KR101406444B1 - Ultra high strength cold rolled steel sheet having excellent elongation and bendability and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a super high strength cold rolled steel sheet mainly used for a self-excavation, that is, an embossed part, and a manufacturing method thereof. More particularly, the present invention relates to a super high strength cold rolled steel sheet which has a carbon content of 0.1% or less, To an ultra high strength cold rolled steel sheet of 980 MPa grade which is excellent in elongation and bending workability by applying a slow cooling heat treatment method and a method of manufacturing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra high strength cold rolled steel sheet having excellent elongation and bending workability and a method of manufacturing the same.

TECHNICAL FIELD The present invention relates to an ultra-high strength cold-rolled steel sheet mainly used for parts for automobiles, particularly, forged parts, and a method of manufacturing the same.

At present, the fuel economy regulation is strengthened as a task for global environmental preservation, and weight reduction of automobile body is actively carried out. As a countermeasure, the weight of automobile material is reduced by strengthening of steel sheet. In addition, in order to improve the fuel consumption or durability of the automotive steel sheet, a steel sheet with higher strength is required, and an ultra-high strength steel sheet having a tensile strength of 980 MPa or more is used as a car body structure or a reinforcing material in view of collision safety and passenger protection.

Recently, as the number of cases of applying high-strength and high-ductility materials to molded parts has been increased, the application of ultra-high strength steels is actively applied to parts requiring a lot of machining.

However, the increase in strength of the steel sheet leads to deterioration of the molding processability and weldability, and development of materials complementary thereto is required.

In order to meet such a demand, it has been necessary to use a transformed structure steel sheet such as dual phase steel (DP steel), Transformation Induced Plasticity Steel (TRIP steel), or complex phase steel Has been developed.

For example, Patent Document 1 discloses a method of manufacturing a steel sheet excellent in formability by controlling the components and the structure, that is, the amount of retained austenite. Patent Document 2 also discloses a method of manufacturing a high strength steel sheet having good press formability by controlling components and microstructure . Patent Document 3 proposes a steel sheet excellent in workability including residual austenite of 5% or more, particularly excellent in local stretching. In Patent Document 4, bainite is a main phase and retained austenite is contained, And a manufacturing method for improving elongation flangeability are proposed.

However, these inventions deteriorate the weldability by adding a large amount of C and Si or Al in order to improve the elongation, and it is difficult to secure the plating quality by adding a large amount of Si and Al and it is difficult to secure the surface quality in steel making and performance, It is necessary to carry out a step of setting the cooling rate to 100 DEG C / s or more to obtain partial deformation of the steel sheet during cooling, and it is difficult to secure the flatness of the steel sheet.

On the other hand, ultra-high-strength steels are applied to parts requiring bending such as sill side, so even if the elongation is excellent, they can not be used as parts if the bendability deteriorates . Therefore, there is a demand for a method of improving the bending workability with a high elongation.

It is generally known that the constitution and ratio of the transformation phase present in the steel are controlled appropriately and the lower the strength ratio of the soft phase and the hard phase is, the better the bending workability is. For this purpose, the steel microstructure should be produced as bainite or tempered martensite instead of martensite. However, these transformation phases have a problem in that the yield strength increases sharply and the elongation is remarkably lowered. Therefore, it is most important to secure the composition ratio of the transformation images appropriately.

On the other hand, when an ultra-high-strength steel sheet having a tensile strength of 980 MPa or more is produced in an actual process, the yield strength is very high along with the tensile strength, so that the cold rolling property is greatly deteriorated due to the high strength of the hot- It is required to apply the heat treatment condition, which causes a problem that the operability is significantly lowered. Furthermore, since these materials change the transformation phases present in the steel very sensitively to the annealing temperature, the types and composition ratios of the transformation phases are changed by a slight change in the annealing temperature, so that the yield strength is remarkably changed and the elongation is decreased.

Therefore, it is necessary to develop a new product capable of securing a stable material in a wide range of annealing temperatures. However, in the known technologies such as Patent Documents 1 to 4 and Patent Document 5, insufficiently studied.

Japanese Laid-Open Patent No. 1994-145892 Japanese Laid-Open Patent Application No. 1994-145788 Japanese Patent Laid-Open No. 1993-070886 Japanese Patent Application Laid-Open No. 2004-292891 Japanese Patent Laid-Open No. 2005-105367

An object of the present invention is to provide an ultra-high strength cold-rolled steel sheet having a high tensile strength of 980 MPa or more and excellent in elongation and bending workability by appropriately controlling the kind and content of the alloy element and the manufacturing conditions thereof and a method for manufacturing the same I want to.

An aspect of the present invention is a ferritic stainless steel comprising 0.070 to 0.095% of C, 0.8 to 1.2% of Si, 2.0 to 2.4% of Mn, 0.001 to 0.10% of P, 0.001 to 0.10% of N, 0.01 to 0.1% of N, 0.5 to 1.0% of Cr, 0.03 to 0.15% of Mo, 0.001 to 0.006% of B and 0.01 to 0.10% of Sb, To 0.08%, and the balance Fe and other unavoidable impurities, Ceq expressed by the following formula (1) satisfies 0.27 or less, and Si, Mn, B and Sb satisfy the following formula The present invention provides an ultra-high strength cold rolled steel sheet excellent in elongation and bending workability.

&Quot; (1) "

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

&Quot; (2) "

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

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising the steps of: reheating a steel slab having the above-described composition system satisfying Ceq = 0.27 or less and satisfying Equation 2; Subjecting the reheated slab to a finish hot rolling in a temperature range of 800 to 950 캜; Rolling the hot-rolled steel sheet at 500 to 750 ° C; Cold rolling the rolled steel sheet at a cold reduction rate of 40 to 70%; Continuously annealing the cold-rolled steel sheet; Cooling the continuous annealed steel sheet; The present invention also provides a method of manufacturing an ultra-high strength cold rolled steel sheet excellent in elongation and bending workability, comprising the step of aggressively treating the cooled steel sheet for 200 to 300 seconds.

According to the present invention, it is possible to provide a steel sheet having a high strength of 980 MPa or higher in tensile strength, a low yield strength, an excellent elongation and bending workability, and a manufacturing method capable of ensuring the operability of such a steel sheet.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a view showing a hole expansion ratio (HER) according to a hardness ratio of a ferrite and a hard phase.
2 is a view showing the hole expansion ratio (HER) according to the elongation.
3 is a graph showing the results of measurement of changes in microstructure due to Si addition.
4 is a graph showing the results of measurement of the generation of hot internal oxides of a steel material satisfying Equation 2 of the present invention (Inventive Example 1 of Table 2) or unsatisfactory (Comparative Example 16 of Table 2).
5 is a graph showing changes in material (yield strength, tensile strength and elongation) with changes in annealing temperature (Inventive Example 1 of Table 2).

The inventors of the present invention have conducted intensive researches to solve the problem that the bending workability is deteriorated and the workability can not be ensured in the conventional ultrahigh strength steel material even when the elongation rate is excellent. As a result, the present inventors have found that the content of the alloy element and the manufacturing conditions It is possible to produce an ultra-high strength cold-rolled steel sheet which can be processed even in an embossed portion having a low yield strength and a sufficient elongation and bending workability, thereby completing the present invention.

Furthermore, even in the two-phase steels, it is possible to secure the weldability, the bending workability and the tensile properties at the same time, and also in a narrow range in which the deviation of the material, particularly the yield strength, does not exceed ± 40 MPa even in a wide range of the annealing temperature range of ± 20 ° C. And proposes the present invention.

Hereinafter, as an aspect of the present invention, an ultra-high strength cold rolled steel sheet excellent in elongation and bending workability will be described.

The bending workability means the ratio of the minimum bending radius to the unit thickness (R / t), where the minimum bending radius ratio (R) means the minimum radius at which no crack occurs in the outer periphery of the plate after bending test do.

The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet comprises 0.070 to 0.095% of C, 0.8 to 1.2% of Si, 2.0 to 2.4% of Mn, 0.001 to 0.10% of P, Of Ti, 0.003 to 0.08% of Nb, and 0.003 to 0.08% of Nb in an amount of 0.5 to 1.0% by mass of Cr, 0.5 to 1.0% of Cr, 0.03 to 0.15% of Mo, 0.001 to 0.006% of B and 0.01 to 0.10% And the balance Fe and other unavoidable impurities.

In the cold-rolled steel sheet of the present invention, the reason for restricting the components as described above will be described in detail.

At this time, the content of the component elements means all the weight percentages.

C: 0.070 to 0.095%

Carbon (C) is a very important element that is added for strengthening the transformational structure. It promotes the strengthening and promotes the formation of martensite in the composite structure steel. That is, as the carbon content increases, the amount of martensite in the steel increases. However, if the content exceeds 0.095%, the hole expandability and weldability decrease. On the other hand, if the content is less than 0.070%, it is very difficult to secure the strength. Therefore, it is preferable to limit the content of C to 0.070 to 0.095%.

Si: 0.8 to 1.2%

Silicon (Si) is an element that promotes ferrite transformation and increases the content of carbon in untransformed austenite to facilitate the formation of a composite structure of ferrite and martensite, and also induces solid solution strengthening effect of Si itself. Such Si is a very useful element for securing the strength and the material, but it is preferable to limit the range because it not only causes surface scale defects in relation to the surface characteristics but also deteriorates the chemical processability.

In the present invention, it is necessary to control Si to 0.8 to 1.2% in order to satisfy an elongation of an ultra-high strength steel having a tensile strength of 980 MPa or more at 15% or more and prevent deterioration of dent and paintability due to hot internal oxide . If the content of Si is less than 0.8%, sufficient ferrite can not be ensured and it is difficult to secure a ductility of 15% or more, which is the target of the present invention. On the other hand, when the content exceeds 1.2%, the elongation is increased. Deterioration may be expected, and the weldability may deteriorate with the decrease of the strength.

Mn: 2.0 to 2.4%

Manganese (Mn) is an element that refines particles without damaging the ductility and precipitates sulfur (S) completely into MnS in the steel to prevent hot brittleness due to the formation of FeS and to strengthen the steel. In the composite structure steel, the martensite phase can be more easily formed by lowering the critical cooling rate at which the martensite phase is obtained. However, when the content is less than 2.0%, it is difficult to secure the desired strength in the present invention. On the other hand, when the content exceeds 2.4%, there is a high possibility that problems such as weldability and hot rolling property are likely to occur. 2.4%.

P: 0.001 to 0.10%

(P) is the substitutional alloying element having the largest effect of solid solution strengthening, and has the role of improving the in-plane anisotropy and improving the strength. However, when the content is less than 0.001%, it is difficult to secure the above-mentioned effect and causes a problem of manufacturing cost. On the other hand, if the content exceeds 0.10%, the press formability may deteriorate and brittleness of steel may occur, The content of P is preferably limited to 0.001 to 0.10%.

S: not more than 0.010%

Sulfur (S) is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. Therefore, when the content exceeds 0.010%, the ductility and weldability of the steel sheet are likely to be deteriorated, so that the content of S is preferably limited to 0.010% or less.

Sol.Al: 0.01 to 0.10%

Aluminum (Al) is a component effective for deoxidation by bonding with oxygen in steel and for improving the martensitic hardenability by distributing carbon in ferrite to austenite like Si. However, when the content is less than 0.01%, it is difficult to secure the above-mentioned effect. On the other hand, when the content exceeds 0.10%, the effect is saturated and the production cost increases, so that the content of soluble Al is 0.01 to 0.10% .

N: not more than 0.01%

Nitrogen (N) is an element that is effective in stabilizing austenite. When the content exceeds 0.01%, the stability of austenite is greatly increased, which hinders formation of bainite to be formed in the present invention. Therefore, it is preferable to limit the content of N to 0.01% or less.

Cr: 0.5 to 1.0%

Chromium (Cr) is a component added to improve hardenability of steel and ensure high strength, and is an element that plays a very important role as a bainite formation promoting element in the present invention. However, when the content is less than 0.5%, it is difficult to secure the above effect. On the other hand, when the content exceeds 1.0%, the effect is saturated and economically disadvantageous. Therefore, it is preferable to limit the content of Cr to 0.5 to 1.0% Do.

Mo: 0.03 to 0.15%

Molybdenum (Mo) is a component added to improve the hardenability of steel and to secure high strength together with Cr, and serves to improve the strength of the ferrite matrix structure by generating Mo-based fine carbides in the steel. Such an effect reduces the difference in phase strength between the transformed structure and the ferrite, and thus has an advantageous effect on the bending workability. When the content is less than 0.03%, it is difficult to obtain the above-mentioned effect. On the other hand, when the content exceeds 0.15%, an excessive increase of the production cost is expected, so it is preferable to limit the Mo content to 0.03-0.15%.

B: 0.001 to 0.006%

Boron (B) is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is added as an element that suppresses ferrite formation and promotes the formation of bainite. If the content of B is less than 0.001%, it is difficult to obtain the above-mentioned effect. On the other hand, if it exceeds 0.006%, B may be excessively concentrated on the surface to deteriorate the plating adhesion, .

Sb: 0.01 to 0.10%

The antimony (Sb) is an element to be added in order to improve the plating ability in addition to the dent resistance of the annealed sheet and the prevention of the oxidation of the hot internal oxidation in the present invention. Sb suppresses the surface enrichment of oxides such as MnO, SiO ₂ and Al ₂ O ₃ , thereby reducing the surface defects caused by the dent, and has an excellent effect in suppressing the coarsening of the surface agglomerates due to the temperature increase and the hot rolling process change have. If the content of Sb is less than 0.01%, it is difficult to secure the above-mentioned effect. On the other hand, if it exceeds 0.10%, the deterioration of material due to grain boundary segregation of Sb, the increase of manufacturing cost and the deterioration of workability may be caused. Therefore, the content of Sb is preferably limited to 0.01 to 0.10%.

Ti or Nb: 0.003 to 0.080%

Titanium (Ti) and niobium (Nb) in the steel are effective elements for increasing the strength and grain size of the steel sheet. If Ti or Nb is added in an amount of less than 0.003%, it is difficult to obtain the above-mentioned effect. On the other hand, if it exceeds 0.080%, ductility can be greatly lowered due to an increase in manufacturing cost and excessive precipitates. Therefore, it is preferable to limit the content of Ti or Nb to 0.003 to 0.080%.

The present invention is composed of the remainder Fe and other unavoidable impurities in addition to the above components.

According to the present invention, when designing an alloy of a steel sheet having the above-mentioned composition range, the value of Ceq expressed by the following formula (1) satisfies 0.27 or less and the alloy composition ratio of Si, Mn, B and Sb is expressed by the following formula Is satisfied.

&Quot; (1) &quot;

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

&Quot; (2) &quot;

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

Equation (1) is obtained as an empirical numerical value of a component relationship capable of securing the weldability of the steel sheet. That is, the elements of C, Mn, Si, P, and S in the steel serve to increase the carbon equivalent, and as is well known, the higher the carbon equivalent, the worse the weldability is. When the super high strength steel according to the present invention is used, a condition in which welding failure of a special plate, which is a welding method to be mainly applied, does not occur will be described through the repeated experiment. If the value calculated by Equation (1) exceeds 0.27, it means that there is a high possibility of welding failure.

In Equation (2), the component relation capable of ensuring surface quality is obtained as an empirical value. That is, Mn, Si, and B in steels are elements having the property of forming a grains on the surface during the annealing operation, and the more the grains of these elements are, the lower the plating characteristics are. Particularly, in the present invention, since Si is added at a high level, it is preferable to control the component system to prevent the problems such as the oxide in the hot-rolled steel sheet and the dent on the annealing and deterioration of the surface quality of the plating. On the other hand, Sb is very advantageous in terms of surface quality because it interferes with the grain boundary diffusion of the above surface enrichment elements. For example, when the value calculated by the equation (2) has a value of 20 to 50, it is possible to secure a good surface quality.

It is preferable that the microstructure of the steel sheet satisfying the above-mentioned component system and equations (1) and (2) includes 35 to 45% of martensite, 15 to 30% of bainite and 30 to 40% of ferrite.

The steel sheet provided in the present invention preferably contains an appropriate amount of bainite, that is, bainite of 15% or more in order to ensure bending workability. However, when the amount of bainite exceeds 30%, ductility deteriorates and yield strength increases . Further, in order to ensure excellent ductility together with the bending workability, it is preferable to contain an appropriate amount of ferrite, that is, 30 to 40% of ferrite.

The steel sheet constituted by the above-mentioned structure has a deviation of the yield strength not exceeding ± 40 MPa, and contains bainite and ferrite at an appropriate level, so that it has not only formability but also excellent bending workability and ductility.

Hereinafter, a method for manufacturing a cold-rolled steel sheet as described above will be described in detail.

The method for manufacturing a cold-rolled steel sheet according to the present invention roughly comprises the steps of heating a slab made of the above-mentioned component system, rolling the heated slab, finishing rolling at a temperature of 800 to 950 ° C, . Thereafter, the steel sheet is cold-rolled at a cold reduction of 40 to 70%, and the cold-rolled steel sheet is subjected to continuous annealing at 770 to 830 ° C, followed by first and second cooling to complete cooling at 350 to 450 ° C, .

Hereinafter, detailed conditions for each step will be described.

Finishing rolling temperature: 800 ~ 950 ℃

After the slab is reheated, hot rolling is carried out. The finish rolling in the hot rolling is preferably performed such that the temperature at the outlet side is between 800 and 950 캜. That is, when the hot-rolling temperature is lower than 800 ° C, there is a high possibility that the hot-deforming resistance will increase sharply, and the top, tail and edge of the hot- The property deteriorates. On the other hand, when the temperature exceeds 950 DEG C, not only a too large oxidation scale is generated but also the microstructure of the steel sheet is likely to be coarsened.

Coiling temperature: 500 ~ 750 ℃

After completion of the hot rolling, the steel sheet is wound at 500 to 750 ° C. If the coiling temperature is less than 500 占 폚, excessive martensite or bainite is generated to cause an excessive increase in strength of the hot-rolled steel sheet, which may cause manufacturing problems such as defects in shape due to load during cold rolling. On the other hand, when the temperature exceeds 750 ° C., surface coagulation due to elements that lower the wettability of hot dip galvanizing such as Si, Mn, and B becomes severe, so that the coiling temperature is preferably limited to 500 to 750 ° C. .

Thereafter, the wound hot rolled sheet is pickled and then subjected to cold rolling.

Reduction rate in cold rolling: 40 to 70%

When cold rolling is carried out, the cold rolling reduction rate is preferably 40 to 70%. When the cold rolling reduction is less than 40%, the recrystallization driving force is weakened and there is a large possibility of obtaining a good recrystallized grain, and the shape correction is very difficult. On the other hand, when the cold rolling reduction ratio exceeds 70%, cracks are likely to occur at the edge of the steel sheet, and the rolling load is rapidly increased. Therefore, it is preferable that the reduction rate in cold rolling is limited to 40 to 70%.

Annealing condition

The cold-rolled sheet thus obtained is subjected to continuous annealing at 770 to 830 占 폚. When the annealing temperature in the continuous annealing is less than 770 캜, it is difficult to form a sufficient austenite, and it is difficult to secure a desired strength in the present invention. On the other hand, when the annealing temperature exceeds 830 캜, the amount of bainite increases sharply due to excessive austenite formation, resulting in an excessive increase in yield strength and deterioration of ductility.

The annealing time at the time of continuous annealing is preferably carried out under generally applicable conditions.

Cooling condition and overexposure treatment

The cooling after the continuous annealing is carried out in primary and secondary. First, the steel sheet subjected to the continuous annealing is first cooled to 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / s. The primary cooling step is to increase the ductility and strength of the steel sheet by securing the equilibrium carbon concentration of ferrite and austenite. When the termination temperature of the primary cooling is less than 650 ° C or more than 700 ° C, It is difficult to secure the desired ductility and strength.

After the primary cooling, the secondary cooling is one of the important control factors in the present invention. It is preferable to perform secondary cooling from 350 to 450 ° C at a cooling rate of 5 to 20 ° C / s, Overflow treatment is performed in the night section. The cooling end temperature condition is a very important condition for ensuring both ductility and bending workability at the same time. When the cooling end temperature is lower than 350 DEG C, it is difficult to secure sufficient bainite amount because of a short staying time in the bainite region during over- It is difficult to obtain 15 to 30% of bainite which is the object of the invention, and the problem of plate warping due to quenching occurs. Further, as a large amount of martensite is produced, a difference in strength between the ferrite and the second phase is increased and the bending workability is deteriorated. On the other hand, when the cooling end temperature exceeds 450 ° C., the time for staying in the bainite region becomes very large mainly during the overflow treatment, resulting in an increase in yield strength and deterioration in ductility due to excessive bainite.

In addition, it is preferable to control the treatment time in the overexposure treatment to 200 to 300 seconds. The amount of bainite to be intended in the present invention can be secured by over-processing the steel sheet during the above-mentioned period.

In addition, the steel sheet is subjected to skin pass rolling in the range of 0.3 to 1.0% after the over-treatment. Generally, when skeletal rolling of a textured steel is performed, a yield strength increase of 50 to 100 MPa or more occurs without increasing the tensile strength in most cases. Therefore, when the reduction ratio is less than 0.3%, it is very difficult to control the shape of the ultra-high strength steel such as the steel of the present invention. On the other hand, when the work is performed in excess of 1.0%, the yield strength target 700 MPa), and the workability is greatly unstable due to the high stretching operation, so that the reduction rate in the skin pass rolling is preferably limited to 0.3 to 1.0%.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

( Example )

The steel slabs prepared as shown in Table 1 below were vacuum-melted and then heated in a heating furnace at a temperature of 1200 DEG C for 1 hour, extracted and hot-rolled, and then wound. At this time, the hot rolling was completed at a temperature range of 880 to 900 ° C, and the coiling temperature was set at 650 ° C. Thereafter, the hot-rolled steel sheet was subjected to pickling treatment and cold-rolled at a cold-reduction rate of 50%. Thereafter, the cold-rolled steel sheet was subjected to continuous annealing under the annealing conditions shown in Table 2, and after primary cooling to 650 to 700 占 폚 at a cooling rate of 1 to 10 占 폚 / s, And cooled. Then, the cold-rolled steel sheet was subjected to a heat treatment for 250 seconds, and skin pass rolling was performed at a reduction ratio of 0.5%.

From the cold-rolled steel sheet thus prepared, a tensile test specimen of JIS No. 5 was prepared and the material thereof was measured. Specifically, the yield strength (YS), tensile strength (TS), elongation (El), yield ratio (YR) and bending workability (R / t <1) of the specimen were measured and are shown in Table 2 below. In addition, the microstructures of the hot-rolled steel sheets were observed for each steel type, and the presence of internal oxides was confirmed. After that, the steel materials of Table 2 were prepared from the GA-coated steel sheet, and then the samples were cut to a size of 20 mm × 50 mm, The results are shown together.

Steel grade C Si Mn P S Sol.
Al Cr Mo Ti Nb N B Sb Equation 1 Equation 2 Remarks One 0.07 1.00 2.3 0.012 0.003 0.040 0.85 0.09 0.020 0.051 0.0038 0.0025 0.03 0.25 27.0 Invention river 2 0.07 1.10 2.3 0.009 0.002 0.050 0.85 0.13 0.025 0.055 0.0050 0.0023 0.02 0.25 41.2 Invention river 3 0.07 0.95 2.2 0.010 0.005 0.035 0.85 0.15 0.022 0.045 0.0030 0.0025 0.03 0.25 26.9 Invention river 4 0.09 1.00 2.3 0.006 0.004 0.060 0.85 0.06 0.050 0.060 0.0040 0.0025 0.04 0.27 20.2 Invention river 5 0.08 1.10 2.2 0.010 0.003 0.070 0.85 0.09 0.030 0.035 0.0056 0.0025 0.02 0.26 43.8 Invention river 6 0.07 0.89 2.2 0.010 0.003 0.060 0.90 0.07 0.020 0.045 0.0055 0.0025 0.03 0.24 26.0 Invention river 7 0.08 0.96 2.2 0.010 0.003 0.040 0.95 0.12 0.025 0.045 0.0047 0.0025 0.02 0.25 40.6 Invention river 8 0.08 1.00 2.2 0.009 0.005 0.055 0.90 0.10 0.035 0.058 0.0070 0.0025 0.03 0.26 27.7 Invention river 9 0.07 0.90 2.2 0.011 0.004 0.055 0.95 0.11 0.045 0.065 0.0060 0.0025 0.03 0.25 26.1 Invention river 10 0.08 0.85 2.2 0.012 0.005 0.060 0.85 0.14 0.052 0.059 0.0055 0.0025 0.02 0.26 38.1 Invention river 11 0.08 1.10 2.5 0.010 0.003 0.050 0.85 0.05 0.040 0.045 0.0041 0.0005 0.04 0.27 12.9 Comparative steel 12 0.07 1.10 2.6 0.011 0.004 0.050 0.90 0.09 0.020 0.050 0.0035 0.0005 0.02 0.27 24.9 Comparative steel 13 0.09 1.00 2.3 0.009 0.005 0.035 0.95 0.06 0.020 0.040 0.0070 0.0010 0.04 0.28 14.6 Comparative steel 14 0.11 1.50 2.6 0.010 0.006 0.040 0.95 0.09 0.010 0.055 0.0050 0.0015 0.02 0.33 40.1 Comparative steel 15 0.13 1.10 2.3 0.009 0.005 0.030 0.90 0.15 0.030 0.050 0.0060 0.0005 0.03 0.32 18.4 Comparative steel 16 0.09 0.10 3.1 0.011 0.006 0.050 0.85 0.08 0.025 0.055 0.0065 0.0020 0.10 0.29 3.30 Comparative steel 17 0.10 0.50 2.7 0.011 0.003 0.060 0.85 0.15 0.040 0.030 0.0050 0.0025 0.15 0.29 3.70 Comparative steel

Steel grade Annealing
Temperature
(° C) Secondary cooling end temperature
(° C) Skin pass rolling rate
(%) YS
(MPa) TS
(MPa) T-El
(%) YR R / t < 1 Internal oxide generation GA material
Plated
Adhesiveness Remarks One 800 270 0.5 585.3 1079.7 16.5 0.54 NG X ◎ Comparative Example 1 800 450 0.5 604.6 1093.4 16.4 0.55 OK X ◎ Inventory 1 2 780 300 0.5 601.2 1030.6 15.7 0.58 NG X ◎ Comparative Example 2 780 450 0.5 616.4 1021.1 16.6 0.60 OK X ◎ Inventory 2 3 800 300 0.5 621.1 1050.2 15.8 0.59 NG X ◎ Comparative Example 3 800 450 0.5 605.8 1090.7 16.3 0.56 OK X ◎ Inventory 3 4 820 270 0.5 611.6 1039.9 16.9 0.59 NG X ◎ Comparative Example 4 820 450 0.5 643.4 1039.6 15.9 0.62 OK X ◎ Honorable 4 5 800 250 0.5 620.9 1022.6 16.4 0.61 NG X ◎ Comparative Example 5 800 450 0.5 617.9 1028.4 16.7 0.60 OK X ◎ Inventory 5 6 800 270 0.5 631.8 1056.6 17.1 0.60 NG X ◎ Comparative Example 6 800 450 0.5 629.4 1028.3 17.5 0.61 OK X ◎ Inventory 6 7 810 300 0.5 614.6 1086.2 15.5 0.57 NG X ◎ Comparative Example 7 810 450 0.5 624.0 1046.7 16.5 0.60 OK X ◎ Honorable 7 8 810 450 0.5 609.4 1090.0 17.6 0.56 OK X ◎ Honors 8 9 780 450 0.5 618.4 1014.5 16.4 0.61 OK X ◎ Proposition 9 10 790 450 0.5 601.3 1065.0 16.8 0.56 OK X ◎ Inventory 10 11 800 450 0.5 670.9 1089.1 14.8 0.62 OK ○ X Comparative Example 8 12 820 450 0.5 700.2 1119.2 13.8 0.63 NG X ◎ Comparative Example 9 13 800 450 0.5 610.5 1025.2 16.5 0.60 OK ○ X Comparative Example 10 14 800 450 0.5 720.9 1107.2 14.5 0.65 NG X ◎ Comparative Example 11 15 790 450 0.5 600.3 1020.1 16.3 0.59 NG ○ △ Comparative Example 12 16 800 450 0.5 762.8 1156.9 9.6 0.66 NG ○ X Comparative Example 13 17 800 450 0.5 730.1 1092.2 12.5 0.67 NG ○ X Comparative Example 14

(The bending workability of the specimen in Table 2 is expressed as 'OK' for the material which does not crack on the surface in the bending test of R / t 1.0 and as 'NG' for the material which cracks. , And the results of observation of the oxide inside the hot-rolled steel sheet were marked with "○" when the internal oxide was present and "X" when the internal oxide was not present.

The plating adhesion was evaluated by applying a 60 ° bend test, then re-stretching the tape to the bent portion, detaching it, and measuring the width of the falling plated layer as follows; ◎: When there is no plating coming off, or when the width is within 1 ~ 3mm, △: When the width of plating is 3 ~ 5mm or less, X: When the width of plating is 5mm or more. Generally, the plating adhesion is judged to be excellent in adhesion property when the plating peeling width is within 3 mm.)

As shown in Tables 1 and 2, the inventive examples (1 to 10 in Table 2) satisfying the composition range and the manufacturing conditions proposed in the present invention were obtained by annealing at 780 to 820 캜, , It was confirmed that the yield strength was in the range of 600 to 650 MPa and the elongation was 15.9 to 17.6%, which satisfied the target yield strength of 700 MPa or less and elongation of 15% or more in the present invention. In addition, from the viewpoint of bending workability, excellent bending workability characteristics are exhibited by satisfying all the conditions that the index R / t is 1.0 or less. These steels were excellent in weldability, dent defects and plating adhesiveness sufficiently satisfying the condition of Equation (2) satisfying the condition that Ceq of Equation 1 representing weldability is 0.27 or less, I could confirm.

On the other hand, in the case where the secondary cooling end temperature does not satisfy the temperature range proposed in the present invention (Comparative Examples 1 to 7 in Table 2) even if the constituent conditions of the inventive steel are satisfied, even if the yield strength is 700 MPa or less, It can be seen that the occurrence of martensite causes an increase in the phase-to-phase strength difference with respect to the ferrite, and the bending workability is deteriorated.

In the case of Comparative Examples 11 to 17, which do not satisfy the expressions (1) and (2) proposed by the present invention, the composition range indicated in the present invention is not satisfied, or Ceq is high even if the material is satisfied, Or the condition of Equation (2) is not satisfied, thereby increasing the internal oxide of the hot-rolled steel sheet, resulting in deterioration of the plating adhesion in the GA material.

As shown in Fig. 5, it can be confirmed that a material with a variation in annealing temperature, in particular, a deviation in yield strength of 50 MPa or less, can secure a very stable material.

INDUSTRIAL APPLICABILITY As described above, the present invention can provide a steel material which is excellent in not only the weldability but also the bending workability, the elongation, and the plating adhesion of the GA material by strictly controlling the composition system and the manufacturing conditions. And it was able to satisfactorily satisfy the conditions required by the customer.

As described above, the inventive steels according to the present invention have a yield strength of 700 MPa or less and a bending workability (R / t) of 1.0 or less. Typically, to improve bendability or hole expandability (HER), the hardness difference between the microstructures in the steel must be reduced. That is, as shown in FIGS. 1 and 2, as the hardness difference between the microstructures decreases, the bending workability and hole expandability are improved. More specifically, for this purpose, it is necessary to reduce the amount of martensite and to produce bainite or tempered martensite.

However, in the present invention, the effect of improving the bending workability is achieved by using a relatively high content of Si. Generally, Si has an effect of increasing the activity of carbon in the steel to disperse the hot-rolled pearlite band. The dispersion effect of the pearlite causes dispersion of the martensite structure in the annealing step and improves the bending workability. In addition, the strength of the final product can be secured by increasing the martensite strength in the annealed sheet by concentrating the carbon in the austenitic system. Furthermore, the elongation is improved due to the cleaning effect of the ferrite base which reduces the amount of the solid carbon in the annealed plate ferrite matrix. Fig. 3 shows the microstructural changes of the hot-rolled steel sheet and the annealed steel sheet according to the addition of Si. In the steel having a higher Si content than the steel having a low Si content, the microstructure of the hot-rolled steel sheet was homogenized. It can be seen that they are uniformly dispersed. The dispersion of the martensite exerts an effect of dispersing the stress on the external load, thereby improving the ductility and the bending workability.

However, when a large amount of Si is added to high-strength steel, various problems occur. In particular, internal oxides are generated by Si, Mn and B in the hot-rolled steel sheet, and these oxides are diffused to the surface in the annealing step, Thereby causing a dent. In addition, such oxides deteriorate the wettability of the plating layer, thereby deteriorating the plating characteristics. Therefore, in order to solve such a problem, in the present invention, when Sb is added to a certain level and the content of Si, Mn and B is strictly controlled in accordance with the condition of Equation (2) with Sb, the oxide inside the hot- thereby preventing dent problems and deterioration of plating properties. FIG. 4 is a graph showing the relationship between the content of oxide-forming elements such as Si and Mn ((a); Comparative Example 16 in Table 2) and (2) (A) shows that there is a very large amount of oxides around the grain boundaries, which is not satisfied in the present invention. These oxides are mainly MnO and SiO ₂ , which adhere to the surface of rolls during annealing, causing dent defects on the steel sheet, and also on the surface of the steel sheet, which reduces the adhesion of the plating during GA plating. On the other hand, in the case of the inventive steel (b) satisfying the formula (2), it can be confirmed that no internal oxide is generated at all, and it can be judged that this characteristic has a very favorable influence on the plating adhesion.

Further, in order to improve the weldability, the content of C, Si, Mn, P, and S in the alloy design satisfies the expression (1) and the carbon content added is designed to be 0.1% or less. Normally, in case of TRIP steel, carbon content is added by 0.1% or more and Si is added by 1.5% or more in order to improve ductility through securing retained austenite in steel. However, adding such a large amount of an alloy element is advantageous in securing elongation. However, in the case of spot welding, which is a welding method which is mainly used in the processing of automotive steel sheets, an increase in strength between the welded portion and the base metal increases, Etc., resulting in frequent welding defects and plating defects due to excessive addition of Si. According to the present invention, not only the plating ability but also the welding performance are improved through a steel material having a low content of carbon and Si as compared with a conventional TRIP steel.

Claims (7)

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, 0.01 to 0.10% of Cr, 0.03 to 0.095% of C, 0.8 to 1.2% % Of Cr, 0.5 to 1.0% of Cr, 0.03 to 0.15% of Mo, 0.001 to 0.006% of B and 0.01 to 0.10% of Sb,
0.003 to 0.080% of Ti, and 0.003 to 0.080% of Nb, the balance Fe and other unavoidable impurities,
Ceq represented by the following formula (1) satisfies 0.27 or less, and Si, Mn, B and Sb satisfy the following formula (2)
Wherein the microstructure is composed of 35 to 45% of martensite, 15 to 30% of bainite and 30 to 40% of ferrite, and is excellent in elongation and bending workability.

&Quot; (1) &quot;
Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

&Quot; (2) &quot;
20 < (Si / Mn + 150B) / Sb < 50
delete The method according to claim 1,
Wherein the steel sheet has a tensile strength of 980 MPa or more, a yield strength of 700 MPa or less, an elongation of 15% or more, and a bending workability (R / t) of 0.1 or less.
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, 0.03 to 0.095% of C, 0.8 to 1.2% Of Ti, 0.003 to 0.08% of Nb, and 0.003 to 0.08% of Nb in an amount of 0.1 to 10% by mass, Cr: 0.5 to 1.0%, Mo: 0.03 to 0.15%, B: 0.0010 to 0.0060% And the remainder is Fe and other unavoidable impurities, and Ceq expressed by the following formula (1) satisfies 0.27 or less, and Si, Mn, B and Sb satisfy the following formula (2) step;
Subjecting the reheated slab to a finish hot rolling in a temperature range of 800 to 950 캜;
Rolling the hot-rolled steel sheet at 500 to 750 ° C;
Cold rolling the rolled steel sheet at a cold reduction rate of 40 to 70%;
Continuously annealing the cold-rolled steel sheet;
Cooling the continuous annealed steel sheet; And
Treating the cooled steel sheet for 200 to 300 seconds,
The cooling step may be performed by first cooling at a cooling rate of 1 to 10 ° C / s to 650 to 700 ° C, second cooling at a cooling rate of 5 to 20 ° C / s, cooling at a temperature of 350 to 450 ° C And then finishing the heat treatment at a temperature higher than the melting point of the cold-rolled steel sheet.

&Quot; (1) &quot;
Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

&Quot; (2) &quot;
20 < (Si / Mn + 150B) / Sb < 50
5. The method of claim 4,
Wherein the continuous annealing is performed at 770 to 830 캜.
delete 5. The method of claim 4,
Further comprising the step of skin pass rolling at a reduction ratio of 0.3 to 1.0% after the above-mentioned overaging treatment.

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