KR101620744B1 - Ultra high strength cold rolled steel sheet having high yield ratio and method for manufacturing the same - Google Patents

Abstract

The present invention provides an ultra-high strength cold rolled steel sheet having a high yield ratio, and a method to manufacture the same. According to an embodiment of the present invention, the ultra-high strength cold rolled steel sheet having a high yield ratio comprises: 0.12-0.17 wt% of carbon (C); 0.3 wt% or less (excluding 0 wt%) of silicon (Si); 2.5-3.2 wt% of manganese (Mn); 0.5-1.2 wt% of chromium (Cr); 0.0010-0.0050 wt% of boron (B); 0.01-0.05 wt% of titanium (Ti); 0.01-0.05 wt% of niobium (Nb); 0.01-0.10 wt% of soluble aluminum (Sol.Al); and the remaining consisting of iron (Fe) and inevitable foreign substances including tempered martensite with a ratio of fraction of area, which is 90% or more (including 100%), wherein tempered martensite has an average grain diameter of 3μm or less (excluding 0μm). The ultra-high strength cold rolled steel sheet has less variation of material, a high yield ratio, and a superior expandability.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra high strength cold rolled steel sheet having a high yield strength,

More particularly, the present invention relates to a high-strength and low-strength high-strength cold-rolled steel sheet and a method of manufacturing the same. More particularly, the present invention relates to a steel sheet having a member, a seat rail, a pillar, a bumper beam, sill side members, and the like, and a method of manufacturing 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 of the structural member than the tensile strength, that is, the higher the yield ratio (yield strength / tensile strength), the more advantageous is the impact energy absorbing ability. Generally, however, as the strength of the steel sheet increases, the elongation rate decreases, and thus the molding processability is lowered. Therefore, there is a need to develop a material that can complement the steel sheet.

Generally, methods of strengthening steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening. However, the strengthening by solid solution strengthening and grain refinement in the above method is disadvantageous in that it is very difficult to produce a high strength steel having a tensile strength of 490 MPa or more.

On the other hand, precipitation-strengthening high-strength steels are produced by adding carbon and nitride forming elements such as Cu, Nb, Ti, V and the like to precipitate carbon and nitride to strengthen the steel sheet or refine the crystal grains by suppressing the growth of grains by fine precipitates, . The above technique has an advantage that high strength can be easily obtained at a low manufacturing cost. However, since the recrystallization temperature is rapidly increased due to micro precipitates, high temperature annealing must be performed in order to ensure sufficient recrystallization and ductility. In addition, there is a problem that it is difficult to obtain a high strength steel of 600 MPa or more in precipitation hardened steel which is burnt on a ferrite base and is strengthened by precipitation of nitride.

On the other hand, the transformation-strengthening high-strength steel is a ferrite-martensite dual phase steel containing a hard martensite in a ferrite base, TRIP (Transformation Induced Plasticity) steel using ferroelectricity of residual austenite, (CP) steel composed of a hard bainite or a martensite structure have been developed. However, the tensile strength that can be achieved in such high strength steel (AHSS) (of course, it is possible to increase the strength by increasing the amount of carbon, but considering the practical aspects such as spot weldability), the level of about 1200 MPa to be. In addition, hot press forming steel, which secures final strength by quenching by direct contact with a die that is water cooled after molding at high temperature, is in the spotlight for application to structural members to secure collision safety. However, The application is not so large because of over-heating and high processing cost.

Recently, in order to further improve the stability of a passenger in the event of a collision, a bumper beam part considering a frontal collision characteristic in a vehicle or a sill side member part advantageous for side collision has been intensified. These parts are mainly manufactured by using the roll forming method instead of the conventional press forming method. The roll forming method with high productivity compared to general press forming and hot press forming is a method of manufacturing a complicated shape through multi-step roll forming. However, the application of ultra high strength steel, which has a low elongation rate,

Patent literatures 1 and 2 are representative techniques of super high strength steels developed for roll forming. Patent Document 1 discloses a method of continuously subjecting a steel material having a carbon content of 0.18% or more to continuous annealing, water-cooling to room temperature, and then performing an overhanging treatment at a temperature of 120 to 300 ° C for 1 to 15 minutes to obtain a martensite volume ratio of 80 to 97% , And a cold-rolled steel sheet having a tensile strength of 1500 MPa or more. However, the steel sheet produced by the above-mentioned Patent Document 1 has poor shape quality due to temperature variations in the width direction and the longitudinal direction during water cooling, and there is a serious disadvantage such as workability deterioration and material deviation in each position during roll forming .

In addition, Patent Document 2 discloses a technique of forming a composite structure of tempered martensite and ferrite with a steel microstructure in order to simultaneously improve strength and ductility of steel. However, the steel sheet produced by the above-mentioned Patent Document 2 is expected to be poor in weldability due to the addition of a large amount of carbon, and there is a possibility of inducing dent in the furnace due to the addition of a large amount of silicon.

Japanese Laid-Open Patent Publication No. 1990-418479 Japanese Unexamined Patent Application Publication No. 2010-90432

One aspect of the present invention is to provide a high-yield and high-specific-strength, ultra-high-strength cold-rolled steel sheet and a method of manufacturing the same, by appropriately controlling the alloy composition and the manufacturing conditions.

In order to achieve the above object, according to one aspect of the present invention, there is provided a ferritic steel comprising 0.12 to 0.17% of C, 0.3% or less of Si (excluding 0%), 2.5 to 3.2% of Mn, Of Ti, 0.01 to 0.05% of B, 0.01 to 0.05% of Nb, 0.01 to 0.10% of Sol.Al, the balance of Fe and unavoidable impurities, and 90% or more of tempered martensite High-strength cold-rolled steel sheet comprising a tempered martensite and an average grain size of the tempered martensite of 3 mu m or less.

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.12 to 0.17% of C, 0.3% or less of Si (excluding 0%), 2.5 to 3.2% of Mn, 0.5 to 1.2% of Cr, Reheating a steel slab containing 0.0050% of Ti, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Nb, 0.01 to 0.10% of Sol.Al, the balance Fe and unavoidable impurities; Hot-rolling the reheated steel slab to obtain a hot-rolled steel sheet; Winding the hot-rolled steel sheet; A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at an annealing temperature (a) of 790 to 820 캜; Cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature of 650 to 700 ° C at a rate of 1 to 10 ° C / sec; And secondarily cooling the primary cold-rolled steel sheet at a rate of 5 to 20 ° C / sec to a secondary cooling end temperature (b) of 250 to 340 ° C, followed by an overbasing treatment, wherein the Mn content , The Cr content, the annealing temperature (a, 占 폚) and the secondary cooling end temperature (b, 占 폚) satisfy the following relational expression (1).

[Relation 1]

15.1? 2.5 [Mn] +1.75 [Cr] + 0.015a-0.013b? 17.2

(Where [Mn] and [Cr] each represent the weight% of the corresponding element)

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.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a high strength cold rolled steel sheet having a small material deviation and excellent yield ratio and hole expandability.

Generally, it is known that a low-temperature structure must be secured in order to improve the yield ratio of a steel sheet. Therefore, attention has been focused on a steel sheet having martensite as a main structure having the highest strength among low-temperature structures. As described above, the most common method of forming the martensite structure as the main structure of the steel sheet is to quench (water-cool) and temper the steel sheet after maintaining a sufficient time to allow formation of austenite during annealing. However, as described above, this method is disadvantageous in terms of shape quality due to temperature variations in the width direction and the longitudinal direction during water cooling, resulting in deterioration in workability in roll forming and serious material disadvantages such as positional deviation.

Accordingly, the present inventors tried to secure martensite by appropriately controlling the kind and content of the alloying element. More specifically, it is possible to secure martensite even at a low cooling rate by adding a large amount of elements improving the hardenability such as manganese and chromium. On the other hand, in this case, problems such as deterioration of weldability and increase of hot-rolled strength occur due to addition of a large amount of alloying elements, and the present inventors tried to minimize the carbon content which has the greatest influence on weldability.

However, as a result of the above alloy design, the yield strength can not be sufficiently increased due to the lack of carbon content in the martensite, and the yield ratio is lowered. As a result, the present inventors have intensively studied to solve this problem. As a result, it has been found that by appropriately adding carbonitride-forming elements and appropriately controlling the production conditions, it is possible to secure a tempered martensite structure of 90% It has been found that a high-yielding, non-super-high-strength steel sheet can be secured by controlling the average grain size of the tempered martensite to 3 탆 or less, and the present invention has been accomplished.

Hereinafter, a high-yield, high-strength, high-strength cold-rolled steel sheet having excellent hole expandability, which is one aspect of the present invention, will be described in detail.

Carbon (C): 0.12 to 0.17 wt%

Carbon is a very important element added for strengthening the metamorphosis. In addition, in the transformed steel such as the cold-rolled steel sheet of the present invention, the formation of martensite is promoted to increase the fraction of martensite in the steel. In order to exhibit such an effect in the present invention, the content is preferably 0.12% by weight or more, more preferably 0.13% by weight or more. On the other hand, if the content is excessive, the weldability of the steel decreases, which may cause welding defects in parts processing of the customer. Therefore, the upper limit of the carbon content is preferably 0.17% by weight, more preferably 0.16% by weight.

Silicon (Si): 0.3 wt% or less (excluding 0%)

Silicon is an element that promotes ferrite transformation, and is not intentionally added in the present invention. Moreover, when the silicon content is excessive, the carbon content in untransformed austenite is increased, the strength of the martensite is lowered, and the composite microstructure of ferrite and martensite is formed by the microstructure of the steel. In addition, surface scale defects are caused to deteriorate the surface quality of the steel sheet and deteriorate the weldability and chemical processability. Therefore, it is preferable to control the content as low as possible, and the upper limit of the silicon content is preferably 0.3 wt%, more preferably 0.2 wt%, and even more preferably 0.15 wt%.

Manganese (Mn): 2.5 to 3.2 wt%

Manganese refines the crystal grains and precipitates S in the steel to completely MnS to prevent hot brittleness due to FeS generation and strengthens the steel by solid solution strengthening without deteriorating ductility. Further, the critical cooling rate for the martensitic transformation can be lowered, so that the martensite structure can be more easily secured. In order to obtain such an effect in the present invention, the content is preferably 2.5 wt% or more, more preferably 2.55 wt% or more, and even more preferably 2.6 wt% or more. However, when the content is excessive, there is a fear that the weldability and the hot rolling property are lowered. Therefore, the upper limit of the manganese content is preferably 3.2 wt%, more preferably 3.1 wt%, and even more preferably 3.0 wt%.

Cr (Cr): 0.5 to 1.2 wt%

Chromium not only improves the strength of the steel but also plays an important role in improving the hardenability of the steel to form martensite, which is a low-temperature transformer. In order to obtain such an effect in the present invention, the content is preferably 0.5% by weight or more, more preferably 0.55% by weight or more, and still more preferably 0.6% by weight or more. However, if the content is excessive, the effect is not only saturated but economically disadvantageous. Therefore, the upper limit of the chromium content is preferably 1.2 wt%, more preferably 1.15 wt%, and most preferably 1.1 wt% More preferable.

Boron (B): 0.0010 to 0.0050 wt%

Boron not only slows the transformation of austenite into pearlite but also suppresses ferrite formation and accelerates the formation of martensite in the process of cooling and annealing the cold rolled steel sheet continuously. In order to obtain such an effect in the present invention, the boron content is preferably 0.0010 wt% or more, more preferably 0.0015 wt% or more, and even more preferably 0.0018 wt% or more. However, if the content is excessive, there is a problem in that an unnecessary increase in cost due to the formation of an excessive iron alloy occurs. Accordingly, the upper limit of the boron content is preferably 0.0050 wt%, more preferably 0.0040 wt%, and even more preferably 0.0030 wt%.

Titanium (Ti): 0.01 to 0.05 wt%

Titanium not only improves the strength of the steel but also plays an important role in refining the grains by forming fine carbons and nitrides with carbon and / or nitrogen in the steel. In order to obtain such an effect in the present invention, the content is preferably 0.01% by weight or more, more preferably 0.015% by weight or more, and even more preferably 0.018% by weight or more. On the other hand, when the content is excessive, not only the economical efficiency is deteriorated but also the ductility of the steel is greatly lowered due to the formation of excessive carbon and nitride. Accordingly, the upper limit of the titanium content is preferably 0.05 wt%, more preferably 0.045 wt%, and even more preferably 0.04 wt%.

Niobium (Nb): 0.01 to 0.05 wt%

Niobium not only improves the strength of steel but also plays an important role in refining the grains by forming fine carbons and nitrides by binding with carbon and / or nitrogen in the steel. In order to obtain such an effect in the present invention, the content is preferably 0.01% by weight or more, more preferably 0.015% by weight or more, and even more preferably 0.018% by weight or more. On the other hand, when the content is excessive, not only the economical efficiency is deteriorated but also the ductility of the steel is greatly lowered due to the formation of excessive carbon and nitride. Therefore, the upper limit of the niobium content is preferably 0.05 wt%, more preferably 0.045 wt%, and even more preferably 0.04 wt%.

Soluble aluminum (Sol.Al): 0.01 to 0.10 wt%

Aluminum (Al) is a strong deoxidizing agent and is an effective element for producing clean steel by lowering oxygen content in molten steel. In addition, it plays a role of reducing dissolved nitrogen by binding with nitrogen in the steel and precipitating AlN. In order to exhibit such an effect in the present invention, the content is preferably 0.01% by weight or more, more preferably 0.015% by weight or more, and even more preferably 0.02% by weight or more. However, if the content is excessive, not only the manufacturing cost increases but also the risk of occurrence of slab cracking due to excessive AlN precipitation increases. Therefore, the upper limit of the soluble aluminum content is preferably 0.10 wt%, more preferably 0.08 wt%, and even more preferably 0.05 wt%.

In addition to the above composition, the balance 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. These impurities are not specifically referred to in this specification, as they are known to one of ordinary skill in the art.

However, since phosphorus, sulfur, and nitrogen are generally referred to as impurities, they will be briefly described as follows.

Phosphorus (P): 0.10% by weight or less (excluding 0% by weight)

Phosphorus is an impurity that is inevitably contained and segregates in grain boundaries to become a main cause of inhibiting toughness. Therefore, it is desirable to control the content as low as possible. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the sulfur content is preferably controlled to 0.10 wt%, more preferably 0.05 wt%, and even more preferably 0.03 wt%.

Sulfur (S): 0.010% by weight or less (excluding 0% by weight)

Sulfur is an inevitably contained impurity, and is an element that is a main cause of deteriorating the ductility and weldability of steel. Therefore, it is preferable to control the content of sulfur to be as low as possible. Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the sulfur content is preferably controlled to 0.01 wt%, more preferably 0.008 wt%, and even more preferably 0.006 wt%.

Nitrogen (N): 0.01 wt% or less (excluding 0 wt%)

Nitrogen (N) in the steel is effective for stabilizing austenite, but it is not intentionally added in the present invention because it is an element that increases the risk of cracking during performance through formation of AlN or the like. Therefore, in the present invention, the content thereof is preferably controlled as low as possible, and the upper limit of the nitrogen content is preferably 0.01 wt%, more preferably 0.008 wt%, and even more preferably 0.007 wt%.

Hereinafter, the preferred microstructure of the cold-rolled steel sheet of the present invention will be described in detail.

In order to secure an excellent yield ratio and hole expandability in an ultra high strength steel having a tensile strength of 1300 MPa as in the present invention, it is important to appropriately control the microstructure, and it is required to secure a single phase martensite. However, among martensite, fresh martensite is not preferable because of its high tensile strength but low yield strength. Tempered martensite is a structure that increases the yield strength by tempering fresh martensite to form carbide by precipitation of carbon. The cold-rolled steel sheet of the present invention preferably has 90% or more (including 100%) of tempered martensite in an area fraction. On the other hand, since bainite is also a low-temperature transformation phase but has a lower strength than martensite, it is more preferable to control it to 10% or less as an areal fraction.

However, since the cold-rolled steel sheet of the present invention has a low carbon content in the steel, the desired tensile strength and yield strength can be ensured only by securing at least 90% of the tempered martensite in the area fraction of the steel microstructure There was no. Accordingly, the present inventors have studied to solve this problem and found that it is important to appropriately control the average particle diameter of the tempered martensite. More specifically, by controlling the average grain size of the tempered martensite to 3 탆 or less (preferably 2.8 탆 or less, and more preferably 2.5 탆 or less), it is possible to secure the desired tensile strength and yield strength without excess addition of alloying elements Could. Here, the average particle diameter means the equivalent circular diameter of the martensite detected by observing the cross section of the steel sheet.

On the other hand, cracks due to external stress generally occur at the interface between the phases. Therefore, if the difference in hardness between phases increases, the possibility of cracking along the boundary between phases increases. The hole expandability has an excellent value when a crack is deformed rather than a specific position when stress is applied. Therefore, when the difference in hardness between phases is lowered, the hole expandability becomes better. Therefore, in order to further improve the hole expandability, it is necessary to appropriately control the interhard hardness ratio. According to the research results of the present inventors, in order to secure a hole expansion ratio (HER) of 30% or more in an ultra high strength steel having a tensile strength of 1300 MPa or more as in the present invention, it is more preferable to control the interphase hardness ratio to 1.5 or less . On the other hand, when ferrite or retained austenite is present in the steel, the interphase hardness ratio can not be equal to or less than 1.5, and it is preferable to control the ferrite or retained austenite so that none of the ferrite or retained austenite exists in the steel.

In the present invention, the interhard hardness ratio means the ratio of the hardness value of the tissue having the highest hardness to the hardness value of the tissue having the lowest hardness among the microstructures. The hardness of the microstructure can be determined by measuring an average value of the remaining values except for the maximum value and the minimum value after measuring 100 points in a square three times with a load of 2 g using a nano-indenter (NT 110) have.

According to an embodiment of the present invention, the cold-rolled steel sheet of the present invention may have a tensile strength of 1300 MPa or more, a yield ratio of 0.75 or more, and a hole expansion ratio (HER) of 30% or more, It can be advantageously applied to automotive structural members such as members, seat rails, pillars, bumper beams, sill side members and the like.

The cold-rolled steel sheet of the present invention described above can be manufactured by various methods, and the manufacturing method thereof is not particularly limited. However, it can be produced by the following method as one embodiment thereof.

Hereinafter, a method of manufacturing a high-yielding, ultra-high-strength cold-rolled steel sheet according to another aspect of the present invention will be described in detail.

First, the steel slab having the above composition is reheated and hot rolled to obtain a hot-rolled steel sheet. Prior to performing the hot rolling of the hot rolling, the temperature condition of the step of reheating the slab is not particularly limited, but it is preferable to control the temperature to the ordinary reheating temperature.

According to an embodiment of the present invention, in the hot rolling, the temperature at the finish rolling exit side may be 870 to 950 캜, and preferably 880 to 920 캜. When the temperature at the finish rolling exit side is less than 870 DEG C, there is a high possibility that the hot deformation resistance will increase sharply and the top, tail and edge of the hot-rolled coil become single-phase regions, There is a possibility of deterioration. On the other hand, when the temperature is higher than 950 占 폚, not only a too large oxidation scale is generated but also the microstructure of the steel sheet may be coarsened.

Thereafter, the hot-rolled steel sheet is wound. According to one embodiment of the present invention, the coiling temperature may be 600 to 750 캜, and preferably 650 to 720 캜. If the coiling temperature is less than 600 ° C, excessive martensite or bainite is generated, which causes an excessive increase in strength of the hot-rolled steel sheet, which may cause manufacturing problems such as defective shape due to load during cold rolling. On the other hand, when the coiling temperature exceeds 750 캜, there is a fear that the pickling property deteriorates due to an increase in the surface scale.

Thereafter, the wound hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet. According to an embodiment of the present invention, the reduction ratio in the cold rolling may be 40 to 70%, and preferably 45 to 55%. When the reduction rate is less than 40%, the recrystallization driving force is weakened and there is a fear that a good recrystallized grain can be obtained, and the shape correction may be very difficult. On the other hand, if it exceeds 70%, not only the rolling load increases sharply but also cracks may occur at the edge of the steel sheet.

Thereafter, the cold-rolled steel sheet is continuously annealed. At this time, the annealing temperature is one of the factors that are importantly managed in the present invention, together with the cooling end temperature (primary and secondary cooling to be described later). The annealing temperature is preferably 790 to 820 ° C and more preferably 790 to 810 ° C in order to secure the yield ratio (YR) of 30 or more and the HER of 0.75 or more proposed in the present invention at the same time. When the annealing temperature is less than 790 占 폚, there is a problem that a large amount of ferrite is formed due to an anomalous reverse annealing, the yield ratio is lowered, and the inter-phase hardness ratio between the ferrite and the transformation phase is increased to decrease hole expandability. On the other hand, when the temperature is higher than 820 占 폚, the packet size of martensite obtained by cooling to be described later increases due to the coarsening of the austenite grains due to the high-temperature annealing. As a result, There is a problem that the site organization can not be secured.

Thereafter, the continuously annealed cold rolled steel sheet is first cooled. The primary cooling step is a step for suppressing ferrite transformation and securing a martensite structure as a main structure of the steel sheet.

At this time, the primary cooling rate is preferably 1 to 5 DEG C / sec. When the primary cooling rate is less than 1 占 폚 / sec, there is a problem that the ferrite transformation occurs partially during cooling and the yield ratio is lowered and the inter-phase hardness ratio is increased to lower the hole expandability. On the other hand, There is a problem that meandering occurs when the coil is passed through.

The primary cooling termination temperature is preferably 650 to 700 ° C, and more preferably 650 to 680 ° C. If the primary cooling end temperature is lower than 650 ° C, there is a possibility that winding of the coil occurs. If the primary cooling end temperature exceeds 700 ° C, there is a fear that the yield rate may be lowered due to the formation of ferrite due to the lowering of the cooling rate.

Thereafter, the primary cooled cold rolled steel sheet is secondarily cooled. This step is one of the control elements that is important in the present invention, and is a step for securing a martensite of 90 area% or more proposed in the present invention by transforming the austenite structure obtained in the continuous annealing process into martensite through quenching.

At this time, the secondary cooling rate is preferably 10 to 20 DEG C / sec. When the secondary cooling rate is less than 10 ° C / sec, sufficient cooling capability necessary for martensitic transformation is not obtained. On the other hand, when the secondary cooling rate exceeds 20 ° C / sec, the coil shape is twisted by quenching, May occur.

The secondary cooling termination temperature is preferably 250 to 340 ° C, more preferably 270 to 320 ° C. The secondary cooling termination temperature is a very important temperature condition for ensuring a high porosity as well as securing the width and longitudinal shape of the coil. When the secondary cooling termination temperature is lower than 250 캜, excessive quenching of the martensite during the over- There is a problem that the yield strength and the tensile strength are excessively increased to deteriorate the ductility, and furthermore, there is a problem that the shape deterioration due to the quenching occurs and the workability in roll forming is deteriorated. On the other hand, if the temperature exceeds 340 ° C, the austenite produced during annealing can not be transformed into martensite, but bainite, granular bainite and the like, which are high-temperature transformation phases, are produced and the yield strength and yield ratio are rapidly deteriorated have.

Thereafter, after completion of the secondary cooling, overheating treatment is performed at the secondary cooling termination temperature. According to an embodiment of the present invention, the time of overexposure may be 100 to 800 seconds, preferably 200 to 600 seconds. If the over-treatment time is less than 100 seconds, the martensite produced by the secondary cooling can not be sufficiently tempered, and it may be difficult to secure tempered martensite of 90% or more by area. On the other hand, However, there is a possibility that the strength is lowered due to excessive tempering treatment.

According to an embodiment of the present invention, when the annealing temperature is a (占 폚) and the secondary cooling end temperature is b (占 폚), a and b satisfy the above-described temperature range, Is satisfied. When the value of 2.5 [Mn] +1.75 [Cr] + 0.015a-0.013b (hereinafter, referred to as 'value of relational expression 1') is excessively low or high, it is difficult to obtain the target yield strength and yield ratio in the present invention.

[Relation 1]

15.1? 2.5 [Mn] +1.75 [Cr] + 0.015a-0.013b? 17.2

(Where [Mn] and [Cr] each represent the weight% of the corresponding element)

According to an embodiment of the present invention, the step of rolling the skin pass may further include a step of rolling the skin pass at a reduction rate of 0.1 to 1.0% after the overexposure treatment. Generally, when skeletal rolling of a textured steel is performed, an increase in yield strength of 50 to 100 MPa or more occurs without increasing the tensile strength in most cases. At this time, if the reduction rate is less than 0.1%, it is very difficult to control the shape in the ultra-high strength steel as in the present invention, while if it exceeds 1.0%, the yield strength increases excessively. In addition, since the workability is greatly unstable due to the high stretching operation, the reduction ratio in the skin pass rolling is preferably 0.1 to 1.0%.

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 )

The steel slab formed as shown in Table 1 below was vacuum-melted and reheated at a heating temperature of 1200 캜 for 1 hour in a heating furnace, and then hot rolled and wound under the conditions shown in Table 2 below. At this time, the coiling temperature was kept constant at 680 캜. Thereafter, the wound hot-rolled steel sheet was pickled, cold rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet, and then subjected to continuous annealing, primary cooling and secondary cooling under the conditions shown in Table 2 below. Then, the secondary cooling was terminated for 350 seconds at the termination temperature. At this time, the primary cooling rate was 2 ° C / sec, the primary cooling termination temperature was 650 ° C, and the secondary cooling rate was 15 ° C / sec.

Then, the phase fraction, the average particle diameter and the inter-phase hardness ratio of the tempered martensite were measured for each of the cold-rolled steel sheets manufactured, and the results are shown in Table 3 below. The tensile strength (YS), tensile strength (TS), yield ratio (YR = YS / TS) and elongation (El) of the test specimen in J direction C were measured and applied to ISO 16630-2009 The hole expandability (HER) was measured and the results are shown in Table 3 below.

At this time, the phase fraction and the average particle size of the tempered martensite were observed with an SEM electron microscope and then measured using an image analyzer. The inter-phase hardness ratio was measured using a nano-indenter (NT110) apparatus at a load of 2 g three times in a square of 100 points, and then the average value of the remaining values except for the maximum value and the minimum value was determined.

Steel grade Alloy composition (% by weight) C Si Mn Cr B Ti Nb Sol.Al P S N Inventive Steel 1 0.15 0.1 2.7 0.7 0.0025 0.02 0.03 0.03 0.01 0.003 0.007 Invention river 2 0.17 0.1 2.8 0.8 0.0025 0.04 0.02 0.025 0.011 0.003 0.005 Invention steel 3 0.16 0.1 2.8 0.7 0.002 0.03 0.02 0.033 0.012 0.003 0.007 Inventive Steel 4 0.16 0.1 3.0 0.6 0.002 0.02 0.03 0.03 0.01 0.005 0.006 Invention steel 5 0.14 0.1 2.8 0.6 0.002 0.03 0.02 0.041 0.012 0.003 0.005 Invention steel 6 0.15 0.1 2.7 1.1 0.002 0.04 0.02 0.041 0.01 0.006 0.004 Invention steel 7 0.13 0.1 2.6 0.8 0.0023 0.03 0.03 0.051 0.011 0.003 0.005 Inventive Steel 8 0.16 0.1 2.8 0.6 0.0025 0.02 0.03 0.042 0.01 0.003 0.003 Invention river 9 0.14 0.1 2.7 0.8 0.0025 0.03 0.03 0.029 0.013 0.003 0.005 Invented Steel 10 0.14 0.1 2.5 0.6 0.0025 0.02 0.02 0.03 0.01 0.003 0.006 Comparative River 1 0.09 0.1 2.8 0.8 0.0023 0.03 0.02 0.03 0.01 0.002 0.005 Comparative River 2 0.14 0.1 2.7 0.3 0.0025 0.03 0.03 0.038 0.012 0.003 0.006 Comparative Steel 3 0.23 0.1 2.6 0.6 0.002 0.04 0.02 0.033 0.01 0.004 0.007 Comparative Steel 4 0.17 0.1 2.1 1.0 0.002 0.04 0.02 0.044 0.011 0.005 0.005 Comparative Steel 5 0.13 0.1 3.6 0.6 0.002 0.04 0.02 0.035 0.01 0.003 0.006 Comparative Steel 6 0.15 0.1 2.6 1.7 0.002 0.04 0.02 0.03 0.01 0.003 0.007

Steel grade FDT (占 폚) SS (℃) RCS (° C) Relationship 1 Remarks Inventive Steel 1 880 800 300 16.08 Inventory 1 880 800 400 14.78 Comparative Example 1 Invention river 2 890 810 300 16.65 Inventory 2 Invention steel 3 880 820 300 16.63 Inventory 3 Inventive Steel 4 880 800 270 17.04 Honorable 4 Invention steel 5 900 810 270 16.69 Inventory 5 900 820 420 14.89 Comparative Example 2 Invention steel 6 880 810 270 17.32 Inventory 6 Invention steel 7 880 810 300 16.15 Honorable 7 880 820 370 15.39 Comparative Example 3 Inventive Steel 8 880 800 300 16.15 Honors 8 880 810 400 15 Comparative Example 4 Invention river 9 880 800 300 16.25 Proposition 9 880 790 450 14.15 Comparative Example 5 880 740 320 15.09 Comparative Example 6 Invented Steel 10 880 790 330 14.86 Comparative Example 7 Comparative River 1 880 800 300 16.5 Comparative Example 8 Comparative River 2 890 800 300 15.38 Comparative Example 9 Comparative Steel 3 890 810 310 15.67 Comparative Example 10 Comparative Steel 4 880 810 310 15.12 Comparative Example 11 Comparative Steel 5 900 810 310 18.17 Comparative Example 12 Comparative Steel 6 890 800 310 17.45 Comparative Example 12 Here, FDT denotes the temperature at the finish rolling exit side, SS denotes the annealing temperature, and RCS denotes the secondary cooling end temperature.

Steel grade Martensite fraction (area%) Martensite average particle diameter (占 퐉) Commodity
Hardness ratio YS
(MPa) TS
(MPa) YR Hand
(%) HER
(%) Remarks Inventive Steel 1 95 2.2 1.2 1085 1353 0.80 7.3 40 Inventory 1 83 4.1 2.1 951 1311 0.73 11.6 21 Comparative Example 1 Invention river 2 96 2.6 1.3 1056 1389 0.76 7.1 43 Inventory 2 Invention steel 3 94 2.4 1.4 1079 1399 0.77 7.9 50 Inventory 3 Inventive Steel 4 98 1.8 1.1 1099 1439 0.76 7.4 39 Honorable 4 Invention steel 5 97 1.9 1.3 1099 1423 0.77 6.9 46 Inventory 5 81 3.8 1.9 912 1344 0.68 10.8 21 Comparative Example 2 Invention steel 6 98 2.5 1.2 1077 1398 0.77 6.5 55 Inventory 6 Invention steel 7 96 2.1 1.4 1062 1346 0.79 7.7 49 Honorable 7 86 4.2 2.1 974 1311 0.74 11.1 23 Comparative Example 3 Inventive Steel 8 96 2.3 1.3 1094 1432 0.76 6.2 41 Honors 8 86 3.9 1.8 932 1352 0.69 9.1 22 Comparative Example 4 Invention river 9 95 2.5 1.4 1054 1368 0.77 7.4 48 Proposition 9 88 3.8 2.2 911 1269 0.72 11.9 24 Comparative Example 5 76 3.5 3.2 863 1362 0.63 9.5 16 Comparative Example 6 Invented Steel 10 89 2.5 1.6 985 1352 0.73 7.6 25 Comparative Example 7 Comparative River 1 86 3.8 1.8 938 1205 0.78 9.2 19 Comparative Example 8 Comparative River 2 82 5.1 2.6 987 1264 0.78 7.7 35 Comparative Example 9 Comparative Steel 3 89 2.2 2.2 950 1589 0.60 6.5 24 Comparative Example 10 Comparative Steel 4 82 4.8 2.1 913 1301 0.70 7.6 22 Comparative Example 11 Comparative Steel 5 88 2.5 1.4 1099 1601 0.69 4.1 12 Comparative Example 12 Comparative Steel 6 93 2.1 1.4 1125 1422 0.79 3.9 18 Comparative Example 13

As shown in Table 3, Examples 1 to 9 satisfying the composition range and manufacturing conditions of the present invention had yield strengths of 1300 MPa or more, yield ratios of 0.75 or more, HER of 30% or more, It can be seen that it is excellent. It is also understood that the inventive steels satisfy at least 95% of the tempered martensite phase fraction, the average grain size of not more than 2.6 탆, and the phase hardness deviation of not more than 1.4, satisfying the requirements of the present invention steel.

On the other hand, in the case of Comparative Examples 1 to 5, the alloy composition satisfied the range proposed by the present invention, but the secondary cooling end temperature exceeded the range proposed by the present invention, and the high temperature transformation phase of bainite, granulabainite granular bainite, etc. were produced. Due to the low phase fraction of martensite and high intermodal hardness ratio, yield ratio decreased and pore dilatability was poor.

In the case of Comparative Example 6, the alloy composition satisfies the range proposed by the present invention, but the annealing temperature is lower than the range suggested by the present invention and annealed in an abnormal range, and thus a large amount of ferrite is formed, Also, the hole hardening ratio was increased and the hole expandability was decreased.

In the case of Comparative Example 7, the alloy composition satisfied the range suggested by the present invention. However, the material of Comparative Example 7 does not satisfy the relational expression 1 shown in the present invention steel, so that the yield strength and the yield ratio are low and the hole expandability is poor. Deterioration occurred.

In addition, in the case of Comparative Examples 8 to 13, material deterioration occurred such that the alloy composition was out of the range proposed by the present invention, the yield strength or the yield ratio was low, and the hole expandability was poor.

Claims (12)

0.1 to 0.15% of Si, 0.1 to 5% of Si, 2.5 to 3.2% of Mn, 0.5 to 1.2% of Cr, 0.0010 to 0.0050% of B, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Nb, 0.01 to 0.10% of Sol.Al, the balance Fe and unavoidable impurities,
(Inclusive of 100%) of tempered martensite, wherein the tempered martensite has an average grain size of 3 탆 or less (excluding 0 탆) and an interphase hardness ratio of 1.5 or less High yield strength cold rolled steel sheet.
The method according to claim 1,
Wherein the cold-rolled steel sheet has an area fraction of 90% or more of tempered martensite and 10% or less of bainite.
delete The method according to claim 1,
The cold rolled steel sheet according to claim 1, wherein the cold rolled steel sheet further comprises a high yield cold rolled steel sheet containing 0.10% or less of P (exclusive of 0%), S of 0.010% or less (excluding 0%) and N of 0.010% or less Steel plate.
The method according to claim 1,
The cold-rolled steel sheet has a yield strength of 1000 MPa or more and a yield ratio of 0.75 or more.
The method according to claim 1,
The cold-rolled steel sheet has a high hole expansion ratio (HER) of 30% or more.
0.1 to 0.15% of Si, 0.1 to 5% of Si, 2.5 to 3.2% of Mn, 0.5 to 1.2% of Cr, 0.0010 to 0.0050% of B, 0.01 to 0.05% of Ti, Reheating a steel slab containing 0.01 to 0.05% of Nb, 0.01 to 0.10% of Sol. Al, the balance Fe and unavoidable impurities;
Hot-rolling the reheated steel slab to obtain a hot-rolled steel sheet;
Winding the hot-rolled steel sheet;
A step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet at an annealing temperature (a) of 790 to 820 캜;
Cooling the continuously annealed cold rolled steel sheet at a rate of 1 to 5 占 폚 / sec to a primary cooling end temperature of 650 to 700 占 폚; And
Cooling the primary cold-rolled steel sheet at a rate of 10 to 20 ° C / sec to a secondary cooling end temperature (b) of 250 to 340 ° C,
Wherein said Mn content, Cr content, annealing temperature (a, 占 폚) and secondary cooling end temperature (b, 占 폚) satisfy the following relational expression (1).
[Relation 1]
15.1? 2.5 [Mn] +1.75 [Cr] + 0.015a-0.013b? 17.2
(Where [Mn] and [Cr] each represent the weight% of the corresponding element)
8. The method of claim 7,
Wherein the temperature at the finish rolling exit side during hot rolling is 870 to 950 占 폚.
8. The method of claim 7,
Wherein the coiling temperature is 600 to 750 占 폚 at the time of winding the superalloy high strength cold rolled steel sheet.
8. The method of claim 7,
A method of producing a high-yielding high-strength and high-strength cold-rolled steel sheet having a reduction ratio of 40 to 70% at the time of cold rolling.
8. The method of claim 7,
A method of producing a high-yielding high-strength and cold-rolled steel sheet having a high yield strength, wherein the over-treatment time is 100 to 800 seconds.
8. The method of claim 7,
Further comprising the step of performing skin pass rolling at a reduction ratio of 0.1 to 1.0% after the above-mentioned overaging treatment.