JP5987675B2 - Steel cooling method and cooling equipment - Google Patents

Description

  The present invention relates to a steel material cooling method and cooling equipment, and more particularly, to a cooling equipment and cooling method used for so-called immersion cooling in which a thick steel material is immersed in a cooling water tank for cooling.

  In producing a thick steel material such as a thick steel plate, in order to obtain high strength and high toughness without adding a large amount of alloy components, it is common to perform heat treatment by roller quenching off-line. However, for example, a steel material such as a very thick steel plate having a plate thickness exceeding 100 mm has a problem in conveyance such as bending of a table roller, so that it is immersed in a cooling water tank and subjected to a quenching process. For example, in a facility as shown in FIG. 5, after the steel material 7 is heated in the heating furnace 1, the steel material 7 is extracted out of the furnace with the carriage 2, and is lifted by the crane 4 and the hanging tool 5 to be water in the cooling water tank 3. The method of immersing in 8 is taken, and the higher the cooling rate during quenching, the higher the strength and toughness of the material will be obtained.

  In order to obtain a high cooling rate, a technique for promoting cooling by causing a jet of high flow velocity to collide with the steel surface is known. This is usually realized by arranging a large number of nozzles in the cooling water tank and injecting a large amount of cooling water over the entire surface of the steel material.

  On the other hand, as a cooling method other than jet collision, a technique of water cooling in a state where a film of oxide or the like is formed on a steel material surface is known.

  In Patent Document 1, a surface film is formed by spraying a powdery surface film material (for example, an oxide ceramic heat-resistant paint or a ceramic heat-resistant paint) from a surface film coating apparatus onto a peeling part of a scale or a thin part of the scale. Thus, a technique is disclosed in which the film thickness including the scale on the steel sheet surface becomes uniform, and the flatness of the steel sheet shape is improved by uniform cooling.

  In Patent Document 2, an oxide film is formed on the surface of a metal member by performing a process for further promoting the generation of an oxide film in addition to the generation of an oxide film inevitably generated in a manufacturing process at a high temperature such as casting and heat treatment. A technique for performing immersion quenching in the above state is disclosed.

Japanese Patent Laid-Open No. 2001-300627 JP 2003-213324 A

  As shown in FIG. 2, the cooling rate of the steel plate thickness center portion at the time of quenching the steel decreases as the steel plate thickness increases. FIG. 2 shows the influence of the plate thickness on the cooling rate at the center of the plate thickness when the thick steel plate heated to 900 ° C. is immersed in water. In FIG. 2, the horizontal axis is the thickness of the thick steel plate, and the vertical axis is the average cooling rate from 800 ° C. to 400 ° C. at the center of the plate thickness. As can be seen from FIG. 2, the average cooling rate from 800 ° C. to 400 ° C. at the center of the plate thickness is, for example, 2 ° C./s when the plate thickness is 100 mm, but is 0.5 when the plate thickness is 200 mm. It becomes very low as ° C / s. Therefore, as the object of the present invention, a measure for increasing the cooling rate is particularly important for heat treatment of a thick steel material that requires heat treatment by immersion cooling.

  By the way, in general, when cooling a high temperature steel material, the surface of the steel material is in a film boiling state in a high temperature region, and the steel material surface is covered with a vapor film, so that the cooling capacity in the film boiling state is small. When the cooling of the steel material further proceeds, the steel material surface temperature decreases and transition boiling in which film boiling and nucleate boiling are mixed occurs, and the cooling capacity increases rapidly.

  Therefore, in order to obtain a high cooling rate, it is effective to cause transition boiling (or nucleate boiling) from the initial stage of cooling by destroying the vapor film. It is known that when a sufficiently high flow velocity jet is made to collide with the steel surface, transition boiling occurs from the initial stage of cooling as shown by a cooling curve B in FIG. 3, and the cooling rate increases. However, for that purpose, in order to realize the state on the entire surface of the steel material, a large number of nozzles capable of forming a high flow velocity jet are arranged in the cooling water tank, and a large amount of cooling water is injected over the entire surface of the steel plate. Therefore, there is a problem that the equipment cost becomes enormous. Furthermore, there is a problem that uneven cooling may occur depending on the arrangement of the nozzles.

  On the other hand, the techniques described in Patent Documents 1 and 2 control the cooling behavior during water cooling in a state where a film such as an oxide is formed on the surface of the steel material. It is noted that it is not always necessary.

  However, the technique of Patent Document 1 is a method of controlling and cooling a thick steel plate after finish rolling of the thick steel plate, and the cooling method is completely different from the quenching treatment of the extra thick steel plate using the cooling water tank targeted by the present invention. The purpose of forming an oxide film is to form an oxide film on a thin part of the scale according to the scale thickness distribution, and the cooling rate of this part is reduced compared to before the oxide film is formed. It is intended to make the speed uniform, improve the flatness of the steel plate shape, and make the material uniform, and is not a technique for actively increasing the cooling capacity of the entire steel material.

  The technique of Patent Document 2 is a technique in which an oxide film is formed on the surface of a steel material and then immersed in a cooling water tank. When immersion and quenching is performed in a state where an oxide film is formed on the surface of a metal member, film boiling is short. The knowledge that the cooling rate will end and the cooling rate will increase will be utilized. And it is described that the cooling rate of a specific part can be raised and the cooling rate of the whole member can be equalize | homogenized by forming an oxide film only in the specific part where the cooling rate at the time of hardening becomes slower than another part.

  However, the technique of Patent Document 2 is a technique for forming an oxide film after applying an oxidizing agent to the surface of a steel material. For this reason, after uniformly applying the oxidizing agent to the steel material surface, if the heating conditions are not strictly controlled, a uniform oxide film cannot be formed, and the uniformity of the oxide film is insufficient. The cooling behavior when immersed in water is also non-uniform, which may adversely affect the uniformity of the material.

  The present invention realizes cooling at a high cooling rate without using a high-speed jet cooling device in the quenching process in which the steel material is immersed and cooled using a cooling water tank, and can obtain a steel material having high strength and high toughness. The object is to provide a method and cooling equipment.

  The present inventor, when forming an oxide film having a predetermined thickness on the steel material surface, does not cause an oxidation reaction after applying an oxidizing agent to form an oxide film, but forms an oxide film on the steel material surface from the beginning. It was conceived that if formed by coating, non-uniformity of the oxide film thickness due to instability of the oxidation treatment can be avoided. Further studies were made to arrive at the present invention.

  The gist of the present invention is as follows.

  [1] A method for cooling a steel material, characterized in that a high-temperature steel material having a film containing a metal oxide or an inorganic oxide on the surface is immersed in a cooling water bath and subjected to an immersion quenching treatment.

  [2] The method for cooling a steel material according to [1], wherein the film containing the oxide has a thickness of 5 mm or less.

  [3] The film containing the oxide is formed at a plurality of locations on the surface of the steel material, and the total area of the film-forming portions containing the oxide is 10% or more of the entire surface of the steel material. The method for cooling a steel material according to [1] or [2], wherein the interval between the formed portions is less than twice the thickness of the steel material.

  [4] The method for cooling a steel material according to any one of [1] to [3], wherein a volume of the cooling water tank is 20 times or more a volume of the steel material.

  [5] A liquid containing a metal oxide or an inorganic oxide is sprayed on the surface of the heated steel material to form a film containing the oxide, and then the steel material is cooled in a water tank for cooling. The method for cooling a steel material according to any one of the above [1] to [4], wherein the steel material is immersed and quenched.

  [6] The above-mentioned [1] to [4], wherein a steel material having a metal oxide or inorganic oxide coated on its surface in a film form is immersed in a cooling water bath after being heated and subjected to immersion quenching treatment The cooling method of the steel materials of any one of Claims 1.

  [7] A heating furnace for heating a steel material, a coating apparatus for spraying a metal oxide or an inorganic oxide on the surface of the steel material before being charged into the heating furnace or after being extracted from the heating furnace, and the entire steel material A cooling equipment for steel material, which is installed close to a cooling water tank that can be immersed in cooling water.

  Since the steel material immersed in the cooling water tank can be cooled at a high cooling rate by using the steel material cooling method and cooling equipment of the present invention, it becomes possible to produce a steel material having high strength and high toughness. Further, since cooling at a high cooling rate is realized without using a high-speed jet cooling device, there is no problem of enormous equipment costs.

It is a figure explaining an example of the coating method to the steel plate of an oxide. It is a figure explaining the relationship between plate | board thickness and the cooling rate of a plate | board thickness center part. It is a figure explaining the relationship between the temperature and time in a quenching process. It is a figure explaining the other example of the application | coating method to the steel plate of an oxide. It is a figure explaining the conventional hardening process using the water tank for cooling. It is a figure explaining the example in which the film | membrane containing an oxide was formed in several places of the steel material surface. It is a figure explaining the example in which the film | membrane containing an oxide was formed in several places of the steel material surface.

In order to cool an extremely thick steel plate at a high cooling rate, a metal oxide (for example, Al ₂ O ₃ , Cr ₂ O ₃ , TiO ₂ , BeO, MgO, ZnO) or inorganic is applied to the surface of the steel plate before heating the extremely thick steel plate. oxide (e.g. _{SiO 2, CaO, SnO 2,} B 2 O 3) high temperature steel having a film containing, may be immersed in immersion quenching treatment to the cooling water tank. Here, the high temperature indicates a temperature equal to or higher than the Ar ₃ transformation point in a general quenching process. In addition, in the case of so-called two-phase quenching in which the austenite-ferrite two-phase region is heated and then cooled, the high temperature indicates a temperature range exceeding the Ar ₁ transformation point and less than the Ar ₃ transformation point.

  In order to form a film containing a metal oxide or an inorganic oxide on the surface of a high-temperature steel material, for example, a metal oxide or an inorganic oxide may be applied to the surface of the steel sheet in the form of a film before heating the very thick steel sheet. it can.

  As a coating method, there is a method in which a liquid containing the oxide is applied to the entire surface of a very thick steel plate placed on a carriage by brushing or the like. Or, for example, as shown in FIG. 4, a liquid containing the oxide is sprayed on one surface of a steel plate (steel material) 7 on a carriage, transported by the carriage 2, lifted by a crane 4, etc., and turned upside down. There is also a method of applying with the spray 6 on the other surface. The oxide in the applied liquid is baked on the surface of the steel plate during heating, and a coating film is formed. When immersion cooling is performed in this state, the cooling capacity (water-cooling heat transfer coefficient) increases five times or more.

  For example, when the plate thickness is 200 mm, the average cooling rate from 800 ° C. to 400 ° C. at the center of the plate thickness increases from the conventional 0.5 ° C./s to 1.0 ° C./s. A tough material can be obtained. With this method, the cooling can be performed at a high cooling rate without using a high-speed jet cooling device or the like, so that there is no problem of enormous equipment costs.

  Also, even before the steel material is heated, even after it is heated, the steel material is applied by spraying the liquid containing the oxide just before being immersed in the cooling water tank 3 as shown in FIG. 7 can be cooled at a high cooling rate. By this method, the oxide can be uniformly applied to the entire front and back surfaces of the steel material, and the entire surface of the steel material can be cooled at a high cooling rate, and a high-quality steel material with high strength, high toughness and small material variation can be obtained. Can be manufactured.

  Next, heating conditions for steel materials, conditions for applying oxides, and the like will be described.

Film thickness of oxide: 5 mm or less If the film thickness of the oxide is 5 mm or less, the cooling rate is increased and a material having high strength and high toughness can be obtained. This is because oxides have good wettability with water, and therefore, by forming an oxide film on the steel material surface, transition boiling or film boiling can be easily realized on the steel material surface. This effect is exhibited when the oxide film thickness is 1 μm or more. Note that the thickness of the oxide is a thickness in a dry state.

  When the film thickness exceeds 5 mm, the oxide is easily peeled off during heating or cooling, and the thermal conductivity of the oxide is lower than that of the steel material, so that the heat conduction from the inside of the steel material is hindered, and the cooling rate is rather high. The problem of deteriorating arises. Therefore, the oxide film thickness is preferably 5 mm or less. More preferably, it is 3 μm or more and 0.5 mm or less. Note that the oxide film thickness is determined in advance by controlling the conditions by obtaining the relationship between the coating conditions and the oxide film thickness described in advance. be able to.

Area ratio of film forming portion: 10% or more Distance between film forming portions: less than twice the thickness of steel material The film forming portion containing the oxide is quenched immediately after water cooling, and its surroundings are also rapidly cooled by heat conduction The Therefore, it is ideal to form an oxide-containing film by coating the oxide on the entire surface of the steel material, but it is not always necessary to form an oxide-containing film on the entire surface of the steel material. The film containing the oxide is formed at a plurality of locations on the surface of the steel sheet, the film forming portion containing the oxide is 10% or more of the entire surface of the steel material, and the distance between adjacent film forming portions is that of the steel material. By making it shorter than twice the wall thickness, the entire steel material can be cooled at a high cooling rate, and a high-strength and high-toughness material can be obtained.

  If the area of the film forming portion is less than 10% of the surface of the steel material, the average cooling rate of the steel material becomes slow, and the cooling rate required to ensure strength and toughness may not be satisfied. Accordingly, the area ratio of the film forming portion is preferably 10% or more of the entire surface of the steel material. More preferably, it is 25% or more of range.

  In addition, when the distance between adjacent film forming portions is twice or more the thickness of the steel material, the difference in the cooling rate within the steel material surface increases, which may cause variations in the material. Therefore, it is preferable that the interval between adjacent film forming portions is less than twice the thickness of the steel material. More preferably, it is within 0.5 times.

  When forming the film | membrane containing an oxide in the several places of the steel plate surface, the shape and magnitude | size are not specifically limited. For example, as shown in the portion 20 where the oxide-containing film in FIGS. 6A to 6D is formed, the rectangular or circular film forming portions can be arranged in a lattice pattern or in a staggered manner.

Cooling tank volume: More than 20 times the volume of steel material Cooling in a cooling water tank that stores water more than 20 times the volume of steel material as a cooling water tank quenching facility will increase the water temperature in the cooling water tank during cooling Since the cooling capacity does not decrease, a material having high strength and high toughness can be secured, which is preferable. When the water in the cooling water tank is less than 20 times the volume of the steel material, the water temperature in the cooling water tank rises during cooling, the cooling capacity decreases, and the strength and toughness deteriorates. The volume is preferably 20 times or more the steel material volume.

  When using the cooling method of the present invention at the time of quenching the steel material, it is preferable to heat the temperature of the steel material before the start of cooling to a temperature at which the entire structure of the steel material is sufficiently austenitized. As a result, the steel is sufficiently fired by subsequent immersion cooling, and a steel material of a uniform material is obtained. If the steel material temperature exceeds 1150 ° C., the austenite grains coarsen during heating, and the finally obtained structure also coarsens, so that the toughness is lowered and a desired material may not be secured. The heating temperature is preferably 1150 ° C. or lower.

  Although the cooling method and the cooling equipment of the present invention exert a great effect when used in the heat treatment process of a thick plate, the present invention is not limited to this, and can be applied to a heat treatment process of general steel materials such as forged products.

  In the present invention, the temperature of the steel material is determined for each steel type, purpose of quenching, etc., when the steel material is transported from the heating furnace to the cooling water tank through the coating apparatus, or from the coating apparatus to the cooling water tank. It is preferable that the heating furnace, the coating device, and the cooling device are close to each other to such an extent that the quenching temperature (the steel material temperature immediately before the cooling treatment) can be secured.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  The heat treatment equipment shown in FIG. 1 was used. The steel plate 7 having a weight of 25 ton, a plate thickness of 150 mm, a plate width of 2500 mm, and a length of 8500 mm was reheated to 900 ° C. in the heating furnace 1 with the carriage 2, and then the steel plate 7 was extracted from the heating furnace 1 with the carriage 2 and the steel plate with the crane 4. 7 was lifted using the lifting tool 5 so that the longitudinal direction thereof was substantially vertical.

Moving the steel plate 7 above the cooling water tank 3, a ratio shown in Table 1 of SiO ₂ on the surface of the steel sheet 7 by spraying by a spray nozzle 6 a liquid containing a SiO ₂ before immersion in the cooling water tank 3 The steel plate 7 was lowered and immersed in the cooling water tank 3 in which 200 m ³ of water was stored, and the whole steel plate 7 was cooled to 100 ° C. or lower. For comparison, an immersion test was also performed when no oxide was applied.

In order to ensure the target material (strength / toughness) in the steel used in this example, the average cooling rate between 800 ° C. and 400 ° C. at the center of the plate thickness is 1.2 ° C./s or more. The average cooling rate between 800 ° C. and 400 ° C. at the center of the plate thickness was set to 1.2 ° C./s or more. Here, the temperature at the center of the plate thickness was measured by a thermocouple attached with a hole drilled to the center of the steel plate.
Test conditions and test results are shown in Table 1.

  In Comparative Example 1, immersion cooling was performed in a cooling water bath with no coated steel sheet surface. The cooling rate at the center of the plate thickness was as low as 0.9 ° C./s, which was lower than the target value. In this way, the target cooling rate was not obtained, so in order to achieve the target strength and toughness under the same cooling conditions, it is necessary to employ steel with a component system added with a large amount of alloy components. I knew it wouldn't be.

In Invention Example 1, before heating, a liquid containing SiO ₂ was sprayed to form a film having a thickness of 0.05 mm and an area of 10% of the steel sheet surface. Here, as a liquid containing a SiO _2, with water containing 1 mass% of SiO ₂ particles having an average particle size of 0.1 [mu] m. A circular full cone spray was used for the coating operation, and circular films were formed in a staggered arrangement as shown in FIG. The total number of coated portions on the front and back surfaces of the steel sheet was 550, and the maximum distance between adjacent coated portions was 250 mm.

The steel plate thus obtained was immersed and cooled in a cooling water tank. Transition boiling occurred at the start of cooling in the SiO ₂ coated part, and the cooling rate at the central part of the plate thickness was as high as 1.5 ° C./s. In other parts as well, a large amount of heat flowed to the nearby SiO ₂ coating part, and as a result of transition boiling soon, the cooling rate at the center part of the plate thickness was 1.2 ° C./s, and the target was achieved. In this case, a material having high strength and high toughness can be obtained over the entire surface of the steel sheet without using a component steel to which a large amount of alloy components are additionally added.

In Invention Example 2, after heating, a liquid containing SiO ₂ was sprayed to apply SiO ₂ to the entire surface of the steel sheet and to cool by immersion in a cooling water tank. Again, as a liquid containing a SiO _2, with water containing 1 mass% of SiO ₂ particles having an average particle size of 0.1 [mu] m. With the start of cooling, transition boiling occurred and the cooling rate was 1.5 ° C./s, achieving the target. Also in this case, a material having high strength and high toughness can be obtained over the entire surface of the steel sheet without using a component steel to which a large amount of alloy components are additionally added.

In Invention Example 3, after heating the steel sheet and before immersion cooling, the liquid containing SiO ₂ was sprayed to form a film having a thickness of 0.05 mm and an area of 10% of the steel sheet surface. Here, as a liquid containing a SiO _2, with water containing 1 mass% of SiO ₂ particles having an average particle size of 0.1 [mu] m. A circular full cone spray was used for the coating operation, and circular films were formed in a staggered arrangement as shown in FIG. The total number of coated portions on the front and back surfaces of the steel sheet was 550, and the maximum distance between adjacent coated portions was 250 mm.

The steel plate thus obtained was immersed and cooled in a cooling water tank. Transition boiling occurred at the start of cooling in the SiO ₂ coated part, and the cooling rate at the central part of the plate thickness was as high as 1.5 ° C./s. In other parts as well, a large amount of heat flowed to the nearby SiO ₂ coating part, and as a result of transition boiling soon, the cooling rate at the center part of the plate thickness was 1.2 ° C./s, and the target was achieved. In this case, a material having high strength and high toughness can be obtained over the entire surface of the steel sheet without using a component steel to which a large amount of alloy components are additionally added.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Carriage 3 Cooling water tank 4 Crane 5 Lifting tool 6 Spray nozzle 7 Steel (steel plate)
8 Water 20 Part where oxide-containing film is formed A Normal cooling B Cooling when transition boiling occurs from the initial stage of cooling

Claims (5)

A film containing a metal oxide or an inorganic oxide is applied to the surface of the heated steel material in a spray form to form a film containing the oxide, and then the steel material is immersed in a cooling water bath. A method for cooling a steel material, characterized by performing immersion quenching treatment.   The method for cooling a steel material according to claim 1, wherein the film containing the oxide has a thickness of 5 mm or less.   The oxide-containing film is formed at a plurality of locations on the surface of the steel material, and the total area of the film-forming portions containing the oxide is 10% or more of the entire surface of the steel material. The method for cooling a steel material according to claim 1 or 2, wherein the interval is less than twice the wall thickness of the steel material.   The method for cooling a steel material according to any one of claims 1 to 3, wherein a volume of the cooling water tank is 20 times or more a volume of the steel material. Close the heating furnace for heating the steel material, a coating apparatus for applying a sprayed form a metal oxide or an inorganic oxide on the steel surface after extracting from a pressurized hot furnace, and a cooling water tank capable of immersing the entire steel to the cooling water Steel cooling equipment, characterized by being installed.

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