KR20190137750A - Cold rolled steel sheet having excellent bake hardenability and corrosion resistance, and method for manufacturing the same - Google Patents

Abstract

Provided is a cold-rolled steel sheet having improved bake hardenability and corrosion resistance, and a manufacturing method therefor. The cold-rolled steel sheet comprises: by wt %, at least one selected from the group consisting of carbon (C) of 0.005-0.08%, manganese (Mn) of 1.25% or less (excluding 0%), phosphorus (P) of 0.03% or less (excluding 0%), sulfur (S) of 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), soluble aluminum (sol.Al) of 0.01 to 0.06%, chromium (Cr) of 1.0% to 2.5%, antimony (Sb) of 0.1% or less (excluding 0%), Ni of 0.3% or less (excluding 0%), Si of 0.3% or less (excluding 0%), Mo of 0.2% or less (excluding 0%), and B of 0.003% or less At (excluding 0%); and the remainder of iron (Fe) and inevitable impurities, and the cold-rolled steel sheet satisfies the following relational expression 1, and contains, by area %, martensite of 1-5% and the remainder of ferrite. [Relational Expression 1] 1.3 <= Mn(wt%)+Cr(wt%)/1.5+Sb(wt%) <= 2.7, where Mn, Cr, and Sb each represent the content (wt%) of the element.

Description

COLD ROLLED STEEL SHEET HAVING EXCELLENT BAKE HARDENABILITY AND CORROSION RESISTANCE, AND METHOD FOR MANUFACTURING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used for automobile exterior panel materials and the like, and more particularly, to a cold rolled steel sheet excellent in hardening hardening, corrosion resistance, and age resistance, and a method of manufacturing the same.

As the impact stability and fuel efficiency of automobiles are emphasized, high-strength steels are being actively used to satisfy both the weight reduction and the high strength of the automobile body.

Currently, 340MPa grade hardened hardened steel is being applied to the exterior of automobiles, but some 490MPa grade steel sheets are also being applied, and are expected to be expanded to 590MPa grade steel sheets in the future.

As such, when the steel sheet having increased strength is applied to the outer plate, the weight reduction and the dent resistance are improved, while the formability is inferior in processability as the strength increases. Therefore, in recent years, the customer demands a steel sheet having a low yield ratio (YR = YS / TS) and excellent ductility in order to compensate for insufficient workability while applying high strength steel to the outer shell.

In addition, it is required to have a certain degree of hardening hardening to a certain level in order to apply to the material for the automotive panel. The hardening hardening phenomenon is a phenomenon in which the yield strength increases due to the solid solution of activated carbon and nitrogen attached to the potential generated during the pressing process. Due to its improved properties, it is ideal for automotive exterior paneling materials. In addition, it is required to have a certain level of aging resistance to guarantee the aging for a certain period or more in order to apply the material for the automotive exterior panel.

Patent documents 1 to 3 and the like are known as prior arts for improving workability in high tensile steel sheets, and Patent Document 1 discloses a steel sheet having a composite structure mainly composed of martensite, and has a particle diameter of 1 to 100 nm in the structure for improving workability. The manufacturing method of the high tension steel plate which disperse | distributed the fine Cu precipitate of this is disclosed. However, this technique needs to add an excess of 2 to 5% of Cu in order to precipitate fine Cu particles, which may cause red brittleness resulting from Cu and excessively increase manufacturing costs.

Patent Document 2 discloses a composite steel sheet comprising ferrite as a main phase, residual austenite as a two phase, and bainite and martensite as a low temperature transformation phase, and a method for improving the ductility and extension flange of the steel sheet. However, this technique has a problem in that it is difficult to secure the plating quality by adding a large amount of Si and Al to secure the retained austenite phase, and it is difficult to secure the surface quality during steelmaking and performance. In addition, due to the metamorphic organic plasticity has a high yield ratio high initial YS value.

Patent Document 3 is a technique for providing a high-strength hot-dip galvanized steel sheet with good workability, a steel sheet comprising a composite of soft ferrite and hard martensite as a microstructure, and to improve its elongation and r value (Lankford value) A manufacturing method is disclosed. However, this technique not only ensures excellent plating quality as a large amount of Si is added, but also causes a problem in that the manufacturing cost increases due to the addition of a large amount of Ti and Mo.

Japanese Laid-Open Patent Publication No. 2005-264176 Japanese Laid-Open Patent Publication No. 2004-292891 Republic of Korea Patent Publication No. 2002-0073564

One preferred aspect of the present invention is to provide a cold rolled steel sheet excellent in hardening hardening, corrosion resistance and aging resistance and a method of manufacturing the same.

One preferred aspect of the present invention is by weight, carbon (C): 0.005 ~ 0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (excluding 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); and Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: It contains at least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities, satisfying the following relation 1, with a microstructure, in area%, 1 to 5% Martens Provided is a cold rolled steel sheet having excellent bake hardening resistance and corrosion resistance including a sight and a residual ferrite.

 [Relationship 1]

1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7

(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)

The cold rolled steel sheet may have a yield strength of 210 to 270 MPa and a yield ratio of less than or equal to 0.6 (YS / TS).

Another preferred aspect of the present invention by weight, carbon (C): 0.005 ~ 0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (excluding 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); and Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: Reheating the slab comprising at least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities and satisfying the following relational formula 1;

[Relationship 1]

1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7

(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)

Hot rolling the reheated slab at a temperature range of 850 to 1150 ° C. to obtain a hot rolled steel sheet;

Cooling the hot rolled steel sheet at an average cooling rate of 10 to 70 ° C / sec to a temperature range of 550 to 750 ° C;

Winding the cooled hot rolled steel sheet in a temperature range of 550 to 750 ° C;

Cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;

Continuously annealing the cold rolled steel sheet at a hydrogen concentration of 3 to 30% by volume in a temperature range of Ac ₁ + 20 ° C to Ac ₃ -20 ° C; And

It provides a method of producing a cold-rolled steel sheet excellent in hardening hardening and corrosion resistance including the step of first cooling the continuous annealing cold rolled steel sheet to an average cooling rate of 2 ~ 10 ℃ / sec to 630 ~ 670 ℃.

The cold rolling reduction rate may be set to 40 to 80%.

The cold rolling may be carried out using a rolling mill composed of 5 or 6 stands, and at this time, the initial stand reduction rate may be set to 25 to 37%.

As one of various effects of the present invention, the cold rolled steel sheet according to a preferred aspect of the present invention is excellent in hardening hardening resistance, corrosion resistance and aging resistance, and can be preferably applied to materials for automobile exterior panels.

The present invention is not only excellent in formability by securing the strength and ductility at the same time to be suitable as a material for automotive exterior panel, but also to provide a cold-rolled steel sheet excellent in hardening hardening, corrosion resistance and aging resistance, and as a result It is based on.

The present invention provides a steel sheet having better corrosion resistance while properly controlling the composition range and the microstructure of the steel sheet to ensure material properties equal to or greater than those of the existing steel sheet.

The present invention provides a steel sheet which further improves plating adhesion and corrosion resistance by inducing Sb segregation at the interface between martensite and ferrite grain boundaries and suppressing surface elution of Mn, Cr, etc. during annealing.

The present invention provides a steel sheet having superior corrosion resistance and plating adhesion as well as excellent bake hardenability and plating adhesion by appropriately adding Sb by optimizing the relative ratio of Mn and Cr, which is an element for improving hardenability.

The present invention provides a steel sheet excellent in strength, ductility, and moldability as well as excellent plating adhesion, baking hardening resistance, and aging resistance by controlling the composition range and manufacturing conditions of the steel sheet appropriately.

Below. The cold rolled steel sheet excellent in the hardening hardening resistance and corrosion resistance according to a preferred aspect of the present invention will be described.

Cold rolled steel sheet excellent in hardening hardening and corrosion resistance according to a preferred aspect of the present invention by weight, carbon (C): 0.005 ~ 0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (excluding 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: At least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities, satisfying the following relation 1, the area%, 1 to 5% of martensite and residual ferrite It includes.

 [Relationship 1]

1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7

(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)

Hereinafter, the alloy component and the preferred content range of the cold rolled steel sheet will be described in detail. It is noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.005-0.08%

Carbon (C) is an essential element added in order to secure the desired composite structure in the present invention, and in general, as the content of carbon increases, martensite is easier to form, which is advantageous for producing a composite structure steel, but its intended strength and yield In order to secure the ratio (yield strength / tensile strength), it is required to manage the proper amount. If the carbon content is less than 0.005%, it may be difficult to secure the strength targeted in the present invention, and it may be difficult to form an appropriate level of martensite. On the other hand, when the content exceeds 0.08%, grain boundary bainite is promoted during cooling after annealing, so that the yield ratio of steel is increased, and bending and surface defects are easily generated during processing into automotive parts. Therefore, in the present invention, the carbon content is set to 0.005 to 0.08%, more preferably 0.007 to 0.06%.

Mn: 1.25% or less (except 0%)

Manganese (Mn) is an element that improves the hardenability in the composite steel, especially an element that plays an important role in forming martensite. However, when the manganese content is increased, Mn oxide is eluted to the surface of the steel sheet during annealing to infer plating adhesiveness, and inferior to weldability when welding after forming parts, and in particular, it is inferior to corrosion resistance due to the inferior adhesion between the base steel and the plating layer. Molten steel can be difficult to apply. The inventors conducted various experiments on hardenable elements such as Mn, Cr, and B for forming fine martensite using ultra low carbon steel, and at room temperature aging due to the formation of fine martensite when forming steel using Cr as a hardening element. It was confirmed that not only the hardened hardened steel which can be guaranteed but also excellent in corrosion resistance was reached.

If the manganese content is greater than 1.25% to form fine martensite, the Cr content is relatively low, so that corrosion resistance is inferior even if the required material properties are secured, resulting in a result different from the object of the present invention. The manganese content is set to less than 1.25% to confirm that the corrosion resistance is improved when manufacturing steel, and the lower limit is not particularly defined (except 0%). Therefore, in the present invention, the content of manganese is controlled to 1.25% or less, more preferably, 0.5 to 1.0%.

P: 0.03% or less (except 0%)

Phosphorus (P) is the most advantageous element to secure the strength without significantly deteriorating the formability, but when excessively added, the possibility of brittle fracture is greatly increased, which greatly increases the possibility of plate breakage of the slab during hot rolling. Since it can inhibit, in the present invention, the phosphorus content is controlled to 0.03% or less.

S: 0.01% or less (except 0%)

Sulfur (S) is an impurity inevitably included in the steel, it is preferable to manage the content as low as possible. In particular, sulfur in the steel increases the possibility of generating red brittle, so the content is controlled to 0.01% or less.

N: 0.01% or less (except 0%)

Nitrogen is an inevitable impurity contained in steel, and it is important to keep the content as low as possible, but for this purpose, the refining cost of the steel is rapidly increased, so the operating conditions are controlled at 0.01% or less, which is possible.

Al (sol.Al): 0.01 ~ 0.06%

Acid soluble aluminum (sol.Al) is an element added to refine the particle size and deoxidation, and when the content is less than 0.01%, aluminum-killed steel cannot be manufactured in a normal stable state. If the amount exceeds 0.06%, it is advantageous to increase the strength due to the effect of grain refinement, but it is not only possible to cause excessive inclusions due to excessive inclusions during steelmaking operation, which leads to a sudden increase in manufacturing cost. have. Therefore, in the present invention, the content of acid soluble aluminum (sol.Al) is controlled to 0.01 to 0.06%.

Cr: 1.0-2.5%

Chromium (Cr) is a component having properties similar to manganese and is an element added to improve the hardenability of the steel and the strength of the steel. In addition, chromium helps to form martensite and yields stretch point yield (YP-El) by forming coarse Cr-based carbides such as Cr ₂₃ C ₆ during hot rolling to precipitate the amount of dissolved carbon in steel below an appropriate level. It is an element that is advantageous for producing a composite tissue steel with low yield ratio. In addition, chromium is an advantageous element for producing high-strength composite tissue steel having high ductility by minimizing ductility drop relative to strength increase. However, when the content is less than 1.0%, the required martensite structure is not formed, and when it exceeds 2.5%, the excessive martensite fraction causes an increase in strength and a decrease in elongation. Control at 2.5%. More preferably, it is 1.3 to 1.8%.

Sb: 0.1% or less (except 0%)

Antimony (Sb) is an element that plays an important role in the present invention. In the present invention, the carbon is as low as possible, preferably 0.005% to 0.04%, and fine M (martensite) using hardenable elements such as Mn and Cr. By distributing the phase in the steel, it is possible to produce a hardened hardened steel excellent in aging resistance.

However, Mn and Cr may cause a problem of plating peeling because Mn and Cr-based oxides are eluted from the surface layer during annealing, thereby inferring adhesion during plating. Thus, a small amount of Sb is added to preferentially segregate at the grain boundaries of the M (martensite) phase, thereby preventing Mn and Cr from moving along the grain boundaries, thereby improving the plating surface quality. Even if a small amount of Sb is added, a sufficient effect can be obtained, and no lower limit is specifically set except for 0%. If the content exceeds 0.1%, excessive Sb is present, which increases the alloy cost and the possibility of surface cracking in hot rolling. High limit to 0.1%. More preferably, it is advantageous to limit to 0.005 to 0.04%.

Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: 0.003% or less (except 0%) More than

Ni: 0.3% or less (except 0%)

Nickel (Ni) is an alloying element commonly used with chromium and is used for reinforcing the base because it makes the structure of steel fine and is well employed in austenite and ferrite. When coexisted with chromium or molybdenum, it shows excellent hardening ability and is useful for improving corrosion resistance. If the content is more than 0.3%, the corrosion resistance is advantageous, but the manufacturing cost increase burden and adversely affect the welding properties, so the upper limit is limited to 0.3% or less. In the present invention, since the Ni effect is beneficial to improve the corrosion resistance by interacting with Cr even if a small amount is added, the lower limit value is not particularly limited, and considering the economic aspect, the addition amount is 0.3% or less (excluding 0%). More preferably, 0.03 to 0.1% is preferable.

Si: 0.3% or less (except 0%)

Silicon (Si) contributes to the increase in strength of the steel sheet by solid solution strengthening, but is not intentionally added in the present invention, even if silicon is not added, there is no major problem in terms of securing physical properties. However, 0% is excluded in consideration of the amount inevitably added during manufacture. On the other hand, when the silicon content exceeds 0.3% there is a problem that the plating surface properties are inferior, in the present invention, the silicon content is controlled to 0.3% or less.

Mo: 0.2% or less (except 0%)

Molybdenum (Mo) is an element added to delay the transformation of austenite into pearlite and to refine the ferrite and improve the strength of the steel. Molybdenum also helps to improve the hardenability of the steel. However, when the content of molybdenum exceeds 0.2%, there is a problem that not only the economic efficiency is lowered due to a sharp increase in manufacturing cost, but also the ductility of the steel is lowered, in the present invention, the content of molybdenum is controlled to 0.2% or less. On the other hand, the lower limit thereof is not particularly limited because the effect is high even when the trace amount is added. More preferably, however, it is set to 0.005 to 0.1%.

B: 0.003% or less (except 0%)

Boron (B) is an element added to prevent secondary work brittleness due to phosphorus in steel, but there is no major obstacle in terms of securing physical properties even without adding boron. On the other hand, if the content of boron exceeds 0.003% may cause ductility degradation of the steel, in the present invention, the content of boron is controlled to 0.003% or less.

In addition to the balance Fe and unavoidable impurities. However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, not all of them are specifically mentioned in the present specification. In addition, addition of an effective component other than the said composition is not excluded.

Mn, Cr, and Sb satisfy the following Equation 1.

 [Relationship 1]

1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7

(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)

 The relational formula 1 is a relational expression obtained by confirming the hardenability according to the steel component in various conditions under various conditions, and the optimum design range was derived by lowering the Mn content as much as possible and increasing the Cr content. Based on the same carbon content, a combination of components that can improve corrosion resistance while securing equivalent mechanical properties to Mn content in steel can secure required properties when Cr is 1.5 times the Mn content. It was confirmed. In particular, it is possible to improve plating adhesion and corrosion resistance while adding Sb while suppressing elution to the surface during annealing of Mn in the steel.

In the above relation 1, there is a problem in that yield ratio is increased because the required martensite cannot be formed and room temperature aging resistance is inferior, and in the case of more than 2.7, the yield strength as well as the increase in yield strength are increased due to the addition of excess components. It is preferable to limit the range to 1.3-2.7, since the possibility of a process crack at the time of forming a part is more preferable, It is 1.8-2.5.

Cold rolled steel sheet excellent in hardening hardening and corrosion resistance according to a preferred aspect of the present invention as the microstructure, the area%, 1 to 5% of the martensite and the residual ferrite.

If the area ratio of martensite is less than 1%, it is difficult to obtain a steel sheet having a low yield ratio due to the formation of a complex structure. Especially, if the martensite (M) content is less than 1%, C contained in the steel does not sufficiently aggregate into the martensite phase. Since it remains in the ferrite phase, it is not only difficult to secure aging resistance at room temperature but also lowers the hardening resistance. On the other hand, if it exceeds 5%, it may be difficult to secure the desired workability due to excessive increase in strength. According to one example, when the content of martensite in the steel sheet is less than 1%, solid solution carbon contained in steel may be present in ferrite first rather than in martensite, thereby making it difficult to secure aging resistance at room temperature. This resulted in an experimental result that the room temperature aging resistance is reduced because the solid solution C present in the ferrite phase is easily transported and fixed to potential, resulting in aging inferiority. Yield strength is excessively increased and cracking increases during processing, so the upper limit is limited to 5%.

Therefore, it is preferable that it is 1-5 area%, and, as for the area ratio of martensite, it is more preferable that it is 1.5-3%.

The cold rolled steel sheet may have a yield strength of 210 to 270 MPa and a yield ratio of less than or equal to 0.6 (YS / TS).

The present invention includes the cold rolled steel sheet and the hot dip galvanized layer formed on the surface of the cold rolled steel sheet.

In the present invention, the composition of the hot dip galvanized layer is not particularly limited, and may be a pure galvanized layer or a zinc-based alloy plated layer containing Si, Al, Mg, or the like. The hot dip galvanized layer may be an alloyed hot dip galvanized layer.

The plated steel sheet may have a yield strength of 210 to 270 MPa and a yield ratio of less than or equal to 0.6 (YS / TS).

The hot-rolled galvanized steel sheet including the cold-rolled steel sheet of the present invention described above and the hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet may be manufactured by various methods, and the production method is not particularly limited. However, as a preferred example, it may be prepared by the following method.

Hereinafter, another preferred aspect of the present invention will be described in detail a method for producing a cold rolled steel sheet excellent in hardening hardening and corrosion resistance and a method for producing a hot dip galvanized steel sheet.

According to another preferred aspect of the present invention, a method for preparing a cold rolled steel sheet having excellent hardening hardening resistance and corrosion resistance is% by weight, carbon (C): 0.005 to 0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (excluding 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); and Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: Reheating the slab comprising at least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities and satisfying the following relational formula 1;

[Relationship 1]

1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7

(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)

Hot rolling the reheated slab at a temperature range of 850 to 1150 ° C. to obtain a hot rolled steel sheet;

Cooling the hot rolled steel sheet at an average cooling rate of 10 to 70 ° C / sec to a temperature range of 550 to 750 ° C;

Winding the cooled hot rolled steel sheet in a temperature range of 550 to 750 ° C;

Cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;

The step of continuously annealing the cold-rolled steel sheet in a hydrogen concentration of 3 to 30% by volume in a temperature range of _{Ac 1 + 20 ℃ ~ A c} 3-20 ℃; And

And primary cooling the continuously annealed cold rolled steel sheet at an average cooling rate of 2 to 10 ° C / sec to 630 to 670 ° C.

Slab reheating stage

First, the slab having the above-described component system is reheated. Slab reheating temperature is preferably set to 1180 ~ 1350 ℃.

This process is performed in order to perform the following hot rolling process smoothly, and to fully acquire the physical property of the target steel plate. In this case, when the reheating temperature is less than 1180 ° C, oxides such as Mn and Cr are not sufficiently re-dissolved, which may cause material deviations and surface defects after hot rolling. Thus, the reheating temperature is preferably 1180 ° C or more and exceeds 1350 ° C. In this case, since the strength is lowered by abnormal grain growth of the austenite grains, it is preferable to limit the temperature to 1180 to 1350 ° C.

Obtaining Hot Rolled Steel Sheets

The reheated steel slab as described above is hot rolled at a temperature in the range of 850 ~ 1150 ℃ to obtain a hot rolled steel sheet. At this time, the hot finish rolling temperature is more than Ar ₃ temperature.

 When the hot rolling is started at a temperature higher than 1150 ° C, the temperature of the hot rolled steel sheet is increased, resulting in coarse grain size and poor surface quality of the hot rolled steel sheet. In addition, when the hot rolling is finished at a temperature lower than 850 ° C., the development and high yield ratio of the stretched grains are obtained due to excessive recrystallization delay, resulting in inferior cold rolling property and poor shearing property.

Cooling and winding stage of hot rolled steel

The hot rolled steel sheet is cooled to an average cooling rate of 10 to 70 ℃ / sec to a temperature range of 500 ~ 750 ℃ and wound in a temperature range of 500 ~ 750 ℃.

At this time, if the hot rolled steel sheet is cooled and wound to a temperature below 500 ° C, bainite phase and martensite phase are formed in the steel to heat the material of the steel. As a result, coarse carbides and nitrides tend to form, resulting in inferior steel materials. In addition, if the average cooling rate is less than 10 ℃ / sec during cooling coarse ferrite grains are formed and the microstructure is non-uniform, if the average cooling rate exceeds 70 ℃ / sec is easy to form bainite phase and plate thickness The microstructure in the direction also becomes nonuniform, leading to inferior shearing workability of the steel.

Steps to Get Cold Rolled Steel Sheet

Cold rolled hot rolled steel sheet cooled and wound as described above is obtained.

In cold rolling, the cold reduction rate may be 40 to 80%. If the cold reduction rate is less than 40%, securing a target thickness may be difficult, and shape correction of the steel sheet may be difficult. On the other hand, when the cold rolling reduction exceeds 80%, cracks may occur at the edge of the steel sheet, and cold rolling load may be caused.

 For example, the cold rolling may be performed using a rolling mill composed of 5 or 6 stands, and at this time, the initial stand reduction rate may be set to 25 to 37%.

If the initial stand reduction rate is less than 25%, the low reduction ratio may not only limit the shape control of the hot rolled steel sheet, but also may not form a uniform martensite in the tissue due to unevenness of the martensite nucleation site upon cooling after annealing. And, if it exceeds 37%, since the initial load can be brought to the facility load due to the increase in the stand down rate, cold rolling mill initial stand down rate can be limited to 25 ~ 37%. It is more preferable to set initial stage stand reduction rate to 30 to 35%.

Continuous Annealing Step of Cold Rolled Steel Sheet

The cold-rolled steel sheet to continuous annealing in a hydrogen concentration of 3 to 30% by volume in a temperature range of _{Ac 1 + 20 ℃ ~ Ac 3} -20 ℃.

This process is performed to form ferrite and austenite at the same time as recrystallization and to distribute carbon.

In the present invention, by managing the fine martensite in the steel in the range of 1 to 5 area% to ensure the aging resistance at room temperature, the annealing temperature is Ac to produce a steel sheet to obtain the baking hardening at 35MPa or more at baking (usually 170 ℃ 20 minutes) temperature _It is prepared by limiting the range of hydrogen concentration in the furnace atmosphere to 3 to 30% under the conditions of ₁ + 20 ° C to Ac ₃ -20 ° C. When the annealing temperature is lower than Ac ₁ + 20 ° C., the austenite fraction is not sufficient at low two-phase (ferrite + austenite) temperature, so that the fine martensite is not sufficiently formed during cooling after final annealing, thereby obtaining the hardening hardening required in the present invention. In case of exceeding Ac ₃ -20 ℃, the austenitic fraction in two-phase annealing is too high, the martensite size is coarse after annealing cooling, the fraction exceeds 5%, and the strength sharply rises, so that the processing crack in forming parts this possibility is increased, the annealing temperature is preferably also limited to _{Ac 1 + 20 ℃ ~ Ac 3} -20 ℃.

Ac ₁ and Ac ₃ may be obtained, for example, as in the following formula (2).

[Relationship 2]

Ac ₁ (° C) = 723-10.7 [Mn] -16.9 [Ni] +29.1 [Si] +16.9 [Cr]

Ac ₃ (° C) = 910-203√C-15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W

(Where [C], [Mn], [Cu], [Cr]. [Ni], [W] and [Mo] each represents the weight percentage of the corresponding element)

On the other hand, when the hydrogen concentration is less than 3% by volume, it is easy to generate surface concentrates of elements having high oxygen affinity such as Si, Mn, and B contained in steel, causing dents and plating defects, while exceeding 30% by volume. In this case, since the defect suppression effect of the elements not only reaches the limit but is disadvantageous in terms of manufacturing cost, the hydrogen concentration is preferably set to 3 to 30% by volume.

Primary cooling stage of continuous annealed cold rolled steel sheet

The continuously annealed cold rolled steel sheet is first cooled to an average cooling rate of 2 to 10 ° C / sec to 630 to 670 ° C.

In the present invention, the higher the primary cooling end temperature or the slower the primary cooling rate, the higher the tendency of uniformity and coarsening of the ferrite, which is advantageous for securing ductility of steel.

In addition, the present invention is characterized by giving a sufficient time for the carbon to diffuse into austenite during the primary cooling, which is very significant in the present invention. In more detail, in the two-phase zone, carbon diffuses and moves to austenite where carbon concentration is high, and as the temperature increases, the diffusion degree increases as the time increases. If the primary cooling end temperature is less than 630 ° C., pearlite or bainite may be formed due to too low temperature, so that the yield ratio is increased, and cracking tends to increase during processing. On the other hand, when the primary cooling end temperature exceeds 670 ℃ ferrite is formed a large amount during cooling and the austenite content for the martensite formation is small, the final martensite content of 1 to 5% can not be effectively controlled.

In addition, when the primary cooling rate is less than 2 ℃ / sec is disadvantageous in terms of productivity and the ferrite fraction is increased to lack the austenite content for martensite formation, whereas, if it exceeds 10 ℃ / sec bainite is formed Increasing yield strength entails material inferiority. In the present invention, it is preferable to minimize the formation of bainite or pearlite in addition to fine martensite.

Hereinafter, the manufacturing method of the hot-dip galvanized steel sheet of the present invention will be described in detail.

The method for producing a hot-dip galvanized steel sheet according to the present invention, in addition to the method for manufacturing a cold rolled steel sheet, the primary cold cold rolled steel sheet is maintained at 440 ~ 480 ℃ at an average cooling rate of 4 ~ 20 ℃ / sec Secondary cooling until immersion in a zinc-based plating bath;

Obtaining a hot dip galvanized steel sheet by immersing the secondary cooled cold rolled steel sheet in a hot dip galvanizing bath maintained at 440 to 480 ° C .; And

Final cooling of the hot-dip galvanized steel sheet at an average cooling rate of 3 ° C./sec or more to (Ms-100) ° C. or less.

Second cooling stage of cold rolled steel

As described above, the cold-rolled cold rolled steel sheet is secondly cooled until being immersed in a hot dip galvanizing bath maintained at 440 ° C to 480 ° C at an average cooling rate of 4 to 20 ° C / sec.

According to the researches of the present inventors, when martensite is formed before passing through the temperature range of 440 to 480 ° C. of a conventional hot dip galvanizing bath, coarse martensite is formed on the finally obtained cold rolled steel sheet to achieve a resistance ratio. I can't.

If the secondary cooling rate exceeds 20 ° C / sec, martensite may be partially generated during the secondary cooling, and a problem such as plate shape distortion may occur due to an increase in the mail speed in terms of productivity. On the other hand, if the secondary cooling rate is less than 4 ℃ / sec, because the too slow cooling rate may form fine bainite, causing the material variation in the width direction, because the plate shape is not good, the secondary cooling rate is 4 ~ 20 It is preferable to control at ° C / sec.

Obtaining Hot Dip Galvanized Steel Sheet

As described above, the cold-rolled cold rolled steel sheet is immersed in a hot dip galvanizing bath maintained at 440 to 480 ° C. to obtain a hot dip galvanized steel sheet.

In the present invention, the composition of the hot dip galvanizing bath is not particularly limited, and may be a pure galvanizing bath or a zinc alloy plating bath containing Si, Al, Mg, or the like.

Final cooling step of hot dip galvanized steel sheet

The hot dip galvanized steel sheet is finally cooled to an average cooling rate of 3 ° C./sec or more to (Ms-100) ° C. or less.

The (Ms-100) ° C. is a cooling condition for martensite formation.

The theoretical temperature of said Ms can be calculated | required, for example by following formula (3).

[Equation 3]

Ms (° C.) = 539-423 [C] -30.4 [Mn] -12.1 [Cr] -17.7 [Ni] -7.5 [Mo]

(Where [C], [Mn], [Cr]. [Ni] and [Mo] each represents the weight percentage of the element)

When the final cooling end temperature is less than (Ms-100) ° C., fine martensite may not be obtained, and a problem of plate shape defect may be caused.

On the other hand, if the average cooling rate is less than 3 ° C / sec martensite is not only irregularly formed in the grain boundary or in the mouth due to the too slow cooling rate, the formation ratio of the grain boundary martensite to the mouth can not be produced low resistance steel. The upper limit value of the average cooling rate is not particularly limited since the facility characteristics are not problematic.

Obtaining an alloyed hot dip galvanized steel sheet

On the other hand, if necessary, prior to final cooling, the step of alloying heat treatment of the hot-dip galvanized steel sheet may further include obtaining an alloyed hot-dip galvanized steel sheet.

In the present invention, the alloying heat treatment process conditions are not particularly limited and may be normal conditions. As an example, an alloying heat treatment process may be performed in a temperature range of 500 to 540 ° C.

Temper Rolling Stage

In addition, if necessary, it may further include the step of temper rolling the final cooled hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet.

In the case of temper rolling, a large amount of dislocations are formed in the ferrite located around martensite, thereby further improving the baking hardenability.

At this time, the reduction ratio is preferably 0.3 to 1.6%, more preferably 0.5 to 1.4%. If the reduction ratio is less than 0.3%, sufficient dislocations are not formed and are disadvantageous in terms of plate shape, and in particular, there is a fear that plating platen defects may occur. On the other hand, when the reduction ratio exceeds 1.6%, it is advantageous in terms of dislocation formation, but side effects such as plate breakage may occur due to the limitation of facility capacity.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only examples for describing the present invention in more detail, and do not limit the scope of the present invention.

(Example)

After preparing the steel slab having the alloy composition shown in Table 1, using the manufacturing process shown in Tables 2 and 3 to prepare a hot-dip galvanized steel sheet (GI steel sheet) or an alloyed hot-dip galvanized steel sheet (GA steel sheet). For reference, Inventive Steels 1, 2, 4 and 5 and Comparative Steels 1 and 2 in Table 1 correspond to alloyed hot-dip galvanized steel sheets, and Invented Steels 3, 6 and 7 correspond to hot-dip galvanized steel sheets. Comparative steel 1 is usually a BH steel using ultra low carbon steel, and comparative steel 2 is a high carbon DP steel series.

For each plated steel sheet prepared as described above, the microstructure was observed, the physical properties were evaluated, and the results are shown in Table 4 below.

In the following Table 4, the microstructure fraction was observed for martensite and bainite through Lepelar corrosion using a first optical microscope at a plate thickness of 1 / 4t (t: sheet thickness), and again using SEM (3,000 times). After observation, the size and distribution of martensite and bainite were measured using the average value of three times through Count Point operation, and the phases except for these tissues were estimated as the ferrite content. In Table 4 below, the tensile test for each test piece was performed in the C direction using a JIS standard. In Table 4, YS represents a yield strength, and YR represents a yield ratio.

On the other hand, baking hardening resistance (L-BH) was measured under baking conditions of (170 ° C × 20 minutes), and evaluated by yield strength difference after 2% pre-strain, and aging resistance [YP-El (%)] was 100 ° C. YP-El (%) was measured in the tensile test after 1 hour retention at

Unplated evaluation was made by visual observation, and the relative evaluation was performed with grades 1 to 5 according to the degree of unplating, and grades 1 and 2 mean securing the quality of exterior plate materials.

Corrosion resistance evaluation was performed by cutting the steel plate into 75 × 150 mm size and then masking the edges with tape for saline spray corrosion resistance evaluation. Then, the specimens prepared in the salt spray tester with spray pressure of 0.098 ± 0.0025 MPa and spraying amount of 1.0 to 2.0ml per hour were compared, and the degree of red rust generation at the time point of 1,000 hours elapsed was compared. .

◎: less than 5% red rust occurrence area, ○: red rust generation area 5 ~ 20%, △: red rust generation area 21 ~ 50%, ×: red rust generation area more than 50%

Steel grade Cold rolled steel composition (% by weight) C Mn Si Cr P S N Sol.Al Ni Sb Mo B Inventive Steel 1 0.006 0.33 0.003 2.31 0.003 0.006 0.003 0.021 0.005 0.005 - 0.0004 Inventive Steel 2 0.007 0.55 0.004 2.18 0.003 0.005 0.003 0.034 0.01 0.009 - 0.0006 Invention Steel 3 0.010 0.72 0.003 1.75 0.003 0.004 0.002 0.045 - 0.018 0.07 - Inventive Steel 4 0.012 0.75 0.021 1.82 0.002 0.004 0.003 0.043 - 0.053 - 0.0021 Inventive Steel 5 0.024 0.82 0.022 1.56 0.001 0.006 0.004 0.052 - 0.061 0.18 0.0031 Inventive Steel 6 0.031 0.93 0.008 1.69 0.003 0.004 0.005 0.026 - 0.01 0.08 0.0008 Inventive Steel 7 0.070 1.15 0.007 1.15 0.005 0.006 0.003 0.041 - 0.04 0.03 0.0009 Comparative Steel 1 0.0018 0.15 0.003 - 0.006 0.004 0.002 0.035 - - - - Comparative Steel 2 0.09 1.85 1.2 - 0.005 0.007 0.004 0.045 - - 0.21 0.0018

Steel grade Hot rolled Cold rolled steel Reheat
Temperature
(℃) Finish rolling temperature (℃) Winding temperature
(℃) Cooling before winding
Speed (℃ / sec) Cold rolling reduction
(%) First cold rolled stand rolling reduction
(%) Inventive Steel 1 1185 886 569 25 54 29 1186 892 552 33 53 26 Inventive Steel 2 1187 904 655 27 65 28 1187 908 646 31 63 23 Invention Steel 3 1214 895 717 43 73 30 1212 896 715 55 76 33 Inventive Steel 4 1195 887 534 81 38 31 1196 928 584 19 75 34 Inventive Steel 5 1224 846 634 18 57 21 1228 914 637 8 42 39 Inventive Steel 6 1187 894 676 37 83 35 1194 895 653 36 74 34 Inventive Steel 7 1206 899 674 45 34 36 1207 888 652 56 63 33 Comparative Steel 1 1189 915 672 38 74 45 Comparative Steel 2 1201 891 535 34 63 37 1207 898 674 28 63 35

Steel grade Annealed Cooling Remarks Annealing Temperature
(℃) Furnace hydrogen concentration
(%) 1st cooling rate
(° C / sec) Primary Cooling End Temperature (℃) 2nd cooling rate (℃ / sec) Secondary Cooling End Temperature (℃) Final cooling rate (℃ / sec) Final Cooling End Temperature (℃) Inventive Steel 1 755 5.1 2.6 640 4.6 460 4.3 Room temperature Inventive Example 1 755 4.5 2.5 640 4.6 460 5.1 Room temperature Inventive Example 2 Inventive Steel 2 776 5.2 3.5 640 3.3 460 6.2 Room temperature Inventive Example 3 790 6.1 3.8 640 3.3 460 6.9 Room temperature Inventive Example 4 Invention Steel 3 813 5.8 4.3 640 5.2 460 9.8 Room temperature Inventive Example 5 812 5.2 9.3 640 6.1 460 9.3 Room temperature Inventive Example 6 Inventive Steel 4 675 6.1 5.6 640 9.1 460 5.3 Room temperature Comparative Example 1 831 3.5 5.8 640 8.4 460 7.8 Room temperature Inventive Example 7 Inventive Steel 5 680 3.5 11.2 640 11.4 460 2.5 Room temperature Comparative Example 2 833 3.1 8.5 640 12.1 460 6.4 Room temperature Comparative Example 3 Inventive Steel 6 841 2.5 7.7 640 8.9 460 5.3 Room temperature Comparative Example 4 834 38.1 15.5 640 7.2 460 8.1 Room temperature Comparative Example 5 Inventive Steel 7 845 3.5 6.7 640 2.8 460 2.7 Room temperature Comparative Example 6 835 3.2 15.5 640 21 460 8.4 Room temperature Comparative Example 7 Comparative Steel 1 843 3.6 4.8 640 6.5 460 5.2 Room temperature Comparative Example 8 Comparative Steel 2 815 4.4 4.9 640 6.5 460 5.3 Room temperature Comparative Example 9 781 4.5 4.1 640 7.8 460 3.8 Room temperature Comparative Example 10

Steel grade ① ② ③ YP-El
(%) L-BH
(MPa) YS
(MPa) YR ④
Unplated ⑤
Corrosion resistance Inventive Steel 1 2.5 0 1.88 0 38 221 0.54 One ◎ Inventive Example 1 2.2 0 1.88 0 42 213 0.52 One ◎ Inventive Example 2 Inventive Steel 2 3.1 0 2.01 0 42 224 0.53 One ◎ Inventive Example 3 3.2 0 2.01 0 38 222 0.57 One ◎ Inventive Example 4 Invention Steel 3 3.2 0 1.9 0 45 232 0.54 One ◎ Inventive Example 5 4.3 0 1.9 0 47 234 0.52 One ◎ Inventive Example 6 Inventive Steel 4 3.2 0 2.02 0.38 53 224 0.52 3 ○ Comparative Example 1 2.1 0 2.02 0 51 223 0.57 One ◎ Inventive Example 7 Inventive Steel 5 6.7 0.4 1.92 0.35 48 256 0.66 3 ○ Comparative Example 2 4.5 0.2 1.92 0.32 46 255 0.66 3 ○ Comparative Example 3 Inventive Steel 6 4.6 0.6 2.07 0.31 42 261 0.68 3 ○ Comparative Example 4 6.2 0 2.07 0.41 47 263 0.62 4 ○ Comparative Example 5 Inventive Steel 7 1.8 0.3 1.96 0.28 47 287 0.58 4 ○ Comparative Example 6 2.1 1.3 1.96 0.18 33 283 0.56 5 △ Comparative Example 7 Comparative Steel 1 0 0 0.15 0 0 181 0.73 2 ◎ Comparative Example 8 Comparative Steel 2 8.3 3.2 1.85 0 45 287 0.65 4 △ Comparative Example 9 9.1 2.5 1.85 0 39 291 0.66 4 △ Comparative Example 10

Here, ① means the martensite area ratio (%), ② means bainite area ratio (%), ③ is the value of the relationship 1 Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt %) And ④ unplated evaluation (1 ~ 2 excellent, 3 ~ 4 average, 5th grade inferior) and ⑤ salt spray corrosion resistance evaluation result.

As shown in Tables 1 to 4, in the case of Inventive Examples 1 to 7 satisfying the alloy composition and manufacturing conditions of the present invention has a yield strength in the range of 210 ~ 270 MPa, the test piece after the heat treatment 100 ℃ × 60 minutes Yield point yield (YP-El) does not appear at all. It can be seen that it shows the best grade in corrosion resistance evaluation.

On the contrary, in Comparative Examples 1 to 10, which do not satisfy at least one of the alloy composition and manufacturing conditions of the present invention, the physical properties of at least one of yield strength, yield ratio, baking hardenability, corrosion resistance, and aging resistance are inferior or It can be seen that it is not enough.

Claims (5)

By weight, carbon (C): 0.005-0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (except 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); and Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: At least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities, satisfying the following relation 1, the area%, 1 to 5% of martensite and residual ferrite Cold-rolled steel sheet excellent in hardening and corrosion resistance, including.
[Relationship 1]
1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7
(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)
[Claim 2] The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a yield strength of 210 to 270 MPa and a yield ratio of less than or equal to 0.6 (YS / TS).
By weight, carbon (C): 0.005-0.08%; Manganese (Mn): 1.25% or less (except 0%); Phosphorus (P): 0.03% or less (excluding 0%); Sulfur (S): 0.01% or less (excluding 0%); Nitrogen (N): 0.01% or less (excluding 0%); Aluminum (sol.Al): 0.01% to 0.06%; Chromium (Cr): 1.0% to 2.5%; Antimony (Sb): 0.1% or less (except 0%); and Ni: 0.3% or less (except 0%), Si: 0.3% or less (except 0%), Mo: 0.2% or less (except 0%) and B: Reheating the slab comprising at least one selected from the group consisting of 0.003% or less (excluding 0%), residual iron (Fe) and unavoidable impurities and satisfying the following relational formula 1;
[Relationship 1]
1.3≤Mn (wt%) + Cr (wt%) / 1.5 + Sb (wt%) ≤2.7
(Where Mn, Cr, and Sb each represent the content (% by weight) of the corresponding element)
Hot rolling the reheated slab at a temperature range of 850 to 1150 ° C. to obtain a hot rolled steel sheet;
Cooling the hot rolled steel sheet at an average cooling rate of 10 to 70 ° C / sec to a temperature range of 550 to 750 ° C;
Winding the cooled hot rolled steel sheet in a temperature range of 550 to 750 ° C;
Cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
The step of continuously annealing the cold-rolled steel sheet in a hydrogen concentration of 3 to 30% by volume in a temperature range of _{Ac1 + 20 ℃ ~ Ac 3 -20} ℃; And
Method for producing a cold-rolled steel sheet excellent in hardening hardening and corrosion resistance comprising the step of first cooling the continuous annealing cold rolled steel sheet to an average cooling rate of 2 ~ 10 ℃ / sec to 630 ~ 670 ℃.
The method for manufacturing a cold rolled steel sheet having excellent hardening hardening resistance and corrosion resistance according to claim 3, wherein the cold rolling reduction rate is 40 to 80%.
The cold rolling of claim 3, wherein the cold rolling may be performed using a rolling mill composed of 5 or 6 stands, and the initial stand reduction rate is set at 25 to 37%. Method of manufacturing steel sheet.