KR101767706B1 - High yield ratio ultra high strength steel cold rolled steel sheet having excellent bendability and method for producing the same - Google Patents

Abstract

An aspect of the present invention is to provide a cold-rolled steel sheet having excellent strength and excellent elongation and molding processability and a method of manufacturing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength, low-strength, high-strength cold-rolled steel sheet excellent in bending workability and a method for manufacturing the same. BACKGROUND ART [0002]

TECHNICAL FIELD The present invention relates to a high yield and high specific gravity ultra high strength cold rolled steel sheet excellent in bending workability and a method for producing the same.

Recently, steel plates for automobiles are required to have higher strength to improve fuel economy and durability by various environmental regulations and energy use regulations. Particularly, as the impact stability regulation of automobiles has been spreading recently, high-strength steels excellent in yield strength have been adopted as structural members such as members, seat rails and pillars in order to improve the impact resistance of the vehicle body. The higher the yield strength value against tensile strength of the structural member, that is, the higher the yield ratio (tensile strength / yield strength), the more favorable the impact energy absorbing ability.

Therefore, a method of using a high strength steel sheet having a yield ratio of 0.75 or more as a structural member or a collision member is being sought. The indispensable properties required for the structural member or the impact member are the spot weldability and the bending characteristics.

Patent Document 1 proposes a technique of ensuring high yield, weldability and ductility for a steel having a tensile strength of 780 MPa or more. However, when the above-described technique is applied to an actual process, it is difficult to control the shape due to the high strength of the hot-rolled steel sheet as an intermediate material, and the cold rolling is greatly reduced due to an increase in the rolling load, and quenching heat- Therefore, there is a problem that the operability such as the shape control of the feldspar and the occurrence of surface defects is greatly deteriorated.

In addition, generally, as the yield ratio of the steel sheet is increased, the strength is increased. As the strength is increased, the elongation rate is decreased and the molding processability is lowered. Therefore, In fact.

Japanese Patent Application Laid-Open No. 2005-105367

An aspect of the present invention is to provide a cold-rolled steel cold-rolled steel sheet having excellent strength and excellent bending workability at high porosity, and a method of manufacturing the same.

The high yield and high specific gravity super high strength cold rolled steel sheet excellent in bending workability which is one aspect of the present invention contains 0.08 to 0.11% of carbon (C), 0.05 to 0.6% of silicon (Si), 2.6 to 3.0 0.01 to 0.1% of aluminum (Al.Al), 0.2 to 0.7% of chromium (Cr), 0.001 to 0.006% of boron (B), 0.001 to 0.1% of antimony (Sb) (Ceq) as defined by the following formula (1) is 0.29 or less, and the microstructure has an area fraction percentage of 70% or less, preferably 0.1 to 0.1%, sulfur (S): 0.01% or less, the balance Fe and other unavoidable impurities. To 95% bainite and balance ferrite and martensite.

[Equation 1]

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

In the above formula (1), C, Mn, Si, P and S represent the content (weight%) of the corresponding element, respectively.

A method for producing a high-yield and high-strength, ultra-high-strength cold-rolled steel sheet excellent in bending workability, which is another aspect of the present invention, comprises 0.08 to 0.11% of carbon (C), 0.05 to 0.6% of silicon (Si) (B): 0.001 to 0.006%, antimony (Sb): 0.001 to 0.1%, phosphorus (B) P): 0.001 to 0.1%, sulfur (S): 0.01% or less, the balance Fe and other unavoidable impurities, reheating the reheated slab to a hot rolling finish temperature of 800 to 950 캜 Rolling the rolled steel sheet at a temperature of 500 to 700 DEG C, cold rolling the rolled steel sheet at a reduction ratio of 40 to 70%, cooling the cold rolled steel sheet at a temperature of 820 to 860 DEG C Continuous annealing at a soaking section temperature (SST), cooling the continuously annealed steel sheet at a first cooling rate of 1 to 10 占 폚 / sec, cooling at a first cooling end temperature of 650 to 700 占 폚 Cooling the first cooled steel sheet at a second cooling rate of 5-20 DEG C / sec and cooling the steel sheet at a second cooling end temperature (RCST) of 250-400 DEG C Pass rolling the second cooled steel sheet at a reduction ratio of 0.1 to 1.0%, wherein the annealing temperature and the second cooling start temperature satisfy the following expression (2): " (2) " .

&Quot; (2) "

0.015SST - 0.013RCST - (0.01SST / RCST) ≥ 7.1

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

The present invention can provide an ultra-high strength cold rolled steel sheet having a bending workability of 1.0 or less as an R / t value, a Ceq value of 0.29 or less as an indicator of weldability, a yield strength of 750 MPa or more and a yield ratio of 0.75 or more.

FIG. 1 is a graph showing the relationship between hardness ratio and hole expansion ratio according to microstructure.
2 is a graph showing the relationship between elongation and hole expandability depending on the microstructure.
Fig. 3 (a) is a photograph showing the microstructure of Inventive Example 1, and Fig. 3 (b) is a photograph showing the microstructure of Comparative Example 1. Fig.
Fig. 4 (a) is a photograph showing the result of the bending test of Inventive Example 1, and Fig. 4 (b) is a photograph showing the result of the bending test of Comparative Example 1. Fig.

The inventors of the present invention have conducted studies to provide a high strength steel, and as a result, have found that it is preferable to use a two phase structure steel.

Generally, in order to produce a steel having a two-phase structure, an appropriate amount of ferrite and austenite are secured by annealing-cracking treatment in a two-phase region between Ar ₁ and Ar ₃ , and then austenite is transformed into martensite through quenching, Securing martensite. The characteristic of these steels is to secure ductility by soft ferrite and to secure the desired level of strength through martensite. However, the two-phase structure steel has a very large difference in strength between the two phases, so cracks can easily occur at the interface of the external deformation. These cracks are a main factor for deteriorating hole expandability or bending workability.

To solve this problem, it is necessary to reduce the difference in phase hardness between two-phase structure steels. As shown in Figs. 1 and 2, it can be seen that the bending workability and hole expansion are improved as the difference in hardness between phases is reduced. In order to obtain such an effect, it is necessary to reduce the amount of martensite and to produce bainite or tempered martensite. It is necessary to increase the annealing temperature to more than Ar ₃ to secure 100% of austenite, After bainite and martensite are secured in the ferrite and the main phase, or annealing is performed in the bimetallic zone, the ferrite and the martensite are secured by quenching, and the carbide is precipitated in the martensite through tempering (tempered martensite) The strength difference can be reduced. However, these studies show that the yield ratio (YR) increases due to an excessive increase in yield strength versus tensile strength, which leads to deterioration of ductility, so that proper phase fraction control is required. In addition, the weldability may deteriorate due to excessive addition of alloying elements.

Accordingly, the inventors of the present invention have conducted studies to solve the above problems, and as a result, it has been found that by controlling precisely the constituent relations of the alloy components and the respective conditions of the manufacturing method, it is possible to provide an ultra- high strength cold- rolled steel sheet excellent in bending workability, And the present invention has been completed.

Hereinafter, a high yield and high specific gravity ultra high strength cold rolled steel sheet excellent in bending workability which is one aspect of the present invention will be described in detail.

The high yield and high specific gravity super high strength cold rolled steel sheet excellent in bending workability which is one aspect of the present invention contains 0.08 to 0.11% of carbon (C), 0.05 to 0.6% of silicon (Si), 2.6 to 3.0 0.01 to 0.1% of aluminum (Al.Al), 0.2 to 0.7% of chromium (Cr), 0.001 to 0.006% of boron (B), 0.001 to 0.1% of antimony (Sb) (Ceq) as defined by the following formula (1) is 0.29 or less, and the microstructure has an area fraction percentage of 70% or less, preferably 0.1 to 0.1%, sulfur (S): 0.01% or less, the balance Fe and other unavoidable impurities. To 95% bainite and balance ferrite and martensite.

[Equation 1]

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

In the above formula (1), C, Mn, Si, P and S represent the content (weight%) of the corresponding element, respectively.

Carbon (C): 0.08 to 0.11 wt%

Carbon is a very important element that is added to reinforce the metamorphosis. It promotes high strength 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, when the content exceeds 0.11% by weight, the hole expandability and weldability are lowered. On the other hand, when the content is lower than 0.08%, it is very difficult to secure the strength. Therefore, it is preferable to limit the content of C to 0.08 to 0.11% by weight.

Silicon (Si): 0.05 to 0.6 wt%

Silicon (Si) promotes ferrite transformation and increases the content of carbon in the untransformed austenite to facilitate formation of a composite structure of ferrite and martensite, and also induces solid solution strengthening effect of Si itself. Although it is a very useful element for obtaining strength and material, it is preferable to limit the range of surface properties due to the surface property defects as well as to deteriorate the chemical treatment. In the steels of the present invention, 0.05 to 0.6% of the steel is preferably used within a range that does not deteriorate the weldability while securing a certain amount of ferrite in order to secure minimum ductility in the high YR steel. When the Si content is less than 0.05% by weight, sufficient ferrite is not ensured and ductility is decreased. When the Si content is more than 0.6% by weight, the weldability deteriorates with the decrease in strength.

Manganese (Mn): 2.6-3.0 wt%

Manganese refines the particles without damaging the ductility and precipitates sulfur into the MnS completely in the steel to prevent hot brittleness due to the formation of FeS and to strengthen the steel. At the same time, the martensite phase is obtained in the composite steel. The martensite can be formed more easily. When the content is less than 2.6% by weight, it is difficult to obtain the desired strength in the present invention. On the other hand, when it exceeds 3.0% by weight, there arises a problem that the weldability and the hot rolling property are deteriorated.

Aluminum (Sol.Al): 0.01 to 0.1 wt%

Aluminum is an element effective to combine with oxygen in steel to deoxidize and to distribute carbon in ferrite to austenite like Si to improve the hardenability of martensite. In order to exhibit the above-mentioned effect, it is preferable to add 0.01 wt% or more. On the other hand, when the amount exceeds 0.1% by weight, not only an increased effect as compared with the amount to be added can not be achieved, but also the production cost is increased and the productivity is lowered.

Cr (Cr): 0.2 to 0.7 wt%

Chromium is a component added to improve hardenability of steel and ensure high strength. In the present invention, bainite formation accelerating element is required to increase the yield strength. In order to exhibit the above-mentioned effects, it is preferable to contain at least 0.2 wt% of chromium. On the other hand, when the content of chromium is excessively added, it is economically disadvantageous with the lowering of the elongation, so that the upper limit is preferably limited to 0.7 wt%.

Boron (B): 0.001 to 0.006 wt%

Boron is a component that delays the transformation of austenite into pearlite during cooling during annealing and is an element that inhibits ferrite formation and promotes the formation of bainite. In order to exhibit the above-mentioned effect, the boron is preferably contained in an amount of 0.001% by weight or more. On the other hand, when the content of boron is excessive, there is a problem that boron is concentrated on the surface to deteriorate the adhesion of the plating, so that the upper limit is preferably limited to 0.006% by weight.

Antimony (Sb): 0.001 to 0.1 wt%

Antimony is an element added in the present invention in order to improve the dent defect in the annealing process. Wherein Sb is sikimyeo to preferentially segregates at the grain boundary during the continuous annealing, as grain boundary segregation element suppressing the surface enrichment of the oxide, such as MnO, SiO _2, Al ₂ O ₃ decreases the surface defects due to the dent, the temperature rises and hot rolling step changes It has an excellent effect in suppressing the coarsening of the surface agglomerates. In order to exhibit the above-mentioned effect, it is preferable that it contains 0.001% by weight or more. On the other hand, when the content of antimony is excessive, problems such as deterioration of workability due to excessive grain boundary segregation and an increase in manufacturing cost are caused, and therefore the upper limit is preferably limited to 0.1 wt%.

Phosphorus (P): 0.001 to 0.1 wt%

Phosphorus is the substitutional alloying element with the greatest solid solution strengthening effect and improves the in-plane anisotropy and improves the strength. In order to exhibit the above-mentioned effect, it is preferable that it contains 0.001% by weight or more. On the other hand, when the content is excessive, press formability deteriorates and steel brittleness is generated, so that it is preferable to limit the upper limit of the phosphorus to 0.1 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. The son impurities are not specifically mentioned in this specification, as they are known to anyone skilled in the art of manufacturing.

However, since sulfur is generally referred to as an impurity, a brief description thereof is as follows.

Sulfur (S): not more than 0.01% by weight

Sulfur is inevitably contained as an impurity and forms a nonmetallic inclusion by bonding with Mn or the like, thereby largely deteriorating the ductility, impact toughness and weldability of steel. Therefore, it is desirable to suppress the content to the maximum. The theoretical sulfur content is advantageous to be limited to 0% but it is inevitably contained in the manufacturing process normally. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.01 wt%.

In addition, the cold-rolled steel sheet of the present invention can further improve the effect of the present invention when 0.003 to 0.05 wt% of titanium (Ti) and niobium (Nb) are additionally added.

Ti or Nb: 0.003 to 0.05%

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 0.05%, ductility may 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%.

According to the present invention, when designing an alloy of a steel sheet having the above-mentioned composition range, it is preferable that the carbon equivalent (Ceq) defined by the following formula (1) satisfies 0.29 or less.

[Equation 1]

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

Here, C, Mn, Si, P, and S in Equation (1) represent the content (weight%) of the corresponding element, respectively.

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 0.29, it means that there is a high possibility that welding failure occurs.

The microstructure of the steel sheet satisfying the above-mentioned component system and the formula (1) preferably contains 70 to 95% of bainite and residual ferrite and martensite in terms of area percentage.

In order to ensure the bending workability of the steel sheet provided in the present invention, it is preferable that the steel sheet contains an appropriate amount of bainite, that is, bainite of 70% or more. When the bainite content exceeds 95%, the steel sheet has a very high strength and a remarkable elongation There is a problem of deterioration. In addition, in order to secure excellent ductility together with the bending workability, it is preferable that ferrite and martensite are included in the remaining structure. As the residual structure, it is more preferable that the martensite is flash martensite, and it is more preferable that the martensite is 3% or less.

That is, in the present invention, it is advantageous to have a ferrite structure mainly in order to secure an elongation of at least a proper level. However, after the annealing, the ferrite and bainite are generated upon cooling and the austenite remaining is finally transformed into martensite. The martensite is transformed into flash martensite by continuous cooling.

The martensite may be divided into flash martensite and tempered martensite. Flash martensite is martensite produced by continuous cooling. When tempered martensite is maintained at a constant temperature, some carbides precipitate The martensite is produced by the case where it is made.

Therefore, in order to secure an excellent tensile strength as in the present invention, it is more effective to secure flash martensite than to temperate martensite.

Furthermore, by including the flash martensite in an amount of 3% or less, there is an effect of suppressing the variation in hardness between phases.

The steel sheet provided in the present invention preferably has a yield strength of 750 MPa or more, a yield ratio of 0.75 or more, a bending workability (R / t: R: minimum bending radius ratio, t: unit thickness) of 0.1 or less and an elongation of 8% or less Do.

Hereinafter, a method for producing a high-yielding ultra-high strength cold-rolled steel sheet excellent in bending workability which is another aspect of the present invention will be described in detail.

A method of producing a high-yield and high-strength, ultra-high-strength cold-rolled steel sheet excellent in bending workability which is another aspect of the present invention comprises 0.08 to 0.11% carbon (C), 0.05 to 0.6% silicon (Mn) (B): 0.001 to 0.006%, antimony (Sb): 0.001 to 0.1%, phosphorus (B) P): 0.001 to 0.1%, sulfur (S): 0.01% or less, the balance Fe and other unavoidable impurities, reheating the reheated slab to a hot rolling finish temperature of 800 to 950 캜 Rolling the hot-rolled steel sheet at 500 to 700 DEG C, cold-rolling the rolled steel sheet at a reduction ratio of 40 to 70%, annealing the cold-rolled steel sheet at 820 to 860 DEG C A continuous annealing step at a soaking section temperature (SST), a step of cooling the continuously annealed steel sheet at a first cooling rate of 1 to 10 占 폚 / sec and cooling at a first cooling start temperature of 650 to 700 占 폚 Cooling the first cooled steel sheet at a second cooling rate of 5 to 20 ° C / sec and cooling the steel sheet at a second cooling start temperature (RCST) of 250 to 400 ° C; And a step of skin pass rolling the second cooled steel sheet at a reduction ratio of 0.1 to 1.0%, wherein the annealing temperature and the second cooling start temperature satisfy the following formula (2): .

&Quot; (2) "

0.015SST - 0.013RCST - (0.01SST / RCST) ≥ 7.1

Reheat step

It is preferable to reheat the slab satisfying the above-mentioned component system at 1180 to 1300 占 폚. If the reheating temperature is less than 1,180 占 폚, there is a problem that the hot rolling load suddenly increases. On the other hand, when the temperature exceeds 1300 DEG C, the amount of surface scale increases, leading to loss of material, and the heating cost is also increased. Therefore, the reheating temperature of the slab is preferably limited to 1180 to 1300 占 폚.

Hot rolling step

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  step

After completion of the hot finishing rolling, it is preferable to wind up. At this time, the winding is preferably performed in a temperature range of 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, cold rolling is preferably performed after pickling the hot-rolled steel sheet produced in the above manner.

Cold rolling step

It is preferable that the hot-rolled steel sheet is cold-rolled. When the 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 step

The cold-rolled cold-rolled steel sheet is preferably annealed. The cold-rolled sheet thus obtained is subjected to continuous annealing at 820 to 860 ° C. If the annealing temperature in the continuous annealing is less than 820 占 폚, a large amount of ferrite exists in the steel and the yield strength is lowered. On the other hand, when the temperature exceeds 860 DEG C, there arises a problem that ductility deteriorates due to an excessive increase in yield strength.

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

Cooling step

It is preferable to cool the annealed steel sheet as described above. At this time, it is preferable to perform cooling of the second or more cooling, more preferably, second cooling.

Primary cooling

It is preferable that the annealed steel sheet is first cooled. The first cooling is preferably performed at a first cooling rate of 1 to 10 ° C / second up to the first cooling termination temperature of 650 to 700 ° C. The primary cooling step is to increase the ductility and strength of the steel sheet by securing the equilibrium carbon concentration of the ferrite and the austenite. When the first cooling termination temperature is lower than 650 ° C or higher than 700 ° C, it is difficult to secure the ductility and strength desired in the present invention. If the first cooling rate is less than 1 ° C / second, the amount of ferrite increases due to excessive slow cooling, and the elongation due to ferrite increases, but the fraction of the transformed structure decreases, so that the strength to be secured in the present invention can not be secured. On the other hand, when the heating temperature is higher than 10 ° C / second, ferrite transformation does not occur due to excessive cooling rate, and the elongation rate is lowered due to a large amount of transformed structure.

Secondary cooling

It is preferable to cool the first cooled steel sheet secondarily. The second cooling is one of the control factors that is important in the present invention. The second cooling is performed at a second cooling rate of 5 to 20 DEG C / sec to 250 to 400 DEG C, which is a rapid soaking cooling temperature (RSCT) It is preferable to perform cold cooling. If the second cooling termination temperature is lower than 250 캜, there is a problem that the yield strength and the tensile strength increase simultaneously due to an excessive increase in the amount of martensite during the over-treatment, and the ductility is extremely deteriorated. On the other hand, when the temperature exceeds 400 ° C, the amount of ferrite transformation until the first and second cooling increases, so that the ductility increases and the yield strength decreases, so that the high yield ratio to be achieved in the present invention can not be secured. When the second cooling rate is less than 5 캜 / second, most of the austenite is transformed into bainite and the yield strength is remarkably increased. On the other hand, when the heating temperature is higher than 20 ° C / second, bending workability is deteriorated due to an increase in deviation between phases of ferrite and martensite due to transformation of martensite rather than bainite transformation.

Further, in the present invention, the relationship between the annealing temperature (SST) and the secondary cooling termination temperature (RCST) is controlled so as to satisfy the following formula (2).

&Quot; (2) "

0.015SST - 0.013RCST - (0.01SST / RCST) ≥ 7.1

By controlling the annealing temperature and the second cooling termination temperature so as to satisfy the formula (2) as described below, it is possible to secure the strength and elongation to be secured in the present invention. When the value of the above formula (2) exceeds 7.1, the change of the material, particularly the change of the yield strength, due to the change of the small amount of the annealing temperature and the quenching start temperature is very large. It is very difficult to secure stable materials.

Skin pass rolling  step

It is preferable that skin pass rolling is performed after the second cooling. The steel sheet is subjected to skin pass rolling in the range of 0.1 to 1.0%. 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 rate is less than 0.1%, it is very difficult to control the shape of the ultra high strength steel such as the steel according to the present invention. On the other hand, when the work ratio exceeds 1.0%, the yield strength target 750 MPa), and since the workability is greatly unstable due to the high stretching operation, the reduction rate in the skin pass rolling is preferably limited to 0.1 to 1.0%.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

The steel slab formed as shown in Table 1 below was vacuum-melted and reheated at a temperature of 1250 占 폚 for one hour in a heating furnace, followed by hot rolling, and then wound. At this time, hot rolling was terminated at a temperature range of 900 ° C and the coiling temperature was set at 650 ° C. Thereafter, cold rolling was performed at a cold reduction rate of 50% after pickling treatment using a hot rolled steel sheet. The cold-rolled steel sheet was subjected to continuous annealing under the annealing conditions shown in Table 2, followed by first cooling to 650 占 폚 at a first cooling rate of 3 占 폚 / sec, followed by a second cooling rate of 15 占 sec Cooling was performed up to the second cooling end temperature (RCST) shown in FIG. Thereafter, skin pass rolling was performed at a reduction ratio of 0.2%.

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), the tensile strength (TS), the elongation (TEl), the yield ratio (YR) and the bending workability (R / t <1) of the specimen were measured and shown in Table 2 below. In addition, the microstructural fractions of cold - rolled steel sheets were observed for each steel type and the results were also shown.

In Table 2, the bending workability of the specimen is indicated by "O" for the material which does not crack on the surface in the bending test of R / t 1.0, and "X" for the material which causes cracking.

Steel grade C Si Mn P S Sol.Al Cr Ti Nb N B Sb Equation 1 Inventive Steel 1 0.09 0.4 2.7 0.01 0.003 0.035 0.5 0.02 0.035 0.005 0.0025 0.03 0.270 Invention river 2 0.09 0.4 2.8 0.009 0.002 0.05 0.4 0.018 0.033 0.005 0.0005 0.02 0.269 Invention steel 3 0.095 0.5 2.7 0.01 0.005 0.025 0.5 0.022 0.035 0.003 0.0025 0.03 0.287 Inventive Steel 4 0.1 0.1 2.6 0.009 0.004 0.06 0.3 0.04 0.035 0.004 0.0025 0.04 0.267 Invention steel 5 0.095 0.1 2.8 0.01 0.003 0.07 0.3 0.03 0.035 0.0056 0.0025 0.02 0.270 Invention steel 6 0.1 0.3 2.7 0.01 0.003 0.05 0.5 0.02 0.035 0.0045 0.0025 0.03 0.277 Invention steel 7 0.1 0.2 2.7 0.01 0.003 0.04 0.6 0.025 0.035 0.0047 0.0025 0.02 0.274 Inventive Steel 8 0.095 0.1 2.9 0.011 0.005 0.055 0.4 0.025 0.035 0.007 0.0025 0.03 0.285 Invention river 9 0.085 0.4 2.7 0.011 0.004 0.045 0.5 0.025 0.045 0.006 0.0025 0.03 0.271 Invented Steel 10 0.085 0.3 2.9 0.012 0.005 0.06 0.4 0.025 0.03 0.0065 0.0025 0.03 0.284 Comparative River 1 0.09 0.6 3.1 0.01 0.003 0.05 0.4 0.02 0.04 0.0041 0.0005 0.04 0.297 Comparative River 2 0.09 0.6 3.3 0.011 0.004 0.05 0.4 0.02 0.045 0.0035 0.0005 0.02 0.313 Comparative Steel 3 0.09 0.6 3.3 0.01 0.005 0.04 0.4 0.03 0.04 0.0055 0.0005 0.05 0.315 Comparative Steel 4 0.1 0.6 3.2 0.012 0.003 0.035 0.4 0.02 0.03 0.006 0.0005 0.03 0.316 Comparative Steel 5 0.09 0.9 2.7 0.009 0.005 0.035 0.4 0.03 0.035 0.007 0.0025 0.04 0.293 Comparative Steel 6 0.12 0.5 2.5 0.01 0.006 0.04 0.4 0.02 0.035 0.005 0.0025 0.02 0.306 Comparative Steel 7 0.15 0.6 2.3 0.011 0.005 0.03 0.4 0.02 0.03 0.006 0.0025 0.03 0.327 Comparative Steel 8 0.09 0.1 3.1 0.011 0.006 0.05 0.4 0.025 0.035 0.0065 0.0025 0.05 0.294 Comparative Steel 9 0.15 0.1 2.7 0.008 0.002 0.04 0.4 0.03 0.035 0.004 0.0025 0.04 0.312 Comparative Steel 10 0.1 0.4 3.5 0.011 0.003 0.06 0.4 0.03 0.03 0.005 0.0025 0.03 0.322

(The components described above are in wt.%)

Steel grade Annealing temperature
(° C) Second cooling termination temperature (캜) Skin pass reduction rate
(%) YS
(Mpa) TS
(MPa) Wire
(%) YR Equation 2 R / t < 1 Bainate fraction
(%) Remarks Inventive Steel 1 830 390 0.7 820.3 1089.7 10.8 0.75 7.36 O 77 Inventory 1 800 460 0.7 674.6 1023.4 14.4 0.66 6.00 X 35 Comparative Example 1 Invention river 2 820 400 0.7 801.2 1030.6 10.7 0.78 7.08 O 75 Inventory 2 780 440 0.7 700.6 1015.1 14.6 0.69 5.96 X 32 Comparative Example 2 Invention steel 3 840 350 0.7 851.1 1070.2 10.6 0.80 8.03 O 82 Inventory 3 790 450 0.7 665.8 1050.7 14.3 0.63 5.98 X 33 Comparative Example 3 Inventive Steel 4 830 300 0.7 861.6 1099.9 8.89 0.78 8.52 O 83 Honorable 4 800 450 0.7 693.4 1029.6 15.1 0.67 6.13 X 36 Comparative Example 4 Invention steel 5 840 400 0.7 800.9 1052.6 10.4 0.76 7.38 O 82 Inventory 5 800 460 0.7 643.9 1051.1 10.7 0.61 6.00 X 38 Comparative Example 5 Invention steel 6 830 400 0.7 821.8 1086.6 10.1 0.76 7.23 O 80 Inventory 6 790 450 0.7 659.4 1008.3 13.6 0.65 5.98 X 31 Comparative Example 6 Invention steel 7 840 370 0.7 794.6 1036.2 10.5 0.77 7.77 O 75 Honorable 7 790 430 0.7 654.0 1040.7 13.2 0.63 6.24 X 32 Comparative Example 7 Inventive Steel 8 830 380 0.7 817.7 1085.5 10.5 0.75 7.49 O 79 Honors 8 800 450 0.7 679.2 1060.0 14.6 0.64 6.13 X 35 Comparative Example 8 Invention river 9 820 400 0.7 811.2 1058.3 11.6 0.77 7.08 O 81 Proposition 9 790 450 0.7 628.6 1025.3 15.2 0.61 5.98 X 39 Comparative Example 9 Invented Steel 10 840 400 0.7 817.7 1079.3 10.5 0.76 7.38 O 78 Inventory 10 780 450 0.7 623.3 1035.0 13.8 0.60 5.83 X 33 Comparative Example 10 Comparative River 1 840 390 0.7 782.3 1093.0 9.6 0.72 7.51 X 71 Comparative Example 11 800 460 0.7 693.1 1051.3 13.8 0.66 6.00 X 39 Comparative Example 12 Comparative River 2 830 400 0.7 851.3 1161.2 7.4 0.73 7.23 X 90 Comparative Example 13 790 450 0.7 700.3 1019.2 11.2 0.69 5.98 X 33 Comparative Example 14 Comparative Steel 3 830 400 0.7 774.4 1101.2 8.8 0.70 7.23 X 69 Comparative Example 15 790 450 0.7 651.3 1090.1 13.6 0.60 5.98 X 22 Comparative Example 16 Comparative Steel 4 840 400 0.7 891.2 1151.1 6.9 0.77 7.38 X 89 Comparative Example 17 780 450 0.7 712.3 1052.3 11.2 0.68 5.83 X 36 Comparative Example 18 Comparative Steel 5 840 400 0.7 753.1 1035.2 11.2 0.73 7.38 X 69 Comparative Example 19 800 450 0.7 600.2 1001.2 15.1 0.60 6.13 X 21 Comparative Example 20 Comparative Steel 6 830 400 0.7 800.3 1074.4 7.1 0.74 7.23 X 79 Comparative Example 21 790 480 0.7 700.5 1105.3 10.5 0.63 5.59 X 36 Comparative Example 22 Comparative Steel 7 840 350 0.7 891.2 1260.3 5.6 0.71 8.03 X 89 Comparative Example 23 790 460 0.7 750.9 1065.1 10.5 0.71 5.85 X 42 Comparative Example 24 Comparative Steel 8 830 350 0.7 792.3 1085.3 8.6 0.73 7.88 X 75 Comparative Example 25 780 450 0.7 710.3 1056.9 10.2 0.67 5.83 X 51 Comparative Example 26 Comparative Steel 9 840 400 0.7 800.1 1250.6 5.5 0.64 7.38 X 83 Comparative Example 27 790 450 0.7 750.1 1150.3 7.6 0.65 5.98 X 56 Comparative Example 28 Comparative Steel 10 830 270 0.7 900.3 1260.3 5.3 0.71 8.91 X 92 Comparative Example 29 790 450 0.7 790.3 1160.1 8.8 0.68 5.98 X 53 Comparative Example 30

As shown in Tables 1 and 2, Inventive Examples 1 to 10 have a yield strength of 750 MPa or more, a yield ratio of 0.75 or more, a bending workability (R / t) of 0.1 or less, And a high yield and high specific gravity super high strength cold rolled steel sheet having an elongation of 8% or less and excellent bending workability can be provided.

On the other hand, in Comparative Examples 1 to 10, even if the composition conditions of the inventive steel were satisfied, the production conditions deviated from the method proposed by the present invention, the yield strength was as low as 700 MPa or less, I am not satisfied. This is because the ferrite fraction increased in the steel and the target amount of bainite exceeded 70% in the present invention steel.

This difference can be seen in more detail in FIG. That is, Inventive Example 1 and Comparative Example 1 show the difference in microstructure according to the annealing temperature and the quenching start temperature difference with respect to the steel. As shown in Fig. 3 (a), in Inventive Example 1, in which the annealing temperature was 840 占 폚, the quenching start temperature was 390 占 폚, and the value of Equation 2 was 7.36, the most of the structure of steel was found to be bainite Martensite. As a result, the yield strength is 820 Mpa and the yield ratio is 0.75, which satisfies the criteria of the present invention. However, in the case of Comparative Example 1, even when having the same system as Inventive Example 1, when the annealing temperature is 800 캜 and the quenching start temperature is 460 캜, and the value of Equation 2 is 6.0, it is out of the range proposed by the present invention, The strength is 670 MPa, and the yield ratio is very low at 0.66.

4, the excellent characteristics of the present invention can be confirmed. Fig. 4 shows the results of bending test after changing the heat treatment conditions for Inventive Example 1 and Comparative Example 1. Fig. That is, as in the case of Inventive Example 1, it can be confirmed that cracks occurred only at 0.5 R in the steel material annealed at 840 캜 and at the quenching start temperature of 390 캜. On the other hand, as in Comparative Example 1, when the annealing temperature was 800 ° C and the quenching start temperature was 460 ° C, cracks were generated on the surface at 1.5R (where R is the minimum bending radius ratio).

In addition, as in Comparative Examples 11 to 30, in the case of having a composition system and manufacturing conditions outside the range proposed by the present invention, even if other materials are satisfied, the carbon equivalent is high to deteriorate the weldability or satisfy the expression (2) proposed by the present invention It is confirmed that there is a problem that the elongation rate is decreased due to excessive strength increase.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (7)

(Cr): 0.08 to 0.11%, silicon (Si): 0.05 to 0.6%, manganese (Mn): 2.6 to 3.0%, aluminum (Sol.Al): 0.01 to 0.1% 0.001 to 0.1% of phosphorus, 0.001 to 0.006% of boron (B), 0.001 to 0.1% of antimony (Sb), 0.001 to 0.1% of phosphorus (P) (Ceq) defined by the following formula (1) is 0.29 or less, and the microstructure contains 70 to 95% of bainite, 3% or less of flash martensite and the remainder ferrite , And having excellent yield strength with a yield strength of 750 MPa or more, a yield ratio of 0.75 or more, a bending workability (R / t) of 0.1 or less, and an elongation of 8% or less.
[Equation 1]
Ceq = C + Mn / 20 + Si / 30 + 2P + 4S
(Where C, Mn, Si, P, and S in the above formula (1) are the contents (weight%
The method according to claim 1,
Wherein the steel sheet further contains 0.003 to 0.05% by weight of titanium (Ti) and niobium (Nb), respectively, and has excellent bending workability.
delete delete (Cr), 0.0 to 0.11% of carbon (C), 0.05 to 0.6% of silicon (Si), 2.6 to 3.0% of manganese (Mn), 0.01 to 0.1% of aluminum (Al) 0.001 to 0.1% of phosphorus, 0.001 to 0.006% of boron (B), 0.001 to 0.1% of antimony (Sb), 0.001 to 0.1% of phosphorus (P) Reheating the steel slab containing impurities;
Hot-rolling the reheated slab at a hot finishing finish temperature of 800 to 950 占 폚;
Rolling the hot-rolled steel sheet at 500 to 750 ° C;
Cold rolling the rolled steel sheet at a reduction ratio of 40 to 70%;
Continuously annealing the cold-rolled steel sheet at a soaking section temperature (SST) of 820 to 860 ° C;
Cooling the continuously annealed steel sheet at a first cooling rate of 1 to 10 占 폚 / sec and cooling to a first cooling finishing temperature of 650 to 700 占 폚;
A second cooling step of cooling the first cooled steel sheet at a second cooling rate of 5 to 20 占 폚 / second and cooling to a second cooling end temperature (RCST) of 250 to 400 占 폚; And
Pass rolling the second cooled steel sheet at a reduction ratio of 0.1 to 1.0%, wherein the annealing temperature and the second cooling initiation temperature satisfy the following formula (2): &quot; (2) &quot; A method for manufacturing an ultra high strength cold rolled steel sheet.
&Quot; (2) &quot;
0.015SST - 0.013RCST - (0.01SST / RCST) ≥ 7.1
6. The method of claim 5,
Wherein the reheating step is performed in a temperature range of 1180 to 1300 占 폚.
6. The method of claim 5,
Wherein the winding step is performed in a temperature range of 500 to 700 占 폚, wherein the high-yield and high-specific-strength super-high strength cold-rolled steel sheet has excellent bending workability.

Images