JP2008308718A - High strength steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a cold-rolled steel sheet and a hot-dip galvanized steel sheet having a tensile strength of 390 MPa or more for an automobile outer panel.
SOLUTION: A precipitate having the following steel composition and containing Nb / Ti and having a diameter of 5 nm or more has a precipitate row distributed in a strip shape in the rolling direction, and the precipitate row is 1 per 10 μm in the plate thickness direction. Present in a range of ˜30 rows, preferably the precipitate row has 5 / μm or more precipitates having a diameter of 5 nm or more containing Nb / Ti in the rolling direction in the rolling direction cross section, and The product having a diameter of 5 nm or more containing Nb or Ti in the portion excluding the precipitate row is 50 pieces / μm ² or less in average number of cross sections in the rolling direction.
C: 0.0005 to 0.010%, Si: 1.0% or less, Mn: 0.30 to 2.5%, P: 0.10% or less, S: 0.02% or less, sol. Al: 0.005 to 0.5%, N: 0.010% or less, Nb: 0.04 to 0.20%, and Ti: 0.05% or less, with the balance being Fe and impurities.
(Nb / 93 + Ti / 48) / (C / 12 + N / 14) ≧ 1.5 (Formula 1)
[Selection] Figure 3

Description

  The present invention relates to a steel sheet having a tensile strength of 390 MPa or higher, such as a cold-rolled steel sheet or a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, an electrogalvanized steel sheet, and a production method thereof. More particularly, the present invention relates to a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a method for producing them, which are suitable for an automotive 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 panel are required to have high strength, formability, secondary work brittleness, and surface properties as outer panel. Yes. For the formability, it is effective to increase the r value and also decrease the yield strength. In recent years, there is a strong demand for improving formability even for high-strength steel sheets. In order to obtain a high r value, it is effective to add a carbonitride-forming element such as Ti or Nb based on an ultra-low carbon steel having a C content of about 30 ppm or less. It is used.

  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, rough surface may occur after pressing and surface properties may deteriorate. In addition, it was difficult to improve the r-value after cold rolling and annealing because the examination of the steel composition suitable for miniaturization 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, the addition of these solid solution strengthening elements is generally expensive and causes an increase in the cost of the steel sheet. For this reason, Patent Document 1 discloses a technique of precipitation strengthening with NbC or TiC for the purpose of reducing solid solution strengthening elements.

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

  In addition, when Ti and Nb are added in combination, C may be precipitated at a high temperature as TiC during cooling, resulting in insufficient generation of NbC fine precipitates, and excellent press formability. The technology for manufacturing steel sheets has not been well established.

  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, when considering application to automotive outer panel such as side panels, doors, hoods, roofs, etc., such steel sheets are prone to wrinkles and surface distortions, have low elongation, and are resistant to secondary processing. Was not enough.
Japanese Patent Laid-Open No. 10-46289 JP 2000-303145 A

As described above, in the prior art, a high-strength cold-rolled steel sheet and a high-strength galvanized steel sheet that satisfy all of the strength, formability, and surface properties with high quality have not yet been obtained.
The present invention is, for example, a high strength cold-rolled steel sheet and hot-dip galvanized steel sheet having a high tensile formability of 390 MPa class and high press formability required for automobile outer panel such as a hood, door, fender, side panel, and the like. An object is to provide a manufacturing method.

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 of automotive parts manufactured by pressing from steel plates, so it is not the slit coil like small parts, it is the maximum coil width. The wide coil of the class, which is blank cut into a nearly rectangular shape, is trimmed slightly.

  Here, the present inventors have focused on, for example, that the side panel has a corner at the four corners of the front door and the rear door opening that are likely to be wrinkled or cracked at 45 ° with respect to the coil rolling direction. In addition, the hood, the door, and the fender are also portions where the wrinkles and cracks are likely to occur near the corners of the four corners, and the positions thereof are located in the 45 ° direction with respect to the coil rolling direction.

Drawing 1 is an explanatory view showing a crack and a wrinkle danger position in a blank material at the time of pressing a side panel of a car.
Automobile outer panel such as side panels, doors, hoods, roofs, etc. are stripped in the direction of 0 ° or 90 ° with respect to the steel plate coil because a large and generally rectangular blank material is used. As a result, as shown in FIG. 1, the 45 ° direction with respect to the rolling direction of the steel sheet coincides with a crack or wrinkle danger position during pressing.

Here, the present inventors have conceived the following in order to obtain a steel plate suitable for such a large press work without requiring excessively high formability of the entire steel plate.
(i) Due to the in-plane anisotropy of the mechanical properties of the steel sheet, if the mechanical properties in the 45 ° direction with respect to the rolling direction are significantly inferior to the 0 ° direction or the 90 ° direction, press defects such as cracks and wrinkles will occur. To be prone to occur, and
(ii) Improving the mechanical properties in the 45 ° direction to improve the formability at corners of side panels, hoods, doors, fenders, etc., which are highly difficult to form and easily wrinkle and crack.

  That is, the inventors have excellent mechanical properties in the 45 ° direction with respect to the rolling direction and good press formability by reducing the mechanical property difference in the 0 °, 45 °, and 90 ° directions. The idea was to make the steel plate optimal for molding large press parts such as automobile side panels, hoods, doors, and fenders.

  In order to solve the above-mentioned problems, the present inventors diligently investigated the steel composition and production conditions, and controlled the precipitation state of Nb and Ti-based fine precipitates, thereby improving the strength and at the same time the features of the present invention. The present inventors have found that an anisotropic balance with excellent mechanical properties in a certain 45 ° direction can be realized, and have completed the present invention.

The present invention is as follows.
(1) Precipitates having a diameter of 5 nm or more containing Nb or Ti have a precipitate row distributed in a strip shape in the rolling direction, and the precipitate row is 1 to 30 rows per 10 μm in the plate thickness direction in the rolling direction section. A steel sheet comprising a steel structure existing in a range.

(2) The precipitate row has 5 / μm or more of precipitates having a diameter of 5 nm or more containing Nb or Ti per unit length in the rolling direction in a cross section in the rolling direction, and the precipitate row is The steel sheet according to (1), wherein precipitates having a diameter of 5 nm or more containing Nb or Ti in an excluded portion are 50 pieces / μm ² or less in average number per unit area of the cross section in the rolling direction.

  (3) By mass%, C: 0.0005 to 0.010%, Si: 1.0% or less, Mn: 0.30 to 2.5%, P: 0.10% or less, S: 0.02 % Or less, sol. Al: 0.005 to 0.5%, N: 0.010% or less, Nb: 0.04 to 0.20%, and Ti: 0 to 0.1%, with the balance being Fe and impurities The steel plate according to (1) or (2) above, which has a steel composition satisfying the following formula 1.

(Nb / 93 + Ti / 48) / (C / 12 + N / 14) ≧ 1.5 (Formula 1)
Here, Nb, Ti, C, and N represent the content (unit: mass%) of each element.
(4) The steel sheet according to (3), wherein the steel composition further contains B: 0.002% or less in terms of mass%.

  (5) The steel composition is one type selected from the group consisting of Cr: 1% or less, Mo: 1% or less, V: 1% or less, Cu: 1% or less, and Ni: 1% or less in mass%. Or the steel plate as described in said (3) or (4) characterized by further containing 2 or more types.

(6) The steel sheet according to any one of (1) to (5) above, wherein a plating layer is provided on the surface of the steel sheet.
(7) A casting process in which the molten steel having the steel composition described in any one of (3) to (5) above is cast into a slab, and the hot slab is hot-rolled into a hot-rolled steel sheet. In the method for producing a steel sheet, comprising: a rolling process, a cold rolling process for cold rolling the hot rolled steel sheet into a cold rolled steel sheet, and an annealing process for annealing the cold rolled steel sheet into a steel sheet. The average cooling rate CR (° C./second) between the liquidus temperature and the solidus temperature in the process is in a range satisfying the following formula 2, and the temperature of the slab used for hot rolling in the hot rolling process is 1000. To 1280 ° C., the finishing rolling temperature is Ar _{3 to} 1000 ° C., the coiling temperature is 400 to 650 ° C., the rolling reduction in the cold rolling step is 50% or more, and the annealing temperature in the annealing step is 780 to 900 ° C. A method for producing a steel sheet, comprising:

0.33 ≦ (t ₂ / t ₁ ) × 770 × CR ^−0.41 ≦ 10 (Formula 2)
Here, t ₁ is IhenAtsu, t ₂ is the cold-rolled steel sheet thickness of the final.
(8) The method for producing a steel sheet according to (7) above, further comprising a plating step of performing plating after the annealing step.

  According to the present invention, steel sheets having excellent press formability, secondary work brittleness resistance, and steel sheet surface quality can be obtained, and when they are applied to automobile outer plates such as side panels, doors, fenders, etc. A material having particularly excellent material properties in the direction of 45 ° with respect to the rolling direction of the main stress direction in which distortion, wrinkles and cracks are likely to occur can be obtained.

As described above, according to the present invention, an optimum high-strength cold-rolled steel sheet having anisotropy at an appropriate position for an appropriate material can be produced, which is extremely useful industrially.
This steel sheet can be applied not only as an automobile outer sheet but also as a cold-rolled steel sheet for processing and a surface-treated steel sheet for processing. Examples of the surface treatment include zinc plating (including alloy system), tin plating, enamel resin coating, and the like. 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.

  Regarding the high-strength steel sheet according to the present invention, the structure, mechanical properties, and reasons for limiting the steel composition defined by the precipitate row will be described below. In the following description, the present invention will be described by taking, for example, a steel sheet of about 0.5 to 2.0 mm as a steel sheet in this specification, and “%” indicating the steel composition is “% by mass” unless otherwise specified. is there.

Steel plate structure Precipitate In the present invention, the yield strength in the 45 ° direction relative to the rolling direction is 300 MPa or less, the r value in the 45 ° direction relative to the rolling direction is 1.50 or more, and the 90 ° direction relative to the rolling direction In order to obtain a steel sheet having a tensile strength of 390 MPa or more, a precipitate row containing Nb or Ti having a diameter of 5 nm or more distributed in a strip shape in the rolling direction, the precipitate row is 1 to 10 μm per 10 μm in the plate thickness direction. It is arranged in the range of 30 rows.

  In the cold rolling according to the present invention, the precipitates generated mainly in the cooling stage of the casting process extend in the rolling direction during the cold rolling and are partially divided to form the precipitate row according to the present invention. .

The precipitate containing Nb or Ti in the present invention is not particularly limited, but TiC, NbC, TiN and their composites, NbC precipitated with MnO as a nucleus, and TiS, Ti ₄ C ₂ S ₂ and those in which MnS is combined, FeTiP, and the like. As can be seen from the above, the term “containing Nb or Ti” in the present invention means that Nb and / or Ti is contained in the form of inclusions, that is, compounds.

  In addition, the precipitation state of the precipitate prescribed | regulated by this invention should just be satisfy | filled in the area | region remove | excluding both the plate thickness 10% from the surface of a steel plate, and the plate thickness 10% from the center of steel plate thickness. Even if the precipitation state of both portions of the plate thickness of 10% from the surface and the plate thickness of 10% from the center is not within the scope of the present invention, if the region excluding them is satisfied, the entire steel plate is an object of the present invention. The following characteristics can be secured.

  In the present invention, Nb and Ti-based precipitates having a diameter of 5 nm or more are specified. This is because it cannot interfere with the effects described below.

  In the present invention, the mechanism that the yield strength in the 45 ° direction with respect to the rolling direction is lowered and the r value is increased is not necessarily clear, but precipitates influence other than the influence of the conventionally known texture. The possibility is guessed. That is, the presence of the above-described precipitate row arranged in the rolling direction and a region with a small amount of precipitates sandwiched between them starts to yield at a low stress in a region where there are few precipitates that hinder the movement of dislocations. Is guessed. Further, since the precipitate row and the region with few precipitates continuously spread along the rolling direction, the direction of sliding in the rolling direction is easier to deform than the other directions, and 45 to 90 with respect to the rolling direction. In the direction of °, the width shrinkage during the tensile test is large, and the possibility that the r value becomes large is estimated.

  When the number of precipitates is less than 1 row per 10 μm in the plate thickness direction, dislocations can easily move in any direction, and when the number of rows exceeds 30 rows, movement of dislocations in the rolling direction is prevented in the same manner as other directions. In either case, the improvement of the mechanical characteristics in the 45 ° direction cannot be obtained. Preferably, the number of precipitates is 5 to 20 per 10 μm in the plate thickness direction.

  In addition, since the distribution in the plate width direction of the precipitate row exists at a larger interval, the distribution in the plate thickness direction is dominant with respect to the influence on the movement of dislocations. Therefore, in the present invention, the number of precipitate rows in the cross section in the plate thickness direction is defined.

Preferably, the precipitate row has Nb or Ti-containing precipitates having a diameter of 5 nm or more per unit length of the cross section in the rolling direction, and the portion excluding the precipitate row is The average number of precipitates containing 5 nm or more in diameter containing Nb or Ti per unit area of the cross section in the rolling direction is preferably 50 / μm ² or less. When the number of precipitates having a diameter of 5 nm or more containing Nb or Ti in the precipitate row is less than 5 / μm, it becomes difficult to perform the function as the precipitate row with respect to the movement of dislocation as described above. Does not contribute to improvement of characteristics. Further, if the portion excluding the precipitate row has Nb or Ti-containing precipitates having a diameter of 5 nm or more in an average number per unit area of the cross section in the rolling direction of more than 50 / μm ² , the dislocation direction is changed to the rolling direction. This movement is also hindered in the same manner as in other directions, and it becomes difficult to improve the mechanical characteristics in the 45 ° direction.

More preferably, the precipitate row has Nb or Ti-containing precipitates having a diameter of 5 nm or more per unit length of the cross section in the rolling direction, and the portion excluding the precipitate row is Nb or Ti-containing precipitates having a diameter of 5 nm or more are preferably 30 pieces / μm ² or less in average number per unit area of the cross section in the rolling direction.

  Precipitates entering the precipitate row and the other precipitates are regarded as a precipitate row where the presence frequency of precipitates per unit length is 10 times or more in the rolling direction in the sheet thickness direction. Identify a precipitate located within 0.1 times the distance from the center line of the target precipitate line to the center line of the adjacent precipitate line as a precipitate included in the precipitate line I decided to.

  The structure of the steel sheet of the present invention is mainly composed of ferrite, but if the average grain size of ferrite is too large, rough surface may occur after pressing and the surface properties may deteriorate, so that it does not matter so much as 30 μm. It is preferable that: On the other hand, if the ferrite average crystal grain size is less than 3 μm, the effect of dislocations being stopped at the grain boundaries is increased, the effect of the precipitate row is relatively reduced, and the mechanical properties in the 45 ° direction are improved. The ferrite average crystal grain size is preferably 3 μm or more because there is a risk that it will not be obtained.

In the present invention, there are no specific means for the analysis of the form and components of the precipitate, but as a standard method, the presence of Ti or Nb is confirmed by analysis by energy dispersive X-ray spectroscopy. The number of precipitates and the like may be observed through a cross section using a transmission electron microscope. The magnification at that time is 100,000.
Mechanical properties In the steel sheet according to the present invention, the yield strength in the 45 ° direction with respect to the rolling direction is 300 MPa or less, and the r value in the 45 ° direction with respect to the rolling direction is 1.50 or more. The tensile strength in the 90 ° direction is 390 MPa or more.

  The control of the form of the precipitate can be performed by first selecting the steel composition, the cooling rate during continuous casting, the reduction rate until reaching the final product, and the like. Since most of the precipitates found in the steel sheet according to the present invention are carbides and nitrides, for example, the amount of precipitates is almost determined by the steel composition, the size of which depends on the grain growth in the solidification process after continuous casting. And the processing method at the processing stage such as rolling, and the processing rate at that time.

As described above, during cold rolling, the precipitates extend in the rolling direction and are partially divided to form a precipitate row according to the present invention.
Steel composition C: 0.0005 to 0.010%
C combines with Nb and Ti to form Nb and Ti-based fine precipitates. It is important to optimize the C content in order to realize the precipitation state of the precipitate row, which is a feature of the present invention. When C is less than 0.0005%, there are too few precipitates, and when it exceeds 0.010%, there are too many precipitates, and the precipitation state characteristic of the present invention cannot be obtained, so 0.0005 to 0.010% And Preferably, it is 0.0005 to 0.004%.

Si: 1.0% or less Si can be added to increase the strength by solid solution strengthening, but if it exceeds 1.0%, the r value is greatly deteriorated. Therefore, it is 1.0% or less.

Mn: 0.30 to 2.5%
Mn is added to increase the strength by solid solution strengthening and to fix S which adversely affects secondary work brittleness resistance. If it is less than 0.30%, the targeted high strength cannot be achieved. Moreover, in order to make secondary work brittleness resistance better, it is desirable to set it as 0.8% or more. If it exceeds 2.5%, the r value and elongation deteriorate, so 0.30 to 2.5% is set.

P: 0.10% or less P can be contained in order to increase the strength by solid solution strengthening. If it exceeds 0.10%, the secondary work brittleness resistance is extremely deteriorated, so the content is made 0.10% or less. Preferably it is 0.07% or less.

S: 0.02% or less S is present as an impurity, but if it exceeds 0.02%, wrinkles are likely to occur during hot rolling and the surface properties deteriorate, so the content is made 0.02% or less.

sol. Al: 0.005 to 0.5%
Al is included for deoxidation. sol. When Al is less than 0.005%, deoxidation is not sufficient, and even if contained over 0.5%, the cost is increased and there is no effect, so 0.005 to 0.5% is made.

N: 0.010% or less If N is excessively contained, it may remain in a solid solution state, resulting in an increase in stretcher strain or yield strength. If the content exceeds 0.010%, TiN precipitates from a high temperature and the precipitate tends to be coarsened, and the precipitation state characteristic of the present invention may not be obtained. Therefore, N is limited to 0.010% or less.

Nb: 0.04 to 0.20%
Nb combines with C and the like to form a composite precipitate of Nb and Ti. Optimizing Nb is extremely important in order to obtain the precipitation state of the precipitate row, which is a feature of the present invention. Nb and Ti are elements suitable for precipitation strengthening in that extremely fine carbonitrides can be obtained, but the present invention further utilizes a small distribution coefficient during solidification in the casting process, It is characterized by microsegregation between the dendrite arms by controlling the cooling condition of the material within an appropriate range. The microsegregation is extended in the rolling process, and Nb and Ti carbonitrides are deposited on the hot run table after hot rolling, after coil winding, and in the annealing process after cold rolling, which is a feature of the present invention. A series of precipitates distributed in a strip shape in the direction is realized.

  In order to effectively obtain a precipitate row, it is essential to use Nb having a smaller distribution coefficient. If it is less than 0.04%, Nb concentration sufficient to obtain a precipitate row cannot be obtained. When Nb is contained exceeding 0.20%, the yield strength increases due to an increase in solid solution Nb and wrinkles are likely to occur during processing.

Ti: 0 to 0.1%
On the other hand, Ti is also an element having a small distribution coefficient, but it is larger than Nb, and the Ti addition has a difference in the precipitate density between the precipitate row and the portion excluding the precipitate row as compared with the case of adding Nb alone. Since it may be small, excessive Ti content should be avoided. Therefore, it may be contained if necessary, and the upper limit that may be contained at that time is 0.1%. Preferably it is 0.05% or less.

(Nb / 93 + Ti / 48) / (C / 12 + N / 14) ≧ 1.5
If (Nb / 93 + Ti / 48) / (C / 12 + N / 14) is less than 1.5, the amount of solute C in the hot rolled steel sheet stage increases, and the r value of the final product decreases, and the side panel Since the deep drawability required for hoods, doors, fenders, etc. may not be obtained, it is specified as described above.

B: 0.002% or less B can be contained in order to prevent secondary processing embrittlement. If the content exceeds 0.002%, the r value and the elongation deteriorate significantly, so the content is made 0.002% or less. If the content is less than 0.0002%, the effect is small.

Cr: 1% or less, Mo: 1% or less, V: 1% or less, Cu: 1% or less, Ni: 1% or less selected from the group consisting of Ni: 1% or less These elements are for securing strength It may be included. 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.
Production method Relationship between average cooling rate CR (° C./second) between liquidus temperature and solidus temperature during casting, slab thickness t ₁ , final cold-rolled steel plate thickness t ₂ : 0.33 ≦ (T ₂ / t ₁ ) × 770 × CR ^−0.41 ≦ 10
In order to obtain a state in which 1 to 30 rows of precipitates in which a precipitate having a diameter of 5 nm or more containing Nb or Ti is distributed in a strip shape in the rolling direction is arranged in 1 to 30 rows per 10 μm in the thickness direction, which is a feature of the present invention. It is very important to control the cooling rate of the hour. In relation to the average cooling rate CR (° C./sec) between the liquidus temperature and the solidus temperature during solidification during casting, the slab thickness t ₁ , and the final steel plate thickness t ₂ , (t ₂ / t ₁ ) When ^X770 × CR- ^0.41 exceeds 10, the interval between the segregation bands of Nb becomes large, and precipitation of Nb and Ti-based precipitates necessary for improving the mechanical properties in the 45 ° direction in the final product Can't get state. On the other hand, if (t ₂ / t ₁ ) × 770 × CR ^−0.41 is smaller than 0.33, the interval between the segregation bands of Nb is too small to improve the mechanical properties in the 45 ° direction in the final product. The necessary precipitation state of Nb and Ti-based precipitates cannot be obtained.

  In addition, the cooling rate at the time of said casting should just be satisfy | filled in the area | region except the both parts of slab thickness 10% from the slab surface, and slab thickness 10% from the center of slab thickness. The object of the present invention is that if the cooling rate of both portions of the slab thickness 10% from the surface and the slab thickness 10% from the center is satisfied in the region excluding them, even if they are out of the scope of the present invention. Characteristics can be secured.

Temperature of slab to be subjected to hot rolling: 1000 to 1280 ° C
In the present invention, a slab having a predetermined component is subjected to rough hot rolling immediately after continuous casting or after reheating. In any case, the temperature used for hot rolling is 1000 to 1280 ° C. When the temperature is less than 1000 ° C., deformation resistance is high and hot rolling is difficult, and when it exceeds 1280 ° C., scales are excessively generated and the surface properties are deteriorated, so the temperature is set to 1000 to 1280 ° C.

In addition, you may heat in the range which does not exceed 1280 degreeC between rough hot rolling and finishing hot rolling, and the effect of this invention is not lost.
Finish rolling _temperature: Ar 3 _~1000 ℃
When the finish hot rolling finish temperature is less than Ar ₃ , the surface layer precipitates become coarse, and the r value is impaired. If the temperature exceeds 1000 ° C., the surface properties deteriorate due to the scale, so the finish rolling temperature is Ar 3 to 1000 ° C.

Winding temperature: 400-650 ° C
If the coiling temperature is less than 400 ° C., carbides are not sufficiently generated after coiling, and the r value is lowered. When it exceeds 650 ° C., coarse precipitates are formed and the strength may be lowered, so the temperature is set to 400 to 650 ° C.

Cold rolling rolling ratio: 50% or more In order to improve the r value, the cold rolling rolling ratio is 50% or more.
Annealing temperature: 780-900 ° C
In order to obtain a high r value, after cold rolling, soaking is performed in a temperature range of 780 to 900 ° C. and annealing is performed.
When the annealing temperature is less than 780 ° C. and exceeds 900 ° C., the r value is decreased and the yield strength is increased.

The cold-rolled steel sheet thus obtained has excellent press formability required for forming an automobile outer panel, and has a tensile strength of 390 MPa or more.
According to the present invention, preferably, the cold-rolled steel sheet may be further plated with a metal such as zinc to form a metal-plated steel sheet. Examples of plating forms include electroplating and molten metal plating. From a practical standpoint, representative examples of such metal-plated steel sheets include electrogalvanized steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets.

  About the manufacturing method of such a plated steel plate, what is necessary is just to follow a conventional method, and it does not restrict | limit in particular in this invention.

Specimen No. containing chemical components shown in Table 1 1 to 30 steel plates were prototyped as follows.
A slab having a slab thickness of 270 mm was formed by continuous casting, and after slab heating, the plate thickness after rough rolling was 40 mm and the plate thickness after finish rolling was 3.2 mm by hot rolling, and then cooled and wound. Further, it was cold-rolled to 0.65 mm and subjected to continuous annealing. Some test materials were further subjected to electrogalvanization or hot dip galvanization with an adhesion amount of 45 g / m ² per side. Further part of the hot dip galvanized steel sheet was alloyed at 470 to 550 ° C. to obtain an alloyed hot dip galvanized steel sheet.

The cold-rolled steel sheet and the electrogalvanized steel sheet were subjected to temper rolling with an elongation rate of 0.9% after the continuous annealing and the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet after plating.
FIG. 2 is an explanatory diagram schematically showing the position of the observation cross section of the test material.

  About the obtained various steel sheets, while observing precipitates by a replica method using a transmission electron microscope with respect to a cross section along the rolling direction, which is indicated by a hatched region in FIG. 2, plating adhesion, secondary work brittleness resistance, The surface properties and moldability were investigated. In FIG. 2, the Y-axis direction in the cross section in the rolling direction indicated by the hatched area is the plate thickness direction, and the X-axis direction is the rolling direction.

For the measurement of the precipitate density in the precipitate row and the precipitate density in the portion excluding the precipitate row, observation was performed for any 10 fields of view of 1 μm square, and the average value was obtained.
FIG. 3 shows an electron micrograph of test number 3 as an example of the results.

From FIG. 3, a string of precipitates distributed in a strip shape in the rolling direction, which is a feature of the steel of the present invention, is observed. As a result of identification by X-ray diffraction, the precipitates in the photograph shown were Nb and Ti based precipitates mainly composed of NbC. The two precipitate rows observed in this field of view are 15 / μm (upper row in the drawing) and 28 / μm (upper row in the drawing) containing Nb or Ti having a diameter of 5 nm or more. , Lower row). On the other hand, the portion excluding the precipitate row has 5 precipitates / μm ² containing Nb or Ti having a diameter of 5 nm or more.

 In the case of FIG. 3, there are two precipitate rows per 1 μm in the thickness direction, so this is performed for 10 fields of view, and by summing them, the number of precipitate rows per 10 μm in the thickness direction The number of columns is calculated.

  The secondary work brittleness resistance is drawn using a cylindrical punch with a diameter of 50 mm and drawn at a drawing ratio of 1.8. After holding at each temperature, it is set on a truncated cone and a weight of 100 kg is dropped from a height of 1 m. From the observation of the fracture surface, the upper limit temperature at which brittle fracture occurs (hereinafter referred to as secondary work embrittlement resistance temperature) was investigated. A material having a secondary work embrittlement resistance of −50 ° C. or less was regarded as a conforming range of the present invention.

The plating adhesion was determined by whether or not the adhesive plating was performed by 180 ° adhesion, cellotape (registered trademark) was adhered to and peeled off from the bent portion, and the amount of peeled plating adhered to the tape satisfied the standard for automotive exterior use.
The surface property was determined by visual observation as to whether or not the appearance of the plating surface satisfies the standards for automotive exterior applications.

As shown in FIG. 4, the moldability was evaluated by using a T-shaped test die simulating the center pillar portion of the side panel at a drawing depth of 25 mm and evaluating the presence or absence of cracks and wrinkles.
Table 1 shows the results of investigating the manufacturing conditions and mechanical properties. The r value of the plated steel sheet was measured after dissolving and removing only the plating layer with hydrochloric acid in order to remove the influence of the plating layer. Even if the r value in the 45 ° direction with respect to the rolling direction is 1.50 or more, if the r value in the other direction is extremely low, cracks are likely to occur in press forming, so the average r value is 1.50. The above is regarded as the applicable range of the present invention. Here, the average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, r ₀ , r ₄₅ , and r ₉₀ are r values of 0 °, 45 °, and 90 ° with respect to the rolling direction, respectively.

The steel sheet of the component range of the present invention is excellent in all of strength, r value, secondary work brittleness resistance and formability, and is optimal for an automobile outer sheet.
FIG. 5 is a graph showing the relationship between the distribution of precipitates and the 45 ° direction r value based on the results shown in Table 1. It can be seen that an r value of 1.50 or more in the 45 ° direction is obtained when the precipitate distribution is within the range of the present invention.

It is a typical explanatory view of a crack and a wrinkle danger position at the time of pressing a side panel of a car. It is a schematic explanatory drawing of the position of the observation cross section of the test material of an Example. It is the transmission electron micrograph by the replica method which shows the example of precipitate observation of a rolling direction cross section (100,000 times). It is a top view of a T-shaped test type. It is a graph which shows the result of an Example.

Claims (8)

Nb or Ti-containing precipitates having a diameter of 5 nm or more have a precipitate row distributed in a strip shape in the rolling direction, and the precipitate row exists in a range of 1 to 30 rows per 10 μm in the plate thickness direction in the rolling direction section. A steel sheet characterized by comprising a steel structure to be used. The precipitate row has 5 / μm or more of precipitates having a diameter of 5 nm or more and containing Nb or Ti per unit length in the rolling direction in a cross section in the rolling direction, and in a portion excluding the precipitate row. ^2. The steel sheet according to claim 1, wherein the number of precipitates containing Nb or Ti and having a diameter of 5 nm or more is 50 / μm ² or less in average number per unit area of the cross section in the rolling direction. In mass%, C: 0.0005 to 0.010%, Si: 1.0% or less, Mn: 0.30 to 2.5%, P: 0.10% or less, S: 0.02% or less, sol. Al: 0.005 to 0.5%, N: 0.010% or less, Nb: 0.04 to 0.20%, and Ti: 0 to 0.1%, with the balance being Fe and impurities The steel plate according to claim 1 or 2, comprising a steel composition satisfying the following formula (1).
(Nb / 93 + Ti / 48) / (C / 12 + N / 14) ≧ 1.5 (Formula 1)
Here, Nb, Ti, C, and N represent the content (unit: mass%) of each element. The steel sheet according to claim 3, wherein the steel composition further contains B: 0.002% or less in mass% instead of part of Fe. The steel composition is, instead of a part of Fe, in mass%, Cr: 1% or less, Mo: 1% or less, V: 1% or less, Cu: 1% or less, and Ni: 1% or less. The steel sheet according to claim 3 or 4, further comprising one or more selected. The steel plate according to claim 1, further comprising a plating layer on a surface of the steel plate. A casting process in which the molten steel having the steel composition according to any one of claims 3 to 5 is cast into a slab, a hot rolling process in which the slab is hot-rolled into a hot-rolled steel sheet, the hot In a method for producing a steel sheet comprising a cold rolling step for cold rolling a rolled steel plate to obtain a cold rolled steel plate, and an annealing step for annealing the cold rolled steel plate to obtain a steel plate, the liquidus temperature in the casting step The average cooling rate CR (° C./second) between the solidus temperature and the solidus temperature is in a range satisfying the following formula 2, and the temperature of the slab used for hot rolling in the hot rolling step is 1000 to 1280 ° C., and finish rolling. The temperature is Ar _{3 to} 1000 ° C., the coiling temperature is 400 to 650 ° C., the rolling reduction in the cold rolling step is 50% or more, and the annealing temperature in the annealing step is 780 to 900 ° C. A method of manufacturing a steel sheet.
0.33 ≦ (t ₂ / t ₁ ) × 770 × CR ^−0.41 ≦ 10 (Formula 2)
Here, t ₁ is IhenAtsu, t ₂ is the cold-rolled steel sheet thickness of the final. The method for manufacturing a steel sheet according to claim 7, further comprising a plating step of performing plating after the annealing step.