Steel-strip heat treatment method

Abstract

Provided is a steel-strip annealing method with which it is possible to efficiently perform manufacturing even with a small lot of material. Provided is a steel-strip heat treatment method which is the annealing method for a steel strip having a sheet thickness of 1.0 mm or more, wherein the surface of the steel strip is subjected to a pretreatment in which the emission rate is locally changed, and the steel strip subjected to the pretreatment is subjected to a heat treatment.

Description

Heat treatment method for steel strip

[0001] The present invention relates to a method for heat treating steel strip.

[0002] Steel sheets are manufactured by hot rolling, cold rolling, etc. of steel slabs, which are raw materials. In this process, it is common to apply heat treatment to the rolled steel strips in order to control the microstructure or texture and improve various properties.

[0003] Annealing is one of the representative heat treatments. For example, Patent Documents 1 and 2 disclose continuous annealing of hot-rolled steel strips and cold-rolled steel strips. The steel strips that have been subjected to continuous annealing are finally wound into a coil and shipped.

[0004] Japanese Patent Application Laid-Open No. 2020-059050

[0005] However, in conventional steel strip manufacturing processes, steel strips are produced in coil units, which makes it difficult to handle small lot orders.

[0006] For example, it is conceivable to manufacture steel strip in coil units, and then cut out the required amount of steel strip from the coil by slitting it for sale. However, in this case, the remaining portion of the coil remains unsold, occupying storage space for a long period of time. As a result, logistics is hindered and overall production efficiency is reduced.

[0007] Disposing of the remaining coils avoids taking up storage space, but it increases product costs and wastes the energy required for production.

[0008] This problem is not limited to continuous annealing, but is a common issue in all heat treatments for steel strips.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a method for heat treating a steel strip that can efficiently produce even small-lot materials.

[0010] As a result of intensive research conducted by the inventors to solve the above problems, they discovered that even small lot materials can be efficiently manufactured by performing a pretreatment to partially change the emissivity of the steel strip surface prior to heat treatment.

[0011] Since the properties of steel are highly dependent on the heat treatment temperature, if one part of a steel strip can be heat treated at a different temperature than the other parts, it is possible to form multiple product parts with different properties within a single steel strip.

[0012] In order to form multiple product portions with different properties within a single steel strip, it is necessary to rapidly change the heating temperature depending on the position within the steel strip. However, because heating in a heat treatment furnace is mainly performed by radiant heating, it has been difficult to perform heat treatment under different heating conditions depending on the position within the steel strip, even if the heating conditions in the heat treatment furnace are controlled.

[0013] On the other hand, the temperature of the steel strip during heat treatment depends on its ability to absorb radiant heat. Therefore, the temperature during heat treatment can be changed by changing the emissivity of the steel strip surface. Therefore, by performing pretreatment to partially change the emissivity of the steel strip surface, the heating temperature can be changed sharply depending on the position within the steel strip, making it possible to form multiple product parts with different properties within a single steel strip.

[0014] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows.

[0015] 1. A method for heat treating a steel strip, comprising: subjecting a surface of the steel strip to a pretreatment that partially changes the emissivity; and subjecting the pretreated steel strip to a heat treatment.

[0016] 2. The method for heat treating a steel strip according to claim 1, wherein the thickness of the steel strip is 1.0 mm or more.

[0017] 3. The method for heat treating a steel strip according to claim 1 or 2, wherein the pretreatment reduces the oxide film in a partial region of the surface of the steel strip.

[0018] 4. The method for heat treating a steel strip according to claim 1 or 2, wherein the pretreatment increases the oxide film in a partial region of the surface of the steel strip.

[0019] 5. A method for heat treating a steel strip according to any one of the above items 1 to 4, wherein the heat treatment is carried out in an atmosphere having an oxygen concentration of 5% by volume or less.

[0020] 6. The method for heat treating a steel strip according to any one of the above items 1 to 5, wherein the heat treatment is carried out in an atmosphere having a dew point of 90°C or less.

[0021] According to the present invention, the temperature during heat treatment can be rapidly changed for each part of the steel strip. As a result, multiple product parts with different properties can be formed within a single steel strip. Therefore, even small lot materials can be efficiently manufactured.

[0022] 1 is a graph showing the results of Example 1. FIG. 2 is a graph showing the results of Example 2. FIG. 3 is a graph showing the influence of oxygen concentration among the results of Example 3. FIG. 4 is a graph showing the influence of dew point among the results of Example 3. FIG. 5 is a schematic diagram showing a test piece used in Example 4. FIG. 6 is a graph showing the results of Example 4. FIG. 7 is a schematic diagram showing a test piece used in Example 5. FIG. 8 is a schematic diagram showing the results of Example 5. FIG. 9 is a schematic diagram showing a test piece used in Example 6. FIG. 10 is a graph showing the results of Example 1. FIG. 11 is a graph showing the results of Example 1. FIG. 12 is a graph showing the results of Example 1.

[0023] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0024] [Steel Strip] The steel strip is not particularly limited and any steel strip can be used. The steel strip may be either a hot-rolled steel strip or a cold-rolled steel strip.

[0025] Furthermore, although the heat treatment method of the present invention can be applied to the manufacture of any product, from the viewpoint of the demand for small-lot materials, it is preferably applied to the manufacture of electrical steel sheets, and particularly to the manufacture of non-oriented electrical steel sheets. Therefore, the steel strip is preferably a steel strip for manufacturing electrical steel sheets. As a steel strip for manufacturing electrical steel sheets, a steel strip containing Si is preferably used. When a Si-containing steel strip is used, the Si content is not particularly limited, but from the viewpoint of increasing the resistivity of the steel sheet, it is preferably 1.0 mass% or more, more preferably 2.0 mass% or more, and even more preferably 2.5 mass% or more. On the other hand, from the viewpoint of suppressing a decrease in magnetic flux density, it is preferably 7.0 mass% or less, more preferably 6.0 mass% or less, and even more preferably 4.5 mass% or less.

[0026] The thickness of the steel strip is not particularly limited and may be any thickness, but is preferably 0.2 mm or more, more preferably 1.0 mm or more, and is preferably 3.0 mm or less.

[0027] [Pretreatment] The heating efficiency during heat treatment is affected by the radiation intensity on the heat-releasing side and the emissivity on the heat-receiving side. To change the radiation intensity, it is necessary to change the furnace wall temperature. However, the heat capacity of the furnace wall is usually very large and controllability is poor, making it difficult to produce a rapid change in the furnace wall temperature.

[0028] Therefore, in the present invention, prior to heat treatment, the surface of the steel strip is subjected to a pretreatment that partially changes the emissivity. By partially changing the emissivity of the steel strip surface, it is possible to achieve a desired temperature by using different heat inputs at different locations even if the furnace temperature is the same. Unlike the furnace temperature, the emissivity of the steel strip surface can be freely changed at different locations, so the method of the present invention makes it easy to adjust the temperature of the steel strip at different locations.

[0029] Generally, various heating methods, such as radiant heating and direct-fire heating, are used for the heat treatment of steel strips. However, even in the case of direct-fire heating, the temperature of the furnace walls rises due to heat from a burner, and as a result, the steel strip is also heated by radiation. Therefore, the method of the present invention is applicable regardless of the type or shape of the heating furnace used for the heat treatment. In other words, the heat treatment of the present invention is not particularly limited, and any heat treatment furnace (heating furnace) can be used, such as a direct-fire furnace, a radiant tube-type heating furnace, or an electric furnace.

[0030] The pretreatment may be carried out on at least one surface of the steel strip, but it is preferable to carry out the pretreatment on both surfaces to enhance the effect.

[0031] The method for changing the emissivity is not particularly limited and any method can be used, but increasing or decreasing the oxide film is preferable. This is because the emissivity of the steel sheet surface varies greatly depending on the state of the oxide film. For example, when the surface of a steel strip is covered with an oxide scale, the emissivity is approximately 0.95. On the other hand, when the surface of the steel strip is mirror-finished without an oxide film, the emissivity decreases to approximately 0.05. Therefore, by increasing or decreasing the oxide film, the emissivity and the heat input amount determined thereby can be adjusted over a wide range. Furthermore, since the state of the oxide film can be abruptly changed for each part of the steel strip surface, the temperature controllability during heat treatment is also excellent.

[0032] The value of the emissivity is not particularly limited, but typically, it may be adjusted to fall within the range of 0.05 to 0.95 as described above.

[0033] When the thickness of the oxide layer is changed, the oxide layer may be decreased or increased in some areas of the surface of the steel strip, or a combination of both may be used, where the oxide layer is decreased in some areas of the surface of the steel strip and increased in other areas.

[0034] (Method for reducing oxide film) An oxide film is usually present on the surface of a steel strip after hot rolling. This oxide film is a scale consisting mainly of iron oxides and is also called mill scale. This scale has the effect of increasing emissivity, so the emissivity of a steel strip after hot rolling (hot-rolled steel strip) is high, at approximately 0.8 to 0.95. Even in cases other than hot-rolled steel strips, if an oxide film is present on the surface of the steel strip, the emissivity will be similarly high.

[0035] Therefore, by pre-treating the steel strip to reduce the oxide film in a certain area of ​​the surface, the emissivity of that area can be selectively reduced, i.e., the pre-treated area will have a relatively low emissivity, while the unpre-treated area will have a relatively high emissivity.

[0036] Furthermore, by adjusting the degree to which the oxide film is reduced, the emissivity in the region can be freely controlled.

[0037] The method for reducing the oxide film is not particularly limited, and any method can be used depending on the target emissivity, convenience in production, etc. The method for reducing the oxide film may be any of a physical method, a chemical method, or a combination thereof. Typical methods include grinding, acid treatment, laser cleaning, shot blasting, air blasting, and a combination thereof.

[0038] (Method for increasing oxide film) On the other hand, an oxide film is usually not present on the surface of a steel strip after cold rolling. Furthermore, even in the case of a hot steel strip, an oxide film is not present on the surface of a steel strip that has been pickled after hot rolling (called pickled steel). The emissivity of such a steel strip without an oxide film on its surface is low, at around 0.05 to 0.4.

[0039] Therefore, by pre-treating the steel strip to increase the oxide film thickness in a specific region of the surface, the emissivity of that region can be selectively increased, i.e., the pre-treated region will have a relatively high emissivity, while the unpre-treated region will have a relatively low emissivity.

[0040] Furthermore, by adjusting the degree to which the oxide film is increased, the emissivity in the region can be freely controlled.

[0041] The method for increasing the oxide film is not particularly limited, and any method can be used as long as it can oxidize the surface of the steel strip to form an oxide film, and the method can be selected depending on the target emissivity, convenience for production, etc. Examples include a method of heating the steel strip surface while blowing oxygen onto it, and a method of blowing heated steam onto it. From an industrial perspective, the method of blowing at least one of oxygen and steam onto the surface of a steel strip heated to 500°C or higher is preferred.

[0042] As described above, in the present invention, the emissivity of the steel strip surface is partially changed by pre-treatment, thereby making it possible to vary the heating temperature during heat treatment at different locations. As a result, multiple regions with different properties can be formed within a single steel strip. By cutting the produced steel strip, multiple products can be obtained from a single steel strip. Therefore, according to the present invention, even small-lot materials can be efficiently produced. Note that the term "product" includes not only final products but also intermediate products such as hot-rolled annealed sheets and cold-rolled annealed sheets.

[0043] (Emissivity) The emissivity value to be used in the pretreatment is not particularly limited. Since the suitable emissivity varies depending on the configuration and atmosphere of the heat treatment furnace used, it is desirable to adjust the emissivity so that the desired heat treatment temperature (ultimate temperature) is achieved under the actual heat treatment conditions. For the adjustment, it is also preferable to determine the target emissivity or oxide film thickness in advance.

[0044] Furthermore, when heat treatment is performed continuously using a continuous furnace such as a continuous annealing furnace, the emissivity of the steel sheet surface can be changed according to the target heat treatment temperature when changing the heat treatment conditions, thereby enabling feedforward control of the target temperature. Generally, when manufacturing multiple types of products using a continuous furnace, a leading steel strip (leading material) and a following steel strip (following material) are welded together for heat treatment. However, the appropriate heat treatment conditions for the leading and following materials may differ. In this case, it is necessary to change the heat treatment conditions around the welded area (hereinafter also referred to as the condition change section). This condition change section often involves changing the heat treatment temperature, but because the furnace temperature cannot be changed rapidly, the actual target temperature may deviate from the ideal heat treatment temperature. Therefore, by changing the emissivity of the steel sheet surface before and after the condition change section, feedforward control of the target temperature can be performed, changing the heat input before and after the condition change section, thereby suppressing temperature deviation.

[0045] (Areas to be pretreated) The areas to be pretreated are not particularly limited and may be selected so as to obtain the desired product. However, from the viewpoint of industrial production, it is desirable to avoid pretreatment in an excessively complicated pattern. Therefore, it is preferable to divide the steel strip into multiple areas using either Pattern 1 or Pattern 2 below, or a combination of both. (Pattern 1) The steel strip is divided into multiple areas in the longitudinal direction (rolling direction). (Pattern 2) The steel strip is divided into multiple areas in the width direction (direction perpendicular to the rolling direction).

[0046] The shape of each of the plurality of regions is not particularly limited, but is preferably approximately rectangular. In other words, it is preferable to divide the steel strip into a plurality of regions by one or both of a line perpendicular to the longitudinal direction of the steel strip and a line parallel to the longitudinal direction of the steel strip.

[0047] Here, "dividing into a plurality of regions" does not mean cutting the steel strip, but means virtually dividing the steel strip into a plurality of regions. In the pretreatment, the emissivity may be changed by performing pretreatment on at least one of the plurality of regions.

[0048] The dimensions of each divided region are not particularly limited. However, if they are too small, the applications are limited. For example, if the length of one side is less than 10 mm, it is difficult to use it for applications such as a motor core. Therefore, it is preferable that the length of the short side of each region is 10 mm or more. On the other hand, there is no particular upper limit to the length of the short side, and it may be, for example, the width of the steel strip.

[0049] The number of the plurality of regions is not particularly limited and can be any number. Typically, the number of regions is preferably 2 or more. On the other hand, the number of regions is preferably 10 or less, and more preferably 5 or less.

[0050] [Heat Treatment] After the pretreatment, the steel strip is heat treated. The present invention can be applied regardless of the heat treatment conditions because the heat input during the heat treatment can be controlled by changing the emissivity through the pretreatment.

[0051] Furthermore, the present invention can be applied to heat treatments performed at any timing during the product manufacturing process. However, from the viewpoint of clearly differentiating the properties depending on the location of the steel strip, the heat treatment is preferably one or both of annealing performed after hot rolling (hot-rolled sheet annealing) and annealing performed between multiple cold rolling passes (intermediate annealing). This is because changing the emissivity during annealing (finish annealing) performed after all cold rolling is completed can cause surface defects, such as increased surface roughness in the emissivity-changed portion. Note that, generally, the thickness of the steel strip is often less than 1.0 mm when all cold rolling is completed. Therefore, the present invention is preferably directed to steel strips with a thickness of 1.0 mm or more.

[0052] Oxygen Concentration The oxygen concentration of the atmosphere during heat treatment is not particularly limited. However, if the oxygen concentration is excessively high, scale may rapidly form on the steel sheet surface during heat treatment, which may result in a change in the emissivity adjusted by pretreatment. Therefore, from the viewpoint of suppressing changes in emissivity and further improving temperature controllability, the oxygen concentration in the atmosphere is preferably 5% or less, more preferably 3% or less, and even more preferably 2% or less. The heat treatment can be performed in a non-oxidizing atmosphere or a reducing atmosphere that is substantially free of oxygen. Therefore, the lower limit of the oxygen concentration is not particularly limited and may be 0%.

[0053] Dew point The dew point during heat treatment is not particularly limited. However, if the dew point is excessively high, scale may rapidly form on the surface of the steel sheet during heat treatment, which may result in a change in the emissivity adjusted by pretreatment. Therefore, from the viewpoint of suppressing changes in emissivity and further improving temperature controllability, the dew point is preferably 90°C or less, more preferably 70°C or less, and even more preferably 40°C or less. On the other hand, since excessively lowering the dew point increases costs, the dew point is preferably -60°C or more.

[0054] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0055] Example 1 The following test was carried out to confirm that the temperature reached during heat treatment can be controlled by changing the emissivity.

[0056] First, molten steel containing 3% by mass of Si was melted by a conventional method and then continuously cast to obtain a steel slab having a thickness of 230 mm. The steel slab was then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2.0 mm. An oxide film (black scale) was formed on the surface of the obtained hot-rolled steel sheet.

[0057] A plurality of test pieces measuring 100 mm in width and 300 mm in length were cut from the hot-rolled steel plate, and the test pieces were subjected to pretreatment. Specifically, both surfaces of the test pieces were ground using an elastic grindstone to create a plurality of samples with different black scale area ratios ranging from 100% to 0%. The black scale area ratio here refers to the area ratio of the black scale remaining after the grinding. A sample with a black scale area ratio of 100% is a sample that was not ground (unground sample), and a sample with a black scale area ratio of 0% is a sample from which all the black scale had been removed by grinding.

[0058] Next, to determine the conditions for annealing as a heat treatment, the time required to heat the unground sample to 1000°C was measured. Specifically, a thermocouple was welded to the center of the unground sample, and annealing was performed in a furnace controlled to an atmosphere with a furnace temperature of 1050°C, an oxygen concentration of 0%, and a dew point of 30°C. The time t required for the sheet temperature to reach 1000°C was measured.

[0059] Next, a thermocouple was welded to the center of each sample with a black scale area ratio of less than 100%, and the sample was annealed under the same conditions as the unground sample. The temperature reached after a certain time t was measured. The measurement results are shown in Table 1 and Figure 1.

[0060] As shown in Table 1 and Figure 1, the lower the black scale area ratio, the lower the reached temperature. This is because lowering the black scale area ratio reduces the emissivity, which in turn reduces the heating efficiency.

[0061]

[0062] Example 2 Next, the following test was carried out to confirm the influence of the thickness of the oxide film on the temperature reached during annealing.

[0063] First, molten steel containing 3% by mass of Si was melted by a conventional method and then continuously cast to obtain a steel slab having a thickness of 230 mm. The steel slab was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3 mm. The obtained hot-rolled steel sheet was pickled to remove an oxide film. Next, the pickled hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.5 mm.

[0064] A plurality of test pieces measuring 100 mm in width and 300 mm in length were cut out from the cold-rolled steel sheet, and the test pieces were subjected to pretreatment. Specifically, an oxidation treatment was performed by spraying heated steam onto the test pieces while maintaining the sheet temperature at 600°C, to form an oxide film on the surface of the test pieces. By varying the oxidation treatment time, oxidation-treated samples with oxide film thicknesses ranging from 0 μm (untreated) to 10 μm were prepared. The thickness of the oxide film was measured by cutting the steel sheet after oxidation treatment and observing the cross section with a scanning electron microscope (SEM). The measurement was performed in 10 randomly selected fields of view, and the average of the 10 measured values ​​was taken as the oxide film thickness.

[0065] Next, to determine the annealing conditions, the time required to heat the untreated sample to 800° C. was measured. Specifically, a thermocouple was welded to the center of the untreated sample, and the sample was subjected to heat treatment in a furnace controlled to an atmosphere with a furnace temperature of 1050° C., an oxygen concentration of 0%, and a dew point of 30° C., and the time t required for the sheet temperature to reach 800° C. was measured.

[0066] Next, a thermocouple was welded to the center of each sample with a scale thickness of more than 0 μm, and the sample was annealed under the same conditions as the untreated sample. The temperature reached after a certain time t was measured. The measurement results are shown in Table 2 and Figure 2.

[0067] As shown in Table 2 and Figure 2, the thicker the oxide film, the higher the temperature reached. This is because increasing the thickness of the oxide film increases the emissivity, which in turn improves the heating efficiency. Therefore, even if annealing is performed under the same conditions, the region with a thicker oxide film will be annealed at a higher temperature.

[0068] As can be seen from Figure 2, when the oxide film thickness is 7 μm or more, the temperature reached hardly changes. This is thought to be because, if the oxide film is sufficiently thick, the emissivity saturates even if it is made thicker.

[0069]

[0070] Example 3 Next, the following test was carried out to confirm the influence of the annealing atmosphere on the temperature controllability.

[0071] First, a hot-rolled steel sheet having a thickness of 2.0 mm was produced under the same conditions as in Example 1. An oxide film (black scale) was formed on the surface of the obtained hot-rolled steel sheet.

[0072] A plurality of test pieces measuring 100 mm in width and 300 mm in length were cut out from the hot-rolled steel sheet, some of which were used as black scale samples (samples with mill scale) as they were, and the remaining parts were pickled to remove oxide scale and used as white scale samples (pickled samples).

[0073] Next, to determine the annealing conditions, the time required to heat the black scale sample to 1000°C was measured. Specifically, a thermocouple was welded to the center of the black scale sample, and the sample was annealed in a furnace controlled to an atmosphere with a furnace temperature of 1050°C, an oxygen concentration of 0%, and a dew point of 30°C. The time t required for the sheet temperature to reach 1000°C was measured.

[0074] Next, the black scale sample and the white scale sample were annealed under the same conditions as above, except that the oxygen concentration and dew point were changed as shown in Table 3. During this annealing, the temperatures reached by the black scale sample and the white scale sample after the time t had elapsed were measured, and the difference therebetween (the difference in the reached temperatures) was calculated.

[0075] The temperature difference can be regarded as an index of temperature controllability during annealing. Therefore, the temperature controllability was evaluated from the temperature difference based on the following criteria. The results are shown in Table 3 and Figures 3 and 4. - Temperature difference of 150°C or more: Excellent - Temperature difference of 100°C or more and less than 150°C: Good - Temperature difference of 50°C or more and less than 100°C: Fair

[0076] As shown in Table 3 and Figures 3 and 4, it can be seen that the temperature controllability gradually decreases with an increase in oxygen concentration and an increase in dew point.

[0077]

[0078] Example 4 Next, the following test was carried out to confirm the effect of pre-treating half of the steel sheet in the sheet width direction.

[0079] First, a hot-rolled steel sheet having a thickness of 2.0 mm was produced under the same conditions as in Example 1. An oxide film (black scale) was formed on the surface of the obtained hot-rolled steel sheet.

[0080] A test piece measuring 200 mm in width and 300 mm in length was cut out from the hot-rolled steel sheet, and half of the test piece in the sheet width direction was masked. Next, the test piece was pickled to remove the oxide film from the unmasked area. Thereafter, the masking was removed. As shown in the schematic diagram of FIG. 5 , the obtained test piece had an oxide film remaining in half of the sheet width direction, and no oxide film was present in the remaining half.

[0081] Next, to determine the annealing conditions, black scale samples with the same shape as the test specimens but without the oxide film removed were prepared, and the time required to heat the black scale samples to 1000°C was measured. Specifically, a thermocouple was welded to the center of the black scale samples, and the samples were annealed in a furnace controlled to an atmosphere with a furnace temperature of 1050°C, an oxygen concentration of 0%, and a dew point of 30°C. The time t required for the sheet temperature to reach 1000°C was measured.

[0082] Next, the test pieces after the pickling were subjected to annealing under the same conditions, and the temperature reached after the time t had elapsed was measured. The temperature reached was measured using thermocouples attached to the longitudinal center of the test pieces at intervals of 25 mm in the sheet width direction, as shown in Figure 5. The temperatures reached by each thermocouple are shown in Table 4 and Figure 6. Note that the temperature measurement positions in Table 4 and Figure 6 are represented by the distance from the center in the sheet width direction. The center in the sheet width direction was taken as 0 mm, the side from which the oxide film had been removed was taken as positive, and the side from which the oxide film had not been removed was taken as negative.

[0083] As shown in Table 4 and Figure 5, the temperatures reached were significantly different between the pre-treated and non-pre-treated sides. It can also be seen that the temperature reached changed sharply at the 0 mm position, i.e., at the boundary between the pre-treated and non-pre-treated regions. Meanwhile, the change in temperature reached was small within each region. Thus, the method of the present invention makes it possible to impart abrupt temperature changes within a single steel strip while annealing each region at a generally uniform temperature.

[0084]

[0085] (Example 5) Next, test pieces were prepared under the same conditions as in Example 4, except that the width x of the portion to be pretreated was changed. As shown in the schematic diagram of Fig. 7, the obtained test piece had no oxide film in a region of width x mm across the entire sheet width direction, but an oxide film remained in the remaining region of width (200-x) mm.

[0086] Next, to determine the annealing conditions, black scale samples with the same shape as the test specimens but without the oxide film removed were prepared, and the time required to heat the black scale samples to 1000°C was measured. Specifically, a thermocouple was welded to the center of the black scale samples, and the samples were annealed in a furnace controlled to an atmosphere with a furnace temperature of 1050°C, an oxygen concentration of 0%, and a dew point of 30°C. The time t required for the sheet temperature to reach 1000°C was measured.

[0087] Next, the test pieces after the pickling were annealed under the same conditions, and the temperature reached after the time t had elapsed was measured. The temperature reached was measured using thermocouples attached to the center of the test piece in the longitudinal direction and to both ends in the sheet width direction, as shown in Figure 7. The temperature reached, T1, measured at the end on the side that had not been pretreated, the temperature reached, T2, measured at the end on the side that had been pretreated, and the difference ΔT between the two (= T1 - T2) are shown in Table 5 and Figure 8.

[0088] As shown in Table 5 and Fig. 8, the difference in temperature achieved ΔT increased as the width x of the pre-treated region increased. Therefore, from the viewpoint of ensuring a sufficient difference in temperature achieved, it is desirable that the width x of the pre-treated region be large, and specifically, 10 mm or more is preferable, and 30 mm or more is more preferable.

[0089]

[0090] Example 6 Next, the following test was carried out to confirm the effect of pre-treating half of the steel sheet in the longitudinal direction.

[0091] First, a hot-rolled steel sheet having a thickness of 2.0 mm was produced under the same conditions as in Example 1. An oxide film (black scale) was formed on the surface of the obtained hot-rolled steel sheet.

[0092] A test piece measuring 100 mm in width and 600 mm in length was cut out from the hot-rolled steel sheet, and half of the test piece in the longitudinal direction was masked. Next, the test piece was pickled to remove the oxide film in the unmasked region. Thereafter, the masking was removed. As shown in the schematic diagram of FIG. 9 , the obtained test piece had an oxide film remaining in half of the region in the sheet width direction, and no oxide film was present in the remaining half of the region.

[0093] Next, to determine the annealing conditions, black scale samples with the same shape as the test specimens but without the oxide film removed were prepared, and the time required to heat the black scale samples to 1000°C was measured. Specifically, a thermocouple was welded to the center of the black scale samples, and the samples were annealed in a furnace controlled to an atmosphere with a furnace temperature of 1050°C, an oxygen concentration of 0%, and a dew point of 30°C. The time t required for the sheet temperature to reach 1000°C was measured.

[0094] Next, the test pieces after the pickling were subjected to annealing under the same conditions, and the temperature reached after the time t had elapsed was measured. The temperature reached was measured using thermocouples attached at the center of the width direction of the test pieces at 50 mm intervals in the longitudinal direction, as shown in Figure 9. The temperatures reached by each thermocouple are shown in Table 6 and Figure 10. Note that the temperature measurement positions in Table 6 and Figure 10 are expressed as positions in the longitudinal direction, in terms of distance from the center in the longitudinal direction. The center in the longitudinal direction was 0 mm, the side from which the oxide film had been removed was positive, and the side from which the oxide film had not been removed was negative.

[0095] As shown in Table 6 and Figure 10, the temperatures reached were significantly different between the pretreated and non-pretreated sides. It can also be seen that the temperature reached changed abruptly at the 0 mm position, i.e., at the boundary between the pretreated and non-pretreated regions. The length of the transition region, where the temperature changed continuously from 1000°C to 800°C, was approximately 100 mm. Meanwhile, the change in temperature reached within each region was small. Thus, according to the method of the present invention, it is possible to impart abrupt temperature changes within a single steel strip while annealing each region at a generally uniform temperature.

[0096]

[0097] Example 7 Next, a test was carried out under the same conditions as in Example 4, except that the oxide film was removed by grinding instead of pickling. The results are shown in Table 7 and FIG.

[0098] As shown in Table 7 and Figure 11, the temperatures reached were significantly different between the pretreated and non-pretreated sides. It can also be seen that the temperature reached changed sharply at the 0 mm position, i.e., at the boundary between the pretreated and non-pretreated regions. Meanwhile, the change in temperature reached was small within each region. Thus, even when the untreated scale is removed by belt grinding, the method of the present invention can impart a sharp temperature change within a single steel strip, while allowing each region to be annealed at a generally uniform temperature.

[0099]

[0100] Example 8 A test was carried out under the same conditions as in Example 4, except that the oxide film was removed by laser cleaning instead of pickling. The results are shown in Table 8 and FIG.

[0101] As shown in Table 8 and Figure 12, the temperatures reached were significantly different between the pretreated and non-pretreated sides. It can also be seen that the temperature reached changed abruptly at the 0 mm position, i.e., at the boundary between the pretreated and non-pretreated regions. Meanwhile, the temperature change within each region was small. Thus, even when the black scale is removed by laser cleaning, the method of the present invention can impart abrupt temperature changes within a single steel strip, while allowing each region to be annealed at a generally uniform temperature.

[0102]

[0103] Example 9 A test was carried out under the same conditions as in Example 4, except that the oxide film was removed by shot blasting instead of pickling. The results are shown in Table 9 and FIG.

[0104] As shown in Table 9 and Figure 13, the temperatures reached were significantly different between the pretreated and non-pretreated sides. It can also be seen that the temperature reached changed sharply at the 0 mm position, i.e., at the boundary between the pretreated and non-pretreated regions. Meanwhile, the change in temperature reached was small within each region. Thus, even when the black scale is removed by shot blasting, the method of the present invention can impart a sharp temperature change within a single steel strip, while allowing each region to be annealed at a generally uniform temperature.

[0105]

[0106] As can be seen from the results of Examples 4 and 7 to 9 above, in the present invention, by performing pretreatment to change the emissivity, the desired effect can be obtained regardless of the specific method used for the pretreatment.

[0107] When the strip temperature is changed by changing the furnace temperature as in the conventional method, it takes several minutes to several tens of minutes for the temperature change to be completed, depending on the controllability of the furnace, and therefore the steel strip that passes through the annealing furnace during that time cannot be annealed at an appropriate temperature. However, according to the method of the present invention, by performing pretreatment in advance to change the emissivity, annealing can be performed at an appropriate temperature.

Claims

1. A method for heat treating a steel strip, comprising: subjecting the surface of the steel strip to a pretreatment that partially changes the emissivity; and subjecting the pretreated steel strip to a heat treatment.

2. The method for heat treating a steel strip according to claim 1, wherein the thickness of the steel strip is 1.0 mm or more.

3. A method for heat treating a steel strip according to claim 1 or 2, wherein the pretreatment reduces the oxide film in a partial area of the surface of the steel strip.

4. A method for heat treating a steel strip according to claim 1 or 2, wherein the pretreatment step increases the oxide film in a partial area of the surface of the steel strip.

5. A method for heat treating a steel strip according to any one of claims 1 to 4, wherein the heat treatment is carried out in an atmosphere having an oxygen concentration of 5% by volume or less.

6. A method for heat treating a steel strip according to any one of claims 1 to 5, wherein the heat treatment is carried out in an atmosphere having a dew point of 90°C or less.

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