JP2009041067A - Cold-rolled steel sheet, hot-dip galvanized steel sheet, and production method thereof - Google Patents

Abstract

An object of the present invention is to provide a cold-rolled steel sheet having an r value of 1.6 or more, an average r value of 1.4 or more, a tensile strength of 390 MPa or more and high production stability in an angle with respect to the rolling direction of 45 °.
SOLUTION: In mass%, C: 0.0005 to 0.01%, Si: 0.7% or less, Mn: 1.0 to 3.0%, P: 0.15% or less, S: 0.02% or less, N: 0.005% or less, sol. A steel plate containing Al: 0.10 to 1.0%, Ti: 0.005 to 0.070% and Nb: 0.02 to 0.20%, with the balance being Fe and impurities, and a decrease in Ac ₃ point due to Mn Suppressed by Al.
[Selection] Figure 1

Description

  The present invention relates to a cold-rolled steel sheet and a hot-dip galvanized steel sheet having a tensile strength of 390 MPa or more, and methods for producing them. In particular, the present invention is suitable as a material for an automobile outer panel such as a side panel, and an r value in a 45 ° direction with respect to a rolling direction is 1.6 or more, and an average r value is 1.4 or more. The present invention relates to cold-rolled steel sheets, hot-dip galvanized steel sheets, and methods for producing them.

1. r value In response to the need for improved automobile crash safety and lighter weight, not only body frame members but also thin steel sheets applied to automobile outer panel such as side panels, hoods, doors, fenders, etc. 390 MPa or more). These thin steel sheets are required to have not only surface appearance quality but also excellent press formability, particularly drawability.

  This drawability has a very good correlation with the Rankford value (r value), which is the plastic strain ratio described in JIS Z 2254. It is known that the higher this r value, the better the drawability. Yes. For this reason, the r value is widely used as an index of drawability and used as an index of material design.

2. Relationship between automotive outer panel and r value Blank material used for automotive outer panel such as side panels, hoods, doors, fenders, etc. is the largest class of automotive parts manufactured by pressing from steel plate. Therefore, it is not from the slit coil used in the case of small parts, but from the wide coil (refers to a steel plate having a width of 1000 mm or more, for example, 1600 mm width, 1800 mm width). Trimmed to a rectangle with a little blank cut, and used for press.

For this reason, for example, when molding a side panel, the molding direction of a part that is difficult to be molded, such as the corners at the four corners of the opening of the front door and the rear door, is positioned at 45 ° with respect to the rolling direction. It will be. In this case, when the r value (r _{45 °} value) in the 45 ° direction is low with respect to the rolling direction of the steel plate as the material, wrinkles and cracks are likely to occur. It is therefore important to increase the r _{45 °} value.

3. Conventional r-value improvement method As a method for obtaining a high r-value to solve these problems, a carbonitride-forming element such as Ti or Nb is added on the basis of an extreme carbon steel having a C content of about 30 ppm or less. It is effective and is generally widely used mainly as mild steel as IF steel. And as a steel plate having a high r value and a high strength, this IF steel is made to contain solid solution strengthening elements such as Si, Mn, and P.

  For example, Patent Document 1 has a chemical composition containing Si: 0.1 to 1.5 mass%, Mn: 0.5 to 3.0 mass%, and P: 0.02 to 0.2 mass%. A hot dip galvanized steel sheet of about 400 MPa or more is described.

  Patent Document 2 discloses a chemical composition containing Si: ≦ 0.7 mass%, Mn: ≦ 1.0 to 2.5 mass%, P: 0.050 to 0.15 mass%, and A 440 MPa cold-rolled steel sheet is described.

Patent Document 3 includes 390 MPa to 500 MPa having a chemical composition containing Si: 0.05% by mass or less, Mn: 0.7 to 3.0% by mass, and P: 0.04 to 0.15% by mass. Less than thin steel sheets are described. In addition, in the thin steel plate described in this literature, when Si exceeds 0.05 mass%, it describes that the chemical conversion property and the adhesiveness of hot dip galvanizing will deteriorate.
JP 2002-155317 A JP 2005-015882 A JP 2001-131691 A

However, according to the investigation by the present inventors, when a large part such as a side panel is press-molded using a wide coil of IF steel containing a solid solution strengthening element such as Si, Mn, or P as described above. In some cases, defective molding may occur while the processing conditions of press molding are constant. The forming defects occur at a relatively high frequency at the corners of the four corners described above, and further investigations have revealed that among the mechanical properties of the steel sheet, the r value as described above, particularly the r _{45 °} value, It has been clarified that molding defects occur due to fluctuations in the width direction, longitudinal direction, and coil unit of the wide coil.

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet or hot-dip galvanized steel sheet that has excellent press formability and is less likely to cause variations in the press formability. In particular, as an excellent press formability, the r value in the 45 ° direction with respect to the rolling direction, which is required for automotive outer panel such as side panels, hoods, doors, and fenders, is extremely high, and the high r value is also high. An object of the present invention is to provide a stably provided cold-rolled steel sheet and hot-dip galvanized steel sheet having a tensile strength of 390 MPa or more and methods for producing them.

The present inventors examined in detail about the influence which the element contained for solid solution strengthening in IF steel has on the characteristic of another steel plate.
First, when Si and P are contained excessively, the surface properties of the steel sheet are deteriorated or the secondary workability is lowered. Moreover, when plating on the surface of a steel plate from a viewpoint of ensuring corrosion resistance, plateability is inhibited. Specifically, a non-plating phenomenon in which plating is not applied, or a decrease in peeling resistance (a decrease in powdering performance) due to a delay in the alloying treatment rate occurs. For this reason, the upper limit of the content of Si and P in IF steel is limited.

  If Cr, Mo, and B are contained as elements other than these elements, the strength can be easily increased. However, if these elements are contained in a large amount, the r value tends to decrease. Therefore, it can be said that Mn is the most effective element for increasing the strength while maintaining a high r value while securing the plating property. For example, the steel sheet described in Patent Document 3 contains almost no Si, but instead contains a large amount of about 2% by mass of Mn.

However, such a steel sheet can achieve both strength and plating properties. However, as described above, in an actual operating environment, the r value, particularly the r _{45 °} value, tends to vary, and the press It is difficult to stably develop the moldability.

As a result of pursuing the cause of the variation of the r value, Si whose content is limited is a ferrite forming element, and Mn contained in a large amount is an austenite forming element. in, Ac ₃ transformation point (hereinafter referred to as "Ac ₃ point".) that is severely degraded been found.

In the recrystallization annealing process in actual operation, there is a variation in the temperature during soaking due to the arrangement of the heating means, etc., and the temperature variation is controlled within a predetermined range by adjusting the output of the heating means. Yes. However, when the Ac ₃ point is lowered as in the case of the steel sheet, the temperature difference between the recrystallization temperature and the Ac ₃ point is reduced, and the temperature range (Ac ₃ point to recrystallization temperature) allowed as the temperature during soaking is reduced. This is a temperature range, and is hereinafter referred to as “allowable temperature range”). In such a case, it is inherently necessary to narrow the control temperature range corresponding to the change in the allowable temperature range, that is, to perform more precise temperature control.

However, conventionally, since the high steel plate Mn content are not known to the allowable temperature range is narrow, such changes in the temperature control range, even if the steel sheet is not particularly performed, in some cases, Ac ₃ It is thought that the allowable temperature range becomes narrower than the controllable temperature range because the point drop is remarkable, and part or all of the steel sheet has been heated beyond the Ac ₃ point during operation. . At this time, a suitable crystal structure formed in the process up to the cold rolling disappears, and as a result, a desired r value cannot be obtained. Moreover, since the recrystallization temperature is also lowered, there is a possibility that the material is deteriorated without being recrystallized during annealing.

  The occurrence of such a phenomenon is not only a steel sheet having the chemical composition described in Patent Document 3, but also a steel sheet in which the Si content is limited to some extent in order to ensure plating properties (for example, a steel sheet described in Patent Document 2). ) Is also a concern.

Therefore, the effect of the contents of Mn and other elements on Ac ₃ points and mechanical properties was investigated in order to increase the strength with Mn while ensuring a high r value. As a result, in the component system containing a large amount of Mn, a new finding was obtained that Al had an effect of increasing the Ac ₃ point without deteriorating mechanical properties.

In addition, the upper limit of the hot rolling completion temperature has a great influence on the r value in the 45 ° direction with respect to the rolling direction, specifically, the completion temperature is set to Ar ₃ points + 50 ° C. or less. The new knowledge that recrystallization behavior preferable for improving the r _{45 °} value is likely to occur at the time of annealing in the steel plate obtained.

Based on the above findings, the inventors have completed the following invention.
(1) By mass%, C: 0.0005 to 0.01%, Si: 0.7% or less, Mn: 1.0 to 3.0%, P: 0.15% or less, S: 0.02 % Or less, N: 0.005% or less, sol. Al: 0.10 to 1.0%, Ti: 0.005 to 0.070% and Nb: 0.02 to 0.20%, the balance having a chemical composition consisting of Fe and impurities, rolling A cold-rolled steel sheet having an r value of 1.6 or more, an average r value of 1.4 or more, and a tensile strength of 390 MPa or more with respect to a direction at an angle of 45 °.

  Here, while satisfying the conditions of the above chemical composition, Mn / Si ≦ 40 (Mn / Si: (mass% of Mn in steel) / (mass% of Si in steel), and so on) is satisfied. In this case, Al acts effectively and the stability of the r value is increased.

  In addition, as long as the above-mentioned “cold-rolled steel sheet” has the above composition and mechanical properties, at least a part of the surface thereof is subjected to surface treatment directly or indirectly. Including. Examples of the surface treatment include electrogalvanization, tin plating, Ni flash plating, phosphoric acid chemical conversion treatment, and coating.

  (2) The cold-rolled steel sheet according to (1), wherein the chemical composition contains B: 0.0020% or less in mass% instead of part of Fe.

  (3) The chemical composition is mass% in place of a part of Fe, Cr: 1% or less, Mo: 1% or less, V: 1% or less, W: 1% or less, Cu: 1% or less and Ni: 1 type or 2 types or more chosen from the group which consists of 1% or less are contained, The cold-rolled steel plate as described in said (1) or (2) characterized by the above-mentioned.

  (4) Any of the above (1) to (3), wherein Si and P of the chemical composition have a chemical composition in which, by mass, Si: 0.5% or less and P: 0.1% or less, respectively A hot-dip galvanized steel sheet comprising a hot-dip galvanized layer on the surface of the cold-rolled steel sheet described in 1.

  The chemical composition related to the above hot-dip galvanized steel sheet is mass%, C: 0.0005 to 0.01%, Si: 0.5% or less, Mn: 1.0 to 3.0%, P: 0.00. 1% or less, S: 0.02% or less, N: 0.005% or less, sol. Al: 0.10 to 1.0%, Ti: 0.005 to 0.05% and Nb: 0.02 to 0.20% are contained as essential components, the balance being Fe and impurities, Optional components, ie, by mass%, B: 0.0020% or less, and Cr: 1% or less, Mo: 1% or less, V: 1% or less, W: 1% or less, Cu: 1% or less, and Ni: One or two or more selected from the group consisting of 1% or less may be included instead of part of Fe.

  In addition, when the above chemical composition conditions are satisfied and 8 ≦ Mn / Si ≦ 40 is satisfied, Al effectively acts, and the stability of the r value is increased and the plating property is decreased. It is possible to avoid fear.

  In addition, said "hot dip galvanized steel plate" is what the hot dip galvanization was given to at least one part of the surface of the cold rolled steel plate which concerns on this invention. The hot dip galvanizing may contain not only pure zinc but also other elements such as aluminum. Furthermore, the alloying process may be performed after plating.

(5) The steel ingot or steel slab having the chemical composition described in any one of (1) to (3) above is subjected to hot rolling at 1100 to 1270 ° C., and Ar ₃ points to Ar ₃ points + 50 ° C. Completing the hot rolling, cooling the steel sheet after the hot rolling and winding it at 400 to 700 ° C., pickling the steel sheet after the winding, performing cold rolling with a reduction rate of 50% or more, A method for producing a cold-rolled steel sheet, comprising subjecting a steel sheet after cold rolling to recrystallization annealing.

  (6) Hot-dip galvanizing is performed on a cold-rolled steel sheet obtained by performing the production method described in (5) above on a steel ingot or steel piece having the chemical composition described in (4) above. A method for producing a hot-dip galvanized steel sheet.

  In addition, after performing said hot-dip galvanization process, it is good also as an alloying hot-dip galvanized steel plate by performing an alloying process succeedingly. Therefore, the term “hot-dip galvanized steel sheet” here includes “alloyed hot-dip galvanized steel sheet”. The alloying treatment temperature is preferably 470 to 550 ° C.

  The cold-rolled steel sheet according to the present invention is excellent in surface appearance and has an r value excellent in press formability, particularly an r value in a 45 ° direction with respect to the rolling direction. In addition, even when manufactured under normal operating conditions, it is difficult for the r value to vary, so that excellent press formability can be stably realized. For this reason, if it is used for automotive outer panel applications such as side panels, doors, and fenders, molding defects are less likely to occur, and it is possible to manufacture desired panels with high productivity. In particular, if the above manufacturing method is employed, it is possible to obtain the cold-rolled steel sheet according to the present invention particularly efficiently and stably.

  Moreover, this steel plate can be applied not only as a cold-rolled steel plate for processing but also as an original plate of a surface-treated steel plate for processing. Examples of the surface treatment include zinc plating (including alloy systems) and tin plating. In addition, the steel sheet of the present invention may be subjected to a special treatment to improve chemical conversion properties, weldability, press formability, corrosion resistance, and the like.

  As described above, when the steel sheet according to the present invention is used as a cold-rolled steel sheet for an automobile outer panel, a hot-dip galvanized steel sheet for an automobile outer panel, or an alloyed hot-dip galvanized steel sheet for an automobile outer panel, its characteristics are It is possible to make the most of it.

  The best mode of the present invention, the range of manufacturing conditions, and the reasons for setting them will be described below. In the present specification, “%” representing the chemical composition is “% by mass” unless otherwise specified.

1. Regarding the Influence of Mn and Al on Mechanical Properties First, the results of research on the influence of Mn and Al as the basis of the present invention on the Ac ₃ point and consequently on the mechanical properties are shown.

(1) Relationship between allowable temperature range and r value in annealing process C: 0.0040%, Si: 0.04%, P: 0.02%, S: 0.003%, Ti at the laboratory level : 0.015%, Nb: 0.06%, B: 0.0005% as basic component, Al content is 0.03%, Mn content is changed in the range of 1.0-3.0% A steel piece having the chemical composition was prepared, subjected to hot rolling and cold rolling, and further subjected to annealing at 860 ° C. to obtain a steel plate.

  As a result of evaluating the mechanical properties (tensile strength, average r value) of the obtained steel plate, as shown by the broken line in FIG. 1, when the Mn content is increased, the tensile strength increases as intended, but r It became clear that the value dropped rapidly.

Such a decrease in the r value is presumed that the annealing process is deeply involved as follows. The relationship between the allowable temperature range, which is the temperature range of Ac ₃ points to the recrystallization temperature, which is the allowable temperature range for the recrystallization annealing, and the Mn content was determined. The results are shown in Table 1.

As shown in Table 1, when the Mn content is increased, the Ac ₃ point is drastically lowered, so that it is difficult to maintain the allowable temperature range of 130 ° C., in particular, the Mn content is 3.4%. In this case, the temperature is 100 ° C. or lower.

Thus, when the allowable temperature range is reduced, the following problems occur.
Even when the Mn content was increased, applying the same annealing temperature as before increasing the Mn content, as in the case where the Mn content shown in Table 1 and FIG. 1 is 3.4%, Obviously, there are cases where the Ac point exceeds ₃ points, and the decrease in the r value becomes significant. Further, even when the annealing temperature does not exceed Ac ₃ point to reality, as if the Mn content shown in Table 1 and Figure 1 are 1.7% and 2.3%, close to the Ac ₃ point At the annealing temperature, annealing is performed in a two-phase state where ferrite and austenite coexist, and since Mn content is high and hardenability is high, C cannot be sufficiently diffused in the subsequent cooling process, and the generated ferrite is not baited. In some cases, the characteristics of the r value may deteriorate.

In order to avoid such a situation, it is necessary to lower the annealing temperature. However, in order to ensure excellent mechanical properties, it is necessary to promote recrystallization as much as possible. Therefore, the annealing temperature is generally set as high as possible within the allowable temperature range. ing. The temperature control of the steel plate in actual operation is performed by adjusting the furnace temperature. However, in the furnace temperature adjustment, the temperature of the steel plate is likely to vary due to thermal inertia and temperature unevenness. Therefore, when the allowable temperature range is narrowed, there is a high possibility that the actual annealing temperature will exceed the Ac ₃ point or near the Ac ₃ point for part or all of the steel sheet in actual operation. In the portion where such a phenomenon occurs, the recrystallization texture preferable for drawability is destroyed or the ferrite becomes bainitic with the transformation to the γ phase, and as a result, the r value of the steel sheet is greatly reduced. End up.

(2) Relationship between allowable temperature range and chemical composition In order to examine means for suppressing such reduction of the allowable temperature range, 0.5% Al is contained in addition to the chemical composition of the basic component, and Mn is contained. Steel slabs whose amount was changed in the range of 1.0 to 3.0% were prepared, and these were also subjected to the same treatment to obtain steel plates. Table 2 is a table showing the relationship between the allowable temperature range and the Mn content when the Al content is 0.5%.

  As shown in Table 2, when the Al content is 0.5%, even if the Mn content is increased, the allowable temperature range does not vary greatly, and it is confirmed that the allowable temperature range of about 130 ° C can be secured. It was done.

  Moreover, when the mechanical characteristic was evaluated about the steel plate obtained through said annealing process, it became clear that the fall of r value was remarkably improved by containing Al (refer the continuous line of FIG. 1).

Regarding the improvement of the r value, it is presumed that the decrease in Ac ₃ point due to the increase in the Mn content was suppressed by containing Al, which is the ferrite forming element, having the same effect as Si. . For this reason, the allowable temperature range is expanded, and the possibility that the steel sheet includes a portion transformed to the γ phase or the ferrite becomes bainitic is low, and a steel sheet that achieves the r value and strength at a high level is obtained. It is thought that it was done.

Since the results in the above table are experimental evaluations, the same results as described above are not always obtained in actual operation. However, the tendency obtained by this experimental evaluation that the allowable temperature range is rapidly narrowed by increasing the Mn content and the decrease in the allowable temperature range by containing Al is suppressed by Ac. _Since it is derived from the fluctuation of _three points, it can also be seen in actual operation.

  Therefore, the new knowledge of increasing the Mn content to increase the tensile strength and increasing the Al content to suppress the deterioration of the r-value characteristics, derived from the above results, is also true for the steel sheets in actual operation. It is fully established and its usefulness is clear.

2. Next, the chemical composition of the steel according to the present embodiment will be described.

C: 0.0005 to 0.01%
C combines with carbide-forming elements such as Nb and Ti to form fine carbonitrides such as TiC, NbC or a composite thereof (Nb, Ti) (C, N). By optimizing the C content, these carbonitrides precipitate at an appropriate volume ratio. At this time, the produced carbonitride provides a great effect of precipitation strengthening, and it is possible to increase the strength without containing a large amount of solid solution elements such as Mn, P, and Si. Moreover, since the solid solution C and N at the time of recrystallization annealing can be reduced by generating carbonitride, there is also an effect of improving the r value of the product.

  If the C content is less than 0.0005%, the secondary work brittleness resistance may be deteriorated, and sufficient tensile strength may not be obtained. Furthermore, since it is necessary to highly decarburize from molten steel, it is not preferable from the viewpoint of economy. On the other hand, if the C content exceeds 0.01%, YS increases and elongation decreases, and the moldability, particularly the r value, decreases. Therefore, the C content is set to 0.0005 to 0.01%. From the viewpoint of further moldability, particularly securing the r value, the upper limit is preferably less than 0.006%.

Si: 0.7% or less Si is an element that increases the strength of a steel sheet by solid solution strengthening at low cost. For this reason, it can be contained for the purpose of improving the strength. However, if the content is large, appearance defects after pickling tend to occur. Therefore, when hot dip galvanization is not performed, the upper limit is set to 0.7%. On the other hand, when hot dip galvanizing is applied to the surface of the steel sheet, the Si content is set to 0.5% or less in order to avoid a decrease in plating quality due to Si. When hot dip galvanization is applied and solid solution strengthening with Si is not required, the preferred upper limit is 0.2% or less. In addition, when applying a hot dip galvanized steel sheet, since Si has the effect of improving peeling resistance which increases the interfacial adhesion strength between the plating and the steel sheet by addition of a trace amount, the lower limit is preferably 0.02% or more.

Mn: 1.0-3.0%
Mn is an important element having an action of increasing the strength of the steel sheet by solid solution strengthening without significantly impairing the plateability. If the Mn content is less than 1.0%, the intended increase in strength may not be achieved. On the other hand, if the Mn content is more than 3.0%, the increase in YS and the deterioration in elongation become remarkable, and wrinkles and cracks are likely to occur during processing. For this reason, Mn content shall be 1.0-3.0%. Thus, even if Mn is present at a high content of 3.0%, since the steel according to the present embodiment contains Al, the deterioration of the r-value characteristics accompanying a decrease in Ac ₃ points. Will not occur. However, in order to further improve the moldability or the plating property, it is preferable that the Mn content is 2.2% or less. This is because Mn has a considerable influence on the plating quality.

In addition, it is especially preferable that Mn / Si ≦ 40 is satisfied as the relationship between Mn and Si. When the ratio of mass% indicated by “Mn / Si” (hereinafter referred to as “Mn / Si ratio”) exceeds 40, the decrease in Ac ₃ points by Mn is excessive, and Al is contained. May not be reflected as an improvement in r value. The lower limit of the Mn / Si ratio is not particularly limited. However, if it is less than 1, the content of Si is relatively high, so a decrease in the Ac ₃ point is difficult to be recognized as a problem. Ac ₃ point increase effect is unlikely to appear. In addition, when hot dip galvanization is performed, it is particularly preferable that 8 ≦ Mn / Si ratio ≦ 40. This is because, when the Mn / Si ratio is less than 8, the Si concentration is relatively high, and thus there is a concern that the deterioration of the plating property may become obvious.

P: 0.15% or less P is a useful element for increasing the strength of a steel sheet by solid solution strengthening while suppressing a decrease in r value, and can be contained for the purpose of improving the strength. However, when the P content exceeds 0.1%, in the case of an alloyed hot dip galvanized steel sheet, the alloying processability is lowered to lower the plating adhesion, or the stripe surface caused by P segregation on the plated surface. May be present. For this reason, when hot-dip galvanizing is performed on the steel sheet surface, the P content is set to 0.1% or less. More preferably, it is 0.06% or less. The lower limit is preferably 0.03% or more because the intended increase in strength may not be achieved. In addition, when not carrying out hot dip galvanization, the upper limit of P content shall be 0.15%. If it exceeds 0.15%, there is a concern about deterioration of secondary work embrittlement resistance.

S: 0.02% or less S is present in the steel sheet as an impurity, but if its content is large, scale wrinkles are likely to occur and the surface appearance may be significantly degraded. Therefore, the content is made 0.02% or less. Preferably it is 0.01% or less.

N: 0.005% or less When N is contained excessively, YS rises and surface distortion tends to occur, or it causes solid solution in Fe and causes surface defects such as stretcher strain. For this reason, N content shall be 0.005% or less. Preferably it is 0.003% or less.

sol. Al: 0.10 to 1.0%
Usually, Al is contained for deoxidation. In the present invention, it is an important element that works to raise the Ac ₃ point simultaneously with deoxidation, and if it is contained in a trace amount only for general deoxidation or the like, a sufficient effect cannot be obtained. In case of containing Mn 1.0% or more, sol to raise the Ac ₃ point. Al content shall be 0.10% or more. However, sol. If the Al content exceeds 1.0%, a large amount of alumina inclusions, which are deoxidation products generated in the steelmaking process, may occur, and material defects may occur frequently. The Al content is 0.10 to 1.0%, preferably 0.15 to 1.0%, particularly preferably more than 0.2% and 1.0% or less.

Ti: 0.005-0.070%
By precipitating Ti as TiN, the rise of stretcher strain and YS due to solute N is suppressed, and surface distortion during processing is less likely to occur. Therefore, the Ti content is set to 0.005% or more. However, if Ti is contained in an amount exceeding 0.070%, the amount of TiC deposited increases and the elongation deteriorates, so that surface distortion and cracking are likely to occur during processing. In the case of a hot dip galvanized steel sheet, there may be a streak pattern caused by uneven alloying on the plated surface. For this reason, Ti content shall be 0.070% or less. Ti is a relatively expensive element and is preferably 0.02% or less from the viewpoint of suppressing the production cost by suppressing the content and suppressing the stripe pattern on the plating surface.

Nb: 0.02 to 0.20%
Nb combines with C in the same way as Ti to produce NbC precipitates and improves mechanical properties. Appropriate inclusion of Nb is important in the present invention, and is an element for increasing the r _{45 °} aimed by the present invention. If the Nb content is less than 0.02%, the increase in r _{45 °} is insufficient. In addition, the amount of NbC deposited is insufficient and solid solution C cannot be fixed, and surface defects such as stretcher strain are likely to occur. Furthermore, it may be difficult to stably secure the tensile strength. For this reason, Nb content shall be 0.02% or more. Preferably, it is 0.05% or more. On the other hand, if the Nb content is more than 0.20%, Nb is excessive as compared with C, so that an increase in YS and a decrease in elongation become remarkable, and wrinkles are likely to occur during processing. For this reason, Nb content shall be 0.20% or less. Preferably it is 0.15% or less.

Furthermore, you may have the following elements as arbitrary components.
B: 0.0020% or less Since B has an effect of preventing secondary work embrittlement, it is preferably contained in place of part of Fe. However, when the B content exceeds 0.0020%, the r value decreases significantly, so the B content is set to 0.0020% or less. Preferably, it is 0.0010% or less. In order to more reliably obtain the effect of preventing secondary work embrittlement due to B, the B content is preferably 0.0001% or more, and more preferably 0.0003% or more.

One or more elements selected from the group consisting of Cr: 1% or less, Mo: 1% or less, V: 1% or less, W: 1% or less, Cu: 1% or less, and Ni: 1% or less These elements May be contained instead of a part of Fe for securing the strength. When the content of each element exceeds 1%, the effect of improving the strength is saturated. Therefore, from the economical viewpoint, the content of each element is set to 1% or less. Preferably, each element is 0.5% or less.

  Components other than the above are Fe and impurities.

3. Manufacturing Method The steel sheet according to the present embodiment has the above-described chemical composition characteristics, the r value in the 45 ° direction is 1.6 or more, the average r value is 1.4 or more, and the tensile strength is 1.4 or more. The manufacturing method is not particularly limited as long as it has mechanical properties of 390 MPa or more. However, if the following manufacturing method is adopted, it is possible to efficiently and stably obtain the steel plate according to the present embodiment.

(1) Hot rolling process In carrying out hot rolling, the conditions to be particularly noted in order to obtain the steel sheet according to the present embodiment efficiently and stably are as follows.

A) Hot rolling start temperature: 1100 to 1270 ° C
Hot rolling is performed after the steel ingot or steel slab having a predetermined steel composition is set to 1100 to 1270 ° C. Here, the steel ingot or steel slab may be reheated to a temperature of less than 1100 ° C. and subjected to hot rolling at 1100 to 1270 ° C., or 1100 after continuous casting when a continuous casting slab is used. You may use for hot rolling, after making it 1100-1270 degreeC, without reducing below 1 degreeC, and when using a steel piece, 1100-1270 without reducing the steel piece after partial rolling below 1100 degreeC. You may use for hot rolling after setting it as ° C.

  When the steel ingot or steel slab to be subjected to hot rolling is less than 1100 ° C, deformation resistance is high and hot rolling may be difficult, and when it exceeds 1270 ° C, an excessive scale is generated until after cold rolling. It may remain and deteriorate the surface properties. Therefore, it is preferable to set the temperature of the steel ingot or steel slab to be subjected to hot rolling to 1100 to 1270 ° C.

B) Hot rolling completion temperature: Ar ₃ points to Ar ₃ points + 50 ° C
When completed the temperature of hot rolling is less than Ar ₃ point, the or cracked during working r value is reduced in the product stage for the surface layer becomes coarse hot rolled tissue and ferritized, the galvanized steel sheet May have a streak pattern on the plating surface.

On the other hand, the upper limit of the hot rolling completion temperature is preferably Ar ₃ points + 50 ° C. from the viewpoint of increasing the r value, particularly the r _{45 °} value. Assuming that the hot rolling completion temperature is just above the Ar ₃ point, the austenite crystal grains that have been stretched and deformed during hot rolling are not sufficiently recrystallized, and the process proceeds to the cooling step. At this time, the ferrite transformation starts from the expanded austenite crystal grains, and the ferrite thus formed affects the recrystallization behavior during annealing. This annealing process improves the mechanical properties especially in the 45 ° direction, and as a result, the r _{45 °} value of the steel sheet is greatly increased. This effect is significant at Ar ₃ point + 50 ° C. or lower. Therefore, the hot rolling completion temperature is preferably Ar ₃ points to Ar ₃ points + 50 ° C. Particularly preferred is Ar ₃ point to Ar ₃ point + 40 ° C.

  In addition, since the preferable range of the hot rolling completion temperature is the narrow temperature range as described above, in order to perform the temperature control in this range, the sheet bar before the completion of rolling is heated by a heating device. Controllability may be improved. At this time, it is desirable to heat the steel strip so that the rear end of the steel strip is at a higher temperature than the tip, thereby reducing the temperature fluctuation over the entire length of the steel strip and improving the uniformity of the characteristics in the coil.

C) Winding temperature: 400-700 ° C
When the coiling temperature is less than 400 ° C., the carbonitride, particularly NbC, after winding may be insufficiently generated. In this case, the effect of NbC intended by the present invention cannot be fully enjoyed, and the r value is lowered, and cracking is likely to occur during processing. On the other hand, when the coiling temperature is higher than 700 ° C., scale may be generated excessively, which may deteriorate the surface properties or reduce the strength. Therefore, a preferable winding temperature is 400 to 700 ° C, and a particularly preferable temperature is 400 to 650 ° C.
In addition, what is necessary is just to implement the cooling to winding after completion | finish of hot rolling in accordance with a conventional method.

(2) Pickling process, cold rolling process, annealing process, surface treatment process The hot-rolled steel sheet obtained by hot rolling is descaled by pickling and subsequently recrystallized after cold rolling. Annealing is performed.

  Pickling may be performed according to a conventional method. In cold rolling, the rolling reduction is preferably 50% or more. In this case, a recrystallized texture that provides excellent drawability in the subsequent recrystallization annealing is easily developed. However, if it exceeds 85%, the amount of deformation increases and there is a concern about the occurrence of cracks. Therefore, the particularly preferable rolling reduction of cold rolling is 50 to 85%.

The recrystallization annealing is preferably performed at a temperature equal to or higher than the recrystallization temperature and less than Ac ₃ points. When the soaking temperature exceeds the Ac ₃ point, the recrystallization texture preferable for drawability is destroyed by transformation, and the r value tends to decrease.

Here, as described above, the allowable temperature range of the annealing process is defined as not less than the recrystallization temperature of the steel sheet and less than Ac ₃ points, but the steel sheet according to the present embodiment has an increased Al content. The allowable temperature range is wider than when the Al content of the steel sheet is less than 0.10% by mass. For this reason, it has been realized that a steel sheet having an excellent r value can be stably produced without particularly strictly controlling the temperature control during soaking in the annealing process. Therefore, although the allowable temperature range of the steel sheet according to the present invention varies depending on not only the chemical composition of the steel sheet but also the manufacturing process up to the annealing process, it is possible to ensure a range of approximately 130 ° C.

  Through the above steps, a cold-rolled steel sheet having excellent mechanical properties that satisfies the object of the present invention can be obtained. A surface-treated steel sheet having excellent corrosion resistance and appearance can be obtained by subjecting the cold-rolled steel sheet as a base material to appropriate surface treatment.

  For example, a hot-dip galvanized steel sheet can be obtained by subjecting the obtained cold-rolled steel sheet to hot dip galvanization as a surface treatment. At this time, the hot dip galvanizing method may be performed according to a conventional method, but it is preferable from the viewpoint of productivity to perform the annealing step and the hot dip galvanizing step continuously using a continuous hot dip galvanizing line. Further, the alloying treatment may be performed after hot dip galvanization. The alloying treatment may be performed according to a conventional method, and an example of the alloying treatment temperature is 470 to 550 ° C. Therefore, the “hot dip galvanized steel sheet” in the present invention includes an alloyed galvanized steel sheet.

  Moreover, as other surface treatment with respect to the obtained cold-rolled steel sheet, electrogalvanization, tin plating, coating, etc. are illustrated. These surface treatment methods may be performed in accordance with ordinary methods. Further, temper rolling may be performed before or after plating.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.
1. Test Method In order to confirm the effect of the present invention, continuous casting was performed using molten steel melted under various test conditions, and the results were evaluated with thin plate products.

  Sample Nos. Containing chemical components shown in Tables 3 and 4 1 to 35 steel plates were produced.

  Using a converter and an RH vacuum degassing apparatus, each was melted at a capacity of about 270 t per heat. The molten steel melted by the above method was supplied to a vertical bending type continuous casting machine and cast into a slab having a thickness of 250 mm and a width of 1500 mm.

After the slab heating, the plate thickness was 40 mm after rough rolling by hot rolling and the plate thickness was 3.2 mm after finish rolling, and then cooled and wound up. Tables 1 and 2 show the start and end temperatures and the winding temperature of finish rolling in this hot rolling. Furthermore rolled (rolling reduction 80%) cold to 0.65 mm, continuous annealing in a continuous hot-dip plating line (annealing temperature see Table 1 and 2.) Was subjected to, single-sided per 45 g / ^{m 2} Galvanized Then, alloying treatment was performed at 470 to 550 ° C., and after cooling, temper rolling with an elongation of 0.6% was performed.

The obtained test materials were examined for mechanical properties and surface properties.
Mechanical properties were obtained by taking a No. 5 test piece specified in JIS Z2201 from a thin steel plate after continuous annealing, and a test method in accordance with JIS Z2241 in three directions with angles to the rolling direction of 0 °, 45 °, and 90 °. YS, TS, EL, YPE, and r values (including average values) were measured.

  The surface properties were visually determined as to whether or not the appearance of the plating surface had a streak pattern, scale wrinkles, or plating defects that would repel the plating.

2. Test results Tables 1 and 2 show the results of investigating the steel components, production conditions, and mechanical properties.

Steel plate No. in the component range of the present invention. 1 to 18 are mechanical properties, in particular, the r value (r _{45 °} ) in the 45 ° direction with respect to the rolling direction is 1.6 or more, the average r value is 1.4 or more, and the surface appearance is also excellent. Suitable for automotive skin panels.

  No. No. 19 has a large amount of P, and in the appearance evaluation of the plating surface, the occurrence of streak patterns and poor alloying was recognized, but it exhibited excellent mechanical properties as a cold-rolled steel sheet, and was particularly defective in appearance before plating. Was not recognized. No. No. 20 had a large amount of Si, and although an unplated portion was observed in the appearance evaluation after plating, the mechanical properties required as a cold-rolled steel sheet were good, and the appearance was good before the plating.

  In contrast, no. 21 is sol. Since Al> 1.0%, there is no problem in mechanical properties, but the quality is poor due to inclusion defects. No. No. 22 of the present invention example. 3 with almost the same components as sol. As a result of changing the amount of Al, sol. Since AL <0.1%, the r value is decreased.

No. Nos. 23 to 30 had poor mechanical properties due to the occurrence of insufficient strength, lack of r value (r _{45 °} , average r value), and yield point elongation due to component failure.
No. In No. 30, not only mechanical properties but also a large amount of Ti was observed, and generation of streak patterns was observed. No. No. 31 had a large amount of S and generation of scale wrinkles was observed.

No. Although 32 to 35 are satisfied in terms of components, they have poor mechanical properties and insufficient r value in terms of operation. No. In No. 33, the finish hot rolling completion temperature was higher than Ar ₃ point + 50 ° C., and in particular, the r value in the 45 ° direction did not improve.

It is a test result which concerns on the knowledge based on this invention, and is a graph which shows the influence of the content rate of Mn and Al which exerts on tensile strength and r value.

Claims (6)

  In mass%, C: 0.0005 to 0.01%, Si: 0.7% or less, Mn: 1.0 to 3.0%, P: 0.15% or less, S: 0.02% or less, N: 0.005% or less, sol. Al: 0.10 to 1.0%, Ti: 0.005 to 0.070% and Nb: 0.02 to 0.20%, the balance having a chemical composition consisting of Fe and impurities, rolling A cold-rolled steel sheet having an r value of 1.6 or more, an average r value of 1.4 or more, and a tensile strength of 390 MPa or more with respect to a direction at an angle of 45 °.   The cold-rolled steel sheet according to claim 1, wherein the chemical composition contains B: 0.0020% or less in mass% instead of a part of Fe.   Instead of a part of Fe, the chemical composition is in mass%, Cr: 1% or less, Mo: 1% or less, V: 1% or less, W: 1% or less, Cu: 1% or less, and Ni: 1 The cold-rolled steel sheet according to claim 1 or 2, comprising one or more selected from the group consisting of% or less.   The cold-rolled steel sheet according to any one of claims 1 to 3, wherein Si and P having the chemical composition each have a chemical composition by mass% of Si: 0.5% or less and P: 0.1% or less. A hot dip galvanized steel sheet comprising a hot dip galvanized layer on the surface of the steel sheet. The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 3 is hot rolled at 1100 to 1270 ° C, and the hot rolling is completed at Ar ₃ points to Ar ₃ points + 50 ° C. The steel sheet after the hot rolling is cooled and wound at 400 to 700 ° C., the steel sheet after the winding is pickled, and then subjected to cold rolling with a reduction rate of 50% or more, and the steel sheet after the cold rolling A method for producing a cold-rolled steel sheet, characterized by subjecting to recrystallization annealing.   A hot-dip galvanizing is applied to a cold-rolled steel sheet obtained by performing the manufacturing method described in claim 5 on a steel ingot or steel slab having the chemical composition described in claim 4. Manufacturing method of galvanized steel sheet.

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