KR101543834B1 - Thin, hot-rolled steel sheet having excellnet workability and anti-aging properties, and method for manufacturing the same - Google Patents

Abstract

An aspect of the present invention is to provide a high-strength hot-rolled steel sheet for use in household appliances, automobiles and the like. More particularly, the present invention relates to a high-strength ultra-thin hot-rolled steel sheet excellent in endurance and moldability by appropriately controlling the weight ratio and manufacturing method between alloy elements and elements by utilizing an ultra-low carbon-based Al-killed steel, .

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra-thin hot-rolled steel sheet excellent in workability and endurance, and a method of manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

The present invention relates to an ultra-fine hot-rolled steel sheet excellent in workability and endurance, and a method for producing the same.

The steel used as a material for household appliances and automobiles is required to have properties such as corrosion resistance, endurance and moldability.

The term "formability" is used herein to mean a degree of molding into a desired shape without fracture (tear-off, neck) or defective shape (such as wrinkle, spring-back, scratch or galling). Such a moldability can be classified according to the deformation mode industrially. The deformation mode is largely classified into four types such as drawing molding, stretching molding, bending molding and stretch-flanging molding, Processing mode.

Among the above-described working modes, stretching molding is a processing mode that is more simple than a dip-drawing molding and has a close relationship with ductility (elongation) of a material because there is almost no inflow of material from the die- Unlike molding, the influence of mold condition is known to be small.

On the other hand, drawing die molding related to the deep-drawing performance is a method in which a workpiece is placed on a drawing die, and a punch is pressed into a die groove under a pressurized condition by a blank holder to form a drawing die. And the outer diameter is reduced. Therefore, it is known that the material properties are largely related to the rank-pod value (Lankford value, hereinafter, r value) expressed as the strain in the width direction with respect to the strain in the thickness direction.

In particular, the average plastic deformation ratio (hereinafter referred to as r-bar value) and the plastic anisotropy value (hereinafter referred to as? R value) measured from the r value measured for each direction with respect to the rolling direction are representative materials expressing drawability Property values, each of which is calculated from the following formula.

r-bar value = (r ₀ + r ₉₀ + 2r ₄₅ ) / 4 (1)

? R = (r ₀ + r ₉₀ - 2r ₄₅ ) / 2 (2)

(Where r _i represents the r value measured on the specimen taken in the direction of i ° from the rolling direction).

The larger the value of r-bar in the above equation, the greater the depth of the molding cup in the drawing process, and the better the deep draw forming formability can be judged. In addition, Planar Anisotropy, one of the important quality characteristics in cup processing, indicates the degree to which the physical / mechanical properties of the material are oriented. The in-plane anisotropy is fundamentally due to the strong orientation of each crystal grains subjected to plastic deformation and the like. If crystal grains undergo processing or the like are present at random, these crystals do not have directionality and the in-plane anisotropy may not be large.

However, in general, the crystal grains in the steel sheet exhibit strong directionality, so that it becomes anisotropy of the firing behavior when machining is performed. If the in-plane anisotropy increases, the anisotropy of the cup after molding increases the earing, which indicates the difference in height of the forming cup in each processing part, resulting in increased processing defects and material loss. It is very important to properly control the value of DELTA r in the drawing process because the strain in all directions is constant and isotropic as the value of? R used as a measure of in-plane anisotropy approaches zero.

As a method for ensuring the durability and workability of steel, conventionally, hot-rolled and cold-rolled steel sheets are subjected to hot-rolling and cold-rolling using low-carbon aluminum-killed steel, By controlling efficiently, the workability is secured.

However, in this case, as the heat treatment takes a long time, not only the productivity is lowered but also the deviation of the material in the coil is increased due to the uneven heating and cooling pattern.

In order to solve the above problems, an element such as titanium (Ti) or niobium (Nb), which is a carbonitride-forming element for precipitating solid-state elements in ultra-low carbon steels, is added Thereby obtaining a desired characteristic.

However, also in this case, there arises a problem that the surface properties of the steel are deteriorated at the same time as the production cost increases due to the addition of the expensive element. In addition, even if these elements are added in steelmaking, it is difficult to ensure workability such as cupping due to the formation of chaotic texture in the hot rolling step.

For this reason, in the case of a working material, a method of forming a desired recrystallized texture by applying a thermal annealing process to a cold annealing process and an annealing process is applied. However, even in the above case, there is a problem that the material cost is increased by the addition of the alloying element and the processing cost is increased due to the necessity of additional processing.

Therefore, there is a growing interest in securing the properties and manufacturing of the processing materials utilizing thermal series in terms of cost reduction and process omission.

Patent Document 1 discloses a continuous continuous working method in which Mn and B are partially added to 0.01 to 0.08% C steel to lower the Ar3 transformation temperature, reheat to 1150 占 폚, Hot rolled steel sheet for processing can be produced by performing the final coiling at 500 DEG C or higher. However, in this case, the thermal elongation percentage was maintained at 45% or more and stretching workability could be secured, but the improvement effect on the drawing processing was not shown.

In Patent Document 2, finishing hot rolling is performed in a single phase of ferrite through continuous continuous hot rolling using ultra low carbon steel to which titanium (Ti) and / or niobium (Nb) is added, , It is suggested that the drawing characteristic can be secured by the self-annealing effect by controlling the difference of the temperature to 100 ° C or less. At this time, however, not only expensive alloying elements such as niobium (Nb) have to be added to fix the solid element in the steel, but strict management of finish rolling temperature and coiling temperature is required for securing the recrystallization lap in the hot rolling step There is a problem that it is difficult to produce stable production in a fishing operation.

Japanese Patent Application Laid-Open No. 9-227950 Japanese Patent Application Laid-Open No. 2-141529

 One aspect of the present invention is to provide a high-strength hot-rolled steel sheet for use in household appliances, automobiles, and the like. More particularly, the present invention relates to a method for producing a carbon black composite material by appropriately controlling an alloy element and an element weight ratio and a manufacturing method thereof by utilizing an ultra-low carbon aluminum-killed steel containing no elements such as carbonitride forming elements Ti and Nb, Strength ultra-thin hot-rolled steel sheet excellent in formability and a method for producing the same.

In one aspect of the present invention, an ultra-thin hot-rolled steel sheet excellent in workability and anti-aging properties comprises 0.0005 to 0.003% of C, 0.4 to 0.8% of C, 0.03% or less of Si (excluding 0% (Gamma) -fiber / alpha (alpha) containing 0.08% of B, 0.0005 to 0.002% of B, 0.0005 to 0.002% of P, 0.03 to 0.05% of P, 0.001 to 0.015% of S, balance Fe and other unavoidable impurities. ) - a fiber assembly texture strength ratio of 4 to 14.

A method of manufacturing an ultra-thin hot-rolled steel sheet excellent in workability and endurance in accordance with another aspect of the present invention is characterized in that it comprises 0.0005 to 0.003% of C, 0.4 to 0.8% of Mn, 0.03% or less of Si (excluding 0%), A steel material containing 0.03 to 0.08% of Al, 0.0005 to 0.002% of B, 0.0005 to 0.002% of P, 0.03 to 0.05% of P, 0.001 to 0.015% of S and the balance Fe and other unavoidable impurities at a temperature of 1100 to 1200 캜 Wherein the hot rolled steel sheet is manufactured in a temperature range of 600 to Ar3, and the hot rolled steel sheet has a coefficient of friction of 0.05 - 0.2 and the hot rolled steel sheet is manufactured by hot rolling so that the ratio (Rt / Rf) of the reduction ratio (Rf) of the two passes at the rear end to the total reduction rate (Rt) of the entire stand is 0.2 to 0.3, Cooling at a cooling rate of 80 to 150 ° C / second, winding the cooled hot-rolled steel sheet, and descaling the rolled- .

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.

According to the present invention, by optimizing the composition range and the manufacturing method, it is possible to provide an ultra-thin hot-rolled steel sheet which is excellent in stretch processability, drawability and endurance, even when no additional heat treatment is performed in hot rolling, Method can be provided. It is also possible to provide a high-strength hot-rolled steel sheet excellent in workability of 35 to 40 kgf / mm < 2 > with an average plastic deformation ratio of 1.3 or more and a plastic anisotropy value of 0.15 or less. That is, according to the present invention, Nb and Ti which are expensive alloying elements can be omitted, and a hot-rolled steel sheet having a workability and an anti-aging property equal to or higher than those of a cold-rolled steel sheet can be provided.

The inventors of the present invention conducted research to produce a hot-rolled steel sheet capable of replacing a cold-rolled steel sheet by securing drawability at the level of a cold-rolled steel sheet and ensuring anticorrosion and anticorrosion resistance. As a result, High strength hot-rolled steel sheet excellent in drawing workability and anti-aging property without additional heat treatment and a manufacturing method thereof have been devised.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the ultra-thin hot-rolled steel sheet for working according to the present invention and a method of manufacturing the same will be described in detail, but the present invention is not limited to the following embodiments. Therefore, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Hereinafter, an ultra-fine hot rolled steel sheet excellent in workability and anti-aging properties, which is one aspect of the present invention, will be described in detail.

In one aspect of the present invention, the hot-rolled steel sheet excellent in workability and anti-aging properties comprises 0.0005 to 0.003% of C, 0.4 to 0.8% of Mn, 0.03% or less of Si (excluding 0%), 0.03 to 0.08 of Al (Gamma) -fiber / alpha (alpha) containing 0.0005 to 0.002% of B, 0.0005 to 0.002% of P, 0.03 to 0.05% of P, 0.001 to 0.015% of S, balance Fe and other unavoidable impurities. - a fiber aggregate texture intensity ratio of 4 to 14;

Carbon (C): 0.0005 to 0.003 wt%

C is an element that is generally added to improve the strength of a steel sheet, and is a representative element that can cause aging when it is present in a steel as a solid element. If the addition amount of C is more than 0.003 wt%, not only the material can be obtained from the final hot-rolled plate due to the increase of the solid carbon in the steel but also adversely affects the aging property of the steel and further deteriorates the drawing processability . On the other hand, when it is less than 0.0005% by weight, the iron price of the alloy increases sharply due to the extreme carbon control in the steelmaking process, and it also causes the steelmaking ability to be significantly deteriorated. Therefore, in order to stably secure the desired workability and anti-aging properties in the present invention, it is preferable to control C to 0.0005 to 0.0030%.

Manganese (Mn): 0.4 to 0.8 wt%

In the case of Mn, it is desirable to add at least 0.40% or more in order to prevent the embrittlement of embrittlement caused by sulfur and to weaken the steel and ensure the desired strength. However, when the content of manganese is excessively high, the drawability is deteriorated by the solid solution element and micro-segregation is caused, which causes the molding property to deteriorate. Therefore, the upper limit of the manganese content is limited to 0.8 wt%.

Silicon (Si): 0.03 wt% or less (excluding 0%)

Si is an element which bonds with oxygen or the like and forms an oxide layer on the surface of the steel sheet to inhibit plating and surface characteristics, and therefore, it is desirable to suppress the content to the maximum. However, considering the steelmaking process, the upper limit is limited to 0.03% by weight.

Aluminum (Al): 0.03 to 0.08 wt%

Al is an element added to aluminum-killed steel for the purpose of preventing deterioration of material due to deoxidizing agent and aging. In order to achieve the above-described effects, it is preferable to add at least 0.03 wt% or more. However, when the addition amount is excessive, the upper limit is limited to 0.08% because saturation of the deoxidation effect and surface inclusions such as aluminum-oxide (Al ₂ O ₃ ) are rapidly increased to deteriorate the surface properties of the hot rolled material.

Boron (B): 0.0005 to 0.002 wt%

B is combined with a solid solution element in the steel to form a B-system nitride precipitate to improve endurance and workability, and also has the effect of refining the ferrite grains by suppressing grain growth of the steel even under high temperature maintenance conditions by the formation of precipitates. In order to exhibit the above-described effects, the present invention preferably contains 0.0005% by weight or more. However, when the boron content is excessive, there is a problem that workability is inhibited. Therefore, the upper limit is limited to 0.002 wt%.

Nitrogen (N): 0.0005 to 0.002 wt%

N is a typical interstitial strengthening element that penetrates into steel and shows reinforcing properties, and is an element that secures the characteristics of the desired strength. In order to exhibit the above-mentioned effect, the present invention preferably contains 0.0005 wt% or more. However, when the content of nitrogen is excessive, the endurance is drastically deteriorated, and the burden of denitrification in the steelmaking step is increased, thereby deteriorating the steelmaking workability. Therefore, the upper limit is limited to 0.002% by weight.

Phosphorus (P): 0.03 to 0.05 wt%

P is an element that improves strength and hardness of steel by causing hardening of employment while being present as a solid element in steel. In order to exhibit the above-mentioned effects in the present invention, it is preferable to include 0.03 wt% or more. However, when the content of phosphorus is more than 0.05% by weight, center segregation occurs at the time of casting and the workability is deteriorated.

Sulfur (S): 0.001 to 0.015 wt%

S is combined with Mn in steel to form a nonmetallic inclusion serving as a corrosion starting point and is a factor of red shortness, so it is desirable to reduce the content as much as possible. Therefore, in the present invention, the lower limit of sulfur is preferably limited to 0.001 wt%. On the other hand, when the sulfur content is excessive, a part of sulfur binds to manganese in the steel to limit the size of the manganese-sulfite-based precipitate, so that the upper limit is limited to 0.015 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.

In order to improve the properties of the material and achieve the target properties through securing the endurance and drawing workability in the steel material formed as described above, it is necessary to control the content ratio of the elements forming the precipitates of the carbide and nitride system in combination with the elements in the steel .

In the present invention, in the hot-rolled steel sheet having the above-described composition range, in the case of Al among the additional elements forming the nitride, the content ratio (AlxB) / N of Al, B and N in order to secure the endurance and drawability of the hot- . More specifically, it is preferable that (Al x B) / N satisfies 0.025 to 0.07. When the content ratio (Al x B) / N of Al, B and N is less than 0.025, the amount of N existing in the solid state in the steel increases to lower the endurance and workability of the final product. On the other hand, when (AlxB) / N is more than 0.07, it is effective from the viewpoint of securing the anti-aging property but increases the recrystallization temperature of the hot-rolled sheet and the amount of the alloy element added at a high price. Therefore, the content ratio (Al x B) / N of Al, B and N is preferably limited to 0.025 to 0.07.

In addition, the carbon added to the steel is present as a carbide precipitate such as cementite or as a solid carbonaceous solid in a ferrite phase which is in a parent phase. Among them, the employment carbon present in the solid phase as a solid serves as the root cause of the aging causing the material change over time. Therefore, it is preferable to control the content of the solid carbon by cooling or precipitation of the solid carbon.

In the present invention, it is preferable to control the solid carbon content of the hot-rolled steel sheet satisfying the above-described composition and component relationship to 5 ppm or less. When the amount of the above-mentioned solid carbon is more than 5 ppm, the endurance is deteriorated by the intramuscular solid element and the workability can not be secured

In addition, the hot-rolled steel sheet preferably has a gamma / alpha-fiber (? /? - fiber) texture.

An array having a certain plane and orientation generated inside a crystal is called a texture, and a pattern in which these aggregate tissues develop into a band in a certain direction is called a collective tissue fiber. These fiber components are also closely related to moldability. Among them, the aggregate texture group of azimuths generated in a direction orthogonal to the (111) plane of the texture texture is referred to as gamma (γ) -fiber and the texture texture group formed of the plane parallel to the <110> - It is called fiber.

That is, the texture representing the aggregation of crystals has a close relationship with the drawing processability. The higher the face strength value of the gamma (γ) -fiber component generated at right angles to the (111) plane in the texture, However, in the present invention, it is clarified that a complex relationship with the surface intensity ratio of α (alpha) -fabric developed in equilibrium with the (100) orientation is directly related to the drawability, and the drawing processability can be ensured by managing the index I suggest a solution.

It was confirmed that the hot-rolled steel sheet of the present invention can secure proper drawing processability by controlling and controlling the aggregate-structure strength ratio of gamma (?) Fiber / alpha (?) Fiber to 4 to 14 levels. When the combined texture strength ratio of gamma / alpha-fiber (? /? - fiber) is less than 4, the formation of aggregate texture on the (111) plane which is advantageous for drawing processing is insufficient and desired drawing processability can not be obtained. On the other hand, when gamma / alpha-fiber (gamma / alpha-fiber) aggregate ratio exceeds 14, the workability is excellent. However, as the anisotropy increases, earing occurs and the material loss increases.

The gamma-fiber aggregate structure is preferably at least one of (111), (111), and (554), and the alpha (alpha) (001) <110>, (112) <110>, and (225) <110>.

The hot-rolled steel sheet of the present invention is capable of securing a tensile strength of 35 to 40 kgf / mm 2, an average plastic deformation ratio of 1.3 or more, and a plastic anisotropy value of 0.15 or less and is capable of achieving an elongation of 40% or more and an aging index of 2 kgf / A steel sheet can be provided.

Further, the thickness of the hot-rolled steel sheet is preferably controlled to be 0.8 to 2.4 mm in order to be produced as an ultra-thin steel sheet.

Hereinafter, a method of manufacturing an ultra-thin hot-rolled steel sheet excellent in workability and endurance, which is another aspect of the present invention, will be described in detail.

A method of manufacturing an ultra-thin hot-rolled steel sheet excellent in workability and endurance in accordance with another aspect of the present invention is characterized in that it comprises 0.0005 to 0.003% of C, 0.4 to 0.8% of Mn, 0.03% or less of Si (excluding 0%), A steel material containing 0.03 to 0.08% of Al, 0.0005 to 0.002% of B, 0.0005 to 0.002% of P, 0.03 to 0.05% of P, 0.001 to 0.015% of S and the balance Fe and other unavoidable impurities at a temperature of 1100 to 1200 캜 And the hot rolled steel sheet is subjected to hot rolling in a temperature range of 600 ° C to (Ar3-50 ° C). In the hot rolling, the hot rolled steel sheet is subjected to hot rolling, Is subjected to hot rolling so as to have a coefficient of friction of 0.05 to 0.2 and a ratio (Rt / Rf) of the reduction ratio (Rf) of the two passes at the rear end to the total reduction ratio (Rt) of the entire stand at 0.2 to 0.3. Cooling the hot-rolled steel sheet at a cooling rate of 80 to 150 ° C / second, winding the cooled hot-rolled steel sheet, Ring.

Reheat step

It is preferable to reheat the aluminum-killed steel satisfying the above-described composition of the components. Since the heating process of the steel material is carried out for the purpose of smoothly performing the subsequent hot rolling process and ensuring the desired physical properties, the heating process should be performed within an appropriate temperature range according to the purpose.

In the present invention, it is preferable that the steel material is subjected to a single phase of austenite capable of coarsening the initial austenite structure at the time of reheating. More specifically, the steel material is preferably reheated in a temperature range of 1100 to 1200 ° C. When the reheating temperature is lower than 1100 ° C, precipitation of aluminum-nitride (AlN) is inhibited. On the other hand, when the reheating temperature is higher than 1200 ° C, abnormal growth of crystal grains occurs due to an increase in the passing time between hot rolling rolls, resulting in deterioration in workability of the product, The possibility increases.

Hot rolling step

The reheated steel material is preferably subjected to hot rolling at a temperature of 600 ° C to (Ar 3 transformation point -50 ° C) in a single phase of ferrite. That is, it is preferable to perform hot rolling at a low temperature ferrite temperature range. When the hot rolling is performed in the ferrite temperature range as described above, it is possible to secure the recrystallized microstructure in the ferrite region in the subsequent cooling process.

More preferably, the hot finish rolling is performed in a temperature range of 600 to 800 ° C. When the hot finish rolling temperature is less than 600 캜, it is advantageous from the viewpoint of securing processability, but it is difficult to secure the coiling temperature in the subsequent winding process, thereby increasing the load of hot rolling and significantly reducing the continuous workability. On the other hand, when the hot rolling temperature is higher than 800 ° C, the processed ferrite fraction in the hot rolling step is lowered and the driving force for recrystallization is decreased, so that it is difficult to secure workability.

It is preferable that the phase on the hot-finished rolling ingress side includes processed ferrite, transformed ferrite and austenite. At this time, it is more preferable that the processed ferrite contains 5 to 20% by area percentage. When the percentage of the area percentage of the processed ferrite is less than 5%, it is difficult to secure the target temperature at the hot rolling rolling exit side, and it is difficult to ensure sufficient workability. On the other hand, when the processed ferrite fraction is more than 20% by area fraction%, there is a problem that workability is remarkably lowered due to an increase in rolling load during hot rolling.

In addition, in the present invention, it is preferable to carry out lubrication rolling during hot rolling in order to ensure drawing processability. In the present invention, a hot rolled steel sheet other than a cold rolled steel sheet is provided as a final product. In order to secure a workability equal to or higher than that of a cold rolled steel sheet, the above-described aggregate structure should be formed.

That is, by developing the formation of texture of gamma (?) Fiber / alpha (?) Fiber, it is possible to secure a high average plastic deformation ratio and low plastic anisotropy, .

Therefore, in order to ensure drawing processability in the present invention, it is preferable to perform lubrication rolling in which the coefficient of friction between the roll and the steel is controlled to 0.05 to 0.2 in hot rolling. When the coefficient of friction between the roll and the steel is less than 0.05, slip phenomenon occurs on the surface of the steel during hot rolling, and proper rolling and surface characteristics can not be secured. On the other hand, when the coefficient of friction between the roll and the steel exceeds 0.2, the fatigue characteristics of the roll are deteriorated to shorten the life of the roll, and a shear band having an unfavorable workability is formed on the surface of the steel sheet, . That is, when the coefficient of friction exceeds 0.2, an alpha (alpha) -fiber shear texture having a (112) <110> component is formed on the surface, and a gamma- The formation of the tissue is suppressed, and therefore the desired drawing processability can not be secured.

Further, in the present invention, it is preferable to control the pressure drop distribution of the rolls in each rolling step in order to ensure the desired drawing processability. The reduction distribution during hot rolling is also closely related to the work rate of the operation, the recovery rate of the steel, and the phase fraction of the steel including the recrystallization behavior.

Therefore, it is preferable to control so that the ratio (Rf / Rt) of the reduction ratio (Rf) of the rear two pass to the total reduction ratio Rt of the entire stand satisfies 0.2 to 0.3. When the reduction ratio (Rf / Rt) is more than 0.3, the rolling load of the rear roll is increased, which makes it difficult to secure a hot-rolled steel sheet having a desired thickness. On the other hand, if the reduction ratio (Rf / Rt) is less than 0.2, there is a problem that the driving force for recrystallization decreases and formation of a desired texture is difficult, and drawing processability is difficult to secure.

Therefore, it is preferable that the total reduction ratio of the two passes in the rear end of the total rolling reduction during hot rolling satisfies 0.2 to 0.3.

By performing hot rolling so as to satisfy all the hot rolling conditions as described above, it is possible to secure a value of plastic deformation ratio of 1.3 or more and a plastic anisotropy value of 0.15 or less which can not be ensured in the conventional hot-rolled steel sheet.

Cooling step

After completion of the hot rolling, it is preferable to cool for precipitation of the solid solution element. It is preferable to optimize the amount of the solid element by performing cooling at a cooling rate of 80 to 150 DEG C / second in a run-out-table (ROT) in order to secure the desired characteristics in the present invention . When the cooling rate is less than 80 DEG C / second, the in-vivo employment effect can not be optimized, and the desired endurance and workability of the present invention can not be secured. On the other hand, when cooling is carried out at a cooling rate exceeding 150 ° C / second, it is advantageous in terms of deposition of a solid solution in a subsequent process, but it is difficult to control the shape of the steel, thereby deteriorating the throughput.

Coiling  step

It is preferable to follow the step of winding after completion of the cooling. In the winding step, recrystallization of the processed ferrite structure and rearrangement of the assembled structure formed in the hot rolling step are performed, and the desired endurance and drawability can be ensured through optimization thereof. More preferably 550 to 650 占 폚. If the coiling temperature is less than 550 占 폚, the precipitation behavior of the solid solution N in the steel is insufficient, and the drawability of the final hot-rolled steel sheet can not be ensured due to the lowering of the endurance of the final hot-rolled steel sheet and the occurrence of some non- On the other hand, when the coiling temperature exceeds 650 ° C, it is advantageous for recrystallization and softening of the material, but an orange-peel ), Thereby lowering the drawability.

In addition, the hot rolled steel sheet subjected to the above-mentioned winding step includes a ferrite phase having a recrystallization ratio of 90% or more and a microstructure in which cementite is precipitated. At this time, the fraction of cementite is preferably 0.1 to 0.8%.

When the ferrite phase recrystallization ratio is less than 90%, the high dislocation density in the hot-rolled steel sheet lowers the workability remarkably, resulting in a problem of work cracking in the drawing process. In addition, when the fraction of the cementite is less than 0.1%, it is difficult to secure the desired drawing and stretching processability in the present invention since the problem of aging due to the solid carbon can not be suppressed. On the other hand, if it exceeds 0.8%, it is advantageous in terms of adhesion of the solid solution element, but the material is uneven, which causes a deterioration of the drawing processability.

Descaling  step

A descaling process is usually performed to remove the oxide layer on the surface of the hot-rolled steel sheet. Therefore, in the present invention, by applying an appropriate compressive stress to the surface of the hot-rolled steel sheet at the same time as removing the oxide layer on the surface of the hot-rolled steel sheet, a ferrite grain having a large dislocation density and a large moving dislocation density is produced, A descaling step is performed in order to reduce the dislocation phenomenon of dislocation and improve the endurance of the material.

In order to achieve the above object, it is preferable to apply mechanical descaling such as shot blasting in the present invention.

It is more preferable to perform shot blasting using a shot ball having a diameter of 0.05 to 0.15 mm. When the diameter of the shot ball is less than 0.05 mm, the mechanical peeling effect of the surface layer of the hot-rolled steel sheet is small, and it is difficult to secure the effect of the residual stress to be secured by the present invention. On the other hand, when the diameter of the shot ball exceeds 0.15 mm, the maximum roughness of the surface of the hot-rolled steel sheet increases sharply, which causes a problem that workpiece cracks occur during processing.

In addition, it is preferable to control the injection speed of shot blasting to 40 to 65 m / s. When the spraying speed is less than 40 m / s, the impact pressure of the shot ball acting on the surface layer of the hot-rolled steel sheet is low, and it is difficult to secure desired endurance and workability. On the other hand, when the injection speed exceeds 65 m / s, the depth of the surface hardened layer is more than 10% in the thickness direction of the hot-rolled steel sheet, which causes a problem of causing nonuniform machining.

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 from them.

(Example)

Each of the steels was subjected to reheating, hot rolling, coiling and descaling in the conditions shown in Table 2 below to prepare final steel sheets having the composition shown in Table 1 below.

The tensile strength, the plastic deformation ratio, the plastic anisotropy, the drawing processability, the stretch processability and the endurance test were measured for the hot-rolled steel sheets prepared according to the respective conditions and shown in Table 3 below.

division Steel grade Chemical composition (% by weight) (Al * B) / N weight ratio C Mn Si S Al P N B Invention river A1 0.0011 0.56 0.009 0.012 0.061 0.046 0.0018 0.0015 0.051 A2 0.0018 0.68 0.015 0.006 0.044 0.037 0.011 0.0009 0.036 A3 0.0015 0.46 0.011 0.008 0.055 0.041 0.0015 0.0018 0.066 Comparative steel A4 0.0021 0.14 0.011 0.015 0.014 0.012 0.0037 0.0002 0.0008 A5 0.0061 0.55 0.020 0.008 0.051 0.044 0.0019 - 0.000 A6 0.0015 0.46 0.011 0.010 0.041 0.039 0.0081 0.0010 0.0005 A7 0.0014 1.25 0.691 0.015 0.010 0.036 0.0015 0.0036 0.096 A8 0.0310 0.48 0.012 0.009 0.143 0.071 0.0012 0.0010 0.119

division Steel grade Reheating temperature (℃) Coefficient of friction Hot finish rolling temperature (℃) Coiling temperature (캜) Cooling rate
(° C / sec) The reduction ratio (Rf / Rt) Short ball diameter (mm) Shot blasting injection speed (℃ / sec) Inventory 1 A1 1140 0.14 700 600 100 26 0.12 52 Inventory 2 A1 1150 0.14 740 600 100 21 0.11 57 Inventory 3 A2 1140 0.10 720 620 110 25 0.10 46 Honorable 4 A2 1140 0.10 740 620 110 25 0.08 60 Inventory 5 A3 1150 0.16 680 580 120 22 0.12 55 Inventory 6 A3 1180 0.16 680 580 120 26 0.08 51 Comparative Example 1 A1 1150 0.35 740 600 90 24 0.08 50 Comparative Example 2 A1 1150 0.14 910 600 100 21 0.08 50 Comparative Example 3 A2 1140 0.10 740 450 110 28 0.12 58 Comparative Example 4 A2 1160 0.10 760 620 50 24 0.11 50 Comparative Example 5 A3 1140 0.16 740 580 90 13 0.12 90 Comparative Example 6 A3 1180 0.16 760 580 90 22 0.00 0 Comparative Example 7 A4 1150 0.12 760 600 90 25 0.08 48 Comparative Example 8 A5 1150 0.16 750 600 90 24 0.12 56 Comparative Example 9 A6 1140 0.16 760 580 90 22 0.10 56 Comparative Example 10 A7 1150 0.16 760 600 90 28 0.10 58 Comparative Example 11 A8 1150 0.16 910 600 90 21 0.12 48

division Processed ferrite fraction (%) Ferrite recrystallization fraction (%) Amount of solid carbon (ppm) The tensile strength Plastic deformation ratio Plastic anisotropy value γ / α-fiber aggregate texture strength ratio Drawing processability Stretching processability Endocytosis Inventory 1 12 98 3 ○ ○ ○ 8.2 ○ ○ ○ Inventory 2 10 96 3 ○ ○ ○ 6.9 ○ ○ ○ Inventory 3 15 100 2 ○ ○ ○ 5.7 ○ ○ ○ Honorable 4 16 99 3 ○ ○ ○ 7.4 ○ ○ ○ Inventory 5 8 95 2 ○ ○ ○ 7.8 ○ ○ ○ Inventory 6 11 97 2 ○ ○ ○ 9.5 ○ ○ ○ Comparative Example 1 4 88 3 ○ X X 1.8 X X ○ Comparative Example 2 0 100 4 ○ X X 1.1 X ○ ○ Comparative Example 3 3 68 11 △ X X 0.9 X X X Comparative Example 4 9 75 9 ○ X X 1.5 X X X Comparative Example 5 3 92 4 ○ X X 2.2 X ○ ○ Comparative Example 6 10 92 7 ○ ○ X 4.9 △ ○ X Comparative Example 7 4 94 10 X X X 2.8 X ○ X Comparative Example 8 3 81 18 ○ X X 1.4 X X X Comparative Example 9 4 74 7 ○ X X 2.1 X X X Comparative Example 10 2 90 4 ○ ○ X 5.9 △ X ○ Comparative Example 11 0 99 27 △ X X 1.1 X X X Tensile strength: O Strength 35 ~ 40kgf / mm ² satisfied, △ 40Kgf / mm ² or more and less than X 35kgf / mm ²
Plastic deformation ratio value: O r-bar value ≥1.3, X r-bar value <1.3
Plastic Anisotropy Value: OΔr ± 0.15 or less, X Δr value is ± 0.15 or more
Drawing processability (Drawing ratio 1.9): Good processability, poor Earing, X cracking
→ The drawing ratio is expressed by the value of (drawing blank diameter) / (diameter of working punch) in drawing processing.
Stretching processability: O elongation = 40%, X elongation <40%
Stability: O Aging index 2 kgf / mm ² Hereinafter, the X senescence index is 2 kgf / mm ² More than

As shown in Tables 1 to 3, in Inventive Examples 1 to 6 that satisfy all of the ranges proposed by the present invention, the phase fraction, the material, and the aggregate texture strength ratio of each condition are secured within the range desired by the present invention It is possible to provide a high-strength hot-rolled steel sheet excellent in all of aging properties, stretch processability and drawing processability. That is, in the case of the steel of the present invention, the strain aging phenomenon is suppressed by the control of the solid element under the manufacturing conditions of the present invention, and the aggregate structure favorable to the drawing processability can be efficiently controlled, It is possible to secure workability.

On the other hand, in spite of using the inventive steel proposed by the present invention, as in Comparative Examples 1 to 6, when the manufacturing method proposed by the present invention is departed from, the high strength hot-rolled steel sheet excellent in resistance to aging, stretchability and drawability Can not be secured.

In addition, as in the case of Comparative Examples 7 to 10, even when manufactured by the manufacturing method proposed in the present invention, when the steel is out of the steel proposed in the present invention, a high strength hot-rolled steel sheet excellent in all of aging resistance, stretchability and drawing workability I can confirm that I can not.

In the case of Comparative Example 11, it can be confirmed that the hot-rolled steel sheet produced by the steel and the manufacturing method out of the range proposed by the present invention secures the hot-rolled steel sheet for heat resistance, stretchability and drawing workability.

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 (14)

(Excluding 0%), Al: 0.03 to 0.08%, B: 0.0005 to 0.002%, N: 0.0005 to 0.002% (Γ) -fiber / alpha (α) -fiber aggregate texture intensity ratio of 4 to 14, which contains P: 0.03 to 0.05%, S: 0.001 to 0.015%, balance Fe and other unavoidable impurities. This excellent ultra hot rolled steel plate.
(Alpha) -fiber assembly texture refers to a group of a group of azimuths generated in a direction orthogonal to the (111) plane of the texture, and the alpha (alpha) -fiber assembly texture is parallel to the < 110 & And a group of aggregated tissues formed by the facets.
The method according to claim 1,
Wherein the Al, B, and N satisfy the following relationship: Al (wt%) * B (wt%) / N (wt%): 0.025 to 0.07.
The method according to claim 1,
An ultra-hot-rolled steel sheet excellent in workability and anti-aging property with a carbon-free amount of 5 ppm or less.
The method according to claim 1,
Wherein the hot-rolled steel sheet has a thickness of 0.8 to 2.4 mm and is excellent in workability and endurance.
The method according to claim 1,
Wherein the average plastic deformation ratio (r-bar) value is 1.3 or more and the plastic anisotropy (DELTA r) value is 0.15 or less.
The method according to claim 1,
An ultra hot rolled steel sheet having excellent elongation of 40% or more and excellent workability and endurance.
(Excluding 0%), Al: 0.03 to 0.08%, B: 0.0005 to 0.002%, N: 0.0005 to 0.002% 0.03 to 0.05% of P, 0.001 to 0.015% of S, the balance Fe and other unavoidable impurities in a temperature range of 1100 to 1200 占 폚;
The hot rolled steel sheet according to claim 1, wherein the hot rolled steel sheet has a coefficient of friction of 0.05 to 0.2 in the hot rolled steel sheet at a temperature of 600 to (Ar3-50) , And a ratio (Rt / Rf) of the reduction ratio (Rf) of the two passes at the rear end to the total reduction ratio (Rt) of the entire stand is 0.2 to 0.3;
Cooling the hot-rolled steel sheet at a cooling rate of 80 to 150 ° C / second;
Winding the cooled hot-rolled steel sheet at 550 to 650 ° C; And
And descaling the wound hot-rolled steel sheet.
8. The method of claim 7,
Wherein said Al, B and N satisfy the following relationship: Al (wt%) * B (wt%) / N (wt%): 0.025 to 0.07.
8. The method of claim 7,
The microstructure of the steel material at the hot rolling ingot side includes 5 to 20% of the processed ferrite in an area fraction%, and the microstructure after the winding is an area percentage of 90% or more in terms of workability and endurance And a method of manufacturing this excellent ultra thin hot rolled steel sheet.
8. The method of claim 7,
Wherein the hot finish rolling temperature is 600 to 800 占 폚.
delete 8. The method of claim 7,
Wherein the hot-rolled steel sheet has a thickness of 0.8 to 2.4 mm and is excellent in workability and endurance.
8. The method of claim 7,
Wherein said descaling is excellent in workability and anti-aging performance by shot blasting.
14. The method of claim 13,
Wherein the shot blasting is performed by spraying a shot ball having a size of 0.05 to 0.15 mm at an injection speed of 40 to 65 m / sec.