JP6349865B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a thin cold-rolled steel sheet used in the field of building materials such as roofing materials, or a cold-rolled steel sheet subjected to surface treatment thereon, in particular, a cold-rolled steel plate or a steel sheet subjected to surface treatment thereon. The present invention relates to a hot-rolled steel sheet and a method for manufacturing the hot-rolled steel sheet that enable stable cold rolling.

  In recent years, demand for building materials has increased with the global population growth and economic development. In particular, a thin cold-rolled steel plate having a thickness of 0.3 mm or less or a surface-treated steel plate plated or painted on a substrate such as plywood is used for the outer wall or roof of a building. In such applications, there is almost no processing on the cold-rolled steel sheet, so workability is not questioned, and steel sheets that have been subjected to surface treatment such as Zn plating are used on steel sheets that have been cold-rolled. Is common.

  In such a steel sheet, although it is desired to reduce the thickness of the steel sheet for cost reduction, there is a problem that the thickness cannot be reduced due to insufficient load during cold rolling. In particular, there is a need for a hot-rolled steel sheet that can produce a thin cold-rolled steel sheet even in a mill with a low rolling capacity in emerging countries.

Thus, there is an increasing demand for a soft hot-rolled steel sheet that can be cold-rolled with a low load as a material for cold rolling.
For example, Patent Document 1 describes a technique related to a hot-rolled steel sheet in which the C content in steel is reduced to 0.010% or less. Patent Document 2 describes a technique related to a hot-rolled steel sheet in which the N content in steel is 0.0020% or less. Further, Patent Document 3 discloses a so-called ferrite zone rolling in which the C content in steel is 0.01-0.10%, the N content is 0.010% or less, and the finish rolling temperature is rolled at 700 ° C. or higher and below the Ar ₃ transformation point. A technique for manufacturing a hot-rolled steel sheet having coarse crystal grains has been proposed.

JP-A-3-79726 Japanese Patent Publication No. 63-30969 JP 2010-77482 A

However, the reduction of the amount of C and N proposed in Patent Documents 1 and 2 needs to reduce the amount of C and N in the atmospheric gas during steelmaking. For that purpose, degassing treatment is performed during steelmaking. This will increase production costs.
In addition, due to demands for manufacturing in emerging countries, it is necessary to keep hot-rolled sheets at least 3 mm or less in order to produce materials of 0.3 mm or less in a cold rolling mill with low rolling capacity. With low steel, rolling beyond the Ar ₃ transformation point is difficult, and a non-uniform microstructure tends to be formed in the thickness direction and width direction, impairing cold rolling properties.

  Further, in the ferrite zone rolling described in Patent Document 3, the strength of the hot-rolled sheet is easily affected by the finish rolling temperature and the winding temperature, and it is difficult to secure a stable soft hot-rolled steel sheet.

  The present invention has been made to solve the above problems, and even when a cold rolling mill having a small rolling capacity is used, a thin steel plate as cold-rolled or a steel plate subjected to surface treatment can be stabilized. An object of the present invention is to provide a hot-rolled steel sheet as a material for cold rolling, which can be manufactured by the manufacturing method, and a manufacturing method thereof.

  As a result of intensive investigation and examination on the correlation between various properties of the cold-rolled steel sheet after cold rolling and surface treatment and the microstructure of the hot-rolled steel sheet used as the raw material, If the material has a structure, the rolling resistance during cold rolling becomes uniform in the coil, and especially the deformation resistance after the cold rolling is reduced, that is, a hot-rolled steel sheet having excellent cold rolling properties is stable. As a result, the present invention was completed. That is, the non-uniformity of the crystal orientation of the hot-rolled sheet in the thickness direction and the width direction has a great influence on the cold rolling property, and it is important to control it.

That is, the gist configuration of the present invention is as follows.
1. In mass% C: 0.015-0.035%,
Si: 0.2% or less,
Mn: 0.05 to 0.35%,
P: 0.02% or less,
S: 0.02% or less,
sol.Al: 0.01 to 0.1% and N: 0.005% or less, with the balance being the composition of Fe and inevitable impurities, 50 mm from the width direction edge of the steel plate to the width direction and the width direction edge As for the texture at the position of 1/4 of the plate width from the plate surface to the depth of 1/4 of the plate thickness, X defined by the following formula (1) satisfies the range of 0.5 to 1.0. Features hot-rolled steel sheet.
X = F1 / (F2 + F3) (1)
However,
F1: ODF strength of {001} <110>
F2: {211} <110> ODF strength
F3: ODF strength of {111} <112>

2. The component composition is further in mass% B: 0.0003 to 0.0030%,
Ti: 0.002 to 0.1%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.1% and
Cr: 0.01-0.5%
2. The hot-rolled steel sheet according to 1 above, containing any one or more selected from among the above.

3. When hot rolling a steel material having the component composition described in 1 or 2 above, the width direction center of the steel sheet and the region extending from the edge in the width direction up to 50 mm in the width direction on the entry side of finish rolling The temperature difference is set within 30 ° C, the finish rolling exit temperature is changed from 870 ° C to 930 ° C, cooling is started within 1 second after the hot rolling, and the average cooling from the finish rolling exit temperature to 750 ° C is started. A method for producing a hot-rolled steel sheet, characterized by winding the steel sheet in a temperature range of 550 ° C to 700 ° C after setting the speed to 10 ° C / s or more.

  According to the hot-rolled steel sheet of the present invention, even a rolling mill having a low rolling capacity can be rolled to 0.3 mm or less, for example, a thin cold-rolled steel sheet or a surface-treated cold-rolled steel sheet used in the building material field such as a roofing material. A hot-rolled steel sheet suitable as a cold rolled material can be provided.

Hereinafter, the hot-rolled steel sheet of the present invention will be described in detail.
First, the reasons for limiting the content of each component in the component composition will be described. In addition, unless otherwise indicated, the "%" display regarding a component means "mass%".
C: 0.015-0.035%
When C exceeds 0.035%, the crystal grain size becomes fine, the carbides increase, the strength of the hot-rolled sheet increases, and the cold rolling property is impaired. On the other hand, when C is less than 0.015%, cementite hardly precipitates, and a large amount of solute carbon remains in the hot-rolled sheet, resulting in an increase in the strength of the hot-rolled sheet. , 0.015% or more. Therefore, the C content is 0.015 to 0.035%, more preferably 0.030% or less.

Si: 0.2% or less
If the Si content is too high, the strength of the hot-rolled sheet will increase, impairing the cold rolling property, and will deteriorate the plating density such as chemical conversion treatment and galvanization, so it is 0.2% or less. To do. In addition, although Si does not add, there is no problem in the material, but since it requires a lot of cost to suppress it to less than 0.005%, the content of 0.005% or more is allowed.

Mn: 0.05-0.35%
Mn is added at 0.05% or more for the main purpose of preventing hot brittleness. However, if the added amount is too large, the hot rolled sheet crystal grains are refined and the strength of the hot rolled sheet is enhanced by the solid solution strengthening action. Rises and hinders cold rollability, so the upper limit is made 0.35%. More preferably, it is 0.10 to 0.20%.

P: 0.02% or less P is set to 0.02% or less because if the addition amount is too large, the strength of the hot-rolled sheet increases and the cold rolling property is impaired. In addition, although there is no problem in the material even if P is not added, since it takes a lot of cost to suppress it to less than 0.0010%, the content of 0.0010% or more is allowed.

S: 0.02% or less S is present as sulfide inclusions in steel. Since the sulfide inclusions extend during cold rolling and become crack starting points during processing, it is desirable to reduce them as much as possible, and the upper limit is made 0.02%. It should be noted that even if S is not added, there is no problem in the material, but since it requires a lot of cost to suppress it to less than 0.0005%, the content of 0.0005% or more is allowed.

sol.Al: 0.01-0.1%
Al is added for the purpose of deoxidation of molten steel, but if the addition amount is less than 0.01% in sol.Al, the effect is poor, while if it exceeds 0.1%, the deoxidation effect is saturated and Al ₂ O ₃ Inclusions increase and cold workability deteriorates. Therefore, the range of the addition amount is 0.01% or more and 0.1% or less with sol.Al.

N: 0.005% or less N forms nitrides with Ti, Nb, Al and the like. From the viewpoint of cold workability, it is more advantageous to reduce the solid solution N by precipitating N as these nitrides as much as possible, and the lower the N content, the better. Therefore, the upper limit is made 0.005% . In addition, even if N is not added, there is no problem in the material, but since it requires a lot of cost to suppress it to less than 0.0003%, the content of 0.0003% or more is allowed.

The basic composition of the hot-rolled steel sheet of the present invention is as described above, and the balance may be Fe and inevitable impurities. However, the following elements may be further added to improve the characteristics.
B: 0.0003 to 0.0030%, Ti: 0.002 to 0.1%, Nb: 0.002 to 0.1%, V: 0.002 to 0.1%, and Cr: 0.01 to 0.5%

  First, when B is added in a small amount, B contributes to increasing the grain size of the hot-rolled sheet structure and improving the cold rolling property. For that purpose, the lower limit is made 0.0003%. However, if added over 0.0030%, the ferrite transformation hardly occurs after hot rolling, the structure becomes finer, the strength of the hot-rolled sheet increases, and the cold rolling property is hindered. And

  Next, Ti, Nb, V, and Cr form charcoal and nitride by adding a trace amount, thereby preventing the solid solution C and N from remaining in the steel sheet and improving the cold rolling property. Therefore, Ti, Nb, V and Cr are added alone or in combination in order to ensure high workability and non-aging properties. When Ti, Nb and V are less than 0.002% and Cr is less than 0.01%, the effect of fixing C and N becomes too small, and it becomes difficult to obtain the desired effect. On the other hand, when Ti, Nb, and V exceed 0.1% and Cr exceeds 0.5%, the microstructure becomes finer and the hardness of the hot rolled sheet increases, and the solid solution carbon decreases and the transformation point further increases. As a result, the structure of the hot-rolled sheet is made non-uniform, so that cold rolling properties are hindered.

Furthermore, in the present invention, it is important to define the texture of the hot-rolled sheet in order to realize stable cold rolling. That is, it is important that the texture in the width direction end region and the width direction internal region of the steel sheet satisfies the range X of 0.5 to 1.0 defined by the following formula (1).
X = F1 / (F2 + F3) (1)
However,
F1: ODF strength of {001} <110>
F2: {211} <110> ODF strength
F3: ODF strength of {111} <112> Here, the texture in the width direction end region of the steel sheet is 50 mm in the width direction from the edge in the width direction of the steel sheet, that is, 50 mm in the center in the width direction. The texture at the depth of 1/4 of the sheet thickness is represented by the texture, and the texture of the inner area in the width direction is a position of 1/4 of the sheet width from the edge in the width direction to the center in the width direction ( Hereinafter, it is represented by a texture at a depth of 1/4 of the plate thickness from the plate surface.

The above ODF (orientation determination function) is a three-dimensional orientation density function, which is obtained from a pole figure of three or more planes by a series expansion method or the like, and a specific crystal orientation specified by three Euler angles is relative to a random orientation. It shows how much it accumulates.
That is, {001} <110> is a crystal orientation that is small in work hardening accompanying cold working and is advantageous for improving cold rolling properties, and in that sense, it is important to accumulate a large amount. However, if the ODF strength of {001} <110> is extremely high, it is a case where the degree of accumulation has increased due to rolling under conditions that make rolling properties unstable, such as ferrite region rolling. It is not preferable from the viewpoint of rollability.

On the other hand, {211} <110> and {111} <112> have a relatively large work-hardening when cold-worked, so it is not preferable to increase their accumulation. However, since these orientations are difficult to generate in the ferrite zone rolling, the accumulation becomes extremely low. In that sense, {211} <110> and {111} <112> are orientations indicating the stability of cold rolling (the higher the accumulation, the higher the stability).
In this way, F1, F2 and F3 have the property that the numerical value appears oppositely as an index of the magnitude of work hardening and as an index of unstable zone rolling, and the ratio X = F1 / ( When the cold rolling property was evaluated by F2 + F3), the necessity of X: 0.5 to 1.0 was found. That is, when X is less than 0.5, work hardening is large, and the rolling load in the latter stage of rolling becomes high. On the other hand, if it exceeds 1.0, the structure becomes non-uniform in the sheet thickness direction and the sheet width direction, making it difficult to control cold rolling.

Here, in order to realize the above-mentioned texture definition, it is advantageous to adjust the finish rolling temperature according to the contents of C and Mn, which will be described in detail later.
In addition, the structure of the hot-rolled steel sheet of the present invention is a ferrite single phase or a ferrite-pearlite phase.

Next, the manufacturing conditions of the above hot-rolled sheet will be described.
A steel material having the above-described composition, such as a slab, is manufactured by hot rolling. At that time, it is necessary to perform hot rolling finish rolling under the following conditions.

[Temperature difference between the center in the width direction of the steel sheet and the area from the edge in the width direction to 50 mm in the width direction on the entry side of finish rolling is within 30 ° C]
In the present invention, since it is intended to provide a hot-rolled sheet as a material for cold rolling capable of stably performing cold rolling, it is necessary to improve the uniformity of the hot-rolled structure. In particular, in a thin steel plate having a low carbon content component composition, the structure changes greatly in the above-described end region due to transformation point cracking. In order to avoid such a situation, it is effective to suppress the temperature drop in the end region. That is, this temperature decrease is caused by the width direction distribution of the entrance side temperature of finish rolling, and a bar heater or the like needs to be used as appropriate in order to suppress the temperature decrease in the end region. When the temperature difference between the center of the width (representative part of the inner area in the width direction in the temperature control and mechanical properties) and the end area becomes lower than 30 ° C, the finish rolling exit temperature between the end area and the center of the width (FDT) cannot be maintained within the proper range.

  Note that the temperature difference between the center of the width and the end region varies somewhat in the longitudinal direction of the coil, so temperature measurement is performed at three positions of the 5 m long portion of the front end portion, the central portion and the rear end portion in the coil longitudinal direction. Then, temperature control is performed with the average value thereof. Moreover, the temperature measurement object is the surface of the steel sheet, and any temperature display including the finishing temperature in the following manufacturing conditions is the temperature of the steel sheet surface.

[Finish rolling delivery temperature: 870 ° C to 930 ° C]
The finish rolling outlet temperature is an important condition for controlling the structure of the hot rolled sheet. When the finish rolling exit temperature is less than 870 ° C., transformation point cracks occur in the width direction of the steel sheet, particularly in the end region, and the microstructure and crystal orientation become nonuniform. If the finish rolling exit temperature is 870 ° C or higher, a uniform microstructure can be obtained, but if it exceeds 930 ° C, scale defects are likely to occur, and the surface quality is adversely affected.
Further, in order to obtain a hot rolled sheet texture satisfying the X defined by the above formula (1) in the range of 0.5 to 1.0, finish rolling out according to the steel sheet composition (particularly the contents of C and Mn). Adjust the side temperature more appropriately within the range of 870 ° C to 930 ° C. For example, if the content of C and Mn is low, the Ar ₃ transformation temperature becomes high. Therefore, if the finish rolling exit temperature is not increased, Ar ₃ transformation point cracking occurs, and the above X may exceed 1.0.
On the other hand, when rolling is performed immediately above the Ar ₃ transformation point, a transformation texture based on the ferrite transformation of austenite rolled in the non-recrystallized region develops, and the above X may be less than 0.5. The transformation texture tends to increase as the Mn content increases, and the F2 texture ({211} <110>) in the transformation texture increases as the C content increases. Therefore, it is necessary to make the content of Mn and C not more than the upper limit of the present application.

[Cooling starts within 1 second after hot rolling]
In order to produce a uniform structure by ferrite transformation after hot rolling, it is necessary to promote ferrite transformation and to start cooling within 1 second after rolling. When the time until the start of cooling exceeds 1 second, the austenite grains before transformation are recrystallized and grains grow, and the ferrite transformation is delayed in part. Although the cooling rate here does not need to be specified, it is preferably performed according to the conditions described later.

[Average cooling rate from finish rolling exit temperature to 750 ℃ is 10 ℃ / s or more]
It is necessary to start cooling within 1 second after hot rolling to avoid staying in a high temperature region, and to cool the temperature region up to 750 ° C. at 10 ° C./s or more. This is because if the average cooling rate up to 750 ° C. is less than 10 ° C./s, the ferrite transformation may be partially uneven and the material may be uneven.

[Winding the steel sheet in the temperature range of 550 ℃ to 700 ℃]
In order to suppress solute C and N in the winding cooling process after hot rolling, it is necessary to promote precipitation of cementite and AlN. This precipitation becomes insufficient when the coiling temperature is lower than 550 ° C., and the hot-rolled sheet is hardened by the solute C and N. On the other hand, if it exceeds 700 ° C., the particle size becomes non-uniform, and since the driving force for precipitation of cementite is small and solid solution C may remain in the carbon amount range of the present invention, it is set to 700 ° C. or less. .

  Using steel melted in the composition shown in Table 1, hot rolling was performed according to the conditions shown in Table 2 to produce a hot-rolled steel sheet having a plate thickness of 2 mm and a plate width of 800 mm. The hot rolled steel sheet thus obtained was examined for microstructure and tensile properties. The tensile test was performed according to JIS Z2241 using a JIS No. 5 test piece.

  Further, the X value of the hot-rolled steel sheet was also measured as follows. That is, at the position of 50 mm from the end in the width direction of the hot-rolled sheet and 1/4 of the sheet width from the same end, a 1/4 part of the plate thickness is exposed from the steel piece punched out with an area of 30 mmφ. After that, the exposed surface was corroded with a nital solution until the macro structure was confirmed, and pole figures were created by reflection method for the three surfaces (110), (220) and (211). The above ODF was calculated from the figure by the series expansion method.

  Furthermore, the obtained hot-rolled steel sheet was subjected to cold rolling to evaluate cold rollability. Here, the cold rolling property is evaluated by the rolling load by the yield strength in the rolling with a reduction ratio of 95%, and the rolling variability is evaluated by the ratio of the yield strength in the end region to the central portion of the steel plate width. Went.

  From Table 2, all the hot-rolled steel sheets according to the present invention have a low yield strength at 95% rolling of less than 830 MPa, a small cold-rolling load, a small strength fluctuation in the width direction, and excellent cold rolling properties. I understand that.

Claims (3)

In mass% C: 0.015-0.035%,
Si: 0.2% or less,
Mn: 0.05 to 0.35%,
P: 0.02% or less,
S: 0.02% or less,
Al: 0.01 to 0.1% and N: 0.005% or less, with the balance being the component composition of Fe and inevitable impurities, from the width direction edge to the width direction of the steel sheet up to 50 mm and from the width direction edge The texture at the position of 1/4 of the sheet width at the depth of 1/4 of the sheet thickness from the sheet surface satisfies that X defined by the following formula (1) is in the range of 0.5 to 1.0. Hot rolled steel sheet.
X = F1 / (F2 + F3) (1)
However,
F1: ODF strength of [001] <110>
F2: ODF strength of [211] <110>
F3: ODF strength of [111] <112> The component composition is further in mass% B: 0.0003 to 0.0030%,
Ti: 0.002 to 0.1%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.1% and
Cr: 0.01-0.5%
The hot-rolled steel sheet according to claim 1, comprising one or more selected from among the above. When hot rolling a steel material having the component composition described in claim 1 or 2, on the entry side of finish rolling, a central portion in the width direction of the steel sheet and a region from the edge in the width direction up to 50 mm in the width direction; The temperature difference between the finishing rolling is set to within 30 ° C., the finish rolling exit temperature is set to 870 ° C. to 930 ° C., cooling is started within 1 second after the hot rolling, and the average from the finish rolling exit temperature to 750 ° C. The method for producing a hot-rolled steel sheet according to claim 1 or 2, wherein the cooling rate is 10 ° C / s or more, and then the steel sheet is wound in a temperature range of 550 ° C to 700 ° C.