METHOD FOR PRODUCING HOT-ROLLED STEEL SHEET, METHOD FOR PREDICTING TEMPERATURE HISTORY OF HOT-ROLLED STEEL SHEET, AND METHOD FOR PREDICTING HARDENED PORTION OF HOT-ROLLED STEEL SHEET

Abstract

A method for producing a hot-rolled steel sheet which enables predicting a temperature history of unevenness of the end face of a coil includes: measuring a surface temperature of hot-rolled strip-shaped steel; calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step; actually coiling the strip-shaped steel after the measurement step; scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

Description

TECHNICAL FIELD

The present invention relates to a method for producing a bot-rolled steel sheet, a method for predicting a temperature history of a hot-rolled steel sheet, and a method for predicting a hardened portion of a hot-rolled steel sheet.

BACKGROUND ART

A hot-rolled steel sheet is produced by: coiling strip-shaped steel being hot-rolled: and cooling a thus obtained coil to around normal temperature. This hot-rolled steel sheet is uncoiled, pickled, and subjected to cold rolling, thereby giving a cold-rolled steel sheet. A problem in producing the cold-rolled steel sheet involves, rupture of the steel in processing. When rupture of the steel occurs, stopping the processing line to conduct a recovery operation is required, leading to an increase in cost for the recovery, as well as impairment of production efficiency. In addition, the rupture of the steel can be a cause of facility failure.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2014-593

Patent Document 2: Japanese Unexamined Patent Application. Publication No. 2010-112958

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

As one cause of rupture during cold rolling of steel having superior quenching performance, a crack at the end of the steel is involved. Generation of a crack at the end of the steel during cold rolling is likely to result in rupture of the steel, due to stress being converged to this crack portion during processing, followed by growing of the crack.

As a cause of the crack, unfavorable coiling shape of a hot-rolled coil is involved. More specifically, when a coil after hot rolling is accompanied by unfavorable coiling, unevenness of the end face of the coil increases. When there is a large protruding part on the coil end face, the protruding part plays a role as a fin, whereby a cooling speed of the protruding part is accelerated during cooling of the coil. Acceleration of the cooling speed of the protruding part is likely to bring about inclusion of hard phases such as bainite and martensite in the protruding part. As a result, a void is generated during cold rolling of the steel, and due to growth of this void, the crack at the end is likely to be generated.

In light of such a viewpoint, the present inventors found that predicting a temperature history of unevenness of the end face of the coil (particularly, a protruding part) enables the possibility of rupture of the steel in the following step to be sensed beforehand.

It is to be noted that Patent Document 1 discloses prediction of uneven temperature on Run-Out-Table (ROT) of a hot strip, and control of conditions for producing a rolled steel sheet before being coiled on a coiler, such that thus predicted uneven temperature is decreased.

Furthermore, Patent Document 2 discloses, in calculation of a telescopic amount of a coil end face by scanning the end face of a metal plate coil over a coil diameter with a distance meter to measure a distance between the distance meter and the coil end face, a technique of distinguishing a measurement value at the coil end portion as to whether being a tongue shape part or in a telescope shape. Patent Document 2 discloses that a coil in which: the difference between measured distances of innermost turn metal plates at both ends on the coil internal diameter; or the difference between measured distances of outermost turn metal plates at both ends on the coil external diameter is each exceeding a threshold value thereof, is considered as suggesting measurement of a tongue shape part of the coil end of the metal plate, and a telescopic amount of the coil is calculated through excluding innermost turn and/or outermost turn distance data/datum of the measurement of this tongue shape part.

However, in Patent Documents 1 and 2, any study on a relationship between unevenness of the coil end face and a crack at the coil end is not made.

The present invention was made in view of the foregoing circumstances, and an object of the present invention is to provide a method for producing a hot-rolled steel sheet which enables predicting a temperature history of unevenness of the end face of a coil, and a method for predicting a temperature history of a hot-rolled steel sheet. A further object of the present invention is to provide a method for predicting a hardened portion of a hot-rolled steel sheet which enables predicting the presence or absence of rupture of the steel in the following step, based on the temperature history of the unevenness of the end face of a coil.

Means for Solving the Problems

The method for producing a hot-rolled steel sheet according to one embodiment of the present invention includes: a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel; a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step; a coiling step of actually coiling the strip-shaped steel after the measurement step; a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

The method for producing a hot-rolled steel sheet enables predicting a temperature history of unevenness of the end face in a natural cooling state, of a coil formed by coiling the strip-shaped steel.

It is preferred that the method for producing a hot-rolled steel sheet further includes: a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step. When the method for producing a hot-rolled steel sheet includes the second calculation step and the second prediction step, predicting the presence or absence of rupture of the steel in the following step is enabled.

In the derivation step, it is preferred that a size of the unevenness is determined by using as a criterion, a median value of measurement values by the displacement meter. When a size of the unevenness is thus determined by using as a criterion, a median value of measurement values by the displacement meter in the derivation step, the temperature history of the unevenness of the end face in a natural cooling state, of the coil is likely to be predicted readily and accurately.

In the derivation step, it is preferred that a size of the unevenness is determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter. When a size of the unevenness is thus determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter in the derivation step, the temperature history of the unevenness of the end face in a natural cooling state, of the coil is likely to be predicted readily and accurately.

The method for predicting a temperature history of a hot-rolled steel sheet according to other one embodiment of the present invention includes: a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel; a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step; a coiling step of actually coiling the strip-shaped steel after the measurement step; a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

The method for predicting a temperature history of a hot-rolled steel sheet enables predicting a temperature history of unevenness of the end face in a natural cooling state, of a coil formed by coiling the strip-shaped steel.

The method for predicting a hardened portion of a hot-rolled steel sheet according to one other embodiment of the present invention includes: a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel; a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step; a coiling step of actually coiling the strip-shaped steel after the measurement step; a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step; a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step.

The method for predicting a hardened portion of a hot-rolled steel sheet enables predicting the presence or absence of rupture of the steel in the following step, based on the temperature history of the unevenness of the end face of a coil.

EFFECTS OF THE INVENTION

As explained in the foregoing, the method for producing a hot-rolled steel sheet according to the one embodiment of the present invention, and the method for predicting a temperature history of a hot-rolled steel sheet according to the other one embodiment of the present invention enable prediction of a temperature history of unevenness of the end face of a coil. In addition, the method for predicting a hardened portion of a hot-rolled steel sheet according to the one other embodiment of the present invention enables predicting the presence or absence of rupture of the steel in the following step, based on the temperature history of the unevenness of the end face of a coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method for producing a hot-rolled steel sheet according to one embodiment of the present invention;

FIG. 2 is a schematic view illustrating a facility for producing a hot-rolled steel sheet used in the method for producing a hot-rolled steel sheet shown in FIG. 1;

FIG. 3 is an explanatory view of a procedure for calculating the temperature history of a virtual coil, by the first calculation step of the method for producing a hot-rolled steel sheet shown in FIG. 1;

FIG. 4 is a schematic view illustrating a position of scanning of the end face of the coil with a displacement meter, in the derivation step of the method for producing a hot-rolled steel sheet shown in FIG. 1;

FIG. 5 is an explanatory view of a procedure for derivation of the size of the unevenness by the derivation step of the method for producing a hot-rolled steel sheet shown in FIG. 1;

FIG. 6 is a flow chart illustrating a method for producing a hot-rolled steel sheet according to an embodiment differing from the embodiment of the method for producing a hot-rolled steel sheet shown in FIG. 1;

FIG. 7 is a graph showing a temperature history in a natural cooling state. starting from immediately after coiling, on a top end face of the virtual coil by the first calculation step of No. 1;

FIG. 8 is a graph showing: a measurement result of a shape of the unevenness of the end face of the coil by a displacement meter in the derivation step of No. 1; and a derivation result of the size of the unevenness of the end face of the coil by the derivation unit;

FIG. 9 is a graph showing: a temperature history in a natural cooling state, starting from immediately after coiling, on an end face of the coil predicted in the first prediction step of No. 1: and an actually measured value of the temperature at the end face;

FIG. 10 is a graph showing a temperature history in a natural cooling state, starting from immediately after coiling, on a top end face of the virtual coil calculated in the first calculation step of No. 2;

FIG. 11 is a graph showing: a measurement result of a shape of the unevenness of the end face of the coil by a displacement meter in the derivation step of No. 2; and a derivation result of the size of the unevenness of the end face of the coil by the derivation unit; and

FIG. 12 is a graph showing a temperature history in a natural cooling state, starting from immediately after coiling, on an end face of the coil predicted in the first prediction step of No. 2; and an actually measured value of the temperature at the end face.

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described in detail below with appropriate reference to the drawings.

First Embodiment

Method for Producing Hot-Rolled Steel Sheet

The method for producing a hot-rolled steel sheet shown in FIG. 1 includes: a step (measurement step S1) of measuring a surface temperature of hot-rolled strip-shaped steel; a step (first calculation step S2) of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face. based on the surface temperature measured in the measurement step S1; a step (coiling step S3) of actually coiling the strip-shaped steel after the measurement step S1; a step (derivation step S4) of scanning with a displacement meter, an end face of a coil formed in the coiling step S3, and deriving a size of the unevenness of the end face over a radius of the coil; and a step (first prediction step S5) of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step S2; and the size of the unevenness derived in the derivation step S4. The measurement step S1, the first calculation step S2, the coiling step S3, the derivation step S4, and the first prediction step S5 constitutes the method for predicting a temperature history of a hot-rolled steel sheet according to one embodiment of the present invention. It is to be noted that “coil (coiled/coiling)” as referred to herein means to be spirally wound in an axial directional view. The “end face of a coil” as referred to herein means a perpendicular face with respect to the center axis in the coil. In other words, the “end face of a coil” as referred to herein means a face formed with ends in a width direction of strip-shaped steel.

According to the method for producing a hot-rolled steel sheet, by predicting a temperature history of the unevenness of the end face of the coil, possibility of rupture of the steel in producing a cold-rolled sheet can be sensed beforehand by using this hot-rolled steel sheet.

Upon describing the method for producing a hot-rolled steel sheet, with reference to FIG. 2 first, a facility for producing a hot-rolled steel sheet 1 (hereinafter, may be also merely referred to as “production facility 1”) which enables the “method for producing a hot-rolled steel sheet” to put into practice is described.

Facility for Producing Hot-Rolled Steel Sheet

The production facility 1 shown in FIG. 2 includes: a hot strip mill 2 having multiple pairs of rolling rolls 2a, roller tables 2b which convey hot-rolled strip-shaped steel X with the rolling rolls 2a, and a down coiler 2c which coils the strip-shaped steel X conveyed to the roller tables 2b, the hot strip mill 2 constituting a hot rolling line; a measurement apparatus 3 which measures a surface temperature of the strip-shaped steel X having been conveyed to the roller table 2b; a calculation apparatus 4 which calculates a temperature history in a natural cooling state of a coil (virtual coil), assuming that the strip-shaped steel X has been coiled without unevenness on an end face, based on the surface temperature of the strip-shaped steel X measured by the measurement apparatus 3; a derivation apparatus 5 constituting a derivation line which derives the size of the unevenness of the end face E of the coil X1 formed by the down coiler 2c over a radius of the coil X1; and a prediction apparatus 6 which predicts a temperature history of the unevenness in a natural cooling state using: the temperature history calculated by the calculation apparatus 4; and the size of the unevenness derived by the derivation apparatus 5. In addition, the production facility 1 includes a natural cooling apparatus 7 which executes natural cooling of the coil X1 after passing through the derivation line. It is to be noted that the production facility 1 may further include: a cold strip mill which executes cold rolling of the coil X1 after the natural cooling by the natural cooling apparatus 7; an annealing apparatus which anneals the strip-shaped steel after the cold rolling by the cold strip mill; and/or the like.

The hot strip mill 2 subjects a thick steel sheet heated by a heating furnace (not shown in the drawing) to coarse rolling and finish rolling, and thereafter, the strip-shaped steel X after rolling is conveyed by the roller table 2b to the down coiler 2c and then coiled by the down coiler 2c. The roller table 2b has, for example, a plurality of convey rollers.

The measurement apparatus 3 has a non-contact temperature sensor 3a such as a thermography. The measurement apparatus 3 measures the surface temperature of the strip-shaped steel X, after being hot rolled and before being coiled by the down coiler 2c. The measurement apparatus 3 measures the temperature of the entire area (the full length and the overall width) of the surface of the strip-shaped steel X.

The calculation apparatus 4 is, for example, constructed from a computer. For example, assuming that a coil (virtual coil) without unevenness on the end face formed by coiling the strip-shaped steel X has a cylindrical shape. the calculation apparatus 4 calculates a temperature history of this virtual coil in a natural cooling state, by means of a two dimensional model of a polar coordinate system.

The derivation apparatus 5 has: a conveyer 5a which conveys the coil X1 formed by the down coiler 2c; a displacement meter 5b with which the end face E of the coil X1 conveyed on the conveyer 5a is scanned, and which measures the shape of the end face E thereof; and a derivation unit 5c which derives the size of the unevenness of the end face E of the coil X1, based on the shape measured with the displacement meter 5b. The displacement meter 5b measures the shape of the end face E of the coil X1 over a radius of the coil X1, preferably measures over the diameter of the same. As the displacement meter 5b, for example, a laser displacement meter is used. The displacement meter 5b has: a laser irradiation unit which irradiates the end face E of the coil X1 with a laser beam; and a light receiving element which receives a part of rays of light reflected on the end face E. The displacement meter 5b reads out the reflected light of the laser beam with which the end face E is irradiated from the laser irradiation unit, by the light receiving element. The displacement meter 5b is configured to enable the measurement of the shape of the end face E of the coil X1 by a triangulation method. The derivation unit 5c is constructed from, for example, a computer. The displacement meter 5b and the derivation unit 5c may be integrally constructed.

The prediction apparatus 6 is constructed from, for example, a computer. The prediction apparatus 6 predicts the temperature history of the unevenness of the end face E of the coil X1, in a case of assuming that the coil X1 actually formed by the down coiler 2c is naturally cooled with the natural cooling apparatus 7 or the like.

The natural cooling apparatus 7 executes natural cooling of the coil X1 after measurement of the shape of the end face E with the derivation apparatus 5. In the production facility 1, the coil X1 after coiling with the down coiler 2c is heated to about 500° C. or higher. The natural cooling apparatus 7 air-cools this heated coil X1 to a normal temperature. In the production facility 1, when a large protruding part is present on the end face E of the coil X1, a cooling speed of this protruding part is likely to be greater than other portions, due to the natural cooling of the coil X1 formed by the down coiler 2c.

Strip-Shaped Steel

The strip-shaped steel X is formed by heating and then hot-rolling a slab. The strip-shaped steel X has a composition of, for example, carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum, and copper, as well as a remainder being iron and inevitable impurities. In a case in which the strip-shaped steel X is subjected to cold rolling, the coiling temperature in the coiling step S3 can be equal to or higher than Ms (martensite transformation start temperature) of the strip-shaped steel X.

The upper limit of a carbon equivalent Ceq of the strip-shaped steel X represented by the following equation (1) is preferably 0.75%, and more preferably 0.70%. When the carbon equivalent Ceq of the strip-shaped steel X is greater than the upper limit, a phase of martensite is more likely to be produced in a case of the cooling speed in natural cooling being so high. On the other hand, the lower limit of the carbon equivalent Ceq is not particularly limited, and may be, for example, 0.55%. When the carbon equivalent Ceq is less than the lower limit, transformation is completed until around the coiling step S3, whereby the phase of martensite may be hardly produced, and thus edge break is less likely to occur in the following step coil X1. Therefore, the method for producing a hot-rolled steel sheet is suitably used, in the case of the carbon equivalent Ceq of the strip-shaped steel X is equal to or greater than the lower limit.

Ceq[%]=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14   (1)

wherein, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] each represent a content (% by mass) of C, Si, Mn, Ni, Cr, Mo, and V.

Measurement Step

The measurement step S1 is carried out by the measurement apparatus 3. In the measurement step S1, the surface temperature of the strip-shaped steel X after hot rolling and before coiling with the down coiler 2c is measured over the entire area (the full length and the overall width) of the surface of the strip-shaped steel X.

First Calculation Step

The first calculation step S2 is carried out by the calculation apparatus 4. In the first calculation step S2, for example, assuming that a coil (virtual coil) without unevenness on the end face formed by coiling the strip-shaped steel X has a cylindrical shape, the calculation of a temperature history of this virtual coil in a natural cooling state is executed, by means of a two dimensional model of a polar coordinate system. The first calculation step S2 may be carried out cither before the coiling step S3, or after the coiling step S3. Alternatively, the first calculation step S2 can be carried out after the derivation step S4.

With reference to FIG. 3, one example of calculation procedures of the temperature history of the virtual coil X2 in the natural cooling state in the first calculation step S2 is described. In the first calculation step S2, the temperature history of the virtual coil X2 in the natural cooling state is calculated by using a two dimensional model of a polar coordinate system in which: the origin O (0, 0) is defined with the coordinate of an intersection of a center axis of the virtual coil X2 with a virtual plane including one end face (top end face in FIG. 3) of the virtual coil X2; and the coordinate in the center axis direction with reference to the origin O is defined as z [m], and the coordinate in the radial direction with reference to the origin O is defined as r [m]. In the first calculation step S2, a plurality of calculation points are provided in each of the center axis direction and the radius direction of the virtual coil X2, and calculation of the temperature history is executed on each calculation point in a natural cooling state. Specifically, in the first calculation step S2, the temperature history of the virtual coil X2 in the natural cooling state is calculated by using: the following equation (2) for internal calculation points (calculation points on the part not exposed to the outside air); or the following equation (3) for calculation points on the part exposed to the outside air of the virtual coil X2, the temperature after “t” hours at the calculation point, starting from immediately after coiling, of the virtual coil X2 is defined as Φ [° C.].

$\begin{array}{cc}\rho \frac{\partial H}{\partial t}={\lambda }_r\left(\frac{{\partial }^2\varphi }{\partial r^2}+\frac{1}{r}\frac{\partial \varphi }{\partial r}\right)+{\lambda }_z\frac{{\partial }^2\varphi }{\partial z^2}& \left(2\right)\end{array}$ $\begin{array}{cc}\rho \frac{\partial H}{\partial t}={\lambda }_r\left(\frac{{\partial }^2\varphi }{\partial r^2}+\frac{1}{r}\frac{\partial \varphi }{\partial r}\right)+{\lambda }_z\frac{{\partial }^2\varphi }{\partial z^2}+\frac{\mathrm{qA}}{V}& \left(3\right)\end{array}$

It is to be noted that in the above equation (2) and the above equation (3), the following meanings are involved:

H: enthalpy [kcal/kg]: ρ: density [kg/m³] of the part corresponding to the calculation point of the strip-shaped steel; λ_r: thermal conductivity in the radial direction [kcal/m/hr/° C.]; λ_z: thermal conductivity in the axial direction [kcal/m/hr/° C]: ε: radiation rate [−]: σ: Stefan-Boltzmann constant [kcal/m²/hr/° C.⁴]: F₁₂: view factor [−] α: natural convection heat transfer coefficient [kcal/hr/m²/° C.] V: volume [m³] of the part corresponding to the calculation point of the strip-shaped steel; and A: surface area [m²] of the part corresponding to the calculation point of the strip-shaped steel.
In addition, in the above equation (3), q represents a boundary condition. This boundary condition is given by the following equation (4) for the inner circumference face of the virtual coil X2, or by the following equation (5) for the end face or the outer circumference face of the virtual coil X2. In the following equation (4) and the following equation (5), the following meanings are involved:
T: surface temperature [° C.] of the part corresponding to the calculation point, measured in the measurement step S1; and T_f: atmospheric temperature [° C.] in natural cooling.

q=εσF₁₂(T⁴−T_ƒ⁴)+α(T−T_ƒ)^1.25   (4)

q=εσ(T⁴−T_ƒ⁴)+α(T−T_ƒ)^1.25   (5)

Coiling Step

In the coiling step S3, the strip-shaped steel X after the measurement of the surface temperature in the measurement step S1 is coiled by the down coiler 2c at a high temperature. The coiling temperature in the coiling step S3 is preferably equal to or higher than the Ms temperature of the strip-shaped steel X, in light of preventing production of a martensite phase. The lower limit of the coiling temperature is preferably 400° C., more preferably 500° C. and still more preferably 560° C. On the other hand, the upper limit of the coiling temperature is preferably 700° C. and more preferably 670° C. When the coiling temperature is below the lower limit, strength of the strip-shaped steel X may be so great that load to the apparatus in the following step(s) such as a cold rolling step may increase. To the contrary, when the coiling temperature is above the upper limit, a scale thickness of the surface of the strip-shaped steel X may be so great. It is to be noted that the “coiling temperature” as referred to herein means the surface temperature of the strip-shaped steel X immediately before coiling.

Derivation Step

The derivation step S4 is carried out by the derivation apparatus 5. As shown in FIG. 2 and FIG. 4, in the derivation step S4, the end face E of the coil X1 conveyed on the conveyer 5a is scanned with the displacement meter 5b, and the shape of the unevenness of the end face E is measured over a radius of the coil X1, and preferably over the diameter thereof. Furthermore, in the derivation step S4, the size of the unevenness of the end face E is determined with the derivation unit 5c, by using a two dimensional coordinate system defined by: a protruding direction (center axis direction of the coil X1) of the unevenness; and a direction of scanning (radius direction of the coil X1) with the displacement meter 5b. Due to the method for producing a hot-rolled steel sheet in which the size of the unevenness of the end face E is determined by using the two dimensional coordinate system, the temperature history of the unevenness of the end face E of the coil X1 in a natural cooling state is likely to be predicted readily and accurately by the first prediction step S5 described later.

In the derivation step S4, it is preferred that the size of the unevenness of the end face E is determined by using as a criterion, a median value of measurement values by the displacement meter 5b. Specifically, in the derivation step S4, it is preferred that after the end face E of the coil X1 is continuously measured over the radius with the displacement meter 5b, the size of the unevenness of the end face E is determined with the derivation unit 5c by using, as a criterion, the median value of the measurement values by the displacement meter 5b. According to the method for producing a hot-rolled steel sheet of the present embodiment, the size of the unevenness of the end face E is determined by using, as a criterion, the median value, whereby even in a case in which a large projecting portion (telescope) resulting from coiling dislocation at the end(s) of the outer circumference side and/or the inner circumference side of the coil X1 has been formed, the size of the unevenness the coil X1 as a whole can be appropriately measured. As a result, the temperature history of the unevenness of the end face E of the coil X1 in a natural cooling state is likely to be predicted readily and accurately by the first prediction step S5.

With reference to FIG. 4 and FIG. 5, one example of derivation procedures of the size of the unevenness of the end face E of the coil X1 in the derivation step S4 is described. First, in the derivation step S4, the end face E of the coil X1 conveyed on the conveyer 5a is scanned with the displacement meter 5b, and the shape of the unevenness of the end face E is measured over a radius of the coil X1. Next, based on a median value of measurement values by the displacement meter 5b, a reference plane R of the end face E is specified. Subsequently, the size of the unevenness of the end face E is determined by using a two dimensional model of a polar coordinate system in which: the origin O (0, 0) is defined with the coordinate of an intersection of the reference plane R with the center axis Z of the coil X1; and the coordinate in the center axis direction with reference to the origin O is defined as z [m], and the coordinate in the radial direction with reference to the origin O is defined as r [m]. Specifically, the coil X1 is divided into a plurality of areas in the radial direction, and then the size of the unevenness is averaged for each area and this average value is fitted to the two dimensional coordinate system to achieve derivation as the size of the unevenness in this area. In this procedure, it is possible not to adjust each area to have the same length in the radial direction of the coil X1. For example, so as to be likely to reflect the unevenness resulting from the telescope, the lengths of a pair of areas positioning at both ends in the radial direction may be set to be smaller than those of the other areas. Further, it is also possible to provide a certain threshold value, and protruding quantities being equal to or less than this threshold value are dealt with as not corresponding to those being unevenness.

First Prediction Step

The first prediction step S5 is carried out by the prediction apparatus 6. First, in the prediction step S5, the temperature history of the unevenness of the end face E of the coil X1 in a natural cooling state, starting from immediately after coiling, is predicted. In the first prediction step S5, the above equations (2) to (5) are used to predict the temperature history of the unevenness of the end face E in a natural cooling state. In this procedure, with respect to the reference plane R, the boundary condition of the above equation (3) is given by the above equation (5).

Advantages

According to the method for producing a hot-rolled steel sheet, the temperature history of the unevenness of the end face E of the coil X1 in a natural cooling state, being formed by coiling the strip-shaped steel X can be predicted. Therefore, the method for producing a hot-rolled steel sheet enables, in producing the cold-rolled sheet by using the strip-shaped steel X, the possibility of rupture of the steel to be sensed beforehand.

According to the method for predicting a temperature history of a hot-rolled steel sheet, the temperature history of the unevenness of the end face E of the coil X1 in a natural cooling state, being formed by coiling the strip-shaped steel X can be predicted. Therefore, the method for predicting a temperature history of a hot-rolled steel sheet enables, in producing the cold-rolled sheet by using the strip-shaped steel X, the possibility of rupture of the steel to be sensed beforehand.

Second Embodiment

Method for Producing Hot-Rolled Steel Sheet

The method for producing a hot-rolled steel sheet shown in FIG. 6 includes: a step (measurement step S11) of measuring a surface temperature of hot-rolled strip-shaped steel; a step (first calculation step S12) of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step S11; a step (coiling step S13) of actually coiling the strip-shaped steel after the measurement step S11; a step (derivation step S14) of scanning with a displacement meter, an end face of a coil formed in the coiling step S13, and deriving a size of the unevenness of the end face over a radius of the coil; a step (first prediction step S15) of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step S12; the size of the unevenness derived in the derivation step S14; a step (second calculation step S16) of calculating a phase transformation rate (ferrite-pearlite transformation rate) using the temperature history predicted in the first prediction step S15; and a step (second prediction step S17) of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step S16. The measurement step S11, the first calculation step S12, the coiling step S13, the derivation step S14, the first prediction step S15, the second calculation step S16, and the second prediction step S17 constitutes the method for predicting a hardened portion of a hot-rolled steel sheet according to one embodiment of the present invention. Regarding the measurement step S11, the first calculation step S12, the coiling step S13, and the derivation step S14, description is omitted since these can be carried out by procedures similar to those of the measurement step S1, the first calculation step S2, the coiling step S3, and the derivation step S4 shown in FIG. 1. It is to be noted that in the first calculation step S12, similarly to the first prediction step S15 described later, the temperature history may be calculated with the following equation (6) in place of the above equation (2), and with the following equation (7) in place of the above equation (3).

First Prediction Step

In the first prediction step S15, the temperature history of the strip-shaped steel is determined by adding the amount of transformation beat generation Q_t [kcal/m³/hr] at each time calculation in the second calculation step S16 described later. Specifically, in the first prediction step S15, the temperature history is predicted with the following equation (6) in place of the equation (2), and with the following equation (7) in place of the equation (3). The first prediction step S15 can be carried out by procedures similar to those of the first prediction step S5 shown in FIG. 1, except that the following equation (6) is used in place of the above equation (2), and the following equation (7) is used in place of the above equation (3).

$\begin{array}{cc}\rho \frac{\partial H}{\partial t}={\lambda }_r\left(\frac{{\partial }^2\varphi }{\partial r^2}+\frac{1}{r}\frac{\partial \varphi }{\partial r}\right)+{\lambda }_z\frac{{\partial }^2\varphi }{\partial z^2}+Q_1& \left(6\right)\end{array}$ $\begin{array}{cc}\rho \frac{\partial H}{\partial t}={\lambda }_r\left(\frac{{\partial }^2\varphi }{\partial r^2}+\frac{1}{r}\frac{\partial \varphi }{\partial r}\right)+{\lambda }_z\frac{{\partial }^2\varphi }{\partial z^2}+\frac{\mathrm{qA}}{V}+Q_1& \left(7\right)\end{array}$

Second Calculation Step

The second calculation step S16 can be carried out with, for example, a computer. In the second calculation step 16, the phase transformation rate is calculated from an isothermal transformation formula involving an influence of a γ particle diameter. Furthermore, in the second calculation step S16, amount of transformation heat generation Q_t is calculated to meet the phase transformation rate thus calculated. Specifically, in the second calculation step S16, a phase transformation rate X [−] is calculated with the following equation (8) and the following equation (9), and further, the following equation (10) is used to calculate an amount of transformation beat generation Q_t [kcal/m³/hr] at time “t”. The amount of transformation heat generation Q_t calculated in the second calculation step S16 is used for prediction of the temperature history in the above first prediction step S15. Also, this amount of transformation heat generation Q, may be used for calculation of the temperature history in the above first calculation step S12.

$\begin{array}{cc}X=1-\exp \left\{-S^m·{\left({\int }_0^{\iota }{K}^{\frac{1}{n}}\right)}^n\right\}& \left(8\right)\end{array}$ $\begin{array}{cc}K=\exp \left\{-{\left(\frac{T-b}{a}\right)}^2-c\right\}& \left(9\right)\end{array}$ $\begin{array}{cc}{Q}_{\iota }=Q_{\mathrm{total}}\Delta X& \left(10\right)\end{array}$

It is to be noted that, in the above equations (8) to (10), the following meanings are involved:

S: nucleation area term; K: temperature dependent term: Q_total: total amount of transformation heat generation [kcal/m³/hr]; and T: temperature [° C.] at the calculation point calculated in the first calculation step S12 or the first prediction step S15. Moreover, a, b, c, m, and n in the above equations (8) to (10) mean constants to be adjusted for each type of the steel. These constants can be determined by, for example, using a hot-rolled crop after coarse rolling to produce TTT diagram (Time Temperature Transformation diagram), followed by adjusting such that the calculated value meets the experimental value. Note, however, that the transformation rate is affected by a structural state before the transformation, such as an austenite particle diameter, thereby leading to variation depending on hot rolling conditions. Thus, the value “c” is adjusted according to each hot rolling condition.

Second Prediction Step

The second prediction step S17 can be carried out with, for example, a computer. In the second prediction step S17, the phase transformation rate calculated in the second calculation step S16 is used to predict the hardened portion of the end face of the coil formed in the coiling step S13. In the second prediction step S17, for example, a relationship between the phase transformation rate and the hardness is previously determined, and the hardened portion is predicted from the phase transformation rate having been calculated. In the second prediction step S17, the presence or absence of rupture of the steel in the following step may be predicted by, for example, previously setting a threshold value of the hardness at which rupture of the steel can occur in the following step such as a cold rolling step, and comparing this threshold value with the phase transformation rate calculated. Alternatively, in the second prediction step S17, the presence or absence of rupture of the steel may be predicted by, for example, previously setting a threshold value of the phase transformation rate at which rupture of the steel can occur in the following step such as a cold rolling step, and comparing this threshold value with a calculated value.

Advantages

By using the phase transformation rate calculated in the second calculation step S16, the method for producing a hot-rolled steel sheet enables predicting the presence or absence of rupture of the steel in the following step. According to the method for producing a hot-rolled steel sheet, by cutting beforehand a hardened portion away which can be the cause of rupture in the following step, the risk of rupture of the steel during processing can be reduced.

By using the phase transformation rate calculated in the second calculation step S16, the method for predicting a hardened portion of a hot-rolled steel sheet enables predicting the presence or absence of rupture of the steel in the following step.

Other Embodiments

The embodiments described above do not limit the constitution of the present invention. Therefore, such omission, replacement, or addition may be made on constitutive elements of each part in the above-described embodiments, based on the descriptions of the present specification and the common technical knowledge, and such omission, replacement, and addition should be construed as falling within the scope of the present invention

For example, in the derivation step, it is also possible to determine the size of the unevenness of the end face, based on the average value, a mode value, etc., of the unevenness of the end face of the coil. However, in light of enabling the size of the unevenness of the coil as a whole to be appropriately measured in the derivation step even in a case in which a large projecting portion such as a telescope is present, as described above, it is preferred that the size of unevenness of the end face of the coil is determined by using as a criterion, a median value of the measurement values.

In the derivation step, it is not necessary to determine the size of the unevenness by using the two dimensional coordinate system defined by: a protruding direction of the unevenness of the end face of the coil; and a direction of scanning with the displacement meter. For example, in the derivation step, the measurement result by the displacement meter may be directly derived as the size of unevenness of the end face of the coil.

EXAMPLES

Hereinafter, the present invention will be explained in more detail by way of Examples. but the present invention is not limited to these Examples.

No. 1

The production facility 1 shown in FIG. 2 was used to produce a hot-rolled steel sheet First, a non-contact temperature sensor 3a (Thermography “CPA-SC7100” manufactured by FLIR) was used to measure the surface temperature of the strip-shaped steel X after hot rolling and before coiling with the down coiler 2c, over the entire area (the full length and the overall width) of the surface of the strip-shaped steel X (measurement step). Next, a temperature history in a natural cooling state of a coil (virtual coil), assuming that the strip-shaped steel X has been coiled without unevenness on an end face was calculated by using the above equations (2) to (5) (first calculation step). In this first calculation step, 10 calculation points were provided in the virtual center axis of coil direction and 50 (total: 10×50 points) calculation points were provided in the radius direction, and the temperature history in the natural cooling state was calculated for each calculation point. The temperature history in the natural cooling state, starting from immediately after coiling, on the top end face being 410 mm away in the radius direction from the center axis in the virtual coil is shown in FIG. 7.

Subsequently, the strip-shaped steel X was actually coiled by the down coiler 2c (coiling step). Next, the end face E of the coil X1 thus coiled was scanned over the diameter with a displacement meter 5b (laser displacement meter “IL-2000” manufactured by Keyence Corporation) to measure the shape of the unevenness of the end face E of the coil X1. Furthermore, based on a median value of measurement values by the displacement meter 5b, a reference plane of the end face E was specified, and then the size of the unevenness of the end face E was derived with the derivation unit 5c by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter 5b (derivation step). It is to be noted that the unevenness of the end face E of the coil X1 was represented by: a plus value in a case of protrusion toward the side of the displacement meter 5b; and a minus value in a case of recession toward the side of the conveyer 5a. In this derivation step, the coil X1 was divided into 13 areas in the radial direction, and the size of the unevenness was averaged for each area and this average value was fitted to the two dimensional coordinate system to achieve derivation as the size of the unevenness in this area. In this derivation step, so as to be likely to reflect the unevenness resulting from the telescope, the lengths of a pair of areas positioning at both ends in the radial direction were set to be smaller than the lengths of the other areas. Specifically, the length of each of the areas positioning at both ends in the radial direction was set half the length of each of the other areas. Furthermore, the threshold value of the unevenness was set to 10 mm, and with respect to the average value of the unevenness of each area, the value of less than 10 mm was rounded down. The measurement result of the shape of the unevenness of the end face E of the coil X1 by the displacement meter 5b in the derivation step, and the derivation result of the size of the unevenness of the end face E of the coil X1 with the derivation unit 5c are shown in FIG. 8.

Subsequently, with respect to the uneven portion of the end face E of the coil X1 derived in the derivation step, the temperature history in the natural cooling state, starting from immediately after coiling, was predicted by using the calculation result in the first calculation step, and the above equations (2) to (5) (first prediction step). The prediction result on the end face E of the coil X1 by the first prediction step, at a position 410 mm away in the radius direction, from the center axis is shown in FIG. 9. In addition, FIG. 9 shows actually measured values of the temperatures after 23 min following immediately after coiling of the end face E of the coil X1 corresponding to the position predicted in the first prediction step.

No. 2

By using the production facility 1 similar to that in No. 1. the measurement step, the first calculation step, the coiling step, the derivation step, and the first prediction step were carried out in a similar manner to No. 1. The temperature history in the natural cooling state, starting from immediately after coiling, on the top end face being 410 mm away in the radius direction from the center axis in the virtual coil is shown in FIG. 10. In addition, the measurement result of the shape of the unevenness of the end face E of the coil X1 with the displacement meter 5b in the derivation step, and the derivation result of the size of the unevenness of the end face E of the coil X1 with the derivation unit 5c are shown in FIG. 11. Furthermore, the prediction result on the end face E of the coil X1 by the first prediction step, at a position 410 mm away in the radius direction from the center axis, and actually measured values of the temperatures after 24 min following immediately after coiling of the end face E of the coil X1 corresponding to the position predicted in the first prediction step are shown in FIG. 12.

As shown in FIG. 7 to FIG. 12, both: No. 1 in which the presence of the unevenness was decided; and No. 2 in which the absence of the unevenness was decided each by the derivation step exhibited the prediction result by the first prediction step and the actually measured values each being substantially identical. Accordingly, it is revealed that both No. 1 and No. 2 enabled sufficiently accurate prediction of the temperature history of the unevenness of the end face E of the coil X1.

No. 3

By using the production facility 1 similar to that in No. 1, the measurement step, the first calculation step, the coiling step, the derivation step, and the first prediction step were carried out. Additionally, in No. 3, the phase transformation rate of the end face of the coil was calculated by using the above equation (8) and equation (9), and the amount of transformation heat generation was calculated by using the above equation (10) (second calculation step). In No. 3, the temperature history was predicted by using the above equation (6) and equation (7) in the first calculation step and the first prediction step. Furthermore, in No. 3, Vickers hardness [Hv] at a position (calculation point) where the phase transformation rate was calculated in the second calculation step was actually measured. The phase transformation rate calculated in No. 3, and the Vickers hardness measured in No. 3 are shown in Table 1.

TABLE 1
	Distance from outer	Phase transformation	Vickers
Calculation	circumference of coil	rate	hardness
point	[mm]	[—]	[Hv]
A	8.9	0.81	300.4
B	17.7	0.95	243.7
C	26.6	0.98	245.8

As shown in Table 1, the Vickers hardness of the calculation point A at which the phase transformation rate was small was greater than those of the calculation points B and C at which the phase transformation rate was great. From these, the phase transformation rate is proven to correlate with the hardness of the coil. Therefore, calculation of the phase transformation rate by the second calculation step enables prediction of the hardened portion of the coil. In addition, prediction as to whether or not rupture of the steel will occur in the following step such as a cold rolling step is enabled by: previous setting of the threshold value of the hardness or of the phase transformation rate at which rupture of the steel can occur in the following step; and comparison of this threshold value with the phase transformation rate thus calculated.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the method for producing a hot-rolled steel sheet according to one embodiment of the present invention is suited for sensing a possibility of rupture of the steel beforehand, in production or the like of a cold-rolled sheet.

Explanation of the Reference Symbols

• • 1 Facility for producing a hot-rolled steel sheet
  • 2 Hot strip mill
  • 2a Rolling roll
  • 2b Roller table
  • 2c Down coiler
  • 3 Measurement apparatus
  • 3a Non-contact temperature sensor
  • 4 Calculation apparatus
  • 5 Derivation apparatus
  • 5a Conveyer
  • 5b Displacement meter
  • 5c Derivation unit
  • 6 Prediction apparatus
  • 7 Natural cooling apparatus
  • X Strip-shaped steel
  • X1 Coil
  • X2 Virtual coil
  • E End face
  • O Origin
  • R Reference plane of a coil end face
  • Z Center axis of coil

Claims

1. A method for producing a hot-rolled steel sheet, the method comprising:

a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;

a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;

a coiling step of actually coiling the strip-shaped steel after the measurement step;

a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and

a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

2. The method for producing a hot-rolled steel sheet according to claim 1, further comprising:

a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and

a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step.

3. The method for producing a hot-rolled steel sheet according to claim 1, wherein in the derivation step, a size of the unevenness is determined by using as a criterion, a median value of measurement values by the displacement meter.

4. The method for producing a hot-rolled steel sheet according to claim 1, wherein in the derivation step, a size of the unevenness is determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter.

5. A method for predicting a temperature history of a hot-rolled steel sheet, the method comprising:

a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;

a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;

a coiling step of actually coiling the strip-shaped steel after the measurement step;

a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and

a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

6. A method for predicting a hardened portion of a hot-rolled steel sheet, the method comprising:

a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;

a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;

a coiling step of actually coiling the strip-shaped steel after the measurement step;

a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil;

a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and

the size of the unevenness derived in the derivation step;

a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and

a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step.

7. The method for producing a hot-rolled steel sheet according to claim 2, wherein in the derivation step, a size of the unevenness is determined by using as a criterion, a median value of measurement values by the displacement meter.

8. The method for producing a hot-rolled steel sheet according to claim 2, wherein in the derivation step, a size of the unevenness is determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter.