JP3755218B2 - Method for producing cold-rolled steel sheet with excellent press formability and rough skin resistance - Google Patents

Description

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【表２】

【００４５】
【表３】

【００４６】
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【発明の効果】
本発明は鋼組成、金属組織及び製造条件を特定することにより、良好な化成処理性と耐肌荒れ性を両立した、特に深絞りなどのプレス成形に好適な冷延鋼板が提供できる。
【図面の簡単な説明】
【図１】本発明の実施例に係る粒径比（Ｄｓ／Ｄｃ）と耐肌荒れ性、化成処理性及びｒ値との関係を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a cold-rolled steel sheet having excellent surface properties after press molding, that is, for example, suitable as an inner and outer plate panel material of an automobile, that is, excellent in surface roughness resistance and chemical conversion treatment.
[0002]
[Prior art]
A cold-rolled steel sheet for automobile inner and outer plates is required to have good chemical conversion treatment as a pre-process of coating in addition to press formability and good surface properties after press forming. Interstitial Free steel (hereinafter referred to as IF steel), in which carbonitride-generating elements such as Ti are added to ultra-low carbon steel with reduced carbon and nitrogen content, is superior in press formability and non-aging, so it is an automobile. It is widely used for deep drawing applications such as panel materials. However, IF steel has a good grain growth property during annealing and therefore tends to cause abnormal grain growth, which may result in rough skin defects after press forming. On the other hand, it is generally known that when Ti is added to a steel sheet, titanium oxide is generated on the surface and the chemical conversion treatment property is deteriorated. Several studies have been conducted to improve the above characteristics.
[0003]
In order to improve the rough skin resistance, as a method for suppressing abnormal grain growth, a method is used in which precipitates are generated by additive elements to suppress grain growth. However, a method in which an additive element is essential is not preferable because it causes an increase in cost and the additive element affects other characteristics. On the other hand, as a method for refining crystal grains by a production method without using an essential additive element, Japanese Patent Laid-Open No. 5-117759 discloses cooling after rolling at Ac ₃ or more and 30% at Ar ₃ (preferably Ar ₁ ) or less. A method is disclosed in which after rolling above, the temperature is raised to Ac ₃ or higher, cooling is performed with or without 50% rolling, and further cold rolling and annealing are performed.
[0004]
As a method for improving chemical conversion properties, for example, Japanese Patent Publication No. 4-61063 discloses an ultra-low carbon steel sheet to which a small amount of Nb defined by the Ti concentration is added in addition to Ti. Japanese Examined Patent Publication No. 4-40412 discloses a method in which ultra-low carbon steel is continuously annealed in an atmosphere having a dew point of −25 ° C. or lower.
[0005]
[Problems to be solved by the invention]
However, according to the method of refining crystal grains according to the additive elements or production conditions shown in the prior art, it is possible to improve the rough skin resistance, but on the other hand, when the crystal grains become fine, the development of a good texture Is suppressed and the r value is lowered. Therefore, it is difficult to achieve both the press formability and the rough skin resistance, which the present invention intends to solve, simply by uniformly miniaturizing the structure. The method disclosed in Japanese Examined Patent Publication No. 4-61063 requires an expensive additive element called Nb, leading to an increase in cost and a decrease in manufacturability. The method for improving chemical conversion treatment disclosed in Japanese Patent Publication No. 4-40412 is difficult to manage the atmosphere in continuous annealing, difficult to operate stably, and has limitations during production.
[0006]
An object of the present invention is to provide a method for producing a cold-rolled steel sheet that achieves both of the two properties of rough skin resistance and press formability required for a steel sheet for automobiles and is excellent in chemical conversion treatment.
[0008]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
(1) The manufacturing method of the cold-rolled steel sheet excellent in press formability and rough skin resistance of the present invention is mass%, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% Hereinafter, P: 0.1% or less, S: 0.003 to 0.02%, Sol. A steel slab containing Al: 0.02-0.1%, N: 0.01% or less, Ti: 0.03-0.08%, B: 0.0001-0.0015% is Ac ₃ or more. A step of heating to a temperature, and when the slab center temperature is Ar ₃ or higher and the surface temperature is Ar ₃ or higher with respect to the heated steel slab, the rolling rate is 30% or higher, the slab center temperature is Ar ₃ or higher and the surface temperature When the ratio is less than Ar _3, a hot rough rolling process in which the rolling rate is 30% or more and the sum of both rolling rates is 70% or more, and the steel slab subjected to the rough rolling is heated at a heating rate of 5 ° C./s or more. After heating to a temperature of Ac ₃ + 50 ° C. or higher, finish rolling at a rolling rate of 51% or higher at a temperature of Ar ₃ or higher, and cold rolling the hot rolled steel sheet that has been finish-rolled, at a temperature of 650 ° C. or higher. recrystallization annealing, the average grain size of the ferrite of 10% plate thickness from the surface A steel sheet is obtained in which the average crystal grain size of ferrite in the central part of the plate thickness excluding 10% or less from the surface is 90% or less and the average crystal grain size of ferrite in the central part of the plate thickness is 10 to 30 μm. And a process .
[0009]
(2) The method for producing a cold-rolled steel sheet having excellent press formability and rough skin resistance according to the present invention is mass%, C: 0.005% or less, Si: 0.1% or less, and Mn: 0.5%. Hereinafter, P: 0.1% or less, S: 0.003 to 0.02%, Sol. A steel slab containing Al: 0.02-0.1%, N: 0.01% or less, Ti: 0.03-0.08%, B: 0.0001-0.0015% is Ac ₃ or more, The step of heating to a temperature of 1150 ° C. or less, and the heated steel slab, when the slab center temperature is Ar ₃ or more and the surface temperature is Ar ₃ or more, the rolling rate is 30% or more, and the slab center temperature is Ar ₃ or more. In addition, when the surface temperature is less than Ar _3, a hot rough rolling process in which the rolling rate is 30% or more and the sum of both rolling rates is 70% or more, and the steel slab subjected to the rough rolling is heated at a rate of 5 ° C./s or more. The entire steel is heated to a temperature of Ac ₃ + 50 ° C. or higher, and then finish rolling at a rolling rate of 51% or higher at a temperature of Ar ₃ or higher, and the hot-rolled steel plate that has been finish-rolled is cold-rolled to 650 ° C. recrystallization annealing at temperatures above, ferrite from the surface of less than 10% of the plate thickness The average crystal grain size of the ferrite is 90% or less of the average crystal grain size of the ferrite at the center of the plate thickness excluding the plate thickness within 10% from the surface, and the average crystal grain size of the ferrite at the center of the plate thickness is 10 to 10%. And a step of obtaining a steel plate having a thickness of 30 μm .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In order to obtain a cold-rolled steel sheet that achieves both of the two properties of rough skin resistance and press formability required for automotive steel sheets, and further has excellent chemical conversion properties, Research on the relationship between the average crystal grain size of ferrite, rough skin resistance, and press formability was repeated, and the metal structure and production conditions of the cold-rolled steel sheet were examined in detail.
[0011]
As a result, the average crystal grain size of ferrite within 10% of the plate thickness from the steel sheet surface is refined to 90% or less of the average grain size of the plate thickness center portion, and the crystal grain size of the plate thickness center portion is limited to 10-30 μm. It was found that by controlling to the range, good skin roughness resistance was obtained while ensuring a high r value. Furthermore, the Ti-B addition IF steel which has the said metal structure also acquired the knowledge that chemical conversion property is very excellent, without adding Nb essential or requiring a special annealing method.
[0012]
Based on the above knowledge, the present inventors adjusted the steel composition, hot rolling conditions and annealing conditions after cold rolling, and controlled the metal structure of the cold rolled steel sheet to the above range, and the rough surface resistance and press The present invention has been completed by finding the cold-rolled steel sheet of the present invention and a method for producing the same, which are compatible with the two properties of formability and further have excellent chemical conversion properties.
[0013]
In other words, the present invention obtains a cold-rolled steel sheet having both of the two properties of rough skin resistance and press formability and excellent chemical conversion treatment properties by limiting the steel composition, metal structure and production conditions to the following ranges. be able to.
Hereinafter, the reason for adding the component, the reason for limiting the component, the reason for limiting the metal structure, and the reason for limiting the manufacturing conditions will be described.
[0014]
(1) Component composition range C: Since C is an element that adversely affects the press formability of a steel sheet, it is desirable that its content be small. In particular, if it exceeds 0.005%, the ductility is extremely lowered, so C is 0.005% or less.
[0015]
Si, Mn, and P: Si, Mn, and P each increase strength by solid solution strengthening, but remarkably deteriorate formability. Therefore, it may be added as necessary, but it is necessary that the content is 0.1, 0.5, 0.1% or less, respectively.
[0016]
sol. Al: Al is added for deoxidation. Its content is sol. If the Al content is less than 0.02%, the above effect cannot be obtained sufficiently. If the Al content exceeds 0.1%, the effect is saturated and uneconomical. Al is 0.02 to 0.1%.
[0017]
S: If S is less than 0.003%, the scale peelability during pickling decreases. On the other hand, if it exceeds 0.02%, the press formability is remarkably deteriorated, so S is 0.003% to 0.02%.
[0018]
N: N reduces formability and generates stretcher strain marks, so it is desirable to reduce it as much as possible. The effect is almost eliminated at 0.01% or less, so N is 0.01% or less.
[0019]
Ti: Ti combines with solute C and N to form carbonitrides and improves formability. If the amount added is less than 0.03%, the effect is not sufficient, and if added over 0.08%, the effect becomes saturated and uneconomical, so Ti needs to be 0.03-0.08%. is there.
[0020]
B: B has an effect of strengthening grain boundaries in IF steel and suppressing secondary work brittleness. In addition, nitrides are formed to suppress abnormal grain growth, and there is an effect of improving the rough skin resistance. If it is less than 0.0001%, the effect is not sufficient, and if it exceeds 0.0015%, the effect is saturated, so B is 0.0001 to 0.0015%.
[0021]
Nb: Nb, like Ti, forms carbonitrides and improves formability, so that the effect of the present invention does not change even if added as necessary.
(2) Metal structure In the metal structure of the present invention, the average crystal grain size of ferrite in the central part of the plate thickness is 10 to 30 μm, and the average crystal grain size of ferrite within 10% of the plate thickness from the steel sheet surface is the central part of the plate thickness. And 90% or less of the average crystal grain size of the ferrite.
[0022]
If this exceeds 90% of the center portion of the plate thickness, both the rough skin resistance and the r value cannot be achieved as in the conventional material. Also, chemical conversion treatment is not excellent. Furthermore, if the average crystal grain size in the center portion of the plate thickness is less than 10 μm, good texture growth is insufficient and a high r value cannot be obtained. If it exceeds 30 μm, rough skin tends to occur.
[0023]
Although the reason why the chemical conversion property is improved by making the surface crystal grains fine is unknown, it is generally considered as follows. For chemical conversion treatment, the elution reaction of Fe on the steel sheet surface is important, and in order to promote this, it is effective to form a local battery on the steel sheet surface. Since the solute atoms are concentrated in the grain boundary and become electrically non-uniform, the finer the grain size, the better the chemical conversion treatment property.
[0024]
Furthermore, if the crystal grain boundary at the center of the plate thickness that does not directly contribute to the formation of the chemical conversion film is reduced by making a difference between the crystal grain size of the surface crystal grain and the center of the plate thickness, the grain boundary at the surface layer and its periphery can be reduced. It is considered that the concentration of solute atoms is promoted and the chemical conversion treatment property is improved. In addition, the rough skin resistance is further improved by the difference in crystal grain size between the surface and the center of the plate thickness. This is because the surface layer is strengthened at the grain boundary and the strength is higher than the center of the plate thickness, so the amount of spring back after press molding is larger on the surface, the residual tensile stress remains on the surface, and the surface is smoothed. This is probably because of this. More desirably, if this ratio is 80% or less, the effect is even greater.
[0025]
By adjusting to the above-mentioned component composition range and metal structure, it is possible to obtain a steel sheet having both of the two properties of rough skin resistance and press formability and having excellent chemical conversion properties.
[0026]
Such a steel plate can be manufactured by the following manufacturing method.
(3) Steel plate manufacturing process After the steel adjusted to the above component composition range is melted in a converter, it is made into a slab by continuous casting and heated to a temperature of Ac ₃ or higher.
[0027]
At this time, if the heating temperature is less than Ac ₃ , rolling in the γ region cannot be performed and good press formability cannot be obtained. Further, by setting the heating temperature to 1150 ° C. or less, the difference between the plate surface crystal grain size and the crystal grain size at the center of the plate thickness, which is a feature of the present invention, is increased, and the rough skin resistance and the r value are further improved.
[0028]
Hot rough rolling is performed at a rolling rate of 30% or more when the slab center temperature is Ar ₃ or more and the surface temperature is Ar ₃ or more, and 30% or more when the slab center temperature is Ar ₃ or more and the surface temperature is less than Ar _3. The total rolling ratio is 70% or more.
[0029]
By rolling at Ar ₃ or more and 30% or more, strain is introduced into the γ phase and the α crystal grains are refined during the γ / α transformation. If the rolling rate is less than 30%, the introduction of strain is insufficient and the effect is small. In addition, the surface layer is rolled at 30% or more with less than Ar ₃ to introduce strain into the α phase, and the crystal grains of the γ phase in the surface layer are refined in the subsequent α / γ reverse transformation at the time of temperature increase. . If the rolling rate is less than 30%, the introduction of strain is insufficient and the effect is small. Further, when the slab center is less than Ar _3, there is no difference in crystal grains between the surface and the plate thickness center, and the r value and the rough skin resistance are not compatible as in the conventional material.
[0030]
The hot rolled material is heated to a temperature of Ac ₃ + 50 ° C. or higher at a heating rate of 5 ° C./s or higher.
This α / γ reverse transformation by heating results in a fine γ phase structure. If the heating rate is less than 5 ° C./s or the heating attainment temperature is less than Ac ₃ + 50 ° C., the number of transformation nuclei decreases, and the effect of miniaturization cannot be obtained.
[0031]
Subsequently, finish rolling is performed at a temperature of Ac ₃ or higher and a rolling rate of 51% or higher.
When the rolling rate is less than 51% or the finishing temperature is less than Ar ₃ , the structure becomes coarse and non-uniform, and the rough skin resistance deteriorates.
[0032]
Further, the hot-rolled steel sheet that has been finish-rolled is cold-rolled and recrystallized and annealed at a temperature of 650 ° C. or higher.
The recrystallization annealing may be either continuous annealing or batch annealing. If the temperature is lower than 650 ° C., the crystal grains are fine and the rough skin is not a problem, but a good texture does not develop, the r value is lowered, and the moldability is deteriorated.
[0033]
In addition, you may process less than 10% with a skin pass, a leveler, etc. as needed. The cold-rolled steel sheet produced according to the present invention is also optimal as a material for surface-treated steel sheets such as hot dip galvanizing and electrogalvanizing.
Examples of the present invention will be given below to prove the effects of the present invention.
[0034]
【Example】
Example 1
First, the results of investigating the relationship between the metal structure defined by the present invention and the r value, which is an index of rough skin properties, chemical conversion properties, and deep drawability, are shown. In the 50 kg vacuum melting furnace, the sample No. A steel ingot having one component is melt cast, and various hot rolling, cold rolling, and annealing processes are combined to produce a 0.7 mm cold rolled steel sheet having various crystal grain sizes and grain size distributions within the plate thickness. did. The results of measuring the average crystal grain size of the surface and the center portion of the plate thickness using these as test materials, and the results of evaluating the rough skin resistance, the chemical conversion treatment properties and the tensile properties are shown in Table 2 (examples of the present invention: symbols a to g, Comparative example: Shown together with symbols h to s).
[0035]
Here, a value obtained by dividing the average crystal grain size (Ds) of ferrite within a plate thickness of 10% from the plate surface by the average crystal grain size (Dc) of ferrite at the center of the plate thickness is defined as a grain size ratio (Ds / Dc). To do.
[0036]
The evaluation of rough skin resistance was made by punching out a 110φ disk-shaped sample, squeezed with a 60φ circular head punch, and measuring the surface roughness of the wall according to JIS B 0601, with an Ra of 1 μm or less. A thing exceeding 1 μm was judged to be unacceptable because of rough skin.
[0037]
The chemical conversion treatment property was evaluated by measuring the adhesion amount and the P ratio after immersing the test piece in a commercially available zinc phosphate treatment solution, washing it with water, and drying it. Here, the P ratio is defined by the composition of iron phosphate crystals (Phosophophyllite) out of zinc phosphate crystals (Hopeite) and iron phosphate crystals (Phosophophyllite) attached by the zinc phosphate treatment, and the measurement was performed by the X-ray diffraction method. In general, a chemical conversion film having a high P ratio has better characteristics.
[0038]
The tensile test used the JIS5 test piece described in JIS Z 2201, and the test was implemented based on the method described in 2241. Similarly, the r value is measured by adding a pre-strain of 15% to the JIS No. 5 test piece in the rolling direction, the r value in each of the 45 ° direction and 90 ° direction, r ₀ , r ₄₅ , r ₉₀ with respect to the rolling direction. After that, the average r value was set to r = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4.
[0039]
FIG. 1 illustrates the relationship between the surface roughness resistance, chemical conversion property, and the crystal grain size (Dc) and grain size ratio (Ds / Dc) at the center of the plate thickness on the r value shown in Table 2. The abscissa indicates the grain size ratio (Ds / Dc), and the marks are distinguished from the crystal grain size (Dc) of the ferrite at the center of the plate thickness from less than 10 μm, 10 to 30 μm, and more than 30 μm. The average crystal grain size of ferrite within a plate thickness of 10% from the plate surface specified by the present invention is 90% or less of the average grain size of ferrite at the plate thickness center portion, and the average crystal particle size of ferrite at the plate thickness center portion is The test materials having a microstructure of 10 to 30 μm have good skin roughness resistance, and the chemical conversion treatment property has an adhesion amount of 2 g / m ² or more, a P ratio of 85% or more, and a high r value of 2 or more. It can be seen that these characteristics are favorable. On the other hand, when the crystal grain size is less than 10 μm at the center of the plate thickness, the rough skin resistance is good, and when the grain size ratio is 0.9 or less, the chemical conversion treatment property is also good. However, the r value is low regardless of the particle size ratio, and the deep drawability is poor. When the crystal grain size exceeds 30 μm at the center of the plate thickness, the r value is good, but both the rough skin resistance and the chemical conversion property are poor regardless of the grain size ratio.
[0040]
(Example 2)
After melting and casting a steel ingot having the chemical composition shown in Table 1 in a 50 kg vacuum melting furnace, hot rolling was performed under the conditions shown in Table 3, followed by cold rolling with a rolling rate of 76%, and the conditions shown in Table 3 Annealing was performed. Here, symbols A to I are manufacturing conditions of the examples of the present invention, and symbols J to S are manufacturing conditions of the comparative example. Table 4 shows the results of investigating the microstructure and tensile properties of the cold-rolled steel sheet having a thickness of 0.7 mm.
[0041]
The samples (symbols A to I) prepared under the chemical components and production conditions of the present invention example all have a ferrite average crystal grain size within 10% of the plate thickness from the plate surface, which is 90% of the ferrite average crystal particle size at the plate thickness center portion. %, The ferrite average crystal grain size at the center of the plate thickness is 10 to 30 μm. Such a material has an r value of 1.8 or more and an elongation of 48% or more, and has no surface roughness after molding. In addition, the chemical conversion treatment property is as good as 2 g / m ² or more of the zinc phosphate adhesion amount and 85% or more of the P ratio. Further, it can be seen that the tests C to I in which the heating temperature of the rough rolling is 1150 ° C. or less are further excellent in r value, elongation, surface properties, and chemical conversion treatment.
[0042]
On the other hand, tests J, K, and L show that the chemical components are outside the scope of the present invention, so that the rough skin resistance and the chemical conversion treatment are poor or the r value and the elongation are insufficient, and the moldability is insufficient. In Test M, since rough rolling was performed at Ar ₃ or more, the surface structure was not refined, and it was found that the rough skin resistance and the chemical conversion treatment were not good. In Test N, the heating temperature is low, and the rolling ratio of the surface in the γ region is not sufficient, so the surface structure is not refined, and the rough skin resistance and the chemical conversion property are not good. In Test O, since the heating temperature for rough rolling is low and rolling in the γ region cannot be performed, a good texture is not generated and the r value is low. Further, since the surface grains are not refined, the rough skin resistance and the chemical conversion treatment are not excellent. In Test P, the heating rate of reheating after rough rolling is low, so abnormal grain growth occurs, and it is understood that the rough skin resistance and the chemical conversion treatment property are not good. Test Q shows that since the rolling ratio of finish rolling is low, the particle size is coarse, and the rough skin resistance and chemical conversion treatment properties are not good. Test R shows that the finishing temperature of finish rolling is Ar ₃ or less, and therefore abnormal grain growth occurs on the surface, and the resistance to rough skin and chemical conversion treatment are poor. In Test S, the recrystallization annealing temperature is less than 650 ° C., so the crystal grain size becomes fine. Therefore, it can be seen that the rough skin resistance is good, but the r value is low and the moldability is poor.
[0043]
[Table 1]

[0044]
[Table 2]

[0045]
[Table 3]

[0046]
[Table 4]

[0047]
【The invention's effect】
The present invention can provide a cold-rolled steel sheet that is suitable for press forming such as deep drawing, which is compatible with good chemical conversion property and skin roughness resistance, by specifying the steel composition, metal structure and production conditions.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship among a particle size ratio (Ds / Dc), rough skin resistance, chemical conversion property, and r value according to an example of the present invention.

Claims (2)

In mass%, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, P: 0.1% or less, S: 0.003-0.02%, Sol. A steel slab containing Al: 0.02-0.1%, N: 0.01% or less, Ti: 0.03-0.08%, B: 0.0001-0.0015% is Ac. _Three Heating to the above temperature;
  For heated steel slabs, the slab center temperature is Ar _Three Above and the surface temperature is Ar _Three At the above time, the rolling rate is 30% or more and the slab center temperature is Ar. _Three Above and the surface temperature is Ar _Three A process of performing hot rough rolling with a rolling rate of 30% or more and a sum of both rolling rates of 70% or more when less than,
  Roughly rolled steel slab is heated at a rate of 5 ° C / s or more, and the entire steel is Ac _Three After heating to + 50 ° C or higher, Ar _Three A step of performing finish rolling at a rolling rate of 51% or more at the above temperature;
  Cold-rolled hot-rolled steel sheet that has been finish-rolled, recrystallized and annealed at a temperature of 650 ° C. or higher, and the average crystal grain size of ferrite within 10% of the thickness from the surface is excluded except within 10% of the thickness from the surface A step of obtaining a steel sheet having 90% or less of the average crystal grain size of ferrite in the center portion of the plate thickness and having an average crystal grain size of ferrite in the center portion of the plate thickness of 10 to 30 μm;
A method for producing a cold-rolled steel sheet excellent in press formability and skin roughness resistance. In mass%, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, P: 0.1% or less, S: 0.003-0.02%, Sol. A steel slab containing Al: 0.02-0.1%, N: 0.01% or less, Ti: 0.03-0.08%, B: 0.0001-0.0015% is Ac. _Three The step of heating to a temperature of 1150 ° C. or lower,
  For heated steel slabs, the slab center temperature is Ar _Three Above and the surface temperature is Ar _Three At the above time, the rolling rate is 30% or more and the slab center temperature is Ar. _Three Above and the surface temperature is Ar _Three A process of performing hot rough rolling with a rolling rate of 30% or more and a sum of both rolling rates of 70% or more when less than,
  Roughly rolled steel slab is heated at a rate of 5 ° C / s or more, and the entire steel is Ac _Three After heating to + 50 ° C or higher, Ar _Three A step of performing finish rolling at a rolling rate of 51% or more at the above temperature;
  Cold-rolled hot-rolled steel sheet that has been finish-rolled, recrystallized and annealed at a temperature of 650 ° C. or higher, and the average crystal grain size of ferrite within 10% of the thickness from the surface is excluded except within 10% of the thickness from the surface A step of obtaining a steel sheet having 90% or less of the average crystal grain size of ferrite in the center portion of the plate thickness and having an average crystal grain size of ferrite in the center portion of the plate thickness of 10 to 30 μm;
A method for producing a cold-rolled steel sheet excellent in press formability and skin roughness resistance.

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