JP2000102813A - Method for manufacturing hot-rolled steel sheet superior in pickling and surface properties - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-rolled steel sheet superior in pickling and surface properties at a low cost. SOLUTION: A cast piece containing 0.005-0.1 wt.% Ni is continuously cast and rolled in such a way that the temperature from the continuous casting to the end of rough rolling does not exceed 1,170 deg.C. After the end of rough rolling, heat-retaining treatment is done by induction heating in the atmosphere of 3% or more oxygen concentration so that the heat retaining temperature, T( deg.C), heat-retaining time, (t) (second) and Ni content, Ni% satisfy the formula, 30>=t>=(3.4×105)/ (Ni%+0.016)T2}. Subsequently, descaling is done under such conditions that the high-pressure water jet pressure, P (kg/cm2) satisfies 160>=P>=2.2×105×(Ni%+0.5)/(log60t+T), provided T is 950-1,170 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an automobile component,
The present invention relates to a method for producing a hot-rolled steel sheet having excellent pickling properties and surface properties used for building materials or cold-rolled materials.

[0002]

2. Description of the Related Art In order to reduce the manufacturing cost of thin steel sheets, it is necessary to increase the production efficiency and to minimize the occurrence of surface defects leading to a reduction in yield. Above all, speeding up the pickling descaling process is an important technology that is key to improving production efficiency, and various methods have been studied. The main methods are 1) a method of reducing the thickness of the scale, 2) a method of making the composition easy to dissolve the scale, and 3) a method of causing cracks in the scale to facilitate dissolution and peeling of the scale during pickling. is there.

As a method of reducing the scale thickness, for example, Japanese Patent Application Laid-Open No. 4-228204 or
No. 266401, a method of suppressing the formation of scale by sealing a hot-rolled steel sheet with a roller from the finish rolling mill outlet side to the winding machine, or by cooling under an inert gas or reducing gas atmosphere. Has been proposed. However, this method requires a large capital investment and a large amount of inert gas and reducing gas, and the cost is enormous. In Japanese Patent Application Laid-Open Nos. 9-295028 and 9-295029, after rough rolling, a heat preservation oxidation treatment is performed by heating a burner to enrich Ni and form a scale. By performing scaling, the subsequent generation of scale is suppressed, and the pickling property is improved. However, with burner heating,
Because heating is performed by burning fuel,
During heating, it produces carbon monoxide and nitrogen oxides. This is caused by the reaction of oxygen in the fuel + air mixture with oxygen from around the burner flame. As a result, oxygen is consumed as carbon monoxide or nitrogen oxide on the surface of the steel sheet near the tip of the flame from the burner, and becomes lower than the oxygen concentration in the atmosphere of the heat-retention furnace. 5-30 for generation and growth
It takes minutes and a long time, and there is a problem in terms of productivity.

[0004] Japanese Patent Application Laid-Open No. 6-39417 discloses that
A technique has been proposed in which the hot rolling finish temperature, the winding temperature, and the time from the end of finish rolling to winding are controlled to specific ranges to improve pickling properties. In this technique, as a method of dissolving the scale into a composition that is easy to dissolve, a winding temperature of 35% is used.
F is controlled in the range of 0 to 550 ° C. and has excellent descaling properties.
By leaving eO, the pickling property is improved. However, when the winding temperature is 550 ° C. or less, A
Since precipitation of 1N is not sufficient and solute N remains even after cooling, there is a problem that the material is significantly deteriorated by aging. In order to prevent this, it is necessary to use high-purity steel, which increases costs. On the other hand, Japanese Patent Application Laid-Open No.
No. 0 discloses that the material to be rolled after the rough rolling is 1000 ° C. or less
r Bending in the temperature range of ₃ points or more to sufficiently precipitate AlN to prevent deterioration of the material due to aging.
A method for improving the pickling property by leaving FeO by winding is proposed below.

[0005]

However, in the method disclosed in Japanese Patent Application Laid-Open No. 2-11720, since the sheet bar is wound by the coil, the bending radius becomes larger toward the outer periphery of the coil, and the amount of distortion is reduced. As a result, the precipitation form of AlN becomes coarser at the outer periphery of the coil and becomes finer at the inner periphery, resulting in a difference in grain growth, and thus there is a problem that the material varies in the longitudinal direction of the coil.

Japanese Patent Application Laid-Open No. 4-72083 discloses a method of forming a crack in a scale to facilitate dissolution and peeling of the scale during pickling. Then, a method has been proposed in which solid particles are caused to collide with the surface of a steel strip by a pressurized gas, and scale is removed in a pickling line. However, also in this case, the cost of an apparatus for colliding the solid particles with a pressurized gas, a subsequent scale treatment, and equipment for circulating the solid particles is large.

On the other hand, various methods for reducing the occurrence of surface defects have been proposed for the most frequently occurring scale defects. For example, Japanese Unexamined Patent Publication No. 2-59108 predicts the amount of secondary scale generated during finish rolling, and when the predicted value is larger than a predetermined reference value, the rolled material is added to the side of the finishing rolling mill. By applying a scale generation inhibitor, scale flaws generated during hot rolling are prevented. However, since the scale generation inhibitor is expensive, the cost is increased, and at the same time, the scale generation inhibitor is mixed into the acid solution in the pickling descaling step, and the pickling treatment becomes difficult. Further, Japanese Patent Application Laid-Open No.
No. 06 discloses a method of estimating the scale thickness of a steel sheet and measuring or estimating the surface roughness of a roll on the rolling mill entering side, and comparing the surface roughness of the roll with a previously determined condition for generating scale flaws. And / or rolling is performed by controlling the scale thickness. In controlling the surface roughness of the roll, roll surface roughness is suppressed by rolling a high-pressure water containing a grindstone or abrasive grains or a soft thick material. However, such suppression not only increases the capital investment but also increases the cost. An object of the present invention is to provide a method for producing a hot-rolled steel sheet which is low in cost, excellent in pickling properties and surface properties in view of such circumstances.

[0008]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The production method of the present invention uses Ni: 0.00% by weight.
A slab containing 5 to 0.1% is continuously cast and rolled so that the temperature from continuous casting to the end of rough rolling does not exceed 1170 ° C. , Heat treatment is performed by induction heating so that the heat retention temperature: T (° C.), the heat retention time: t (second), and the Ni content: Ni% satisfy the following formula (1). A method for producing a hot-rolled steel sheet having excellent pickling properties and surface properties, wherein descaling is performed under the condition that the pressure: P (kg / cm ² ) satisfies the following expression (2).

30 ≧ t ≧ (3.4 × 10 ⁵ ) / {(Ni% + 0.016) T ² } (1) 160 ≧ P ≧ 2.2 × 10 ⁵ × (Ni% + 0.5) / (Log60t + T) (2) where T: 950 to 1170 ° C

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors added a small amount of Ni and increased the temperature from continuous casting to the end of rough rolling by 1170.
℃ or less to suppress surface oxidation, and after rough rolling, heat treatment is performed by induction heating under appropriate conditions (oxygen concentration in atmosphere, heat retention temperature, heat retention time) to suppress oxidation. By performing descaling before finish rolling using a controlled high-pressure water jet, the scale thickness to be formed can be reduced and productivity can be greatly improved without reducing productivity. It has been found that excellent surface properties can be obtained because the surface condition can be prevented.

That is, the present inventors investigated in detail the effects of trace elements and oxidation conditions on the oxidation behavior or descaling properties of steel, and the effect of the descaling pressure of a high-pressure water jet. Then, the temperature during the period from continuous casting to the end of rough rolling is properly controlled. By controlling the descaling of the high-pressure water jet, a Ni-enriched layer is formed on the steel sheet surface in a short time, and the generation and growth of scale during subsequent rolling and on the run-out table is significantly suppressed. From that
A method for producing a hot-rolled steel sheet that can produce a thinner scale without drastically reducing productivity, greatly improving pickling properties, and preventing the occurrence of scale defects, and obtaining excellent surface properties. And completed the present invention.

That is, the present invention can provide a method for producing a hot-rolled steel sheet which is low in cost and excellent in pickling properties and surface properties by specifying the steel composition and the production conditions within the following ranges.

The reasons for adding the components of the present invention, the reasons for limiting the components, and the reasons for limiting the manufacturing conditions will be described below. (1) Component composition range Ni: 0.005 to 0.1% Ni is an element that is harder to oxidize than Fe in steel, and is concentrated at the interface between the scale and the base iron at the time of high-temperature oxidation, and the adhesion of the scale is increased. Has been previously shown to improve. For example, JP-A-51-40322, JP-A-60-63319, and JP-A-4-157.
In Japanese Patent Publication No. 134, Ni is enriched between the scale and the ground iron by adding a small amount of Ni to improve the adhesion of the scale. The present inventors have found that, in addition to such an effect of improving adhesion, Ni has an effect of further suppressing the generation and growth of scale and improving pickling properties. However, in order to obtain the effects of scale generation and growth suppression, it is necessary to optimize the descaling conditions before induction heating and finish rolling to leave a Ni-enriched layer on the steel surface after scale removal, as described later. all right. However, if the Ni content is less than 0.005%, it takes an extremely long time to concentrate, while if it exceeds 0.1%, the strength increases, ductility decreases, and the cost increases, so the upper limit is 0. 0.1%.

In the present invention, other components are not particularly limited. That is, addition within a range that does not inhibit the effects of the present invention is permitted. By adjusting to the above component composition range, it becomes possible to obtain a hot-rolled steel sheet which is low in cost and excellent in pickling properties and surface properties.

A steel sheet having such characteristics can be manufactured by the following manufacturing method. (2) Hot rolled steel sheet manufacturing process (Manufacturing method) Steel adjusted to the above component composition range is melted in a converter, cast pieces are continuously cast, and the temperature from continuous casting to the end of rough rolling is 1170 ° C. After completion of rough rolling, in an atmosphere having an oxygen concentration of 3% or more, heat retention temperature: T (° C.), heat retention time: t (second), and Ni content:
Heat treatment is performed by induction heating so that Ni% satisfies the following equation (1), and then the pressure of the high-pressure water jet: P
(Kg / cm ² ) is characterized by performing descaling under a condition satisfying the following equation (2).

30 ≧ t ≧ (3.4 × 10 ⁵ ) / {(Ni% + 0.016) T ² } (1) 160 ≧ P ≧ 2.2 × 10 ⁵ × (Ni% + 0.5) / (Log 60t + T) (2) where T: 950 to 1170 ° C. a. Reheating temperature or reheating temperature: 1170 ° C or less When re-heating the slab after continuous casting, it is necessary to set the reheating temperature of the slab to a low temperature of 1170 ° C or less. In this case, the slab cooled to room temperature may be reheated, or may be reheated without cooling to room temperature. Alternatively, when the slab is continuously rolled as it is continuously cast, the recuperation temperature is set to 1170 ° C. or less, and the
It is necessary to roll without exceeding ℃. Temperature 117
If the temperature exceeds 0 ° C., the distribution of the Ni-enriched layer becomes uneven, and the scale / ground iron interface becomes complicated and complicated, so that the descaling property before rough rolling and before finishing rolling is poor, and scale is completely removed. Can not do it. For this reason, scale locally remains and becomes a scale defect, so the upper limit is 1170 ° C. By setting the reheating temperature or the reheating temperature of the slab to 1170 ° C. or lower in this manner, a scale with poor descaling properties is not generated, and therefore, the descaling in the rough rolling step may be performed by a normal method.

B. Heat retention by induction heating In induction heating, since there is no generation of combustion emissions because it is not a heating method based on combustion, Ni remaining on the base iron side also concentrates in a short time. Further, since the outermost surface of the steel sheet is heated to a temperature higher and higher than the set temperature in the heat retaining furnace, the diffusion of Ni is promoted, and the Ni-enriched layer is formed in a shorter time.

C. Heat retention temperature: 950 to 1170 ° C If the heat retention temperature exceeds 1170 ° C, the concentration of Ni at the scale / base iron interface is promoted. However, as in the case of the reheating temperature and the reheating temperature, the Ni-rich layer Distribution becomes uneven,
The upper limit is 1170 ° C. because the descaling property before the finish rolling is poor and causes a scale defect. Also,
When the heat retention temperature is lower than 950 ° C., the scale growth is slow and the scale thickness is insufficient, so that the descaling property before finish rolling is deteriorated, and it takes a long time for Ni to be concentrated, which is not practical. . Therefore, the lower limit of the heat retention temperature is 950 ° C.
It is.

D. Heat retention atmosphere: oxygen concentration of 3% or more If the oxygen concentration is less than 3%, a Ni-enriched layer is formed before the scale grows sufficiently, and the scale growth is remarkably delayed, resulting in descaling before finish rolling. Decrease.
For this reason, even if the pressure of the high-pressure water jet at the time of descaling before finish rolling is increased, scale remains locally, which leads to an increase in the incidence of scale defects.

E. Heat retention time t (second): 30 ≧ t ≧ (3.
4 × 10 ⁵ ) / {(Ni% + 0.016) T ² }
(1) However, T: 950-1170 ° C. Since the growth of the scale and the Ni concentration state change depending on the Ni content and the heat retention temperature, the heat retention time must satisfy the above formula (1). When the heat retention time falls below the lower limit of the heat retention time defined by the equation (1), as in the case where the heat retention temperature is low,
Since the concentration of Ni is insufficient and the scale thickness is insufficient, the descaling property before finish rolling is deteriorated, which leads to the generation of scale defects. On the other hand, when the heat retention time exceeds 30 seconds, the production efficiency is significantly reduced. It is not always necessary to keep the heat at a constant temperature, and within the temperature range defined by the present invention, the same effect can be obtained irrespective of the temperature raising or lowering process. T (° C)
Can be replaced by the average temperature.

F. Descaling condition after heat retention: pressure P (kgf / cm ² ) of high-pressure water jet, 160 ≧ P ≧ 2.
2 × 10 ⁵ × (Ni% + 0.5) / (log60t +
T) (2), provided that T: 950 to 1170 ° C. The descalability of the scale improves as the scale thickness increases. Therefore, the pressure of the high-pressure water jet must be changed according to the heat retention temperature, the heat retention time, and the Ni content, which affect the scale thickness, so as to satisfy the above equation (2). If the pressure is lower than the value determined by the expression (2), descaling failure occurs to cause a scale defect, and the surface quality is significantly deteriorated. From such a viewpoint, it is desirable that the pressure of the descaling high-pressure water jet is high. However, if the pressure exceeds 160 kgf / cm ² , the Ni concentrated layer is destroyed or removed irrespective of the heat retention conditions, and the subsequent scale formation and growth Since the suppression effect is lost, it is 160 kgf / cm ² or less. The descalability and the ease with which the Ni-enriched layer remains are affected by the temperature of the steel sheet at the time of descaling, but do not significantly change within the temperature range at which ordinary descaling is performed. The effect of can be obtained.

Incidentally, the Ni-enriched layer is also generated at the time of reheating and reheating, and it is conceivable that the Ni-enriched layer affects the heat retention conditions later. Therefore, the effect of the reheating time of the continuous cast slab on the heat retention time was investigated. Alloy component (0.03C-0.01
Si-0.2Mn-0.01P-0.01S-0.04
0Al-0.0035N-0.02Ni) steel as a slab by continuous casting, and 1 hour at a reheating temperature of 1150 ° C.
FIG. 1 shows the results of investigating the heat retention time at which no scale remains in the pickling step after holding for r, 3 hr and 5 hr.
In addition, as other manufacturing conditions, heat retention temperature: 1050 ° C.,
The oxygen concentration in the internal atmosphere of the induction heating was 15%, and the pressure of the high-pressure water jet in the descaling before the finish rolling after the heat retention was 120 kgf / cm ² . From FIG. 1, as the reheating time of the slab becomes longer, the heat retention time for obtaining a good descaling result with no scale residue becomes shorter.
Compared with the conditions of the present invention, the difference is slight, and there is no great difference. This is explained as follows. The scale and the Ni-enriched layer are formed even when the slab is reheated or reheated, but only the scale is removed by the descaling on the entry side of the rough rolling step, and the remaining Ni-enriched layer and during the subsequent rough rolling are performed. The resulting scale is stretched during rolling to the final roughing mill exit side. As a result, the scale and the Ni-enriched layer formed at the stage after the rough rolling are much thinner than those formed in the heat retaining step. That is, the scale and the Ni-enriched layer formed when the slab is reheated and reheated do not affect the heat retention conditions defined in the present invention.

The effect of the present invention is not affected by manufacturing conditions other than those described above, and the winding temperature can be wound in the most appropriate temperature range in terms of the material. That is, 640 ° C
In addition to normal winding, a high-temperature winding of, for example, about 680 ° C. may be performed to soften the material and increase ductility. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0024]

EXAMPLES (Example 1) Steels having the chemical compositions shown in Table 1 (A to
C, EG, IK, MO: Examples of the present invention, D, H, L,
P: Comparative Example) was cast into a slab by continuous casting, directly sent to a rough rolling step at 1150 ° C., and then kept within a range of the present invention by induction heating in a temperature range of 1050 to 1100 ° C. for 30 seconds.
Here, the oxygen concentration in the internal atmosphere of the induction heating was adjusted to 15% by inserting an inert gas. The measurement of the oxygen concentration in the internal atmosphere of the induction heating was performed at the center in the longitudinal direction of the induction heating. The heat-retained steel sheet was subjected to descaling before finish rolling at a pressure of a high-pressure water jet: 150 kgf / cm ² , and subsequently rolled to a sheet thickness of 2.8 mm by a finish rolling mill and wound around a coil. Next, the obtained coil was subjected to a descaling test in a hydrochloric acid pickling line. The pickling solution in the hydrochloric acid pickling line was 10% HCl at a liquid temperature of 85 ° C.

Table 2 shows the winding temperature (CT) of each steel, the scale thickness of the hot rolled sheet, and the pickling line speed (A to C,
EG, IK, MO: Examples of the present invention, D, H, L, P:
Comparative example). The scale thickness was measured using a cross-sectional microphotograph. The pickling line speed is the limit line speed at which no scale residue occurs (maximum 300 mpm).
It is. In Table 2, steels satisfying the present invention (A to A)
(C, EG, IK, MO)), the scale was thin, and no scale residue was generated up to the maximum pickling line speed of 300 mpm.

On the other hand, steel (D, H, L, P) having a composition outside the scope of the present invention has a large scale and poor pickling properties. Needed.

[0027]

[Table 1]

[0028]

[Table 2]

(Example 2) A, F, K
After performing continuous casting to rough rolling under the conditions shown in Tables 3, 4 and 5 using the steel (Example of the present invention), the sheet bar was subjected to induction heating at various temperatures or temperature ranges to obtain various sheet bars. The time was maintained, and at the same time, the oxygen concentration was also changed by changing the amount of inert gas inserted. For comparison, heat retention by burner heating was also performed. Subsequently, descaling before finish rolling was performed by changing the pressure of the high-pressure water jet in various ways, and subsequently rolling was performed to a sheet thickness of 2.8 mm by a finish rolling mill, and wound around a coil. The resulting coil was subjected to a descaling test in a hydrochloric acid pickling line. The pickling solution in the hydrochloric acid pickling line was 10% HCl at a liquid temperature of 85 ° C.

Table 6 shows the scale thickness, pickling line speed, and surface properties of the hot-rolled sheet under the production conditions of Tables 3, 4, and 5 (Nos. 1-3, 11-14, 22-25). :
Example of the present invention, 4 to 10, 15 to 21, 26 to 31:
Comparative example). The scale thickness was measured using a cross-sectional microphotograph. Further, the pickling line speed is a limit line speed (maximum 300 mpm) at which no scale residue occurs.

In Table 6, those satisfying the production conditions of the present invention (Examples of the present invention Nos. 1-3, 11-14, 22-25)
Has a thin scale, excellent pickling properties, and good surface properties.

On the other hand, production conditions out of the range of the present invention (Nos. 4 to 10, 15 to 21, 26 to 3)
In Comparative Example 1), oxidation after descaling before finish rolling was not suppressed, and the scale grew remarkably and the pickling property was poor, or scale defects were generated due to insufficient descaling before finish rolling. As a result, the surface properties became poor. In the case of the burner heating, under the same conditions as the induction heating, the scale becomes thick and the pickling property is poor, and sufficient pickling property cannot be obtained even if the heat retention is extended.

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

[0037]

As described above, according to the present invention,
By specifying the steel composition and the manufacturing conditions, the scale thickness to be formed can be reduced. Therefore, by increasing the pickling property, the pickling time can be significantly reduced, and an improvement in productivity can be expected. At the same time, since the occurrence of scale defects can be prevented, a hot-rolled steel sheet having excellent surface properties can be manufactured. Speed up pickling descaling process and improve production efficiency,
In addition, since the yield can be improved by preventing the occurrence of surface defects having scale properties, a hot-rolled steel sheet having excellent pickling properties and surface properties can be manufactured at low cost, which is an industrially extremely useful invention.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of the heat retention time after rough rolling and the reheating time of a slab on the occurrence of residual scale in the pickling step according to the embodiment of the present invention.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) C21D 9/52 101 C21D 9/52 101 H05B 6/10 381 H05B 6/10 381 // C22C 38/00 301 C22C 38 / 00 301W 38/08 38/08 (72) Inventor Yoshimasa Funagawa 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Masaaki Yamamoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Yoichi Honashiki 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term inside Nippon Kokan Co., Ltd. 3K059 AB26 AC33 AC35 AC54 AD03 AD07 AD35 AD40 CD44 4K032 AA01 AA02 AA04 AA05 AA16 AA21 AA22 AA23 AA27 AA29 AA31 AA35 BA01 CA02 CB02 4K043 AA01 AB01 AB02 AB03 AB04 AB15 AB20 AB21 AB22 AB25 AB26 AB27 AB29 BA01 BA06 CA04 FA03 FA09 HA02 HA07

Claims (1)

[Claims] 1. Ni: 0.005 to 0.1% by weight
Is continuously cast and rolled so that the temperature from continuous casting to the end of rough rolling does not exceed 1170 ° C., and after the end of rough rolling, the heat retention temperature: T in an atmosphere having an oxygen concentration of 3% or more. (° C.), heat retention time: t (second) and Ni content:
Heat treatment is performed by induction heating so that Ni% satisfies the following equation (1), and then the pressure of the high-pressure water jet: P
A method for producing a hot-rolled steel sheet having excellent pickling properties and surface properties, wherein descaling is performed under the condition that (kg / cm ² ) satisfies the following expression (2). 30 ≧ t ≧ (3.4 × 10 ⁵ ) / {(Ni% + 0.016) T ² } (1) 160 ≧ P ≧ 2.2 × 10 ⁵ × (Ni% + 0.5) / (log60t + T) ... (2) where T: 950 to 1170 ° C