JP2011202231A - Method for producing hot rolled steel sheet having excellent pickling property and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet which has satisfactory pickling properties and also has satisfactory mechanical properties upon the production of the hot rolled steel sheet.SOLUTION: When a steel having a composition comprising, by mass, 0.001 to 0.30% C, 0.05 to 0.40% Si, ≤1.0% Mn, ≤0.04% P, ≤0.02% S, 0.005 to 0.10% acid soluble Al, and the balance Fe with inevitable impurities is subjected to continuous casting, and is thereafter subjected to hot rolling, the temperature on the outlet side in a hot rolling finish rolling mill is controlled to an Arpoint or above, coiling temperature is controlled to 550 to 700°C, and an Fe-Si based oxide is formed on the boundary between the scale in the surface of the coiled steel sheet and base iron.

Description

  The present invention relates to a method for producing a hot-rolled steel sheet (hereinafter referred to as a hot-rolled steel sheet) having a good surface property, which is used for automobile parts, building members, general processing, or various plated steel sheets. .

A hot-rolled steel sheet is generally manufactured with equipment having a configuration as shown in FIG. In FIG. 1, the conveying direction of the hot-rolled steel sheet is from the right to the left in the figure.
A slab having a thickness of about 200 mm, which is a material, is heated to, for example, 1200 to 1300 ° C. in a heating furnace (not shown). Thereafter, the scale generated on the surface is descaled by a high-pressure water descaling device (not shown), and then thinned by the rough rolling mill 1 to a thickness of about 25 to 45 mm. Subsequently, after finishing rolling with a finish rolling mill 2 to a thickness of about 1.2 to 6.0 mm, cooling is performed on a hot run table 4 and winding is performed by a coiler 5 to obtain a hot-rolled steel sheet. ing. A high-pressure water descaling device 3 is also provided on the entry side of the finish rolling mill 2.
The wound hot-rolled steel sheet is then pickled after introducing cracks (fine cracks) into the scale on the surface of the hot-rolled steel sheet by light rolling (skin pass) or leveler with an elongation of about 0.5 to 3%. It is common to remove the scale at.

The scale generated on the surface of the steel strip in the hot rolling process includes a primary scale generated by oxidizing the surface of the slab in the heating furnace, a secondary scale generated during rough rolling, and further during finish rolling and There is a cubic scale to be generated later.
The primary scale is removed by a high-pressure water descaling device (not shown) installed on the entry side of the roughing mill 1. The secondary scale generated thereafter is removed in most cases by the high-pressure water descaling device 3 installed on the entry side of the finish rolling mill 2, and a uniform tertiary scale is generated on the surface of the rolled material.

However, depending on the raw material components and the temperature of the rolled material, the secondary scale may not be completely removed by the high-pressure water descaling device before finish rolling, and may remain in that case. It may be reduced by the roll of the machine and a red scale may occur.
The red scale is a secondary scale that has not been removed by high-pressure water descaling and remains on the surface of the steel strip in the form of strips or stripes and is observed as a pattern.
On the other hand, the tertiary scale generated during finish rolling is rolled together with the steel sheet, but when the steel sheet surface temperature is high, the tertiary scale peels off during finish rolling, and the second row stand roll of the finish rolling mill. In some cases, the steel sheet is wound around the surface of the steel sheet, resulting in scale wrinkles on the surface of the steel sheet and rough surface of the steel sheet.

Here, the thickness of the scale of the hot-rolled steel sheet is preferably as thin as possible. This is because a long processing time is required to dissolve and remove a thick scale with an acid solution, and in the pickling line where the length of the pickling tank is fixed, the plate passing speed is reduced to reduce the processing time. This is because the productivity is lowered because of securing.
As mentioned above, cracks (fine cracks) are introduced into the scale formed on the surface by applying light rolling (skin pass) or leveler to the hot-rolled steel sheet. This is for promoting the descaling efficiently.

Thus, several methods for producing hot-rolled steel sheets with good pickling properties have been proposed.
For example, in Patent Document 1, after carrying out descaling before the start of finish rolling, by controlling the finish rolling end temperature, the winding temperature (550 ° C. or less), and the time from the finish rolling end to winding to a specific range, A method for producing a hot-rolled steel sheet with a thin scale (6 μm or less) and excellent pickling properties is proposed.

Patent Document 2 discloses a hot-rolled steel sheet having a scale on the surface layer of the ground iron, the average particle size L of the surface iron surface layer being 20 μm or less, the scale thickness d being 10 μm or less, and satisfying L × d ² ≦ 1000. In the production of a hot-rolled steel sheet characterized by the above, and a hot-rolled steel sheet having a scale in the surface layer of the ground iron, finish rolling is finished at Ar ₃ points + 50 ° C. or less, and 80 seconds at a cooling rate of 100 ° C./s or more within 1 second. A method of manufacturing a hot-rolled steel sheet is proposed in which the scale of the surface layer is controlled so as to satisfy the above-described conditions by winding at 400 ° C. or higher after cooling at or above C.
However, these have a drawback that the material varies in the coil longitudinal direction and width direction.

In Patent Document 3, C: 0.001 to 0.20%, Si: 0.01 to 0.5%, Mn: 0.05 to 2.0%, P: 0.05% or less, S: 0 .05% or less, sol. A steel material containing Al: 0.01 to 0.10%, N: 0.02% or less, with the balance being Fe and inevitable impurities, is roughly rolled after heating to Ac ₃ points or more, and then the collision pressure is Perform ultra-high pressure descaling under the condition of ^{2 to 5} kgf / mm ² or more and a liquid density of 0.002 liter / cm ² or more, and subsequently finish rolling at a rolling reduction rate of 80% or more and a rolling end temperature Ar of ₃ points or more. A method for producing a thin-scale hot-rolled steel sheet with a scale thickness of 5 μm or less, which starts within a second and winds at 700 ° C. or less is proposed.

In Patent Document 4, the finish rolling start temperature is set to 920 ° C. to 1050 ° C., and a tension of 1 to 5 kgf / mm ² is applied between the rolling mill immediately before the final rolling mill and the final rolling mill. both surfaces subjected to Desuke and atmosphere shutoff by injecting 20 kg / cm ² or more high-pressure water, the final finish rolling temperature so as not to touch the atmosphere steel sheet back surface right under the final rolling roll bite while keeping the above Ar ₃ point The manufacturing method of the ultra-thin scale steel plate based on cooling and winding up to 400-550 degreeC with the cold speed of 30 degrees C / sec or more is proposed.

  In addition, Patent Document 5 discloses a technique in which a scale layer is cracked by a skin pass, a leveler, or the like to improve the permeability of the acid solution. However, there is a possibility that the base material is work hardened to deteriorate the workability.

JP-A-6-39418 JP 2001-329344 A JP-A-9-67648 JP-A-7-268456 JP-A-4-59116

In the production of a hot-rolled steel sheet having a scale on the surface, if the coiling temperature is lowered, the scale thickness is likely to be reduced, and the pickling property is improved. However, the mechanical properties of the hot-rolled steel sheet deteriorate due to the low winding temperature. In particular, the elongation deteriorates and the workability becomes poor.
On the other hand, if high-temperature winding is performed in order to emphasize workability, the scale tends to grow thick, which is disadvantageous in terms of pickling properties.
The present invention has been devised in order to solve such problems. When producing a hot-rolled steel sheet, the hot-rolled steel sheet has good pickling properties and good mechanical properties. It aims at providing the manufacturing method which can obtain a rolled steel plate.

In order to achieve the object, the method for producing a hot-rolled steel sheet having excellent pickling properties and workability according to the present invention, in mass%, C: 0.001 to 0.30%, Si: 0.05 to 0.00. 40%, Mn: 1.0% or less, P: 0.04% or less, S: 0.02% or less, acid-soluble Al: 0.005 to 0.10%, the balance being Fe and inevitable When hot rolling after continuously casting a steel having an impurity composition, the temperature at the exit side of the hot rolling finish rolling mill is set to Ar ₃ points or more, the coiling temperature is 550 to 700 ° C. A method for producing a hot-rolled steel sheet having excellent pickling properties and workability, characterized by forming an Fe—Si-based oxide at an interface.
The composition of the hot-rolled steel sheet is further Cu: 0.01 to 0.30%, Ni: 0.01 to 0.20%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0 20%, B: 0.0005 to 0.005% of one or more may be contained.

  In the method for producing a hot-rolled steel sheet according to the present invention, in a hot-rolled steel sheet containing a predetermined amount of a component such as Si, the steel sheet temperature and the coiling temperature on the outlet side of the hot finish rolling mill are defined, and the hot-rolling after winding Fe-Si-based oxides are formed at the scale of the steel plate and the base iron interface. For this reason, it becomes possible to manufacture a hot-rolled steel sheet with improved pickling property by improving the penetrability of the acid solution to the surface of the base iron during pickling while maintaining excellent workability.

Schematic of a hot rolling apparatus to which the hot rolling method of the present invention is applied The figure which is one Example of this invention, Comprising: The figure which shows the relationship between pickling completion time and the amount of Si which a hot-rolled steel plate contains

1: Coarse rolling mill 2: Finish rolling mill 3: High-pressure water descaling device 4: Hot run table 5: Coiler

The inventors prepared slabs having various compositions, manufactured hot-rolled steel sheets at various coiling temperatures, and evaluated the pickling properties of these test steels. As a result, it has been found that the pickling property is improved even with an equivalent scale thickness as the amount of Si in the hot-rolled steel sheet increases.
In addition, as a result of detailed analysis of the scale / base iron interface by EPMA, it was found that after the finish rolling after the primary scale was descaled according to the amount of Si contained in the hot-rolled steel sheet, Fe- It was found that a Si-based oxide was formed.

This Fe—Si-based oxide generally exists in the form of firelite (Fe ₂ SiO ₄ ) that is stable at room temperature, and is said to be brittle at room temperature, just like wustite (FeO). From this, it is considered that the rate at which the acid solution permeates the surface of the iron core through cracks in the scale layer is increased, and the pickling property is improved.
Therefore, when hot-rolling a slab containing a predetermined amount of Si, the steel plate temperature and the coiling temperature on the outlet side of the hot finish rolling mill are specified, and the scale of the hot-rolled steel sheet after winding and the interface It was presumed that pickling efficiency would be improved if a Fe—Si-based oxide was positively formed, and the present invention was achieved.

The details will be described below.
In addition, this invention relates to the surface property of the cold rolled steel plate and plated steel plate which used the hot rolled steel plate and this raw material as base steel, and does not specify the kind of the annealing method or plating method of a cold rolled steel plate.

The effects of the chemical components of the hot-rolled steel sheet according to the present invention, the reason for limiting the content, and the hot rolling conditions will be described individually as follows.
C: 0.30% or less C is an element effective for securing strength. As the amount added increases, the workability decreases and the scale peeling becomes non-uniform and wrinkles are likely to occur, so the upper limit was set to 0.30%.

Si: 0.05-0.40%
Si is generally added for deoxidation and strengthening of the steel. As the amount of Si increases, the concentration of Fe-Si-based oxides generated at the interface between the scale layer and the ground iron advances, and the pickling property is improved. If it is 0.05% or less, the concentration of the Fe—Si-based oxide is insufficient and the pickling property is inferior.
On the other hand, when the amount of Si increases, the surface skin becomes dampened by scale bites or red scale, so that the pickling property deteriorates, so the upper limit value was made 0.40% or less.

Mn: 1.0% or less Mn is an element added to improve the strength. However, even if the Mn content exceeds 1.0%, the workability deteriorates although the strength increases with the addition amount. Therefore, 1.0% was made the upper limit.

P: 0.04% or less P is an alloy element effective for increasing the strength. As P increases, Fe-Si-P-based oxides are generated and concentrated at the interface between the scale layer and the ground iron. Since the P-based oxide lowers the eutectic temperature and increases the liquid phase, the scale peelability is good. However, even if the content exceeds 0.04%, the effect on the peelability of the scale is saturated, and grain boundary embrittlement easily occurs and the workability deteriorates. Therefore, 0.04% was made the upper limit.

S: 0.02% or less Since a large amount of S impairs cold or hot workability, it is preferably as small as possible. There is no problem above.
Al: 0.005-0.10%
Al is added as a deoxidizer, but in order to obtain a sufficient deoxidation effect, 0.005% or more of acid-soluble Al needs to be added. The effect of Al deoxidation is saturated at 0.10%, and even if it is added more than that, the cost of the steel is increased.

Cu: 0.01 to 0.30%
Since Cu diffuses from the ground iron into the scale interface and the scale layer and has a low melting point, the peelability of the scale is improved during descaling. For this reason, it adds as needed. However, if the added amount is less than 0.01%, the concentrated layer is not sufficiently concentrated on the side of the ground iron interface. However, if it exceeds 0.30%, it tends to become brittle during hot rolling.

Ni: 0.01-0.20%
Ni is concentrated at the interface between the iron base and the scale, and has the effect of preventing embrittlement of the slab due to Cu melting during slab heating, so it is added as necessary. If the Ni content is less than 0.01%, the concentrated layer will not be sufficiently concentrated on the side of the ground iron interface. However, if it exceeds 0.20%, it becomes easy to become brittle during hot rolling, and the surface properties deteriorate.

Ti: 0.01-0.20%
Ti combines with C, S and N to form precipitates, and is an element effective for increasing the strength of steel strip by precipitation strengthening. Further, these precipitates suppress the recovery of processing strain in the weld heat affected zone and can prevent softening of the heat affected zone due to solid solution and reprecipitation during welding heating. If the addition amount is less than 0.01%, a remarkable effect is not exhibited. However, even if added over 0.20%, the effect is saturated and the manufacturing cost is increased.

Nb: 0.01-0.20%
Nb combines with C in the same way as Ti to form precipitates, and is an element effective for increasing the strength of the steel strip by precipitation strengthening, and refines the metal structure of the steel sheet to improve the strength. Furthermore, in the weld zone, as well as the effect of Ti, it is possible to suppress the recovery of the work strain of the weld heat affected zone by precipitates and to prevent softening of the heat affected zone by solid solution and reprecipitation. . If the addition amount is less than 0.01%, a remarkable effect is not exhibited. However, even if added over 0.20%, the effect is saturated and the manufacturing cost is increased.

B: 0.0005 to 0.005%
B is added as necessary in order to improve aging and hardenability. If the addition amount of B is less than 0.0005%, there is no effect of addition, and if it exceeds 0.005%, the effect is saturated and economically disadvantageous.

Hot rolling finish delivery temperature: Ar ₃ points or more The delivery temperature of hot rolling finish rolling greatly affects the scale thickness. Therefore, in order to reduce the thickness of the scale, it is desirable that the finish delivery temperature is as low as possible. On the other hand, if the finish rolling exit temperature is less than the Ar ₃ point, the rolling becomes a two-phase (gamma + ferrite) region, the ferrite grains become coarse, the surface skin deteriorates, and the workability deteriorates.

Winding temperature: 550-700 ° C
From the viewpoint of scale thickness, the lower the winding temperature, the better. However, when the winding temperature is lowered, the mechanical properties are deteriorated. In particular, since the workability deteriorates due to the decrease in elongation, the lower limit of the winding temperature is set to 550 ° C. or higher. The upper limit is set to 700 ° C. or lower because mechanical properties are deteriorated by the growth of ferrite grains and carbides.

  Steel slabs having the components shown in Table 1 were prepared, heated to 1230 ° C. in a heating furnace, rough rolling was terminated at about 1120 ° C., the scale was removed, and rolling was performed with a finish rolling mill. At that time, the descaling temperature at the entrance of the finish mill was about 1050 ° C., and the exit temperature of the finish mill was 840 ° C. to 920 ° C. Then, it water-cooled on the hot run table, and produced the hot-rolled board with a plate | board thickness of 2.0 mm at coiling temperature 550 degreeC-650 degreeC. Table 2 shows the steel plate temperature and the coiling temperature on the exit side of the finish rolling mill.

From the obtained hot-rolled steel sheet, cross-sectional observation was performed to measure the scale thickness, and the formation of Fe—Si-based oxides on the steel sheet surface scale and ground iron interface was examined by EPMA. Moreover, the pickling property test piece of 50 mm length x 35 mm width was cut out from the obtained hot-rolled steel sheet to evaluate the pickling property.
As a test method, the test piece was immersed in hydrochloric acid having a concentration of 10% heated to 60 ° C., and the time when the residual area ratio of the scale became 0% by changing the immersion time was defined as the pickling completion time.

Table 2 shows the cross-sectional observation result and the pickling completion time together.
Steel type no. 1, no. In steel with a small Si addition amount of 2, the pickling completion time was very long at 120 seconds and 75 seconds. In contrast, no. Ten steel has been pickled in 30 seconds. In these three types of steel, the scale thickness is almost the same. As a result of EPMA analysis, no. No. 1 steel and No. 1 steel. In each of the two steels, generation of Fe-Si-based oxide on the steel plate surface scale and the iron interface was not observed. In Steel No. 10, Fe—Si-based oxides were observed.
Steel type no. 3-No. For the No. 9 Si addition amount of 0.05 to 0.40%, the pickling completion time was as short as 30 to 40 seconds.
In these EPMA analysis results, the formation of Fe—Si-based oxides was observed on the scale of the steel sheet surface and the iron base interface. The scale thickness was 6.0 to 10 μm.

Table 3 shows the relationship between the hot rolling conditions and the mechanical properties of the hot rolled steel sheet.
Steel type No. shown in Table 1 3, no. 4, no. 7 was used to produce a hot-rolled steel sheet by the above-described experimental process. Of the hot rolling conditions, the temperature on the exit side of the finish rolling mill was the same, and the coiling temperature was changed by two levels.
A JIS No. 5 tensile test piece was collected from the produced hot-rolled steel sheet and subjected to a tensile test based on JIS Z 2241 to measure yield strength (YS), tensile strength (TS), and total elongation. As a result, the total elongation of the hot-rolled steel sheet produced within the scope of the present invention was 4 to 6% higher than that of the low-temperature winding material at 550 ° C. or lower.

Claims (2)

In mass%, C: 0.001 to 0.30%, Si: 0.05 to 0.40%, Mn: 1.0% or less, P: 0.04% or less, S: 0.02% or less, When hot-rolling after continuously casting steel having a composition of acid-soluble Al: 0.005 to 0.10% and the balance being Fe and inevitable impurities, Hot rolling excellent in pickling property and workability, characterized in that the temperature is Ar ₃ points or more, the coiling temperature is 550 to 700 ° C., and Fe—Si-based oxides are formed at the scale and the iron base interface on the steel sheet surface A method of manufacturing a steel sheet.   The composition of the hot-rolled steel sheet is further Cu: 0.01 to 0.30%, Ni: 0.01 to 0.20%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20. %, B: 0.0005-0.005% of 1 type or 2 types or more, The manufacturing method of the hot-rolled steel plate excellent in the pickling property and workability of Claim 1.

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