JP4325326B2 - Manufacturing method of high-tensile steel sheet - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet having a tensile strength of 570 MPa or higher, in which an accelerated cooling steel sheet is tempered online by an induction heating device installed on the same line as a rolling mill and an accelerated cooling device. It is about.

  In recent years, with the increase in the size of steel structures, development of stronger steel has been demanded. Normally, steel with a tensile strength of about 570 MPa or more is mainly intended to improve toughness by causing martensite or bainite transformation by quenching and precipitating supersaturated solute carbon and alloy elements as carbides by subsequent tempering treatment. And put to practical use. Such a method of manufacturing a quenched / tempered steel sheet has been problematic in that it takes a long time to manufacture, the productivity is low, and the manufacturing cost is high.

  Therefore, as a quenching method, a direct quenching technique in which quenching is performed as it is after rolling has been developed, which is effective in terms of reducing manufacturing costs. Such a production method is described in, for example, Japanese Patent Publication No. 55-49131 and Japanese Patent Publication No. 58-3011.

  However, with regard to the tempering process of these techniques, a great amount of time is consumed in the heat treatment for heating and holding, and therefore, the heat treatment process must be performed on a production line different from the production line for rolling and direct quenching. There is a problem of consuming time for transporting the steel sheet at the time, and there is room for improvement in terms of productivity and manufacturing cost.

  As a method for solving such a problem, there is a method described in Japanese Patent No. 3015923. This is achieved by using a tempering device installed directly on the same production line as rolling and direct quenching, and heating and tempering the steel sheet more rapidly than in the prior art. This makes it possible to significantly increase the productivity of the steel sheet.

Japanese Patent Publication No.55-49131 Japanese Patent Publication No.58-3011 Japanese Patent No. 3015923

  As disclosed in Patent Document 3, the idea of in-line heat treatment using a rapid heating apparatus has existed in the past, but has not yet been put into practical use. The reason for this was not only hardware problems such as insufficient induction heating capacity, but also soft problems. In order to perform the heat treatment, it is necessary to uniformly heat the steel plate without causing a temperature difference in the longitudinal direction and the thickness direction. For this purpose, it is necessary to accurately estimate the internal temperature of the steel sheet during induction heating, and it is necessary to obtain the power required for heating using this temperature estimation model. Moreover, since the electric power at the time of heating differs depending on the temperature at the time of heating, it is necessary to perform these processes online. Moreover, it is necessary to determine a conveyance speed and the frequency | count of a heating so that rolling efficiency may not be inhibited. Furthermore, it is required to minimize the amount of power supplied from the viewpoint of manufacturing cost.

  However, there are no literatures on how to calculate the power and how to determine the steel plate conveyance speed that give a clear answer to these problems.

  Therefore, the object of the present invention is to control the amount of power supplied to the induction heating device and the steel sheet conveyance speed during online rapid tempering using the induction heating device, thereby achieving a tensile strength of 570 MPa or more which is excellent in strength and toughness. An object of the present invention is to provide a method for producing a high-strength steel plate capable of producing a high-strength steel plate having strength with high efficiency.

  As a result of intensive studies to overcome the above problems, the inventors of the present application have accurately estimated the surface temperature and the internal temperature of the steel sheet during induction heating in order to perform in-line heat treatment using an induction heating device. In addition, it was found that it is important to set the power and the steel plate conveyance speed that satisfy the target value of the heating temperature.

  The present invention has been made on the basis of the above findings, and is characterized by the following.

The invention according to claim 1 is, in mass%, C: 0.06-0.18%, Si: 0.05-0.6%, Mn: 0.5-1.6%, Al: 0.005. -0.1%, N: 0.0005-0.008%, P: 0.03% or less, S: 0.03% or less, the balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., a rolling mill and a direct quenching device In the method of manufacturing a steel sheet that performs tempering using a solenoid induction heating device installed on the same manufacturing line, the data after induction heating is obtained from data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device. Satisfying the calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness, and the temperature condition where the maximum temperature of the surface of the steel plate is in the range of 500 to 720 ° C. and the temperature at the center of the plate thickness is in the range of 500 to 700 ° C. A temperature condition determination step for determining whether or not to perform, and if the temperature condition is not met, the power is corrected and the calculation step and the temperature condition determination step are repeated. And a power supply step for supplying power used for the calculation to the induction heating device when the temperature condition is met.

Invention of Claim 2 is the mass%, C: 0.06-0.18%, Si: 0.05-0.6%, Mn: 0.5-1.6%, Al: 0.005 -0.1%, N: 0.0005-0.008%, P: 0.03% or less, S: 0.03% or less, the balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., a rolling mill and a direct quenching device In the method of manufacturing a steel sheet that performs tempering using a solenoid induction heating device installed on the same manufacturing line, the data after induction heating is obtained from data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device. Satisfying the calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness, and the temperature condition where the maximum temperature of the surface of the steel plate is in the range of 500 to 720 ° C. and the temperature at the center of the plate thickness is in the range of 500 to 700 ° C. A temperature condition determination step for determining whether or not to perform, and if the temperature condition is not met, the power is corrected and the calculation step and the temperature condition determination step are repeated. And when the temperature condition is satisfied, based on the electric power used for the calculation, the total value of the respective electric energy of the induction heating device used for heating the steel sheet is not more than a predetermined value. A power determination step for determining whether the power condition is satisfied, and a power supply step for supplying the power used for the calculation to the induction heating device when the temperature condition is satisfied. It has a special feature.

Invention of Claim 3 is the mass%, C: 0.06-0.18%, Si: 0.05-0.6%, Mn: 0.5-1.6%, Al: 0.005 -0.1%, N: 0.0005-0.008%, P: 0.03% or less, S: 0.03% or less, the balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., a rolling mill and a direct quenching device In the method of manufacturing a steel sheet that performs tempering using a solenoid induction heating device installed on the same manufacturing line, the data after induction heating is obtained from data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device. Satisfying the calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness, and the temperature condition where the maximum temperature of the surface of the steel plate is in the range of 500 to 720 ° C. and the temperature at the center of the plate thickness is in the range of 500 to 700 ° C. A temperature condition determination step for determining whether or not to perform, and if the temperature condition is not met, the power is corrected and the calculation step and the temperature condition determination step are repeated. And when the temperature condition is satisfied, the calculation step, the temperature condition determination step, and the determination processing step are not adapted to the temperature condition by using the new transfer speed obtained by increasing the transfer speed. The extraction step for extracting the power and the conveyance speed used in the final calculation conforming to the temperature condition, and the steel plate was extracted while being conveyed by the conveyance speed extracted by the extraction step. And an extraction power supply step for supplying electric power to the induction heating device.

Invention of Claim 4 is the mass%, C: 0.06-0.18%, Si: 0.05-0.6%, Mn: 0.5-1.6%, Al: 0.005 -0.1%, N: 0.0005-0.008%, P: 0.03% or less, S: 0.03% or less, the balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., a rolling mill and a direct quenching device In the method of manufacturing a steel sheet that performs tempering using a solenoid induction heating device installed on the same manufacturing line, the data after induction heating is obtained from data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device. Satisfying the calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness, and the temperature condition where the maximum temperature of the surface of the steel plate is in the range of 500 to 720 ° C. and the temperature at the center of the plate thickness is in the range of 500 to 700 ° C. A temperature condition determination step for determining whether or not to perform, and if the temperature condition is not met, the power is corrected and the calculation step and the temperature condition determination step are repeated. And when the temperature condition is satisfied, based on the electric power used for the calculation, the total value of the respective electric energy of the induction heating device used for heating the steel sheet is a predetermined value. A power determination step for determining whether or not the following power condition is satisfied; and, if the temperature condition is satisfied, the calculation step, the temperature condition determination step, and a new transfer speed obtained by increasing the transfer speed; The determination processing step is repeatedly executed until the temperature condition is not met, and an extraction step for extracting the power and the conveyance speed used in the final calculation that meets the temperature condition is extracted by the extraction step. An extraction power supply step of supplying the extracted power to the induction heating device while conveying the steel sheet at a conveyance speed. Than is.

Invention of Claim 5 is a manufacturing method of the high-tensile steel plate of Claim 3 or 4, Each step from the said calculation step to the said extraction step until the heating start temperature in the said induction heating apparatus is decided. The power and the conveyance speed are extracted in advance, and when the heating start temperature is determined, based on the amount of change from the heating start temperature predicted in the calculation step to the determined heating start temperature. The previously extracted transport speed is corrected to a newly extracted transport speed, and the power obtained by recalculating the power at the newly extracted transport speed is newly extracted. The extracted power supply step is performed using the newly extracted transport speed and the newly extracted power. The invention according to claim 6 is the invention according to any one of claims 1 to 5 , wherein the tempering waits for another steel plate to be processed next to the steel plate to be tempered in a step before the heat treatment step. The present invention is characterized in that it includes a heating pass number setting step for setting the number of times of heating pass of the steel sheet so that it can be completed within the time that is not required or the waiting time is the shortest.

The invention described in claim 7 is characterized in that, in the invention described in claim 6 , the steel sheet is reciprocated to heat the induction heating device through three or more passes.

The invention according to claim 8 is the invention according to any one of claims 1 to 7 , wherein the mass% is Cu: 1% or less, Ni: 4% or less, Cr: 1% or less, Mo: It is characterized by containing at least one of 1% or less.

The invention according to a ninth aspect is the invention according to any one of the first to eighth aspects, wherein the mass% further includes Nb: 0.05% or less, V: 0.5% or less, Ti: 0.00. It is characterized by containing at least one of 03% or less.

The invention according to claim 10 is the invention according to any one of claims 1 to 9 , wherein at least one of B: 0.003% or less and Ca: 0.01% or less is further contained by mass%. It is characterized by containing.

  According to this invention, the tensile strength of 570 MPa or more excellent in strength and toughness is achieved by controlling the amount of power supplied to the induction heating device and the steel sheet conveyance speed during online rapid tempering using the induction heating device. It is possible to produce a high-tensile steel plate having high efficiency.

  First, the reasons for limitation in the present invention will be described. In addition, all% which shows a chemical component composition ratio is the mass%.

(C: 0.06-0.18%)
C is included to ensure the strength, but if less than 0.06%, the effect is insufficient, while if it exceeds 0.18%, the toughness of the base metal and the weld heat affected zone deteriorates, The weldability is significantly deteriorated. Therefore, the C content is limited to a range of 0.06 to 0.18%.

(Si: 0.05-0.6%)
Si is contained as a deoxidizing material and a strength improving element in the steelmaking stage. However, if it is less than 0.05%, the effect is insufficient. On the other hand, if it exceeds 0.6%, the toughness of the base material and the weld heat affected zone is insufficient. Deteriorates and weldability deteriorates remarkably. Therefore, the Si content is limited to a range of 0.05 to 0.6%.

(Mn: 0.5-1.6%)
Mn is contained in order to ensure the strength, but if it is less than 0.5%, the effect is insufficient. On the other hand, if it exceeds 1.6%, the toughness of the weld heat affected zone deteriorates and weldability is reduced. Deteriorates significantly. Therefore, the Mn content is limited to a range of 0.5 to 1.6%.

(P, S: 0.03% or less)
P and S are both impurity elements, and if it exceeds 0.03%, it becomes impossible to obtain a sound base material and a welded joint. Therefore, the P and S contents are limited to 0.03% or less.

(Al: 0.005 to 0.1%)
Al is added for deoxidation. However, when the content is less than 0.005%, the effect is not sufficient. On the other hand, when the content exceeds 0.1%, surface flaws of the steel sheet are likely to occur. Accordingly, the A1 content is limited to a range of 0.005 to 0.1%.

(N: 0.0005 to 0.008)
N has the effect of forming precipitates with Al, Nb, etc. to refine the structure and improving the toughness of the base metal, and ensuring the strength by precipitation strengthening by forming precipitates with Nb, V, etc. during tempering It is added to have the effect of. However, if the addition is less than 0.0005%, precipitates necessary for refining the structure and securing the strength are not formed, while addition exceeding 0.008% impairs the toughness of the base metal and the welded joint. Therefore, the N content is limited to a range of 0.0005 to 0.008%.

(Cu: 1% or less)
Cu has an effect of improving strength by solid solution strengthening and precipitation strengthening. However, when the Cu content exceeds 1%, the toughness decreases. Therefore, when Cu is added, its content is limited to 1% or less, preferably 0.50% or less.

(Ni: 4% or less)
Ni has an effect of improving toughness and hardenability. However, if the Ni content exceeds 4%, the economy is inferior. Therefore, when Ni is added, its content is limited to 4% or less, preferably 0.50% or less.

(Cr: 1% or less)
Cr has an effect of improving strength and toughness. However, if the Cr content exceeds 1%, the weldability deteriorates. Therefore, when adding Cr, the content is limited to 1% or less, preferably 0.50% or less.

(Mo: 1% or less)
Mo has the effect | action which improves hardenability and intensity | strength. However, if the Mo content exceeds 1%, the economy is inferior. Therefore, when adding Mo, the content is limited to 1% or less, preferably 0.50% or less.

(Nb: 0.05% or less)
Nb is added as a microalloying element to improve the strength. However, if it exceeds 0.05%, the toughness of the weld heat affected zone is deteriorated. Therefore, when adding Nb, the content is limited to 0.05% or less.

(V: 0.5% or less)
V is added as a microalloying element in order to improve the strength. However, if it exceeds 0.5%, the toughness of the weld heat affected zone is significantly deteriorated. Therefore, when V is added, its content is limited to 0.5% or less, preferably 0.1% or less.

(Ti: 0.03% or less)
Ti produces | generates TiN at the time of rolling heating or welding, refines | miniaturizes an austenite grain, and improves the toughness of a base material toughness and a weld heat affected zone. However, if the content exceeds 0.03%, the toughness of the weld heat affected zone is deteriorated. Therefore, when adding Ti, the content is limited to 0.03% or less.

(B: 0.0030% or less)
B has the effect | action which improves hardenability. However, if it exceeds 0.0030%, the toughness is deteriorated. Therefore, when adding B, the content is limited to 0.0030% or less.

(Ca: 0.01% or less)
Ca is an element indispensable for controlling the form of sulfide inclusions. However, addition over 0.01% leads to a reduction in clean purity. Therefore, when adding Ca, the content is limited to 0.01% or less.

(Pcm: 0.30% or less)
Pcm is an index representing the susceptibility to weld cracking. The higher this value, the higher the tensile strength. However, in order to omit the preheating step or reduce the preheating temperature in welding in a normal environment, the Pcm value is 0. Limited to 30% or less. When the tensile strength is 570 MPa class, the Pcm value is desirably 0.25% or less.

  In the production of the steel sheet, a steel melting method such as a converter method and an electric furnace method, and a slab production method such as a continuous casting method and an ingot forming method can be selected as appropriate.

(Heating temperature before hot rolling: 1000-1300 ° C.)
It is necessary to set the heating temperature to 1000 ° C. or higher in order to achieve homogenization of the alloy elements and solid solution of Nb effective for expanding the non-recrystallized region. However, if the heating temperature exceeds 1300 ° C., the toughness of the base material cannot be ensured due to the coarsening of the structure. Therefore, the heating temperature before hot rolling is limited to a range of 1000 to 1300 ° C.

(Hot rolling)
The process of hot rolling the steel of the present invention that has been uniformly heated to a predetermined plate thickness may be performed under normal conditions. The rolling finish temperature must be above the Ar ₃ transformation point. This is because if the rolling is finished at a temperature lower than the Ar ₃ transformation point, quenching becomes incomplete in the subsequent direct quenching, and good base material characteristics cannot be ensured.

(Direct quenching)
After hot rolling is completed, it is necessary to quench the steel sheet having a temperature equal to or higher than the Ar ₃ transformation point by forced cooling in order to ensure the base material strength and base material toughness. The reason for directly quenching until reaching 400 ° C. or less is to complete the transformation from austenite to martensite or bainite and to maintain the strength of the base material.

(Tempering)
Tempering was performed by a solenoid induction heating apparatus installed directly on the same production line as the rolling mill and the direct quenching apparatus. This is because direct connection makes it possible to eliminate additional time required for conveyance between rolling / direct quenching and tempering, and the like, and to significantly improve productivity.

(Maximum temperature reached on steel sheet during tempering)
If the maximum temperature reached on the steel sheet surface during tempering is less than 500 ° C., the heating temperature is too low, and the recovery of toughness deteriorated by direct quenching is insufficient. On the other hand, when the maximum temperature reached on the surface of the steel sheet exceeds 720 ° C., the toughness and strength of the surface layer portion are significantly deteriorated due to reverse transformation and the like. Therefore, the maximum temperature reached on the steel sheet surface during reheating is limited to a range of 500 to 720 ° C.

(Maximum temperature reached at the center of thickness during tempering)
If the maximum temperature reached at the center of the plate thickness during tempering is less than 500 ° C., the heating temperature is too low, and the recovery of toughness deteriorated by direct quenching is insufficient. On the other hand, when the maximum temperature reached in the center of the steel sheet exceeds 700 ° C., the upper limit of the maximum temperature reached on the steel sheet surface is limited as described above. Production efficiency is significantly reduced. Accordingly, the maximum temperature reached at the center of the plate thickness at the time of reheating is limited to a range of 500 to 700 ° C.

  Next, a method for controlling the induction heating device at the time of reheating by the sonolide type induction heating device will be described.

  Heating methods using an induction heating device include surface layer heating and uniform heating. In both cases, the surface and the inside of the steel plate are heated so as to have different target temperatures. The internal temperature includes an average temperature in the thickness direction (average temperature), a temperature at the center of the thickness (center temperature), and the like. In the case of surface heating, heating is performed so that the surface temperature at the end of heating becomes the target temperature. In that case, it heats so that the internal temperature in a heating process may not exceed upper limit temperature.

  In the case of uniform heating as in the present invention, the internal temperature at the end of heating is heated to the target temperature. In that case, heating is performed so that the surface temperature during the heating process does not exceed the upper limit temperature. In order to perform uniform heating, it is necessary to prepare a plurality of induction heating devices and gradually heat them while repeating the heating process and the cooling process.

  Moreover, you may heat by reciprocating the induction heating apparatus group several times. In order to increase the conveyance speed, it is necessary to increase the heating capacity (maximum power) and to prepare a large number of induction heating apparatuses, because this increases the apparatus cost and installation space.

  Furthermore, even under the same manufacturing conditions, the steel plate temperature after accelerated cooling differs depending on the operating conditions.Therefore, rather than preparing power in advance with a table or the like, the steel plate temperature after accelerated cooling is measured, and based on this measured temperature. It is necessary to build a mechanism that requires electricity online.

  In order to accurately perform the heat treatment online with an induction heating apparatus, there are the following problems.

(1) The internal temperature of the steel sheet during induction heating is accurately estimated.
(2) The power and the conveyance speed that satisfy the target and limit of the heating temperature are obtained.

Furthermore, in practical use,
(3) Reduce power consumption as much as possible.
(4) Heat treatment is performed at a conveyance speed that does not impede operation.
(5) The steel plate temperature before heating is measured, and the heating power, the conveyance speed, etc. are determined based on the measured values.

  The present invention uses the following means in order to solve the above problems.

  (1) In order to accurately estimate the internal temperature of the steel sheet during induction heating, a differential equation in the thickness direction is adopted, the permeability and penetration depth are estimated from the steel sheet temperature and power, and the induced current in the thickness direction of the steel sheet. Obtain the distribution and calculate the calorific value.

  (2) In order to obtain the heating power setting, there are a plurality of temperature conditions, a plurality of manipulated variables (power), and the model is nonlinear. As a result, the surface temperature and the internal temperature are not independent variables, but can be regarded as independent to some extent by heating with a plurality of induction heating devices, and it is possible to set targets separately.

  This is expressed as follows.

Variable: Electric power P applied to each induction heating device = [p1,..., Pi,.
Internal temperature condition: | Ti-Tr | <c (c is a constant)
Surface temperature condition: Tu-Ts> 0
Objective function: Sum of power consumption (kWh) Σ (pi * v)
Where Ts: Maximum surface temperature
Tr: Heating target temperature
Tu: Maximum temperature
Ti: Maximum internal temperature
v: Steel plate conveyance speed

  (3) By using the objective function of nonlinear programming as the sum of power consumption, the power that minimizes the power consumption within the temperature condition can be obtained.

  (4) By obtaining the power setting at a certain speed, and repeatedly performing the calculation while changing the transportation speed within a heatable range, a transportation speed that does not hinder the operation is obtained while satisfying the temperature condition.

  (5) In order to obtain the power and conveyance speed online, the power is set in advance in the initial setting to simplify the optimization calculation. Further, by performing correction calculation of the electric power and the conveyance speed based on the actually measured temperature at the end of the acceleration cooling, it is possible to perform control with higher accuracy.

  FIG. 1 is a schematic configuration diagram of a heat treatment apparatus according to the present invention.

  The steel material 1 is heated while moving through a plurality of induction heating devices 2 arranged in series. At the entrance of each induction heating device 2, a temperature detector 3 for detecting the temperature of the steel material 1 is provided. The temperature signal obtained by the temperature detector 3 is input to the control device 4. The control device 4 calculates the power to be supplied to each induction heating device 2 based on the temperature of the steel material 1 or the scheduled heating start temperature and the steel plate conveyance speed, and outputs the value to the power supply device 5. The power supply device 5 controls the output of the induction heating device 2 so that the supplied power becomes a value given from the control device 4.

Next, how to determine the heating power and the conveyance speed will be described.
In the present invention, a mathematical model for estimating the temperature distribution in the thickness direction of the steel material is used. Further, using the mathematical model, power setting calculation and speed setting calculation are performed.

First, the temperature distribution of the steel material by induction heating is obtained as follows.
The current distribution inside the steel material is represented by the penetration depth. The penetration depth is expressed by a frequency and a relative magnetic permeability as shown in Equation (1).

  When the penetration depth is large, the induced current flows to the inside of the steel material, but when the penetration depth is small, the induced current is concentrated on the surface, so that heating is also concentrated on the surface. It will be heated by heat transfer. Therefore, even when the same electric power is applied, the surface heating temperature varies depending on the penetration depth. Therefore, the penetration depth is obtained from the relative permeability, the current distribution inside the steel material is calculated from the penetration depth, and the temperature distribution inside the steel material is obtained from the current distribution.

  In general, the relationship between the distance z from the steel material surface and the induced current I (z) is expressed by Expression (2). α is a constant.

  Therefore, the ratio of the power consumption at the distance z from the steel material surface is expressed by Expression (3).

  That is, Equation (3) can be considered as the power distribution during induction heating.

  Next, the temperature change of the steel material in the heating process using the induction heating apparatus is expressed by a mathematical formula. Equation (4) is obtained from the difference equation of the heat conduction equation.

Q _{1 in the} equation (4) includes heat transfer with the atmosphere, which is a boundary condition, and the amount of heat supplied from the heating device.

By using the equation (4) to (7), the temperature distribution of the steel after heating _{(x 1, j x 2,} j x nb-1, j x nb, j) can be obtained. This flow is shown in FIG. The calculation ends when the steel material passes through the induction heating device.

  Next, FIG. 3 shows a flow of how to obtain the heating power using this temperature model.

First, an appropriate initial value power (u _{k, J} ) is given, and the heating temperature distribution (x _{i, J} ) on the induction heating device outlet side is calculated. And the heating temperature and temperature conditions in each induction heating apparatus are compared, and it is determined whether the temperature conditions are satisfied. If the temperature condition is met, the calculation ends with the heating power as the final heating power. If they do not match, a new induction heating power is applied and the temperature calculation is performed again. The method for giving the new heating power (u _{k, J} ) may be a general method such as linear programming or nonlinear programming. If the temperature condition is feasible, it converges with a finite number of calculations.

Furthermore, the sum of the power consumption in each induction heating apparatus is given as an objective function. After determining whether or not the steel material temperature after heating meets the temperature condition, it is also determined whether or not the power consumption in each induction heating device is minimized. That is, the heating power required by this process minimizes the power consumption in the induction heating device. Also in this case, a method for giving a new heating power (u _{k, J} ) may be a general method such as linear programming or non-linear programming. This flow is shown in FIG.

  To determine the transport speed, the number of passes is determined first, and convergence calculation as shown in the flow of FIG. 5 is performed. Start with an appropriate initial speed and perform power setting calculations. Depending on the speed, heating may not be possible depending on the upper limit of the power capacity and temperature conditions. In that case, the power setting calculation is performed at a reduced speed. If heating is possible, increase the transfer speed and calculate the power setting. Find the fastest speed within the heatable range.

  And based on the conveyance speed decided here, heating electric power is calculated | required using the electric power setting calculation of the preceding clause. The steel material is heated using the obtained heating power and conveyance speed.

  The heating power and transfer speed obtained in this way can be calculated in advance and stored in a table, etc., or used online, or calculated online when the accelerated cooling of the steel has been completed and the heating start temperature has been determined. Can also be requested.

  However, if calculated in advance, the temperature at the end of the accelerated cooling may be different from the schedule. In addition, since the convergence calculation is repeatedly performed using a complicated model, the calculation amount is enormous, and the calculation may not be in time on-line. In such a case, the following correction method is effective. In this method, the heating power and the conveyance speed are calculated in advance, the conveyance speed is corrected with the actual temperature after the completion of the accelerated cooling, and the power is recalculated. This is shown below.

  First, after calculating in advance the conveyance speed according to the flow of FIG. 5, an influence coefficient of the conveyance speed when the heating start temperature is changed is obtained. The flow of this procedure is shown in FIG. When the heating start temperature is (Ti), the heating start temperature change amount is (ΔTi), and the heating start temperature is (Ti + ΔTi), the coefficient of how much the conveyance speed should be changed is obtained. Starting from the influence coefficient of 1, the influence coefficient is adjusted so that heating is possible and the processing time is the shortest. When this value is (q), the conveyance speed (v ′) when the actual heating start temperature is (Ti + ΔT) is obtained by the following equation (8).

  When the accelerated cooling is finished and the actual temperature is detected, the conveyance speed is corrected. Further, the power calculation shown in FIG. 3 or 4 is performed again at the corrected speed. If only the convergence calculation of the power calculation, it will not take much time. By using the correction method, the most efficient conveyance speed can be obtained, and the heating power can be set with high accuracy.

  Next, an example will be described in which a steel sheet is heated by reciprocating an induction heating device group a plurality of times. In order to perform heat treatment by installing an induction heating device on the line, it is necessary to perform heat treatment with high efficiency so as not to impair rolling efficiency. In other words, it is necessary to complete the heat treatment within a time during which another steel plate to be processed next to the steel plate to be tempered does not have to wait in a step prior to the heat treatment step such as rolling or accelerated cooling. Even if it is necessary to wait, the condition is set so that the waiting time becomes the shortest.

  In order to achieve this, in the present invention, the number of heating passes through which the induction heating device group passes is set. In other words, from the dimensions of the steel material and the required temperature rise, the conveyance speed and power setting in several passes and the heat treatment time are obtained, and as a result, the number of passes with the shortest heat treatment time is selected and heated. To do.

  When heating in one pass or heating in three passes by reciprocating a steel plate, the variables are the conveyance speed and the power in the induction heating device, the constraint conditions are the target temperature of the steel plate surface and the center of the plate thickness, and the objective function Solves an optimization problem consisting of processing time and power consumption. In this case, it can be solved by using an optimization method such as linear programming or nonlinear programming, or by appropriately changing the variables, the processing time becomes the shortest, and the conveyance speed and power at which power consumption is reduced are minimized. It can also be solved by obtaining a combination.

  Next, the present invention will be described in further detail with reference to examples.

  Steels A to D having chemical components shown in Table 1 were melted and cast into slabs, heated to 1150 ° C. in a heating furnace, and then hot-rolled. After rolling, it was directly quenched and then tempered continuously using three induction heating devices arranged in series on the line.

  Table 2 shows the quenching conditions, heating conditions, and characteristics of the steel sheets manufactured under these manufacturing conditions.

  The tensile strength was measured by a full thickness tensile test piece, and the toughness was evaluated by vTrs obtained by a Charpy impact test using a test piece taken from the surface layer portion and the plate thickness center portion. The target value of the material property was set to vTrs: −50 ° C. or less in the surface layer portion and the plate thickness center portion.

  As is apparent from Table 2, Example No. of the present invention. The internal temperature after reheating in 1 to 6 was heated to the target temperature, and the surface temperature was also within the range of 500 to 720 ° C. of the present invention. As a result, the tensile strength, the surface layer portion, and the plate thickness center portion vTrs satisfied the target values. Invention Example No. 1 and No. 2 in comparison with Example No. 2 of the present invention in which the steel material temperature at the start of heating was corrected by actual measurement. In the correction processing method 2, the internal temperature after reheating was closer to the target temperature. Moreover, in any case, it was possible to perform the heat treatment without waiting the steel material to be processed next in a step before the induction heating apparatus.

  In contrast, Comparative Example No. 7-10, the conveyance speed and power setting were inappropriate, the surface temperature after reheating exceeded 720 ° C. (Comparative Examples No. 7 and 10), or the internal temperature did not reach the target temperature ( Comparative examples No. 8 and 9). As a result, any of the tensile strength, the surface layer portion, or the plate thickness center portion vTrs became a value less than the target value.

It is a schematic block diagram of the heat processing apparatus which concerns on this invention. It is a flowchart which shows how to obtain | require temperature distribution. It is a flowchart which shows how to obtain | require electric power. It is a flowchart which shows optimization of power consumption. It is a flowchart which shows speed optimization. It is a flowchart which shows how to obtain | require an influence coefficient.

Explanation of symbols

1: Steel material 2: Induction heating device 3: Temperature detector 4: Control device 5: Power supply device

Claims (10)

% By mass
C: 0.06-0.18%,
Si: 0.05 to 0.6%,
Mn: 0.5 to 1.6%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.008%,
P: 0.03% or less,
S: 0.03% or less,
The balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., and a rolling mill and a direct quenching device In the manufacturing method of a steel sheet that performs tempering using a solenoid type induction heating device installed on the same manufacturing line,
A calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness after induction heating from the data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device,
A temperature condition determination step for determining whether or not a temperature condition in which the maximum temperature of the surface of the steel sheet is in a range of 500 to 720 ° C. and the temperature in the center of the plate thickness is in a range of 500 to 700 ° C .;
If the temperature condition is not met, a determination processing step of correcting the power and repeatedly executing the calculation step and the temperature condition determination step;
A method for producing a high-strength steel sheet, comprising: a power supply step for supplying power used for the calculation to the induction heating device when the temperature condition is met. % By mass
C: 0.06-0.18%,
Si: 0.05 to 0.6%,
Mn: 0.5 to 1.6%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.008%,
P: 0.03% or less,
S: 0.03% or less,
The balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., and a rolling mill and a direct quenching device In the manufacturing method of a steel sheet that performs tempering using a solenoid type induction heating device installed on the same manufacturing line,
A calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness after induction heating from the data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device,
A temperature condition determination step for determining whether or not a temperature condition in which the maximum temperature of the surface of the steel sheet is in a range of 500 to 720 ° C. and the temperature in the center of the plate thickness is in a range of 500 to 700 ° C .;
If the temperature condition is not met, a determination processing step of correcting the power and repeatedly executing the calculation step and the temperature condition determination step;
If the temperature condition is met, the total power amount of the induction heating device used for heating the steel sheet is less than a predetermined value based on the power used for the calculation. A power determination step for determining whether to
A method for producing a high-strength steel sheet, comprising: a power supply step for supplying power used for the calculation to the induction heating device when the temperature condition is met. % By mass
C: 0.06-0.18%,
Si: 0.05 to 0.6%,
Mn: 0.5 to 1.6%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.008%,
P: 0.03% or less,
S: 0.03% or less,
The balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., and a rolling mill and a direct quenching device In the manufacturing method of a steel sheet that performs tempering using a solenoid type induction heating device installed on the same manufacturing line,
A calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness after induction heating from the data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device,
A temperature condition determination step for determining whether or not a temperature condition in which the maximum temperature of the surface of the steel sheet is in a range of 500 to 720 ° C. and the temperature in the center of the plate thickness is in a range of 500 to 700 ° C .;
If the temperature condition is not met, a determination processing step of correcting the power and repeatedly executing the calculation step and the temperature condition determination step;
When the temperature condition is met, the calculation step, the temperature condition determination step, and the determination processing step are repeatedly performed until the temperature condition is not met using a new conveyance speed obtained by increasing the conveyance speed, An extraction step for extracting the power and the conveying speed used in the final calculation meeting the temperature condition;
A method for producing a high-strength steel sheet, comprising: an extraction power supply step for supplying the extracted power to the induction heating device while conveying the steel sheet at the conveyance speed extracted in the extraction step. % By mass
C: 0.06-0.18%,
Si: 0.05 to 0.6%,
Mn: 0.5 to 1.6%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.008%,
P: 0.03% or less,
S: 0.03% or less,
The balance: Fe and inevitable impurities, and
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
A steel having a Pcm value defined by ≦ 0.30% is heated to 1000 to 1300 ° C. and hot-rolled, and then directly quenched from an Ar ₃ point to 400 ° C., and a rolling mill and a direct quenching device In the manufacturing method of a steel sheet that performs tempering using a solenoid type induction heating device installed on the same manufacturing line,
A calculation step for estimating the surface temperature of the steel plate and the temperature at the center of the plate thickness after induction heating from the data including the conveyance speed of the steel plate and the power to be supplied to the induction heating device,
A temperature condition determination step for determining whether or not a temperature condition in which the maximum temperature of the surface of the steel sheet is in a range of 500 to 720 ° C. and the temperature in the center of the plate thickness is in a range of 500 to 700 ° C .;
If the temperature condition is not met, a determination processing step of correcting the power and repeatedly executing the calculation step and the temperature condition determination step;
When the temperature condition is met, based on the power used for the calculation, the power condition is such that the total amount of power of each of the induction heating devices used for heating the steel sheet is equal to or less than a predetermined value. A power determination step for determining whether it conforms; and
When the temperature condition is met, the calculation step, the temperature condition determination step, and the determination processing step are repeatedly performed until the temperature condition is not met using a new conveyance speed obtained by increasing the conveyance speed, An extraction step for extracting the power and the conveying speed used in the final calculation meeting the temperature condition;
A method for producing a high-strength steel sheet, comprising: an extraction power supply step for supplying the extracted electric power to the induction heating device while conveying the steel sheet at the conveyance speed extracted in the extraction step. In the manufacturing method of the high-tensile steel sheet according to claim 3 or 4, before the heating start temperature in the induction heating device is determined, each step from the calculation step to the extraction step is performed, Extracted in advance based on the amount of change from the heating start temperature predicted in the calculation step to the determined heating start temperature when the conveyance speed is extracted and the heating start temperature is determined The transport speed is corrected to the newly extracted transport speed, and the power obtained by recalculating the power at the newly extracted transport speed is used as the newly extracted power. A method for producing a high-strength steel sheet, wherein the extracted power supply step is performed using the extracted conveyance speed and newly extracted power. The heating path of the steel sheet so that tempering is completed within the time when other steel sheets to be processed next to the steel sheet to be tempered do not have to wait in the process before the heat treatment process, or the time in which the standby time is the shortest The method for producing a high-strength steel sheet according to any one of claims 1 to 5 , further comprising a heating pass number setting step for setting the number of times. The method for producing a high-strength steel sheet according to claim 6 , wherein the induction heating apparatus is heated by reciprocating three or more passes by reciprocating the steel sheet. In mass%,
Cu: 1% or less,
Ni: 4% or less,
Cr: 1% or less,
Mo: At least 1% or less is contained, The manufacturing method of the high-tensile steel plate as described in any one of Claim 1 to 7 characterized by the above-mentioned. In mass%,
Nb: 0.05% or less,
V: 0.5% or less,
Ti: 0.03% or less,
The method for producing a high-strength steel sheet according to any one of claims 1 to 8 , wherein the steel sheet contains at least one of the following. In mass%,
B: 0.003% or less,
Ca: 0.01% or less,
The method for producing a high-strength steel sheet according to any one of claims 1 to 9 , wherein the steel sheet contains at least one of the following.

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