KR20210078912A - Steel sheet having excellent surface properties for panel of vehicle and method manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material for an outer panel of a vehicle having an excellent surface characteristic and a manufacturing method thereof. More specifically, the steel material for an outer panel of a vehicle includes: more than 0% and no more than 0.003 wt% of C; no more than 0.2 wt% of Si; 0.01-0.2 wt% of Mn; 0.005-0.02 wt% of P; no more than 0.008 wt% of S; no more than 0.005 wt% of N; no more than 0.1 wt% of S.Al; 0.02-0.07 wt% of Ti; 0.002-0.03 wt% of Nb; no more than 0.0035 wt% of B; and the remaining of Fe and inevitable impurities. In regard to a density function (ODF) {Φ1,Φ,Φ2} of a three-dimensional crystal orientation of a surface part, the strength of ODF {0°,0°,45°} is no more than 2.0, and the strength of ODF {30°,55°,45°} is no less than 5.5 and no more than 9.5.

Description

Steel material for vehicle exterior with excellent surface properties and manufacturing method thereof {STEEL SHEET HAVING EXCELLENT SURFACE PROPERTIES FOR PANEL OF VEHICLE AND METHOD MANUFACTURING THE SAME}

The present invention relates to a steel material for a vehicle exterior panel having excellent surface properties and a method for manufacturing the same, and more particularly, to a steel material that can be preferably applied as a material for exterior panel material such as an automobile panel and a method for manufacturing the same.

Materials such as ultra-low carbon steel to which carbonitride forming elements are added have been used in steel materials used as materials for automobile exterior panels such as doors, hoods, and fenders to prevent processing defects due to aging.

In particular, in the case of a steel sheet for a vehicle, excellent formability and workability are essentially required. In addition, in order to prevent the steel sheet from being oxidized by external conditions, as well as to obtain a surface color desired by the user, organic materials such as paint must be well coated, so the control of the surface properties of the steel sheet is an important management factor in terms of paintability. .

In order to improve the workability of steel sheets for vehicles, Ti or Nb is added alone or in combination to ultra-low carbon cold-rolled steel sheets to precipitate solid solution elements such as C, N, and S in the form of carbides and nitrides to increase elongation and plastic deformation ratio, There is so-called IF steel (Interstitial Free Steel) that improves formability. Therefore, conventionally, the aging phenomenon caused by the solid solution element is limited by adding a carbonitride forming element such as Ti, which can fix the solid solution element, while achieving high purity in the steelmaking step to precipitate the solid solution element.

In general, for the development of ND//<111> texture, the manufacturing technology of IF steel refines the grain size of the hot-rolled structure through a rapid cooling facility immediately after finishing rolling to develop ND//<111> texture, so that it shows excellent workability in the deformation of the deep drawing mode. However, in the case of ultra-low carbon steel with Ti and Nb alone or complex addition, the behavior of recrystallization temperature according to the size and fraction of the (Ti,Nb)C composite carbide changes, so that the non-recrystallized structure of the surface during light processing such as loops and hoods is non-uniform. It is a continuous problem, such as causing inferiority in paintability due to the occurrence of surface curvature.

Japanese Patent Laid-Open No. 1992-280943

According to one aspect of the present invention, it is an object of the present invention to provide a steel material for a vehicle exterior plate having high strength and excellent surface formability of the steel plate and a method for manufacturing the same.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention, by weight, C: more than 0% 0.003% or less, Si: 0.2% or less, Mn: 0.01 to 0.2%, P: 0.005 to 0.02%, S: 0.008% or less, N: 0.005% or less, S.Al: 0.1% or less, Ti: 0.02 to 0.07%, Nb: 0.002 to 0.03%, B: 0.0035% or less, the remainder including Fe and other unavoidable impurities,

In the density function (ODF) {φ1, φ, φ2} of the three-dimensional crystal orientation of the surface part, the strength of the ODF {0°, 0°, 45°} is 2.0 or less, and the ODF {30°, 55°, 45° } strength of 5.5 or more and 9.5 or less, to provide a steel for vehicle exterior.

In addition, another aspect of the present invention, after reheating the steel slab having the above composition, hot rolling at a finish rolling temperature of 900 ~ 950 ℃ to obtain a hot-rolled steel sheet;

obtaining a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 70 to 83%;

continuously annealing the cold-rolled steel sheet at a temperature increase rate of 4-7°C/s at 780-850°C; and

It provides a method of manufacturing a steel for a vehicle exterior, comprising the step of temper rolling the continuous annealed cold-rolled steel sheet at a reduction ratio of 0.3 to 1.6%.

According to the present invention, it is possible to provide a steel material for a vehicle exterior plate having high strength and excellent surface formability and a method for manufacturing the same.

1 shows the φ2=45° cross section of the density function of the crystal orientation for the major texture components developed in the steel.
2 shows a graph analyzing the φ2=45° cross section of the density function of the EBSD observation photograph of the surface portion and the crystal orientation for Inventive Steel 1. FIG.
3 shows a graph analyzing the φ2=45° cross section of the density function of the EBSD observation photograph of the surface portion and the crystal orientation for Comparative Steel 1.

The present inventors have studied in depth to solve the problems of the prior art that cause inferior paintability due to the occurrence of flexure due to the unevenness of the texture of the surface during processing. As a result, by optimizing the steel component and manufacturing conditions, the surface formability is excellent for vehicle exterior panels. It was confirmed that it was possible to provide a steel material, and came to complete the present invention.

Specifically, by adding an appropriate amount of titanium (Ti) and/or neobium (Nb), which are strong carbonitride forming elements in steel, solid solution elements such as carbon (C), nitrogen (N), sulfur (S) are removed and At the same time, by controlling the finish rolling temperature, the reduction ratio during cold rolling, the temperature of continuous annealing, the temperature increase rate of continuous annealing, the reduction ratio of temper rolling, etc., the density function (ODF) of the three-dimensional crystal orientation of the surface part (ODF) {φ1,φ, In φ2}, the strength of the ODF {0°, 0°, 45°} is 2.0 or less, and the strength of the ODF {30°, 55°, 45°} is 5.5 or more and 9.5 or less. It was discovered that steel could be manufactured.

Specifically, one aspect of the present invention, by weight, C: more than 0% 0.003% or less, Si: 0.2% or less, Mn: 0.01 to 0.2%, P: 0.005 to 0.02%, S: 0.008% or less, N : 0.005% or less, S.Al: 0.1% or less, Ti: 0.02 to 0.07%, Nb: 0.002 to 0.03%, B: 0.0035% or less, balance Fe and other unavoidable impurities,

In the density function (ODF) {φ1, φ, φ2} of the three-dimensional crystal orientation of the surface part, the strength of the ODF {0°, 0°, 45°} is 2.0 or less, and the ODF {30°, 55°, 45° } strength of 5.5 or more and 9.5 or less, to provide a steel for vehicle exterior.

Hereinafter, the reason for adding each alloy component constituting the steel composition, which is one of the main features of the present invention, and an appropriate content range thereof will be described first.

Carbon (C): 0.003% or less (excluding 0%)

As an interstitial solid solution element, C has a great influence on the formation of texture of the steel sheet during cold rolling and annealing. When the amount of dissolved carbon in the steel increases, the growth of crystal grains having a {111} gamma (γ)-fiber texture, which is advantageous for drawing processing, is suppressed, and the growth of grains having a {110} and {100} texture is promoted and annealing. The drawability of the plate deteriorates. Furthermore, when the content of C exceeds 0.003%, the content of Ti and Nb required for precipitating it as carbide increases, which is disadvantageous in terms of economic feasibility, as well as a large amount of fine TiC precipitates distributed in the steel to rapidly reduce the drawability. Since there is a problem, the content of C is preferably limited to 0.003% or less.

Silicon (Si): 0.2% or less (including 0%)

Si is an element contributing to an increase in strength due to solid solution strengthening, and is not intentionally added in the present invention. However, when the Si content exceeds 0.2%, there is a problem in that the plating surface properties are deteriorated by inducing surface scale defects.

Manganese (Mn): 0.01~0.2%

Mn as a solid solution strengthening element not only contributes to the increase in strength, but also serves to precipitate S in the steel as MnS. When the content of Mn is less than 0.01%, MnS cannot be effectively precipitated and drawability is deteriorated. On the other hand, when the content of Mn exceeds 0.2%, since surface problems may occur due to oxides, the content of Mn is preferably limited to 0.01 to 0.2%.

Phosphorus (P): 0.005-0.02%

P is the most effective element for securing the strength of steel without significantly impairing the solid solution effect and drawability. When the content of P is less than 0.005%, it is impossible to secure the desired strength. On the other hand, when the content of P exceeds 0.02%, since surface streak defects may occur due to P segregation, the content of P is preferably limited to 0.005 to 0.02%.

Sulfur (S): 0.008% or less and Nitrogen (N): 0.005% or less

S and N are unavoidably added as impurities present in steel, and in order to secure excellent welding properties, it is desirable to control the content as low as possible. In the present invention, the ?t amount of S is controlled to 0.008% or less, and the N content is managed to 0.005% or less.

Aluminum (Al): 0.1% or less (excluding 0%)

Al contributes to improving the drawability and ductility of steel by precipitating AlN. However, when the Al content exceeds 0.1%, there is a problem in that internal defects of the steel sheet due to excessive formation of Al inclusions during the steelmaking operation occur, so the Al content is preferably controlled to 0.1% or less.

Titanium (Ti): 0.02~0.07%

Ti is an element that greatly contributes to the improvement of the drawability of a steel sheet by reacting with solid solution carbon and solid solution nitrogen during hot rolling to precipitate Ti-based carbonitrides. When the Ti content is less than 0.02%, the carbonitride cannot be sufficiently precipitated and the drawability is inferior. On the other hand, when it exceeds 0.07%, it is preferable to limit the content of Ti to 0.02 to 0.07% because it is difficult to manage inclusions during the steelmaking operation and thus defects in inclusion properties may occur.

Niobium (Nb): 0.002 to 0.03%

Nb is an element that facilitates the formation of a texture during annealing by precipitating a coarse (Ti,Nb)C composite carbide of solid solution carbon during hot rolling, thereby improving the drawability in the rolling direction and the 45 degree direction. When the content of Nb is less than 0.002%, most of the solid solution carbon in the steel is precipitated as TiC, and the amount of coarse (Ti,Nb)C composite carbide precipitated is small, resulting in poor drawability. On the other hand, when the Nb content exceeds 0.03%, most of the solid solution carbon in the steel is precipitated as NbC, and the (Ti,Nb)C composite carbide precipitation amount is small, as well as material deterioration due to the recrystallization temperature rise. there is Therefore, the content of Nb is preferably limited to 0.002 to 0.03%. In this case, when very fine TiC or NbC precipitates of several nm are mainly precipitated at grain boundaries, the development of gamma (γ)-fibers, which are advantageous for workability during annealing recrystallization, is inhibited, so that a coarser ( Precipitation with Ti,Nb)C is preferable.

Boron (B): 0.0035% or less (excluding 0%)

B is an element added to prevent secondary processing embrittlement caused by the addition of P in steel, but when its content exceeds 0.0035%, it entails a decrease in ductility of the steel sheet, so the content of B is limited to 0.0035% or less This is preferable.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The present invention can provide a steel material for a vehicle exterior panel excellent in surface formability only when the above-described component system is satisfied. That is, if the above-mentioned component system is not satisfied, it is impossible to obtain a steel material for a vehicle exterior panel having excellent surface formability as in the present invention.

Meanwhile, according to one aspect of the present invention, since the steel material corresponds to an ultra-low carbon steel having a C content of 30 ppm or less, the steel material may have a ferrite-oriented structure. That is, according to one aspect of the present invention, the steel material may be formed of a ferrite single-phase structure, but may include some other structures that are inevitably generated.

In addition, according to one aspect of the present invention, the steel material may have an average particle diameter of ferrite 5 to 50㎛.

According to one aspect of the present invention, by setting the average particle diameter of ferrite to 5 μm or more, there is an effect of securing stable strength, and by setting the average particle diameter of ferrite to 50 μm or less, the effect of securing a uniform particle size and minimizing non-uniform deformation there is

In addition, according to one aspect of the present invention, in the steel material, the area ratio of ferrite having a particle diameter of less than 50 μm may be 50 to 80% with respect to the area ratio of the total ferrite.

That is, according to one aspect of the present invention, by setting the occupied area ratio of ferrite having a particle diameter of less than 50 μm to 50% or more with respect to the occupied area ratio of the entire ferrite, the size of the entire structure can be controlled to a specific size or more. It is possible to secure the required elongation.

In addition, according to one aspect of the present invention, by setting the occupied area ratio of ferrite having a particle diameter of less than 50 μm to 80% or less with respect to the occupied area ratio of the total ferrite, the size of the entire structure can be controlled to a specific size or less. You can get the strength you want.

In addition, according to one aspect of the present invention, in the steel, the crystal grains up to 0.2 mm in the thickness direction from the surface must be recrystallized.

That is, according to one aspect of the present invention, the microstructure up to 0.2 mm in the thickness direction from the surface of the steel material may be made of a single-phase ferrite structure having an average particle diameter of 20-50 μm.

In addition, according to one aspect of the present invention, the occupied area ratio of ferrite having a particle diameter of less than 50 µm in the thickness direction from the surface of the steel to 0.2 mm in the thickness direction is the occupied area ratio of all ferrites in the thickness direction from the surface of the steel to 0.2 mm. It can be 50-80% with respect to

On the other hand, by setting the occupied area ratio of ferrite having a particle diameter of less than 50 μm from the surface to 0.2 mm in the thickness direction to 80% or less, surface bending defects are prevented during processing of vehicle exterior plates such as roofs and doors, thereby securing excellent surface properties There is an effect that can be done.

In addition, by setting the occupied area ratio of ferrite having a particle diameter of less than 50 μm from the surface to 0.2 mm in the thickness direction to 50% or more, there is an effect of securing a stable material and surface quality through a uniform particle size.

Further, according to one aspect of the present invention, the steel material may include precipitates in ferrite grains, and the precipitates may be TiC precipitates.

In addition, according to another aspect of the present invention, the average size of the TiC precipitates included in the ferrite grains may be 5 nm or more, and more preferably 5 nm or more and 70 nm or less.

Meanwhile, in the present specification, the size of the TiC precipitates represents an average value of values measured for the longest length passing through the center of the precipitates.

In addition, according to an aspect of the present invention, the steel material may include TiC precipitates having a size of 50 nm or more in an amount of 0.2 pieces/㎛ ² or less per unit area.

On the other hand, according to one aspect of the present invention, by setting the number of TiC precipitates having the size of 50 nm or more to ^{0.2 or less per unit area (㎛ 2} ) of the steel, the stored energy is relatively low due to recrystallization delay. It is possible to secure excellent surface quality by preventing the problem of non-uniformity of grains on the surface.

In addition, according to one aspect of the present invention, for the purpose of promoting recrystallization and uniform grain distribution intended in the present invention, the number of TiC precipitates having a size of 50 nm or more is 0.2 pieces/㎛ ² or less per unit area.

That is, according to one aspect of the present invention, it is possible to develop a texture of gamma (γ)-fibers through control of the size of TiC precipitates and promotion of recrystallization in the region from the surface of the steel to 0.2 mm in the thickness direction, thereby Formability can be improved.

In addition, according to an aspect of the present invention, in the steel material, in the density function (ODF) {φ1, φ, φ2} of the three-dimensional crystal orientation of the surface part, the strength of the ODF {0°, 0°, 45°} is 2.0 or less, and the strength of ODF {30°, 55°, 45°} is 5.5 or more and 9.5 or less.

Meanwhile, in the present specification, the surface portion represents a region within 200 μm in the thickness direction from the surface.

In the present specification, the strength of the ODF {0°, 0°, 45°} is a density function of the three-dimensional crystal orientation of the surface part when φ is 0°, φ1 is 0°, and φ2 is 45° ( ODF) is shown, and the strength of the ODF {30°, 55°, 45°} is a three-dimensional crystal orientation of the surface portion when φ is 55°, φ1 is 30°, and φ2 is 45°. represents the intensity as measured in the density function (ODF) of .

That is, the present inventors have repeatedly studied to solve the problem of poor surface formability due to tissue non-uniformity of the surface portion during processing, and as a result, the density function (ODF) of the three-dimensional crystal orientation of the surface portion described above {φ1, φ, φ2 }, the strength of ODF{0°,0°,45°} and ODF{30°,55°,45°} satisfies a specific value, so that the texture of gamma (γ)-fiber can be developed and , it has been found that, due to this, not only the surface formability is excellent, but also it can be suitably used as a steel material for the exterior plate of a vehicle.

In addition, according to one aspect of the present invention, the steel material may include γ-fiber texture as an area fraction, preferably 50% or more, more preferably 60% or more and 80% or less.

According to one aspect of the present invention, the steel material, in the density function (ODF) {φ1, φ, φ2} of the three-dimensional crystal orientation of the surface portion described above, the strength of the ODF {0 °, 0 °, 45 °} 2.0 or less, and the strength of ODF {30 °, 55 °, 45 °} satisfies the characteristics of 5.5 or more and 9.5 or less, so that the γ-fiber texture that can improve the surface formability can be effectively generated at 50% or more .

That is, according to one aspect of the present invention, by including 50% or more of the γ-fiber texture, excellent moldability can be exhibited.

On the other hand, when one or more of the alloy composition and manufacturing conditions of the present invention are not satisfied, the γ-fiber texture cannot be effectively generated, and the rotated cube texture, which is a texture that deteriorates the surface formability, is rather formed excessively.

That is, according to one aspect of the present invention, the steel material can be controlled to form a small amount of the rotated cube texture that can deteriorate the formability, and more specifically, the rotated cube texture as an area fraction, 10% It can be controlled to include less than, and more preferably can be controlled to include less than 5%.

On the other hand, the lower limit of the steel material according to an aspect of the present invention may not be specifically limited because it is preferable to control the content of the rotated cube texture less.

After all, according to one aspect of the present invention, it is possible to provide not only a steel material having excellent surface formability, but also a high formability steel material that can be suitably used as a steel material for a vehicle exterior plate.

In addition, the steel material according to an aspect of the present invention may have a yield strength of 120 MPa or more and 180 MPa or less.

In addition, the steel material according to an aspect of the present invention may have a tensile strength of 270 MPa or more and 340 MPa or less.

In addition, the steel material according to an aspect of the present invention may have a breaking elongation of 53% or less.

In addition, according to an aspect of the present invention, by providing a steel material that satisfies a specific range of yield strength, tensile strength, and/or elongation, it can be particularly preferably used as a hood or roof, which is an exterior plate of an automobile.

On the other hand, another aspect of the present invention,

After reheating the steel slab having the above composition, hot rolling at a finish rolling temperature of 900 ~ 950 ℃ to obtain a hot-rolled steel sheet;

obtaining a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 70 to 83%;

continuous annealing of the cold-rolled steel sheet at 780 to 850° C. at a temperature increase rate of 4 to 7° C./s; and

It provides a method of manufacturing a steel for a vehicle exterior, comprising the step of temper rolling the continuous annealed cold-rolled steel sheet at a reduction ratio of 0.3 to 1.6%.

First, according to an aspect of the present invention, after reheating a steel slab having the above-described composition, hot-rolling to a finish rolling temperature of 900 to 950° C. to obtain a hot-rolled steel sheet. If the finish rolling temperature is less than 900 ℃, when rolling in an ideal region where austenite and ferrite coexist, non-ideal coarse grains are generated in the surface portion and linear bending defects due to non-uniform deformation are generated on the surface during final product processing. , if it exceeds 950°C, it may result in inferiority due to surface scraping due to hot rolling scale.

Next, in the present invention, the hot-rolled steel sheet is cold-rolled at a reduction ratio of 70 to 83% to obtain a cold-rolled steel sheet. When the rolling reduction during the cold rolling is less than 70%, there is a problem in that the {111} texture is not sufficiently grown and the formability is poor. On the other hand, when the rolling reduction during the cold rolling exceeds 83%, there is a problem because the roll load is very severe during on-site manufacturing and the shape deteriorates. Accordingly, the rolling reduction during the cold rolling may be 70 to 83%, preferably 74 to 83%, more preferably 77 to 82%.

In addition, in the present invention, the cold-rolled steel sheet obtained as described above is continuously annealed in a temperature range of 780 to 850°C. If the temperature of the continuous annealing is less than 780 ℃, recrystallization does not occur sufficiently, there is a problem of surface curvature. On the other hand, when the temperature of the continuous annealing exceeds 850° C., there are problems such as a material defect due to a decrease in strength due to grain growth and an excess of elongation. Therefore, in the present invention, the temperature of the continuous annealing is preferably limited to 780 ~ 850 ℃, more preferably limited to 800 ~ 830 ℃.

On the other hand, according to one aspect of the present invention, it is preferable to manage the temperature increase rate in the continuous annealing process in the range of 4 ~ 7 ℃ / s. That is, when the temperature increase rate is less than 4°C/s in the continuous annealing process, as the fraction of TiC fine precipitates increases, recrystallization may be delayed. In addition, when the temperature increase rate exceeds 7° C./s in the continuous annealing process, nucleation is not sufficient, so that a non-recrystallized structure may exist. Therefore, it is more preferable that the temperature increase rate in the continuous annealing process is limited to 4 to 7° C./s.

In addition, according to one aspect of the present invention, the continuously annealed cold-rolled steel sheet is temper-rolled at a reduction ratio of 0.3 to 1.6%. That is, by setting the rolling reduction to 0.3% or more during temper rolling, there is an effect of removing the yield elongation point at which the surface orange peel phenomenon may occur, and by setting the rolling reduction during temper rolling to 1.6% or less, there is a beautiful surface effect.

As described above, through the control of manufacturing conditions such as finish rolling temperature, cold rolling reduction rate, annealing temperature, annealing temperature increase rate, temper rolling reduction rate, etc., as well as appropriate mixing of alloy components, the object of the present invention It is possible to obtain a steel material having the following characteristics.

That is, if the above-described alloy composition and manufacturing method of the present invention are satisfied, in the density function (ODF) of the three-dimensional crystal orientation of the surface portion {φ1, φ, φ2}, the ODF {0°, 0°, 45°} It is possible to effectively provide cold-rolled steel sheets with excellent surface formability with a strength of 2.0 or less and an ODF {30°, 55°, 45°} strength of 5.5 or more and 9.5 or less.

In addition, in the steel material according to an aspect of the present invention, TiC precipitates having a size of 50 nm or ^{more are distributed at 0.2 or less per unit area (㎛ 2} ), and ferrite, which is a recrystallized structure in the region from the steel sheet surface to 0.2 mm in the sheet thickness direction. It is possible to provide a cold-rolled steel sheet excellent in surface formability that can be uniformly distributed without a non-recrystallized structure with a size of 20-50 μm.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

A steel slab having a thickness of 250 mm having an alloy composition shown in Table 1 was reheated to 1250° C., and hot-rolled at a finish rolling temperature of 900 to 950° C. Then, cold rolling, annealing, and temper rolling were performed under the conditions shown in Table 2 to prepare a cold rolled steel sheet. After measuring the precipitate distribution and mechanical properties of the cold-rolled steel sheet thus prepared, the results are shown in Tables 3 to 5 below. In Table 5, YS, TS, and T-EI mean yield strength, tensile strength, and elongation at break, respectively, and the tensile test was performed with specimens collected in accordance with JIS No. 5 standard.

Specifically, after measuring the width of the specimen, the length and thickness of the parallel part, the specimen was mounted on a tensile tester and waited until the specimen was destroyed, and then the yield strength, tensile strength, and breaking elongation of the specimen were measured. Yield strength is the limiting stress when elastic deformation occurs, and is usually expressed through a 0.2% offset, tensile strength is the maximum load divided by the distal surface, and elongation at break is the amount of deformation of the specimen after fracture from the tensile test in %. will be.

In addition, the density function (ODF) intensity value of the crystal orientation of the pole surface portion (region within 200 μm in the thickness direction from the surface) was obtained through EBSD. Pretreatment of specimens for EBSD observation is very important. During the pretreatment through mechanical polishing, a considerable depth of the surface layer is cut away, so an error may be made in determining the structure of the actual pole surface portion. Therefore, it is necessary to refrain from mechanical polishing as much as possible and conduct observation through EBSD (electron backscatter diffraction) by using electrolytic polishing to avoid cutting the depth of the surface as much as possible.

In addition, for each Example and Comparative Example prepared under the conditions of Tables 1 and 2, the texture in the surface portion (region within 200 μm in the thickness direction from the surface) of each specimen, especially the γ-fiber texture and With respect to the texture of the rotated cube, the area fraction of each texture was calculated by electron back scattered diffraction (EBSD), and the values are shown in Table 3.

In addition, after applying 3% of mini press for surface bending, grades 1 to 4 were classified as follows, and grade 2 for bending grades is allowed as a good plate.

1: Difficulty in confirming curvature

2: Micro-confirmation of curvature

3: Identification of curvature

4: Identification of curvature

steel grade C Si Mn P S Sol. Al Ti Nb B Invention lecture 1 0.0019 0.002 0.096 0.0085 0.003 0.021 0.0467 0.008 0.001 Invention lecture 2 0.0021 0.001 0.088 0.0076 0.0033 0.026 0.0232 0.006 0.003 Invention Lesson 3 0.0021 0.001 0.088 0.008 0.0033 0.045 0.0342 0.007 0.001 Invention lecture 4 0.0022 0.002 0.108 0.0102 0.003 0.022 0.03 0.015 0.002 Comparative Steel 1 0.0031 0.002 0.07 0.009 0.003 0.04 0.0846 - 0.001 Comparative Steel 2 0.0025 0.001 0.09 0.01 0.0029 0.038 - 0.032 0.001

steel grade division Cold rolling reduction (%) Annealing temperature (℃) Temperature increase rate during annealing (℃/s) During temper rolling
reduction rate Invention lecture 1 Example 1 75.7 825 4.9 0.8 Comparative Example 1 75.7 762 5.1 1.0 Invention lecture 2 Example 2 80 828 5.5 0.8 Comparative Example 2 80 760 4.6 0.2 Invention Lesson 3 Example 3 82 800 5.8 1.2 Invention lecture 4 Example 4 78 805 6.2 1.0 Comparative Example 3 75.7 770 2.1 0.8 Comparative Steel 1 Comparative Example 4 78 828 2.7 0.7 Comparative Steel 2 Comparative Example 5 78 780 3.1 0.6

steel grade division {0°,0°,45°} intensity {30°,55°,45°} intensity Presence of unredetermined organization γ-fiber
fraction
(%) Rotate cube fraction
(%) Invention lecture 1 Example 1 0.7 8.7 radish 62 5 Comparative Example 1 2.09 5.6 U 40 11 Invention lecture 2 Example 2 0.89 7.6 radish 55 4 Comparative Example 2 2.18 4.7 U 41 21 Invention Lesson 3 Example 3 0.68 8.7 radish 59 4 Invention lecture 4 Example 4 0.71 9.2 radish 61 5 Comparative Example 3 3.07 5.4 U 34 18 Comparative Steel 1 Comparative Example 4 3.21 4.8 U 38 20 Comparative Steel 2 Comparative Example 5 3.43 5.1 U 41 21

division Average size of TiC precipitates (nm) Average number of TiC precipitates with a size of 50 nm or more (pieces/㎛ ² ) Surface average grain size (㎛) Example 1 25 0.1 27 Comparative Example 1 10 10 62

division YS(MPa) TS(MPa) T-El (%) Surface curvature (grade/condition) Example 1 163 287 49 2 / good Comparative Example 1 135 260 53 3 / bad Example 2 159 291 48 1 / good Comparative Example 2 137 288 51 4 / bad Example 3 152 285 47 2 / good Example 4 150 284 48 2 / good Comparative Example 3 164 282 49 3 / bad Comparative Example 4 152 270 45 4 / bad Comparative Example 5 160 310 45 4/ bad

As can be seen from Tables 1 to 5, in the case of Examples 1 to 4 that satisfy the alloy composition and manufacturing conditions proposed in the present invention, the density function (ODF) of the three-dimensional crystal orientation of the pole surface portion {φ1, Φ, φ2 }, the strength of the ODF {0°,0°,45°} when Φ is 0°, φ1 is 0°, and φ2 is 45° is 2.0 or less, and Φ is 55°, φ1 is 30°, φ2 is Since the strength of the ODF {30°, 55°, 45°} at 45° has a range of 5.5 or more and 9.5 or less, it satisfies the range stipulated in the present invention, and thus the surface curvature can be secured up to grade 2 confirmed that there is.

In addition, in the case of Examples 1 to 4 of the present application, the γ-fiber texture in the surface portion was all 50% or more in area fraction, and the rotated cube texture that could deteriorate the moldability was all produced in 10% or less. was confirmed.

On the other hand, with respect to Examples 1 to 4 of the present application, not only the surface portion but also the entire area of the steel material with respect to the γ-fiber texture and the texture of the rotated cube by back scattering electron diffraction (EBSD), each The area fraction of the texture was calculated, and it was confirmed that the γ-fiber texture was more than 50% and the texture of the rotated cube was less than 10% in the entire area of the steel.

However, in the case of Comparative Examples 1 to 3, although the alloy composition of the present invention is satisfied, the temperature increase rate, the annealing temperature, and/or the rolling reduction during temper rolling controlled by the present invention are not satisfied, so the pole surface part 3 As a density function (ODF) of dimensional crystal orientation {φ1,Φ,φ2}, the intensity of ODF{0°,0°,45°} when Φ is 0°, φ1 is 0°, and φ2 is 45° is 2.0. exceeds, and/or the strength of the ODF {30°,55°,45°} when φ is 55° and φ1 is 30° and φ2 is 45° is less than 5.5, which is outside the range stipulated in the present invention, As a result, the strength was lowered, the elongation exceeded 51%, and/or the surface curvature was poor.

On the other hand, in the case of Comparative Examples 4 and 5, examples are shown that do not satisfy the alloy composition prescribed in the present invention. Specifically, in Comparative Example 4, the recrystallization temperature was increased because the content of Ti exceeds the range specified in the present invention, and at the same time, Ti-based carbides are finely distributed in large amounts, and tissue non-uniformity is caused due to recrystallization delay, which is inferior in surface curvature showed up On the other hand, Comparative Example 5 was an example in which Ti was not added, and the carbonitrides were not sufficiently precipitated, so that the drawability was inferior, and it was also inferior in terms of surface curvature.

On the other hand, Figure 1 shows the φ2 = 45° cross section of the density function of the crystal orientation for the major texture components developed in the steel. In addition, (a) of FIG. 2 shows the EBSD observation photograph of the surface of the invention steel 1, and FIG. 2 (b) shows the result of analyzing the density function φ2=45° cross section of the crystal orientation for the invention steel 1. it has been shown

That is, according to (a) of FIG. 2, it can be confirmed that the recrystallization showing the polygonal ferrite form is uniformly distributed as a result of observation of the structure of the surface part, and according to FIG. 2(b), the crystal orientation of It can be confirmed that the ODF {30°, 55°, 45°} strength of the density function φ2 = 45° cross section, that is, the γ-fiber texture, which is advantageous for formability, is well developed.

In addition, (a) of FIG. 3 shows the surface EBSD observations for comparative steel 1, and FIG. 3 (b) is the result of analyzing the density function φ2=45° cross section of the crystal orientation for comparative steel 1. it has been shown

That is, according to (a) of FIG. 3, it can be confirmed that the non-recrystallized structure exists, and according to (b) of FIG. 3, the ODF {0 °, 0 °, It can be confirmed that the strength of 45°}, that is, the rotated cube texture, which is unfavorable to moldability, is developed.

Claims (11)

By weight %, C: more than 0% 0.003% or less, Si: 0.2% or less, Mn: 0.01 to 0.2%, P: 0.005 to 0.02%, S: 0.008% or less, N: 0.005% or less, S.Al: 0.1 % or less, Ti: 0.02 to 0.07%, Nb: 0.002 to 0.03%, B: 0.0035% or less, the remainder including Fe and other unavoidable impurities, the density function (ODF) of the three-dimensional crystal orientation of the surface part (φ1, φ, In φ2}, the strength of ODF {0°, 0°, 45°} is 2.0 or less, and the strength of ODF {30°, 55°, 45°} is 5.5 or more and 9.5 or less.
The method of claim 1,
The surface portion is a region within 200㎛ in the thickness direction from the surface, the vehicle exterior steel.
The method of claim 1,
The steel material includes a γ-fiber texture in an area fraction of 50% or more, a vehicle exterior plate steel.
The method of claim 1,
The steel includes a rotated cube texture as an area fraction, 10% or less, a vehicle exterior plate steel.
The method of claim 1,
The steel material is a vehicle exterior panel steel containing TiC precipitates having a size of 50 nm or more in an amount of 0.2 pieces/㎛ ^{2 or less per unit area.}
The method of claim 1,
The yield strength of the steel is 120 MPa or more and 180 MPa or less, a steel for vehicle exterior.
The method of claim 1,
The tensile strength of the steel is 270 MPa or more and 340 MPa or less, a steel for vehicle exterior.
The method of claim 1,
The breaking elongation of the steel is 53% or less, the steel for the vehicle exterior.
The method of claim 1,
The microstructure from the surface of the steel to 0.2 mm in the thickness direction is made of a ferrite single-phase structure having an average particle diameter of 20-50 μm, a steel for vehicle exterior panels.
By weight %, C: more than 0% 0.003% or less, Si: 0.2% or less, Mn: 0.01 to 0.2%, P: 0.005 to 0.02%, S: 0.008% or less, N: 0.005% or less, S.Al: 0.1 % or less, Ti: 0.02 to 0.07%, Nb: 0.002 to 0.03%, B: 0.0035% or less, after reheating the steel slab containing the remainder Fe and other unavoidable impurities, hot rolling at a finish rolling temperature of 900 to 950 ° C. obtaining a hot-rolled steel sheet;
obtaining a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 70 to 83%;
continuous annealing of the cold-rolled steel sheet at 780 to 850° C. at a temperature increase rate of 4 to 7° C./s; and
A method of manufacturing a steel for a vehicle exterior, comprising the step of temper rolling the continuously annealed cold-rolled steel sheet at a reduction ratio of 0.3 to 1.6%.
11. The method of claim 10,
In the temper rolling steel sheet, in the density function (ODF) {φ1, φ, φ2} of the three-dimensional crystal orientation of the surface part, the strength of the ODF {0°, 0°, 45°} is 2.0 or less, and the ODF {30°, 55 °, 45 °} strength of 5.5 or more and 9.5 or less, a method of manufacturing a steel for vehicle exterior panel.

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