot-dip galvanized steel sheet (GI) according to Example 1-2 has yield strength (YP): 600 MPa or more, tensile strength (TS): 1000 to 1180 MPa, elongation (EL): 11% or more, and bendability (R/t) : 0.6 or less, cross-tension strength (CTS) as the strength of the welded portion: satisfies the range of 1000 kgf/spot or more, and the fraction of the Fe-Al compound present in the interface region between the base steel sheet and the plating layer (inhibition layer coverage) It is 90% or more, and the plating peeling property of the plating layer is excellent.

The alloyed hot dip galvanized steel sheet (GA) according to Example 1-3 has yield strength (YP): 600 MPa or more, tensile strength (TS): 1000 to 1180 MPa, elongation (EL): 11% or more, and bendability (R/t ): 0.6 or less, the cross tension strength (CTS) as the strength of the welded portion: satisfies the range of 1000 kgf/spot or more, and the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer is 95% or more.

Experimental example division Inhibition Layer Coverage
(%) GA alloying
Area fraction
(%) Size
Ratio YP
(MPa) TS
(MPa) EL
(%) 90 ^o
Bendability
(R/t) Plating peeling CTS
(Kgf/spot) Example 1-1 CR - - 0.70 811 1070 13.1 0.4 - 1520 Example 1-2 GI 97 - 0.68 760 1093 12.3 0.4 OK 1492 Example 1-3 GA - 100 0.65 650 1030 15.2 0.4 - 1489 Comparative Example 1-1 GI 70 - 0.68 770 1089 12.6 0.4 NG 1482 Comparative Example 2-1 GI 22 - 0.66 774 1079 12.5 0.4 NG 1521 Comparative Example 2-2 GI 23 - 0.62 769 1082 12.9 0.4 NG 1534 Comparative Example 2-3 GI 21 - 0.64 768 1089 12.7 0.4 NG 1537 Comparative Example 3-1 GA - 16.3 0.62 671 1045 14.2 0.4 - 1395 Comparative Example 4-1 CR - - 0.71 890 1136 8.7 0.6 - 1486 Comparative Example 4-2 GI 21 - 0.64 841 1143 7.5 0.6 NG 1477 Comparative Example 4-3 GA - 100 0.60 744 1081 9.0 0.8 - 1501 Comparative Example 5-1 CR - - 0.31 689 1001 15.1 1.4 - 1492 Comparative Example 5-2 GI 97 - 0.29 679 1008 14.9 1.4 OK 1472 Comparative Example 5-3 GA - 100 0.22 582 978 15.2 1.6 - 1469 Comparative Example 6-1 GA - 100 0.23 620 1021 14.4 1.8 - 926 Comparative Example 7-1 CR - - 0.62 902 1165 10.1 1.4 - 1500 Comparative Example 7-2 GI 98 - 0.65 910 1172 9.9 1.4 OK 1498 Comparative Example 7-3 GA - 100 0.72 850 1142 10.9 1.4 - 1518 Comparative Example 8-1 CR - - 0.67 692 1012 14.2 1.0 - 1543 Comparative Example 8-2 GI 98 - 0.62 686 997 14.9 1.0 OK 1531 Comparative Example 8-3 GA - 100 0.72 582 978 16.2 1.2 - 1528 Comparative Example 9-1 CR - - 0.77 912 1182 8.2 1.6 - 1522 Comparative Example 9-2 GI 38 - 0.68 905 1179 9.8 1.6 NG 1499 Comparative Example 9-3 GA - 100 0.61 702 1102 10.2 1.8 - 1527 Comparative Example 10-1 GI 97 - 0.72 774 1082 13.2 0.42 NG 1499

On the other hand, the comparative examples in Table 3 do not satisfy all of the above-described characteristics, specifically, the hot-dip galvanized steel sheet according to Comparative Example 1-1, Comparative Example 2-1, Comparative Example-2, and Comparative Example-3. (GI) does not satisfy the percentage of Fe-Al compound present in the interface region between the base steel sheet and the plating layer (inhibition layer coverage): 90% or more, and does not have excellent plating peeling properties of the plating layer. The alloyed hot-dip galvanized steel sheet (GA) according to Comparative Example 3-1 does not satisfy the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer: 95% or more. The cold-rolled steel sheet (CR) according to Comparative Example 4-1 does not satisfy the range of elongation (EL): 11% or more. In the hot-dip galvanized steel sheet (GI) according to Comparative Example 4-2, the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the base steel sheet and the plating layer (inhibition layer coverage): 90% or more, elongation (EL): 11% or more It does not satisfy the range, and the plating peeling property of the plating layer is not excellent. The alloyed hot-dip galvanized steel sheet (GA) according to Comparative Example 4-3 does not satisfy the range of elongation (EL): 11% or more and bendability (R/t): 0.6 or less. The cold-rolled steel sheet (CR) according to Comparative Example 5-1 does not satisfy the range of SR (size ratio): 0.5 or more, and bendability (R/t): 0.6 or less, which is the ratio of the minimum size and maximum size of crystal grains. The hot-dip galvanized steel sheet (GI) according to Comparative Example 5-2 does not satisfy the range of SR (size ratio): 0.5 or more, and bendability (R/t): 0.6 or less, which is the ratio of the minimum size and maximum size of crystal grains. can not do it. The alloyed hot-dip galvanized steel sheet (GA) according to Comparative Example 5-3 is SR (size ratio), which is the ratio of the minimum size and maximum size of crystal grains: 0.5 or more, yield strength (YP): 600 MPa or more, tensile strength (TS): 1000 to 1180 MPa, and bendability (R/t): does not satisfy the range of 0.6 or less. The alloyed hot dip galvanized steel sheet (GA) according to Comparative Example 6-1 is SR (size ratio), which is the ratio of the minimum size and maximum size of crystal grains: 0.5 or more, bendability (R/t): 0.6 or less, and crosswise as the weld strength. Tensile strength (CTS): It does not satisfy the range of 1000 kgf/spot or more. Elongation (EL) of the cold-rolled steel sheet (CR) according to Comparative Example 7-1, the hot-dip galvanized steel sheet (GI) according to Comparative Example 7-2, and the alloyed hot-dip galvanized steel sheet (GA) according to Comparative Example 7-3: 11 % Or more and bendability (R/t): It does not satisfy the range of 0.6 or less. The cold-rolled steel sheet (CR) according to Comparative Example 8-1 does not satisfy the range of bendability (R/t): 0.6 or less, and the hot-dip galvanized steel sheet (GI) according to Comparative Example 8-2 and Comparative Example 8-3 The alloyed hot-dip galvanized steel sheet (GA) does not satisfy the range of tensile strength (TS): 1000 to 1180 MPa and bendability (R/t): 0.6 or less. Elongation (EL) of the cold-rolled steel sheet (CR) according to Comparative Example 9-1, the hot-dip galvanized steel sheet (GI) according to Comparative Example 9-2, and the alloyed hot-dip galvanized steel sheet (GA) according to Comparative Example 9-3: 11 % Or more and bendability (R/t): It does not satisfy the range of 0.6 or less. In addition, the hot-dip galvanized steel sheet (GI) according to Comparative Example 9-2 and the hot-dip galvanized steel sheet (GI) according to Comparative Example 0-1 do not have excellent plating peeling properties of the plating layer.

So far, the results of the experimental examples of the present invention have been described. In particular, in the present invention, antimony (Sb) was added to improve the plating properties of the steel sheet by suppressing the formation of surface oxides or nitrides.

Figure 3 is a graph showing the relationship of the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the substrate and the plating layer according to the content of antimony in the experimental example of the present invention, Figure 4 is an experiment of the present invention In the example, these are pictures of observing the plating peeling characteristics according to the content of antimony.

3, plating peeling occurs when the content of antimony (Sb) is 0% by weight and 0.01% by weight, whereas plating peeling occurs when the content of antimony (Sb) is 0.03% by weight and 0.05% by weight. It can be seen that it is not.

Referring to FIG. 4, when antimony (Sb) is not added, the fraction of the Fe-Al compound present in the interface region between the base steel sheet and the plating layer is relatively low, whereas plating peeling occurs, antimony In the case of containing 0.03% by weight of (Sb), it can be seen that the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the holding steel sheet and the plating layer is relatively high, and plating peeling does not occur.

Meanwhile, according to Comparative Example 10-1 of Table 3, it was confirmed that plating peeling occurred in a hot-dip galvanized steel sheet (GI) having an antimony (Sb) content of 0.063% by weight.

Considering all these results, it is preferable to limit the amount of antimony (Sb) added to 0.02 to 0.05% by weight of the entire steel sheet in order to secure plating properties by suppressing the generation of surface oxides and nitrides in the steel sheet according to the embodiment of the present invention. I can understand.

5 are photographs illustrating an area fraction of a Zn-Fe alloyed portion on the surface of a plating layer in an experimental example of the present invention.

Referring to Figure 5 (a), the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer of the alloyed hot dip galvanized steel sheet (GA) according to Comparative Example 3-1 among the experimental examples of the present invention is 16.3%, whereas, Referring to FIG. 5B, it can be seen that the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer of the alloyed hot-dip galvanized steel sheet (GA) according to Example 1-3 among the experimental examples of the present invention is 100%. .

6 and 7 are diagrams illustrating a comparison of a size ratio (SR), which is a ratio between a minimum size and a maximum size of a steel plate. In order to calculate the SR (size ratio), which is the ratio of the minimum and maximum sizes of the grains of the steel plate, the diameter of the grains is measured when the grains are circular, the length of the long axis is measured in the case of an ellipse, and the longest axis in the case of polygons. The diagonal length was measured, and if there was a band, the band length was measured.

Referring to FIG. 6, the size ratio (SR), which is the ratio of the minimum and maximum crystal grain sizes of the steel sheet, has a value of 0.08 since the minimum size of the crystal grains is 3 μm and the maximum size of the crystal grains is 36 μm. On the other hand, referring to FIG. 7, the size ratio (SR), which is the ratio of the minimum and maximum sizes of the crystal grains of the steel sheet, has a value of 0.6 since the minimum size of the crystal grains is 3 μm and the maximum size of the crystal grains is 5 μm. The smaller the size ratio (SR) is, the larger the difference in the size of the tissue is, indicating that the tissue is uneven, and the closer the size ratio (SR) is to 1, the smaller the difference in the size of the tissue is, indicating that the tissue is uniform.

In the above, the embodiments of the present invention have been described mainly, but various changes or modifications may be made at the level of those skilled in the art. These changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims (15)

In% by weight, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus (P): more than 0 and not more than 150 ppm, sulfur (S): 0 More than 30ppm, aluminum (Al): 0.2 ~ 0.5%, chromium (Cr): 0.8 ~ 1.2%, molybdenum (Mo): 0.05 ~ 0.1%, titanium (Ti): 0.03 ~ 0.06%, boron (B): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05%, the balance consists of Fe and inevitable impurities,
The steel sheet has a yield strength (YP) of 600 MPa or more, a tensile strength (TS) of 1000 to 1180 MPa, an elongation (EL) of 11% or more, and a bendability (R/t) of 0.6 or less,
Grater. The method of claim 1,
The steel plate is characterized in that the cross tension strength (CTS) as the strength of the weld is 1000 kgf/spot or more,
Grater. The method of claim 1,
The steel sheet has a carbon equivalent defined as [C]+[Mn]/20+[Si]/30+2[P]+4[S] (the above [C], [Mn], [Si], [P] And [S] is characterized in that the weight ratio of carbon, manganese, silicon, phosphorus and sulfur, respectively) is 0.25 or less,
Grater. The method of claim 1,
The microstructure of the steel sheet includes ferrite, martensite and bainite,
Grater. The method of claim 4,
The steel sheet is characterized in that the fraction of ferrite is less than 60%, the fraction of martensite is 40% or more, and the fraction of bainite is 5% or less,
Grater. The method of claim 4,
The steel sheet is characterized in that the average size of ferrite is 10 μm or less, the average size of martensite is 10 μm or less, and the average size of bainite is 10 μm or less,
Grater. The method of claim 1,
The steel plate is characterized in that the SR (size ratio), which is a ratio of the minimum size and the maximum size of crystal grains, is 0.5 or more. (a) By weight%, carbon (C): 0.05 to 0.09%, silicon (Si): 0.3 to 0.7%, manganese (Mn): 2.0 to 2.8%, phosphorus (P): more than 0 and not more than 150 ppm, sulfur (S ): greater than 0 and less than 30ppm, aluminum (Al): 0.2 to 0.5%, chromium (Cr): 0.8 to 1.2%, molybdenum (Mo): 0.05 to 0.1%, titanium (Ti): 0.03 to 0.06%, boron (B ): 10 ~ 30ppm, antimony (Sb): 0.02 ~ 0.05%, the balance of preparing a hot-rolled steel sheet having an alloy composition consisting of Fe and inevitable impurities;
(b) cold-rolling the hot-rolled steel sheet to prepare a cold-rolled steel sheet;
(c) heat-treating the cold-rolled steel sheet by heating and maintaining it at a temperature of 790 to 860°C; Containing,
Method of manufacturing a steel plate. The method of claim 8,
The step (a),
(a-1) preparing a steel slab having the alloy composition;
(a-2) reheating the steel slab at 1180°C or higher;
(a-3) manufacturing a hot-rolled steel sheet by hot finish rolling the reheated steel slab at 880 to 920°C; And
(a-4) winding the hot-rolled steel sheet at 560 ~ 640 °C; containing,
Method of manufacturing a steel plate. The method of claim 8,
The step (b) is characterized in that the cold rolling reduction ratio is 40 to 80%,
Method of manufacturing a steel plate. The method of claim 8,
In the step (c), the cold-rolled steel sheet is heated at a rate of 2 to 8°C/s, heated and maintained at a temperature of 790 to 860°C, and then maintained at a temperature of 240 to 360°C at a rate of 3 to 30°C/s. Including the step of quenching heat treatment,
Method of manufacturing a steel plate. The method of claim 8,
In step (c), the cold-rolled steel sheet is heated at a rate of 2 to 8°C/s, heated at a temperature of 790 to 860°C, and maintained, and then at a rate of 3 to 30°C/s to a temperature of 460 to 520°C. Including the step of quench heat treatment,
After the step (c), forming a plating layer on the steel sheet by dipping the steel sheet in a plating bath of 450 ~ 470 ℃; further comprising,
Method of manufacturing a steel plate. The method of claim 12,
Characterized in that the fraction (inhibition layer coverage) of the Fe-Al compound present in the interface region between the steel plate and the plating layer is 90% or more,
Method of manufacturing a steel plate. The method of claim 8,
In step (c), the cold-rolled steel sheet is heated at a rate of 2 to 8°C/s, heated at a temperature of 790 to 860°C, and maintained, and then at a rate of 3 to 30°C/s to a temperature of 460 to 520°C. Including the step of quench heat treatment,
After the step (c), dipping the steel sheet in a plating bath of 450 to 470 °C to form a plating layer on the steel sheet and then performing and maintaining alloying heating to 500 to 540 °C; further comprising,
Method of manufacturing a steel plate. The method of claim 14,
Characterized in that the area fraction of the Zn-Fe alloyed portion on the surface of the plating layer is 95% or more,
Method of manufacturing a steel plate.

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