KR101536409B1 - Ultra high strength cold rolled steel sheet having exellent ductility and exelent bendability and method for manufacturing the same - Google Patents

Abstract

0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.010 to 0.10% of Sn, 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% % Of at least one element selected from the group consisting of Nb and Ti, at least one element selected from the group consisting of Cr and Fe, 0.001 to 0.0060%, B: 0.0010 to 0.0060% and Sb: 0.015 to 0.05% Includes unavoidable impurities,
Characterized in that the main structure is made of ferrite and martensite and the sum of bainite and tempered martensite or their sum is not more than 20% This excellent ultra high strength cold rolled steel sheet and its manufacturing method are provided.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra high strength cold rolled steel sheet having excellent ductility and bending workability and a method of manufacturing the same. [0002]

The present invention relates to an ultra-high strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, which is applicable to an embossed part, and a method of manufacturing the same.

Recently, fuel economy regulations have been strengthened as a task to preserve the global environment, and the weight of automobile body is being actively actively carried out. One measure is to reduce the weight of automotive materials by increasing the strength of steel plates. In order to improve fuel efficiency and durability, steel plates for automobiles are required to have higher strength. In terms of collision safety and passenger protection, ultra high strength steel plates with a tensile strength of 980 MPa or more are increasingly used as body structures and reinforcements. High-strength steels are actively applied to parts requiring high machining by machining machined parts using high-strength and ductile materials. However, the high strength of the steel sheet causes deterioration of the formability and weldability, and development of a material complementary thereto is desired. To meet such demands, composite structure steels such as TRIP steels have been developed using dual phase (DP) and transformed organic plasticity.

On the other hand, the areas where the ultrahigh strength steel of 980 MPa class or higher is actually used are mostly processed by bending like side sills, so that even if the ductility is excellent, when the bendability deteriorates, none. The bending workability (R / t) means the ratio of the minimum bending radius (R) to the unit thickness (t), where the minimum bending radius ratio (R) It does not mean the minimum radius.

Patent Document 1 discloses a method of manufacturing a steel sheet excellent in moldability by controlling the chemical composition and the amount of retained austenite in the steel sheet. Patent Literatures 2 and 3 disclose a method of controlling the chemical composition and the microstructure of a steel sheet, A method of manufacturing a high-strength steel sheet is disclosed. Patent Document 4 discloses a steel sheet having excellent workability including residual austenite of 5% or more, particularly excellent in local stretching. Patent Document 5 discloses a steel sheet having bainite as a main phase and a retaining austenite- Discloses a method of manufacturing a steel sheet with improved oily properties. However, the above-mentioned inventions have a problem that the weldability is deteriorated due to excessive addition of carbon, silicon or aluminum in order to improve the elongation rate, and the plating quality is poor due to excessive addition of silicon or aluminum, there was. Further, since the cooling rate is set to 100 deg. C / sec or more in order to secure high strength, partial deformation of the steel sheet during cooling is caused, and it is difficult to secure the flatness of the steel sheet.

Japanese Laid-Open Patent Publication No. 1994-145892 Japanese Patent Publication No. 2660644 Japanese Patent Publication No. 2704350 Japanese Patent Publication No. 3317303 Japanese Patent Application Laid-Open No. 2004-292891

One aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility, bending workability, plating ability, and weldability by appropriately controlling the composition and the manufacturing method of the steel sheet.

In order to achieve the above object, according to one aspect of the present invention,

0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.010 to 0.10% of Sn, 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% % Of at least one element selected from the group consisting of Nb and Ti, at least one element selected from the group consisting of Cr and Fe, 0.001 to 0.0060%, B: 0.0010 to 0.0060% and Sb: 0.015 to 0.05% Includes unavoidable impurities,

(Bainite), tempered martensite, or a mixture thereof, having a ductility and bending workability of 20% or less at a cross-sectional area ratio A cold-rolled steel sheet is provided.

According to another aspect of the present invention,

0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.010 to 0.10% of Sn, 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% % Of at least one element selected from the group consisting of Nb and Ti, at least one element selected from the group consisting of Cr and Fe, 0.001 to 0.0060%, B: 0.0010 to 0.0060% and Sb: 0.015 to 0.05% After reheating the slab containing unavoidable impurities, rolling to obtain a hot rolled steel sheet so that the temperature at the finish rolling exit side becomes 800 to 950 占 폚;

Winding the hot-rolled steel sheet at a temperature of 500 to 620 캜;

Rolling the rolled hot-rolled steel sheet at a reduction ratio of 40 to 70% to obtain a cold-rolled steel sheet;

Continuously annealing the cold-rolled steel sheet at a temperature of 770 to 830 캜;

Cooling the cold-rolled steel sheet continuously annealed to a temperature of 650 to 700 占 폚 at a cooling rate of 0.5 to 5 占 폚 / sec; And

And secondarily cooling the primary cooled cold-rolled steel sheet to a temperature of 300 to 450 ° C at a cooling rate of 5 to 20 ° C / sec. The present invention also provides a method of manufacturing an ultra-high-strength cold-rolled steel sheet excellent in ductility and bending workability.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to the present invention, there is provided an ultrahigh-strength cold-rolled steel sheet having an elongation of not less than 15%, a bending workability (R / t) of not more than 0.1 and a tensile strength of not less than 980 MPa, .

Fig. 1 is a photograph showing Comparative Example 1 which does not satisfy Relation 1 and Observation of cross-sectional structure of hot-rolled steel sheet of Inventive Example 5 satisfying Relational Expression 1. Fig.
2 is a photograph showing the microstructure of the steel sheet according to the silicon content.
FIG. 3 is a photograph showing the surface enrichment, that is, the internal oxide formation behavior, of the cold-rolled steel sheet according to the antimony content.
4 is a photograph showing the microstructure of the steel sheet of Comparative Example 11 in which the slow cooling heat treatment was not performed and the steel sheet of Example 5 in the slow cooling heat treatment.
FIG. 5 is a photograph showing internal oxides according to the coiling temperature of a steel to which 1% of silicon is added. FIG.

It is generally known that the hardness difference between phases must be reduced in order to improve the bending workability or hole expandability (HER) of a steel sheet. Therefore, attention has been focused on a composite structure steel having bainite or tempered martensite as a main structure. However, such a transformation phase has a problem that the yield strength is very high and the ductility is significantly lowered.

Silicon (Si), on the other hand, increases the activity of carbon in the steel and disperses the pearlite band during hot rolling. The dispersed pearlite band acts as a nucleus site of the martensite structure in the annealing process and causes dispersion of the martensite structure to disperse the stress externally applied, thereby improving the bending workability of the steel sheet . In addition, the strength of the steel sheet is improved because the carbon is concentrated in the austenite to increase the martensite strength. In addition, ductility is improved due to the cleaning effect of the ferrite base which reduces the amount of the used carbon in the ferrite matrix.

Therefore, the present inventors have found that a cold-rolled steel sheet having a high silicon content is subjected to a cold and heat treatment to form a main structure of ferrite and martensite, whereby an ultra-high strength cold-rolled steel sheet having excellent ductility and bending workability and having a tensile strength of 980 MPa or more Can be produced, leading to the present invention.

Hereinafter, an ultra-high strength cold-rolled steel sheet excellent in ductility and bending workability which is one aspect of the present invention will be described in detail.

A super high strength cold rolled steel sheet excellent in ductility and bending workability, which is one aspect of the present invention, comprises 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% of Mn, 0.001 to 0.10% of P, 0.001 to 0.10% of P, : 0.010% or less, Sol.Al: 0.01 to 0.10%, N: 0.010% or less, Cr: 0.2-0.7%, B: 0.0010-0.0060% and Sb: 0.015-0.05% And 0.005 to 0.05% of the total of at least one selected from the group consisting of Fe and unavoidable impurities,

Ferrite and martensite as main structures, and bainite, tempered martensite, or a mixture thereof is 20% or less in cross-sectional area ratio.

First, the alloy composition of the cold-rolled steel sheet of the present invention will be described in detail.

Carbon (C): 0.08 to 0.12 wt%

Carbon (C) is a very important element added to strengthen the metamorphosis. Carbon promotes the strengthening of the steel and promotes the formation of martensite in the composite structure steel. As the carbon content increases, the amount of martensite in the steel increases. If the carbon content is less than 0.08 wt%, it is difficult to ensure strength. On the other hand, if the carbon content exceeds 0.12%, weldability and hole expandability are deteriorated. Therefore, the carbon content is preferably limited to 0.08 to 0.12% by weight.

Silicon (Si): 0.8 to 1.2 wt%

Silicon (Si) is an element added for strength enhancement by solid solution strengthening. Silicon is an element effective for promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element and increasing the carbon content in untransformed austenite to form a complex structure of ferrite and martensite. When the content of silicon is less than 0.8% by weight, a sufficient ferrite structure can not be secured. On the other hand, if the content of silicon exceeds 1.2% by weight, surface scale defects are caused to result in deteriorating the quality of the surface of the steel sheet and deteriorating chemical conversion treatment, plating ability and weldability. Therefore, the content of silicon is preferably limited to 0.8 to 1.2% by weight.

Manganese (Mn): 2.6-3.0 wt%

Manganese (Mn) fine grains and precipitates sulfur (S) in the steel as MnS to prevent hot brittleness due to the formation of FeS, and strengthens the steel by solid solution strengthening without deterioration of ductility. In addition, in a composite structure steel, a martensite phase can be obtained and a martensite can be more easily formed by lowering a critical cooling rate. When the content of manganese is less than 2.6% by weight, it is difficult to obtain a desired strength in the present invention. On the other hand, when the content of manganese exceeds 3.0% by weight, weldability and hot rolling property are deteriorated. Is preferably limited to 2.6 to 3.0% by weight.

Phosphorus (P): 0.001 to 0.10 wt%

Phosphorus (P) is a substitutional alloying element having the largest solid solution strengthening effect and is an effective element for improving in-plane anisotropy and improving the strength of steel. In order to exhibit such effects in the present invention, it is preferable that the content of phosphorus is 0.001% or more. On the other hand, if the content of phosphorus exceeds 0.10% by weight, press formability may deteriorate and brittleness of steel may be generated. Therefore, the content of phosphorus is preferably limited to 0.001 to 0.10% by weight.

Sulfur (S): 0.010 wt% or less

Sulfur (S) is an impurity present in the steel and is preferably controlled as low as possible. If the sulfur content exceeds 0.010%, there is a problem that ductility and weldability of the steel sheet are impaired. Therefore, it is preferable that the content of S is limited to 0.010% or less.

Aluminum (Sol.Al): 0.01 to 0.10 wt%

Aluminum (Al) is an element added mainly for deoxidation, and is an element effective for improving the hardenability of martensite by distributing carbon in ferrite to austenite like silicon. In order to exhibit such effects in the present invention, it is preferable that the aluminum content is 0.01% or more. On the other hand, if the content of aluminum exceeds 0.1 wt%, the effect is not only saturated but also economically disadvantageous, so that the content of soluble aluminum is preferably limited to 0.01 to 0.1%.

Nitrogen (N): 0.010 wt% or less (excluding 0%)

Nitrogen (N) is an element useful for stabilizing austenite. When it exceeds 0.010% by weight, nitrogen (N) is useful for improving the stability of austenite. However, since it is accompanied by material deterioration such as elongation decrease due to formation of a large amount of nitride, .

Cr (Cr): 0.2 to 0.7 wt%

Chromium (Cr) is an effective element for improving hardenability and strength of steel. If the content of chromium is less than 0.2%, it is difficult to obtain the desired strength in the present invention. On the other hand, if the content of chromium exceeds 0.7%, the effect is saturated and economically disadvantageous. To 0.7%.

Boron (B): 0.0010 to 0.0060 wt%

Boron (B) is a typical element for improving hardenability of steel. It is a component that delays transformation of austenite into pearlite during cooling after cracking during annealing process even in a slight amount. It suppresses ferrite formation, And the like. In order to exhibit such effects in the present invention, it is preferable that the boron content is 0.0010% or more. On the other hand, when the content of boron exceeds 0.0060%, excessive boron is concentrated on the surface and deterioration of the plating adhesion is caused, so that the content of boron is preferably limited to 0.0010 to 0.0060%.

Antimony (Sb): 0.015 to 0.05 wt%

Antimony (Sb) prevents the generation of internal oxides by silicon, manganese and boron which are surface enrichment elements in the steel during hot rolling, thereby preventing induction of dent during annealing, and additionally, to improve the plating ability of cold- . The antimony suppresses the surface enrichment of oxides such as MnO, SiO2, and Al2O3 to suppress the surface defects caused by the dent, and has an excellent effect in suppressing the coarsening of the surface enrichment due to the temperature increase and the hot rolling process change. In order to exhibit such effects in the present invention, it is preferable that the content of antimony is 0.015% or more. On the other hand, when the content of antimony exceeds 0.05%, deterioration of materials due to grain boundary segregation of antimony, deterioration of workability, and increase of manufacturing cost are disadvantageous. Therefore, the content of the antimony is preferably limited to 0.015 to 0.05% by weight.

On the other hand, it is preferable that the above-mentioned components include at least one selected from the group consisting of Ti and Nb. In this case, the sum of the contents of Ti and Nb is preferably limited to 0.005 to 0.05 wt% in total.

Ti and Nb in steel are effective elements for increasing the strength and fineness of the steel sheet. When the content of Ti and Nb is less than 0.005%, it is difficult to secure such effect. When the content of Ti and Nb exceeds 0.05%, the manufacturing cost is increased and the ductility is greatly lowered due to excessive precipitates. Therefore, it is preferable to limit the content to 0.005 to 0.05%.

The remainder includes Fe and unavoidable impurities. Addition of an effective component other than the above-mentioned composition is not excluded.

On the other hand, the cold-rolled steel sheet produced according to the present invention satisfies the above-described composition conditions, and its microstructure is composed of ferrite and martensite as main structures, and bainite, tempered martensite (Tempered Martensite) or a sum thereof is preferably not more than 20% at a cross-sectional area ratio. If the sum of the bainite and the tempered martensite exceeds 20% as a cross-sectional area ratio, there is a problem that the yield strength is excessively increased and the ductility is deteriorated.

According to the present invention, it is preferable that the alloy composition ratio of Si, Mn, B, Sb, C, P and S satisfies the following relational expressions 1 and 2 in designing an alloy of a steel sheet having the above-

[Relation 1]

20 < (Si / Mn + 150B) / Sb < 50

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S? 0.30

Relation 1 shows a component relationship capable of ensuring surface quality. During the hot rolling, internal oxides are generated by silicon, manganese and boron which are surface enrichment elements in the steel, and these internal oxides diffuse to the surface during annealing to cause a dent to the steel sheet. In addition, there is a problem that the wettability of the plating layer is deteriorated to lower the plating property. To solve this problem, a certain level of antimony which is a function of interfacial diffusion of the surface enrichment elements is added. Conditions under which dent generation and deterioration of the plating characteristics do not occur in the cold-rolled steel sheet according to the present invention are derived through repeated experiments. When the content of silicon, manganese, boron and antimony is strictly controlled under the condition of the above-mentioned relational expression 1, It is possible to prevent the problem of dent during annealing and the deterioration of the plating ability.

On the other hand, the relationship (2) shows a component relationship capable of ensuring weldability. That is, carbon, manganese, silicon, phosphorus and sulfur in steel have a role of increasing the carbon equivalent (Ceq), and as is well known, the higher the carbon equivalent, the worse the weldability. The conditions under which welding defects, which are the welding method used mainly for welding the cold-rolled steel sheet according to the present invention, do not occur are obtained through repeated experiments. The content of carbon, manganese, silicon, It is confirmed that the welding failure can be prevented by strict control under the condition.

Meanwhile, the cold-rolled steel sheet according to the present invention has a yield ratio of less than 0.70. When the yield ratio is more than 0.70, the ductility is deteriorated due to a high yield strength, and there arises a problem that machining defects such as cracks are generated during component forming.

Meanwhile, the hot-rolled steel sheet according to the present invention as described above can secure an impact energy of 50 J or more at -20 캜, and can be suitably applied to automobile chassis parts and structural members.

Hereinafter, a method of manufacturing an ultra-high strength cold rolled steel sheet having an excellent impact resistance elongation, which is another aspect of the present invention, will be described in detail.

A method of producing an ultra-high strength cold-rolled steel sheet excellent in ductility and bending workability, which is another aspect of the present invention, comprises 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% of Mn, 0.10% or less of S, 0.01 to 0.10% of Sol.Al, 0.010% or less of N, 0.2 to 0.7% of Cr, 0.0010 to 0.0060% of B and 0.015 to 0.05% of Sb, Ti and the balance Fe and inevitable impurities in a total amount of 0.005 to 0.05%, and then rolling the slab so that the temperature at the finish rolling exit side is 800 to 950 ° C to obtain a hot-rolled steel sheet step; Winding the hot-rolled steel sheet at a temperature of 500 to 620 캜; Rolling the rolled hot-rolled steel sheet at a reduction ratio of 40 to 70% to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at a temperature of 770 to 830 캜; Cooling the cold-rolled steel sheet continuously annealed to a temperature of 650 to 700 占 폚 at a cooling rate of 0.5 to 5 占 폚 / sec; And secondarily cooling the primary cooled cold rolled steel sheet to a temperature of 300 to 450 캜 at a cooling rate of 5 to 20 캜 / sec.

Step of obtaining hot-rolled steel sheet

After the slab satisfying the above composition is reheated, it is hot-rolled to obtain a hot-rolled steel sheet. At this time, the temperature at the outlet side during finish rolling is preferably 800 to 950 占 폚. When the temperature at the outlet side during the finish rolling is less than 800 ° C, there is a high possibility that the hot deformation resistance is rapidly increased, and the top, bottom and tail of the hot-rolled coil become single- There is a problem of deterioration. On the other hand, when the temperature at the outlet side during the finish rolling exceeds 950 DEG C, there is a problem that not only a too large oxidation scale is generated but also the microstructure of the steel sheet is likely to be coarsened.

Winding  step

The hot-rolled steel sheet is wound at 500 to 620 占 폚. In the present invention, a high content of silicon is added in order to ensure excellent ductility with an elongation of 15% or more and excellent bending workability. When the coiling temperature exceeds 620 캜, Si-based inner oxide is generated, ) On the surface of the substrate. On the other hand, when the coiling temperature is less than 500 ° C, excessive production of martensite or bainite causes an excessive increase in the strength of the hot-rolled steel sheet, resulting in problems such as poor shape due to high load during cold rolling . Therefore, the winding temperature is preferably limited to a very narrow range of 500 to 620 占 폚.

Step of Obtaining Cold Rolled Steel Sheet

The rolled hot-rolled steel sheet is cold-rolled at a reduction ratio of 40 to 70% after pickling. When the reduction rate is less than 40%, the recrystallization driving force is weakened and it is difficult to obtain good recrystallized grains, and the shape correction is very difficult. On the other hand, if the reduction rate exceeds 70%, there is a high possibility that a crack occurs at the edge of the steel sheet, and the rolling load rapidly increases. Therefore, the reduction rate is preferably limited to 40 to 70%.

Continuous annealing  step

The cold-rolled steel sheet is continuously annealed at 770 ° C to 830 ° C. When the annealing temperature is lower than 770 占 폚, austenite is not sufficiently formed and it is difficult to secure the desired strength in the present invention. On the other hand, if the annealing temperature exceeds 830 ° C, the bainite phase fraction increases sharply due to excessive austenite formation, resulting in an excessive increase in yield strength and deterioration of ductility.

Cooling step

The cold-rolled steel sheet thus subjected to the continuous annealing is first cooled to 650 to 700 占 폚 at a cooling rate of 0.5 to 5 占 폚 / sec. The primary cooling step is to secure an equilibrium carbon concentration of ferrite and austenite to ensure excellent ductility and strength. When the primary cooling end temperature is lower than 650 ° C or higher than 700 ° C, It is difficult to secure ductility and strength. When the cooling rate of the primary cooling step is less than 0.5 캜 / sec, most of the austenite generated in the continuous annealing step is transformed into ferrite, and the fraction of the transformation phase produced in the secondary cooling is decreased. Therefore, it is very difficult to obtain a desired strength in the present invention. On the other hand, when the cooling rate of the primary cooling step exceeds 5 DEG C / sec, the austenite produced in the continuous annealing step is not transformed into ferrite due to a high cooling rate. Therefore, since the austenite produced by the change of the annealing temperature changes into the transformation state during cooling without ferrite transformation, the transformation phase fraction varies depending on the annealing temperature, so that the fluctuation of the material becomes very large. In addition, there is a problem in that there is a great possibility that a fatal problem such as breakage of a coil due to winding of a coil during annealing or the like occurs.

The primary cooled cold rolled steel sheet is secondarily cooled to a temperature of 300 to 450 DEG C at a cooling rate of 5 to 20 DEG C / s. Secondary cooling is one of the controlling factors which is important in the present invention. When the secondary cooling end temperature is lower than 300 ° C, plate-like twisting occurs due to quenching, and ferrite due to excessive martensite generation and secondary phase The bending workability deteriorates due to an increase in the strength difference between the two. On the other hand, when the secondary cooling end temperature exceeds 450 ° C., the time for staying in the bainite region during overflow treatment increases, yield strength increases and ductility deteriorates due to generation of bainite Occurs. If the cooling rate of the secondary cooling step is less than 5 占 폚 / sec, there is a problem that the desired strength can not be obtained in the present invention steel because the transformation amount is low. On the other hand, when the cooling rate is more than 20 占 폚 / sec, The martensite transformation of martensite is not only caused but also the strength is increased due to an increase in carbon concentration in martensite and the difference in strength between ferrite and martensite is very large and the bending workability is deteriorated.

On the other hand, according to the above-described method, skin pass rolling can be further performed on the cold-rolled steel sheet that has been subjected to the secondary cooling at a reduction rate of 0.1 to 0.5%. Generally, when skeletal rolling of a textured steel is performed, there is little increase in tensile strength, and a yield strength increase of at least 50 to 100 Mpa or more occurs. When the reduction rate is less than 0.1%, it is difficult to control the shape of the super high strength steel having a tensile strength of 980 MPa according to the present invention. On the other hand, when the reduction rate exceeds 0.5%, the yield strength is excessively increased, There is a problem that this becomes unstable.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

Table 1 below shows the composition of steel slabs having the composition of inventive steels and comparative steels according to the present invention. Table 2 shows the coiling temperature, the annealing temperature, the cooling rate in the first and second cooling, and the end temperature for the steel types shown in Table 1. The reheating temperature was 1200 占 폚, the temperature at the outlet side during finish rolling was 880 占 폚, the coiling temperature was 600 占 폚, the reduction rate during cold rolling was 50%, and the skin pass rolling rate was constant at 0.2%.

Table 3 shows the mechanical properties (yield strength, tensile strength, elongation and bending workability) of the inventive steel and the comparative steel, whether or not oxide is formed in the hot-rolled steel sheet, and the evaluation results of the adhesion of the GA re-plating. In Table 3, YS, TS, T-El, and YR mean yield strength, tensile strength, fracture elongation, yield ratio (YR = YS / TS). JIS No. 5 tensile test specimens were prepared from the cold-rolled steel sheets and their mechanical properties were measured. The bending workability was evaluated as "OK" for the material which did not crack on the surface in the bending test of R / "NG" in the material.

In addition, by observing the microstructure of the hot-rolled steel sheet cross-section for each steel type, it was confirmed whether or not the oxide was inside the hot-rolled steel sheet. As for the observation result, "O" Respectively.

On the other hand, in the evaluation of the plating adhesion, the steel sheets of Table 2 were prepared by using GA-coated steel sheet, the plate was cut into 20 mm × 50 mm, the 60 ° bend test was performed, and the width of the plated layer, And evaluated on the same basis.

◎: No falling off plating or width within 1 ~ 3mm

△: The falling plating width is 3 to 5 mm or less

X: Plating width of off 5mm or more

In the above-mentioned criterion, the plating adhesion is judged to be excellent when the plating peeling width is 3 mm or less.

Inventive Examples 1 to 14 satisfying the composition and the production method of the present invention all exhibit excellent bending workability with a yield strength of 700 MPa or less, an elongation of 15% or more, and R / t of 1.0 or less. It is also confirmed that the conditions of the relational expressions 1 and 2 are satisfied and the weldability and the plating adhesion are very excellent.

On the other hand, Comparative Examples 1 to 7 did not satisfy Relation 2, and weldability was shown to be on the order of heat. In Comparative Examples 1, 3, and 5 to 7, relation 1 was also unsatisfactory and internal oxides were generated in the hot- , And the adhesion of plating was poor in GA material. On the other hand, FIG. 1 shows the cross-sectional structure of the hot-rolled steel sheet of Inventive Example 5 satisfying Relational Example 1 and Relational Expression 1 which do not satisfy Relational Expression 1. In Comparative Example 1 not satisfying Relational Expression 1, In the presence of oxygen. These oxides are mainly attached to the surface of the roll during annealing with MnO and SiO ₂ , causing dent defects in the steel sheet, and they are present on the surface of the steel sheet, which reduces the adhesion of the plating during GA plating. However, in Inventive Example 5, which satisfies the relational expression 1, it can be confirmed that no internal oxide is generated at all, which confirms that the plating adhesion is very excellent.

In Comparative Example 8, the silicon content was so low that sufficient ferrite structure could not be secured, and bending workability, tensile strength and ductility were poor. 2 is a graph showing the microstructure of steel according to the content of silicon. Figs. 2 (a) and 2 (b) show the hot-rolled structure and annealed structure of Comparative Example 8 and Inventive Example 1, respectively. The hot-rolled structure of Inventive Example 1 is homogenized compared to the hot-rolled structure of Comparative Example 8, and the annealed structure of Inventive Example 1 shows that martensite is dispersed very uniformly.

In Comparative Example 9, the inner oxide was generated in the hot-rolled steel sheet because the antimony content was rare, and the adhesion of the plating was poor in the GA material. On the other hand, Comparative Example 10 exhibited poor workability (bending workability) due to excessive antimony content. 3 (a), 3 (b), 3 (c) and 3 (d) are graphs showing the surface enrichment, that is, the internal oxide formation behavior, of the cold rolled steel sheet according to the antimony content. 2, Example 3 and Comparative Example 10, respectively. As shown in FIG. 3, it can be seen that fine granular oxides are increased by increasing the content of antimony. These spherical surface oxides help form a uniform plating layer on the surface and also have the effect of inhibiting the formation of internal oxides formed in the grain boundaries. This makes it possible to suppress the generation of surface defects such as dents in the annealing process.

In Comparative Examples 11 to 19, the alloy composition was included in the range of the present invention, and all of the relational expressions 1 and 2 were satisfied, showing excellent weldability and plating adhesion.

However, in Comparative Example 11, the cooling rate to 650 to 700 占 폚 was 8 占 폚 / sec, which exceeded the range controlled by the present invention. As a result, ferrite transformation was not performed, and the carbon content in the austenite was decreased. As a result, bainite having a cross-sectional area ratio of 55% or more was generated in the microstructure, and a high yield strength of 820 MPa or more and a low elongation of 10.8% were exhibited. FIG. 4 is a photograph showing the microstructure according to whether or not the sample is subjected to a cold / hot treatment. FIGS. 4 (a) and 4 (b) are photographs of microstructure observed in Inventive Example 5 and Comparative Example 11, respectively. As shown in FIG. 4, it can be confirmed that a large amount of bainite was generated in the microstructure of Comparative Example 11 in which the heat treatment was not carried out.

On the other hand, in Comparative Examples 12 and 14 to 19, the secondary cooling termination temperature was out of the range of the present invention, and the martensite was excessively formed to increase the phase-to-phase strength deviation with respect to the ferrite, thereby deteriorating the bending workability .

In Comparative Example 13, the coiling temperature was 650 ° C, which was outside the range of the present invention, and a considerable amount of internal oxide was generated in the steel, which resulted in poor plating adhesion in the GA material. 5 (a), 5 (b) and 5 (c) are graphs showing the internal oxides according to the coiling temperature of the steel to which 1% of silicon was added, 13 &lt; / RTI &gt; As shown in FIG. 5, in Comparative Example 13 in which the coiling temperature was outside the scope of the present invention, it was confirmed that a considerable amount of internal oxide was generated in the steel.

Claims (6)

0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.010 to 0.10% of Sn, 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% , 0.001 to 0.0060% of B, 0.015 to 0.05% of Sb, and at least one selected from the group consisting of Nb and Ti in a total amount of 0.005 to 0.05% And the balance Fe and unavoidable impurities,
Characterized in that the main structure is made of ferrite and martensite and the sum of bainite and tempered martensite or their sum is not more than 20% This excellent super high strength cold rolled steel sheet.
The method according to claim 1,
Wherein the content of C, Si, Mn, P, S, B, and Sb satisfies the following relational expression (1) and (2).
[Relation 1]
20 < (Si / Mn + 150B) / Sb < 50
[Relation 2]
C + Mn / 20 + Si / 30 + 2P + 4S? 0.30
(C, Si, Mn, P, S, B and Sb 'in the relational expressions 1 and 2 mean the weight% of the alloy component)
3. The method according to claim 1 or 2,
The cold-rolled steel sheet has a yield ratio of 0.70 or less, a bending workability (R / t) of 0.1 or less, and an elongation of 15% or more.
(Where R is the minimum bending radius and t is the unit thickness).
0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.010 to 0.10% of Sn, 0.08 to 0.12% of C, 0.8 to 1.2% of Si, 2.6 to 3.0% , 0.001 to 0.0060% of B, 0.015 to 0.05% of Sb, and at least one selected from the group consisting of Nb and Ti in a total amount of 0.005 to 0.05% And reheating the slab containing the remainder Fe and unavoidable impurities, followed by rolling to obtain a hot rolled steel sheet at a temperature at the finish rolling exit side of 800 to 950 ° C;
Winding the hot-rolled steel sheet at a temperature of 500 to 620 캜;
Rolling the rolled hot-rolled steel sheet at a reduction ratio of 40 to 70% to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet at a temperature of 770 to 830 캜;
Cooling the cold-rolled steel sheet continuously annealed to a temperature of 650 to 700 占 폚 at a cooling rate of 0.5 to 5 占 폚 / sec; And
Cooling the primary cold-rolled steel sheet to a temperature of 300 to 450 DEG C at a cooling rate of 5 to 20 DEG C / sec,
The secondary cold-rolled steel sheet has ferrite and martensite as its main structure, and the bainite, tempered martensite or the sum thereof is not more than 20% in cross-sectional area ratio Wherein the steel sheet has excellent ductility and bending workability.
5. The method of claim 4,
Wherein the content of C, Si, Mn, P, S, B, and Sb satisfies the following relational expression (1) and (2).
[Relation 1]
20 < (Si / Mn + 150B) / Sb < 50
[Relation 2]
C + Mn / 20 + Si / 30 + 2P + 4S? 0.30
(C, Si, Mn, P, S, B and Sb 'in the relational expressions 1 and 2 mean the weight% of the alloy component)
The method according to claim 4 or 5,
Further comprising the step of skin pass rolling the secondary cooled cold rolled steel sheet at a reduction ratio of 0.1 to 0.5%.

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