JP2007039812A - Steel sheet having excellent surface property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent surface properties and free from surface defects caused by the concentration of impurity elements on a slab surface layer by scale formation at high temperature. <P>SOLUTION: The steel sheet comprises, by mass, ≤0.05% Ni, ≤0.1% Cu, ≤0.005% As, ≤0.005% Ge, ≤1% Si and ≤0.1% P, and is obtained by heating a cold slab, and thereafter performing hot rolling. The concentration of Ni in the surface layer of the steel sheet after scale removal is ≤0.9%, and also, the contrast ratio of the Ni concentration in the surface layer is <2 times; wherein the contrast ratio of the Ni concentration in the surface layer denotes (the Ni concentration in the Ni-concentrated part of the surface layer)/(the Ni concentration in the Ni-nonconcentrated part of the surface layer). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel sheet having excellent surface properties.

  Steel sheets used for home appliances and automobile outer plates are required to have a high degree of surface quality in terms of design. Nevertheless, impurity elements such as Ni and Cu that are less likely to be oxidized than Fe before hot rolling are unevenly concentrated on the surface layer of the slab steel as the oxide scale is formed. After pickling or after electrogalvanizing, a mountain-shaped dark color pattern (hereinafter referred to as a mountain-shaped pattern) as shown in FIG. 1 appears, deteriorating the surface properties of the product.

For example, in Patent Document 1, as a means for preventing a pattern defect on the surface of a steel sheet, the amount of Ni is reduced, and the upper limit of the Ni concentration of the steel sheet surface layer portion is limited in Patent Document 1, for example. A method for preventing the occurrence is disclosed.
JP-A-8-337842

  However, in reality, the Ni content in the steel is as low as about 0.01%, and the Ni concentration in the surface layer of the steel sheet is so small that the difference between the pattern part and the normal part cannot be detected even by Auger electron spectroscopy analysis. Even in this case, a chevron pattern may occur on the surface of the steel plate after pickling, and the technique described in Patent Document 1 cannot sufficiently reduce pattern defects.

  The present invention has been made in view of such circumstances, and there is no surface defect caused by concentration of impurity elements on the slab surface layer due to scale generation at high temperature, and it is intended to provide a steel sheet having excellent surface properties. Objective.

  The inventors of the present invention focused on the concentration of the surface layer Ni of the steel plate after scale removal, and as a result of intensive research to obtain a steel plate having excellent surface properties, the maximum Ni concentration of the surface layer and the (Ni concentration portion of the surface layer) The surface defects caused by the concentration of impurity elements in the slab surface layer by regulating the surface layer Ni concentration contrast ratio represented by (Ni concentration in the non-concentrated surface layer) to a predetermined level or less. It has been found that a steel sheet having an excellent surface texture without any problem can be obtained.

The present invention has been made based on such knowledge, and has the following configuration.
mass: Ni: 0.05% or less, Cu: 0.1% or less, As: 0.005% or less, Ge: 0.005% or less, Si: 1% or less, P: 0.1% or less A steel sheet obtained by hot rolling after heating a cold piece slab, wherein the Ni concentration of the steel sheet surface layer after scale removal is 0.9% or less, and the surface layer Ni concentration contrast ratio is less than twice A steel sheet with excellent surface properties, characterized by being However, the surface layer Ni concentration contrast ratio is (Ni concentration of Ni concentrated portion of surface layer) / (Ni concentration of Ni non-concentrated portion of surface layer).
In addition, in this specification, all% which shows the component of steel is mass%.

  According to the present invention, a steel sheet having excellent surface properties can be obtained.

The details of the present invention will be described below together with the reasons for limitation.
First, in order to analyze in detail a chevron pattern on the surface of a steel plate that has occurred in a conventional steel plate, the present inventors have performed a surface layer Ni of a hot-rolled steel plate after pickling under normal conditions and removing the scale. Is measured under the conditions of a probe diameter of 5 μm and an acceleration voltage of 15 kV using an EPMA analyzer, and the Ni / Fe count number ratio in the analysis region of 50 μm × 50 μm is calculated in the chevron pattern portion and the normal portion, The ratio of (Ni / Fe count number ratio of the chevron pattern portion) / (Ni / Fe count number ratio of the normal portion) was evaluated as the contrast ratio. Further, among the hot-rolled steel sheet chevron pattern portions after pickling, surface Ni concentration analysis was performed at five points that look dark visually, and the maximum value was taken as the maximum Ni concentration of the surface layer. In addition, it analyzed similarly about three places which are not recognized as a chevron pattern visually. That is, 10 analysis areas of 50 μm × 50 μm are selected arbitrarily, the Ni / Fe count number ratio is calculated, the maximum value of these is the maximum Ni concentration of the surface layer, and the ratio of the maximum value to the average value is the surface layer Ni concentration contrast Ratio. The result is shown in FIG. In FIG. 2, those recognized as chevron patterns are indicated as surface properties x: NG, and those not recognized are indicated as surface properties ◯: OK.
Here, the EPMA analyzer is an apparatus capable of analyzing a specific element with high accuracy by wavelength dispersive X-ray spectroscopy, and can perform analysis with an accuracy about one digit higher than Auger electron spectroscopy analysis.

According to FIG. 2, when the Ni concentration of the surface layer exceeds 0.9% and there is Ni concentration more than twice as high as the contrast ratio compared to the normal part, it is recognized as a chevron pattern and the surface properties deteriorate. You can see that When the Ni concentration on the surface of the steel sheet is more than a certain level of the Ni concentration ratio between the chevron pattern portion and the normal portion (in the case of using an EPMA analyzer, the contrast ratio compared with the normal portion is more than twice the Ni concentration). Concentration), it was found to be recognized as a chevron pattern on the steel sheet surface.
Furthermore, as a result of investigating the concentration behavior of other elements in the chevron pattern part in detail, the surface properties can be obtained by concentrating the component elements other than Ni (Cu, As, Ge) in the chevron pattern part as well. It has been found that the chevron pattern can be drastically improved only when the concentration control of these elements is also performed.

  For the above reasons, first, in the present invention, the maximum Ni concentration in the surface layer of the hot-rolled steel sheet after scale removal is set to 0.9% or less. From the above analysis results, if there is a portion with a large amount of Ni concentration, the portion becomes a chevron pattern and deteriorates the surface properties. Therefore, to effectively prevent the surface pattern, the surface concentration of Ni is 0.9%. It is necessary to do the following.

Moreover, the surface layer Ni concentration contrast ratio of the hot-rolled steel sheet after scale removal is set to less than 2 times. Here, the contrast ratio is (Ni concentration of Ni concentrated portion of surface layer) / (Ni concentration of Ni non-concentrated portion of surface layer), more specifically, contrast ratio when using an EPMA analyzer. : (Ni / Fe count number ratio of chevron pattern portion) / (Ni / Fe count number ratio of normal portion).
From the above analysis results, even when the Ni concentration in the surface layer is small, if the Ni concentration ratio between the chevron pattern portion and the normal portion is greatly different, the contrast of the light and shaded color is recognized in the steel plate surface layer, and the surface properties deteriorate. Therefore, the contrast ratio needs to be less than twice in order to effectively prevent the chevron pattern on the surface layer. It is desirable that the surface Ni is not concentrated, that is, the contrast ratio is 1.

Next, the reasons for limiting the components of the steel sheet in the present invention will be described.
Ni is an element that unevenly concentrates in the surface layer portion of the slab and develops a chevron pattern. The greater the content, the more easily the contrast of surface concentration. Therefore, Ni is set to 0.05% or less in order to prevent uneven concentration and suppress the expression of the chevron pattern.

  Cu is an element that is particularly easily mixed when iron scrap is used as a raw material, and similarly to Ni, it is unevenly concentrated on the surface portion of the slab and causes a chevron pattern to deteriorate the surface properties. Therefore, in order to prevent uneven concentration and suppress the expression of the chevron pattern, Cu is made 0.1% or less.

  As and Ge, as with Ni, are concentrated unevenly on the surface of the slab, causing a chevron pattern and deteriorating the surface properties. Therefore, in order to prevent uneven concentration and suppress the expression of the chevron pattern, As and Ge are each 0.005% or less.

  Si is an element having a solid solution strengthening effect of steel. However, if the added amount exceeds 1%, the descaling property at the time of descaling is deteriorated, and the Ni concentrated layer becomes deeper by increasing the surface roughness at the time of slab heating. Therefore, Si is preferably 1% or less.

  P is an element contributing to solid solution strengthening of steel. However, addition exceeding 0.1% generates Si and a low-melting-point oxide during slab cold piece heating, increases the surface roughness, and deepens the Ni-enriched layer of the surface iron surface layer. % Or less is preferable.

Moreover, it is preferable that the steel plate in this invention contains the component of the following ranges.
C can be added in an amount of about 0.1% or less in order to improve the strength of the steel. However, if the addition amount exceeds 0.1%, the workability of the steel sheet deteriorates, which is not preferable.
Mn is an element that improves the strength of the steel, and since it has the effect of fixing S and reducing the surface slab surface, addition of 0.1% or more is preferable. However, if the addition amount exceeds 2%, workability is remarkably deteriorated.
The upper limit of S is 0.015%. If it exceeds 0.015%, the slab surface flaw increases, which is not preferable.

  In addition, within the range where the effect of the present invention is obtained, for example, Ti, Nb, and V are each about 0.1% or less for the purpose of improving the strength, for example, Cr and Mo are each set to 0 for the purpose of improving the hardenability. About 2% or less can be added. In addition, within a range not impeding the effects of the present invention, for example, Ca and REM can be added to each about 0.005% or less for the purpose of improving workability.

Next, the manufacturing method of the steel plate excellent in the surface property of this invention is demonstrated.
The steel sheet having excellent surface properties according to the present invention can be obtained by casting the steel having the above-described component composition, performing rough rolling, hot rolling, pickling, and the like.
Here, the present inventors conducted a hot rolling test and analysis of surface defects. As a result, it was found that the appearance of chevron patterns in the steps after pickling can be greatly reduced by optimizing the thermal history of the slab by the following method, more preferably by optimizing the descaling conditions.

  That is, hot rolling is performed at a slab surface maximum heating temperature of 1200 ° C. or lower and a scale-off amount during heating of 2 mm or lower when the slab is heated and used for rolling. When the maximum heating temperature of the slab exceeds 1200 ° C., the oxidation of Fe is promoted, and hardly oxidizable elements such as Ni and Cu are easily discharged to the surface layer of the steel. Even if the amount of scale-off during heating exceeds 2 mm, the amount of Fe oxidation increases, so that concentration of Ni or the like proceeds and the surface properties deteriorate.

  Subsequently, descaling is preferably performed before the first rough rolling pass. Here, the rough rolling is horizontal rolling with a rolling reduction of 20% or more, for example. Therefore, light rolling horizontal rolling may be performed for width adjustment for width adjustment or shape correction of the slab end before descaling. The descaling method is preferably jetting for 0.01 seconds or more with a water flow having a collision pressure of 1 MPa or more. At this time, the descaling medium is not particularly defined, but, for example, high-pressure water can be used. By performing descaling by jetting for 0.01 seconds or more with a water flow with an impact pressure of 1 MPa or more, the effect of scraping off the slab surface layer, and effectively removing the scale and exposing the surface of the iron bar to the atmosphere after descaling There is an effect of facilitating removal of the non-uniformly concentrated layer such as Ni which is the basis of the chevron pattern by scaling off.

  As described above, a steel sheet having excellent surface properties can be obtained by the present invention, and the obtained steel sheet can be applied to pickling materials, hot-rolled base EG materials, cold-rolled base EG materials, Cr-plated materials, and the like.

  The steel slab (A) having the chemical composition shown in Table 1 is heated from the cold piece to the temperature shown in Table 2, the scale-off amount and the descaling conditions are the conditions shown in Table 2, and the first pass is a horizontal with a rolling rate of 22%. Rough rolling was performed, followed by normal hot rolling and pickling. One or two steel plates were obtained.

得られた熱延鋼板について、表層最大Ｎｉ濃度および表層Ｎｉ濃度コントラスト比を求め、山型模様の評価を行った。ここで、表層最大Ｎｉ濃度および表層Ｎｉ濃度コントラスト比の測定は、通常の条件にて酸洗しスケールを除去した後の熱延鋼板の表層Ｎｉの濃度を、ＥＰＭＡ分析装置を用いて、プローブ径５μｍ、加速電圧１５ｋＶの条件にて測定し、次いで、５０μｍ×５０μｍの分析領域のＮｉ／Ｆｅカウント数比を山型模様部および正常部で算出し、(山型模様部のＮｉ／Ｆｅカウント数比)／(正常部のＮｉ／Ｆｅカウント数比)をコントラスト比として評価した。また、酸洗後の熱延鋼板山型模様部のうち、目視で濃く見える箇所５点の表層Ｎｉ濃度分析を行い、その最大値を表層のＮｉ濃度とした。なお、山型模様が発生していないものについては、５０μｍ×５０μｍの分析領域を任意に１０領域選定し、Ｎｉ／Ｆｅカウント数比を算出し、その最大値を表層のＮｉ濃度とした。また、最大値と平均値の比を表層Ｎｉ濃度コントラスト比とした。
結果を表３に示す。なお、表３において、山型模様の評価は通常条件にて酸洗した熱延鋼板を目視にて判定し、山型模様が発生していないものを○、発生しているものを×とした。

About the obtained hot-rolled steel sheet, the surface layer maximum Ni concentration and the surface layer Ni concentration contrast ratio were determined, and the chevron pattern was evaluated. Here, the measurement of the maximum surface layer Ni concentration and the surface layer Ni concentration contrast ratio is performed by measuring the surface layer Ni concentration of the hot-rolled steel sheet after pickling under normal conditions and removing the scale using an EPMA analyzer. Measurement was performed under the conditions of 5 μm and acceleration voltage of 15 kV, and then the Ni / Fe count number ratio of the analysis region of 50 μm × 50 μm was calculated for the chevron pattern part and the normal part, and (Ni / Fe count number of the chevron pattern part) Ratio) / (ratio of Ni / Fe count of normal part) was evaluated as a contrast ratio. Further, among the hot-rolled steel sheet chevron pattern portions after pickling, surface Ni concentration analysis was performed at five points that look dark, and the maximum value was taken as the Ni concentration of the surface layer. In addition, for the case where no chevron pattern was generated, 10 analysis regions of 50 μm × 50 μm were arbitrarily selected, the Ni / Fe count number ratio was calculated, and the maximum value was taken as the Ni concentration of the surface layer. The ratio between the maximum value and the average value was defined as the surface Ni concentration contrast ratio.
The results are shown in Table 3. In Table 3, the evaluation of the chevron pattern was made by visually observing a hot-rolled steel sheet pickled under normal conditions. .

表３より、比較例であるＮｏ．１のようにスケールオフ量が大きい場合は、山型模様により表面性状が劣化しているが、本発明例であるＮｏ．２では、表面性状は良好であった。

From Table 3, No. which is a comparative example. When the scale-off amount is large as in No. 1, the surface texture is deteriorated due to the chevron pattern. In 2, the surface properties were good.

  Since the steel sheet obtained by the present invention has very few surface patterns and excellent surface properties, it can be applied to pickling materials, hot-rolled base EG materials, cold-rolled base EG materials, Cr-plated materials, etc. It is optimal as a steel plate used for the outer plate of automobiles and home appliances.

The schematic diagram of the chevron pattern which generate | occur | produces on the steel plate surface of a prior art. The figure which shows the relationship between the maximum Ni density | concentration of surface layer, surface layer Ni density | concentration contrast ratio, and surface property.

Claims (1)

mass: Ni: 0.05% or less, Cu: 0.1% or less, As: 0.005% or less, Ge: 0.005% or less, Si: 1% or less, P: 0.1% or less A steel sheet obtained by hot rolling after the cold slab is contained, the Ni concentration of the steel sheet surface layer after scale removal is 0.9% or less, and the surface Ni concentration contrast ratio is less than twice A steel sheet with excellent surface properties, characterized by being However, the surface layer Ni concentration contrast ratio is (Ni concentration of Ni concentrated portion of surface layer) / (Ni concentration of Ni non-concentrated portion of surface layer).

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