JP2011231391A - STEEL SHEET HAVING HIGH Si CONTENT AND EXCELLENT IN SURFACE PROPERTY, AND METHOD FOR PRODUCTION THEREOF - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet having high Si content, excellent in pickling property and having good surface property, which is obtained by suppressing the grain boundary oxidation of a steel sheet surface layer as much as possible, and to provide a useful method for producing such steel sheets having high Si content.SOLUTION: The production method includes, when hot-rolling the high-Si steel sheet in which the chemical component composition is properly controlled, winding the sheet at a temperature of ≥550°C but <750°C, and thereafter cooling to ≤500°C in the atmosphere satisfying the relationship of formula (1): (0.075×T-38)×(-1.5a+3.75)/(2.45)≤10+1.92×10×exp{-15,462/(T+273)}×(0.1566b+0.506)×60/3.6 (wherein 10≤b≤40; a is a value of ratio ([Si]/[Mn]); and T(°C) is a winding temperature).

Description

  The present invention suppresses the grain boundary oxidation of the steel sheet surface layer as much as possible, thereby improving the pickling property and improving the surface properties, and the high Si-containing steel plate. The present invention relates to a useful method for manufacturing.

  In order to manufacture a thin steel plate by hot rolling, a slab is heated in a heating furnace, then rolled to a predetermined thickness by rough rolling and finish rolling, and further on a hot run table in which a water cooling zone (water cooling zone) is arranged. Water-cool to a predetermined temperature and wind in a coil.

  In recent years, in a high-strength steel plate that is widely used mainly for automobile applications, it is common that a relatively large amount of Si is added to ensure strength. It is known that when normal hot rolling is performed on a steel sheet containing a large amount of Si, grain boundary oxidation occurs in the surface layer portion. This grain boundary oxidation occurs at a depth of several μm to several tens of μm, but cannot be removed by ordinary pickling, and the grain boundary oxidized portion of the steel sheet peels off during cold rolling after pickling, and the peeled steel Pushing occurs by the piece, which deteriorates the surface properties of the steel sheet. In addition, problems such as deterioration of workability occur due to microcracks in the grain boundary oxide layer.

  Grain boundary oxidation is often generated or promoted during cooling of the coil, and oxidizable Si is oxidized by oxygen in the atmosphere or scale. Grain boundary oxidation is more likely to occur at higher temperatures, and grain boundary oxidation occurs significantly when held at a high temperature for a long time, such as when the coiling temperature is high or the cooling rate is slow.

  For these reasons, various techniques have been proposed so far in order to suppress grain boundary oxidation. As such a technique, for example, in Patent Document 1, a steel slab in which the contents of C, Si, and Mn are defined is completed by transformation between finish rolling and winding, and is wound at a predetermined temperature. It is disclosed that a hot-rolled steel sheet having excellent surface properties without grain boundary cracking is produced. Patent Document 2 discloses that the yield of hot-rolled steel sheets is controlled by disposing solid carbon on or near the steel material during heating, and heating the steel material at a specific temperature to suppress surface oxidation of the steel material and grain boundary oxidation. Technologies for improving quality have been proposed.

  Patent Document 3 proposes a technique for preventing the occurrence of grain boundary oxidation and preventing the occurrence of ear cracks during the processing of steel sheets by combining the application of an antioxidant to the surface of the steel slab and the coating on the surface of the steel sheet. Has been. Patent Document 4 also proposes a technique of cooling at a cooling rate of 30 ° C./second or more after hot rolling and winding at 450 to 580 ° C. to reduce the grain boundary oxidation depth of the hot rolled steel sheet to 5 μm or less. ing. Furthermore, in Patent Document 5, an alloy steel material containing a predetermined amount of Cr or Mo is heated and subjected to rough rolling, and then the grain boundary oxide layer depth in the hot rolled steel sheet is estimated from the heating and rough rolling conditions. Then, the grain boundary oxide layer depth is set as the necessary scale thickness in the hot-rolled steel sheet, and hot rolling is performed so that the required scale thickness satisfies a predetermined relationship with the finish rolling finish temperature, and then winding is performed. Technologies have also been proposed.

  Various techniques proposed so far employ a means such as manufacturing a steel sheet at a specific cooling rate or winding temperature, or applying an antioxidant. However, as a technique for suppressing grain boundary oxidation caused by Si, such a means is not necessarily sufficient.

JP-A-01-087716 JP-A-62-013520 Japanese Patent No. 1571951 JP 2008-231493 A JP-A-2005-060768

  The present invention has been made paying attention to the circumstances as described above, and the purpose thereof is to suppress the grain boundary oxidation of the steel sheet surface layer as much as possible, thereby making it excellent in pickling properties and surface properties. It is in providing the useful method for manufacturing the high Si content steel plate which becomes favorable, and such a high Si content steel plate.

The production method of the high-Si content hot-rolled steel sheet of the present invention that has achieved the above object is C: 0.02 to 0.30% (meaning mass%; the same applies to the chemical components of steel), Si : 1.0-3.0%, Mn: 1.0-3.5%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%) And Al: 0.15% or less (not including 0%), respectively, and the ratio of the Si content [Si] to the Mn content ([Si] / [Mn]) is 0.5 to 2 0.0, and when hot-rolling a high-Si-containing steel plate, the balance of which is iron and inevitable impurities, after coiling at a coiling temperature of 550 ° C. or more and less than 750 ° C., the water vapor concentration in the atmosphere is b ( % By volume, provided that 10 ≦ b ≦ 40), the ratio ([Si] / [Mn]) is a, and the winding temperature is T (° C.). To have the gist in that cooling to 500 ° C. or less in an atmosphere which satisfies the following relationship (1).
(0.075 × T-38) × (−1.5a + 3.75) ² /(2.45) ² ≦
10 + 1.92 × 10 ⁶ × exp {−15462 / (T + 273)} ×
(0.1566b + 0.506) × 60 ^0.5 /3.6 (1)

  In the manufacturing method of this invention, it is preferable that the said atmosphere is controlled by adding water vapor | steam to air | atmosphere.

  In the present invention, the target steel sheet has the above basic components, but if necessary, (a) Cr: 1.0% or less (excluding 0%), (b) Cu: 0.5% or less (Not including 0%) and / or Ni: 1.0% or less (not including 0%), (c) Ti: 1.0% or less (not including 0%), V: 1.0% or less (Not including 0%) and Nb: one or more selected from the group consisting of 1.0% or less (not including 0%), (d) B: 0.1% or less (not including 0%), (E) Mo: 1.0% or less (not including 0%), (f) Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (including 0%) And the like, and the properties of the steel sheet are further improved depending on the components contained.

  The production method of the present invention basically defines the steps up to hot rolling, but the high-Si content hot-rolled steel sheet obtained by this method is excellent in surface properties by further cold rolling. A cold-rolled steel sheet will be obtained.

  According to the present invention, by appropriately controlling the winding temperature and the cooling atmosphere after winding, the form of the oxide scale layer formed on the surface of the high Si-containing steel sheet is made appropriate, and the grain boundary oxidation is performed. Such a steel sheet is excellent in surface properties.

It is explanatory drawing which showed typically the structure of the scale in a hot-rolled steel plate surface. It is a graph which shows the relationship between the value of ratio ([Si] / [Mn]) and the grain boundary oxidation depth. It is a graph which shows the relationship between coiling temperature and grain boundary oxidation depth. It is a graph which shows the relationship between coiling temperature and inner oxide layer thickness. It is a graph which shows the relationship between a water vapor | steam density | concentration and an inner oxide layer thickness.

The present inventors examined the generation mechanism of grain boundary oxidation occurring on the surface of a high Si content steel sheet from various angles. In the hot rolling (hot rolling) step, a scale having a structure as shown in FIG. 1 (schematic diagram) is formed on the surface of the hot rolled steel sheet. An outer oxide layer composed of iron-based oxides hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and wustite (FeO), and a Si-containing oxide firelite (Fe ₂ SiO ₄ ) A surface scale is formed on the surface of the steel sheet by the inner oxide layer mainly composed of.

As a result of experiments conducted by the inventors to simulate the hot rolling cooling process, oxygen in the atmosphere diffuses (inward diffusion) into the steel sheet through the outer oxide layer and the inner oxide layer, It became clear that Si diffused in the grain boundary on the steel plate surface was oxidized to form a grain boundary oxide layer (see FIG. 1) made of SiO ₂ . In addition, the inward diffusion of oxygen through the inner oxide layer depends on the properties of the inner oxide layer, and the degree of inward diffusion of oxygen depends on the amount of firelite (Fe ₂ SiO ₄ ) in the inner oxide layer. In other words, it was found that when the amount of firelite capable of inhibiting the diffusion of oxygen is small, the inward diffusion of oxygen into the steel sheet increases and the grain boundary oxide layer becomes thick.

Further, according to the study by the present inventors, the amount of firelite generated increases as the value of the ratio of Si content [Si] to Mn content ([Si] / [Mn]) increases. Increasing the ratio ([Si] / [Mn]) increases the ratio of firelite (Fe ₂ SiO ₄ ) and decreasing the ratio ([Si] / [Mn]) decreases Mn ₂ SiO It was also found that the ratio of ₄ would increase.

Firelite (Fe ₂ SiO ₄ ) exists in the form of a dense film near the interface between the steel plate and the scale and inhibits oxygen diffusion. However, Mn ₂ SiO ₄ is formed in a granular form, so that it has a diffusion inhibiting ability. Is low. Therefore, when the value of (ratio [Si] / [Mn]) decreases, a large amount of granular Mn ₂ SiO ₄ is generated, and as a result of impairing the diffusion inhibition by firelite (Fe ₂ SiO ₄ ), a large amount of oxygen diffuses. It is thought that the grain boundary oxidation becomes deep.

Based on the above findings, for various steel sheets with various ratio ([Si] / [Mn]) values, grain boundaries generated in the atmosphere at a coiling temperature of 550 ° C. or higher and lower than 750 ° C. An experimental prediction formula was obtained for the relationship between the oxidation depth and the ratio ([Si] / [Mn]) value. As a result, when the value of the ratio ([Si] / [Mn]) is in the range of 0.5 or more and 2.0 or less, the value a of the ratio ([Si] / [Mn]) and the grain boundary oxidation It is understood that the grain boundary oxidation depth (= inner oxide layer thickness + grain boundary oxide layer thickness) A (μm) including the thickness of the influencing inner oxide layer is expressed by the following equation (2). It was.
A = (0.075 × T−38) × (−1.5a + 3.75) ² /(2.45) ² (2)
However, T represents a coiling temperature (° C.).

  An example of the experimental result that led to the relationship of the above equation (2) is shown in FIGS. In this experiment, a Pt marker layer was formed by sputtering on the surface of various Si-containing steel samples before oxidation treatment, and the Pt marker portion (inward oxidation) was observed by optical microscope observation of the sample cross-sections oxidized at various temperatures. The distance from the outermost surface of the layer) to the tip of the grain boundary oxide layer (the deepest part of the grain boundary oxide layer) is defined as “grain boundary oxidation depth” and plotted for each oxidation condition.

  FIG. 2 is a graph showing the relationship between the ratio ([Si] / [Mn]) value a and the grain boundary oxidation depth, and FIG. 3 is a graph showing the relationship between the coiling temperature and the grain boundary oxidation depth. .

  As is clear from these results, the grain boundary oxidation depth is determined by factors such as the value a of the ratio ([Si] / [Mn]) and the coiling temperature. Therefore, the present inventors control the atmosphere by adding water vapor to the atmosphere in order to reduce the grain boundary oxidation depth determined by the value a of the ratio ([Si] / [Mn]) and the coiling temperature. Further studies were made to form an inner oxide layer by oxidizing the inside of the steel sheet and to scale the grain boundary oxidized portion.

As a result of measuring the atmosphere control by adding water vapor to the atmosphere and the thickness of the inner oxide layer for various high Si content steel sheets having various ratios ([Si] / [Mn]) value a, 550 ° C. or higher, 750 ° C. In the coiling temperature range below, it was found that the inner oxide layer thickness B (μm) can be expressed by the following equation (3) when the water vapor concentration is b (volume%).
B = 1.92 × 10 ⁶ × exp {-15462 / (T + 273)} × (0.1566b + 0.506) × 60 ^0.5 /3.6 (3)
However, T represents a coiling temperature (° C.).

  An example (FIGS. 4 and 5) of the experimental results that led to the relationship of the above expression (3) is shown. In this experiment, a Pt marker layer was formed by sputtering on the surface of various Si-containing steel samples before the oxidation treatment, and the inner oxide layer thickness was determined by optical microscope observation of the sample cross-sections that were oxidized at various temperatures. A plot was made with respect to the oxidation conditions.

  FIG. 4 is a graph showing the relationship between the coiling temperature and the inner oxide layer thickness, and FIG. 5 is a graph showing the relationship between the water vapor concentration and the inner oxide layer thickness.

Next, when the thickness of the grain boundary oxide layer that causes poor pickling was examined, it was found that if the grain boundary oxide layer thickness was 10 μm or more, pickling failure occurred. Therefore, if the grain boundary oxidation depth represented by the above formula (2) is smaller than the thickness of the inner oxide layer represented by the above formula (3) plus 10 μm, that is, the following (1 It was found that the thickness of the grain boundary oxide layer can be made 10 μm or less if the relationship defined by the formula is satisfied, and the present invention has been completed.
(0.075 × T-38) × (−1.5a + 3.75) ² /(2.45) ² ≦
10 + 1.92 × 10 ⁶ × exp {−15462 / (T + 273)} ×
(0.1566b + 0.506) × 60 ^0.5 /3.6 (1)
Where a: ratio [Si] / [Mn] value, b: water vapor concentration, T: coiling temperature (° C.)

  Next, requirements defined in the present invention will be described.

[Ratio ([Si] / [Mn]) Value a: 0.5 to 2.0]
The Si content and the Mn content in the steel sheet are set within a predetermined range in order to exhibit basic characteristics as a steel sheet, but the ratio of these ([Si] / [Mn]) The value a affects the production amount of Fe ₂ SiO ₄ having diffusion inhibition and Mn ₂ SiO ₄ having little diffusion inhibition. When the value a is less than 0.5, since the amount of oxygen diffusing into the steel plate through the surface scale is overwhelmingly larger than the amount of Si diffusing from the inside of the steel plate to the surface, Si is oxidized inside the steel plate. To form a granular oxide (that is, Mn ₂ SiO ₄ ). For this reason, when the value a is less than 0.5, grain boundary oxidation does not occur in the first place. Therefore, the present invention does not cover steel sheets having such a composition. On the other hand, when the value a exceeds 2.0, Fe ₂ SiO ₄ having a high diffusion inhibiting ability is formed thick, so that the grain boundary oxidation is suppressed, but the pickling property is extremely deteriorated. In addition, the preferable minimum of this value a is 0.7 (more preferably 0.8), and a preferable upper limit is 1.5 (more preferably 1.2).

  In carrying out the method of the present invention, hot rolling may be performed according to a conventional method, but the heating temperature when heating the steel slab (slab) is preferably 1000 to 1300 ° C. from the viewpoint of securing the finishing temperature. . Moreover, the finishing temperature of hot rolling is set to a temperature range of 800 to 950 ° C. so that a texture that impairs workability is not formed, and the cooling rate after finishing rolling is 30 to 120 ° C./second in order to suppress the formation of pearlite. It is preferable to be (more preferably about 50 to 100 ° C./second). However, the coiling temperature affects the form of the oxide layer and must be strictly controlled.

[Winding temperature: 550 ° C or higher and lower than 750 ° C]
When the coiling temperature is less than 550 ° C., hematite with low acid solubility is easily generated on the surface scale, and grain boundary oxidation cannot be removed even by pickling. On the other hand, when the coiling temperature is 750 ° C. or higher, the surface layer scale becomes thick, and even when pickling is performed, the surface layer scale itself cannot be sufficiently removed, and grain boundary oxidation remains. From such a viewpoint, the coiling temperature is set to 550 ° C. or higher and lower than 750 ° C. Preferably, they are 570 degreeC or more (more preferably 600 degreeC or more) and 730 degreeC or less (more preferably 700 degreeC or less).

[Cooling atmosphere control]
It is necessary to appropriately control conditions (cooling end temperature, cooling atmosphere) for cooling after winding. Regarding the cooling stop temperature, since the temperature at which grain boundary oxidation is not generated is about 500 ° C., it is necessary to cool to 500 ° C. or lower in a steam-added atmosphere to suppress grain boundary oxidation. In addition, the water vapor concentration in the atmosphere needs to be 10 to 40% by volume from the viewpoint of proceeding inward oxidation. The water vapor concentration is preferably 15% by volume or more (more preferably 20% by volume or more) and 38% by volume or less (more preferably 35% by volume or less), but it is necessary to satisfy at least the relationship of the formula (1). There is. Note that the atmosphere is preferably controlled by adding water vapor to the atmosphere from the viewpoint of ease of control.

  In the present invention, by appropriately controlling the value a of the ratio ([Si] / [Mn]) and the production conditions, a steel sheet having excellent surface properties is obtained. Any material that satisfies the characteristics as a high-strength steel sheet may be used. From such a viewpoint, as basic components including Si and Mn, C: 0.02 to 0.30%, Si: 1.0 to 3.0, Mn: 1.0 to 3.5%, P: 0.00. Examples include those containing not more than 03% (not including 0%), S: not more than 0.03% (not including 0%), and Al: not more than 0.15% (not including 0%). The reason for adding each element is as follows.

[C: 0.02 to 0.30%]
C is an element effective for increasing the strength of a steel material (that is, a steel sheet), and is an element that affects elongation and stretch flangeability by changing the amount and transformation of a low-temperature transformation product. If the C content is less than 0.02%, it will not be possible to meet the needs of high strength for automobiles. On the other hand, if the C content exceeds 0.30%, the weldability will deteriorate. become. Preferable C content is 0.04% or more (more preferably 0.06% or more) and 0.25% or less (more preferably 0.2% or less).

[Si: 1.0-3.0%]
Si is an important element for securing the strength of the steel material. In the steel plate which is the subject of the present invention, the content was set to 1.0% as the minimum amount of Si necessary for securing the strength. However, when the Si content is excessive, the ductility may be deteriorated, and the content is set to 3.0% or less. A preferable Si content is 1.2% or more (more preferably 1.5% or more), 2.8% or less (more preferably 2.6% or less).

[Mn: 1.0 to 3.5%]
Mn is an element useful for securing the strength of the steel material, and in order to obtain the characteristics as a high-strength steel plate with excellent workability, it is necessary to contain at least 1.0% or more. However, if the Mn content is excessive, the elongation is reduced and the carbon equivalent is increased, and the weldability is deteriorated. Preferable Mn content is 1.2% or more (more preferably 1.5% or more) and 3.0% or less (more preferably 2.5% or less).

[P: 0.03% or less (excluding 0%)]
P is an effective element for obtaining a high-strength steel sheet. However, if it exceeds 0.03%, P is likely to cause uneven plating and difficult to alloy, so it is mixed as an inevitable impurity. In that case, it is necessary to limit the upper limit to 0.03%. The P content is preferably 0.01% or less. In addition, it is difficult to make P content in steel materials 0% on industrial production.

[S: 0.03% or less (excluding 0%)]
S is an element that causes hot cracking during hot rolling and significantly impairs spot weldability. In steel materials, it is fixed as precipitates, but if the amount increases, elongation and stretch flangeability are deteriorated. Therefore, when mixed as an unavoidable impurity, the upper limit must be limited to 0.03%. The S content is preferably 0.01% or less (more preferably 0.008% or less). In addition, it is difficult to make S amount in steel materials 0% on industrial production.

[Al: 0.15% or less (excluding 0%)]
Al is an element useful as a deoxidizer in the steelmaking stage. However, when the Al content is excessive, not only the production cost is increased, but also the surface properties are deteriorated, so 0.15% is made the upper limit. Preferably it is 0.1% or less (more preferably 0.05% or less). The minimum with preferable Al content for exhibiting the said effect is 0.01% or more (more preferably 0.02% or more).

  Components other than the above component composition are substantially iron. In the case where the balance is substantially iron, it is naturally allowed that inevitable impurities (for example, impurities (O, N, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc.) are contained in the steel sheet. The steel material to be used in the present invention may contain various selective elements as necessary, and the characteristics of the steel material are further improved according to the kind of the contained element. The contents and reasons for limitation when these elements are contained are as follows.

[Cr: 1.0% or less (excluding 0%)]
Cr is an element effective in enhancing the hardenability of the steel sheet and strengthening the structure. Moreover, Cr not only concentrates C in austenite, increases stability, and facilitates the formation of martensite, but also affects the plating properties by forming an oxide on the steel sheet surface. However, if the Cr content exceeds 1.0%, the effect is saturated and the cost is disadvantageous. More preferably, it is 0.8% or less. A more preferable lower limit of the Cr content for exhibiting the above effect is 0.03% or more (more preferably 0.05% or more).

[Cu: 0.5% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%)]
Cu and Ni are effective elements for improving the strength of the steel material itself. In particular, the amount of oxygen diffusing into the steel material can be further reduced by uniformly concentrating Cu and Ni, which are harder to oxidize than Fe, on the surface. However, excessive inclusion is not economically compatible and the workability deteriorates, so it should be 0.5% or less for Cu and 1.0% or less for Ni. A preferable content when these elements are contained is 0.003% or more. A more preferable upper limit is 0.3% or less for Cu and 0.7% or less for Ni.

[Ti: 1.0% or less (not including 0%), V: 1.0% or less (not including 0%) and Nb: 1.0% or less (not including 0%) One or more
Ti, V, and Nb are all effective elements for forming carbides and increasing the strength of steel. Ti also fixes C and N and exhibits the effect of increasing the workability (for example, r value) of the steel sheet. However, if any of these contents exceeds 1.0% and becomes excessive, the cost increases and processability is deteriorated. A more preferable upper limit of these is 0.8% or less. In addition, in order to exhibit said effect, it is preferable to contain all 0.003% or more (more preferably 0.005% or more).

[Mo: 1.0% or less (excluding 0%)]
Mo is an element effective in achieving solid solution strengthening of steel materials. However, if the Mo content exceeds 1.0% and becomes excessive, the manufacturing cost will be increased. In addition, in order to exhibit such an effect, it is preferable to contain Mo 0.003% or more (more preferably 0.01% or more). In addition, the more preferable upper limit of Mo content is 0.7% or less.

[B: 0.1% or less (excluding 0%)]
B is an element that has the effect of improving the weldability of the steel material and improving the hardenability. However, when the B content exceeds 0.1% and becomes excessive, these effects are not only saturated, but ductility is deteriorated and workability is lowered. In order to exert such effects, B is preferably contained in an amount of 0.0002% or more (more preferably 0.0005% or more). In addition, the upper limit with more preferable B content is 0.07% or less.

[Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (not including 0%)]
Ca and Mg have the effect | action which controls the form of an inclusion, raises ductility, and improves workability. However, if the content of Ca exceeds 0.005% and Mg exceeds 0.01%, the inclusions in the steel increase and the ductility deteriorates and the workability deteriorates. In addition, in order to exhibit such an effect, it is preferable to contain 0.0005% or more (more preferably 0.0007% or more) in any case. In addition, a more preferable upper limit of these contents is 0.003% or less for Ca and 0.007% or less for Mg.

  The hot-rolled steel sheet obtained by the method of the present invention has good pickling properties and a thin grain boundary oxide layer. When such a hot-rolled steel sheet is cold-rolled, good cold-rollability is exhibited, and a cold-rolled steel sheet having excellent surface properties with no surface defects (pushing) is obtained.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
Steel slabs (steel types A to V) having the chemical composition shown in Table 1 are melted and heated to 1250 ° C. (temperature control is measured by simultaneously heating a general slab with a measurement slab embedded with a thermocouple). Depending on the method), finishing temperature: 870-900 ° C., thickness: 2.6 mm, hot rolling, then cooling at an average cooling rate: 40 ° C./second, and then various windings shown in Tables 2 and 3 below Winding was performed at a temperature, and then the water vapor concentration in the atmosphere was changed (atmosphere control by adding water vapor to the atmosphere), and then cooled to 500 ° C. or lower.

  The scale properties (grain boundary oxidation depth, inner oxide layer thickness, grain boundary oxide layer thickness) of the obtained hot-rolled steel sheet (coil) were measured with an optical microscope. For these, samples are taken from each of the front end, center and rear end of the coil, cross-section samples are prepared from arbitrary three locations of each sample, measured with an optical microscope, and the average value of the whole is obtained. The scale properties (grain boundary oxidation depth, inner oxide layer thickness, grain boundary oxide layer thickness) under each condition were used.

  Next, the coil after performing normal pickling (15% hydrochloric acid, 70 ° C.) is cold rolled to a thickness of 1.4 mm at a cold rolling rate of 46% to produce a cold rolled steel sheet, Grain boundary oxide layer thickness and surface properties were evaluated. At this time, the thickness of the grain boundary oxide layer was measured by taking a sample from each of the front end portion, the central portion, and the rear end portion of the coil, preparing cross-sectional samples from arbitrary three locations of each sample, and using an optical microscope. And the average value of the whole was calculated | required and it was set as the grain boundary oxide layer thickness.

  Also, regarding the evaluation of the surface properties, the steel plate surface after cold rolling is visually observed, and the case where there is no drop of the grain boundary oxidation part is `` good '', and the case where the drop of the grain boundary oxidation part occurs is `` bad '' evaluated. The results are shown in Tables 4 and 5 below.

  From this result, it can be considered as follows. First, those satisfying the requirements specified in the present invention (Test Nos. 1, 2, 5, 7, 9, 11, 13, 15-34) are improved in pickling properties of hot-rolled steel sheets and steel sheets ( It can be seen that the grain boundary oxide layer depth in the hot-rolled steel sheet and the cold-rolled steel sheet) can be reduced, and excellent surface properties are obtained. In particular, in cold-rolled steel sheets, the grain boundary oxide layer depth can be reduced to less than 5 μm, and good surface properties are obtained.

  On the other hand, those that do not satisfy the requirements defined in the present invention (test Nos. 3, 4, 6, 8, 10, 12, 14) have a thick grain boundary oxide layer thickness (that is, (( 1) The relationship of the formula is not satisfied), and it is understood that excellent surface properties are not obtained.

Claims (9)

C: 0.02 to 0.30% (meaning mass%, the same applies to the chemical components of steel), Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%) and Al: 0.15% or less (not including 0%), respectively, Hot-rolling a high-Si steel sheet having a content ratio [Si] and a content of Mn ([Si] / [Mn]) of 0.5 to 2.0, with the balance being iron and inevitable impurities In this case, after winding at a coiling temperature of 550 ° C. or more and less than 750 ° C., the water vapor concentration in the atmosphere is b (volume%: 10 ≦ b ≦ 40), the ratio ([Si] / [Mn]). When the value of a is a and the coiling temperature is T (° C.), cooling to 500 ° C. or lower in an atmosphere satisfying the relationship of the following equation (1) Method for producing a high Si-containing hot-rolled steel sheet having excellent surface properties according to symptoms.
(0.075 × T-38) × (−1.5a + 3.75) ² /(2.45) ² ≦
10 + 1.92 × 10 ⁶ × exp {−15462 / (T + 273)} ×
(0.1566b + 0.506) × 60 ^0.5 /3.6 (1)   The method for producing a high-Si content hot-rolled steel sheet according to claim 1, wherein the atmosphere is controlled by adding water vapor to the atmosphere.   The method for producing a high-Si content hot-rolled steel sheet according to claim 1 or 2, wherein the steel sheet further contains Cr: 1.0% or less (not including 0%).   The steel sheet further contains Cu: 0.5% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%). The manufacturing method of the high Si content hot-rolled steel sheet as described in 2.   The steel sheet further includes Ti: 1.0% or less (excluding 0%), V: 1.0% or less (not including 0%), and Nb: 1.0% or less (not including 0%). The manufacturing method of the high Si content hot-rolled steel plate in any one of Claims 1-4 containing 1 or more types chosen from the group which consists of.   The said steel plate is a manufacturing method of the high Si content hot-rolled steel plate in any one of Claims 1-5 which contains B: 0.1% or less (excluding 0%) further.   The said steel plate is a manufacturing method of the high Si content hot-rolled steel plate in any one of Claims 1-6 which contains Mo: 1.0% or less (0% is not included) further.   The steel sheet further contains Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (not including 0%). The manufacturing method of the high Si content hot-rolled steel sheet as described in 2.   A high-Si content cold-rolled steel sheet having excellent surface properties, which is obtained by cold rolling the hot-rolled steel sheet obtained by the production method according to claim 1.

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