JPH06226569A - Manufacture of stainless steel belt having excellent surface quality - Google Patents

Abstract

PURPOSE:To prevent generation of surface defect such as crack flaw or peeling flaw so as to manufacture a heat roll plate at high product yield, in heat rolling of stainless steel and the like of which surface quality is attached with importance. CONSTITUTION:When a stainless steel continuous casting piece is heat-rolled, the maximum crack length existing in the whole continuous casting piece is estimated from the measured value of the crack length on the surface (Z-face) of a sample cut out from the continuous casting piece by the use of an extreme value statiscal method, and the maximum crack depth of the continuous casting piece to be heat-rolled is predicted from the estimated maximum crack length. A grinding quantity of the surface of the continuous casting piece is decided based on the predicted maximum crack depth. After grinding the surface of the continuous casting piece by the grinding quantity, hot rolling is performed. Consequently, by grinding the surface layer containing defect by a necessary minimized quantity, the heat roll plate having no defect such as crack flaw or peeling flaw is obtained at high product yield.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hot rolling continuous cast stainless steel slabs, where surface quality is particularly important, with a high product yield without causing surface defects such as cracks and dents. .

[0002]

2. Description of the Related Art When stainless steel is hot-rolled, mountain-shaped dent defects, which are referred to as burrow defects, may occur. This defect remains even after cold rolling. In many cases, even fine baldness defects that cannot be found in hot-rolled steel strip are clearly recognized after cold rolling. Surface defects such as bald spots are fatal particularly in stainless steel where surface quality is important. If a steel strip having no surface defects is to be obtained, the yield will be reduced and the cost will be significantly increased. In addition, the proportion of rejects that do not match the order size also increases. Although the mechanism of defect generation has not been fully clarified, it is generally considered to be caused by austenite intergranular cracking during solidification by casting. That is, in the austenite grain boundary,
It is presumed that sulfides, oxides, etc. that promote intergranular cracks are generated and small cracks are generated during casting. In addition, the cause is also the biting of the mold bower and the accumulation of inclusions.

In Japanese Patent Laid-Open No. 55-92255, an upward dipping nozzle and a Ni electromagnetic mold are used in order to reduce surface flaws and accumulated matter appearing on the surface of the continuous cast slab.
Further, in Japanese Patent Laid-Open No. 58-141835, the surface depth of the slab is not manipulated by controlling the maximum depth-width ratio of the oscillation marks during casting to produce a slab having good surface properties. A method of hot rolling is proposed. However, with these methods, it is difficult to industrially produce them stably, and at the same time, the beard defects are not completely eliminated.

In order to obtain a hot-rolled steel strip free from surface defects, it is required to grind and remove the surface layer portion of the continuous cast slab having defects such as cracks and inclusions before hot rolling. Since it is impossible to measure the crack depth of the entire continuous cast slab, surface grinding before hot rolling is usually set to a grinding amount of about 2-3% of the slab thickness based on experience. ing.
However, since the crack depth of the slab surface varies depending on the manufacturing conditions, the amount of grinding may be insufficient, and it has not yet been possible to prevent the burr after the hot rolling. Further, setting a large grinding amount in order to suppress the occurrence of defects causes a reduction in product yield.

As a method for preventing surface defects after hot rolling, JP-A-57-16153 proposes to control the amount of δ ferrite. In this method, the calculated value δcal of δ ferrite determined by the following equation is set to 4.0% or less. δcal = 3 (Cr + Mo + 1.5Si + 0.5Nb) -2.8 (Ni + Mn / 2 + Cu / 2) -84 (C + N) -19.8 Further, depending on the product of molten steel heating temperature (superheat) ΔT and N during continuous casting. There is also known a method of adjusting the heating temperature during hot rolling (JP-A-57-127506), a method of adding a special component to improve hot strength, and the like.

[0006]

The method for suppressing the calculated value δcal of δ ferrite to be 4.0% or less limits the alloy design of the steel to be manufactured and lacks versatility for many steel types. For example, in stainless steel alloy-designed in consideration of mechanical properties and corrosion resistance, suppressing δcal to 4.0% or less limits the types of stainless steel that can be manufactured. In addition, it is also difficult to make 100% suitable for the target setting component during the melting of stainless steel. The ΔT and N contents change during continuous casting. In addition, since a plurality of slabs are heated at the same time in the heating furnace operation, it is not easy to control the heating temperature during hot rolling for all the slabs. Therefore, the method of adjusting the heating temperature during hot rolling according to the product of ΔT and N also involves difficulties in actual operation.

In the method of improving the hot strength by adding a special component, for example, Ti is used as an additive. However, inclusion of Ti increases the number of inclusions and thus increases the number of stringer defects. Further, since the special component is expensive in price, there is a problem that the cost is significantly increased.
The present invention has been devised in order to solve such a problem, in place of the amount of grinding conventionally set empirically, prior to hot rolling, a statistical method is used to determine the size of defects. By predicting and determining the grinding amount and removing the surface layer part containing defects with a specific grinding amount, cracks and hair defects that occur when hot rolling stainless steel continuous cast pieces where surface quality is particularly important It is an object of the present invention to prevent such surface defects as described above, improve the product yield and reduce the load of the post-process, and manufacture a stainless steel strip having excellent surface properties.

[0008]

In order to achieve the object, the method for producing a stainless steel strip according to the present invention has a crack of the entire continuous cast piece hot-rolled from the maximum crack length on the inspection surface of the stainless steel continuous cast piece. Is predicted, and the surface of the continuous cast piece is ground by the grinding amount calculated based on the predicted maximum depth of the crack, and then the continuous cast piece is hot-rolled.

[0009]

[Operation] The inventors of the present invention have experimentally determined that there is a close relationship between the actual measurement value of the maximum crack length on the surface (Z plane) of the continuous casting slab and the maximum depth of cracks existing in the entire slab. Found in. Therefore, the surface of the continuous casting slab (Z
The maximum crack length on the surface) is measured, the maximum depth of cracks existing in the entire slab is predicted from the measured value, and when the slab is ground with a grinder, a cutting machine, etc. with a grinding amount corresponding to the predicted value, defects are detected. The containing surface layer portion is removed. The continuous cast piece whose surface has been adjusted in this manner has a high quality without surface defects such as cracks and dents in the state of being rolled into a strip by hot rolling.

Innumerable cracks and flaws are present on the surface (Z surface) of a continuous cast piece such as a slab. A sample is cut out from this continuous cast piece, and the maximum crack length on the surface (Z plane) of the continuous cast piece is measured. The measurement of the crack length is preferably repeated n times so that the inspection portions do not overlap. For example, the measured n crack lengths are rearranged in ascending order as shown in the equation (1) to form a set l _j (j = 1 to n). l ₁ <l ₂ <l ₃ ... <l _j ... <l _max ... (1) For set l _j (j = 1 to n), cumulative distribution function F _j (%)
Alternatively, the standardized variable y _j is calculated. _{F j (%) = j /} (n + 1) * 100 ···· (2) y j = -l n [-l n (j / (n + 1)] ···· (3)

Cumulative distribution function F _j (%) or standardized variable y _j
In either case, the maximum crack length can be obtained from the measured value l _j . Here, a method of estimating the maximum crack length using the standardized variables y _j will be described. From the relationship between the maximum crack length and the standardized variable y _j , the maximum crack length distribution straight line of the equation (4) is obtained by the least square method. In addition, a and b in the equation (4) are constants determined by actual measurement of the inspection sample. l _j = ay _j + b (4) Further, the relationship between the inspection area S ₀ and the predicted area S is expressed by the equation (5).
It is represented by. T = (S + S ₀ ) / S ₀ (5) The predicted area ratio F for T is expressed by the formula (6), and the standardized variable y for the predicted area ratio F is calculated by the formula (7). . F = (T-1 / T ) ···· (6) y = -l n (-l n · F) ···· (7)

The maximum crack length can be obtained by substituting the value of the standardized variable y for the predicted area ratio F obtained by the equation (7) into the equation (4). On the other hand, regarding the relationship between the crack length and the crack depth on the surface of the continuous cast piece, the surface cutting method for obtaining the relationship between the length and the depth by stepping the surface, the surface of the continuous cast piece (Z
C-section cutting method for cutting the C-section of the same crack as the length of the surface) and measuring the crack depth, crack-cutting for measuring the crack length and crack depth after cutting the crack of the continuous cast slab and tensile rupturing with a tensile tester Method, ultrasonic microscopy, X-ray transmission test, etc. By substituting the crack length obtained by the equation (4) into the obtained crack length and crack depth,
The maximum crack depth of the continuous cast piece to be rolled is predicted.

The predicted maximum crack depth value is 5-1
The surface (Z surface) of the continuous cast piece is ground and trimmed with a grinder, a grinder, etc., with an amount of grinding added about 0%. Thus, in the present invention, a sample is cut out from a continuous cast piece such as a slab and the maximum crack length on the surface (Z plane) of the continuous cast piece is measured. Based on the measured value, the maximum length of cracks existing in the entire hot-rolled continuous cast piece is estimated by the extreme value statistical method, and the maximum crack depth of the entire slab is predicted from the estimated maximum crack length. For this prediction, the relationship between the crack length and the crack depth obtained in advance is used. Based on the predicted crack depth, the grinding amount of the continuously cast slab to be hot rolled is determined. The continuous cast piece is hot-rolled after being ground by the determined grinding amount. The obtained hot-rolled sheet and the cold-rolled sheet produced by cold-rolling the hot-rolled sheet are steel strips having no defects such as surface defects.

[0014]

Example Austenitic stainless steel SUS430N having the composition shown in Table 1 was melted in a normal atmospheric melting furnace and cast into a continuous cast slab.

[Table 1]

A sample having an area of the Z plane of 0.02 m ² was cut out for each 7 m ² of the surface (Z plane) of the slab. The sample is further divided into 30 pieces and each surface (Z plane)
The crack length was measured with a microscope. Then, the maximum crack length in each small piece was obtained, and the pieces were rearranged in the ascending order of the crack length to be l _j (j = 1 to 30). Table 2 shows the rearranged crack lengths.

[Table 2]

Further, for j (j = 1 to 30), the standardized variable y _j was calculated according to the equation (2). Further, the maximum crack length distribution straight line of the equation (8) was obtained from the maximum crack length l _j and the standardized variable y _j . y _j = 0.826l _j −0.921 (8) Next, in the area to be predicted (S = 7 m ² ) of the entire slab from the inspection area (S ₀ = 0.02 m ² ) of the sample. Predict the maximum crack length. The predicted area ratio F of the area to be predicted (S = 7 m ² ) is F = 0.99715 from the expressions (4) and (5). Equation (6) is used when normalizing the area ratio to be predicted. That is, y _j is determined from the inspection area and the predicted area, and the predicted area (S == ₀ ) is calculated from the inspection area (S ₀ = 0.02 m ² ).
7m ² ) is predicted and standardized to be 5.86. Substituting the obtained value of y _j into the maximum crack length distribution straight line of the formula (8), the maximum crack length of 5.979 mm is obtained.

After the cracks in the sample were cut out and tensilely broken by a tensile tester, the crack length and crack depth were investigated, and the relationship shown in FIG. 1 was established between them. From Fig. 1, the crack length y (m
m) is the maximum crack depth d in the C cross section of the slab
It has been found that there is a relation of (9) with (mm). d = 1.049y (9) By predicting the maximum crack depth from the maximum crack length using the formula (9), the maximum depth of 6.27 mm can be measured on the surface 7 m ² of the continuous casting slab. The presence of cracks was predicted. Therefore, the surface of the continuous cast slab was ground by a grinder with a grinding amount of 7.0 mm so that the crack having the maximum depth of 6.27 mm was ground and removed. The continuous cast slab after grinding was hot-rolled, and the state of occurrence of bald spot defects on the surface of the obtained hot-rolled sheet was investigated. The results of the investigation are shown in Table 3. In Table 3, slab 1 corresponds to the calculation example,
In the conventional method, the grinding amount was set to a constant value of 6.0 mm.

[Table 3]

As is clear from Table 3, the occurrence of burnt spots was not detected in the hot-rolled sheet produced by rolling the continuous cast slab ground according to the present invention. The product yield was also stable at 80 to 85%. On the other hand, in the hot-rolled sheet obtained from the continuous casting slab that was ground while keeping the grinding amount at a constant value of 6.0 mm, the yield of the product was significantly decreased along with the occurrence of bald defects.

[0019]

As described above, according to the present invention, the grinding amount can be more specifically determined as compared with the method of determining the grinding amount based on the conventional experience. Therefore, the cutting yield is improved without the need for extra cutting, and the occurrence of burn marks on the hot-rolled sheet, the steel strip, and further the cold-rolled steel sheet and the steel strip is prevented. Further, the manufacturing method itself is not significantly different from the conventional one, and no extra cost is required. In this way, it becomes possible to inexpensively manufacture a stainless steel strip having excellent surface properties.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between crack length and crack depth of stainless steel SUS304N.

Claims (1)

[Claims] 1. The maximum depth of cracks in the entire hot-rolled continuous cast piece is predicted from the maximum crack length on the inspection surface of the stainless steel continuous cast piece, and is calculated based on the predicted maximum depth of cracks. A method for producing a stainless steel strip having excellent surface properties, comprising grinding the surface of a continuous cast slab with a grinding amount and then hot rolling the continuous cast slab.