JP2005133158A - Decarburized layer depth estimating method, decarburized layer depth control method, and steel rolling method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a decarburized layer depth estimating method capable of estimating the depth of a decarburized layer in a heat-treated rolled steel by obtaining the depth by a formula for estimation, a method for controlling the depth of the decarburized layer on the surface of the rolled steel, and a steel rolling method for controlling the temperature of a heating furnace based on the estimated depth of the decarburized layer on the surface of the rolled steel. <P>SOLUTION: A charged steel is heat-treated in a heating furnace 1 under the conditions of the steel temperature T, and the heating time t at the steel temperature T. The heat-treated charged steel is transported to a rolling mill 2 and rolled, and drawn as a rolled steel. A decarburized layer generated in the heating furnace is observed as the decarburized layer depth (Dm) in the rolled steel after the rolling, and the decarburized layer depth (Dm) is approximated by the formula of estimation. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a decarburization layer depth prediction method for predicting a depth of a decarburization layer generated on a steel surface when a steel slab is heated and rolled, and a decarburization layer on a steel surface of a rolled steel material predicted by the decarburization layer depth prediction method. A method of controlling the temperature of the decarburization layer on the surface of the rolled steel by controlling the temperature of the heating furnace based on the predicted depth value and the temperature of the heating furnace based on the predicted value of the decarburization layer depth on the steel surface of the rolled steel The present invention relates to a steel rolling method to be controlled. In particular, the present invention relates to a method for predicting a decarburized layer depth of a steel bar and a wire rod, such as medium-high carbon steel, carburized steel, and high-speed steel, for which strict control is required for a decarburized layer on a steel surface, a decarburized layer depth control method, and a steel material rolling method.

  In the heat treatment process of heating the steel (feed steel) as a pre-treatment before rolling the steel, a decarburized layer is generated on the surface of the incoming steel, and the decarburized layer is also formed on the surface of the steel (rolled steel) after rolling. Remain as The decarburized layer is generated when carbon on the surface portion of the steel material is oxidized to form carbon dioxide gas and escape from the surface of the steel material. Due to the presence of the decarburized layer, the desired hardness cannot be obtained, and the fatigue strength is greatly reduced.

  In order to reduce the influence of the decarburized layer, research on the decarburized layer is advanced, and for example, methods for evaluating the decarburized layer have been proposed (for example, refer to Patent Documents 1 and 2). Moreover, as a method for suppressing the generation of the decarburized layer, a heating method for shortening the heating time at a temperature equal to or higher than the temperature of the A1 transformation point at which the generation of the decarburized layer is promoted, a heating method for reducing the maximum heating temperature as much as possible, a rolling process A heating method for reducing the temperature of the steel material sent to the process is known.

However, in the conventional heating method, it has been difficult to predict how much decarburization layer is produced when the steel material in the heating furnace becomes the rolled steel material after the rolling treatment. Usually, rolling steel is sampled, and the heating temperature, heating time, extraction temperature, etc. are set empirically based on the results of the decarburization test (decarburization layer depth measurement). The depth was only controlled (suppressed).
JP 2002-5921 A JP 2002-22733 A

  As described above, in the conventional heating method, the depth of the decarburized layer is obtained by sampling the rolled steel material, and based on the result, the heat treatment condition is controlled to control (suppress) the depth of the decarburized layer. From the control of the steel material temperature, the heating time, the size of the steel material before and after rolling, the depth of the decarburized layer before the heat treatment, the influence of the amount of scale generated in the heat treatment process, etc. There is a problem that it is difficult to control the heating conditions.

  The present invention has been made in view of such a problem, and in order to enable immediate control, the decarburization layer depth determined by the steel material temperature and the heating time during the heat treatment, and the size of the steel material before and after the rolling treatment. The decarburization layer depth in rolled steel can be easily and accurately determined using a prediction formula that takes the product of the change in size (ratio of size before and after rolling) and the correction coefficient having a predetermined value as the depth of decarburization layer in rolled steel. An object of the present invention is to provide a method for predicting the depth of decarburized layer.

  Furthermore, the present invention provides a decarburization layer depth control method and a steel material rolling method capable of keeping the depth of the decarburization layer of the rolled steel material below the specification value in a state in which the heating temperature capable of rolling is ensured. Objective.

  The decarburized layer depth prediction method according to the first aspect of the present invention is a decarburized layer depth for predicting the depth of a decarburized layer in a rolled steel material formed by heating and rolling the incoming steel material at a temperature equal to or higher than the A1 transformation point. In the prediction method, the prediction formula for predicting the depth of the decarburized layer is the product of the value of the calculation term including the steel material temperature and the heating time during the heat treatment, the ratio of the size before and after rolling, and the correction coefficient having a predetermined value. The correction coefficient is obtained in advance as a function of the size of the rolled steel material.

  In the first invention, the value of the calculation term obtained from the steel material temperature and the heating time during the heat treatment in the heating furnace, the ratio of the size by the rolling treatment, and the correction coefficient (decarburization coefficient) having a predetermined value, Is the prediction formula (predicted value) of the decarburized layer depth in the rolled steel (rolled steel), and the correction factor is determined in advance as a function of the size of the rolled steel. Therefore, it is possible to provide a decarburization layer depth prediction method capable of easily and accurately predicting the decarburization layer depth of the rolled steel material in parallel with the heat treatment regardless of the sampling.

  The decarburized layer depth prediction method according to the second invention is the first invention, wherein the heating furnace for performing the heat treatment is divided into a plurality of heating zones, and the sum of the values calculated by applying the calculation terms for each heating zone is calculated. The value of the operation term is used.

  In the second invention, the heating furnace to be heated is divided into a plurality of heating zones, and the sum of the values calculated by applying the calculation terms for each heating zone is set as the value of the calculation term. It is possible to provide a decarburization layer depth prediction method capable of predicting the decarburization layer depth in a rolled steel material more accurately and precisely in accordance with the heating condition (heating profile).

  The decarburized layer depth prediction method according to the third aspect of the present invention is the first or second aspect of the present invention, wherein the prediction formula includes the influence on the decarburized layer depth of the rolled steel material due to the decarburized layer depth of the incoming steel material. The variation of the decarburized layer depth due to the scale layer generated during the heat treatment is added.

  In the third invention, the influence of the decarburization layer depth on the rolled steel material due to the decarburization layer depth of the incoming steel material existing before rolling and the variation of the decarburization layer depth due to the scale layer generated during the heat treatment Therefore, it is possible to provide a decarburization layer depth prediction method capable of predicting the decarburization layer depth in the rolled steel material more accurately and precisely.

  The “size of steel material” in the present invention is mainly the size of the cross section perpendicular to the rolling direction in the case of rolled steel materials and the direction of rolling in the case of infeed steel materials. It can be represented by a cross-sectional dimension, that is, an angular dimension in a square steel slab, a width and thickness in a plate steel, a diameter (radius) in a round steel slab or round steel, and the like. It can also be represented by the square root of the cross-sectional area in order to eliminate the influence of the cross-sectional shape. In the first to third inventions, the steel material temperature to which the prediction formula is applied may be equal to or higher than the temperature of the A1 transformation point. Since the decarburization reaction mainly occurs at a temperature equal to or higher than the temperature of the A1 transformation point, the decarburization layer depth can be predicted quickly with sufficient accuracy.

  In addition, if the correction coefficient is calculated in advance by applying the actual measurement value of the decarburized layer depth of the rolled steel to the prediction formula, the decarburized layer depth in the rolled steel material can be calculated more accurately and accurately. A decarburized layer depth prediction method that can be predicted can be provided.

  In addition, the correction coefficient is a constant when the size of the rolled steel is greater than or equal to a predetermined value, and when it is less than the predetermined value, it is defined by a simple equation as a linear function that decreases in proportion to the size of the rolled steel. By doing so, it is possible to provide a decarburization layer depth prediction method that can easily calculate the prediction formula, and can more easily and quickly predict the decarburization layer depth in the rolled steel material.

  The decarburized layer depth control method according to the fourth aspect of the present invention compares the decarburized layer depth of the rolled steel material predicted based on the decarburized layer depth predictive method of any of the first to third aspects of the invention with a specification value, and predicts it. If the decarburized layer depth is smaller than the specified value, the furnace temperature is controlled so that the steel temperature becomes the target steel temperature within the range where the decarburized layer depth is less than the specified value, and the predicted decarburized layer depth is Is larger than the specification value, the decarburization layer of the rolled steel material is controlled by controlling the temperature of the heating furnace so that the decarburization layer depth is not more than the specification value within the range where the steel material temperature is not less than the lower limit value of the target steel material temperature. The depth is controlled.

  In the fourth invention, the predicted decarburized layer depth of the rolled steel material is compared with the specification value. If the predicted decarburized layer depth is smaller than the specified value, the furnace temperature is adjusted so that the steel material temperature becomes the target steel material temperature (the steel material temperature that can be rolled) within the range where the decarburized layer depth can be maintained below the specified value. When the temperature is controlled and the predicted decarburized layer depth is greater than the specified value, the furnace temperature is set so that the decarburized layer depth is less than the specified value within the range where the steel material temperature is equal to or higher than the lower limit of the target steel temperature. Therefore, it is possible to provide a decarburization layer depth control method capable of controlling the depth of the decarburization layer of the rolled steel material to be equal to or lower than the specification value while ensuring a heating temperature at which rolling is possible.

  In the fourth invention, if the lower limit value of the target steel material temperature is further defined by the lowest temperature at which rolling after heating is possible, the heating temperature at which rolling is possible (steel material temperature) is reliably maintained. It is possible to provide a decarburized layer depth control method capable of controlling (suppressing) the depth of the decarburized layer of the rolled steel material to be equal to or lower than the specification value.

  A steel material rolling method according to a fifth aspect of the present invention is a steel material rolling method in which a steel material to be rolled is subjected to heat treatment after heating the incoming steel material to the A1 transformation point or higher in a heating furnace, and the decarburized layer depth of the rolled steel material is predicted. Compare the predicted decarburized layer depth of the rolled steel with the specification value.If the predicted decarburized layer depth is smaller than the specified value, the steel material temperature should be within the range where the decarburized layer depth is less than the specified value. If the temperature of the furnace is controlled so that the temperature is the same, and the predicted decarburized layer depth is greater than the specified value, the decarburized layer depth is below the specified value within the range where the steel material temperature is equal to or higher than the lower limit of the target steel material temperature. The temperature of the heating furnace is controlled so that

  In the fifth invention, the decarburization layer depth of the rolled steel material is predicted, the predicted decarburization layer depth of the rolled steel material is compared with the specification value, and when the predicted decarburization layer depth is smaller than the specification value, The furnace temperature is controlled so that the steel temperature becomes the target steel temperature within the range where the decarburized layer depth is not more than the specified value.If the predicted decarburized layer depth is larger than the specified value, the steel temperature is the target. Since the temperature of the heating furnace is controlled so that the depth of the decarburized layer is not more than the specified value within the range of the lower limit value of the steel material temperature, the decarburized layer of the rolled steel material in a state where the heating temperature capable of rolling is ensured. It is possible to provide a method of rolling steel that can control the depth of the steel to a specification value or less.

  In the present invention, the decarburized layer depth determined by the steel material temperature and the heating time during the heat treatment, the change in size of the steel material before and after the rolling treatment (size ratio), and the correction coefficient having a predetermined value. The depth of the decarburized layer in the rolled steel material can be predicted easily, accurately, and accurately using a prediction formula in which the product of is the depth of the decarburized layer in the rolled steel material.

  In the present invention, the decarburized layer depth in the rolled steel material predicted using the prediction formula is compared with the specification value, and the heat treatment conditions (decarburized layer depth by controlling the heating temperature) are determined according to the situation of the comparison result. Of the decarburization layer depth that can keep the depth of the decarburization layer of the rolled steel below the specified value in a state where the heating temperature capable of rolling is ensured. Can control.

  Hereinafter, the present invention will be described with reference to the drawings illustrating embodiments thereof.

<Embodiment 1>
FIG. 1 is an explanatory diagram showing a concept of a decarburized layer depth prediction method when the steel material temperature is single. The incoming steel material is heated in the heating furnace 1 under the conditions of the steel material temperature T (absolute temperature K) and the heating time t (minute) at the steel material temperature T. The heat-treated incoming steel material is conveyed to the rolling device 2, subjected to the rolling treatment, and extracted as a rolled steel material (product) after rolling. The decarburized layer generated in the heating furnace is observed as a decarburized layer depth Dm (mm) in the rolled steel material after rolling, and this decarburized layer depth Dm (mm) is approximated by the expression (1).
Dm = k (a / A) (√t) exp (−Q / RT) (1)
However,
Dm (mm): Decarburized layer depth T (absolute temperature K): Steel material temperature of the incoming steel material in the heating furnace 1 t (min): Heating time at the steel material temperature T Q (cal / mol): Carbon diffusion Activation energy R (1.987 cal / (mol · K)): gas constant a: size of rolled steel (for example, diameter a (mm) in the case of round steel)
A: Size of incoming steel (for example, square dimension A (mm) in the case of a square steel piece)
a / A: Ratio of sizes before and after rolling Q / RT: 8787 / T
k: Correction coefficient (mm / √min)

  That is, the depth of the decarburized layer in the rolled steel material includes the calculation term ((√t) exp (−Q / RT)) including the steel material temperature T and the heating time t during the heat treatment, and the size of the rolled steel material. It is approximated by the product of the ratio a / A of the size determined by the ratio of a to the size A of the incoming steel material and the correction coefficient k having a predetermined value. This approximate expression is a prediction expression for predicting the depth of the decarburized layer in the rolled steel material after rolling. At this time, the correction coefficient is specified in advance as a function of the size A of the rolled steel material. The unit of each value can be changed as appropriate, and in that case, it can be adjusted by changing the value of the correction coefficient k as appropriate. In addition, the steel material temperature can be easily predicted with sufficient accuracy by applying the steel material temperature to a temperature at which a decarburized layer is easily generated, that is, a temperature equal to or higher than the temperature of the A1 transformation point (723 ° C. in the case of a steel material). Note that all calculations here can be performed quickly by a computer, which is particularly effective when the steel material temperature is controlled in the heat treatment. Such characteristics of calculation by the computer can be similarly obtained in the following calculation.

  As described above, in the formula (1), the steel material temperature T, the heating time t at the steel material temperature T, the carbon diffusion activation energy Q, the gas constant R, the rolled steel material size a, and the incoming steel material size A If the correction coefficient k is substituted, the decarburized layer depth Dm in the rolled steel after the heat treatment and rolling treatment steps can be predicted. Since the correction coefficient k is obtained in advance for the heating furnace 1, the depth of the decarburized layer of the rolled steel can be predicted easily and accurately without sampling. The “size of steel material” in the present invention is mainly the size of the cross section perpendicular to the rolling direction in the case of rolled steel materials and the direction of rolling in the case of infeed steel materials. It can be represented by a cross-sectional dimension, that is, an angular dimension in a square steel slab, a width and thickness in a plate steel, a diameter (radius) in a round steel slab or round steel, and the like. It can also be represented by the square root of the cross-sectional area in order to eliminate the influence of the cross-sectional shape.

<Embodiment 2>
FIG. 2 is an explanatory diagram showing a concept of a decarburized layer depth prediction method when a plurality of steel material temperatures are set. According to the situation in which the steel material temperature T changes in the heating furnace 1, it is divided into a plurality of (heating) zones Zi (i = 1 to N), and a state in which the operation term of the expression (1) is applied to each zone. Show. That is, the decarburized layer depth in the zone Zi is expressed by the formula (2).
Dmi = k (a / A) (√ti) exp (−Q / RTi) (2)
Ti (absolute temperature K): Steel material temperature of the incoming steel material in zone Zi of heating furnace 1 ti (min): Heating time at steel material temperature Ti

Equation (2) is basically the same as Equation (1) except that the decarburized layer depth is obtained for each zone. The part of k (a / A) is the same as that of Formula (1), and the part of the calculation term ((√ti) exp (−Q / RTi)) is different. Decarburized layer depth of each zone determined by equation (2) (Decarburized layer depth of zone Z1: Dm1, Decarburized layer depth of zone Z2: Dm2, ... Decarburized layer depth of zone Zi: Dmi, ... -The sum of the zone ZN decarburization layer depth: DmN) is defined as the decarburization layer depth Dm of the rolled steel material. That is, when the zone Zi is divided into N, the decarburized layer depth Dm is a predictive formula (3).
Dm = Dm1 + Dm2 + ... + Dmi + ... + DmN (3)
In the actual calculation, the k (a / A) portion is common, and therefore k (a / A) is the sum of only the calculation term ((√ti) exp (−Q / RTi)) portions. Multiply by

  As described above, in Formula (2), if the steel material temperature Ti in each zone of the heating furnace 1 and the heating time ti at the steel material temperature Ti are respectively substituted and summed, the removal in the rolled steel material after the heat treatment and the rolling treatment is performed. The coal bed depth Dm can be predicted. Since the correction coefficient k is obtained in advance for the heating furnace 1, the depth of the decarburized layer of the rolled steel can be predicted easily and accurately without sampling. Furthermore, the decarburized layer depth in the rolled steel material can be predicted more accurately and accurately according to the heating state (heating profile) in the heating furnace 1.

<Embodiment 3>
The depth of the decarburized layer in the rolled steel material includes not only the decarburized layer generated by the heat treatment, but also the decarburized layer already present before the incoming steel material is heat-treated, and further the decarburized layer generated on the surface by the heat treatment. Coal seam reduction will also have an effect. The influence on the decarburization layer depth of the rolled steel material due to the decarburization layer depth Db already possessed before the incoming steel material is heat-treated is the ratio of the size in the rolling process (a / A). The decarburized layer depth Dmb can be expressed by the formula (4). Further, the variation Dms in the depth of the decarburized layer due to the scale layer can be obtained as Equation (5) using the scale loss ratio α.
Dmb = (a / A) Db (4)
Dms = − (a / A) Db√ (1−α) (5)

Therefore, what added Formula (4) (5) to the decarburized layer depth Dm (Formula (3)) in the rolled steel materials which generate | occur | produced by heat processing becomes the prediction formula of the decarburized layer depth D in rolled steel materials, It can be shown as equation (6).
D = Dm + Dmb + Dms (6)

  As described above, in Formula (6), the decarburized layer depth Dm of the rolled steel material generated by the heat treatment is set to the decarburized layer depth Db of the incoming steel material (the decarburized layer depth already present before the heat treatment). Add the value converted to the decarburized layer depth Dmb after rolling, and add the variation Dms of the decarburized layer depth in the rolled steel due to the scale layer generated during the heat treatment (decrease by the scale layer) By using the prediction formula, the depth of the decarburized layer in the rolled steel can be predicted accurately and precisely. When the decarburized layer depth Dmb and the variation Dms of the decarburized layer depth are small compared with the decarburized layer depth Dm and can be ignored, Equation (6) is a simplified prediction formula such as D = Dm. You can also.

<Embodiment 4>
FIG. 3 is a graph for explaining a correction coefficient determination method. The horizontal axis shows the relative size a (for example, the diameter of the round steel) of the rolled steel material on an arbitrary scale, and the vertical axis shows the correction coefficient k relatively on the arbitrary scale. As described above, the size a of the rolled steel material can be represented by various materials depending on the round steel or the plate steel. In this case, it goes without saying that the value of the correction coefficient k is different. The correction coefficient is obtained by substituting a predetermined value for the prediction formula of formula (6) and back-calculating from the measured value of the decarburized layer depth of the rolled steel (corresponding to D of formula (6) which is the prediction formula). It is a thing. The black circles on the graph indicate individual correction coefficients obtained from actual measurement values, and the solid lines indicate approximate expressions for the correction coefficient k. Since the individual correction coefficient obtained from the actual measurement value is determined by the furnace characteristics of the heating furnace, it is obtained for each heating furnace. An approximate expression indicating the upper limit of all the calculated correction coefficients k is calculated, and the approximate expression is adopted as the correction coefficient k.

  By adopting an approximate expression (maximum value of the correction coefficient k) indicating the upper limit of all the correction coefficients k as the correction coefficient k in the expression (6) as a prediction expression, the decarburized layer depth is equal to or less than the value calculated by the prediction expression. Can be reliably suppressed. From the graph of the correction coefficient obtained here, for example, when the size of the rolled steel material is not less than a predetermined value of 70 (for example, diameter (mm)), the correction coefficient k is a fixed value (constant) of 22.4. . Further, when the size of the rolled steel material is smaller than a predetermined value of 70, the correction coefficient k is represented by 59.85-0.54a. That is, when the size a of the rolled steel material is less than a predetermined value, it is a linear function that decreases in proportion to the size a of the rolled steel material. By defining the correction coefficient k by a simple characteristic formula of a constant and a linear function, the calculation of the prediction formula can be made simpler, and the decarburization layer depth in the rolled steel can be predicted more easily and quickly. Although an approximate expression indicating the upper limit of all the correction coefficients k is used, an approximate expression including, for example, about 95% of the correction coefficient k can be used if some errors are allowed.

<Embodiment 5>
FIG. 4 shows the relationship between the zone division when the heating furnace is divided into a plurality of zones, and the steel material temperature and heating time in each zone. FIG. 4A shows the zone division state of the heating furnace 1, and FIG. 4B shows the heating time ti and the steel material temperature Ti in each zone (Zi). The heating furnace 1 includes a heating region 1a and a soaking region 1b. The incoming steel material is transported along the transport path 1c, and is heated at the steel material temperature (Ti) set in the zone (Zi) for the set heating time (ti). Here, the heating area 1a is divided into zones Z1 to Z3, and the soaking area 1b is divided into zones Z4 to 6. In the zone Z1, the steel material temperature T1, the heating time t1, the zone Z2 as the steel material temperature T2, the heating time t2, and the zone Z6 as the steel material temperature T6 and the heating time t6. The steel material temperature Ti in each zone Zi is the higher temperature (exit side temperature) having the greatest influence. The heating time ti is calculated in consideration of the rolling efficiency in the rolling process performed after the heating process. The steel material temperature Ti can be measured in the heating furnace 1 by an appropriate means, but can also be predicted from the temperature before feeding the steel material into the heating furnace and the temperature condition in the heating furnace.

  The steel material temperature Ti and the heating time ti are applied to Equation (6), which is the prediction equation described above, and the decarburized layer depth D of the rolled steel material is calculated and predicted. The calculation of the prediction formula can be performed more accurately by performing it for each heated steel material. The calculation result (predicted value) is compared with the decarburized layer depth of the rolled steel material. When the predicted decarburized layer depth D is smaller than the specification value Dspec, the steel material temperature (T6) at the outlet of the heating furnace becomes the target steel material temperature within a range where the decarburized layer depth D is equal to or less than the specified value Dspec. The temperature of the heating furnace 1 is controlled (for example, temperature rise control). When it is predicted that the decarburization layer depth D is larger than the specification value Dspec by the temperature increase control, the temperature of the heating furnace 1 is again controlled (for example, temperature decrease control) so that the decarburization layer depth D falls within the specification value Dspec. . That is, by calculating and predicting the decarburized layer depth of the rolled steel material based on the prediction formula during the heat treatment, the decarburized layer depth D is compared with the specified value Dspec while the decarburized layer depth D is compared with the specified value Dspec. By controlling the temperature of the heating furnace 1 so as to be within the range, the decarburized layer depth D of the rolled steel material can be controlled.

  Further, when the predicted decarburized layer depth D is larger than the specification value Dspec, the steel material temperature (T6) at the outlet of the heating furnace is appropriately defined as a lower limit value of the target steel material temperature (a value equal to or higher than the minimum temperature that can be rolled). The temperature of the heating furnace 1 is controlled (for example, temperature drop control) so that the decarburized layer depth D is equal to or less than the specification value Dspec within the above range. Even when the steel material temperature (T6) is equal to or lower than the lower limit value of the target steel material temperature, when the decarburized layer depth D exceeds the specification value Dspec, the temperature of the heating furnace 1 is controlled so as to maintain the lower limit value of the target steel material temperature.

  By predicting the decarburized layer depth D of the rolled steel during the heat treatment process described above, and controlling the temperature of the heating furnace 1 according to the result, the decarburized layer depth D of the rolled steel can be controlled. A decarburized layer depth control method can be used. Although the number of divisions has been described here as six, it can be further subdivided and calculated within the range of computer processing capability (calculation capability). The accuracy improves as the data is subdivided, but the accuracy more than necessary is unnecessary, so the appropriate number of divisions is determined in consideration of product use.

  FIG. 5 is a graph showing the state of occurrence of defective decarburized layer when the decarburized layer depth control method in the fifth embodiment is applied. A horizontal axis is a rolling month, the month after applying the decarburization layer depth control method in Embodiment 5 is shown as 1-3, and the month before application is shown as -1-3. The amount of defective products before application was 40 in the month of -3, 92 in the month of -2, and 25 in the month of -1, but the amount of defective products generated was almost 0 after application.

It is explanatory drawing which shows the concept of the decarburization layer depth prediction method in case steel material temperature is single. It is explanatory drawing which shows the concept of the decarburization layer depth prediction method at the time of setting steel material temperature to multiple. It is a graph for demonstrating the determination method of a correction coefficient. The relationship between the zone division | segmentation at the time of dividing a heating furnace into a some zone, the steel material temperature in each zone, and heating time is shown. It is a graph which shows the generation | occurrence | production condition of the decarburized layer defect product at the time of applying the decarburized layer depth control method in Embodiment 5. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Rolling apparatus A Size of incoming steel materials a Size of rolled steel materials T Steel material temperature Ti Steel material temperature in zone Zi t Heating time ti Heating time in steel material temperature Ti Zi zone (i = 1 to N)

Claims (5)

In the decarburization layer depth prediction method for predicting the depth of the decarburization layer in the rolled steel formed by heat-treating and rolling the incoming steel at a temperature equal to or higher than the A1 transformation point,
The prediction formula for predicting the depth of the decarburized layer is the product of the value of the calculation term including the steel material temperature and the heating time during the heat treatment, the ratio of the size before and after rolling, and the correction coefficient having a predetermined value,
The decarburized layer depth prediction method, wherein the correction coefficient is obtained in advance as a function of the size of the rolled steel material.   The heating furnace for performing the heat treatment is divided into a plurality of heating zones, and a sum of values calculated by applying the calculation terms for each heating zone is set as the value of the calculation terms. Coal seam depth prediction method.   The prediction formula is characterized by adding the influence on the decarburization layer depth of the rolled steel due to the decarburization layer depth of the incoming steel material and the fluctuation of the decarburization layer depth due to the scale layer generated during the heat treatment. The decarburized layer depth prediction method according to claim 1 or 2. The decarburized layer depth of the rolled steel material predicted based on the decarburized layer depth predicting method according to any one of claims 1 to 3 is compared with a specification value,
When the predicted decarburized layer depth is smaller than the specification value, the temperature of the heating furnace is controlled so that the steel material temperature becomes the target steel material temperature within the range where the decarburized layer depth is not more than the specified value.
If the predicted decarburized layer depth is greater than the specified value, the temperature of the heating furnace is controlled so that the decarburized layer depth is below the specified value within the range where the steel material temperature is not less than the lower limit of the target steel material temperature. A decarburization layer depth control method characterized by controlling the depth of a decarburization layer of rolled steel.   This is a rolling method of steel material that is subjected to rolling treatment after heating the incoming steel material to the A1 transformation point or higher in a heating furnace, predicting the decarburized layer depth of the rolled steel material, and predicting the decarburized layer depth of the rolled steel material. When the predicted decarburized layer depth is smaller than the specified value compared to the specification value, the furnace temperature is controlled so that the steel material temperature becomes the target steel temperature within the range where the decarburized layer depth is less than the specified value. When the predicted decarburized layer depth is larger than the specified value, the temperature of the heating furnace is controlled so that the decarburized layer depth is not more than the specified value within the range where the steel material temperature is not less than the lower limit value of the target steel material temperature. A method for rolling steel.

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