JP2019157207A - Steel plate - Google Patents

Abstract

To provide a steel plate preventing generation of scale flaw and excellent in surface quality by suppressing generation of red scale.SOLUTION: There is provided a steel plate having a scale layer with thickness of 5 to 50 μm on a surface of a base material, in which a chemical composition of the base material has, by mass%, C: over 0.04% and 0.20% or less, Si: 0.1 to 1.0%, Mn: 0.3 to 2.5%, P: 0.01% or less, S: 0.01% or less, Sn: 0.01 to 0.50%, Al: 0.001 to 0.01%, N: 0.0005 to 0.01%, W: 0 to 0.5%, Mo: 0 to 0.5%, Ti: 0 to 0.5%, and the balance: Fe with impurities, the scale layer contains a layer mainly containing magnetite, half width of a peak appearing around 650 cmin a Raman spectroscopic analysis of the layer mainly containing magnetite is 25 to 80, and hue of the steel plate is L=2 to 20, a=-20 to 10, and b=-10 to 20 in a Lab color space.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate, and more particularly, to a steel plate excellent in scale surface properties.

In the hot rolling process of the steel plate, the slab as a raw material is heated to a high temperature region, so that an oxide film called scale is generated on the surface of the steel plate. This scale is mainly composed of iron oxide (Fe ₂ O ₃ , Fe ₃ O ₄ , FeO). At this time, a scale exhibiting an abnormal red color (hereinafter referred to as “red scale”) may be generated in a scale exhibiting normal black color (hereinafter referred to as “black scale”).

  If a red scale exists on the surface of the steel sheet, the design properties are significantly deteriorated. Normally, descaling with high-pressure water is performed to remove the scale, but if the scale does not peel and remains as a red scale, a scale flaw called red scale flaw occurs on the surface of the rolled steel sheet. There are things to do.

  Further, if the red scale is generated non-uniformly on the steel plate surface, a difference occurs between the portion where the red scale is generated and the normal portion in the thickness of the scale generated during the subsequent rolling. As a result, when the steel sheet is cooled after rolling, the cooling rate of the steel sheet varies due to the difference in scale thickness. If the cooling rate becomes uneven, not only will the final mechanical properties of the steel plate vary, but if the temperature difference between the hot and cold parts exceeds a certain limit during the cooling process, This causes a shape defect such as an ear wave or a slack while being cooled down.

  Furthermore, when the steel plate is stored outdoors or in a high humidity environment, the red scale has a small barrier effect of the corrosive environmental factor, and thus corrosion may occur by forming a local battery between the scale and the base material. By forming the local battery between the scale and the base material, the steel material is corroded in a pit shape. Therefore, the base material may become defective during descaling during painting.

For example, Patent Document 1 discloses a structural steel plate having good scale adhesion, having a scale with a thickness of 10 μm or less and a composition ratio of Fe ₃ O ₄ of 70% or more on the surface of the steel plate. In Patent Document 2, the scale thickness is 10 μm or less, and the scale color of the surface layer is L * a * b * colorimetry, 15 ≦ L * ≦ 40, −2 ≦ a * ≦ 2, −5 ≦ b A dark purple hot-rolled steel sheet with * ≦ -3 is disclosed.

Japanese Patent Laid-Open No. 5-195055 JP 2002-327240 A

  By the way, Sn containing steel may be used for the steel plate used in the atmospheric environment containing chloride, and an acidic environment. As a result of investigation by the present inventor, the reason is not clear, but it was found that when Sn is contained in steel, a red scale tends to be generated.

  In Patent Documents 1 and 2, the problem of red scale generation associated with Sn addition has not been studied, and there remains room for improvement.

  An object of this invention is to provide the steel plate which is excellent in the surface property which prevented generation | occurrence | production of the scale flaw by suppressing the production | generation of a red scale.

  The present invention has been made in order to solve the above-described problems, and provides the following steel sheet.

(1) A steel sheet in which a scale layer having a thickness of 5 to 50 μm is formed on the surface of a base material,
The chemical composition of the base material is mass%,
C: more than 0.04% and 0.20% or less,
Si: 0.1 to 1.0%,
Mn: 0.3 to 2.5%
P: 0.01% or less,
S: 0.01% or less,
Sn: 0.01 to 0.50%,
Al: 0.001 to 0.01%
N: 0.0005 to 0.01%,
W: 0 to 0.5%
Mo: 0 to 0.5%,
Ti: 0 to 0.5%,
Balance: Fe and impurities,
The scale layer includes a layer mainly composed of magnetite,
The full width at half maximum of the peak appearing in the vicinity of 650 cm ^{−1 in} Raman spectroscopic analysis of the layer mainly composed of magnetite is 25 to 80,
The color of the steel sheet is L ^* = 2 to 20, a ^* = − 20 to 10, and b ^* = − 10 to 20 in the Lab color space.
steel sheet.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of a red scale can be suppressed, deterioration of the surface property of a steel plate and generation | occurrence | production of a scale flaw can be prevented, and the improvement of a product external appearance and product surface quality can be aimed at.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.

  In the scale layer formed on the surface of the base material, a layer mainly composed of magnetite (also referred to as “magnetite layer” in the following description) is formed. As a result of investigations by the present inventors, it has been found that the crystallinity of magnetite contained in the magnetite layer greatly affects the color tone.

Specifically, as a result of observing the magnetite layer in each region where the red scale and the black scale were generated using Raman spectroscopy, there was a Fe ₃ O ₄ type scale with good crystallinity in the red scale. It was found that there was a Fe ₃ O ₄ type scale with poor crystallinity in the black scale.

  In addition, it is important to optimize the manufacturing conditions in order to suppress the generation of red scale. In particular, the crystallinity of the magnetite in the magnetite layer can be adjusted by strictly controlling the conditions in the finish rolling process.

  The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition of base material Reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: Exceeding 0.04% and 0.20% or less C is an element necessary for securing the strength as a material, and needs to be contained exceeding 0.04%. However, if the content exceeds 0.20%, the weldability is significantly reduced. In addition, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in an environment where the pH is lowered increases, so the corrosion resistance decreases. For this reason, the C content is more than 0.04 and 0.20% or less. The C content is preferably 0.05% or more. Further, the C content is preferably 0.18% or less, and more preferably 0.16% or less.

Si: 0.1 to 1.0%
Si forms iron silicon oxide such as firelite in the scale in a high temperature environment. When the firelight is formed in the scale, not only the adhesion of the scale is improved, but also the scale thickness is prevented from becoming excessive, and as a result, the occurrence of red scale is suppressed. Therefore, the Si content needs to be 0.1% or more. However, if the amount of firelite present in the scale becomes excessive, a large amount of highly crystalline Fe ₃ O ₄ and Fe ₂ O ₃ formed therethrough is formed, resulting in the generation of red scale. . Further, since Si is easily oxidized and reduces the oxygen partial pressure in the scale, if the Si content is excessive, the adhesiveness is lowered. Therefore, the Si content is 0.1 to 1.0%. The Si content is preferably 0.2% or more, and preferably 0.85% or less.

Mn: 0.3 to 2.5%
Mn is an element necessary for increasing the strength of the steel sheet. When the Mn content is less than 0.3%, it is difficult to produce a high-strength steel plate. On the other hand, when the content exceeds 2.5%, it becomes difficult to maintain processability. For this reason, the Mn content is set to 0.3 to 2.5%. The Mn content is preferably 0.4% or more, and preferably 2.0% or less.

P: 0.01% or less P is an element effective for increasing strength and beneficial for improving corrosion resistance, and has been conventionally used for corrosion-resistant steel sheets. However, if the P content exceeds 0.01%, slab embrittlement (cracking) occurs during the production of the steel sheet. Therefore, the P content is 0.01% or less. The P content is preferably 0.008% or less.

S: 0.01% or less S combines with Mn to form MnS which is a sulfide. This sulfide is easily deformed, is stretched by rolling and exists in the steel material, and deteriorates the bendability and workability of the steel material. In particular, in high-strength steel materials, the S content needs to be reduced as much as possible in order to increase crack sensitivity. Therefore, the S content is 0.01% or less. The S content is preferably 0.008% or less.

Sn: 0.01 to 0.50%
Sn is an element having an effect of greatly improving the corrosion resistance in an atmospheric environment and an acidic environment containing chloride. In order to obtain these effects, the Sn content needs to be 0.01% or more. In a high temperature environment, when Sn is contained in the base material, a scale containing Sn oxide is generated on the surface of the steel material. As a result, Sn is concentrated at the base material interface in the scale, which becomes a factor of reducing the adhesion of the scale. Further, when Sn is contained in the base material, a red scale tends to be easily generated. Therefore, the Sn content is set to 0.01 to 0.50%. In consideration of the compatibility between corrosion resistance and prevention of red scale, the Sn content is preferably 0.05% or more, and preferably 0.20% or less.

Al: 0.001 to 0.01%
Al is an element that improves the corrosion resistance of steel. Moreover, like P, it has the effect of suppressing the formation of firelight and consequently preventing the occurrence of red scale. In order to obtain these effects, the Al content needs to be 0.001% or more. On the other hand, when the Al content exceeds 0.01%, the above effect is saturated. Therefore, the Al content is 0.001 to 0.01%. In addition, when a large amount of Al is contained, not only the steel sheet is easily embrittled but also the corrosion resistance is deteriorated. Therefore, the Al content is preferably 0.008% or less.

N: 0.0005 to 0.01%
N dissolves as ammonia and has an effect of improving the corrosion resistance of the steel sheet in a salt environment by suppressing the pH drop due to the hydrolysis of Fe ^{3+ in} an environment where the amount of incoming salt is large. In order to obtain this effect, the N content needs to be 0.0005% or more. On the other hand, when the N content exceeds 0.01%, not only the effect is saturated, but also the toughness of the steel sheet is deteriorated. Therefore, the N content is set to 0.0005 to 0.01%.

W: 0 to 0.5%
Mo: 0 to 0.5%
Ti: 0 to 0.5%
Since W, Mo, and Ti all have an effect of improving the corrosion resistance, they may be contained as necessary. However, if the content of these elements exceeds 0.5%, the effect is saturated and the cost increases, so the content of each element is set to 0.5% or less. In order to obtain the effect of improving the corrosion resistance, it is preferable to contain 0.01% or more of one or more selected from the above elements. When two or more elements are contained, the total content is preferably 1.0% or less.

  The base material of the steel sheet according to the present invention has a chemical composition composed of the above-described elements from C to Ti, the remainder Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

(B) Scale layer As described above, the scale layer formed on the surface of the base material includes a magnetite layer. The magnetite layer in the black scale, which is a normal scale, has a relatively broad Fe ₃ O ₄ type scale having poor crystallinity, whereas the magnetite layer in the red scale, which is an abnormal scale, is present. Has an Fe ₃ O ₄ type scale with good crystallinity.

When Raman spectroscopic analysis was performed using various materials, the half-value width of the peak near 650 cm ⁻¹ characteristic of Fe ₃ O ₄ in the magnetite layer was observed on a scale where the red scale was observed partially or entirely. Was less than 25. On the other hand, at a scale where no red scale was observed, the half-value width of the peak near 650 cm ⁻¹ in Raman spectroscopic analysis was 25 or more. In the case of Raman spectroscopic analysis, detection is difficult if the half width of the peak exceeds 80. Therefore, the half width of the peak appearing in the vicinity of 650 cm ⁻¹ in the Raman spectroscopic analysis of the magnetite layer in the present invention is set to 25 to 80.

The scale layer formed on the surface of the base material serves as a lubricant during rolling. If the thickness of the scale is less than 5 μm, the formation of the scale becomes uneven and the role as a lubricant becomes insufficient. On the other hand, when the thickness of the scale exceeds 50 μm, cracking is likely to occur, resulting in the exposure of the steel sheet substrate. Furthermore, the thicker the scale, the easier it is to obtain a Fe ₃ O ₄ type scale with good crystallinity, resulting in the problem of red scale. Therefore, the thickness of the scale layer is 5 to 50 μm.

(C) Color of Steel Plate The color of the steel plate according to the present invention is Lab color space, and L ^* = 2 to 20, a ^* = − 20 to 10, and b ^* = − 10 to 20. By preventing the generation of the red scale, the color of the steel plate can be adjusted to the above range, and a steel plate having excellent design properties can be obtained.

(D) About the plate thickness of a steel plate Although there is no restriction | limiting in particular about the plate | board thickness of the steel plate which concerns on this invention, It is desirable that it is 6-30 mm.

(E) Production conditions Although there is no particular limitation on the production method of the steel sheet according to the present invention, for example, the steel having the chemical composition described above is heated and hot-rolled under the following conditions. Can be manufactured by. This will be described in detail below.

  Steel having the above chemical composition is melted in a converter, electric furnace or the like to produce molten steel. If necessary, processing such as vacuum degassing may be subsequently performed. Then, it is made into a steel slab (slab) by a known method, for example, a continuous casting method or a method of forming a steel ingot and then rolling it into pieces. Moreover, you may use methods, such as what is called strip casting, which manufactures a steel plate directly from molten steel. At this time, the component segregation of the steel ingot increases the dispersion of the carbide particle size, and therefore it is preferable to employ a method of reducing solidification segregation such as electromagnetic stirring under unsolidified region pressure.

  Next, the slab manufactured by the above method is heated in a furnace having a temperature of 1200 ° C. or higher for 1 hour or longer, and rough-rolled in a temperature range of 950 ° C. or higher. Following the rough rolling, rolling is performed at a rolling reduction of 15 to 20%. Descaling is performed at a water pressure of 20 to 50 Pa between the rolling passes. And in the final three passes of finish rolling, rolling is performed at a reduction rate of 5 to 10%, and descaling is not performed. In addition, the holding time between passes in the final three passes is 5 to 10 seconds.

  The finish rolling temperature is 760 to 850 ° C. After finish rolling, cooling is performed at an average cooling rate of 10 to 30 ° C./s up to 350 ° C. Thereafter, the steel sheet is overlaid on the cooling floor or covered with an aluminum heat insulating sheet to cool from 350 ° C. to room temperature at an average cooling rate of 0.1 ° C./s or less. Thereby, generation | occurrence | production of the scale swelling or a crack by heat contraction can be prevented.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel plates having the chemical composition shown in Table 1 were manufactured under the conditions shown in Table 2 to obtain a steel plate having a plate thickness of 6 mm and a plate width of 1000 mm. Test pieces were collected from the above steel plates, and the half width measurement of the scale, the color measurement of the steel plates, and the corrosion test shown below were performed.

<Measurement of half width of scale>
A test specimen having a length of 20 mm, a width of 20 mm, and a thickness of 6 mm was cut out from the steel plate, embedded in a resin, and a micro sample for observing the cross section of the scale layer was produced. Microscopic Raman scattering spectroscopy was performed to identify the magnetite layer in the scale layer. Microscopic Raman scattering spectroscopic analysis was performed using HORIBA Jobin Yvon HR800 LD laser (532 nm). The laser intensity was 80 μW, and the exposure time was 10 seconds or less. The full width at half maximum of the peak near 650 cm ⁻¹ characteristic of Fe ₃ O ₄ was measured from the Raman scattering spectrum.

<Measurement of color of steel sheet>
The color of the steel sheet was measured using a color difference meter CR-5 manufactured by Konica Minolta Co., Ltd., using the above-mentioned test piece before performing the microscopic Raman scattering spectroscopic analysis measurement. And the value in Lab color space was calculated | required by analyzing using color management software CM-S100w by Konica Minolta.

<Corrosion test>
A test piece having a length of 100 mm, a width of 60 mm, and a thickness of 6 mm was cut out from the steel sheet, and then the cut end portion was coated with Neo Gosei made by Shinto Paint Co., Ltd. to be anticorrosive. A scale defect portion having a length of 30 mm and a width of 1 mm was provided in the center of the test piece with a mini milling machine. Thereafter, the test piece is held for 4 hours in a humid environment of 50 ° C. and 100% RH, and then irradiated with a UVA lamp (0.77 W / m ² / nm, 340 nm) in the same environment of 50 ° C. and 100% RH for 4 hours. The cycle test was carried out for 9 cycles and 72 hours in accordance with ISO 11507.

  Subsequently, a salt spray test was performed for 72 h in accordance with ISO 09227 on the test piece after the cycle test was completed. Furthermore, it hold | maintained for 24 hours in the environment of -20 degreeC after that. The above test for a total of one week was regarded as one cycle, and 25 cycles were performed.

  After the corrosion test, the corrosion depth at the scale defect was measured. Moreover, the rusting from other than a defect part was observed and the rusting area was computed by the binarization process.

  As can be seen from Tables 1 to 3, test No. 1 satisfying the provisions of the present invention. In 1-10, while having the outstanding design property, it became a result excellent in corrosion resistance. On the other hand, test No. which is a comparative example. In 11-28, design nature deteriorated by generation | occurrence | production of the red scale. In addition, test No. whose chemical composition deviates from the definition of the present invention. In 23-26, it was a result inferior to corrosion resistance.

  According to the present invention, generation of red scale can be suppressed, product surface property deterioration and generation of scale wrinkles can be prevented, and product appearance and product surface quality can be improved.

Claims (1)

A steel plate in which a scale layer having a thickness of 5 to 50 μm is formed on the surface of a base material,
The chemical composition of the base material is mass%,
C: more than 0.04% and 0.20% or less,
Si: 0.1 to 1.0%,
Mn: 0.3 to 2.5%
P: 0.01% or less,
S: 0.01% or less,
Sn: 0.01 to 0.50%,
Al: 0.001 to 0.01%
N: 0.0005 to 0.01%,
W: 0 to 0.5%
Mo: 0 to 0.5%,
Ti: 0 to 0.5%,
Balance: Fe and impurities,
The scale layer includes a layer mainly composed of magnetite,
The full width at half maximum of the peak appearing in the vicinity of 650 cm ^{−1 in} Raman spectroscopic analysis of the layer mainly composed of magnetite is 25 to 80,
The color of the steel sheet is L ^* = 2 to 20, a ^* = − 20 to 10, and b ^* = − 10 to 20 in the Lab color space.
steel sheet.