JP2008080344A - Method for manufacturing steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel sheet, which method can improve productivity and can reduce a manufacturing cost without taking excessive conditioning of a slab before rolling. <P>SOLUTION: In manufacturing the steel sheet by rolling a slab, the rolling work is carried out after removing the skin of the slab within the range from 1.5 mm to 2.5 mm when the ratio (D) of the target thickness of the steel sheet after rolling to the thickness of the slab is within the range from 0.018 to 0.6, and the surface temperature (T) of the slab just after taking out from a heating furnace is within the range from 1335°K to 1483°K, and the following equation (1) is satisfied, D≥-19.334×(T/1000)<SP>2</SP>+57.297×(T/1000)-41.898, where 0.018≤D≤0.60, and 1335°K≤T≤1483°K. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a slab pre-processing technique in the case of manufacturing a steel sheet by rolling a slab manufactured in a continuous casting process.

Slabs cast by continuous casting are susceptible to scratches (hereinafter referred to as “longitudinal cracks”) in the longitudinal direction (casting direction) (non-patent document 1), and slabs having longitudinal cracks. If the material is rolled as it is, scratches remain on the surface of the product, resulting in a defective product, resulting in a decrease in product yield.
It is known that vertical cracks on the surface of a slab occur due to concentration of internal stress in a solidification delay portion caused by temperature nonuniformity of steel in a continuous casting process. Conventionally, in order to achieve uniform temperature of steel in the continuous casting process, improvement of mold powder and cooling method at the time of casting, etc. have been carried out, and by reducing the occurrence of vertical cracks in the slab to some extent, no maintenance rolling and HCR (Hot charge rolling) has been attempted.

However, it is difficult to eliminate vertical cracks on the surface of the slab even if the mold powder is improved and the cooling method during casting is improved. In particular, the carbon concentration is 0.08 to 0.16% by weight ( In the continuous casting of subperitectic medium carbon steel, the occurrence of fine surface cracks (longitudinal small cracks) with a crack depth of 2 mm or less was remarkable (FIG. 4).
Therefore, in order to determine whether or not the surface treatment of the slab is necessary before the rolling process, the determination is a measure for determining the amount of deoxidation product (aluminum oxide) generated calculated from the refining process operation data. The scale of inclusion floating in the tundish obtained from the scale, the amount of molten steel retained in the tundish and the temperature in continuous casting, and the judgment scale of the amount of mold powder entrained in the mold are checked against the acceptance criteria of the slab. In the technology (Patent Document 1), continuous casting of Nb-containing and V-containing medium carbon steel for thick plates, the Ti / N ratio is calculated from the analytical value of the molten steel in the tundish. Technology that predicts that it will not occur and sends it to the rolling process as it is without care. If Ti / N ratio <3, it adds the additional weight for surface care and sends it to the rolling process after carrying out surface care such as scarfing ( Permissible literature 2) and surface care method and operation method for eliminating surface defects of bottom slab (slab at the start of casting), molten steel heating in the tundish, presence or absence of molten steel stirring, and ground Under the casting conditions determined by the amount of gold adhesion, the pick-up amount of the total oxygen concentration of the slab at the start of casting and the separation rate of oxide inclusions are measured in advance, and the subsequent casting starts in light of this measurement result. A technique is disclosed in which the total oxygen concentration of the slab and the separation rate of oxide inclusions are estimated and determined based on this estimated value (Patent Document 3).
Iron and Steel (Japan Iron and Steel Association), 86th (1982) No. 13, pp. 1744-1770 JP 2004-306085 A JP 2002-283021 A JP 2000-61591 A

  In the technique disclosed in Patent Document 1, “deoxidation product (aluminum oxide) generation amount”, in the technique disclosed in Patent Document 2, “Ti / N ratio”, and in the technique disclosed in Patent Document 3, “casting”. Conventionally, it has often been done using a component of molten steel to determine whether or not the surface treatment of the slab is necessary, such as the pickup amount of the total oxygen concentration of the slab at the start and the separation rate of oxide inclusions . For this reason, there is a risk that a result component may be unpredictable due to a factor that is not managed and observed in the casting process, or an operation abnormality may occur, and thus the necessity of slag surface maintenance may be erroneously determined.

  The present invention has been made in view of the above-mentioned problems, and appropriately determines the necessity of slag surface maintenance, and avoids excessive slab maintenance before rolling while ensuring good quality. It aims at providing the manufacturing method of the steel plate which can aim at the improvement and reduction of manufacturing cost.

In order to solve the above-mentioned problems, the present inventor tried unmaintained rolling of medium carbon steel for thick plates and evaluated the relationship between vertical small cracks and rolled product defects. As a result, it was found that even if the crack occurrence situation on the slab surface was the same, the defect occurrence situation of the rolled product varied greatly depending on the rolling conditions (the slab surface temperature at the time of taking out the heating furnace, the ratio of the product thickness to the slab thickness).
That is, in the case of the vertical small crack, when the slab surface temperature and the product thickness / slab thickness at the time of taking out the heating furnace satisfy a specific relationship, no product defect occurs even if the vertical small crack occurs. On the other hand, when it deviates from a specific relationship, small vertical cracks remain as product surface defects and become defective products.

Based on these findings, the following technical means were taken in the present invention in order to achieve the object.
The method for producing a steel plate according to the present invention is a method for producing a steel plate by rolling a slab, and the ratio of the target product thickness to the slab thickness of the steel plate after rolling (D = target product thickness ÷ slab thickness) is 0. When the surface temperature (T) of the slab when it is taken out from the heating furnace is 1335K or more and 1483K or less and further satisfies the formula (1), the surface of the slab is 1.5 mm or more and 2 or less. Roll after removing in a range of 5 mm or less.

D ≧ −19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −41.898 (1)
(0.018 ≦ D ≦ 0.60, 1335 (K) ≦ T ≦ 1483 (K))
Another method of manufacturing a steel sheet according to the present invention is a method of manufacturing a steel sheet by rolling a slab, wherein the ratio of the target product thickness of the steel sheet after rolling to the slab thickness (D = target product thickness ÷ slab thickness). Is not less than 0.018 and not more than 0.6 and the surface temperature (T) of the slab when taken out from the heating furnace is not less than 1335K and not more than 1483K and further satisfies the formula (2), the surface of the slab is not maintained. Roll as it is.

D <−19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −42.033 (2)
(0.018 ≦ D ≦ 0.60, 1335 (K) ≦ T ≦ 1483 (K))

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the steel plate which can avoid the maintenance of the excess slab before rolling and can aim at the improvement of productivity and reduction of manufacturing cost can be provided.

FIG. 1 shows an outline of a thick plate rolling apparatus 1, which has a heating furnace 2 for heating a slab on the upstream side, and a pair of workpieces on the downstream side of the heating furnace 2. A rough rolling mill (coarse mill) 5 including a roll 3 and a pair of backup rolls 4 is provided. A cooling device 6 that adjusts the temperature by spraying water or air on the slab is provided on the downstream side of the roughing mill 5, and a pair of work rolls 3 and a pair of backup rolls are provided on the downstream side of the cooling device 6. 4, a finish rolling mill (finish mill) 7 is provided.
Between the heating furnace 2 and the roughing mill 5, a scarfer 8 for polishing the slab as necessary is provided.

FIG. 2 is a flowchart from a continuous casting process to a rolling process in the production of a steel sheet.
In FIG. 2, the molten steel flows out of the mold by a continuous casting machine and solidifies as a strip-shaped steel ingot (# 11), and is cut into a slab having a predetermined length by a gas cutter (# 12).
The slab is heated in a heating furnace in preparation for rolling (# 13). The heating furnace is a batch type or a continuous type. The slab is heated for a predetermined time in a heating furnace set to a predetermined temperature and then removed from the heating furnace, and the surface temperature is measured (# 14).
The surface temperature is measured by the apparatus and method described in the specification and drawings attached to the application of the patent application (Japanese Patent Application No. 5-14561) by the present applicant (Japanese Patent Laid-Open No. 6-229833). The surface temperature of the slab can also be easily measured with a portable radiation thermometer or the like.

  By the way, the surface of the slab is oxidized by heating in the heating furnace, the oxidized surface is peeled off by the flame, and the fine vertical cracks generated in the continuous casting process disappear. Moreover, the higher the heating temperature, the greater the amount of peeling from the surface, which is advantageous in eliminating fine vertical cracks. However, an increase in the amount of delamination from the surface leads to a decrease in product yield, and when the heating temperature is increased, the structure and strength of the slab change, such as the crystal state, and the thickness of the rolled product. It may be difficult to increase the heating temperature depending on the requirements of the device. Further, when rolling is performed at a high pressure ratio, fine vertical cracks are likely to disappear, and a rolled product with a smooth surface is easily obtained.

  Therefore, using the reduction ratio obtained from the target product thickness and the surface temperature of the heated slab as an index, it is predicted whether or not scratches will remain on the product surface when the slab after taking out from the heating furnace is rolled (# 15 ). When it is predicted that scratches will remain on the product surface when rolled (Yes in # 15), the surface of the slab is cleaned (# 16). The prediction method will be described later. “Surface care” means that the entire surface of the slab is ground (melted) with a scurfer, and the remaining cracks and scarf fins are partially ground with a grinder or a partial scurfer.

By reducing the heat flux in the mold by controlling the composition of the mold powder, most cracks generated on the slab surface are suppressed to a depth of 2 mm or less. Therefore, in surface care, if the amount of surface grinding is 1 mm or more, the generation of scratches on the product can be substantially prevented. In consideration of the variation in the amount of cutting by the scurfer and the fact that the yield loss increases when excessive grinding is performed, the surface grinding amount is preferably 1.5 mm or more and 2.5 mm or less.
In addition, the control of the amount of cutting is `` Naoichi Yamamura et al., Automation of hot scarf operation and yield improvement, materials and processes, VOL. 9, No. 5, 1996, p. 1035 '', `` Yoshimitsu Kato et al., Steel slab cutting amount measuring device, material and process, VOL.5, No.2, 1992, p.364 ”and“ Iron and steel, Vol.78, No.8, p.T137-T140 ” Depending on how you are.

After the slab has been ground (surface care) (# 16), or after it is predicted that no scratch will remain even if it is rolled without maintenance (without grinding) (No in # 15), the rolling process Is rolled into a steel plate product (# 17).
Next, a method for predicting whether scratches remain on the product surface when the slab after being taken out from the heating furnace is rolled using the temperature and reduction ratio after heating as an index will be described (# 15).
Table 1 summarizes the evaluation of scratches on the product surface after rolling under the following rolling conditions without care, together with the product thickness, product width, and the like. FIG. 3 is a graph showing the relationship between the surface temperature and rolling ratio at the time of taking out the heating furnace in Table 1 and the state of scratches on the product.
[Rolling conditions]
Slab thickness: 280mm
Rolled product thickness: 5 to 168 mm
Product thickness / slab thickness (= 1 / rolling ratio) (D): 0.018 to 0.6
Surface temperature of slab at the time of taking out the heating furnace (T): 1335-1483K
Evaluation of the scratch of the product after rolling of each point (key) in FIG. 3 is based on the following criteria.

Δ: No scratch on the surface (◯ in Table 1)
□: Depth of scratch less than 0.1 mm (repairable, Δ in Table 1)
◇: Scratch depth of 0.1 mm or more (repair impossible, x in Table 1)
("Repair" refers to scratch removal work (surface care) performed on rolled products.)
Hereinafter, D represents the product thickness / slab thickness for both the rolling performance and the target before rolling.

As shown in FIG. 3, the presence or absence of scratches on the surface of the product after rolling and the state of the scratches are the surface temperature T of the slab at the time of taking out the heating furnace (hereinafter sometimes referred to as “surface temperature T”) and the product thickness / slab. In relation to the thickness D, the three areas defined by the formulas (1) and (2) are divided into “no scratches on the surface”, “scratches that can be repaired”, and “scratches that cannot be repaired”, respectively. It is done.
D ≧ −19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −41.898 (1)
D <−19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −42.033 (2)
That is, in the rolling performed in the range where the surface temperature T is in the range of 1335K to 1483K and the product thickness / slab thickness D is in the range of 0.018 to 0.6, the rolling condition in the region 1 defined by the equation (1) is almost Unacceptable scratches were observed on the surface of all rolled products. In other words, under rolling conditions that satisfy equation (1), it is predicted that scratches will occur on the product surface without maintenance (# 15).

Further, under the rolling conditions in the region 3 defined by the expression (2), almost all rolled products did not have scratches on the surface, and the surface state was a level that could be shipped as a product. Therefore, in the rolling operation by the combination of the surface temperature T in the region 3 and the target product thickness / slab thickness D, it is predicted that no scratch will occur on the product surface even if the slab is rolled without care (# 15). By performing maintenance-free rolling in accordance with the above, it is possible to increase yield, reduce costs, and shorten the work period.
Under the rolling conditions in the region 2 that does not correspond to either of the formulas (1) and (2), minor scratches that can be repaired often remain on the product surface. Under such rolling conditions, it is preferable to perform rolling after surface care so as to ensure the surface quality of a good product more reliably.

In this way, if the surface temperature T and the target product thickness / slab thickness D, which are rolling conditions, are applied to the formulas (1) and (2), the formula (1) is satisfied or the formula (2) is satisfied? By determining whether or not both are satisfied, the degree of scratches on the product surface when rolling without maintenance can be predicted. This prediction result is useful for determining whether to perform maintenance before rolling.
The surface temperature T of the slab applied to the equations (1) and (2) is preferably measured values. However, when the heating conditions of the heating furnace and the measured values of the surface temperature T have a high correlation, heating is performed. You may apply the surface temperature estimated from conditions.

In Table 1, sample No. 1 to 21 are the conditions included in the region 1 in FIG. 22 to 74 are the conditions included in the region 2 and the sample Nos. 74 to 164 are the results of rolling under the conditions included in region 3.
Table 2 shows the scratches on the surface of the product after rolling when the entire surface of the slab is ground with a scarf so that the amount of scarf is 1.5 mm before rolling under the conditions in regions 2 and 3. It is a table | surface which shows the generation | occurrence | production situation.

As shown in Table 2, if the rolling conditions belong to region 1 and region 2 and it is predicted that the product surface will be flawed without maintenance according to equations (1) and (2), the product is cleaned before rolling. By carrying out the above, it is possible to eliminate surface scratches in the product and prevent the product from being rejected.
In Table 2, sample No. 151 to 171 are the conditions included in region 1 in FIG. 172 to 221 are the results of rolling under the conditions included in region 2.

In the steel sheet manufacturing method shown in FIG. 2, it is possible to predict the presence or absence of scratches on the product surface from the rolling conditions, particularly in a situation where rolling must be performed under conditions according to the product quality required for each slab. What can be done is extremely beneficial in minimizing the amount of slab care before rolling and optimizing costs and delivery times. By selecting slabs that require maintenance, it is possible to significantly reduce costs and shorten delivery times.
In the above-described embodiment, another process may be included between the continuous casting process and the rolling process. For example, you may provide the removal process of fusing noro before a heating process.

  In addition, continuous casting (# 11), cutting (# 12), heating (# 13), surface temperature measurement (# 14), surface maintenance necessity judgment (# 15), surface care (# 16) and rolling (#) Each step of 17) can be appropriately changed in accordance with the gist of the present invention.

  The present invention can be used for slab pre-processing before rolling a slab manufactured in a continuous casting process.

It is the schematic of a thick plate rolling apparatus. It is a flowchart from the continuous casting process in the manufacture of a steel plate to a rolling process. It is a figure which shows the relationship between the surface temperature and reduction ratio at the time of taking out a heating furnace, and the state of the crack of a product. Sectional drawing of the vertical small crack of the slab surface.

Explanation of symbols

D Ratio of the target product thickness of the steel sheet after rolling and the slab thickness T Surface temperature of the slab when taken out from the heating furnace

Claims (2)

A method of rolling a slab to produce a steel plate,
The ratio of the target product thickness of the rolled steel sheet to the slab thickness (D = target product thickness / slab thickness) is 0.018 or more and 0.6 or less, and the surface temperature (T) of the slab when taken out from the heating furnace is 1335K. More than 1483K and satisfying the formula (1),
Rolling after removing the surface of the slab in the range of 1.5 mm or more and 2.5 mm or less.
D ≧ −19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −41.898 (1)
(0.018 ≦ D ≦ 0.60, 1335 (K) ≦ T ≦ 1483 (K)) A method of rolling a slab to produce a steel plate,
The ratio of the target product thickness of the rolled steel sheet to the slab thickness (D = target product thickness / slab thickness) is 0.018 or more and 0.6 or less, and the surface temperature (T) of the slab when taken out from the heating furnace is 1335K. More than 1483K and satisfying the formula (2),
A method for producing a steel sheet, comprising rolling the surface of the slab without care.
D <−19.334 × (T ÷ 1000) ² + 57.297 × (T ÷ 1000) −42.033 (2)
(0.018 ≦ D ≦ 0.60, 1335 (K) ≦ T ≦ 1483 (K))