JP6717151B2 - Cooling method for coil steel sheet - Google Patents

Description

The present invention relates to a method for producing a high-tensile steel sheet containing Si, Mn, and Al by hot rolling, and particularly to a coil winding method.

Since high-tensile steel is hard at low temperatures and difficult to cold-roll, it is generally wound at high temperatures.
However, the high-tensile material wound up at a high temperature has a very poor pickling property, and there is a problem that productivity is impaired.
The following are possible reasons for poor pickling properties. In the high-tensile steel material containing Si, Mn, and Al, internal oxidation of Si, Mn, and Al occurs in a wound state at high temperature. The internal oxidation is a state in which oxides of Si, Mn and Al are dispersed inside the steel in a metallic state in the vicinity of the surface of the steel material. The oxides of Si, Mn, and Al may be individual oxides or may be oxides in which two or more of them are combined. These oxides may be distributed at the crystal grain boundaries of the steel or may be distributed within the crystal grains. The case of distribution at the crystal grain boundaries is sometimes called grain boundary oxidation, but here it is collectively referred to as internal oxidation including grain boundary oxidation. This internal oxidation occurs when the oxide scale serves as an oxidation source and oxidizes Si, Mn, and Al in the steel (see FIG. 1). These oxides take the form of internal oxidation generated inside the steel. The internal oxide layer containing such an oxide is inside the steel deeper than the oxide scale on the surface, and long-term pickling is required to remove it. For example, stripping at low speed or pickling twice or more is required.

Various studies have been conducted so far regarding the internal oxidation generated in hot-rolled steel sheets.
Patent Document 1 is intended to improve the chemical conversion treatment property and hot dip galvanizing property of hot-rolled steel sheet and hot-dip hot-rolled steel sheet. However, the cause of plating failure is Si on the surface of hot-rolled steel sheet after pickling. It is thought that the essential cause is that oxides such as Mn and P remain, and in order to solve this, an internal oxide layer is formed in the vicinity of the steel surface layer, and Si of the steel surface is formed inside the internal oxide layer. By encapsulating Mn, P, etc., the plating property and the like are improved. That is, Patent Document 1 proposes to positively generate the internal oxide layer. In addition, specifically, in Patent Document 1, the hot-rolled steel sheet is heat-treated at 650 to 950° C. in an atmosphere in which reduction does not occur, for example, an atmosphere of 100% nitrogen, while keeping the black scale.

Patent Document 2 relates to a method for producing a hot-rolled steel sheet for hot-dip galvanizing for improving the appearance after plating and the adhesion of the plating over the entire surface of the steel sheet. It is considered to be thick (if it is too thin, the surface oxidation of Si cannot be suppressed, and if it is too thick, the internal oxide layer cannot be removed completely, which causes flaws on the steel plate surface), and the thickness of the internal oxide layer is from 1 μm to 4 μm. Proposes to control in range. Specifically, the hot-rolled steel sheet is wound around a coil at 430 to 560° C., then the coil is loaded into a temperature holding facility, and the coil is cooled at a cooling rate slower than atmospheric cooling, whereby an internal oxide layer is formed. The thickness is made uniform within the above range.

JP 2000-309847 A JP, 2015-28200, A

As described above, the high-tensile material wound at a high temperature has a problem that the pickling property is extremely poor and the productivity is impaired. In Patent Documents 1 and 2, the internal oxide layer is positively formed or the thickness thereof is controlled. It is an object of the present invention to provide a method capable of suppressing the generation of internal oxidation of a high-tensile steel sheet, reducing the load of pickling, and improving productivity by a completely new method.

According to the present invention, the following means are provided.
[1]
0.5% by mass% Si≦3.5%, 0.5%≦Mn≦5.0%, 0.5%≦Al≦5.0%, or a total of 0 or more of them. A steel sheet containing 0.5% or more and 5.0% or less and further containing 0.05 to 0.8% by mass of C is hot-rolled and wound into a coil in a temperature range of 600 to 900°C. When cooling, the coil is brought into a pressure state of 10 kPa or less in absolute pressure within 30 minutes after winding, and is cooled in the pressure state until the temperature of the outermost steel plate becomes 500° C. or less. For cooling sheet steel.
[2]
Item 2. The method according to Item 1, wherein the absolute pressure is 1 kPa or less.

According to the present invention, it is possible to suppress the generation of internal oxidation of high-tensile steel sheet, reduce the load of pickling, and improve the productivity.

FIG. 1 is a diagram conceptually showing the progress of internal oxidation of Si, Mn, and Al.

1% or more of 0.5%≦Si≦3.5%, 0.5%≦Mn≦5.0%, 0.5%≦Al≦5.0% in mass% according to the present invention. In the range of 0.5% to 5.0% in total, and further hot rolled a steel sheet containing 0.05 to 0.8% by mass of C in the temperature range of 600 to 900° C. When the coil is wound and cooled in a circular shape, the coil is brought to a substantially vacuum pressure state such as 10 kPa or less or 1 kPa or less in absolute pressure within 30 minutes after the coiling until the steel plate temperature at the outermost periphery becomes 500° C. or less. A method for cooling a coiled steel sheet is provided, which is characterized in that cooling is performed under the pressure or less.

The steel sheet targeted by the present invention contains one or more of 0.5 to 3.5 mass% Si, 0.5 to 5 mass% Mn, and 0.5 to 5 mass% Al. Here, the total content of Si, Mn, and Al is 0.5 to 5.0 mass%, and is a steel plate containing 0.05 to 0.8 mass% C, which is a so-called high-tensile steel. .. From the viewpoint of the quality of high-tensile steel, the content of each element of Si, Mn and Al is defined, while these elements can cause internal oxidation. That is, Si is an element added to secure strength, but it produces internal oxidation with a Si content of 0.5% or more by mass %. Further, if Si exceeds 3.5%, the workability is deteriorated, so the Si content is set to 3.5% by mass or less. Mn is also an element added for improving the strength, but it produces internal oxidation at a Mn content of 0.5% or more by mass %. Further, if Mn is added in excess of 5.0 mass %, the workability deteriorates, so the Mn content is made 5.0 mass% or less. Al is also an element added to improve strength and corrosion resistance, but it causes internal oxidation with an Al content of 0.5% or more by mass %. Further, if Al is added in an amount of more than 5.0% by mass, workability deteriorates, so the amount of Al is set to 5.0% by mass or less. Further, Si, Mn and Al are elements capable of forming a complex oxide and forming an internal oxide layer. Therefore, the total content of these three elements is also specified. If the total content of Si, Mn and Al is less than 0.5% by mass, no complex oxide is formed and no internal oxide layer is formed. Further, when the total content of Si, Mn and Al exceeds 5.0 mass %, an outer oxide layer made of an oxide of Si, Mn and Al is formed and an inner oxide layer is not formed. Therefore, in the present invention, 0.5 to 3.5 mass% of Si, 0.5 to 5.0 mass% of Mn, and 0.5 to 5 mass% of Al are contained in one kind or two or more kinds. The target is a steel plate in which the total content of Si, Mn, and Al is 0.5 to 5.0 mass %.

Further, in the present invention, steel having a C concentration of 0.05% or more and 0.8% or less is targeted. As will be described later in detail with respect to C, it is considered that internal oxidation is caused by oxidation of Si, Mn, and Al in steel by using an oxide scale (FeO etc.) as an oxidation source. The inventors of the present application have found that internal oxidation occurs not through oxidation but through CO or CO ₂ gas. CO and CO ₂ are generated when C in the steel sheet reacts with oxygen (O) in the scale, and when the C concentration in the steel sheet is 0.05% or more, CO and CO ₂ are remarkably generated, and internal oxidation occurs. Therefore, the steel having a C concentration of 0.05% or more is the subject of the present invention. If the C concentration exceeds 0.8%, it will fall within the C concentration range of the hyper-eutectoid steel, and it will not be in the region of the Si, Mn, Al-containing high-tensile steel that is the subject of the present invention, so it is not the subject of the present invention.

The steel containing Si, Mn, Al and C in the above contents is a so-called high-tensile steel, which is hard at low temperature and difficult to cold-roll, and therefore is rapidly wound after hot rolling. Therefore, the winding temperature is 600°C to 900°C. If the temperature is lower than 600°C, the steel plate may become hard and winding may be difficult. In general, the higher the winding temperature, the softer the steel sheet and the easier the winding. The winding may be performed at 650° C. or higher to facilitate the winding. Even if the winding temperature is too high, the ease of winding reaches a ceiling and the heating energy is wasted. Therefore, the upper limit of the winding temperature is set to 900°C.

In the above winding temperature range, Si, Mn and Al are easily oxidized when oxygen is supplied. Oxide scale is present on the surface of the steel sheet, and oxygen in the oxide scale diffuses inward into the steel and reacts with Mn, Si and Al in the steel to form oxides (see FIG. 1). .. The layer containing this oxide is an internal oxide layer, and pickling is not easy. This has been known in the past, and Patent Documents 1 and 2 which are conventional techniques also attempt to control this internal oxide layer.

The present inventor has surprisingly discovered that the progress of internal oxidation is via CO and CO ₂ gas. The mechanism of the progress of internal oxidation will be described.
In a steel material with scale attached at 600° C. or higher, oxygen (O) contained in the scale (including FeO etc.) and carbon (C) contained in the steel react at the scale/steel interface, and CO or CO ₂ Gas is generated. The CO gas and the CO ₂ gas generated at the interface pass through the scale and stay between the hot-rolled steel sheets wound in a coil shape.
These CO and CO ₂ gases reach the following chemical equilibrium with FeO, which is the main composition of the scale, while exchanging oxygen (O) with the scale.
CO+FeO=CO ₂ +Fe
Here, since Fe (steel material) and FeO (scale) are sufficiently present, Fe and FeO reach chemical equilibrium, and accordingly, CO and CO ₂ also reach chemical equilibrium. That is, the ratio of the partial pressure of CO gas and the partial pressure of CO ₂ gas is the ratio of the CO ₂ partial pressure and the CO partial pressure at which FeO and Fe are in equilibrium. Through this chemical equilibrium reaction, the exchange of oxygen (O) is stably performed, and the oxygen partial pressure or oxygen potential is maintained high.
Owing to this high oxygen partial pressure or oxygen potential, oxygen (O) diffuses inward into the steel, reacts with Si, Mn and Al in the steel, and deep internal oxidation proceeds.

The present inventor has conceived that, in the above mechanism of the progress of internal oxidation, the progress of internal oxidation can be suppressed by forcibly removing CO and CO ₂ .
When CO and CO ₂ are not present, oxygen (O) is not supplied via CO and CO ₂ , and oxygen (O) is directly supplied from the scale (FeO) to the steel. Si, Mn, and Al existing in the steel react with oxygen (O) supplied from the scale (FeO), that is, oxygen (O) is consumed, and the oxygen partial pressure or oxygen potential at the scale-steel interface is consumed. Becomes low. In such a state, Fe can exist stably, and reduced Fe is distributed at the scale/steel interface. Fe does not have oxygen (O) as a matter of course, and cannot supply oxygen (O). As a result, internal oxidation does not proceed. As described above, the present inventor has found that the internal oxidation can be suppressed by discharging the gas of CO and CO ₂ accumulated between the plates of the coiled steel plate to the outside of the system.

In order to discharge and remove the CO and CO ₂ gas accumulated between the plates wound in a coil shape, the atmosphere around the coil may be reduced in pressure. If the atmospheric pressure around the coil is lower than the gas pressure between the plates of the coil, the gas between the coil plates is released from between the plates. By utilizing this pressure reducing action, CO and CO ₂ gas accumulated between the plates of the steel plate coil are discharged and removed, thereby reducing the oxygen partial pressure or oxygen potential at the scale/steel interface and generating internal oxidation. The inventor of the present application has found that the above can be suppressed, and completed the present invention.

The invention of the present application is based on a new finding that CO and CO ₂ gas accumulated between plates of a coiled steel plate may be discharged out of the system in order to suppress internal oxidation. The following configuration embodies the emission of CO and CO ₂ gases and the accompanying suppression of internal oxidation.

In the present invention, when the hot rolled steel sheet is wound into a coil at a winding temperature of 600° C. to 900° C. and cooled, the coil is in a substantially vacuum state within 30 minutes after winding, and the absolute pressure is 10 kPa or less or 1 kPa. The pressure is set to the following, and cooling is performed at the pressure or lower until the temperature of the steel plate on the outermost periphery of the winding coil becomes 500° C. or lower.

The winding coil may be brought into a substantially vacuum state by housing the coil in a container and reducing the internal pressure of the container using a vacuum pump. By making the atmospheric pressure around the coils lower than the gas pressure inside the plates of the coils, CO and CO ₂ gas between the coil plates is released from between the plates, and internal oxidation is suppressed. In order to significantly suppress the internal oxidation of the steel sheet, the pressure around the coil is set to 0.01 MPa (10 kPa) or less, which is about 1/10 of atmospheric pressure. By further reducing the pressure around the coil, the effect of suppressing internal oxidation can be further enhanced. When the pressure is reduced to 0.001 MPa (1 kPa) or less, which is about 1/100 of the atmospheric pressure, generation of internal oxidation is no longer recognized.

The presence or absence of internal oxidation can be determined as follows. After the coil has reached normal temperature, it is collected, a sample piece is cut out from the coil, the cross section of the sample piece is observed with a SEM (Scanning electron microscope), and EDX (Energy Dispersive X-ray) analysis is performed to analyze C, Si, O (oxygen). ), Mn, Al, and Fe can be mapped and quantitatively analyzed to determine the thickness of the internal oxide layer. In the present specification, when the thickness of the internal oxide layer is 1 μm or less, the internal oxide layer is evaluated as having no problem.

It is preferable to use a container having a closed structure as a container for accommodating the coil so as to maintain a reduced pressure state. Further, since the coil accommodates the coil wound at a temperature of 600° C. to 900° C., it is preferable to use a container having heat resistance applicable to the temperature range.

Prior to winding the steel sheet into a coil, the surface of the steel sheet is usually exposed to the atmosphere, and it is considered that internal oxidation is limited. In the air, oxygen in the atmosphere produces scales made of iron oxide on the surface of steel materials. During scale growth, the steel surface is constantly oxidized at the scale/steel interface, and the steel surface is sequentially consumed by oxidation. This is because iron, which is the main component of steel, is transformed into scale by oxidation. In such a state, internal oxidation does not occur on the steel surface. This is because the surrounding steel is lost by the oxidation immediately even if the internal oxidation is about to be generated. Therefore, it is considered that the internal oxidation mainly proceeds in the wound coil. This is because the atmosphere hardly penetrates between the plates in the wound coil and the growth of the scale almost stops. Then, as described above, CO and CO ₂ are retained between the plates in the coil (unless the coil is placed in a substantially vacuum atmosphere), the oxygen partial pressure or oxygen potential is maintained high, and internal oxidation proceeds.

In general, the higher the temperature of the steel sheet in the wound coil, the faster the rate of progress of internal oxidation, and particularly the faster the rate of internal oxidation at a steel sheet temperature of 650°C or higher. When the steel sheet temperature is lower than 650°C, the progress of internal oxidation slows down, and when it is lower than 600°C, the internal oxidation does not progress. On the other hand, in a coiled steel material, there may be a temperature distribution in the coil for several hours after winding, and the temperature of the hottest point where the temperature of the steel material is highest and the coldest point of the temperature where the steel material is the lowest The temperature difference may be about 100°C. Generally, the hottest point is near the center of the coil and the coldest point is the outermost circumference of the coil. Therefore, the pressure reducing process is performed until the outermost circumference of the coil becomes 500° C. or less. As a result, the temperature at the hottest point can also be set to 600° C. or lower, and the subsequent generation of internal oxidation can be suppressed. The temperature of the coil such as the coldest point or the hottest point may be measured using a contact type thermocouple, a non-contact type infrared radiation thermometer, or the like.

As described above, the normal hot-rolled steel sheet, especially the high-tensile steel, may be difficult to wind because the steel sheet becomes hard at low temperature, and therefore, the hot-rolled steel sheet is wound at a temperature of 600°C to 900°C. On the other hand, since the coiled steel sheet has a coil shape, the cooling rate is slow, and if normal natural cooling (cooling) is performed, it will be cooled to a temperature (less than 600°C) at which internal oxidation does not proceed. It takes a long time of about 30 minutes to 10 hours. That is, the temperature is 600° C. or higher for about 30 minutes to 10 hours after winding, and internal oxidation may proceed. In the present invention, the coil is exposed to an atmosphere of substantially vacuum (10 kPa or less) within 30 minutes after winding the steel material. With this time, the progress of internal oxidation is within the allowable range. After that, CO and CO ₂ gas accumulated between the plates of the steel plate coil are discharged and removed, whereby the oxygen partial pressure or oxygen potential at the scale/steel interface is reduced, and as a result, the generation of internal oxidation is suppressed. .. The coil may be naturally cooled in the container, but the coil may be forcibly cooled to shorten the time until the outermost circumference of the coil becomes 500° C. or lower. It is considered that the time in the temperature range where internal oxidation can occur is shortened, which is considered to be more useful for suppressing internal oxidation. As a cooling method, a cooling facility such as a water-cooling cooling facility may be provided in the coil storage container, and the storage container may be placed in a low temperature environment.

Hereinafter, the present invention will be described using examples. However, the present invention should not be limitedly interpreted by the examples.

A steel slab containing 0.2 mass% C, 1.0 mass% Si and 2.3 mass% Mn is hot rolled into a hot rolled steel sheet, and the hot rolled steel sheet is coiled at the temperatures shown in Table 1. It was wound into a shape, and then the coil was placed in a container capable of reducing the pressure. Next, under the conditions shown in Table 1, the inside of the container was depressurized by a vacuum pump, and the coil was cooled in the depressurized state. Generation of internal oxidation by changing the pressure in the container during depressurization, the time from winding the coil to the start of depressurization, and the time for cooling while depressurization (in other words, the outermost peripheral temperature at the end of depressurization) The situation was compared. The outermost temperature of the coil was measured by placing the coil in a container and then placing a contact thermocouple on the outermost circumference of the coil. Regarding the state of internal oxidation, after the coil is at room temperature, a sample piece is cut out from the coil, the cross section of the sample piece is observed with a SEM (scannning electron microscope), and C, Si, EDX (Energy Dispersive X-ray) analysis is performed. Mapping and quantitative analysis of O (oxygen), Mn, Al, and Fe were performed to determine the thickness of the internal oxide layer. The results are shown in Table 1. When the thickness of the internal oxide layer was 1 μm or less, the surface quality was evaluated as “good”. When the internal oxidation exceeded 1 μm, the surface quality was considered to be poor and evaluated as x. If none of the internal oxide layers were found, it was set to "none". No. In Example 4, the outer circumference of the decompression container was water-cooled for the purpose of promoting cooling of the coil in the decompression container. By doing so, the coil cooling time can be shortened even when the coil is placed in the vacuum container, and high productivity can be secured.

Comparative Example No. In the coil steel sheet in which the container internal pressure of 5 was exposed to a reduced pressure state of 20 kPa, the reduced pressure was not sufficient, that is, the removal of CO and CO ₂ was not sufficient, and an internal oxide layer with a thickness of 15 μm was observed. Comparative Example No. In No. 6, after the coil was placed in the container, a contact thermocouple was placed on the outermost circumference of the coil to cool while measuring the temperature of the outermost circumference of the coil. When the temperature of the outermost circumference of the coil reached 550°C, the depressurization process (1 kPa) was stopped. At this point, the hottest point temperature inside the coil exceeded 600° C., and it is considered that internal oxidation proceeded, and an internal oxide layer having a thickness of 20 μm was observed. Comparative Example No. In No. 7, 40 minutes after winding the coil, the coil was exposed to a reduced pressure state (1 kPa), and it is considered that the internal oxidation proceeded before the exposure to the reduced pressure state, and an internal oxide layer with a thickness of 10 μm was observed. It was
Inventive Example No. In Nos. 1 to 4, the conditions of temperature, time and pressure specified in the present invention were satisfied, and internal oxidation was not observed.

Claims (2)

0.5% by mass% Si≦3.5%, 0.5%≦Mn≦5.0%, 0.5%≦Al≦5.0%, or a total of 0 or more of them. A steel sheet containing C in an amount of 0.5% or more and 5.0% or less and further containing 0.05 to 0.8 mass% of C is hot-rolled and wound into a coil in a temperature range of 600 to 900°C. When cooling, the coil is brought into a pressure state of 10 kPa or less in absolute pressure within 30 minutes after winding, and is cooled in the pressure state until the temperature of the outermost steel plate becomes 500° C. or less. For cooling sheet steel. The method according to claim 1, wherein the absolute pressure is 1 kPa or less.

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