JP4987786B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet suitable for applications that require a beautiful surface, such as an automobile.

  Hot-rolled steel sheets used for automobiles and industrial machines are generally manufactured through a rough rolling process and a finish rolling process. In the production process of a hot-rolled steel sheet, first, a slab obtained by continuously casting molten steel adjusted to a predetermined composition is rolled by a roughing mill, and then hot rolled by a finish rolling mill constituted by a plurality of rolling stands. Rolled into a hot-rolled steel sheet having a predetermined thickness.

  In such a method for producing a hot-rolled steel sheet, a scale made of an oxide is generated on the surface of the rough-rolled steel sheet after the rough rolling. Since the scale causes wrinkles (scale wrinkles) in the subsequent finish rolling, high pressure water is sprayed from the nozzle of the descaling device toward the rough rolled steel sheet in the conventional hot rolled steel sheet manufacturing method. Thus, after removing the scale on the surface of the steel plate, finish rolling is performed.

  However, even if descaling is performed after rough rolling to remove the scale on the surface, the scale is generated again in the subsequent finish rolling process, and there is a problem that scale wrinkles occur in the product. Specifically, the scale generated during the hot rolling of the first stage rolling stand (hereinafter also referred to as the first stand) is the first stand and the second stage rolling stand (hereinafter referred to as the second stand). It also grows between.

  The scale on the grown surface partially swells and peels off or cracks, and is pushed into the steel plate during rolling at the second and subsequent rolling stands, which causes scale defects and poor appearance. Cause.

For this reason, conventionally, for example, in Patent Document 1, cooling water is sprayed onto the surface of the hot-rolled steel sheet on each of the entrance side and exit side of the hot-roll stand, suppressing the generation of scale between the stands, Improvement techniques have been proposed. Incidentally, in the disclosed technique in Patent Document 1, the rolling reduction per rolling stand in the finish rolling mill row is controlled within a predetermined range, and at least three stands from the entrance side of the finishing rolling mill row are cooled. Water is sprayed at an average water volume of 5 to 50 l / min / m ² for 0.3 to 4 seconds, and on the exit side of each stand, cooling water is injected at an average water volume of 10 to 100 l / min / m ² to 0.1 to 1.. Spray for 5 seconds.

However, in the disclosed technique of Patent Document 1, the cooling water in each stand is at least an average water amount of 5 l / min / m ² , and the cooling water amount itself is very large, so that the surface temperature of the hot-rolled steel sheet is greatly reduced. In addition, temperature unevenness is likely to occur, and the mechanical properties of the steel material may vary due to the temperature unevenness. In particular, when manufacturing a product with a thin finish plate thickness, it is necessary to set the surface temperature of the hot-rolled steel sheet higher in the finish rolling process. In order to cool too much, when trying to roll the hot-rolled steel sheet thinner, there is a problem that the material is lowered.

Further, in Patent Document 2, there is a manufacturing facility in which a plurality of descaling devices having an injection pressure of 20 kg / cm ² or more and less than 100 kg / cm ² are arranged along the running direction of the hot-rolled steel sheet above and below the run-out table. It is disclosed. In the technology disclosed in Patent Document 2, scale is removed by spraying high-pressure water onto a hot-rolled steel sheet that has been thinned by hot rolling, but the scale thickness of the product can be reduced. However, there was a problem that it was not possible to prevent the occurrence of surface flaws due to scale swelling generated between rolling stands.
JP 2002-086209 A JP 2006-247707 A

  The present invention has been devised in view of the above-described problems, and provides a method for producing a hot-rolled steel sheet capable of improving the surface shape by suppressing the occurrence of scale wrinkles during hot rolling. For the purpose.

The method for producing a hot-rolled steel sheet according to claim 1 comprises a rough rolling step of roughly rolling a slab produced by continuous casting to obtain a rough rolled steel sheet, and a finish rolling mill comprising the rough rolled steel sheet comprising a plurality of rolling stands. A finish rolling step in which hot rolling is performed to obtain a hot rolled steel plate, and the finish rolling step is performed in all rolling stands in the surface temperature T (° C.) of the steel plate after passing through each rolling stand and the surface of the steel plate. relationship of the oxygen concentration in the atmosphere in contact with the M (% by volume) is, meets the following formula (1), and the oxygen concentration M is 10% by volume or more (except for 10% by volume) so that the Adjusting the surface temperature T of the steel sheet and the oxygen concentration M M ≦ 36.4−0.022 × T (1)

The method for producing a hot-rolled steel sheet according to claim 2 is the invention according to claim 1, wherein the finish rolling step injects nitrogen gas to the steel sheet after passing through the rolling stand, thereby It is characterized by controlling M (volume%).

The method for producing a hot-rolled steel sheet according to claim 3 is the invention according to claim 1, wherein the finish rolling step is performed by using 1.5 to 4.5 l of cooling water for the steel sheet after passing through the rolling stand. The oxygen concentration M (volume%) is controlled by spraying in the form of a mist at a rate of / m ² / min.

  The method for producing a hot-rolled steel sheet according to claim 4 is characterized in that, in the invention according to claim 3, the water droplet size is sprayed in a mist shape of 0.1 mm or less.

  The method for producing a hot-rolled steel sheet according to claim 5 is the invention according to any one of claims 1 to 4, wherein, in the finish rolling step, a scale growth rate between the rolling stands is 1.2 μm / second. It is characterized by suppressing to the following.

  In the present invention having the above-described configuration, the occurrence of scale swelling and peeling can be prevented, and as a result, the surface properties of the steel sheet can be improved.

  Hereinafter, the best mode for carrying out the present invention will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  The present inventors performed the following experiment in order to find out the conditions for preventing scale swelling between rolling stands in the hot-rolled steel sheet after the finish rolling process.

After heating the steel plate to a temperature of 800 ° C. to 1100 ° C. in a nitrogen atmosphere, put it in a box with an oxygen concentration changed, hold it for 3 to 10 seconds, and immediately put it in a box in a nitrogen atmosphere. Cooled in non-oxidized state. As a result of investigating the swelling state of the scale on the surface of the steel sheet after cooling, as shown in FIG. 1, the scale when the temperature T (° C.) and the oxygen concentration M (volume%) of the steel sheet satisfy the following formula (1). I found that no blistering occurred.
M ≦ 36.4−0.022 × T (1)

  Furthermore, the occurrence of scale bulge is more affected by the growth rate of the scale than the thickness of the scale. By suppressing the scale growth rate to 1.2 μm / second or less, the bulge of the scale can be prevented. As a result, it discovered that the surface property of the steel plate could be improved.

Also, by injecting nitrogen gas against the steel sheet after passing through the rolling stand, it has been found that it is effective to control the oxygen concentration M (% by volume).

Further, the finish rolling process for steel sheet after passing through a rolling stand, water droplet size of the cooling water at a rate of 1.5~4.5l / m ² / min is injected below the mist 0.1mm Thus, it has been found that it is effective to control the oxygen concentration M (volume%).

  Hereinafter, the detail of the finish rolling mill for implementing the manufacturing method of the hot-rolled steel plate to which this invention is applied is demonstrated. As shown in FIG. 2, nitrogen gas injection cylinders 22 are formed in a plurality of stages between a first stage rolling stand 21a and a second stage rolling stand 21b. This nitrogen gas injection cylinder 22 injects nitrogen gas to the hot-rolled steel sheet 24 rolled by the rolling stand 21a. By injecting nitrogen gas to the hot-rolled steel sheet 24, the oxygen concentration in the atmosphere around the hot-rolled steel sheet 24 can be reduced.

  Further, the finish rolling mill 13 may be provided with, for example, a cooling water spray cylinder 23 as an alternative to disposing the nitrogen gas injection cylinder 22 shown in FIG. The cooling water spray cylinder 23 can reduce the oxygen concentration in the atmosphere around the hot-rolled steel sheet 24 by spraying the cooling water in a mist form.

Moreover, these nitrogen gas injection cylinders 22 and cooling water spray cylinders 23 should just be arrange | positioned after the 1st stage rolling stand 21a .

  In addition, in the manufacturing method of the hot-rolled steel sheet in this embodiment, steel composition is C: 0.01-0.3%, Si: 0.001-1.5%, Mn: 0.05-2.5%. , P: 0.001 to 0.1%, S: 0.001 to 0.04%, Al: 0.005 to 1.5%, N: 0.001 to 0.015%, necessary Accordingly, Cr: 0.01 to 0.5%, Cu: 0.1 to 1.5%, Ni: 0.1 to 1.5%, Ti: 0.02 to 0.15%, Nb: 0.0. 01-0.1%, V: 0.01-0.1%, Mo: 0.01-0.1%, Ca: 0.001-0.05%, B: 0.0003-0.005% Is preferably added.

  Hereafter, the preferable range of the component of steel composition is demonstrated.

C: 0.01 to 0.3%
C is an element necessary for ensuring the strength of the steel sheet. However, when the C content is less than 0.01%, it is not possible to meet the demand for high strength, and the appearance is good without generating scale wrinkles without using this method. On the other hand, if the C content exceeds 0.3%, the generation of wrinkles due to the scale becomes severe, which is not preferable for aesthetic reasons. For this reason, C content shall be 0.01 to 0.3%.

Si: 0.001 to 1.5%
Si is an element necessary for ensuring the strength of the steel sheet. In addition, as Si is added, the adhesion of the scale is improved and the appearance is improved. If the Si content is less than 0.001, the manufacturing cost increases, which is not preferable. On the other hand, when the Si content exceeds 1.5%, the amount of Si oxide increases, which causes the appearance to deteriorate. For this reason, Si content shall be 0.001-1.5%.

Mn: 0.05 to 2.5%
Mn has the effect of ensuring the strength of the steel sheet and preventing cracking during hot rolling due to S in the steel. However, when the Mn content is less than 0.05%, cracking during hot rolling cannot be prevented, and the appearance properties are deteriorated. Further, when the Mn content exceeds 2.5%, the effect is saturated, and on the contrary, wrinkles due to scale become remarkable. For this reason, the Mn content is set to 0.05 to 2.5%.

P: 0.001 to 0.1%
P is an element inevitably contained in steel as an impurity. If the P content is less than 0.001, the production cost increases, which is not preferable. Moreover, when P content exceeds 0.1%, a streaky pattern will generate | occur | produce on the steel plate surface by local segregation, and external appearance property will fall. For this reason, the P content is set to 0.001 to 0.1%.

S: 0.001 to 0.04%
S is an element contained as an impurity like P, and is not an element added intentionally. If the S content is less than 0.001, the production cost increases, which is not preferable. Moreover, when this S exceeds 0.04%, roughening will arise in the steel plate surface at the time of hot rolling. For this reason, S content shall be 0.001-0.04%.

Al: 0.005 to 1.5%
Al is necessary as a deoxidizing element for producing steel, and is used for preventing ductility deterioration due to N and ensuring the workability of the steel sheet. If the Al content is less than 0.005%, the above-described effects cannot be exhibited sufficiently. If the Al content exceeds 1.5%, the effects are saturated. For this reason, Al content shall be 0.005-1.5%.

N: 0.001 to 0.015%
N is an element that is inevitably mixed in steel because nitrogen in the air is taken in during the treatment of molten steel. Since this N causes the ductility fall of a steel plate, the one where it is smaller is desirable. However, if this N is less than 0.001, the crystal grains of the steel sheet may locally grow abnormally. In this case, there are disadvantages such as a local decrease in strength. Moreover, when this N exceeds 0.015%, the ductility fall of a steel plate will become remarkable. For this reason, N content shall be 0.001-0.015%.

Cr: 0.1 to 0.5%
Cr is effective for increasing the strength, but if added in excess, the toughness is lowered, so the upper limit is made 0.5%. Moreover, since the effect is not exhibited at 0.1% or less, the lower limit is set to 0.1%.

Cu: 0.1 to 1.5%
Cu has the effect of improving the degree of adhesion of the scale as the strength of the steel sheet increases. If this Cu exceeds 1.5%, the surface of the steel sheet in hot rolling becomes rough, so the upper limit is made 1.5%. Moreover, since the effect is not exhibited at 0.1% or less, the lower limit is set to 0.1%.

Ni: 0.1 to 1.5%
Ni is an element useful for improving hardenability and preventing low-temperature embrittlement, and is an element effective for improving the adhesion of the scale. However, since the effect is saturated when Ni exceeds 1.5%, the upper limit is set to 1.5%. Moreover, since the effect is not exhibited at 0.1% or less, the lower limit is set to 0.1%.

Ti: 0.02 to 0.15%
Addition of 0.02% or more of Ti is effective in improving toughness through fine graining. Further, Ti has a strong affinity for N and precipitates as TiN during solidification, and is used for preventing ductility deterioration due to N. However, if this Ti exceeds 0.15%, the effect is saturated, so the upper limit is made 0.15%.

Nb: 0.01 to 0.1%
When Nb is added in an amount of 0.01% or more, the toughness is improved by making the structure finer. Nb also plays a role of preventing ductility deterioration due to N, but its effect is saturated when the Nb content exceeds 0.1%. For this reason, the upper limit of Nb is set to 0.1%.

V: 0.01 to 0.1%
V is an element that increases the strength of the base material and improves toughness by adding 0.01% or more. However, when the V content exceeds 0.1%, the effect is saturated. For this reason, the upper limit of V is set to 0.1%.

Mo: 0.01 to 0.1%
Mo is not only useful for improving the strength of steel, but also greatly improves toughness. However, when the Mo content exceeds 0.1%, the effect is saturated, so the upper limit is set to 0.1%. Moreover, since the effect is not exhibited at 0.01% or less, the lower limit is made 0.01%.

Ca: 0.001 to 0.05%
Ca is added to prevent a reduction in strength of the steel sheet due to MnS generation. In order to exert such an effect, it is necessary to contain at least 0.001%, but when the Ca addition amount exceeds 0.05%, the effect is saturated, so the upper limit is made 0.05%. .

B: 0.0003 to 0.005%
Since B also has an effect of fixing N as BN, it plays a role of preventing a decrease in ductility of the steel sheet due to N, and also has an effect of improving brazing strength. However, if this B content exceeds 0.005%, the effect is saturated, so the upper limit is made 0.005%. Moreover, since the effect is not exhibited at 0.0003% or less, the lower limit is made 0.0003%.

  Next, the conditions of the manufacturing method of the hot rolled steel sheet to which the present invention is applied will be described in detail.

  The temperature of the steel sheet surface after passing through each rolling stand of the finishing mill is defined as T (° C.). Moreover, it is set as the oxygen concentration M (%) in the atmosphere which contacts a steel plate surface. The atmosphere in contact with the steel sheet surface here means a region within 2 cm from the steel sheet surface. Specifically, this oxygen concentration M represents the oxygen concentration at a position 10 mm away from the steel plate surface when nitrogen gas or cooling water is sprayed onto the steel plate surface. The oxygen concentration M is measured by injecting nitrogen gas or cooling water onto the surface of the hot rolled steel sheet heated to various temperatures, and measuring the oxygen concentration at a position 10 mm away from the steel sheet surface with an oxygen sensor.

At this time, the steel sheet surface temperature T and the oxygen concentration M are adjusted so as to satisfy Equation (1).
M ≦ 36.4−0.022 × T (1)

  Incidentally, the relationship of this mathematical formula (1) needs to be established in all sections from the rolling stand 21a to the rolling stand 21g. For this reason, in the above-described example, the nitrogen gas injection cylinder 22 is not limited to the case where it is provided only between the first stage rolling stand 21a and the second stage rolling stand 21b. It may be disposed between any rolling stands 21 within a range satisfying the relationship of Expression (1).

  Actually, in order to control the oxygen concentration M in the mathematical formula (1), in the finish rolling process, nitrogen gas is injected to the hot-rolled steel sheet 24 after passing through the rolling stand 21 through the nitrogen gas injection cylinder 22. To do. By injecting this nitrogen gas, it is possible to increase the nitrogen concentration in the atmosphere in contact with the surface of the hot-rolled steel sheet 24 and thus to decrease the oxygen concentration. Actually, in order to control the oxygen concentration M, an oxygen sensor (not shown) is arranged in the vicinity of the steel plate surface, and the amount of nitrogen gas discharged from the nitrogen gas injection cylinder 22 is controlled while measuring the oxygen concentration. .

  Further, as an alternative to providing the nitrogen gas injection cylinder 22, when the cooling water spray cylinder 23 is provided, the cooling water is sprayed in a mist form on the hot-rolled steel sheet 24 after passing through the rolling stand 21. Fine water droplets can be uniformly sprayed onto the hot-rolled steel sheet 24, and the water droplets or the heat of the steel sheet becomes water vapor to cover the surface of the hot-rolled steel sheet 24. As a result, the oxidation rate can be reduced without reducing the temperature of the hot-rolled steel sheet 24.

  As an alternative to providing the nitrogen gas injection cylinder 22, even when the cooling water spray cylinder 23 is provided, in order to control the oxygen concentration M, an oxygen sensor (not shown) is arranged in the vicinity of the steel plate surface to measure the oxygen concentration. The amount of water and the droplet size of the mist-like cooling water sprayed from the cooling water spray cylinder 23 are controlled.

Incidentally, in the method of manufacturing a hot-rolled steel sheet to which the present invention is applied, the cooling water to be injected through the cooling water spray cylinder 23 is sprayed in a mist at a rate of 1.5 to 4.5 l / m ² / min. It is stipulated as an essential component requirement. As the reason, when the amount of water is less than 1.5 l / m ² / min, the amount of water vapor is insufficient, and the oxygen concentration is lowered in the atmosphere in contact with the hot-rolled steel sheet 24. As a result, the scale growth rate in the hot-rolled steel sheet 24 increases, so that swelling occurs, and the surface properties after hot rolling are reduced. On the other hand, when the amount of water exceeds 4.5 l / m ² / min, the temperature of the hot-rolled steel sheet 24 is excessively cooled, and the mechanical properties of the steel material may vary due to temperature unevenness. That is, since the surface temperature of the hot-rolled steel sheet 24 is excessively cooled, when the thickness of the hot-rolled steel sheet 24 is reduced, the material is lowered. For this reason, in the manufacturing method of the hot-rolled steel plate to which the present invention is applied, it is a requirement that the cooling water is sprayed in a mist at a rate of 1.5 to 4.5 l / m ² / min.

  In the present invention, the water droplet size is required to be sprayed in a mist shape of 0.1 mm or less. If the water droplet size exceeds 0.1 mm, the sprayed water droplet does not immediately become water vapor when it comes into contact with the hot-rolled steel plate 24, and remains on the surface of the hot-rolled steel plate 24 as a water droplet. As a result, a large amount of water droplets adhere to the surface of the hot-rolled steel sheet 24, so that a region having a high oxygen concentration is partially generated. In this region, the scale growth rate is increased and the surface properties are deteriorated. It is because it will do. In addition, the water droplet size referred to here is sprayed on a plastic resin plate such as an acrylic plate fixed with a mist-like cooling water sprayed from the cooling water spray cylinder 23 separated from the surface of the hot-rolled steel plate 24 by 150 mm, This is the diameter of a water droplet measured by enlarging it with an optical microscope.

  In the finish rolling process, the scale growth rate between the rolling stands 21 is suppressed to 1.2 μm / second or less. This scale growth rate is set to be non-oxidized by removing the steel plate that has been non-oxidized and heated to the same temperature as the finish rolling in a nitrogen atmosphere into the atmosphere and simultaneously spraying spray water under various conditions and then immediately placing it in a box with a nitrogen atmosphere. The scale thickness produced by cooling, cutting the steel sheet and observing the cross section with a microscope was measured. By suppressing the scale growth rate to 1.2 μm / second or less, generation of scale bulge can be prevented, and as a result, the surface properties of the steel sheet can be improved. On the other hand, when the scale growth rate cannot be suppressed to 1.2 μm / second or less, a part of the generated scale swells and easily peels off.

  The present invention is not limited to the embodiment described above. Of course, any means may be used as means for achieving the condition of the mathematical formula (1) other than injecting nitrogen gas to the steel plate or injecting cooling water in the form of mist.

Examples of the present invention will be described below. First, as an example of the present invention, a hot-rolled steel sheet was produced based on the above-described method for producing a hot-rolled steel sheet. Specifically, after heating a slab having a thickness of 250 mm to 1210 ° C. with the composition shown in Table 1 below, the slab was roughly rolled to a thickness of 40 mm by a rough rolling machine 12 to prepare a rough rolled steel sheet. Next, water is sprayed at a pressure of 15 MPa by the descaling equipment 16 toward the rough rolled steel sheet, and the scale generated on the surface is removed, and then immediately passed between the seven rolling stands 21a to 21g. The hot rolled steel sheet having a thickness of 4.5 mm was manufactured by performing finish rolling to hot rolling. At this time, the surface temperature T (° C.) of the hot-rolled steel sheet after passing through the rolling stand 21 a and the oxygen concentration M in the atmosphere in contact with the steel sheet surface are adjusted via the discharge amount of nitrogen gas from the nitrogen gas injection cylinder 22. The surface properties of the hot-rolled steel sheet from the finish rolling mill 13 were observed. Judgment criteria for the surface properties are the observation of the appearance of the hot-rolled steel sheet with the scale attached, the width of the steel sheet at an arbitrary position x length of 2 m, the width of the scale color different from the surroundings is 1 mm or more, and the length is 5 mm or more Those having 5 or more size regions were regarded as having poor surface properties, and those having less than that were considered to have good surface properties. The results are summarized in Table 2 below. Table 2 below also shows the surface temperature T (° C.) and the oxygen concentration M (volume%) in each example and comparative example .

  The balance in the steel composition shown in Table 1 below is Fe and inevitable impurities. Moreover, the underline in the following Table 2 shows that it is outside the scope of the present invention. Moreover, the bold type in Table 1 has shown the upper limit or lower limit in each component.

  Invention Examples 1 to 10 shown in Table 2 satisfied the formula (1), and the surface properties were all good. On the other hand, Comparative Examples 1-4 did not satisfy Formula (1), and none of the surface properties was good.

  In addition, the oxygen concentration M in the atmosphere in contact with the steel sheet surface is adjusted by spraying the cooling water in a mist form with the cooling water spray cylinder 23, and the surface properties of the hot-rolled steel sheet from the finish rolling mill 13 are adjusted. The observation results are shown in Table 3.

In Table 3, the surface temperature T (° C.), the cooling water injection rate (l / m ² / min), the water droplet size (mm), the oxygen concentration M (%), and the formula in each example and comparative example are shown. The value defined by (1) is also shown.

  Invention Examples 11 to 24 shown in Table 3 satisfied the mathematical formula (1), and the surface properties were all good. On the other hand, Comparative Examples 5 to 11 did not satisfy Expression (1), and none of the surface properties were good.

It is a figure for demonstrating verification of Numerical formula (1) equivalent to the technical feature of this invention. It is a figure for demonstrating the detail of a finish rolling mill. It is another figure for demonstrating the detail of a finish rolling mill.

Explanation of symbols

13 Finishing Roller 22 Nitrogen Gas Injection Tube 23 Cooling Water Spray Tube 24 Hot Rolled Steel Sheet

Claims (5)

A rough rolling step of roughly rolling a slab produced by continuous casting to obtain a rough rolled steel sheet,
A finish rolling step for obtaining a hot-rolled steel sheet by hot rolling the rough rolled steel sheet with a finish rolling mill comprising a plurality of rolling stands;
In the finish rolling process , the relationship between the surface temperature T (° C.) of the steel sheet after passing through each rolling stand and the oxygen concentration M (volume%) in the atmosphere in contact with the steel sheet surface is expressed by the following formula. (1) meets, and oxygen concentration M is 10% by volume or more (except for 10% by volume) such that, M ≦ 36.4 adjusting the oxygen concentration M with the surface temperature T of the steel sheet -0.022 × T (1)
A method for producing a hot-rolled steel sheet. 2. The hot rolled steel sheet according to claim 1, wherein the finish rolling step controls the oxygen concentration M (volume%) by injecting nitrogen gas to the steel sheet after passing through the rolling stand . Production method. In the finish rolling step, the oxygen concentration M (volume) is obtained by spraying cooling water in a mist at a rate of 1.5 to 4.5 l / m ² / min on the steel sheet after passing through the rolling stand. %) Is controlled. The method for producing a hot-rolled steel sheet according to claim 1. The method for producing a hot-rolled steel sheet according to claim 3, wherein the water droplet size is sprayed into a mist having a size of 0.1 mm or less. In the said finish rolling process, the scale growth rate between the said rolling stands is suppressed to 1.2 micrometers / second or less. The manufacturing method of the hot-rolled steel plate in any one of Claims 1-4 characterized by the above-mentioned.

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