JP6533648B2 - Heat treatment method of heat treated high tensile steel sheet - Google Patents

Description

  The present invention relates to a heat treatment method for a tempered high-tensile steel sheet excellent in material uniformity in the thickness direction.

  Steel plates used in the manufacture of steel structures such as buildings, bridges, storage tanks, pressure vessels, etc. should be excellent not only in strength and toughness but also in formability at the time of molding and resistance to hydrogen-induced cracking. Required When the hardness of the surface layer portion of the steel plate is increased, the workability is reduced and the resistance to hydrogen induced cracking is also reduced. Therefore, the hardness of the surface layer portion of the steel sheet is preferably as low as possible, as long as the strength of the entire steel sheet can be maintained. However, in general, in the heat treated steel sheet, the cooling rate on the surface of the steel sheet is faster than the inside of the steel sheet in cooling after heat treatment, so the hardness of the surface layer portion of the steel sheet is higher than the hardness of the inside of the steel sheet.

  For this reason, in the heat treated steel sheet, an increase in the hardness of the surface layer portion of the steel plate is suppressed to make the hardness about the same as the inside of the steel plate, and a steel plate having uniform material in the thickness direction with few differences in hardness It is important in order to balance the strength of the steel sheet and the processability or hydrogen-induced cracking resistance.

  As a technique for suppressing an increase in hardness of the surface layer portion of the steel sheet, for example, Patent Document 1 proposes a method of accelerating only the surface layer portion after accelerated rolling and induction heating to soften only the surface layer portion.

Further, Patent Document 2 discloses a steel plate whose hardness of the surface layer portion of the steel plate is lower than that of the central portion of the steel plate by more than 15 HV. As a manufacturing method thereof, only the surface layer portion of the steel plate after rolling and cooling is Ac ₁ transformation point or more. A method of heating has been proposed. Furthermore, as a specific method of heating only the surface layer portion of the steel plate, when heating the steel plate in a heating furnace, a method of extracting from the heating furnace in a state in which the surface layer portion is at a higher temperature than the inside Alternatively, a method has been proposed in which the surface layer portion of the steel plate is intensively heated by the induction heating device, or a method in which the surface of the steel plate is heated by flames of burners arranged in the width direction of the steel plate.

Patent No. 3666458 gazette JP, 2011-195961, A

  However, the above-mentioned prior art has the following problems.

  In the method of intensively heating the surface layer portion of the steel plate by induction heating, which is disclosed in the cited documents 1 and 2, it is necessary to newly install a strong induction heating device, which increases the equipment cost. Furthermore, the heating by induction heating takes time, and the manufacturing efficiency is reduced.

  Also in the method of heating the surface layer portion of the steel plate by the heating burner disclosed in FIG. 2 of the cited document 2, similarly to the method by induction heating, it is necessary to newly install a large number of powerful heating burners. It takes Furthermore, the heating by the heating burner takes time, and the manufacturing efficiency is reduced.

  A method disclosed in Patent Document 2 (paragraph 0054) for extracting a steel sheet from a heating furnace in a state where the surface layer portion is at a higher temperature than the inside without heating time being sufficiently long when heating the steel plate in the heating furnace There is no need to install new equipment, and it is a simple and easy method, but it uses the temperature difference between the surface layer and the inside of the steel sheet being heated by the heating furnace, so the temperature distribution in the steel sheet and the heat treatment temperature Unless time can accurately predict the influence on the properties of the steel sheet, it is not possible to stably produce a steel sheet with desired properties. However, Reference 2 does not disclose any specific measures for this purpose.

  The present invention has been made to solve the above-mentioned problems, and a technology capable of stably manufacturing a tempered high-tensile steel sheet excellent in material uniformity in the thickness direction even when new equipment is not used. To provide.

  Usually, when producing a steel plate by quenching and tempering, the quenched steel plate is heated and tempered to take out (extract) the steel plate from the heating furnace after the entire steel plate becomes uniformly at a predetermined temperature. .

  As shown in FIG. 1, the hardness distribution of the steel sheet after quenching is such that the hardness of the surface layer is high and the hardness of the central portion is low. This is because the surface layer has a faster cooling rate than the central portion, and more complete baking can occur. As described above, when the steel plate having such hardness distribution is tempered until the entire steel plate reaches a uniform temperature, the surface layer hardness decreases to a predetermined hardness, but the hardness at the center of the steel plate Is too low, the strength of the entire steel plate is insufficient (high temperature tempering in FIG. 1), or the strength of the whole steel plate is sufficient, but the surface hardness does not decrease below the predetermined hardness (low temperature in FIG. 1). There is a problem such as tempering).

  Then, the present inventors acquired the following knowledge, as a result of repeating earnest research, in order to solve the said subject.

 In the tempering process, a temperature difference can be provided between the surface layer portion of the steel plate and the inside by taking out the steel plate from the heating furnace during heating. By utilizing this, it is possible to temper only the surface layer portion at a high temperature to make the surface layer hardness less than a desired hardness, lower the tempering temperature inside the steel plate, and prevent strength reduction due to tempering to ensure the strength of the entire steel plate. .

 In order to stably manufacture a steel plate excellent in material uniformity in the thickness direction by such a method, accurately predicting the temperature during heating up at each position in the thickness direction of the steel plate, and heat treatment temperature It is necessary to accurately predict the influence of time on the properties of the steel sheet.

 In conventional heat treatment, a target temperature is reached and then held for a certain period of time in order to make the entire steel sheet have a uniform temperature. Therefore, the tempering parameters (tempering parameters) are determined only by the target temperature and the holding time at that temperature.

 However, since the present invention takes out the steel plate from the heating furnace during heating, unlike the conventional heat treatment, there is no time to hold the steel plate at a constant temperature. Therefore, in order to predict the material in the thickness direction, it is necessary to combine the temporal change of the temperature of each part of the steel sheet by the heat conduction calculation and the evolved tempering parameter in consideration of the continuous heating process.

 The present invention has been completed based on the above findings, and specifically, the present invention provides the following production method.

[1] In a heat treatment method in which a steel plate surface portion is extracted from the heating furnace at a temperature higher than that of the inside of the steel plate by heating the steel plate in the heating furnace, time change of the temperature of each portion in the thickness direction of the steel plate The time change of this temperature is divided into time intervals, and from the temperature T _i (° C.) in the time interval and the length t _i (h) of the time interval, the evolved tempering parameter λ is obtained by the following equations 1 and 2. Calculate the hardness of each part in the thickness direction of the steel sheet from the relationship between the developed tempering parameter λ and the hardness, and refine the high tension to make the hardness distribution in the thickness direction of the steel sheet desired. Method of manufacturing steel plate.
λ = log (10 ^{λ 1} +10 ^λ 2 +...)...
λ _i = log (t _i ) −A / T _i +50...
However, A is a constant determined by the steel composition.

  [2] The method for producing a tempered high-tensile steel sheet according to [1], wherein the set temperature of the heat treatment furnace and the extraction temperature of the steel sheet are set so that the hardness distribution in the thickness direction of the steel sheet is desired.

  In addition, since the manufacturing method of the steel plate of this invention is what provides a temperature difference in the surface layer of a steel plate, and plate | board thickness center in a heat treatment process, application is difficult if it is a thick steel plate to some extent. For this reason, the manufacturing method of this invention is suitably applicable to the steel plate of 10 mm-80 mm of plate | board thickness. Moreover, the manufacturing method of the steel plate of this invention is applicable also to normalizing etc. besides hardening-tempering.

  By using the method for producing a steel plate of the present invention, it is possible to reduce the surface layer hardness while securing the strength of the steel plate, and stably produce a steel plate excellent in the material uniformity in the thickness direction. This steel plate is excellent in bending workability and resistance to hydrogen induced cracking. Further, the manufacturing method of the present invention can use an existing heating furnace, and it is not necessary to provide new equipment.

It is a schematic diagram which shows plate thickness direction hardness distribution of a hardening-tempered steel plate. It is a figure which shows typically the calculation method of an evolutionary type tempering parameter. It is a figure explaining the setting method of heat processing temperature and offset temperature. It is a figure which shows plate thickness direction hardness distribution of the steel plate as hardening and after tempering.

  Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

Prediction of the hardness of each part in the thickness direction of the steel sheet The time change of the temperature of each part in the thickness direction of the steel sheet when heating the steel sheet in the heating furnace is the heat transfer from the atmosphere in the heating furnace to the steel sheet It is obtained by solving the conduction equation numerically. Furthermore, the time change curve of the temperature of each part is divided into an appropriate short time section (section length t _i ), and the temperature T _i in that section is determined (the section temperature is the average temperature of the section or the end of the section The temperature of the part can be taken, but if the section is made sufficiently short, sufficient accuracy can be obtained at any temperature of the section, so any temperature in the section is appropriately selected).

FIG. 2 shows an example of the temperature curve at the surface layer and the thickness 1/2 position (1 / 2t) and an example where the temperature curve at 1 / 2t is divided into time intervals. The tempering parameter λ _i in this divided section is obtained from the section length t _i of the divided section _i and the temperature T _i in that section using the above-mentioned equation (2). Then, the tempering parameters in the divided sections are added for the entire period in which the steel plate is heated in the heating furnace using the equation 1 to obtain an evolved tempering parameter λ. In addition, constant A in Formula 2 is calculated | required by experiment about each steel type. Further, since the tempering parameter λ _i of the section becomes extremely small when the section temperature is low, the temperature range of the section temperature is sufficiently lower than the extraction temperature (usually about 100 ° C.) is added to the calculation of the evolutionary tempering parameter λ. It is not necessary.

  The hardness distribution in the thickness direction of the steel sheet after quenching is determined by measurement. Further, the relationship between the developed tempering parameter λ and the hardness after tempering under the same conditions as the heat treatment condition for obtaining the evolved tempering parameter λ is determined by experiment. Since the evolution type tempering parameter λ of each part of the steel plate is determined as described above, the evolution type tempering parameter λ that makes each part of the steel sheet after quenching have a desired hardness is determined, and tempering is performed under the conditions when this λ is obtained. You can determine the hardness after

Determination of Heat Treatment Furnace Set Temperature and Extraction Temperature The hardness of each part of the steel plate is predicted by the above method. Since the hardness distribution of the thickness direction of the steel plate after heat treatment is known by this, the set temperature of the heat treatment furnace and the extraction temperature of the steel plate are determined so as to obtain a desired hardness distribution.

  Generally, the surface hardness of the steel sheet and the tensile strength at 1⁄4 position (1⁄4 t) of the thickness are often required. Therefore, as an example, an example will be described in which the set temperature of the heat treatment furnace and the extraction temperature of the steel plate are determined such that the surface hardness of the steel plate and the tensile strength at 1/4 t are desired. Here, the set temperature of the heat treatment furnace is the atmosphere temperature of the heat treatment furnace, and the extraction temperature of the steel plate is the temperature of 1/2 t of the steel plate at the time of extraction. The temperature of 1 / 2t of the steel plate at the time of extraction is hereinafter referred to as offset temperature (the temperature of 1 / 2t is a value by heat transfer-heat conduction calculation).

  First, the offset temperature is temporarily set. The offset temperature is the temperature at 1 / 2t during extraction (normal heat treatment for maintaining the temperature of the steel plate until the temperature of the steel plate becomes uniform) at which the desired tensile strength (at 1 / 4t) can be obtained In order to hold until uniform, it is set to the same temperature as the set temperature of the heat treatment furnace.

  Next, the set temperature of the heat treatment furnace is set. The set temperature of the heat treatment furnace is set higher than that of the normal heat treatment. According to the set temperature of the heat treatment furnace, as shown in FIG. 3, the time change of the surface layer temperature is calculated, and the time change of the surface layer hardness is calculated from the time change of the surface layer temperature. Also, at the same time, the time change of hardness at 1/4 t is calculated by the same procedure (the time change of 1/4 t temperature is not shown). Also, the time change of temperature at 1 / 2t is also calculated. The point in time at which the temperature at 1⁄2 t reaches the temperature at 1⁄2 t during extraction in normal heat treatment is taken as the timing for extracting the steel sheet from the heat treatment furnace.

  The surface hardness and the hardness at 1⁄4 t at this extracting timing can be understood from the above calculation. Since the hardness can be easily converted to tensile strength, the offset temperature set in the above calculation and the set temperature of the heat treatment furnace are set so as to obtain desired characteristics if the hardness is in the desired range. .

  If these hardnesses do not fall within the desired range, the offset temperature or the setting temperature of the heat treatment furnace is reset, the calculation is repeated, and the calculation is repeated until the desired characteristics are obtained. According to the above-described procedure, the offset temperature and the set temperature of the heat treatment furnace are determined in order to obtain a steel plate having desired characteristics. In addition, conversion from hardness to tensile strength may be based on a conversion table (such as SAE J 417) or a conversion formula may be used.

  The steel plate (35 mm in thickness) having the component composition shown in Table 1 was heated to 930 ° C. and then quenched by water cooling.

The hardness distribution of the as-quenched steel plate was measured using the steel plate thus quenched. Moreover, tempering was performed on various conditions using this hardened steel plate, and hardness was measured.

From this data, the value of constant A in equation 2 was determined. In addition, the hardness distribution is predicted using the evolved tempering parameter λ, and the surface hardness is Brinell hardness, HBW 225 or less (Vickers hardness conversion 237 HV or less), 1 / 4t tensile strength is 550 MPa or more As heat treatment conditions which become, the preset temperature of a heat treatment furnace: 700 ° C, offset temperature: 680 ° C were set up. Under the heat treatment conditions, tempering was performed using the above-described hardened steel sheet. The hardness distribution of the steel plate after tempering was measured, and the tensile strength at 1/4 t was measured. Hardness measurement and tensile properties were measured by the following methods.
[Hardness measurement]
After the cross section of the steel plate was polished until it became a mirror surface, Vickers hardness (weight: 98 N) was measured from the position of 1 mm from the steel plate surface to the position of 1 mm from the back of the steel plate at 1 mm intervals in the thickness direction.
[Tensive properties]
Round bar tensile test pieces (parallel part diameter 6.0 mm × distance between marks 25 mm) are collected in the direction of 90 ° (direction C) with respect to the rolling direction, and the crosshead speed 10 mm / in accordance with JIS Z 2241. A tensile test was performed at min to measure tensile strength (TS).

  FIG. 4 shows the thickness direction distribution of hardness of the as-quenched steel plate and the steel plate after tempering under the conditions of the present invention (set temperature: 700 ° C., offset temperature: 680 ° C.). In the drawing, as a comparison, the distribution of hardness in the thickness direction of the steel plate tempered by the conventional method (setting temperature: 680 ° C.) is also shown. Compared with the conventional method, the hardness in the vicinity of the surface is reduced by about 30 HV in the tempering according to the conditions of the present invention, and it is understood that the target HBW 225 or less (Vickers hardness conversion 237 HV or less) is satisfied.

  Table 2 shows predicted hardness values and actual hardness values by calculation after tempering (set temperature: 700 ° C., offset temperature: 680 ° C.) of the conditions of the present example.

The calculated values (forecasts) and the actual values agree well. According to the method of the present invention, it was possible to produce a steel plate compatible with the surface layer hardness HBW of 225 or less and the tensile strength of 1⁄4 t of 550 MPa or more, which were impossible by tempering by the conventional method.
Moreover, although 25 sheets of steel plates were tempered under the above-described tempering conditions, all of the 25 sheets satisfied the above-described conditions for the surface layer hardness and the tensile strength of 1/4 t. It is clear that the method of the present invention stably provides the desired properties.

Claims (4)

Heat conduction in heat treatment where the steel plate is heated in the heating furnace and the temperature of the surface layer of the steel plate reaches the set temperature of the heating furnace while heating up and the surface layer of the steel plate is extracted from the heating furnace at a higher temperature than the inside of the steel plate The time change of the temperature of each part in the thickness direction of the steel plate is calculated by calculation, and the time change of this temperature is divided into time intervals, and the temperature T _i (° C.) in that time interval and the length t _{i of the} time interval h) calculate the evolutionary tempering parameter λ according to the following equations 1 and 2 and predict the hardness of each part in the thickness direction of the steel plate from the relationship between the evolutionary tempering parameter λ and the hardness obtained by experiment , A heat treatment method of a tempered high tensile steel sheet, wherein a heat treatment condition of the heating furnace is set based on the prediction result, and a hardness distribution in a thickness direction of the steel sheet is made as desired.
However, the hardness of the surface layer in the hardness distribution in the thickness direction of the steel plate is higher than the hardness at the half thickness position.
λ = log (10 ^{λ 1} +10 ^λ 2 +...)...
λ _i = log (t _i ) −A / T _i +50...
However, A is a constant determined by the composition of steel.   The heat treatment method according to claim 1, wherein the heat treatment conditions are a set temperature of a heating furnace and an extraction temperature of a steel plate. The extraction temperature of the steel plate is a temperature at a thickness 1/2 position of the steel plate when extracting the steel plate,
The heat treatment method of the heat treated high tensile steel sheet according to claim 2, wherein the set temperature of the heating furnace is set higher than the extraction temperature of the steel sheet.   The heat treated steel plate has a surface hardness of HBW 225 or less and a tensile strength at a quarter position of the thickness of the steel plate of 550 MPa or more. The heat treatment of the tempered high tensile steel plate according to any one of claims 1 to 3. Method.

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