JP2019183267A - Hot rolled steel sheet excellent in scale adhesion and manufacturing method therefor - Google Patents

Abstract

To provide a hot rolled steel sheet having excellent scale adhesion uniformly in a longer direction even in a hot rolled steel sheet with larger sheet thickness, and a manufacturing method therefor.SOLUTION: There is provided a hot rolled steel sheet excellent scale adhesion uniformly having a scale layer on a surface and having a prescribed component composition, the scale layer has a magnetite layer from a ground iron side, a magnetite particle and/or an eutectoid transformation structure of iron and magnetite over the magnetite layer, the average particle diameter of the magnetite and/or average block size of the eutectoid transformation structure is 3 μm to 8 μm, and mass fraction of wustite contained in the scale layer is 10% or less.SELECTED DRAWING: None

Description

  The present invention relates to a hot-rolled steel sheet excellent in scale adhesion, and a method for producing the same, used for automobiles, home appliances, building materials, and the like, and particularly suitable as a material for building machine parts subjected to laser cutting. The present invention relates to a thick hot-rolled steel sheet having excellent scale adhesion and a method for producing the same.

  Usually, a scale (black skin) inevitably forms on the surface of a hot-rolled steel sheet. When this scale-attached black skin hot-rolled steel sheet is subjected to temper rolling, bending, press molding, laser cutting, etc., part of the scale peels off, resulting in processing defects, processing line contamination, and post-processing. This may cause surface defects in the product. In order to avoid such a situation, a hot-rolled steel sheet with excellent scale adhesion on the surface of the steel sheet has been demanded, and the demand is increasing.

  In addition, the scale of the hot-rolled steel sheet has a side face that the larger the thickness of the hot-rolled steel sheet, the greater the strain generated in the scale during deformation and the easier it is to peel off, and the degree of processing such as bending and press molding is large. It becomes easier to peel off during molding. On the other hand, the needs for black-skin hot-rolled steel sheets with a large thickness have been increasing in recent years. For example, for hot-rolled steel sheets with a thickness greater than 5.0 mm, there is a strong demand for improved scale adhesion. It has been.

  Furthermore, if the adhesion of the scale is uneven in the longitudinal direction of the steel sheet and there is a part with poor adhesion, it must be used except for that part. Therefore, from the viewpoint of improving yield, there is a strong demand for a hot-rolled steel sheet having a uniform scale in the longitudinal direction and excellent scale adhesion.

The scale structure of the hot-rolled steel sheet is composed of a fine magnetite (Fe ₃ O ₄ ) layer (hereinafter referred to as magnetite seam) generated by recrystallization from the interface between the base iron and the scale in order from the base iron side. Magnetite layer generated from the interface and iron (Fe) and magnetite eutectoid transformation structure layer formed by eutectoid transformation of wustite (FeO), magnetite layer grown from scale surface layer, and hematite (Fe ₂ O ₃ ) layer . A wustite layer that remains untransformed at room temperature may be formed.

  Conventionally, various proposals have been made for improving the adhesion of the scale. For example, in Patent Document 1, in mass%, C: 0.01 to 0.4%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0%, P: 0.05 % Or less, S: 0.05% or less, Al: 0.3% or less, N: 0.01% or less, the slab having a component composed of Fe and inevitable impurities is heated and rough-rolled Then, finish rolling is started at 1100 ° C. or lower, finished at 1000 ° C. or lower, the temperature range from the finish rolling finish temperature to 800 ° C. is cooled at 80 ° C./s or more, and wound at a temperature range of 450 ° C. to 550 ° C. The thickness of the scale formed on the surface of the steel sheet is 20 μm or less by taking out, cooling to a temperature range of 400 to 450 ° C. within 60 minutes after winding, and holding in the temperature range of 400 to 450 ° C. for 90 minutes or more. Steel plate and wustite, magnetite and hemata The ratio of the length in the rolling direction of the interface where the ground iron and the magnetite are in contact to the length in the rolling direction of the interface where the scale having the contact is 80% or more, and the average particle size of the magnetite Has been proposed, which is a hot-rolled steel sheet excellent in scale adhesion, characterized by being 3 μm or less.

  In Patent Document 2, as mass%, C: 0.02 to 0.20%, Mn: 0.1 to 2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03-0.3%, Cu: 0.04-0.5%, Cr: 0.03-0.3%, the balance Fe and inevitable impurities After heating the slab made of 1100 ° C. or higher, the hot rolling is finished in the temperature range of 800 to 950 ° C., and the surface roughness of the steel plate surface scale and the steel plate interface is obtained by scraping at 400 ° C. to 650 ° C. For example, a hot-rolled steel sheet having excellent tight-scale properties has been proposed in which the number of irregularities of 0.5 μm or more per inch is 300 or more.

  Further, in Patent Document 3, in mass%, C: 0.3% or less, Si: 0.1% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.02% Hereinafter, in a finish rolling process of a hot rolling process, a dew point is 50 ° C. or higher in a finish rolling process for a steel material having a composition including Al: 0.10% or less and the balance of Fe and inevitable impurities. It includes an oxidation treatment that is held for 1.0 to 10 seconds in the atmosphere, and finish rolling finish temperature: 700 to 900 ° C. After the finish rolling is finished, it is cooled and wound at a winding temperature of 450 to 650 ° C. According to the process, a hot-rolled steel sheet excellent in scale adhesion has been proposed, wherein the scale layer includes pores of 0.10 to 3.0% by area ratio and has a thickness of 10 μm or less. .

Japanese Patent No. 5799913 Japanese Patent No. 4153734 Japanese Patent No. 5679112

  In the technique described in Patent Document 1, the temperature range from the finish rolling to 800 ° C. is cooled at 80 ° C./s or more, and the thickness of the scale is controlled to 20 μm or less, but the temperature range of 800 ° C. or less. The cooling rate is not described, and the scale thickness, the scale composition, the average particle diameter of the magnetite, and the like vary depending on the subsequent cooling conditions, and it may be difficult to obtain stable scale adhesion. In addition, the evaluation of scale adhesion is evaluated by the scale peeling state of the inner periphery of the bend after performing a 90-degree bending test under the condition that the bending radius is 25 mm, and in the examples, the evaluation is performed with a plate thickness of 2 to 5 mm. However, when the thickness of the hot-rolled steel sheet becomes larger, the processing of the inner periphery of the bending becomes more severe, and there is a concern that the scale adhesion may be lowered. Moreover, when the plate | board thickness of a hot-rolled steel plate is large, since a deformation | transformation amount becomes large in a bending outer peripheral part compared with a bending inner peripheral part, we are anxious about the fall of scale adhesiveness. Furthermore, no mention is made of improving the adhesion of a uniform scale in the longitudinal direction.

  The technique described in Patent Document 2 is a hot-rolling rolling of steel to which a predetermined amount of Ni, Cu, and Cr is added, and the tightness is controlled by controlling the surface roughness of the steel plate surface scale and the steel plate ground iron interface to a predetermined range. A hot-rolled steel sheet excellent in resistance has been proposed. However, although the adhesion at the interface between the scale layer and the ground iron is improved, there is a concern that scale peeling occurs in the scale surface layer or inside the scale, resulting in a decrease in adhesion. Moreover, when the plate | board thickness of a hot-rolled steel plate becomes larger, we are anxious about the fall of the adhesiveness of a scale. Furthermore, no mention is made of improving the adhesion of a uniform scale in the longitudinal direction.

  In the technique described in Patent Document 3, in order to increase the dew point in the finish rolling mill, it is necessary to carry out an oxidation treatment in which water is sprayed or water that is in contact with the surface of the hot-rolled sheet is steamed. However, it is difficult to stably perform such processing, and there is a concern about a decrease in manufacturing stability. Moreover, when the plate | board thickness of a hot-rolled steel plate becomes larger, we are anxious about the fall of the adhesiveness of a scale. Furthermore, no mention is made of improving the adhesion of a uniform scale in the longitudinal direction.

  An object of the present invention is to solve the above problems and to provide a hot-rolled steel sheet having excellent scale adhesion uniformly in the longitudinal direction and a method for producing the same even in a hot-rolled steel sheet having a larger thickness.

  The present inventors diligently studied a method for solving the above problems. As a result, using a steel material having a predetermined composition, by adjusting the finish rolling exit temperature during hot rolling, the cooling rate after rolling, and the coiling temperature, the magnetite on the base iron side in the scale layer is adjusted. It was found that the average particle size of the magnetite grains in the upper layer of the layer and / or the average block size of the eutectoid transformation structure of iron and magnetite were optimized, and the adhesion of the scale was improved. Further, in order to improve the adhesion of the scale, it is more preferable to control the finish rolling entry temperature during hot rolling or the temperature difference between the finish rolling entry temperature and the exit temperature. Furthermore, the knowledge that the adhesiveness of a scale improves uniformly in a longitudinal direction by controlling the temperature of the longitudinal direction in the steel plate immediately after finish rolling was acquired.

The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
[1] A hot-rolled steel sheet having a scale layer on the surface, wherein the hot-rolled steel sheet is in mass%, C: 0.01 to 0.3%, Si: 0.20% or less, Mn: 0.01 -2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01-2.0%, the balance being Fe and inevitable impurities Having a composition consisting of
The scale layer has a magnetite layer from the ground iron side, and a magnetite grain and / or a eutectoid transformation structure of iron and magnetite in the upper layer of the magnetite layer, and an average particle diameter of the magnetite grain and / or the eutectoid transformation A hot-rolled steel sheet excellent in scale adhesion, wherein the average block size of the structure is 3 μm or more and 8 μm or less, and the mass fraction of wustite contained in the scale layer is 10% or less.
[2] The scale according to [1], further comprising at least one of Cu: 1.0% or less and Ni: 0.50% or less as a component composition by mass% Hot rolled steel sheet with excellent adhesion.
[3] As a component composition, further, by mass, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01 % Or less, Sb: One or more of 0.03% or less is contained, The hot rolled steel sheet having excellent scale adhesion according to [1] or [2].
[4] A method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein the steel material is roughly rolled, descaled, and finish rolling temperature: 800 to 950 ° C. Then, finish rolling satisfying the following expression (1) is performed, and after cooling at an average cooling rate of 30 ° C./s or more from the finish rolling to the start of winding, winding at a winding temperature of 430 to 580 ° C. A method for producing a hot-rolled steel sheet having excellent scale adhesion, which is characterized.
| T ₂ −T ₁ | ≦ 50 ° C. and | T ₃ −T ₂ | ≦ 50 ° C. (1)
However, in the above equation (1),
T ₁ : Temperature (° C.) of 30 m from the front end in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₂ : Temperature (° C.) of the center in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₃ : Temperature (° C.) of 30 m from the tail end in the longitudinal direction and the center in the width direction in the steel sheet after finish rolling.
It is.
[5] A method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein the steel material is subjected to rough rolling, descaling, and finish rolling temperature: 800 to 950 ° C. Then, finish rolling satisfying the following expression (1) is performed, and after cooling at a mean cooling rate from 3 ° C./s to 80 ° C./s after finishing rolling, a winding temperature: 430 to 580 ° C. A method for producing a hot-rolled steel sheet excellent in scale adhesion, characterized in that it is wound up by a roll.
| T ₂ −T ₁ | ≦ 50 ° C. and | T ₃ −T ₂ | ≦ 50 ° C. (1)
However, in the above equation (1),
T ₁ : Temperature (° C.) of 30 m from the front end in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₂ : Temperature (° C.) of the center in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₃ : Temperature (° C.) of 30 m from the tail end in the longitudinal direction and the center in the width direction in the steel sheet after finish rolling.
It is.

  ADVANTAGE OF THE INVENTION According to this invention, the hot rolled steel plate excellent in scale adhesiveness can be manufactured easily and cheaply, and there exists a remarkable effect on industry. Further, according to the present invention, scale peeling can be prevented, and there is an effect that the surface quality of the product is improved, the processing defect of the product is prevented, and the working environment is greatly improved. Furthermore, problems such as a decrease in the adhesion of the scale accompanying an increase in the thickness of the hot-rolled steel sheet and a variation in the adhesion in the longitudinal direction can be solved.

  In addition, the plate | board thickness of the hot-rolled steel plate in this invention shall be 2.0 mm or more and 25 mm or less, Preferably it exceeds 2.0 mm and 25 mm or less, More preferably, you may be 5.0 mm and 25 mm or less.

  Hereinafter, the hot-rolled steel sheet and the method for producing the same according to the present invention will be described in detail. Note that “%”, which is a unit of content of the component composition, means “mass%” unless otherwise specified.

1) Component composition C: 0.01 to 0.3%
C is an element useful for ensuring strength. If the amount is less than 0.01%, the effect of securing the strength is small, so the C amount is 0.01% or more. If the C content exceeds 0.3%, CO gas is generated at the interface between the scale and the base iron, causing peeling of the scale during rolling and causing scale flaws. And From the viewpoint of scale adhesion, it is preferably 0.2% or less.

Si: 0.20% or less Si is an element that acts as a deoxidizing material. In order to obtain such an effect, it is preferable to contain 0.01% or more. However, when Si content exceeds 0.20%, Si is concentrated at the interface between the scale and the ground iron, and a Si oxide layer is formed. Scale separation is likely to occur at the interface between the Si oxide layer and the scale layer formed thereon. For this reason, the amount of Si shall be 0.20% or less. Preferably, it is 0.10% or less.

Mn: 0.01 to 2.0%
Mn is an element that is effective in improving strength in addition to detoxifying the solid solution S that causes embrittlement during hot working as MnS. If the amount is less than 0.01%, the effect is small. On the other hand, if the content exceeds 2.0%, the toughness tends to be lowered. For this reason, the amount of Mn is made 0.01 to 2.0%. Preferably, it is 0.05 to 1.0%.

P: 0.10% or less P is an element that is desirably reduced as much as possible because it adversely affects grain boundary embrittlement. Further, P forms a very brittle oxide layer at the interface between the scale and the ground iron, and reduces the adhesion of the scale. If the amount of P exceeds 0.10%, these adverse effects increase, so the content is made 0.10% or less. Preferably it is 0.05% or less.

S: 0.10% or less S is an element that significantly deteriorates hot workability and toughness. In addition, S is concentrated at the interface between the scale and the ground iron to reduce the adhesion of the scale. If the amount of S exceeds 0.10%, these adverse effects increase, so the content is made 0.10% or less. Preferably it is 0.05% or less.

Al: 0.10% or less Al is an element that acts as a deoxidizer. The Al content may be 0.00%, but in order to obtain such an effect, it is preferable to contain 0.01% or more. On the other hand, when it contains exceeding 0.10%, an oxide type inclusion will increase and a cleanliness will fall. For this reason, the amount of Al is made 0.10% or less. Preferably it is 0.06% or less.

Cr: 0.01 to 2.0%
Cr has the effect of increasing strength, hardenability, and corrosion resistance. In addition, Cr is concentrated at the interface between the scale and the base iron, and the scale bites into the base iron due to the unevenness of the interface, thereby improving the adhesion of the scale. When the Cr content is less than 0.01%, the above effect is small. On the other hand, when the content exceeds 2.0%, the above effect is saturated, so the Cr content is 0.01 to 2.0%. . Preferably, it is 0.01 to 1.0%, more preferably 0.07 to 1.0%, still more preferably 0.12 to 1.0%.

  The said chemical component is an essential component of the hot-rolled steel plate of this invention. In addition to the above chemical components, the hot-rolled steel sheet of the present invention contains at least one of Cu: 1.0% or less and Ni: 0.50% or less as necessary for improving various properties. It may be contained.

Cu: 1.0% or less Cu is an element that concentrates at the interface between the scale and the base iron to promote grain boundary oxidation and enhance the adhesion of the scale. In order to acquire such an effect, it is preferable to contain 0.01% or more of Cu. However, if the content exceeds 1.0%, the grain boundary oxidation becomes excessively significant, and this promotes the unevenness of the scale-base metal interface. In addition, the adhesion of the scale is excessively increased, and the scale cannot be completely removed by descaling after rough rolling. As a result, the remaining scale is pushed into the scale by subsequent finish rolling, which causes surface defects. For this reason, when it contains Cu, it is 1.0% or less.

Ni: 0.50% or less Ni, like Cu, is an element that concentrates at the interface between the scale and the base iron to promote grain boundary oxidation and enhance the adhesion of the scale. In order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. However, if the amount of Ni exceeds 0.50%, grain boundary oxidation becomes excessively significant, and this promotes unevenness at the interface between the scale and the base iron. In addition, the adhesion of the scale is excessively increased, and the scale cannot be completely removed by descaling after rough rolling. As a result, the remaining scale is pushed into the scale by subsequent finish rolling, which causes surface defects. For this reason, when it contains Ni, it is 0.50% or less.

  In the present invention, if necessary, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, Sb: One or more of 0.03% or less may be contained.

Mo: 1.0% or less Mo has an effect of improving strength and hardenability and suppressing softening associated with tempering. In order to acquire such an effect, it is preferable to contain Mo 0.1% or more. On the other hand, if the content exceeds 1.0%, the strength is excessively increased and the toughness and formability deteriorate, so when Mo is contained, the amount is made 1.0% or less.

Nb: 0.1% or less Nb is an element that improves the strength and toughness of the base material, and in order to obtain such effects, it is preferably contained in an amount of 0.003% or more. On the other hand, if the content exceeds 0.1%, the toughness may be lowered. Therefore, when Nb is contained, the amount is made 0.1% or less.

V: 0.1% or less V is an element that improves the strength and toughness of the base material. In order to obtain such an effect, V is preferably contained in an amount of 0.003% or more. On the other hand, if the content exceeds 0.1%, the toughness may be lowered. Therefore, when it contains V, the quantity shall be 0.1% or less.

Ti: 0.03% or less Ti is an element that improves the strength and toughness of the base material, and is effective in securing toughness in the weld heat affected zone. In order to acquire these effects, it is preferable to contain Ti 0.001% or more. On the other hand, if the content exceeds 0.03%, the toughness may be lowered. Therefore, when Ti is contained, the amount is set to 0.03% or less.

B: 0.01% or less B is an element that enhances the hardenability of steel, and this effect can increase the strength. In order to acquire such an effect, it is preferable to contain B 0.0005% or more. On the other hand, when the content exceeds 0.01%, this effect is saturated. Therefore, when B is contained, the amount is made 0.01% or less.

Sb: 0.03% or less Sb has a function of concentrating on the surface layer when the material is heated, and suppressing a decrease in the amount of C in the surface layer during heating. In order to acquire such an effect, it is preferable to contain Sb 0.001% or more. On the other hand, if it exceeds 0.03%, it becomes a liquid metal when the material is heated, erodes the prior austenite grain boundaries, and reduces the adhesion of the scale. For this reason, when it contains Sb, it is 0.03% or less.

  The balance other than the chemical components described above is composed of Fe and inevitable impurities. As inevitable impurities, O: 0.005% or less and Mg: 0.003% or less are acceptable.

2) Microstructure of scale layer The scale layer of the hot-rolled steel sheet of the present invention has a magnetite layer (magnetite seam) from the ground iron side, and magnetite grains and / or an eutectoid transformation structure of iron and magnetite on the upper layer of the magnetite seam. The magnetite seam and the magnetite grain are generated at different positions as described above, and the particle size of the magnetite seam is remarkably finer. Therefore, the magnetite seam and the magnetite grain can be easily distinguished by observation of the structure.

  The average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite is 3 μm or more and 8 μm or less, preferably 3 μm or more and 7 μm or less. When the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite exceeds 8 μm, the adhesion of the scale is reduced. This is because the grain of the magnetite seam directly above the ground iron is fine, so the difference between this fine magnetite and the grain size of the upper layer of the magnetite seam is large, so cracks occur during deformation and the scale tends to peel off. This is probably because of this. In addition, if the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite is less than 3 μm, the adhesion of the scale may decrease due to the increase in the hardness of the scale itself. 3 μm or more.

  In addition to the magnetite seam, the magnetite grains and the eutectoid transformation structure, wustite may remain untransformed at room temperature. Wustite is more brittle than magnetite at room temperature, and the adhesion of the scale is impaired by cracks in the scale. Therefore, the mass fraction of wustite contained in the scale layer is 10% or less. Preferably, it is 7% or less. Further, in addition to wustite, a hematite layer may be formed in the outermost layer portion of the hot-rolled steel sheet. However, since the formation of the hematite layer does not impair the effect of the present invention, it may be contained. Since the hematite layer causes surface defects such as red scale, the mass fraction of hematite is preferably 10% or less.

The mass fraction of wustite and hematite was measured using an X-ray diffractometer, the integrated intensity of the diffraction peak of each phase in the scale using a CoK _α- ray source, and standard samples (Fe, FeO (wustite), It can be obtained by using the following equation (2) from the ratio of the integrated intensity of each phase in the test sample and a mixture of Fe ₂ O ₃ (hematite) and Fe ₃ O ₄ (magnetite) in equal weight.
Phase mass fraction of _{A = (I A / R A} ) × 100 / ((I Fe / R Fe) + (I FeO / R FeO) + (I Fe2O3 / R Fe2O3) + (I Fe3O4 / R Fe3O4)) ... (2)
However, in the above equation (2),
I _A : Integrated intensity of phase A in test sample R _A : Integrated intensity of phase A in standard sample A: Fe, FeO, Fe ₂ O ₃ , or Fe ₃ O ₄
It is.

3) Manufacturing conditions Next, the manufacturing method of the hot-rolled steel sheet of the present invention will be described in detail. In the present invention, a step of heating a steel material such as a slab having the above-described component composition to a predetermined heating temperature (heating step), a step of performing hot rolling consisting of rough rolling and finish rolling to form a hot rolled sheet (rolling step) ), Slab heating temperature of steel unless otherwise specified, manufactured in a process of accelerated cooling of the hot-rolled sheet at a predetermined cooling rate (accelerated cooling process) and a process of winding at a predetermined winding temperature (winding process) The temperatures such as the entrance side and exit side temperatures of the finish rolling, the cooling stop temperature, and the coiling temperature are the surface temperatures and can be measured with a radiation thermometer or the like. The average cooling rate is ((cooling start temperature−cooling stop temperature) / cooling time) unless otherwise specified.

In the present invention, the method for producing a steel material having the above component composition is not particularly limited, and any conventional method can be applied. For example, it is desirable that molten steel having the above-described component composition is melted in a converter, electric furnace, or the like and used as a steel material such as a slab by a casting method such as a continuous casting method. In addition, there is no problem even if it uses the ingot-making-slab rolling method. Usually, a steel material is hot-rolled after being heated. This heating is sufficient as long as the solution is sufficiently dissolved, and the heating is preferably performed to Ac ₃ points or more. Specifically, a normal slab heating temperature range of 1060 ° C to 1300 ° C is appropriate. In the case of a slab manufactured by a continuous casting method, direct feed rolling may be applied in which it is rolled as it is or for the purpose of suppressing a temperature drop.

  A hot rolling process consists of rough rolling and finish rolling. In rough rolling, it is sufficient that the sheet bar has a predetermined size, and the conditions for rough rolling need not be particularly limited. Moreover, since finish rolling is performed at a predetermined temperature, the material to be rolled may be heated by a heating means such as a sheet bar heater in the middle. Before the finish rolling, the scale generated on the surface of the sheet bar is removed by descaling due to high water pressure or the like on the entrance side of the finish rolling mill.

  Next, finish rolling is performed. When the finish rolling entry temperature exceeds 1100 ° C., the thickness of the scale increases and the adhesion of the scale decreases. Therefore, the finish rolling entry temperature is preferably 1100 ° C. or less, more preferably 1050 ° C. or less. To do. On the other hand, if the finish rolling entry temperature is less than 950 ° C., the rolling load is significantly increased, and the productivity may be lowered. In addition, since the thickness on the finish rolling entry side increases as the product sheet thickness increases, for example, when the product sheet thickness exceeds 5.0 mm, it takes a long time to start the finish rolling. It may cause a decrease. Accordingly, the finish rolling entry temperature is preferably 1100 ° C. or less, and more preferably 1050 ° C. or less. Moreover, it is preferable that the minimum of finish rolling entrance temperature shall be 950 degreeC or more.

  The finish rolling exit temperature is 800 to 950 ° C. When the finish rolling outlet temperature is less than 800 ° C., cracks are generated in the scale, and this crack may cause a decrease in the adhesion of the scale. In addition, the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite becomes less than 3 μm, and the scale adhesion may decrease due to the increase in the hardness of the scale itself due to this refinement. . On the other hand, when the finish rolling outlet temperature exceeds 950 ° C., the scale thickness may increase due to the growth of the scale, and the adhesion of the scale may decrease. Further, the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite may exceed 8 μm, and the adhesion of the scale may decrease.

  Moreover, although it is not necessary to specifically limit, it is preferable to control the temperature difference between the finish rolling entry temperature and the finish rolling exit temperature within a predetermined range. The temperature difference between the finishing rolling entry temperature and the finishing rolling exit temperature is mainly controlled by the amount of cooling water in the finishing mill (cooling water amount between stands). If the temperature difference exceeds 180 ° C, the heat of the scale The generation of cracks due to shrinkage becomes significant, and the adhesion of the scale may be reduced. Therefore, the temperature difference between the finish rolling entry temperature and the finish rolling exit temperature is preferably 180 ° C. or less, and more preferably 130 ° C. or less.

Furthermore, finish rolling is performed under conditions that satisfy the following formula (1).
| T ₂ −T ₁ | ≦ 50 ° C. and | T ₃ −T ₂ | ≦ 50 ° C. (1)
However, in the above equation (1),
T ₁ : Temperature (° C.) of 30 m from the front end in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₂ : Temperature (° C.) of the center in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling
T ₃ : Temperature (° C.) of 30 m from the tail end in the longitudinal direction and the center in the width direction in the steel sheet after finish rolling.
It is. T ₁ , T ₂ , and T ₃ are temperatures on the finish rolling exit side at the respective positions.

When | T ₂ −T ₁ | or | T ₃ −T ₂ | exceeds 50 ° C., the scale adhesion in the longitudinal direction varies, and uniform scale adhesion in the longitudinal direction cannot be obtained.

  In addition, control of the temperature of the longitudinal direction in the steel plate after finish rolling (control of Formula (1)) can be performed by measuring finish rolling entrance temperature and adjusting the amount of cooling water in the finish rolling mill.

  From finish rolling to winding, the temperature is 3 ° C./s or more and 80 ° C./s or less, and cooling is preferably performed at an average cooling rate of 30 ° C./s or more. If the average cooling rate is less than 3 ° C./s, the scale thickness may increase due to the growth of the scale, and the adhesion of the scale may decrease. In addition, the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite may become coarse particles, which may reduce the adhesion of the scale. On the other hand, when the average cooling rate exceeds 80 ° C./s, the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite becomes fine and the scale adhesion decreases. There is a case. Accordingly, the average cooling rate from finish rolling to the start of winding is 3 ° C./s or more and 80 ° C./s or less. More preferably, it is 5 ° C./s or more and 50 ° C./s or less.

  Further, the cooling after finish rolling is preferably performed at an average cooling rate of 45 ° C./s or more in a temperature range of 800 ° C. to 700 ° C. and a temperature range of 700 ° C. to 600 ° C. of 35 ° C./s or more. This is because the scale grows faster in the high temperature region, and the adhesion of the scale decreases as the scale thickness increases.

  After the cooling, the steel sheet is wound at a winding temperature of 430 to 580 ° C. When the coiling temperature is less than 430 ° C., the eutectoid transformation of the scale layer does not occur sufficiently, and wustite remains excessively at room temperature, and the adhesion of the scale decreases. When the coiling temperature exceeds 580 ° C., the average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite becomes larger than 8 μm, and the adhesion of the scale decreases. . Therefore, the coiling temperature is 430 to 580 ° C., preferably more than 450 ° C. and 540 ° C. or less.

  In addition, it is preferable to insert the coil after winding into a coil box, or to cover the coil and cut off from the atmosphere to suppress oxidation of the outermost periphery and the edge portion.

  Furthermore, in a hot rolled steel sheet wound up in a coil shape, the steel sheet may be deformed by a roller leveler, a tension leveler, or the like to perform shape correction processing. For example, in a hot-rolled steel sheet having a thickness of 12 mm, two upper rolls and three lower rolls with a diameter of 250 mm may be arranged, and the shape correction process may be performed under the condition of a pushing amount of 2 mm.

  Steel having the composition shown in Table 1 was melted and cast to obtain a steel material.

  These steel materials were subjected to hot rolling, accelerated cooling, and winding under the conditions shown in Table 2 to obtain hot rolled coils having a thickness of 3 to 23 mm. The obtained hot-rolled coil was subjected to shape correction treatment by leveler processing, and then cut into a predetermined length to obtain a hot-rolled sheet. A test piece was collected from each part of the obtained hot rolled sheet, and the microstructure and adhesion were evaluated.

  The microstructure is the rolling direction with respect to each part of the collected specimen (30 m from the longitudinal tip and the center in the width direction, the center in the longitudinal direction and the center in the width direction, 30 m from the tail in the longitudinal direction and the center in the width direction). After being embedded in a conductive resin so that the cross-section becomes the surface to be observed, and after polishing, a reflected electron image of the cross-section of the scale was taken using a scanning electron microscope. The observation magnification was 3000 times. Further, the mass fraction of wustite and hematite in the scale was determined by the measurement method by X-ray diffraction. When the scale layer has a magnetite layer (magnetite seam) from the ground iron side, a magnetite grain and / or a eutectoid transformation structure of iron and magnetite in the upper layer of the magnetite seam, and the mass fraction of wustite is 10% or less, A When there is no magnetite seam or when the mass fraction of wustite exceeds 10%, it is shown in B and Table 2.

  In addition, the particle size of the magnetite grains in the upper layer of the magnetite seam or the block size of the eutectoid transformation structure of iron and magnetite is identified by the EBSD (Electron Backscatter Diffraction Pattern) method to identify the layer and crystal orientation in the cross-sectional structure of the scale in the rolling direction. The point at which the crystal orientation difference between adjacent measurement points is 15 ° or more is defined as a crystal grain boundary, and the circle equivalent diameter of each crystal grain is measured. In the eutectoid transformation structure of iron and magnetite, the magnetite belonging to the same block shows the same orientation when measured by the EBSD method. By treating the magnetite with this same orientation as one magnetite grain, the eutectoid of iron and magnetite The block size of the metamorphic tissue can be determined. The average particle size of the magnetite grains in the upper layer of the magnetite seam or the average block size of the eutectoid transformation structure of iron and magnetite was calculated by area-averaging the average diameter of each crystal grain in one field of view from the crystal orientation measurement results.

  In addition, specimens were collected from each part of the hot-rolled sheet after leveler processing (30 m from the longitudinal tip and the center in the width direction, the center in the longitudinal direction and the center in the width direction, 30 m from the tail in the longitudinal direction and the center in the width direction). Then, the adhesion of the scale was evaluated. For the evaluation of adhesion, a tape was attached to the surface of the steel sheet, the scale was peeled off, the tape with the scale attached was attached to a transparent sheet, and then captured by a scanner, and the amount of peeling of the scale was measured by image processing. Table 2 shows the case where the peeled area of the scale is 10% or less at all positions and the case where the peeled area exceeds 10%.

  Production conditions and evaluation results are shown in Table 2.

  As shown in Table 2, Invention Example No. 1, 3, 5, 7, 9, 11, 13, 15, 17, 18, 19, 25, and 26 all exhibited excellent scale adhesion at each part in the longitudinal direction.

  In contrast, Comparative Example No. 2 and no. In No. 4, since the finish rolling exit temperature exceeded 950 ° C., the scale particle size exceeded 8 μm and the adhesion of the scale was inferior.

  Comparative Example No. In No. 6, since the finish rolling exit temperature was less than 800 ° C., 10% by mass or more of wustite was generated in the scale layer, and the adhesion of the scale was inferior.

Comparative Example No. Nos. 8 and 10 are portions where the average particle size or average block size of the scale layer exceeds 8 μm because | T ₂ −T ₁ | or | T ₃ −T ₂ | exceeds 50 ° C. Existed.
Comparative Example No. In No. 12, since the average cooling rate after finish rolling was less than 3 ° C./s, the average particle size or average block size of the scale layer exceeded 8 μm, and the adhesion of the scale was inferior.
Comparative Example No. In No. 14, since the winding temperature exceeded 580 ° C., the average particle size or average block size of the scale layer exceeded 8 μm, and the scale adhesion was inferior.
Comparative Example No. In No. 16, since the winding temperature was less than 430 ° C., wustite exceeding 10% by mass was generated in the scale layer, and the adhesion of the scale was inferior.
Comparative Example No. In 20, 21, 22, and 23, any one of C, Si, Cr, Cu, and Ni among the chemical components was out of the scope of the present invention, and thus the adhesion of the scale was deteriorated.
Comparative Example No. In No. 24, since the average cooling rate after finish rolling was higher than 80 ° C./s, the average particle size or the average block size of the scale layer was less than 3 μm, and the adhesion of the scale was inferior.

Claims (5)

A hot-rolled steel sheet having a scale layer on the surface, wherein the hot-rolled steel sheet is in mass%, C: 0.01 to 0.3%, Si: 0.20% or less, Mn: 0.01-2. 0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01 to 2.0%, the balance being Fe and inevitable impurities Have
The scale layer has a magnetite layer from the ground iron side, and a magnetite grain and / or a eutectoid transformation structure of iron and magnetite in the upper layer of the magnetite layer, and an average particle diameter of the magnetite grain and / or the eutectoid transformation A hot-rolled steel sheet excellent in scale adhesion, wherein the average block size of the structure is 3 μm or more and 8 μm or less, and the mass fraction of wustite contained in the scale layer is 10% or less.   The component composition further comprises at least one of Cu: 1.0% or less and Ni: 0.50% or less in terms of mass%. Excellent hot-rolled steel sheet.   As the component composition, in addition, by mass, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, The hot-rolled steel sheet having excellent scale adhesion according to claim 1 or 2, characterized by containing one or more of Sb: 0.03% or less. It is a manufacturing method of the hot-rolled steel plate in any one of Claims 1-3, Comprising: A steel raw material is rough-rolled and descaling is performed, Finishing rolling exit temperature: 800-950 degreeC, following (1 ) After finishing rolling satisfying the formula, after cooling at an average cooling rate from the finish rolling to the start of winding: 30 ° C./s or more, and then winding at a winding temperature: 430-580 ° C. A method for producing a hot-rolled steel sheet having excellent adhesion.
| T ₂ −T ₁ | ≦ 50 ° C. and | T ₃ −T ₂ | ≦ 50 ° C. (1)
However, in the above equation (1),
T ₁ : Temperature (° C.) of 30 m from the front end in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₂ : Temperature (° C.) of the center in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₃ : Temperature (° C.) of 30 m from the tail end in the longitudinal direction and the center in the width direction in the steel sheet after finish rolling.
It is. It is a manufacturing method of the hot-rolled steel plate in any one of Claims 1-3, Comprising: A steel raw material is rough-rolled and descaling is performed, Finishing rolling exit temperature: 800-950 degreeC, following (1 ) After finishing rolling that satisfies the formula, after cooling at an average cooling rate from 3 ° C./s to 80 ° C./s after finishing rolling, winding at a winding temperature of 430 to 580 ° C. A method for producing a hot-rolled steel sheet having excellent scale adhesion, which is characterized.
| T ₂ −T ₁ | ≦ 50 ° C. and | T ₃ −T ₂ | ≦ 50 ° C. (1)
However, in the above equation (1),
T ₁ : Temperature (° C.) of 30 m from the front end in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling.
T ₂ : Temperature (° C.) of the center in the longitudinal direction and the center in the width direction of the steel sheet after finish rolling
T ₃ : Temperature (° C.) of 30 m from the tail end in the longitudinal direction and the center in the width direction in the steel sheet after finish rolling.
It is.