KR101560876B1 - Formable hot-rolled steel sheet having excellent shape fixability and formability, and method for manufacturing the same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet used as a material for home appliances, automobiles, and the like, and more particularly, to a hot-rolled steel sheet for processing having excellent shape-formability and workability and a method for manufacturing the same.
To this end, the present invention can provide a hot-rolled steel sheet excellent in shape-formability and processability by optimizing alloying elements and manufacturing conditions thereof by utilizing aluminum killed steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for machining,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet used as a material for home appliances, automobiles, and the like, and more particularly, to a hot-rolled steel sheet for processing having excellent shape-formability and workability and a method for manufacturing the same.

Steel materials used as materials for household appliances and automobiles are required to have physical properties such as shape crystallization, endurance and moldability. In order to improve shape-crystallinity in the final product and improve productivity by improving the manufacturing process, In addition to fluting prevention, various processing characteristics are required together.

Here, formability is used to indicate the extent to which a desired shape is formed without fracture (tear-off, neck) or defective shape (wrinkle, spring-back, scaratch, galling, etc.).

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 a rank-pod value (Lankford value, hereinafter referred to as "r value") expressed as a strain in the width direction with respect to the strain in the thickness direction.

In particular, an average plastic deformation ratio (hereinafter referred to as "r-bar value") measured by the following equation (1) from the r value measured for each direction with respect to the rolling direction and a plastic anisotropy value Hereinafter, referred to as 'Δr value') is a typical material characteristic value expressing drawability.

r-bar value = (r0 + r90 + 2r45) / 4 Equation (1)

? R = (r0 + r90 - 2r45) / 2 Equation (2)

(Where ri is 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 formula (1), the greater the depth of the molding cup in the drawing process, so that the deep draw molding formability can be judged to be good.

Planar Anisotropy, which is 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 indicates that each of the grains subjected to plastic deformation, If the crystal grains undergo processing or the like are present at random, these crystals do not have directionality, so that the in-plane anisotropy may not be large.

However, in general, since the crystal grains in the steel sheet exhibit strong directionality, the anisotropy of the firing behavior becomes apparent when the processing is performed. When the in-plane anisotropy increases, There is a problem that an earing phenomenon indicating a height difference is increased, resulting in an increase in processing defects and material losses.

Therefore, as the value of? R used as a measure of in-plane anisotropy approaches zero, the strain in all directions is constant and isotropic so that it is very important to appropriately manage the? R value in the drawing process .

In addition, the shape freezing property, which is a characteristic that maintains a constant shape even after steel forming, has a close relationship with the yield strength and the material thickness. Generally, since the forming thickness of the material is fixed, the yield strength, that is, (YR, Yield Ratio), which is the ratio of tensile strength to yield strength, is more advantageous. Therefore, control of the yield ratio is required in order to secure the shape-pendency.

On the other hand, as a method for securing the shape-shape crystallinity, endurance and workability of a steel, conventionally, hot-rolling and cold-rolling are carried out using low-carbon aluminum-killed steel, The processability was secured by efficiently controlling carbon and nitrogen.

However, in this case, a long period of time is required in the heat treatment operation, which not only lowers the productivity but also causes a problem that the deviation of the material in the coil increases due to uneven heating and cooling.

In order to solve this problem, 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, And the like.

In this case, however, an increase in the production cost due to the addition of the expensive alloying element is inevitable, and at the same time, the surface characteristics of the steel are deteriorated. In addition, even if these elements are added in steelmaking, it is difficult to ensure workability such as cupping performance because disordered texture is formed in the hot rolling step.

Accordingly, a method of forming a recrystallized texture structure favorable to workability by cold rolling and annealing using a hot-rolled steel sheet as a raw material for manufacturing a working material has been applied. However, in this case, There is a problem that the processing cost increases depending on the passage of the process.

Accordingly, there is a growing interest in securing the properties and manufacturing of the materials for processing utilizing thermal series in terms of cost reduction and omission of additional processes.

Patent Document 1 discloses a method of reducing the Ar3 transformation temperature by adding a certain amount of manganese (Mn) and boron (B) to a steel containing 0.01 to 0.08% of carbon (C) and reheating the steel at 1150 占 폚, The present invention relates to a method of manufacturing an ultra thin hot rolled steel sheet by a continuous continuous working method in which a cold rolled steel sheet is joined and joined together. However, in this case, the elongation of the hot-rolled sheet can be maintained at 45% or more, so that the desired stretchability can be secured, but the improvement effect on the drawing can not be shown.

Further, 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 proposed that the drawing characteristic can be ensured by the self-annealing effect by controlling the difference between the coiling temperature and the coiling temperature to be 100 DEG C or less. However, in this case, in addition to adding expensive alloying elements such as niobium in order to fix the solid element in the steel, strict management of the finish rolling temperature and winding temperature is required to secure the recrystallization lap in the hot rolling step, There is a problem that stable production is difficult.

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

One aspect of the present invention provides a method of manufacturing a hot-rolled steel sheet for machining that has excellent shape-formability and workability without adding expensive alloying elements such as Ti and Nb by controlling the composition of the steel and the manufacturing conditions I would like to.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.001 to 0.010% carbon, 0.3 to 0.8% manganese (Mn), 0.03% (V): 0.03 to 0.08% Boron (B): 0.0005 to 0.0020% N: 0.0005 to 0.0020% P: 0.02 to 0.06% S: 0.001 to 0.015% : 0.03 to 0.09%, balance Fe and other unavoidable impurities, and having a gamma (?) - fiber / alpha (?) - fiber aggregate texture intensity ratio of 4 to 14 do.

According to another aspect of the present invention, there is provided a method of manufacturing a steel material, comprising: reheating a steel material having the above-described composition; Subjecting the reheated steel material to finish hot rolling at a hot finish temperature of 600 ° C to (Ar3-50 ° C) to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 550 to 650 ° C; And descaling the wound hot-rolled steel sheet, wherein a coefficient of friction between the rolling roll and the steel during the hot rolling is 0.05 to 0.2, and a reduction ratio of the two passes at the rear end with respect to the total reduction rate (Rt) (Rf / Rt) of from 0.2 to 0.3, wherein the ratio (Rf / Rt) is in the range of 0.2 to 0.3.

According to the present invention, it is possible to provide a hot-rolled steel sheet which is excellent in both stretch formability, drawability and endurance, as well as alloy constituents of steel, as well as manufacturing conditions, and is excellent in shape freezing and is suitable for application as a working material.

Particularly, the present invention provides a very thin hot-rolled steel sheet which can replace conventional cold-rolled steel sheets.

The inventors of the present invention have conducted intensive studies to provide a hot-rolled steel sheet capable of replacing cold-rolled steel sheets by securing drawability at the level of conventional cold-rolled steel sheets while securing endurance. As a result, It is possible to produce a hot-rolled steel sheet excellent in workability and shape durability without performing a heat treatment subsequent to the control of the steel sheet. The present invention has been accomplished based on this finding.

Hereinafter, embodiments of the hot-rolled steel sheet according to the present invention and the method for producing 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, the present invention will be described in detail.

A hot-rolled steel sheet excellent in shape-formability and workability according to one aspect of the present invention is characterized by containing 0.001 to 0.010% of carbon (C), 0.3 to 0.8% of manganese (Mn) and 0.03% or less of silicon (Si) (B): 0.0005 to 0.0020%, nitrogen (N): 0.0005 to 0.0020%, phosphorus (P): 0.02 to 0.06%, sulfur (S) (Gamma) -fiber / alpha (alpha) -fiber aggregate texture intensity ratio of from 4 to 14, and the balance Fe and other unavoidable impurities, in the range of from 0.001 to 0.015%, 0.03 to 0.09% vanadium (V) .

Hereinafter, the reason for restricting the alloy component in the hot-rolled steel sheet of the present invention will be described in detail. At this time, the content of each component means weight%, unless otherwise specified.

C: 0.001 to 0.010%

Carbon (C) is an element added to improve the strength of a steel sheet, but it is a typical element that can cause aging when it is present as a solid element in steel.

If the content of C exceeds 0.010%, not only the desired processability in the final hot-rolled steel sheet can not be secured due to the increase of the solid carbon in steel but also the aging property of the steel is adversely affected. There is a problem that the drawing processability is remarkably deteriorated. On the other hand, if the content of C is less than 0.001%, the cost of steelmaking increases drastically with the control of the carbon content in the steelmaking process, and the shape freezing of the product and the steelmaking performance are significantly reduced.

Accordingly, in the present invention, it is preferable to limit the content of C to 0.001 to 0.010% in order to stably secure the desired processability and endurance.

Mn: 0.3 to 0.8%

The manganese (Mn) is an element to be added to prevent the red brittleness caused by sulfur (S), and it is preferable to add Mn of 0.3% or more for the purpose of securing the target shape crystallization and workability together with the above effect . However, if the content of Mn exceeds 0.8%, not only the drawing processability is deteriorated due to the remnant of the solid-solution elements, but also micro-segregation is caused in the steel, thereby deteriorating the formability.

Therefore, in the present invention, the content of Mn is preferably limited to 0.3 to 0.8%.

Si: 0.03% or less (excluding 0%)

Silicon (Si) is an element that bonds with oxygen or the like to form an oxide layer on the surface of the steel to deteriorate the plating property and the surface property, 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% or less.

Al: 0.03 to 0.08%

Aluminum (Al) is an element added for the purpose of preventing deterioration of material by aging agent and aging in Al-killed steel. In order to obtain the above-mentioned effect, it is necessary to add Al at a content of 0.03% or more. However, if it is added too much, not only the deoxidation effect is saturated but also the surface inclusions such as aluminum-oxide (Al ₂ O ₃ ) There is a problem that the surface characteristics are deteriorated. Therefore, it is preferable to limit the upper limit of the content to 0.08%.

B: 0.0005 to 0.0020%

Boron (B) combines with the solid element in the steel to form a boron (B) -based precipitate to improve processability and endurance and to suppress grain growth of the steel even under high temperature maintenance conditions by precipitate formation, .

In order to obtain the above-mentioned effect, B is preferably added in an amount of not less than 0.0005%, but if the content of B is too large, the workability is deteriorated. Therefore, the upper limit of the content is preferably limited to 0.0020%.

N: 0.0005 to 0.0020%

Nitrogen (N) is a typical interstitial strengthening element that penetrates into steel and exhibits strengthening properties, and is an element favorable for securing a desired strength characteristic.

In order to obtain the above-mentioned effect, N is preferably added in an amount of 0.0005% or more. However, if the content of N is excessively high, not only the endurance is deteriorated drastically but also the burden on denitration is increased in the steel making step, Therefore, it is preferable to limit the upper limit of the content to 0.0020%.

P: 0.02 to 0.06%

Phosphorus (P) is an element which is useful for improving the strength and hardness of steel by causing hardening of employment while existing as a solid element in steel. In order to obtain such an effect, it is necessary to add P at 0.02% or more. However, if the content exceeds 0.06%, center segregation occurs at the time of casting, and the workability is deteriorated. Therefore, in the present invention, the content of P is preferably limited to 0.02 to 0.06%.

S: 0.001 to 0.015%

Sulfur (S) is combined with Mn in the steel to form a nonmetallic inclusion serving as a corrosion starting point, and is a factor of red shortness. Therefore, it is desirable to reduce the content as much as possible. However, considering the steelmaking process, the lower limit of S is limited to 0.001%. In addition, if the content of S is excessively large, there is a problem that a part of S binds with Mn in the steel to coarsen the size of the manganese-sulfite-based precipitate. Therefore, the upper limit of the content is limited to 0.015%.

V: 0.03 to 0.09%

Vanadium (V) is an element that has the effect of delaying the recrystallization of ferrite when added at a certain amount and has an effect of improving the strength of steel by precipitating in combination with C, N and the like in the steel.

It is preferable to add V to 0.03% or more in order to ensure the desired strength and shape crystallinity together with the above effects. However, if the content is excessively over 0.09%, there is a problem that the production cost is increased and the hot rolling workability is lowered. Therefore, in the present invention, it is preferable to limit the content of V to 0.03 to 0.09%.

The remainder of the present invention is iron (Fe). However, in the ordinary steel 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 mentioned in this specification, as they are known to any person skilled in the art of steel making.

It is desirable to control the content ratio of the elements forming precipitates of carbide and nitride system in combination with the elements in the steel in order to improve the properties of the material and obtain the target properties by securing the shape crystallization and workability in the steel material formed as described above Do.

In the present invention, in the hot-rolled steel sheet having the above-mentioned composition range, in the case of Al among the additional elements forming the nitride, the content ratio of Al, B and N is defined by the following relational expression 1 in order to ensure the anti- It is preferable to control the atomic ratios of V and C, which are carbide forming elements, to satisfy the following relational expression 2 in order to secure the shape-conformity and workability of the hot-rolled steel sheet.

[Relation 1]

0.025? (Al x B) / N? 0.07

(In the above relational expression 1, the unit of Al, B, and N is% by weight.)

[Relation 2]

1.9? V / C? 3.2

(The numerical value of the above relational expression 2 means the atomic ratio of V and C.)

In the present invention, when (Al x B) / N is less than 0.025, the amount of N in the solid solution in the steel increases to decrease the endurance and workability of the final product, while (Al x B) / N is 0.07 However, the recrystallization temperature of the hot-rolled sheet is imagined and the amount of the expensive alloying element is increased, which increases the manufacturing cost. Therefore, in the present invention, it is preferable to limit the content ratio of Al, B and N ((Al x B) / N) to 0.025 to 0.07 in order to secure excellent endurance and drawability.

In the present invention, when the (V / C) atomic ratio is less than 1.9, it is difficult to secure breakability and workability by the expression of a solid solution element. On the other hand, However, there is a problem that surface defects and productivity are deteriorated due to an excessive addition of V. Therefore, in the present invention, it is preferable to limit the atomic ratio (V / C) of V and C to 1.9 to 3.2 in order to secure excellent shape crystallization and workability.

Further, in the present invention, the desired drawing processability can be secured by controlling the surface strength ratio of the aggregate structure fibers that are closely related to the steel formability.

Generally, an array having a certain plane and orientation generated inside a crystal is called a texture, and an aspect in which these collective tissues develop into a band in a certain direction is called a collective tissue fiber. 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.

As described above, the texture representing the aggregation of crystals has a close relationship with the drawing processability. The higher the face strength value of the gamma (?) - fiber component generated at right angles to the (111) However, in the present invention, it has been found out that a complex relationship with the surface intensity ratio of alpha (alpha) -fabricated in equilibrium in the <110> orientation is directly related to the drawability, and by managing them, .

More specifically, in the hot-rolled steel sheet according to one aspect of the present invention, a proper drawing workability can be ensured by controlling the gamma (?) - fiber / alpha (alpha)

If the gamma (gamma) -fiber / alpha (alpha) -fiber aggregate texture strength ratio is less than 4, the formation of aggregate texture to the (111) plane which is advantageous for drawing processing is insufficient and the desired drawing processability can not be ensured, (γ) -fiber / alpha (α) -fiber aggregate structure strength ratio exceeding 14, the workability is excellent. However, as the anisotropy increases, the earing phenomenon becomes serious and the material loss increases.

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

As described above, it is preferable that the hot-rolled steel sheet of the present invention, which satisfies the component composition, the component relationship, and the surface strength ratio of the aggregate structure fiber, contains the ferrite having an area fraction of 90% or more in its microstructure. If the ferrite phase is less than 90%, the workability is remarkably lowered due to the high dislocation density in the hot-rolled steel sheet, which causes a problem of work cracking in the drawing process.

The hot-rolled steel sheet of the present invention may contain some precipitated cementite in addition to ferrite.

The hot-rolled steel sheet of the present invention can have an average plastic deformation ratio (r-bar) value of 1.3 or more and a plastic anisotropy (DELTA r) value of 0.15 or less and a yield ratio of 70% or more, an elongation of 40% ^The hot-rolled steel sheet has an aging index of not more than ² , and is excellent in workability and endurance.

In addition, in order to produce the hot-rolled steel sheet of the present invention as an ultra-thin steel sheet, it is preferable to control its thickness to 0.8 to 2.4 mm.

Hereinafter, a method of manufacturing a hot-rolled steel sheet according to one aspect of the present invention will be described in detail.

First, the hot-rolled steel sheet according to the present invention can be manufactured by a reheating-hot-rolling-winding-descaling process that satisfies the composition of the composition proposed in the present invention. .

(Reheating step)

It is preferable to reheat the aluminum-killed steel material satisfying the above-mentioned compositional composition. In this case, the reheating step is carried out for the purpose of smoothly performing the subsequent hot rolling process and securing the desired physical properties, . &Lt; / RTI &gt;

In the present invention, it is preferable that the steel material be subjected to a single phase of austenite which can coarsen the initial austenite structure as much as possible during 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, the precipitation of aluminum-nitride (AlN) is suppressed. On the other hand, when the reheating temperature exceeds 1200 ° C, the passing time between hot rolling rolls increases Abnormal growth of crystal grains may occur, resulting in deterioration of processability of the product and increase of the surface scale, thereby increasing the possibility of occurrence of surface defects.

(Hot rolling process)

The reheated steel material may be hot-rolled and rolled to produce a hot-rolled steel sheet.

At this time, the hot rolling is preferably performed in a single-phase ferrite phase at 600 ° C to (Ar 3 transformation point -50 ° C). That is, it is preferable to perform hot rolling at a low temperature ferrite temperature range.

Thus, when hot rolling is performed in the ferrite temperature range, it is possible to secure recrystallized microstructure in the ferrite region in the subsequent cooling process.

More preferably, the hot finish rolling is preferably performed at 600 to 800 ° C. If the hot finish rolling temperature is less than 600 ° C, it is advantageous in terms of securing processability, but it is difficult to secure the coiling temperature in the following winding process, There is a problem that the load is increased and the continuous workability is remarkably lowered. On the other hand, when the hot rolling temperature exceeds 800 캜, the processed ferrite fraction in the hot rolling step is lowered, and the driving force for recrystallization is decreased, which makes it difficult to secure workability.

Particularly, in the present invention, it is preferable that the microstructure at the inlet side in the hot rolling step includes processed ferrite, transformed ferrite, and austenite, and more preferably 5 to 20% of the processed ferrite in the area fraction.

If the fraction 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 secure sufficient workability. On the other hand, if it exceeds 20%, there is a problem that workability is remarkably lowered due to an increase in rolling load during hot rolling.

In addition, in order to provide a hot-rolled steel sheet having workability equal to or higher than that of a conventional cold-rolled steel sheet, it is preferable to form the above-described aggregate structure. In other words, the formation of gamma (?) - fiber / alpha (alpha) - fiber aggregate structure is developed to secure a high average plastic strain ratio (r-bar) value and low plastic anisotropy So as to control the rolling process so as to facilitate manufacture of a desired part.

More specifically, in the present invention, it is preferable to carry out lubrication rolling during hot rolling in order to secure drawing processability, and it is preferable to control the coefficient of friction between the rolling roll and the steel at 0.05 to 0.20.

If the coefficient of friction between the roll and the steel is less than 0.05, slipping 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 steel exceeds 0.20, The properties are deteriorated to shorten the life of the roll, and a shear band having a poor processability is formed on the surface of the steel sheet, resulting in a problem of deteriorating workability of the product. That is, when the coefficient of friction exceeds 0.20, 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 the targeted drawing processability can not be secured.

The desired rolling processability can be more advantageously secured by controlling the rolling roll and the coefficient of friction of the steel material and controlling the rolling distribution of the rolls in each rolling step in the present invention. 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.

More specifically, 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 is 0.2 to 0.3.

If the reduction ratio (Rf / Rt) is more than 0.3, it is difficult to obtain a hot-rolled steel sheet having a desired thickness due to an increase in the rolling load of the rear roll, If the ratio (Rf / Rt) is less than 0.2, there is a problem that the driving force for recrystallization decreases and it is difficult to form a desired texture, and it is difficult to ensure drawing processability.

By performing the final hot rolling under the hot rolling conditions as described above, it is possible to secure an average plastic deformation ratio (r-bar) value of 1.3 or more and a plastic anisotropy (DELTA r) value of 0.15 or less which can not be secured in the conventional hot- have.

(Winding step)

The hot rolled steel sheet produced by the hot rolling can be wound. In the present invention, the winding step is a process in which rearrangement of the assembled structure constructed in the recrystallization and hot rolling of the processed ferrite structure occurs. Therefore, the desired endurance and drawability can be secured by optimizing the winding process.

Preferably, the winding is carried out at 550 to 650 ° C.

If the coiling temperature at the time of coiling 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 The drawability is deteriorated.

The hot-rolled steel sheet of the present invention which has been subjected to the above-mentioned winding step includes a ferrite phase having a recrystallization ratio of 90% or more, and may partially contain precipitated cementite. If the recrystallization ratio of the ferrite phase is less than 90%, the workability is remarkably lowered due to the high dislocation density in the hot-rolled steel sheet, which may cause a crack in the drawing process.

(Descaling step)

Generally, a descaling process is performed to remove the oxide layer on the surface of the hot-rolled steel sheet.

In the case of the present invention, besides the effect of controlling the oxide layer on the surface of the hot-rolled steel sheet produced as described above, a suitable compressive stress is introduced to the hot-rolled steel sheet surface to generate ferrite grains having a dislocation density, , Descaling is performed in order to reduce the phenomenon of dislocation of the dislocations by the hiring element and to improve the endurance of the material.

At this time, the descaling process is preferably a mechanical descaling method. For example, shor blasting may be used.

The shot blasting may be performed by spraying a shot ball having a diameter of 0.05 to 0.15 mm at an injection speed of 25 to 50 m / s.

If the diameter of the shot ball is less than 0.05 mm, the mechanical peeling effect of the hot-rolled steel sheet surface layer is small, and it is difficult to secure the residual stress effect as desired in 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 sharply increases, which causes a problem of workpiece cracking during processing.

In addition, if shot blasting speed is less than 25 m / s, impact pressure of short ball acting on the surface layer of hot-rolled steel sheet is low and it is difficult to secure desired endurance and workability. On the other hand, There is a problem in that the depth is formed in the thickness direction of the hot-rolled steel sheet by 10% or more, which causes 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 )

The steel having the composition shown in Table 1 below was subjected to reheating, hot rolling, coiling and descaling under the conditions shown in Table 2 below to obtain a final hot-rolled steel sheet.

Then, the microstructure, plastic deformation ratio, plastic anisotropy, texture ratio, shape dynamics, drawing processability, stretch processability and endurance of each hot-rolled steel sheet were measured and the results are shown in Table 3 below .

Steel grade Component composition (% by weight) (Al x B) / N V / C division C Mn Si S Al P N B V A1 0.0037 0.46 0.008 0.011 0.046 0.038 0.0014 0.0012 0.036 0.0394 2.289 Invention river A2 0.0043 0.51 0.011 0.007 0.054 0.051 0.0012 0.0007 0.051 0.0315 2.791 Invention river A3 0.0051 0.69 0.016 0.009 0.065 0.049 0.0018 0.0016 0.064 0.0578 2.953 Invention river A4 0.0031 0.24 0.011 0.012 0.015 0.011 0.0039 0.0003 0.021 0.0012 1.594 Comparative steel A5 0.0061 0.65 0.020 0.008 0.056 0.034 0.0015 - 0.044 - 1.697 Comparative steel A6 0.0045 0.46 0.009 0.011 0.042 0.049 0.0065 0.0013 0.039 0.0084 2.039 Comparative steel A7 0.0074 1.15 0.591 0.019 0.009 0.096 0.0014 0.0032 0.121 0.0206 3.847 Comparative steel A8 0.0310 0.09 0.011 0.009 0.123 0.035 0.0016 0.0009 0.051 0.0692 0.387 Comparative steel

(In Table 1, (Al x B) / N represents the weight ratio, and (V / C) represents the atomic ratio.

The (V / C) atomic ratio is calculated as [(V weight ratio) / 51] / [(C weight ratio) / 12]

Steel grade Reheating Hot rolling Coiling Descaling division
Temperature
(° C) friction
Coefficient Wrap-up
Rolling temperature (캜) Reduction ratio
(Rf / Rt) Temperature
(° C) Short ball diameter
(mm) Injection speed
(° C / s) A1 1130 0.13 720 0.25 620 0.12 45 Inventory 1 A1 1160 0.16 730 0.22 620 0.11 48 Inventory 2 A2 1150 0.11 730 0.26 600 0.10 35 Inventory 3 A2 1150 0.15 740 0.26 600 0.08 41 Honorable 4 A3 1140 0.11 660 0.24 560 0.12 30 Inventory 5 A3 1180 0.15 660 0.25 560 0.08 38 Inventory 6 A1 1130 0.38 740 0.23 620 0.08 40 Comparative Example 1 A1 1130 0.13 920 0.21 620 0.08 40 Comparative Example 2 A2 1150 0.09 720 0.29 400 0.12 42 Comparative Example 3 A2 1150 0.12 760 0.51 620 0.11 40 Comparative Example 4 A3 1140 0.15 740 0.11 580 0.12 70 Comparative Example 5 A3 1180 0.18 760 0.21 580 - - Comparative Example 6 A4 1160 0.14 760 0.25 620 0.08 35 Comparative Example 7 A5 1160 0.15 750 0.23 620 0.12 46 Comparative Example 8 A6 1150 0.15 760 0.25 580 0.10 26 Comparative Example 9 A7 1150 0.15 760 0.26 600 0.10 38 Comparative Example 10 A8 1150 0.16 910 0.22 600 0.12 28 Comparative Example 11

division Microstructure r-Bar
value
△ r
value γ / α
Fiber
Organized organization
Surface Intensity Ratio Properties Processing
ferrite Redetermination
ferrite shape
Frost formation Drawing processability Stretching processability Endocytosis Inventory 1 10 95 ○ ○ 9.6 ○ ○ ○ ○ Inventory 2 14 97 ○ ○ 7.5 ○ ○ ○ ○ Inventory 3 12 99 ○ ○ 6.8 ○ ○ ○ ○ Honorable 4 9 95 ○ ○ 8.7 ○ ○ ○ ○ Inventory 5 15 98 ○ ○ 10.4 ○ ○ ○ ○ Inventory 6 16 94 ○ ○ 11.2 ○ ○ ○ ○ Comparative Example 1 3 86 × × 2.1 ○ × × ○ Comparative Example 2 0 100 × × 1.5 × × ○ ○ Comparative Example 3 4 65 × × 1.1 ○ × × × Comparative Example 4 10 71 × × 3.2 × × × × Comparative Example 5 2 91 × × 1.9 × × ○ ○ Comparative Example 6 9 92 ○ × 5.1 ○ △ ○ × Comparative Example 7 3 93 × × 3.1 × × ○ × Comparative Example 8 4 82 × × 2.2 × × × × Comparative Example 9 4 71 × × 2.6 ○ × × × Comparative Example 10 One 92 ○ × 6.0 ○ △ × ○ Comparative Example 11 0 100 × × 1.2 × × × × - plastic deformation ratio (r-bar) value:?; r-bar value &gt; = 1.3, x; r-bar value <1.3
- plastic anisotropy (DELTA r) value: o; Δr value within ± 0.15, ×; Δr value ± 0.15 or more
- Shape crystallization: ○; Yield ratio ≥ 70%, ×; Yield ratio <70%
(Yield ratio (yield strength / tensile strength) * 100)
- Drawability (Drawing ratio 1.9): ○; Good workability,?; Earing bad, ×; Machining crack generation
(The drawing ratio is expressed by the value of (drawing blank diameter) / (diameter of machining punch) in drawing processing)
- Stretch processability: ○; Elongation &gt; 40%, x; Elongation <40%
- Endosseous: ○; Aging index 2 kgf / mm ² or less, x: Aging index 2kgf / mm ² or more

As shown in Tables 1 to 3, in the case of Inventive Examples 1 to 6 manufactured according to the manufacturing method proposed by the present invention using a steel material satisfying all of the component ranges proposed in the present invention, the fraction, the material The plastic deformation ratio and the plastic anisotropy value) and the aggregate structure surface strength ratio can be ensured within the aimed range of the present invention, and a hot-rolled steel sheet excellent in both shape crystallinity, resistance to aging, stretchability and drawability can be provided .

That is, the steel of the present invention suppresses strain aging under the manufacturing conditions of the present invention and efficiently controls the aggregate structure favorable to the drawing processability, thereby satisfying the control range of the surface strength ratio and the microstructure fraction, Formation and workability can be secured.

On the other hand, as in the case of Comparative Examples 1 to 6, in spite of using the inventive steel proposed by the present invention, when the manufacturing method proposed by the present invention is not satisfied, shape durability, resistance to aging, stretchability and drawability One or more of them fell. That is, it can be confirmed that a hot-rolled steel sheet having both excellent shape-formability and workability can not be secured.

In addition, even when manufactured by the manufacturing method proposed in the present invention, as in Comparative Examples 7 to 10, even when the composition composition is out of the range proposed by the present invention, any of the shape crystallinity, the resistance to aging, the stretchability, It can be confirmed that a hot-rolled steel sheet having both excellent shape-formability and workability can not be obtained.

In addition, it can be confirmed that, in the case of Comparative Example 11, the hot-rolled steel sheet for both the shape fixability, the anti-aging property, the stretch processability and the drawing processability was obtained when the composition and the manufacturing conditions proposed by the present invention were not all satisfied.

Claims (9)

(Si): 0.03% or less (excluding 0%), aluminum (Al): 0.03 to 0.08%, boron (B) (B) of 0.0005 to 0.0020%, nitrogen (N) of 0.0005 to 0.0020%, phosphorus (P) of 0.02 to 0.06%, sulfur (S) of 0.001 to 0.015%, vanadium (V) Fe and other unavoidable impurities, the relationship between Al, B and N satisfies the following relational expression 1, the relationship between V and C satisfies the following relational expression 2,
Gamma (γ) - Fiber / Alpha (α) - Fiber Aggregate Structural surface strength ratio of 4 ~ 14.
(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.
[Relation 1]
0.025? (Al x B) / N? 0.07
(In the above relational expression 1, the unit of Al, B, and N is% by weight.)
[Relation 2]
1.9? V / C? 3.2
(The numerical value of the above relational expression 2 means the atomic ratio of V and C.)
delete The method according to claim 1,
Wherein the hot-rolled steel sheet has a microstructure and includes an area fraction of 90% or more of ferrite.
(Si): 0.03% or less (excluding 0%), aluminum (Al): 0.03 to 0.08%, boron (B) (B) of 0.0005 to 0.0020%, nitrogen (N) of 0.0005 to 0.0020%, phosphorus (P) of 0.02 to 0.06%, sulfur (S) of 0.001 to 0.015%, vanadium (V) Fe and other unavoidable impurities, and reheating the steel material having a relationship of Al, B and N satisfying the following relational expression 1 and satisfying the following relationship (2):
Subjecting the reheated steel material to finish hot rolling at a hot finish temperature of 600 ° C to 800 ° C to produce a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at 550 to 650 ° C; And
And descaling the wound hot rolled steel sheet,
Wherein a friction coefficient between the rolling roll and the steel during the hot rolling is 0.05 to 0.2 and a ratio (Rf / Rt) of the reduction ratio (Rf) of the rear two passes to the total reduction ratio (Rt) A method for manufacturing a hot-rolled steel sheet for machining having excellent crystallinity and workability.
[Relation 1]
0.025? (Al x B) / N? 0.07
(In the above relational expression 1, the unit of Al, B, and N is% by weight.)
[Relation 2]
1.9? V / C? 3.2
(The numerical value of the above relational expression 2 means the atomic ratio of V and C.)
delete delete 5. The method of claim 4,
The descaling is performed by shot blasting,
Wherein the shot blasting is carried out by spraying a shot ball having a diameter of 0.05 to 0.15 mm at an injection speed of 25 to 50 m / s, and having excellent shape-formability and processability.
5. The method of claim 4,
Wherein the microstructure of the steel at the inlet side during the final hot rolling step includes processed ferrite having an area fraction of 5 to 20%.
5. The method of claim 4,
Wherein the microstructure of the hot-rolled steel sheet after the winding includes ferrite having an area fraction of 90% or more, and having excellent formability and processability.