JP5470912B2 - Annealing method that can prevent temper color - Google Patents

Description

  The present invention relates to an annealing method capable of preventing a temper color suitable for preventing a temper color generated during BAF annealing in which recrystallization annealing of a tight coil steel plate is performed by Box Annealing Furnace (hereinafter referred to as BAF).

  A steel sheet with a tempered grade of T-1 to T-3 as defined in JIS is a soft steel sheet with low Rockwell hardness, and induces cracks such as deep drawing and overhanging. It is used for severe processing applications that are easy to perform. On the other hand, steel sheets having a tempering degree of T-4 to T-6 have high Rockwell hardness and are used for light processing applications such as bending.

  The preparation of these tempering degrees is mainly based on the selection of the annealing method. In order to set the tempering degree to T-1 to T-3, the BAF annealing is performed, and the tempering degree is set to T-4 or more. Is used for continuous annealing. The reason why a soft steel sheet can be obtained by BAF annealing is derived from the fact that the manufacturing period is much longer than that of continuous annealing, and in particular, in BAF annealing, the holding time of several hours or more is applied to the soaking process from recrystallization to grain growth. Is spent. This is an extremely long holding time compared to a holding time of about 1 minute in continuous annealing.

  Such a difference in holding time in the annealing process appears as a difference in the appearance of the steel sheet surface. Specifically, the steel sheet surface after continuous annealing is clean and has a strong metallic luster, while the steel sheet surface after BAF annealing becomes dull because the oxide film grows thick. When the oxide film on the surface of the steel sheet after the BAF annealing becomes thick enough to be visually recognized, this becomes a surface defect called a temper color.

  For example, Non-Patent Document 1, Patent Document 1, and Patent Document 2 described below are disclosed as the causes of the temper color and methods for preventing the temper color.

  In Non-Patent Document 1, additive elements such as Mn, Si, Cr, Al, and P are easily segregated on the surface of the steel sheet, and since these are elements that are more easily oxidized than iron, they are likely to be present on the steel sheet surface as oxides. Furthermore, for example, when a complex oxide of iron, Mn, and Si is formed as a film on the surface, this color develops and a temper color is generated. Regarding measures against temper color, Non-Patent Document 1 describes that there is a means for limiting the amount of an element that causes temper color.

  Patent Document 1 discloses a method for preventing a temper color caused by Mn, Al, and Si oxides. In the disclosed technique of Patent Document 1, the temperature of the coil steel plate is temporarily held in the temperature range of 350 to 430 ° C. during the temperature increase to the soaking plate temperature, and when the dew point of the atmospheric gas becomes −20 ° C. or less. It is characterized in that the temperature rise to the soaking plate temperature is restarted, and this is a technique that can suppress the temper color caused by Mn and the like.

  Patent Document 2 discloses a technique capable of suppressing a temper color generated by strong oxidation of iron (hereinafter referred to as Fe) on a steel sheet surface. The disclosed technique of Patent Document 2 controls the dew point when the steel plate temperature becomes 300 ° C. or higher in the temperature raising process to a dew point derived from a predetermined formula consisting of the plate temperature of the coil steel plate and the hydrogen concentration of the atmospheric gas. This makes it possible to prevent temper color.

JP 2002-256337 A JP-A-8-157743

Published by Mitsunobu Abe, “Series of Iron and Steel Technology, Series 2 Volume 4 Steel Sheet Manufacturing Technology”, issued February 9, 2000. Japan Steel Association Publication, P210-P225

  By the way, the disclosed techniques described in the above-mentioned documents have the following problems.

  The technology disclosed in Non-Patent Document 1 takes measures to limit the amount of each element that causes this to prevent temper color, and each element that causes this controls the material of the steel sheet. Therefore, such means has not been preferable for preventing the temper color while securing the material of the steel sheet.

  The disclosed techniques of Patent Document 1 and Patent Document 2 mainly control the dew point when the plate temperature of the coil steel sheet is in the temperature range of 300 to 430 ° C. during the temperature increase to the soaking plate temperature. There is no special mention about the control of the dew point when the plate temperature is in the temperature range thereafter. As will be described later, according to experiments conducted by the present inventor, when the plate temperature of the coil steel sheet reaches a temperature range of 650 ° C. or more which is a recrystallization and grain growth temperature range, It has been found that temper color cannot be sufficiently prevented only by controlling the dew point. That is, after the plate temperature of the coil steel plate reaches, for example, 650 ° C., as disclosed in Patent Document 1, it is insufficient to prevent the temper color only by setting the dew point of the atmospheric gas to −20 ° C. or less. . Further, according to the predetermined formula disclosed in Patent Document 2, for example, the dew point is −30 ° C. or lower under the condition of a hydrogen concentration of 5 vol% and the coil steel plate temperature of 300 ° C., but the condition of the plate temperature of 580 ° C. or higher. In this case, the dew point becomes + 5 ° C. or lower, and annealing with a humid atmosphere gas is allowed, which is insufficient to prevent temper color.

  In other words, conventionally, there has not been proposed an annealing method that can sufficiently prevent the generation of a temper color while controlling the material of the annealed steel sheet.

  Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to effectively prevent the generation of a temper collar when annealing a tight coil steel plate by BAF. An object of the present invention is to provide an annealing method capable of preventing the temper color which can secure the material of the BAF annealed steel sheet and can effectively solve the above-mentioned problems of the prior art.

As shown in the background art, in order to prevent the temper color in the BAF annealing, it is important to cut off the oxygen supply source in the atmospheric gas so as not to generate an oxide film on the steel plate surface. Examples of the oxygen supply source include (a) moisture adhering to the steel sheet surface and (b) moisture generated by causing a reduction reaction with hydrogen during the annealing of the oxide on the steel sheet surface. It is constantly occurring during annealing. In order to avoid the formation of an oxide film on the surface of the steel plate, it is necessary to target moisture generated from the start to the end of annealing. However, the disclosed technologies in Patent Document 1 and Patent Document 2 are only 300 to 430 ° C. Only countermeasures against moisture generated up to this point are taken into consideration, and no particular consideration has been given to moisture generated in the temperature range thereafter.

  According to the experiment conducted by the inventor this time, the temperature range in which the generation of the Fe oxide film that causes the temper color is easily promoted is a temperature of 300 ° C. to 430 ° C., which is mentioned in the disclosure techniques of Patent Document 1 and Patent Document 2. It was found that it exists not only in the region but also in a temperature range of 650 ° C. or higher corresponding to the recrystallization and grain growth temperature range. In order to completely suppress the temper color, the present inventor can prevent the temper color caused by the Fe oxide film by suppressing the soaking plate temperature to 630 ° C. and controlling the dew point to −40 ° C. I found out. However, the present inventor has also found that such a decrease in the soaking temperature causes the steel sheet to become hard, and there is a problem that the tempering degree of the annealed steel sheet is higher than T-1. As a result of studying conditions that can achieve both the contradictory temper color suppression and the tempering degree T-1, the present inventor has added a predetermined amount of B to low-carbon aluminum killed steel, and S is 0.009%. It has been found that if the temperature is regulated below, an annealed steel sheet having a tempering degree T-1 can be obtained even if the soaking plate temperature decreases.

  The present invention has been devised based on the above circumstances, and the gist of the present invention is as follows.

The annealing method capable of preventing the temper color according to claim 1 is mass%, C: 0.02 to 0.06%, Si: 0.020% or less, Mn: 0.10 to 0.30%, P : 0.020% or less, S: 0.009% or less, Al: 0.020 to 0.060%, N: 0.008% or less, B: 0.0008 to 0.008%, When the B content in mass% is [B] and the N content is [N], [B] / [N] satisfies the relationship of 0.6 or more and less than 1.0, with the balance being Fe and Tight coil steel sheet obtained by subjecting a low carbon aluminum killed hot-rolled steel sheet having a steel composition of inevitable impurities to rolling at a cold rolling ratio of 85 to 95% and electrolytic cleaning is subjected to recrystallization annealing by Box Annealing Furnace. At the time, nitrogen gas concentration: 90-97 vol%, hydrogen gas concentration: -10 vol% of mixed atmosphere gas is supplied, and moisture generated from the tight coil steel plate is removed while vaporizing during the temperature rising process, and the dew point when the soaking plate temperature reaches 580 to 630 ° C is -40 ° C or less An annealed steel sheet having a tempering degree T-1 is obtained by retaining the tight coil steel sheet in a reducing atmosphere.

  According to the first aspect of the present invention, it is possible to prevent the temper collar caused by the Fe oxide film during the BAF annealing.

According to the invention which concerns on Claim 2, it becomes possible to obtain the annealed steel plate of the refining degree T-1 while preventing a temper color.

It is a schematic diagram which shows an example of the BAF annealing furnace used at the time of implementation of the search experiments 1-4 performed for this invention and this invention. It is the figure which showed transition of the plate temperature and dew point in the annealing furnace in the search experiment 1 performed for this invention in 60 hours from heating to cooling. It is the figure showing as a schematic diagram the generation | occurrence | production condition of the temper color which arose in the tight coil steel plate of the search experiment 1 conducted for this invention. It is the figure which showed transition of the plate temperature and the dew point in the annealing furnace of the search experiment 3 conducted for this invention in 60 hours from heating to cooling. It is the figure which showed the relationship between the amount of fine nitrides of the search experiment 4 conducted for this invention, and hardness. It is the figure which showed the relationship between the amount of fine nitride derived | led-out from the data of an Example, and hardness.

  First, the basic research content that led to the completion of the present invention will be described. In the following, the mass% described for components and compositions is simply referred to as%.

(Search Experiment 1)
The present inventor conducted the following search experiment 1 in order to clarify the actual state and the cause of the occurrence of temper color. In Search Experiment 1, the coil steel sheet was annealed under the BAF operating condition of tempering degree T-1 where temper color frequently occurs, the coil steel sheet after annealing was unwound and the occurrence of temper color was confirmed, and annealing was performed. Samples were taken from the discolored portion and the undiscolored portion of the coil steel plate later, and the cause of these differences was investigated.

  The test material used in this experiment was a low carbon aluminum killed steel slab, and the main components were C: 0.05%, Si: 0.009%, Mn: 0.25%, P: 0.014%, S : 0.011%, Al: 0.020%, N: 0.0048%, Cr: 0.064%. The specimen was slab heated at a heating temperature of 1235 ° C., further rolled to a plate thickness of 3.0 mm at a hot rolling temperature of 900 ° C., and then wound up at a winding temperature of 550 ° C. Subsequently, the obtained hot-rolled steel sheet was pickled and descaled, and then cold-rolled to a sheet thickness of 0.301 mm, wound after electrolytic degreasing, and the tight coil obtained was placed in an annealing furnace.

  FIG. 1 is a schematic view showing a state in which a plurality of stages of tight coil steel plates are arranged in an inner cover 11 of an annealing furnace. The tight coil steel plates are arranged in three stages in a furnace, and a convector 4 for promoting heat transfer is arranged between the tight coil steel plates at each stage. The annealing furnace is configured such that the atmospheric gas is supplied to the inside through the air supply pipe 12, and the atmospheric gas is exhausted from the inside through the exhaust pipe 13. A dew point control mechanism (not shown) is further provided so as to be able to control the dew point in the furnace by adjusting the flow rate of the atmospheric gas flowing in and out from the service pipe 13. The measurement of the dew point of the atmosphere gas, the measurement of the plate temperature of the upper coil 1, the measurement of the plate temperature of the middle coil 2 and the lower coil 3, as shown in FIG. Various measuring instruments were installed at the upper coil measurement point 16 and the middle / lower coil plate temperature measurement point 15.

  The atmosphere gas supplied into the BAF annealing furnace was adjusted such that the nitrogen gas concentration and the hydrogen gas concentration were 90 to 97: 3 to 10 in volume ratio (vol%).

FIG. 2 is a graph showing the transition of the plate temperature of the upper coil 1 in the furnace and the transition of the dew point of the atmospheric gas in the furnace in the annealing process. In the annealing process, the atmospheric temperature in the furnace is 750 ° C., the upper coil 1
The soaking plate temperature of 680 ° C. and the soaking plate temperature between the lower coil 3 and the middle coil 2 were heated and raised to 630 ° C., and then kept at that soaking plate temperature for 8 hours. Was recrystallized. Then, after slow cooling until the plate temperature of each tight coil steel plate reached 100 ° C., each tight coil steel plate was defurcated.

  The dew point of the atmosphere gas in the furnace becomes a high dew point of 0 ° C. at a plate temperature of 100 ° C. immediately after ignition of the annealing furnace, and then decreases by gas replacement, but even if the plate temperature of the upper coil 1 exceeds 300 ° C., −20 ° C. The above high dew point.

  After completion of the annealing process, each decoiled tight coil steel sheet is rolled with a temper rolling mill at a reduction ratio of 1% to a sheet thickness of 0.300 mm, and each tight coil steel sheet is unwound to remove a temper color (hereinafter, discoloration). The hardness at normal temperature was measured with a Rockwell hardness meter, and the tempering degree as a tin plate and tin-free steel plate was confirmed. FIG. 3 is a schematic view showing a state in which the obtained tight coil steel plate 20 is unrolled.

The following points were confirmed by the experiment of the search experiment 1.
(1) As shown in FIG. 3, a discoloration portion 23 having a deep color change is formed closer to the end in the width direction of the tight coil steel plate 20, and the undiscolored portion 21 in which no discoloration is observed remains in the center in the width direction. It was. (2) The discoloration portion 23 is more likely to be generated as the stack is higher, and the width of the upper coil 1 is discolored in the range extending over the entire length in the longitudinal direction.
(3) Discoloration of the lower coil 3 and the middle coil 2 of the soaking plate temperature of 630 ° C. was mild, and no discoloration as in the upper coil 1 was observed.
(4) The Rockwell hardness measurement value of the upper coil 1 was HR30T: 49, and the tempering degree was T-1 (49 ± 5). The measured value of Rockwell hardness of the middle coil 2 is HR30T: 51, the measured value of Rockwell hardness of the lower coil 3 is HR30T: 52, and each tempering degree is higher by T-1 (49 ± 5). The value was close to the refining degree T-2 (53 ± 5).

  The above (1) indicates that discoloration appears strongly at the end that is easily in contact with the atmospheric gas having a high dew point, and (2) and (3) indicate that discoloration progresses as the annealing temperature increases. Thus, it was confirmed that the moisture in the atmospheric gas and the soaking plate temperature of the tight coil steel plate were the operating factors for the occurrence of discoloration. Moreover, according to said (4), in the composition like the test material used by this experiment, unless it is high temperature annealing of 650-680 degreeC, the annealing steel plate of low tempering degree like tempering degree T-1 The fact that it is difficult to obtain was confirmed.

Next, a sample is taken from the discolored portion 23 at the widthwise end of the upper coil 1 and the undiscolored portion 21 (hereinafter referred to as a normal portion) at the center in the width direction.
As a result of surface analysis by Spectroscopy, the following points were confirmed.
(5) No surface enrichment of Si, Mn, P, Al, Cr, etc. was observed in the normal part 21 and the discoloration part 23.
(6) On the other hand, the normal portion 21 and the discolored portion 23 have an Fe oxide film whose composition is mainly Fe3O4, and the discolored portion 23 is thicker than the normal portion 21.

  In these (5) and (6), the cause of discoloration (temper color) is not the segregation of additive elements as shown in Non-Patent Document 1, but the Fe oxide film as shown in (6). It shows that it is in shading. In this Fe oxide film, water vapor that is generated immediately after ignition in the annealing furnace is generated as an oxygen supply source, and the amount of water vapor can be controlled by the dew point of the atmospheric gas. However, in the above search experiment 1, the relationship between the dew point generated by the Fe oxide film and the soaking plate temperature of the tight coil steel sheet could not be clarified.

(Experiment 2)
Therefore, the present inventor planned and promoted the following search experiment 2 in order to clarify the relationship between the dew point generated by the Fe oxide film and the soaking plate temperature of the tight coil steel plate. The same low carbon aluminum killed steel slab as the test material used in search experiment 1 was used as the test material used in search experiment 2. The test conditions before annealing, such as hot rolling and cold rolling, for the test materials used in Search Experiment 2 were all the same as those in Search Experiment 1.

In search experiment 2, a small amount of a cut plate sample was collected from a tight coil obtained after electrolytic degreasing. The obtained sample was adjusted so that the nitrogen gas concentration and the hydrogen gas concentration were 90 to 97: 3 to 10 by volume, and the dew point of the atmospheric gas in the furnace was −20, −30, −40, It was placed in each electric furnace adjusted to -50 ° C. In each electric furnace, each sample was held at the plate temperature for 8 hours after heating until the soaking plate temperature reached 580 ° C. while maintaining the dew point adjusted in advance. Thereafter, the inside of the furnace was gradually cooled to room temperature, each sample was taken out, and the discolored state of the steel sheet was observed. Further, the same experiment was performed under the same conditions except that only the soaking plate temperature during annealing was changed to 600 ° C, 630 ° C, and 650 ° C. As a result, the following knowledge was obtained.
(7) If the dew point is kept at -40 ° C. or lower and the soaking plate temperature is 630 ° C. or lower, no visual discoloration is observed.

(Experiment 3)
Next, the present inventor conducted the following search experiment 3 in order to try to apply the annealing condition shown in (7) obtained in the search experiment 2 to an actual machine. The test material used in Search Experiment 3 is the same low carbon aluminum killed steel tight coil as that used in Search Experiment 1, and hot rolling and cold rolling of the test material used in Search Experiment 3 are used. Test conditions before annealing, etc., rolling conditions with a temper rolling mill after de-furnace, observation of discoloration, and test conditions after annealing such as confirmation of tempering degree are all the same as in Search Experiment 1. Condition. In the search experiment 3, the tight coil obtained after electrolytic degreasing was arranged in three stages in an annealing furnace under the conditions shown in FIG.

  FIG. 4 is a graph showing the transition of the plate temperature of the upper coil 1 in the furnace and the transition of the dew point of the atmospheric gas in the furnace in the annealing process. In the annealing step, the atmospheric temperature in the furnace is set to a first stage of 600 ° C. and the temperature of the upper coil 1 is increased to 300 ° C. Then, the atmospheric temperature in the furnace is set to 300 ° C., the same as the plate temperature of the upper coil 1. Then, the plate temperature was maintained for 10 hours, and the temperature increase was interrupted until the plate temperature between the lower coil 3 and the middle coil 2 was 300 ° C. and the dew point was −30 ° C. After that, the atmosphere temperature in the furnace is set to the second stage 750 ° C., the soaking plate temperature of the upper coil 1 is 630 ° C., and the soaking plate temperature between the lower coil 3 and the middle coil 2 is 600 ° C. again. In a state where the temperature was raised and the dew point was −40 ° C., the atmosphere temperature in the furnace was set to about 630 ° C. which was the same as the soaking plate temperature of the upper coil 1, and the plate temperature was maintained for 8 hours (hereinafter referred to as “the plate temperature”). This condition is referred to as “600 ° C. annealing”). Then, after slow cooling until the plate temperature of each tight coil steel plate reached 100 ° C., each tight coil steel plate was defurcated.

The following points were confirmed by the experiment of the search experiment 3.
(8) It was confirmed that annealing at 600 ° C. was possible by industrial means.
(9) Discoloration at the end in the width direction of all the tight coil steel plates was eliminated by annealing at 600 ° C., and good surface gloss was obtained.
(10) On the other hand, the Rockwell hardness was hardened to 55, and the tempering degree was T-2.5 (55 ± 5).

From the above search experiment 3, after the annealing furnace was ignited, the dew point was lowered early in the temperature range where the oxidation reaction on the steel sheet surface was slow, and the soaking plate temperature was lowered to 630 ° C. or lower. And then
It was confirmed that discoloration could be suppressed by lowering the dew point at the soaking plate temperature. In particular, by lowering the soaking plate temperature, the rate of temperature increase per unit time decreases, and the rate at which moisture evaporates can be suppressed. As a result, gas replacement is performed while suppressing a rapid increase in dew point. It becomes possible. On the other hand, according to the above (10), it was confirmed that with the composition like the test material used in this experiment, the material hardening occurred in the BAF annealed steel sheet and the tempering degree T-1 could not be obtained. The cause of the material hardening is that annealing was performed at a lower temperature than in the search experiment 1, and this caused precipitation hardening due to excessive remaining fine Al-N-based precipitates (hereinafter referred to as nitrides). I guessed that.

(Search Experiment 4)
Next, the present inventor clarifies the relationship between the amount of fine nitride and the tempering degrees T-1 to T-3, and obtains a BAF annealed steel sheet having a tempering degree T-1 under conditions of 600 ° C. annealing. In order to clarify the component conditions of the BAF-annealed steel sheet that makes possible the following search experiment 4 was performed.

In search experiment 4, test materials (steel Nos . 1 to 5; No. 1 and 2 are comparative examples) having different components shown in Table 1 below were used. Steel No. B-added steel low carbon aluminum killed steel was used for the test materials 1 to 3. Each test material was subjected to rolling, annealing, etc. under the test conditions before annealing, annealing conditions, and post-annealing test conditions described in Search Experiment 3 to produce a product plate having a thickness of 0.300 mm.

  The product plate was subjected to Rockwell hardness measurement at room temperature and subjected to an artificial aging treatment at 210 ° C. for 30 minutes, and then the yield point elongation value was measured by a tensile test. The yield point elongation value is zero in all samples, and solid solution C and solid solution N do not exist in the product plate. This sample was measured by a dissolution method using bromine ester, the difference between the nitride contained in the residue and the total N amount of the product plate was calculated, and this difference value was defined as the amount of fine nitride. The relationship between the amount of fine nitrides and the tempering degrees T-1 to T-3 is as shown in Table 2 and FIG.

The following points were confirmed by the experiment of the search experiment 4.
(11) An annealed steel sheet having a tempering degree T-1 (49 ± 5) is obtained by adjusting the amount of fine nitride to around 0.0003%.
(12) Fine nitride amount: Starting from 0.0003%, there is a linear relationship in which the refining degree is incremented by 1 every time the fine nitride amount is increased by 0.0013%. For example, if the amount of fine nitride is 0.0016%, an annealed steel sheet having a tempering degree T-2 is obtained, and if 0.0029%, an annealed steel sheet having a tempering degree T-3 is obtained.
(13) All annealed steel plates have no discoloration.

  Here, the composition of each test material is devised as follows. Since low carbon aluminum killed steel that is melted industrially inevitably contains N and has a high N content, the amount of fine nitride is 0.0003% in order to obtain a BAF annealed steel sheet with a tempering degree T-1. It is difficult. Steel No. In the test materials 1 to 3, B is added to the low carbon aluminum killed steel in advance, and a part of N is precipitated as BN in the hot rolling stage, so that the amount of solute N in the steel sheet before BAF annealing is extremely low. A means for N is taken, and this makes it easy to secure the tempering degree T-1 by suppressing the amount of fine nitride. In addition to this, steel no. In the test materials of 1 to 5, paying attention to S that is not a material control element in the disclosed technique of each document shown in the background art, and reducing S (≦ 0.009%) in the annealing process By promoting the growth of recrystallized grains, the refining degree T-1 is easily secured.

  The present invention has been devised based on the knowledge obtained as described above. Hereinafter, specific components of the present invention will be described.

  The method for preventing temper color of a BAF annealed steel sheet according to the present invention is performed using an annealing furnace as shown in FIG. 1, and before the annealing process, first, a plurality of stages of tight coil steel sheets are placed in the annealing furnace. Keep it. In addition, the structure of the annealing furnace used at the time of BAF annealing and the arrangement | positioning aspect of a tight coil steel plate are not specifically limited, A well-known thing may be used elsewhere.

  Thereafter, a mixed atmosphere gas obtained by mixing nitrogen gas and hydrogen gas is supplied into the annealing furnace while raising the temperature in the annealing furnace. In the temperature rising process until each tight coil steel plate reaches the soaking plate temperature, or in the holding process of keeping at the soaking plate temperature, etc., for example, while supplying an atmospheric gas having a low dew point of −60 ° C. or less, The dew point of the atmospheric gas in the annealing furnace is lowered by gas replacement for exhausting the atmospheric gas having a high dew point. Thus, the temperature of the tight coil steel plate is raised or held while the moisture generated from the tight coil steel plate is vaporized and removed during the temperature raising process or the holding process.

  After each tight coil steel plate reached a soaking plate temperature of 580 to 630 ° C., each tight coil steel plate was held for 2 to 8 hours in a reducing atmosphere in which the dew point of the atmospheric gas was −40 ° C. or less. Then, after slow cooling until the plate temperature of each tight coil steel plate reaches about 100 degrees, each tight coil steel plate is de-furnace, and the annealing process according to the present invention is completed.

  In the present invention, in order to prevent temper color, it is necessary to adjust the numerical conditions during BAF annealing as follows. The reason for limiting the numerical conditions will be described below.

<Hydrogen gas concentration in atmospheric gas: 3 to 10 vol%>
<Nitrogen gas concentration in atmospheric gas: 90 to 97 vol%>
If the hydrogen gas concentration of the atmospheric gas supplied into the annealing furnace is too low, the steel plate surface is oxidized during annealing and a temper color is generated. Therefore, the lower limit is set to 3 vol%, but the temper color is more reliable. Therefore, the lower limit is preferably 4 vol%. Further, the hydrogen gas concentration is not particularly adversely affected from the viewpoint of suppressing the generation of temper color even if the upper limit is not particularly defined, but if it is too much, the cost is increased, so the upper limit is set to 10 vol%. The nitrogen gas concentration of the atmospheric gas only needs to be adjusted to 100 vol% together with the hydrogen gas concentration, and the lower limit and upper limit thereof are 90 vol% and 97 vol%, respectively, according to the hydrogen gas concentration.

<Soaking plate temperature: 580 to 630 ° C.>
This is one of the most important conditions in the annealing conditions of the present invention. This temperature range is a recrystallization temperature range of a low carbon aluminum killed steel sheet with a cold rolling rate range of 85 to 95% and a low carbon aluminum killed steel sheet to which B is added. By holding the tight coil steel sheet in this temperature range, Recrystallization of tight coil steel sheet is promoted. If the soaking plate temperature is higher than 630 ° C., a temper color is generated in the tight coil steel plate at the time of holding, so the upper limit is set to 630 degrees. Further, if the soaking plate temperature is less than 580 ° C., recrystallization is not promoted and the toughness of the annealed steel plate is lowered, so the lower limit is set to 580 ° C.

<Dew point when reaching soaking plate temperature: -40 ° C. or lower>
This is one of the most important conditions in the annealing conditions of the present invention. In the temperature range of 580 to 630 ° C. which is the temperature range of the soaking plate temperature of the tight coil steel plate, an Fe oxide film is generated with a very small amount of water, so the dew point of the atmospheric gas needs to be kept as low as possible. . For this reason, the upper limit of atmospheric gas was -40 degreeC. The lower limit of the dew point of the atmospheric gas is not particularly defined.

  In the temperature rising process up to the soaking plate temperature, the temperature distribution of the plate temperature varies due to the shape, size, number of stages, etc. of the tight coil steel plate, and the temperature rising rate also varies. End up. For this reason, when the rate of temperature rise to the soaking plate temperature is fast, the dew point suddenly rises at the part where the temperature rise rate is high due to variations in the temperature distribution of the plate temperature. There is a possibility that a dew point when the plate temperature is reached does not become -40 ° C. or less, and a temper color is locally generated.

For this reason, at the time of annealing of the present invention, if necessary, after raising the temperature at the coldest point of the tight coil steel plate to 300 ° C. in the temperature raising process to the soaking plate temperature, It is preferable to restart the temperature rise after holding the plate temperature at a point for a predetermined time. As a result, the temperature distribution of the plate temperature is made uniform in a temperature range where temper color is unlikely to occur, and then the temperature rise is started in that state, and the dew point when the soaking plate temperature is reached is determined as the tight coil steel plate. It becomes possible to make it stably -40 degrees C or less as a whole. The time for keeping the plate temperature at the coldest point is uniform in the temperature distribution variation caused by the shape, size, number of stages, temperature rise temperature, etc. of the tight coil steel plate placed in the annealing furnace. For example, it is preferable that the time be 8 hours or longer. In addition, the coldest point here means the point with the lowest temperature of the tight coil steel plate heated in the annealing furnace. Generally, since the central portion in the radial direction of the tight coil steel plate is the coldest point, in the present invention, the temperature measured at the central portion in the radial direction is treated as the coldest point.

  Next, the reason for adding each component constituting the BAF annealed steel sheet and the reason for limiting the numerical values, which are desirable for obtaining a BAF annealed steel sheet having a tempering degree T-1, will be described.

<C: 0.02 to 0.06%>
C is an element that strongly affects the material of the BAF-annealed steel sheet. When the content is less than 0.02%, the amount of solute C increases, age hardening occurs, and the tempering degree T-1 cannot be obtained and the strain aging depends on the aging. Since material variation with time progresses, the lower limit of the content is set to 0.02%. On the other hand, if C exceeds 0.06%, it is excessively hardened and the tempering degree T-1 cannot be obtained, and precipitation of coarse cementite particles increases, which increases the risk of cracking during steel sheet processing. The upper limit of the content is 0.06%. In addition, in order to ensure T-1 stably on industrial production, it is further more preferable that the C content range is 0.02 to 0.04%.

<Si: 0.020% or less>
Si deteriorates the corrosion resistance of the tin plate and tin-free steel plate, and excessive inclusion has a bad effect such as deterioration of the workability of the steel sheet due to inclusion formation. That is, in the present invention, Si is an unnecessary element, and it is desirable that Si is not contained, so the lower limit of the Si content is not particularly defined. On the other hand, regarding the upper limit of Si, the ASTM standard defines 0.020% as the upper limit, and since there is no problem in obtaining the predetermined effect of the invention, the upper limit is set to 0.020%. Note that Si does not need to be added at the time of melting, and since it is necessary to stop SiO2 in the refractory material by reducing it with Al in the molten steel, the upper limit of its content is less than 0.01%. It is further desirable to use a trace amount.

<Mn: 0.10 to 0.30%>
Mn segregates on the surface of the steel sheet during BAF annealing, oxidizes and causes discoloration, and is an expensive alloy element. Further, in a steel that can reduce the amount of S as in the present invention, even if the amount of Mn is reduced, it is described later It is desirable to make the content small because the effect of preventing the occurrence of ear cracks can be sufficiently obtained, and the upper limit is made 0.30%. On the other hand, when Mn is added in a predetermined amount, it forms MnS and has an effect of preventing the occurrence of ear cracks in the hot rolled coil. The effect of preventing the occurrence of ear cracks can be sufficiently obtained if Mn is added so as to satisfy the condition represented by [Mn] / [S] ≧ 10. Since it is 009%, the lower limit of Mn is 0.10%. In addition, [Mn] and [S] here show content in the mass% of Mn and S in a steel plate, respectively.

<P: 0.020% or less>
P is an element that makes the steel plate material brittle and excessively segregates at the grain boundaries to cause cracks in the steel plate processing and deteriorates the corrosion resistance of the tin plate and tin-free steel plate. Therefore, the upper limit of P is 0.020% so as not to hinder the effects of the invention, but is preferably less than 0.010%. In addition, since P is an unnecessary element in this invention, the minimum in particular is not defined.

<S: 0.009% or less>
S, like B described later, is a main chemical component for the present invention, and it is important to set the upper limit to 0.009% or less in the B-added low carbon aluminum killed steel. By setting the upper limit of S to 0.009% or less, grain growth is promoted during BAF annealing, and the tempering degree T-1 of the annealed steel sheet can be easily secured. On the other hand, S, like P, is an element that deteriorates the corrosion resistance of the tin plate and tin-free steel plate, and is also a factor causing cracks in the hot rolled coil. Since it is an element, the lower limit is preferably 0.001% so as not to increase the melting cost. In addition, since corrosion resistance improves so that S is very trace amount, it is preferable to set it as 0.003% or less of content.

<Al: 0.020 to 0.060%>
Al acts as a deoxidizer at the time of melting, but its effect is insufficient if it is less than 0.020%. For this reason, when ensuring the cleanliness of the steel sheet, the lower limit of Al is 0.020%. On the other hand, when the content of Al is large, it becomes AlN during continuous casting and precipitates at the grain boundary to cause slab cracking, and may impair the N reduction effect due to the addition of B during hot rolling. 0.060%. In addition, since it is not necessary to leave Al as metal Al substantially in this invention, it is preferable to suppress the content as much as possible, and it is desirable to make the content into the range of 0.02-0.04%.

<N: 0.008% or less>
N is a major chemical component in the present invention. If the content of N is large, the material becomes brittle, and if it is excessively contained, it becomes hard and it is difficult to obtain a tempering degree T-1. Further, N is an element which causes a steel sheet processing crack and also causes a material variation with time due to strain aging. Therefore, the upper limit of N is set to 0.008% so as not to hinder the predetermined effect of the invention, but preferably 0.003% or less. Note that N is desirably a small element because it is a harmful element in securing T-1, and the lower limit is not particularly defined.

<B: 0.0008 to 0.008%, [B] / [N]: 0.6 or more and less than 1.0>
B, together with S, is the main chemical component of the present invention. B precipitates N as BN in the hot rolling stage to reduce the amount of solid solution N to a very low N, thereby suppressing precipitation hardening of fine nitride in BAF annealing and ensuring a tempering degree T-1. It has a function to make it easier. The lower limit of B is determined by [B] / [N] = 0.6, but when the amount is less than 0.0008%, the effect disappears, so the lower limit is set to 0.0008%. On the other hand, if the content of B exceeds 0.008%, the steel sheet is hardened and the tempering degree T-1 cannot be secured, so the upper limit is made 0.008%. Stoichiometrically, the composition ratio in which all N contained in the hot-rolled steel sheet is BN is [B] / [N] = 0.8. However, since hot rolling is performed in a short time, BN is quick. Therefore, it is desirable to increase [B] / [N], so [B] / [N] is set to less than 1.0. Further, if [B] / [N] is 0.6 or more, the amount of fine nitride deposited by BAF annealing is increased, but T-1 can be sufficiently secured by the combined use with S. Therefore, [B] / [N] was set to 0.6 or more. In addition, [B] and [N] here show content in the mass% of B and N in a steel plate, respectively.

<Other chemical components>
In the present invention, in order to obtain an annealed steel sheet having a tempering degree T-1, it is desirable to use a steel sheet having the composition as described above, but in addition to this, it is generally used in known steel sheets such as thin steel sheets for containers. The component element which exists in may be contained. That is, for example, Cr: 0.10% or less, Cu: 0.20% or less, Ni: 0.15% or less, Mo: 0.05% or less, Ti, Nb, Zr, V, etc. Various component elements can be contained according to the purpose so that the above is 0.3% or less, or Ca: 0.01% or less.

  By the way, obtaining the steel of the above-mentioned component range is easily realized through secondary refining such as primary refining performed using a converter and vacuum degassing treatment performed as necessary after primary refining. be able to.

  Further, in the present invention, in addition to the steel sheets having the above-described component ranges, a steel plate made of the same material as the tin plate original plate specified in JIS G 3303 or the tin-free steel base plate specified in JIS G 3315 is subjected to BAF annealing. It is good to do.

  Next, manufacturing conditions such as hot rolling conditions and cold rolling conditions for the tight coil steel plate before BAF annealing will be described. These manufacturing conditions are not essential components in the present invention, but are preferably satisfied in order to stably obtain an annealed steel sheet having a tempering degree T-1 while preventing a temper color.

<Hot rolling temperature: 860-950 ° C.>
The slab obtained through the primary refining or the secondary refining is preferably hot-rolled in a temperature range of 860 to 950 ° C. after being heated to a temperature at which rolling is possible. The lower limit of the finish rolling temperature is set to 860 ° C. This temperature is in the temperature range above the Ar3 transformation point, and if this temperature is maintained and rolling is performed, BN precipitation occurs rapidly, and the formation of nitride in BAF is suppressed. This is because the shape of the steel plate is improved and the productivity of the BAF steel plate is improved. On the other hand, the upper limit of the finish rolling temperature is set to 950 ° C. The reason why rolling exceeding this temperature is that there is significant secondary scale generation, and there is a high risk of frequent scale flaws, and the roll is accompanied by surface deterioration of the roll. This is to shorten the life.

<Taking-up temperature: 500-720 degreeC>
The hot-rolled steel sheet obtained after hot rolling is preferably wound up in a temperature range of 500 to 720 ° C. The lower limit of the coiling temperature is set to 500 ° C. in order to prevent the rolling structure from remaining due to self-annealing after coiling. This improves the primary cold rolling property and provides a desirable steel plate shape for BAF annealing. is there. On the other hand, the upper limit of the coiling temperature is set to 720 ° C. because the descalability in pickling deteriorates at a temperature higher than this, and the risk of frequent occurrence of scale wrinkles is extremely high.

<Pickling>
There are no particular restrictions on the conditions of pickling performed on the hot-rolled steel sheet obtained by winding at the winding temperature described above, and pickling with hydrochloric acid or sulfuric acid as normal conditions is possible.

<Cold rolling cold rolling ratio: 85-95%>
The hot-rolled steel sheet obtained after hot rolling is cold-rolled before BAF annealing. During the cold rolling, the cold rolling rate is preferably 85 to 95%. The tandem cold rolling mill has an upper limit for the practical cold rolling rate, and if the cold rolling rate exceeds 95%, the risk of the steel sheet breaking is extremely high. 95%. The reason why the lower limit of the cold rolling rate is 85% is that when the cold rolling rate is less than 85%, non-recrystallized grains are generated in the annealing method according to the present invention. In addition, this range is a high cold rolling rate compared with the cold rolling rate of 60 to 80% given to a general cold rolled steel sheet. This is because the limit of rolling is reached when a BAF annealed steel sheet having a thickness of less than 0.20 mm is manufactured.

<Electrolytic cleaning>
There are no particular restrictions on the conditions of electrolytic cleaning performed on the cold-rolled steel sheet obtained after the cold rolling described above, and degreasing with sodium orthosilicate as a normal condition is possible. After degreasing by electrolytic cleaning, the cold rolled steel sheet is wound with tension applied to form a tight coil steel sheet.

<Refined rolling: 1%>
The BAF annealed steel sheet obtained after the annealing process is preferably subjected to temper rolling. As for the rolling rate in this temper rolling, it is desirable to suppress work hardening as a dry 1% rolling rate required for shape correction and surface roughness imparting. In particular, when a BAF annealed steel sheet having a tempering degree T-1 is obtained, the obtained annealed steel sheet is the softest steel sheet, and therefore it is more preferable to perform temper rolling to obtain these effects.

  Hereinafter, the effect of the present invention will be described in more detail with reference to examples. In this example, a slab was obtained by continuous casting after melting steel having the composition shown in Table 3 below. In addition, the balance of steel of the composition shown in Table 3 is Fe and inevitable impurities. In this case, when adjusting C to 0.02% or less, vacuum degassing treatment was performed. The obtained slab was heated to 1235 ° C., then rolled at a hot rolling temperature of 900 ° C., and then wound at a winding temperature of 550 ° C. The obtained hot-rolled steel sheet was subjected to acidic descaling and then cold-rolled to a thickness of 0.301 mm at a cold rolling reduction rate shown in Table 4. The obtained cold-rolled steel sheet was wound up after degreasing by electrolytic cleaning to obtain a tight coil steel sheet.

The obtained tight coil steel plates were arranged in three stages in an annealing furnace as shown in FIG. The atmosphere gas supplied into the annealing furnace was adjusted so that the nitrogen gas concentration and the hydrogen gas concentration had the volume ratio (vol%) shown in Table 4. In the annealing process, the atmosphere temperature in the furnace is set to the first stage of 600 ° C., and the temperature of the upper coil 1, the middle coil 2 and the lower coil 3 is raised to 300 ° C., and then the holding treatment conditions shown in Table 4 The temperature was interrupted until the dew point reached the value of 300 ° C. dew point shown in Table 4. After that, the atmosphere temperature in the furnace is set to the second stage 750 ° C., and the soaking plate temperature of the upper coil 1 is 630 ° C., and the soaking plate temperature between the lower coil 3 and the middle coil 2 is again heated to 600 ° C. After raising the temperature, the atmosphere temperature in the furnace was set to about 630 ° C., which is the same as the soaking plate temperature of the upper coil 1, and the soaking plate temperature was maintained for 8 hours to recrystallize each tight coil steel plate. The dew point in the annealing process is a high dew point of about 0 ° C. at a plate temperature of 100 ° C. immediately after ignition of the annealing furnace, and the dew point at the time when the upper, middle and lower coil plate temperatures are maintained at 300 ° C. by proceeding with gas replacement. In addition, the dew point at the time when each tight coil steel plate reached the soaking plate temperature was adjusted so as to have the values shown in Table 4 (600 ° C. annealing condition). Then, after slow cooling until the plate temperature of each tight coil steel plate reached 100 ° C., each tight coil steel plate was defurcated.

  After completion of the annealing process, each decoiled tight coil steel sheet is rolled with a temper rolling mill to a reduction rate of 1% to a sheet thickness of 0.300 mm, and each tight coil steel sheet is unwound and visually observed for occurrence of discoloration. did. In addition to this, samples for material investigation were collected from each tight coil steel sheet, measured for Rockwell hardness at room temperature, subjected to artificial aging treatment at 210 ° C. for 30 minutes, and yield point elongation value by tensile test Was measured. The results are shown in Table 4. In addition, the Rockwell hardness measurement was performed based on the measurement method prescribed | regulated to JISZ2245, and the tensile test was done based on the test method prescribed | regulated to JISZ2241.

  In Table 4, “No discoloration” described in “Discoloration presence / absence” of “Product board defect occurrence / non-occurrence” indicates that the temper color was not confirmed, and “With discoloration” indicates that the temper color was confirmed. “Hard” described in “Other” in “Product plate defect occurrence” indicates that Rockwell hardness is higher than 49 ± 2 (hardness near T-1), and “Non-recrystallized” indicates unrecrystallized by observation. "Corrosion resistance" indicates that the steel sheet is easily corroded by excessive Si addition, and "surface defects" indicate defects such as steel sheet cracking due to excess or deficiency of Al or excess S. It is shown that. Moreover, the underline in Table 3 and Table 4 has shown what has deviated from the numerical range of the invention component mentioned above. In Table 4, “room temperature hardness” is 53 or more, and “tempering degree” is T-2 or more.

  Examples included within the scope of the present invention include steel no. Annealed steel sheets of A1 to A20 and B1 to B13 are mentioned. In any of these annealed steel sheets, the composition ratio of the atmospheric gas, the soaking plate temperature, and the dew point at the time of soaking are included in the numerical range of the above-described constituent elements of the invention, so that no temper color is generated. Steel No. Since all of the annealed steel sheets A1 to A20 included the cold rolling ratio during cold rolling and the composition of the steel sheet within the above-described numerical ranges, no temper color was produced, and the tempering degree T- 1 is obtained. Steel No. In the annealed steel sheets B1 to B13, since the composition of the steel sheet is not included in the numerical range described above, the temper color is not generated, but the tempering degree T-1 is not obtained.

  Examples not included within the scope of the present invention include steel no. Examples include C1-C8 annealed steel sheets. These annealed steel sheets do not contain any of the composition ratio of the atmospheric gas, the soaking plate temperature, and the dew point during soaking within the numerical range described above. 1 is not obtained.

  FIG. 6 summarizes the relationship between the fine nitride amount and the tempering degrees T-1 to T-3 using the data of the examples shown in Tables 3 and 4. The annealing condition 1 in FIG. 6 indicates data in which the soaking plate temperature in Table 4 is 630 ° C. or less, and the annealing condition 2 indicates data in which the soaking plate temperature is more than 630 ° C. In addition, the measurement of the amount of fine nitride is “corporate corporation. This was carried out using the bromine ester method described in “3rd edition Steel Handbook Vol. IV Steel Material Testing and Analysis” P330 to P331 ”published by Maruzen Co., Ltd., edited by Japan Iron and Steel Institute.

Also in this example, the mechanism for determining the T-1 to T-3 tempering degree is functioning similarly to the knowledge of the exploratory experiment 4, and the tempering degree T- without the temper color is made of the low S + B added low carbon aluminum killed steel.
An annealed steel sheet of No. 1 is a low carbon aluminum killed steel without B, and an annealed steel sheet with a tempering degree T-2 to T-3 without a temper color can be produced.

  One of the features of the present invention is that, conventionally, the tempering degree T-1 requiring high temperature annealing can be easily obtained by low temperature annealing at the recrystallization temperature level. This temperature is also a condition that can surely suppress the occurrence of temper color, and it is considered that the effect on the productivity and yield improvement of the tinplate, tin-free steel, or the surface treatment original plate is extremely large. Further, the temper color suppression mechanism elucidated by the present invention has an expectation that it can be used for, for example, a steel plate for building materials or a steel plate for automobiles, which has a step of BAF annealing of cold-rolled steel plates.

Claims (1)

% By mass, C: 0.02 to 0.06%, Si: 0.020% or less,
Mn: 0.10 to 0.30%, P: 0.020% or less, S: 0.009% or less, Al: 0.020 to 0.060%, N: 0.008% or less, B: 0.00. 0008 to 0.008%, and when the B content in mass% is [B] and the N content is [N], [B] / [N] is 0.6 or more and 1.0. It is obtained by subjecting a low carbon aluminum-killed hot-rolled steel sheet having a steel composition of Fe and inevitable impurities to the balance, and rolling the steel sheet at a cold rolling rate of 85 to 95%, followed by electrolytic cleaning. When recrystallization annealing of a tight coil steel sheet by Box Annealing Furnace, a mixed atmosphere gas having a composition of nitrogen gas concentration: 90 to 97 vol% and hydrogen gas concentration: 3 to 10 vol% is supplied, and the tight coil is heated in the temperature rising process. Evaporates moisture generated from steel plates Eliminate, and characterized by obtaining the annealed steel sheets of soaking plate temperature 580-630 By retaining the tight coil steel dew point during ° C. reached in the reducing atmosphere and -40 ℃ less temper grade T-1 An annealing method that can prevent temper color.

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