JP2007231371A - Cold-rolled steel sheet, galvanized steel sheet, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet which has high press-formability suitable for an automotive outer panel such as a hood, a door, a fender and a side panel, does not form a streaky defect thereon after having been plated, shows no poor appearance in the cold-rolled state, and has a tensile strength of a 340 MPa grade or higher, though there have not been such a high-strength cold-rolled steel sheet and a galvanized steel sheet as to satisfy all properties of strength, formability, brittleness resistance in secondary forming and surface properties (surface roughening resistance and uniform platability), in a high grade, and to provide a manufacturing method therefor. <P>SOLUTION: This steel sheet has a chemical composition comprising, by mass%, 0.0005% or more but less than 0.004% C, 0.5% or less Si, 0.1-2.5% Mn, 0.06% or less P, 0.05% or less S, 0.1% or less sol. Al, 0.006% or less N, (N/14×48×0.5)% to (0.02+N/14×48)% Ti, and 0.04-0.20% Nb, while controlling the contents of Ti and Nb so as to satisfy the relational expression of 2≤Nb/Ti≤20, and the balance Fe with unavoidable impurities; and has an (r) value of which the minimum value is 1.0 or more, and an (r) value of 1.5 or more in a direction of 45 degrees by angle with respect to a rolling direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cold-rolled steel sheet and a hot-dip galvanized steel sheet having a tensile strength of 340 MPa or more and a method for producing them. In particular, the present invention relates to a high-strength galvanized steel sheet having a strength of 340 MPa or higher, a cold-rolled steel sheet, and a method for producing the same, which are suitable for an automobile outer panel such as a side panel.

  In response to the need for high strength and light weight of automobiles, steel sheets applied to automobile outer plates and the like are required to have properties such as formability, secondary work brittleness resistance, and surface properties in combination with strength. For formability, it is effective that the r value is high and the yield strength (YS) is low. In recent years, there is a high demand for improving formability even for high-strength steel sheets. In order to obtain a high r value, it is effective to add carbonitride-forming elements such as Ti and Nb based on an extremely carbon steel having a C concentration of about 30 ppm or less, and it is generally widely used as IF steel. Yes.

  However, IF steel tends to be coarse in crystal grains. Therefore, when the annealing temperature is set to a high temperature in order to improve the r value, roughening may occur after pressing and the surface properties may deteriorate. In addition, it was difficult to improve the r-value after cold rolling and annealing because the study of the steel composition suitable for refinement of the hot-rolled steel sheet was insufficient.

  On the other hand, as a steel plate having a high r value and a high strength, a steel plate to which solid solution strengthening elements such as Mn and P are added based on IF steel has been developed. However, since the solid solution strengthening element is generally expensive and causes an increase in the cost of the steel sheet, Patent Document 1 discloses a technique for precipitation strengthening with NbC or TiC for the purpose of reducing the solid solution strengthening element.

However, since TiC is generated at a high temperature of Ar ₃ or higher, the size of the precipitate is large, the effect of improving the r value by refining the hot-rolled steel sheet structure, precipitation strengthening and surface properties by refining the cold-rolled steel sheet structure The effect of improvement is small.

  In addition, when Ti and Nb are added in combination, C may be precipitated as TiC at a high temperature during cooling, resulting in insufficient generation of NbC fine precipitates. Therefore, a technique for producing a steel sheet excellent in surface properties and press formability due to the effect of the unique fine precipitation of NbC when Nb is added alone has not been sufficiently established.

  Furthermore, when hot dip galvanizing is performed using steel containing Ti as a base material, streaky plating unevenness occurs on the surface of the steel sheet, which may cause problems when used for automobile outer plates. there were.

  In Patent Document 2, for the purpose of solving the above-mentioned problem, Nb is appropriately added to a steel sheet containing C: 0.0040 to 0.01%, thereby generating fine precipitates of NbC and fine grains of the structure. Steel sheets that improve the surface properties and mechanical properties are disclosed.

However, considering application to side panels, doors, hoods, roofs, and other automobile outer panel, wrinkles and surface distortion are likely to be manifested, the elongation is low, and the secondary work brittleness resistance is not sufficient.
Japanese Patent Laid-Open No. 10-46289 JP 2000-303145 A

  As described above, conventionally, there has been no high-quality cold-rolled steel sheet and galvanized steel sheet that satisfy all of the strength, formability, secondary work brittleness resistance and surface properties (skin roughness resistance, plating unevenness). Therefore, the present invention has a high press formability required for automobile outer panel such as hood, door, fender, side panel, etc., has excellent skin roughness resistance, and has a streak pattern defect or cold rolling in a plated steel sheet. An object of the present invention is to provide a TS340 MPa grade or higher high-strength cold-rolled steel sheet that exhibits excellent surface properties that do not cause appearance defects on the steel sheet, its galvanized steel sheet, and methods for producing them.

The present inventors first examined high press formability required for automobile outer panel such as side panels, hoods, doors, and fenders.
The blank material used for these applications is the largest class size of automotive parts manufactured by pressing from steel plates, so it is not the slit coil like small parts, but the coil width is the largest class. A wide coil, which is blank-cut into a substantially rectangular shape, is trimmed slightly and used for pressing.

  For example, if the side panel is a side panel, the corners of the four corners of the front door and the rear door opening that are likely to be wrinkled and cracked are 45 ° with respect to the rolling direction of the steel sheet, The hood, the door, and the fender also focused on the fact that wrinkles and cracks are likely to occur in the vicinity of the four corners, and their positions are located in the 45 ° direction with respect to the coil rolling direction.

  FIG. 1 is a plan view showing the direction of a side panel that is press-formed from a steel plate blank. Not only the illustrated side panel but also an automobile outer panel such as a door, a hood, and a roof are large and generally rectangular steel plate blanks. Therefore, the sheet is usually taken in the direction of 0 ° or 90 ° with respect to the steel plate coil.

As a result, the 45 ° direction of the steel sheet coincides with a crack or wrinkle danger position during pressing.
Then, due to the in-plane anisotropy of the mechanical properties of the steel sheet, if the mechanical properties in the 45 ° direction are significantly inferior to the 0 ° direction or 90 ° direction with respect to the rolling direction, presses such as cracks and wrinkles during press forming Improves the mechanical properties in the 45 ° direction and improves the formability at the corners of side panels, hoods, doors, fenders, etc., which are highly difficult to form and easily wrinkle and crack. Thus, the present inventors have conceived of obtaining a steel plate suitable for such a large press work without requiring excessively high formability of the entire steel plate.

  Therefore, the inventors have excellent mechanical properties in the 45 ° direction with respect to the rolling direction, and the lowest r value in the 0 °, 45 °, and 90 ° directions is 1.0 or more. It was conceived that a steel plate optimal for molding large press parts such as automobile side panels, hoods, doors, fenders, etc., having good press formability was obtained.

  In order to solve the above-mentioned problems, the present inventors have intensively studied the steel composition and production conditions, control the C content to 0.0005 to 0.0040%, and optimize the balance of Nb and Ti. As a result, the crystal grain size of the hot-rolled steel sheet is refined due to the fine precipitation effect of NbC, the r value is improved, and the rough surface resistance during press forming is improved. Further, precipitation strengthening of NbC and solid solution Nb Thus, it was found that high strength can be achieved without adding a large amount of Mn, P, and Si. A large amount of Mn, P, and Si deteriorates appearance quality, secondary work brittleness resistance, and plating quality.

Furthermore, it has been found that by properly controlling the balance between Nb and Ti, an anisotropic balance having excellent mechanical properties in the 45 ° direction, which is an object of the present invention, can be realized.
It was also confirmed that high workability can be secured without lowering workability by controlling C to 0.0005% or more and less than 0.0040% and strengthening the insufficient strength with Mn, Nb or the like.

  Applying steel sheets with excellent press formability, secondary work brittleness resistance, and steel sheet surface quality according to the present invention to automobile outer panels such as side panels, doors, fenders, etc., surface distortion, wrinkles, and cracks occur during press processing of these parts. The material characteristics in the direction of 45 ° with respect to the rolling direction of the main stress direction, which is likely to occur, are particularly excellent, making it possible to manufacture optimal high-strength cold-rolled steel sheet products with anisotropy in the right place for the industry. , Extremely beneficial.

  The steel sheet according to the present invention can be applied not only as a cold-rolled steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface treatment include galvanization (including alloy system) and tin plating. The steel sheet of the present invention may be subjected to special treatment after annealing or galvanization to improve chemical conversion properties, weldability, press formability, corrosion resistance, and the like.

In the present invention, the reason why the component composition range is defined as described above is as follows. In this specification, “%” indicating the steel composition is “% by mass” unless otherwise specified.
C: 0.0005% or more and less than 0.0040% C combines with Nb to form fine carbides of NbC. It is preferable to optimize the C content in order to precipitate fine NbC at an appropriate volume ratio and to improve the moldability. Finely precipitated NbC has the effect of reducing the crystal grain size of the hot rolled steel sheet and improving the r value after annealing of the cold rolled steel sheet. In addition, since NbC can be precipitated very finely, a great effect of precipitation strengthening can be obtained, and the strength can be increased without requiring the addition of a large amount of solid solution elements such as Mn, P, and Si.

  If the C content is less than 0.0005%, the secondary work brittleness resistance may deteriorate. On the other hand, when the C content is 0.0040% or more, NbC is excessively generated and the effect of suppressing the grain growth of the annealed sheet becomes excessive, YS rises and the elongation decreases, and surface distortion and cracking are likely to occur during processing. Become. Therefore, the C content is 0.0005% or more and less than 0.0040%. A preferable lower limit is 0.0007% or more, and a preferable upper limit is 0.0035% or less.

Si: 0.5% or less Si is an element contained as an impurity, but it is a useful element for increasing the strength of a steel sheet by solid solution strengthening at a low cost, so it can be contained for the purpose of improving the strength. . However, when the Si content exceeds 0.5%, scale wrinkles are likely to occur. Moreover, YS becomes high and elongation deteriorates, and surface distortion and cracking are likely to occur during processing. Therefore, the Si content is 0.5% or less. When more excellent appearance properties are required, the Si content is preferably set to 0.25% or less. On the other hand, since Si has the effect | action which improves plating adhesiveness, it is preferable to make it contain 0.002% or more.

Mn: 0.1 to 2.5%
Mn has the effect of increasing the strength of the steel sheet by solid solution strengthening. If the Mn content is less than 0.1%, the intended increase in strength may not be achieved. On the other hand, if the Mn content exceeds 2.5%, YS increases and elongation deteriorates, and wrinkles and cracks are likely to occur during processing. Therefore, the Mn content is set to 0.1 to 2.5%. In order to further improve the workability, the Mn content is preferably set to 2.0% or less. In order to stably secure a tensile strength of 390 MPa or more, the Mn content is preferably 0.5% or more.

P: 0.06% 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. When the P content exceeds 0.06%, in the case of an alloyed hot-dip galvanized steel sheet, the alloying processability may be lowered to lower the plating adhesion, or the surface of the plating may be striped. . For this reason, the P content is set to 0.06% or less. In order to stably secure a tensile strength of 390 MPa or more, the P content is preferably 0.03% or more.

S: 0.05% or less S is present in the steel sheet as an impurity, but if the content is large, scale wrinkles are likely to occur and the surface appearance may be significantly deteriorated, so the content is 0.05% or less. And Preferably it is 0.03% or less.

sol. Al: 0.005 to 0.1%
Al is sol. An amount of 0.1% or less of Al is contained. sol. If the Al content is less than 0.005%, deoxidation becomes insufficient, and if it exceeds 0.1%, the steel sheet may be strengthened by solid solution strengthening of Al and ductility may be lowered. 0.005 to 0.1%.

N: 0.006% or less When N is excessively contained, YS rises and surface distortion is likely to occur, or it may cause solid defects in Fe and cause surface defects such as stretcher strain marks. For this reason, N content shall be 0.006% or less. Preferably it is 0.0035% or less.

Nb: 0.04 to 0.20%
Nb combines with C to produce NbC fine precipitates, thereby refining the structure and improving mechanical properties and surface properties. Nb is the most preferable element among the carbide-forming elements as a means for obtaining the intended effect of the present invention in that carbides are precipitated just below the Ar ₃ point, so that very fine precipitates are obtained. On the other hand, Ti, which is the same carbide-forming element, precipitates carbide at a high temperature of Ar ₃ or higher, so that the precipitate becomes coarse and is inappropriate as a means for obtaining the intended effect of the present invention.

  Therefore, it is important in the present invention to appropriately contain Nb. If the Nb content is less than 0.04%, the amount of NbC deposited may be insufficient, and it may be difficult to stably secure a tensile strength of 340 MPa or more. In addition, solid solution C may remain and surface defects such as stretcher strain are likely to occur. On the other hand, if the Nb content exceeds 0.20%, Nb becomes excessive as compared with C, so that YS increases and elongation decreases, and wrinkles are likely to occur during processing. Therefore, the Nb content is set to 0.04 to 0.20%. Preferably, it is 0.06 to 0.10%.

Ti: (N / 14 × 48 × 0.5)% or more, (0.02 + N / 14 × 48)% or less Ti suppresses the rise of stretcher strain and YS due to N by precipitating N as TiN. This makes it difficult for surface distortion to occur during processing. Moreover, since the characteristic difference (anisotropy) of the r value in the three directions of 0 °, 45 °, and 90 ° with respect to the rolling direction can be reduced, the generation of cracks and wrinkles during pressing is suppressed. Can do. In order to obtain these effects, the Ti content is set to [N (48/14) × 0.5], that is, (N / 14 × 48 × 0.5)% or more.

  However, if Ti is contained in excess of (0.02 + N / 14 × 48)%, the amount of TiC deposited increases and the elongation deteriorates, and surface distortion and cracking are likely to occur during processing. Moreover, in the case of a hot dip galvanized steel sheet, a stripe pattern may be exhibited on the plating surface. Therefore, the Ti content is set to [0.02 + N (48/14)], that is, (0.02 + N / 14 × 48)% or less. Preferably, it is (0.01 + N / 14 × 48)% or less.

Nb / Ti: 2 or more and 20 or less The optimum value for molding large press parts such as automobile side panels, hoods, doors, and fenders, which is the object of the present invention, is an r value of 1 at an angle with respect to the rolling direction of 45 °. In order to make it 0.5 or more, the ratio of the content of Nb and Ti (Nb / Ti) is made 2 or more. If the r value in the 45 ° direction is less than 1.5, the material properties of the corner part, which is the most severely molded, in the side panels, hoods, doors, fenders and the like are insufficient. Moreover, in order to compensate for this, it is necessary to use a higher grade steel plate. If Nb / Ti is less than 2, the r value in the 45 ° direction may be less than 1.5.

On the other hand, if Nb / Ti is excessively high, the minimum r value may be 1.0 or less, so the upper limit of Nb / Ti is 20 or less.
B: 0.0020% or less B is preferably contained because it has an effect of preventing secondary work embrittlement. However, if the B content exceeds 0.0020%, YS increases and elongation decreases, and wrinkles and cracks are likely to occur during processing. For this reason, B content shall be 0.0020% or less. Preferably it is 0.0010% or less. In order to reliably obtain the above action, the B content is preferably 0.0001% or more, and more preferably 0.0003% or more.

One or more selected from Cr: 1% or less, Mo: 1% or less, V: 1% or less, and Ni: 1% or less. These elements may be contained for securing strength. If the content of each element exceeds 1%, the effect of improving the strength is saturated and the cost increases, so the content of each element is set to 1% or less. Preferably it is 0.5% or less.

The steel composition of the cold-rolled steel sheet according to the present invention is the balance Fe and impurities, but as impurities, Cu, Ca, REM, etc. are allowed to be 0.1% or less in total.
The mechanical properties of the cold rolled steel sheet having such a steel composition are as follows.

Minimum value of r value: 1.0 or more The r value mentioned here is an r value in three directions at angles of 0 °, 45 °, and 90 ° with respect to the rolling direction, and the minimum value of the r value is in each direction. This is the minimum value among the r values.

  In the molding of automobile side panels, hoods, doors, fenders, etc., the minimum r value in the three directions of 0 °, 45 ° and 90 ° with respect to the rolling direction is less than 1.0. In some cases, cracks may occur at the site where the main processing is applied in a certain direction. Accordingly, the minimum value of the r value in the three directions of 0 °, 45 ° and 90 ° with respect to the rolling direction is set to 1.0 or more. Preferably it is 1.2 or more.

R value when the angle to the rolling direction is 45 °: 1.5 or more If the r value is less than 1.5 when the angle to the rolling direction is less than 1.5, it is a corner when molding automobile side panels, hoods, doors, fenders, etc. Wrinkles and cracks are likely to occur in the part. Therefore, the r value when the angle with respect to the rolling direction is 45 ° is 1.5 or more.

Next, the suitable manufacturing method for manufacturing the high intensity | strength cold-rolled steel plate and high-strength hot-dip galvanized steel plate demonstrated above is demonstrated below.
(1) Hot rolling process Hot rolling start temperature: 1100 to 1280 ° C
Hot rolling is performed after the steel ingot or steel slab having the above steel composition is set to 1100 to 1280 ° 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 1280 ° C., or after continuous casting when a continuous casting slab is used. You may use for hot rolling, after making it 1100-1280 degreeC, without reducing it to less than 1100 degreeC, and when using a steel piece, 1100 without reducing the steel piece after partial rolling to less than 1100 degreeC. You may use for hot rolling after setting it as 1280 degreeC.

  When the steel ingot or steel slab to be subjected to hot rolling is less than 1100 ° C, deformation resistance may be high and hot rolling may be difficult, and when it exceeds 1280 ° C, an excessive scale is generated until after cold rolling. It may remain and deteriorate the surface properties.

For this reason, it is preferable that the temperature of the steel ingot or steel slab used for hot rolling shall be 1100-1280 degreeC.
Hot rolling completion temperature: Ar ₃ points to 1000 ° C
When the hot rolling completion temperature (hereinafter also referred to as “finishing temperature” or “FT”) is less than Ar ₃ points, the surface layer becomes ferritic and the hot rolled structure becomes coarse, so the r value in the product stage decreases. As a result, cracking may occur during processing, or the hot dip galvanized steel sheet may exhibit a streak pattern on the plated surface.

  On the other hand, when the hot rolling completion temperature exceeds 1000 ° C., it is difficult to refine the hot-rolled structure, so the r value in the product stage is lowered and cracking occurs during processing, or the surface properties are deteriorated by the scale. Sometimes.

Therefore, it is preferable that the hot rolling completion temperature is Ar ₃ points to 1000 ° C.
Winding temperature: 400-650 ° C
When the coiling temperature is less than 400 ° C., NbC is not sufficiently formed after winding, and the r value may be lowered during processing because the NbC intended by the present invention cannot be fully enjoyed. Cracks are likely to occur.

On the other hand, when the coiling temperature exceeds 650 ° C., the r value decreases.
Therefore, the winding temperature is preferably 400 to 650 ° C.
(2) Pickling step, cold rolling step, annealing step A hot rolled steel sheet obtained by hot rolling is descaled by pickling and is annealed after cold rolling. The high-strength hot-dip galvanized steel sheet is further subjected to hot-dip galvanization, and an alloying treatment is performed as necessary.

Pickling may be performed by a conventional method.
In cold rolling, the rolling reduction is set to 50% or more in order to improve the r value after annealing.
Annealing is performed at an annealing temperature of 780 to 900 ° C. in order to obtain a high r value. When the annealing temperature is less than 780 ° C., YS increases and the r value becomes insufficient, and surface distortion and cracking are likely to occur during processing. When it exceeds 900 ° C., YS increases and surface distortion and cracking may occur during processing. is there. For this reason, an annealing temperature shall be 780-900 degreeC.

(3) Hot dip galvanizing process When manufacturing a hot dip galvanized steel sheet, hot dip galvanization is performed after that. Although the method of hot dip galvanization may be a conventional method, 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. Alloying treatment may be performed by a conventional method. In addition, the galvanized steel sheet in the present invention includes an alloyed galvanized steel sheet.

  Thus, according to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a tensile strength of 340 MPa or more, which have excellent formability and excellent surface properties such as streak patterns.

  Moreover, the cold-rolled steel sheet in the present invention includes an electrogalvanized steel sheet, a tin-plated steel sheet, a coated steel sheet, and other surface-treated steel sheets obtained by performing a surface treatment thereafter. These surface treatment methods may be conventional methods.

  Steel No. which is a test material containing chemical components shown in Table 1. Trial steel plates 1 to 39 were manufactured, and under the conditions shown in Table 2, after slab heating, hot rolled to a thickness of 40 mm after rough rolling and a thickness of 3.2 mm after finish rolling, and then cooled and wound up. Further, cold rolling was performed to 0.65 mm, continuous annealing was performed in a continuous annealing line, and after cooling, temper rolling with an elongation of 0.6% was performed.

  The obtained specimens were investigated for secondary work brittleness resistance (longitudinal crack initiation temperature), surface properties, and moldability.

The results are also summarized in Table 2.
Secondary work brittleness resistance is obtained by collecting a 90 mmφ test piece from the obtained cold-rolled steel sheet, performing drawing with a squeezing ratio of 1.8 using a cylindrical punch with a diameter of 50 mm, and holding at various temperatures for 5 minutes or more. Then, a 52.6 kg weight was dropped from a height of 1.901 m by setting on a truncated cone, and the upper limit temperature at which brittle fracture occurred was determined from the fracture surface observation and evaluated as the vertical crack initiation temperature.

The surface properties were determined by visual observation of whether or not the appearance of the cold-rolled steel sheet surface satisfies the standards for automotive exterior applications, that is, the presence or absence of streaks or scale defects. The case where there was no defect was defined as “OK”.
As shown in FIG. 2, the formability was evaluated by using a T-shaped test piece simulating the center pillar portion of the side panel at a drawing depth of 25 mm and evaluating the presence of cracks, wrinkles, and surface distortion. . The case where there was no surface defect was also “OK”. In addition, the number in FIG. 2 shows a dimension (mm).

Moreover, steel No. which is a test material containing the chemical components shown in Table 1. 1 to 39 steel plates were manufactured, and under the conditions shown in Table 3, after slab heating, hot rolled to a thickness of 40 mm after rough rolling and a thickness of 3.2 mm after finish rolling, and then cooled and wound up. Furthermore, after cold rolling to 0.65 mm and continuous annealing in a continuous hot dipping line, 45 g / m ² hot dip galvanizing is applied per side, alloying is performed at 470 to 550 ° C., and after cooling Then, temper rolling with an elongation of 0.6% was performed.

  The obtained specimens were investigated for alloying degree, plating adhesion, secondary work brittleness resistance, surface properties and formability.

The results are also summarized in Table 3.
The degree of alloying is indicated by the Fe content (% by mass) of the plating layer.
For plating adhesion, a 90 mmφ test piece was taken from the plated steel sheet, cup-formed with a 50 mmφ punch, and cello tape (registered trademark) was adhered to and peeled from the vertical wall. Judgment was made based on whether or not the standard was satisfied as an application. “OK” refers to the case where the peel amount is 25 mg / piece or less.

The surface properties were determined by visual observation of whether the appearance of the plating surface satisfies the standards for automotive exterior applications, that is, the presence or absence of streaks or scale defects. The case where there was no defect was defined as “OK”.
Secondary processing brittleness resistance (vertical split generation temperature) and formability were evaluated by the same method as in Example 1.

The steel sheet of the component range of the present invention is excellent in all of strength, r value, and surface appearance, and is optimal for an automobile outer plate.
No. No. 24 had a high C content, so YS was high and elongation was low, and surface distortion occurred during molding.

No. No. 25 had a poor surface property due to the scale because of its high Si content. Further, the plating adhesion was unsatisfactory.
No. In No. 26, since the Mn content was high, YS was highly elongated and the r value was low, so that cracking occurred during molding.

No. Since No. 27 had a high P, a streak pattern occurred on the surface of the steel sheet. Further, the plating adhesion was unsatisfactory.
No. No. 28 had a poor surface property due to the scale because of its high S content.

No. No. 29 has a low Ti value, so the r value in the 0 ° direction was low, and cracking occurred during molding.
No. Since No. 30 had a high Ti content, a streak pattern was generated on the surface of the steel sheet.

No. No. 31 had a low Nb content, so the r value in the 45 ° direction was as low as less than 1.5, and cracking occurred during molding.
No. Since No. 32 had a high Nb content, YS was high and elongation was low, so that cracking occurred in molding.

No. Since No. 33 had a high N content, YS was high, and surface distortion occurred during molding.
No. Since No. 34 has a high B content, the r value in the 0 ° direction is remarkably lower than the r value in the other directions, and the elongation is also low.

No. Nos. 35 to 37 have a low Nb / Ti, so that the r value in the 45 ° direction was less than 1.5, and cracking occurred during molding.
No. Nos. 38 and 39 had high Nb / Ti, so the r value in the 0 ° direction was low, and cracking occurred in molding.

It is an abbreviated top view which shows the direction of cutting from a steel plate blank material, and a crack and a wrinkle danger position at the time of pressing a side panel of a car. It is a top view which shows the shape of the test piece which evaluates a moldability.

Claims (6)

In mass%, C: 0.0005% or more and less than 0.004%, Si: 0.5% or less, Mn: 0.1 to 2.5%, P: 0.06% or less, S: 0.05% Hereinafter, sol. Al: 0.1% or less, N: 0.006% or less, Ti: (N / 14 × 48 × 0.5)% or more, (0.02 + N / 14 × 48)% or less, and Nb: 0.04 to It has a chemical composition that contains 0.20%, the balance is Fe and impurities, and the content of Ti and Nb satisfies the relational expression of 2 ≦ Nb / Ti ≦ 20, and the minimum value of r is 1. A high-strength cold-rolled steel sheet having a tensile strength of 340 MPa or more, wherein the tensile strength is 340 MPa or more. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet according to claim 1 contains B: 0.0020% or less in mass% instead of part of the Fe. Instead of a part of the Fe, it contains one or more selected from the group consisting of Cr: 1% or less, Mo: 1% or less, V: 1% or less, and Ni: 1% or less in mass%. The high-strength cold-rolled steel sheet according to claim 1 or 2. A high-strength hot-dip galvanized steel sheet comprising a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1. The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 3 is subjected to hot rolling at 1100 to 1280 ° C, and hot rolling is completed at Ar _{3 to} 1000 ° C, at 400 to 650 ° C. A method for producing a high-strength cold-rolled steel sheet, which is wound around a steel strip, pickled, cold-rolled at a reduction rate of 50% or more, and then annealed at 780 to 900 ° C. A method for producing a high-strength hot-dip galvanized steel sheet, comprising subjecting a high-strength cold-rolled steel sheet obtained by the production method according to claim 5 to hot dip galvanizing.

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