METHOD FOR HEATING STEEL PLATE AND METHOD FOR MANUFACTURING HOT-PRESSED PRODUCT

Abstract

A steel plate to be heated is a blank having a first region and a second region adjacent to the first region. The blank is heated by direct resistance heating. A jet of cooling medium is applied to the first region during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature. To provide a clear boundary between the first and second regions, the jet of cooling medium is applied along a slant direction such that the cooling medium expands along the boundary between the first and second regions. Alternatively, a partition member is provided along the boundary between the first and second regions. The heated blank is then press-formed and cooled in a press die to obtain a hot-pressed product.

Description

TECHNICAL FIELD

The present invention relates to a method for heating a steel plate and a method for manufacturing a hot-pressed product.

BACKGROUND ART

Hot-pressed products are used in, for example vehicles such as automobiles, from the viewpoints of increase in strength and weight reduction. Hot-pressed products are obtained by hot-pressing a sheet of steel blank and quenching it by cooling it under a pressed condition together with a pressing die. The blank is heated by, for example, direct resistance heating in which electric current passed through the blank.

Hot-pressed products may be formed to partially have one or more unquenched regions. Unquenched regions are subjected to post-processing such as piercing, trimming, or welding. According to a related art, a jet of cooling gas is applied to a selected region of a blank during the direct resistance heating, so that the temperature of the selected region is kept lower than a quenching temperature (see, e.g., U.S. Pat. No. 6,903,296B2).

In this related art, the jet of cooling gas is applied to both sides of the selected region perpendicularly and at a central part of the selected region. The jet of cooling gas applied to the selected region in this manner is dispersed around the selected region along the front surface and the back surface, suppressing the temperature increase also in the area around the selected region. With the rapid cooling of the blank after the heating, the blank is not quenched in the selected region in which the temperature is kept lower than the quenching temperature, whereas the blank is quenched in the area around the selected region where the temperature is increased to be equal to or higher than the quenching temperature. However, a desired hardness distribution may not be obtained sometimes, due to an expansion of a transition area between the unquenched region and the quenched region resulting from the suppression of the temperature increase in the area around the selected region.

SUMMARY

Illustrative aspects of the present invention provide a method for heating a steel plate with a clear boundary between a region where the temperature is increased to be equal to or higher than a quenching temperature and a region where the temperature is kept lower than the quenching temperature, and also provide a method for manufacturing a hot-pressed product with a clear boundary between a quenched region and an unquenched region.

According to an illustrative aspect of the present invention, a method for heating a steel plate is provided. The steel plate is a blank having a first region and a second region adjacent to the first region. The method includes heating the blank by direct resistance heating, and applying a jet of cooling medium to the first region on at least one of a front surface and a back surface of the blank during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature. The jet of cooling medium is applied along a slant direction that is inclined toward the second region from a direction perpendicular to the at least one of the front surface and the back surface in the first region such that the jet of cooling medium expands along a boundary between the first region and the second region.

According to another illustrative aspect of the present invention, another method for heating the steel plate is provided. The method includes heating the blank by direct resistance heating, and applying a jet of cooling medium to the first region on at least one of a front surface and a back surface of the blank during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature. A partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank.

According to another illustrative aspect of the present invention, a method for manufacturing a hot-pressed product is provided. The method includes heating the blank by one of the methods described above, press-forming the heated blank by a press die, and cooling the blank inside the press die to quench the second region.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of an example of a blank, illustrating an example of its heating pattern.

FIG. 2 illustrates a method for heating the blank with the heating pattern illustrated in FIG. 1.

FIG. 3 illustrates the heating method together with FIG. 2.

FIG. 4 is a graph illustrating an example of temperature variations in first and second regions of the blank when it is heated by the heating method illustrated in FIGS. 2 and 3.

FIG. 5 illustrates a modification of the heating method illustrated in FIGS. 2 and 3.

FIG. 6 is a plan view of another example of a heating pattern of the blank.

FIG. 7 illustrates a method for heating the blank with the heating pattern illustrated in FIG. 6.

FIG. 8 illustrates the heating method together with FIG. 7.

FIG. 9 illustrates an example of a method for manufacturing a hot-pressed product, according to an embodiment of the present invention.

FIG. 10 is a plan view of another example of a blank and its heating pattern.

FIG. 11A illustrates a method for heating the blank with the heating pattern illustrated in FIG. 10.

FIG. 11B illustrates the heating method together with FIG. 11A.

FIG. 11C illustrates the heating method together with FIGS. 11A and 11B.

FIG. 12A illustrates the heating method together with FIGS. 11A to 11C.

FIG. 12B illustrates the heating method together with FIGS. 11A to 12A.

FIG. 12C illustrates the heating method together with FIGS. 11A to 12B.

FIG. 13 is a diagram illustrating a control of a movement speed of a first electrode and an amount of electric current when heating the blank to be in a prescribed temperature range with the method illustrated in FIGS. 11A to 12C.

FIG. 14 is a graph showing an example of the control of the movement speed of the first electrode and the amount of electric current in the method illustrated in FIGS. 11A to 12C.

FIG. 15 is a graph showing another example of the control of the movement speed of the first electrode and the amount of electric current in the method illustrated in FIGS. 11A to 12C.

FIG. 16 is a plan view illustrating another example of a heating pattern of the blank.

FIG. 17A illustrates a method for heating the blank with the heating pattern illustrated in FIG. 16.

FIG. 17B illustrates the heating method together with FIG. 17A.

FIG. 17C illustrates the heating method together with FIGS. 17A and 17B.

FIG. 18A illustrates the heating method together with FIGS. 17A to 17C.

FIG. 18B illustrates the heating method together with FIGS. 17A to 18A.

FIG. 18C illustrates the heating method together with FIGS. 17A to 18B.

FIG. 19 illustrates another method for heating the blank with the heating pattern illustrated in FIG. 1.

FIG. 20 illustrates the heating method together with FIG. 19.

FIG. 21 illustrates a modification of the heating method illustrated in FIGS. 19 and 20.

FIG. 22 illustrates another method for heating the blank with the heating pattern illustrated in FIG. 6.

FIG. 23 illustrates the heating method together with FIG. 22.

FIG. 24A illustrates another method for heating the blank with the heating pattern illustrated in FIG. 10.

FIG. 24B illustrates the heating method together with FIG. 24A.

FIG. 24C illustrates the heating method together with FIGS. 24A and 24B.

FIG. 25A illustrates the heating method together with FIGS. 24A to 24C.

FIG. 25B illustrates the heating method together with FIGS. 24A to 25A.

FIG. 25C illustrates the heating method together with FIGS. 24A to 25B.

FIG. 26A illustrates a method for heating the blank with the heating pattern illustrated in FIG. 16.

FIG. 26B illustrates the heating method together with FIG. 26A.

FIG. 26C illustrates the heating method together with FIGS. 26A and 26B.

FIG. 27A illustrates the heating method together with FIGS. 26A to 26C.

FIG. 27B illustrates the heating method together with FIGS. 26A to 27A.

FIG. 27C illustrates the heating method together with FIGS. 26A to 27B.

DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates an example of a blank 1 and its heating pattern.

The blank 1 illustrated in FIG. 1 is a rectangular steel plate having a constant (including substantially constant) sectional area along the longitudinal direction of the blank 1. The blank 1 is for manufacture of a hot-pressed product and is to be subjected to quenching.

The heating pattern of the blank 1 illustrated in FIG. 1 has two first regions A1 which are side areas located on the two respective sides in the width direction and extending in the longitudinal direction excluding both end regions in the longitudinal direction and a second region B1 which is a central area between the two first regions A1. The blank 1 is to be heated so that its temperature is increased to an Ac3 transformation point or higher in the second region B1 while being kept lower than an Ac1 transformation point in the first regions A1.

The Ac1 transformation point is a temperature at which ferrite and pearlite of steel of which the blank 1 is made start to undergo transformation to austenite, and the Ac3 transformation point is a temperature at which ferrite and pearlite of the steel of which the blank 1 is made complete the transformation to austenite.

FIGS. 2 and 3 illustrate a method for heating the blank 1 with the heating pattern illustrated in FIG. 1.

Electrodes 2 are fixed at respective longitudinal ends of the blank 1, and the blank 1 is heated as electric current is passed through the blank 1 in its longitudinal direction between the two electrodes 2. During the direct resistance heating, a jet of cooling medium is applied to at least one of the front surface and the back surface of each of the first regions A1. As a result, the temperature of the blank 1 is increased to be equal to or higher than the Ac3 transformation point in the second region B1 while being kept lower than the Ac1 transformation point in the first regions A1.

In the example illustrated in FIGS. 2 and 3, coolant dischargers 3, each configured to discharge the cooling medium, are disposed on a side of the front surface of the blank 1 and the jet of cooling medium is applied only to the front surfaces of the first regions A1. Alternatively, the coolant dischargers 3 may be disposed on a side of the back surface of the blank 1 so that the jet of cooling medium is applied only at the back surfaces of the first regions A1. As a further alternative, the coolant dischargers 3 may be disposed on both sides of the blank 1 so that the jet of cooling medium is applied to the front and back surfaces of the first regions A1. The cooling medium is not particularly limited. The cooling medium is, for example, air.

Each coolant discharger 3 extends along the associated side edge of the blank 1 and has a plurality of nozzles 4 arranged at intervals in the extending direction of the coolant discharger 3. The center axis of each nozzle 4 is inclined toward the second region B1 from the direction perpendicular to the front surface of the first region A1. The cooling medium emitted from the nozzles 4 is directed in the slant direction that is inclined toward the second region B1 from the direction perpendicular to the front surface of the first region A1, and is applied to the front surface of the first region A1 such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A1 and the second region B1. Instead of the nozzles 4, the coolant discharger 3 may have one or more slits extending in the extending direction of the coolant discharger 3. The nozzles 4 or the slits may be arranged in a plurality of rows.

The jet of cooling medium applied to the front surface of the first region A1 flows along the front surface of the first region A1. Being directed in the slant direction that is inclined toward the second region B1 from the direction perpendicular to the front surface of the first region A1, the cooling medium flows off the edge of the blank 1 in the width direction. In other words, the cooling medium is prevented from flowing into the second region B1 from the first region A1. Thus, an area C1 of the second region B1 adjoining the first region A1 is prevented from being cooled by the cooling medium so that the entire second region B1, including the area C1, can be heated to be equal to or higher than the Ac3 transformation point. As a result, clear boundaries can be formed between the second region B1 where the blank 1 is heated to be equal to or higher than the Ac3 transformation point and the first regions A1 where the temperature of the blank 1 is kept lower than the Ac1 transformation point.

FIG. 4 illustrates an example of temperature variations in the first regions A1 and the second region B1 of the blank 1 when it is heated by the heating method illustrated in FIGS. 2 and 3.

In the example illustrated in FIG. 4, direct resistance heating of the blank 1 is started at t₀, application of a jet of cooling medium to the first region A1 of the blank 1 is started at t₁, after a given period of time from to, and the direct resistance heating of the blank 1 is finished at t₂.

The temperatures in the first regions A1 and the second region B1 increase approximately in the same manner from the start of direct resistance heating (t₀) to the start of application of the cooling medium (t₁). A temperature T₁ in the first regions A1 and the second region B1 at the start of application of the cooling medium is higher than room temperature and lower than the Ac1 transformation point.

In the interval between the time of the start of the application of the cooling medium (t₁) and the time of the end of the direct resistance heating (t₂), the portions of the blank 1 in the first regions A1 are cooled by the cooling medium and the temperature in the first regions A1 is not increased from the temperature T₁ at the start of the application of the cooling medium, that is, is kept lower than the Ac1 transformation point. On the other hand, the temperature in the second region B1 continues to increase and becomes higher than the Ac3 transformation point at the end of the direct resistance heating (t₂).

Although the application of the cooling medium may be started at the same time as the start of the direct resistance heating, the difference between the temperature in the first regions A1 and the temperature in the second region B1 in the period from the start of the application of the cooling medium to the end of the direct resistance heating can be reduced by starting the application of the cooling medium after a given period of time from the start of the direct resistance heating. As a result, heat transfer from the second region B1 to the first regions A1 can be suppressed and clearer boundaries can be formed between them.

Since resistivity depends on temperature, the resistivity of the blank 1 in the first regions A1 where the temperature is relatively low is smaller than in the second region B1 where the temperature is relatively high. Thus, a relatively large current tends to flow through the conduction path extending along the first region A, that is, in the longitudinal direction of the blank 1. But this current difference is made smaller by reduction of the difference between the temperature in the first regions A1 and the temperature in the second region B1. This serves to suppress overheating in areas D1 (see FIG. 2) that are located in the second region B1 and adjoining the first regions A1 in the current flow direction.

From the viewpoints of suppressing the heat transfer from the second region B1 to the first regions A1 and the overheating in the areas D1 adjoining the first regions A1 in the current flow direction, it is preferable to keep the temperature in the first regions A1 between 300° C. and 700° C. in the period from the start of application of the cooling medium to the end of the direct resistance heating. The temperature in the first regions A1 can be adjusted as appropriate by controlling, for example, the temperature of the cooling medium, the flow rate of the cooling medium, and/or discharging method (e.g., continuous or intermittent) of the cooling medium.

FIG. 5 illustrates a modification of the heating method illustrated in FIGS. 2 and 3.

In the heating method illustrated in FIGS. 2 and 3, the blank 1 is supported in such a manner that its two end portions in the longitudinal direction are held by the respective electrodes 2. In this case, the blank 1 may be bent, for example, due to its thermal expansion in the longitudinal direction caused by the direct resistance heating or pressure produced by receiving the jet of cooling medium. If the blank 1 is bent, the relative positions of the first regions A1 of the blank 1 and the respective coolant dischargers are changed, so that the application of the cooling medium onto the first regions A1 of the blank 1 becomes less effective.

In view of the above, in the example illustrated in FIG. 5, the jet of cooling medium is applied to the front surfaces of the first regions A1 in a state in which the back surfaces, opposite to the front surfaces, of the first regions A1 are supported by support members 5. With this configuration, the bend of the blank 1 is suppressed, whereby the jet of cooling medium can be applied to the first regions A1 in a desired manner and hence clearer boundaries can be formed between the second region B1 and the first regions A1.

Either of the front surface and the back surface of the blank 1 or both of the front surface and the back surface of the blank 1 may be supported by support members 5 as appropriate so as to attain the purpose of suppressing the bend of the blank 1, irrespective of whether the jet of cooling medium is applied to the front surface and/or the back surface of the blank 1.

It is preferable that the support members 5 be members that support the portions of the blank 1 in the first regions A1 at points, such as pins. This makes it possible to suppress heat transfer from the portions in the first regions A1 of the blank 1 to the support members 5 and to prevent obstruction of flows of the cooling medium in the case where the blank 1 is supported by support members 5 at the surface to which the jet of cooling medium is applied. To support the portion of the blank 1 in each first region A1, one or more support members 5 are provided according to the size of the first region A1.

The heating pattern of the blank 1 is not limited to the example illustrated in FIG. 1. FIG. 6 illustrates another example of the heating pattern in which a first region A2 is provided in the middle of the blank 1 as a closed region surrounded by a second region B2. Although in the example illustrated in FIG. 6 the first region A2 is a circle, the shape of the first region A2 is not limited to it and may be a rectangle or the like. Further, a plurality of first regions A2 may be provided.

FIGS. 7 and 8 illustrate a method for heating the blank 1 with the heating pattern illustrated in FIG. 6.

Electrodes 2 are fixed at respective longitudinal ends of the blank 1, and the blank 1 is heated as electric current is passed through the blank 1 in its the longitudinal direction between the two electrodes 2. During the direct resistance heating, a jet of cooling medium is applied to the front surface of the first region A2. As a result, the temperature of the blank 1 is increased to the Ac3 transformation point or higher in the second region B2 while being kept lower than the Ac1 transformation point in the first region A2.

A coolant discharger 13 has an annular configuration. The cooling medium emitted from the coolant discharger 13 flows in slant directions that are inclined toward the second region B2 from the direction perpendicular to the front surface of the first region A2, and is applied to the front surface of the first region A2a such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A2 and the second region B2.

The jet of cooling medium applied to the front surface of the first region A21 flows along the front surface of the first region A2. Being directed in the slant direction that is inclined toward the second region B2 from the direction perpendicular to the front surface of the first region A2, the cooling medium flows from the circumference of the first region A2 toward its center. In other words, the cooling medium is prevented from flowing into the second region B3 from the first region A2. Thus, an area C2 of the second region B2 adjoining the first region A2 is prevented from being cooled by the cooling medium so that the entire second region B2, including the area C2, can be heated to be equal to or higher than the Ac3 transformation point. As a result, a clear boundary can be formed between the second region B2 where the blank 1 is heated to be equal to or higher than the Ac3 transformation point and the first region A2 where the temperature of the blank 1 is kept lower than the Ac1 transformation point.

In the heating method illustrated in FIGS. 7 and 8, the cooling medium is applied only to the front surface of the blank 1. However, the cooling medium may be applied only to the back surface of the blank 1 or both the front surface and the back surface of the blank 1. Likewise, the blank 1 may be supported only at the front surface of the blank 1, only at the back surface of the blank 1 or both the front surface and the back surface of the blank 1.

The first regions A1 illustrated in FIG. 1 and the first region A2 illustrated in FIG. 6 may be formed in the single blank 1. In this case, the heating method illustrated in FIGS. 2 and 3 and the heating method illustrated in FIGS. 7 and 8 are performed in parallel.

The blank 1 whose temperature has been kept lower than the Ac1 transformation point in the first region A1, A2 and increased to be equal to or higher than the Ac3 transformation point in the second region B1, B2 is press-formed by a press die 20 and then cooled inside the press die 20 (see FIG. 9), so that the second region B1, B2 is quenched. A clear boundary is formed between the first region A1, A2 where the temperature has been kept lower than the Ac1 transformation point and the second region B1, B2 where the temperature has been increased to be equal to or higher than the Ac3 transformation point, that is, a clear boundary is formed between the unquenched region (first region) and the quenched region (second region).

A method of heating a steel plate and a method for manufacturing a hot-pressed product have been described so far in connection with the rectangular blank 1 having a constant (includes approximately constant) sectional area along the longitudinal direction of the blank 1. However, the blank is not limited to this example. FIG. 10 illustrates another blank 101 and an example of its heating pattern.

The blank 101 illustrated in FIG. 10 is a non-rectangular steel plate having a constant thickness and a gradually decreasing width from one end R to the other end L along the longitudinal direction of the blank 101. Thus, in the blank 101, the area of the cross section taken orthogonally to the longitudinal direction decreases monotonously and hence the resistance per unit length in the longitudinal direction increases monotonously as the position goes from the relatively wide end R to the relatively narrow end L. The blank 101 is used for manufacture of a hot-pressed product and is subjected to quenching.

The “sectional area increases or decreases monotonously” means that the sectional area increases or decreases as the position comes close to one end in the longitudinal direction without occurrence of an inflection point. The sectional area can be regarded as increasing or decreasing monotonously unless a partial low-temperature portion or high-temperature portion that would cause a problem in practical use occurs due to excessive non-uniformity in the current density in the width direction during direct resistance heating.

The heating pattern of the blank 101 illustrated in FIG. 10, which is similar to that of the blank 1 illustrated in FIG. 1, has two first regions A3 which are side areas located on the two respective sides in the width direction and extending in the longitudinal direction excluding both end regions in the longitudinal direction and a second region B3 which is a central area between the two first regions A3. The blank 101 is to be heated so that its temperature is increased to the Ac3 transformation point or higher in the second region B3 while being kept lower than the Ac1 transformation point in the first regions A3.

FIGS. 11A to 12C illustrate a method for heating the blank 101 with the heating pattern illustrated in FIG. 10.

First, as illustrated in FIG. 11A to 11C, a first electrode 102a and a second electrode 102b are placed adjacent to the relatively wide end R of the blank 101.

Then, as illustrated in FIGS. 12A to 12C, while current is caused to flow through the blank 101 between the first electrode 102a and the second electrode 102b, the first electrode 102a is moved toward the end L of the blank 101 and the distance between the first electrode 102a and the second electrode 102b is thereby increased gradually. Current flows through the region between the first electrode 102a and the second electrode 102b and this region is heated. This direct resistance heating of the blank 101 is finished after the first electrode 102a reaches the end L.

Coolant dischargers 103, each configured to discharge cooling medium, are disposed on the front surface side of the blank 101. As illustrated in FIG. 11B, at a start of direct resistance heating, a space through which the first electrode 102a can pass exists between the front surface of the blank 101 and the coolant dischargers 103. As illustrated in FIG. 12B, after the first electrode 102a has passed the first regions A3 of the blank 101, the interval between the front surface of the blank 101 and the coolant dischargers 103 is decreased by moving the coolant dischargers 103 toward the front surface of the blank 101 and the application of the jet of cooling medium to the front surfaces of the first regions A3 is started. As a result, the temperature of the blank 1 is increased to the Ac3 transformation point in the second regions B3 while being kept lower than the Ac1 transformation point in the first regions A3.

A description will now be made of a method for heating the blank 101 such that the entire blank 101 becomes within a prescribed temperature range with a temperature distribution that can be regarded substantially uniform, assuming that the cooling medium is not applied to the first regions A3. As illustrated in FIG. 11A, the blank 101 is divided into n segments w1, w2, . . . , wn each having a length A1. With Ii (A) being electric current that flows when the first electrode 102a passes an ith segment wi, and ti (sec) being a time in which the first electrode 102a passes the ith segment wi, since the ith segment wi is heated after the first electrode 102a has passed the ith segment wi, a temperature increase θi of the ith segment wi is given by the following equation, where ρe being the resistivity (Ω·m), ρi being the density (kg/m³), c being the specific heat (J/kg·° C.), and Ai being the sectional area (m²) of the ith segment wi.

$\begin{array}{cc}{\theta }_i=\frac{{\rho }_e}{C\rho }\frac{1}{A_t^2}\underset{i}{\stackrel{n}{\Sigma }}\left(I_i^2\times t_i\right)& \left[\mathrm{Math}.1\right]\end{array}$

The movement speed of the first electrode 102a and the current flowing through the blank 101 are controlled by a control unit (not shown) from a start to an end of current flow through the blank 101. This makes it possible to control the quantities of heat that are generated in the respective strip-shaped segments w1, w2, . . . , wn which are obtained by dividing the blank 101 imaginarily in the longitudinal direction.

In particular, where the first electrode 102a is moved in the longitudinal direction of the blank 101 and the sectional area of the blank 101 decreases monotonously in the movement direction of the first electrode 102a, it is possible to heat the blank 101 so that the entire blank 101 will be in such a prescribed temperature range that the temperature distribution can be regarded substantially uniform. FIG. 13 is a conceptual diagram for description of how the movement speed of the first electrode 102a and the current to flow through the blank 101 should be controlled to heat the blank 101 to within a prescribed temperature range.

The temperature increase of the ith segment wi of the case that the blank 101 is divided into the n segments w1-wn having the length A1 is given by the foregoing equation. The temperature increases θ1-θn of the respective segments w1-wn are made identical (θ1=θ2= . . . =θn) by controlling the current Ii and the time ti (electrode movement speed Vi) so that the following equation is satisfied:

$\begin{array}{cc}\frac{1}{A_1^2}\underset{i=1}{\stackrel{n}{\Sigma }}\left(I_i^2\times t_i\right)=\frac{1}{A_2^2}\underset{i=2}{\stackrel{n}{\Sigma }}\left(I_i^2\times t_i\right)=\cdots =\frac{1}{A_n^2}\underset{i=n}{\stackrel{n}{\Sigma }}\left(I_i^2\times t_i\right)& \left[\mathrm{Math}.2\right]\end{array}$

Where the second electrode 102b is fixed at the end R of the blank 101 and the first electrode 102a is moved from the end R to the end L of the blank 101, the w1-wn are different from each other in energization time and the energization time increases as the position comes closer to the end R. If the same current is caused to flow through segments on the side of the end R and segments on the side of the end L for the same time, a smaller quantity of heat is generated in the segment that is closer to the end R (the resistance per unit length decreases). In view of this, the blank 1 can be heated so as to be in a prescribed temperature range by adjusting the quantity of heat generated in each segment wi by controlling one or both of the movement speed of the first electrode 102a and the current to flow through the blank 101 according to the variation of the resistance per unit length.

FIGS. 14 and 15 illustrate examples relationships between the position X of the first electrode 102a in the longitudinal direction and the temperature T of the blank 101 at the end of the direct resistance heating, the current I flowing through the blank 101, the movement speed V of the first electrode 102a, and the elapsed time t from the start of the direct resistance heating. In FIGS. 14 and 15, the position X of the first electrode 102a is the distance from the origin (close to the end R of the blank 101) that is the initial position of the first electrode 102a at the start of the direct resistance heating.

In the example illustrated in FIG. 14, adjustments are made so that the first electrode 102a is moved at a constant speed from the end R to the end L of the blank 101 and the current flowing through the blank 101 decreases gradually. For a prescribed time after arrival of the first electrode 102a at the end L, the first electrode 102a is held at the end L and the flow of the same current as at the time of the arrival of the first electrode 102a at the end L is continued. With this current adjustment, the blank 1 can be heated so as to be in a prescribed temperature range.

In the example illustrated in FIG. 14, adjustments are made so that a constant current flows through the blank 101 and the first electrode 102a is moved from the end R to the end L of the blank 101 in such a manner that its movement speed increases gradually. For a prescribed time after arrival of the first electrode 102a at the end L, the first electrode 102a is held at the end L and a constant current is caused to flow through the blank 101. With this speed adjustment, the blank 1 can be heated so as to be in a prescribed temperature range.

Again referring to FIGS. 12A to 12C, though it is possible to heat the blank 101 so that the entire blank 101 is in a prescribed temperature range that is higher than or equal to the Ac3 transformation point, the temperature of the portions of the blank 101 in the first regions A3 is kept lower than the Ac1 transformation point by applying the jet of cooling medium to the front surfaces of the first regions A3. Each coolant discharger 103 configured to discharge a jet of cooling medium extends alongside the associated side edge of the blank 1, and has a plurality of nozzles 104 arranged at intervals in the extending direction of the coolant discharger 103. The center axis of each nozzle 104 is inclined toward the second region B3 from the direction perpendicular to the front surface of the first region A3. The cooling medium emitted from the nozzles 104 is directed in the slant direction that is inclined toward the second region B3 from the direction perpendicular to the front surface of the first region A3, and is applied to the front surface of the first region A3 such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A3 and the second region B3.

The jet of cooling medium applied to the front surface of the first region A3 flows along the front surface of the first region A3. Being directed in the slant direction that is inclined toward the second region B3 from the direction perpendicular to the front surface of the first region A3, the cooling medium flows off the edge of the blank 101 in the width direction. In other words, the cooling medium is prevented from flowing into the second region B3 from the first region A3 to the second region B3. Thus, the area C3 of the second region B3 adjoining the first region A3 is prevented from being cooled by the cooling medium so that the entire second region B3, including the area C3, can be heated to be equal to or higher than the Ac3 transformation point. As a result, clear boundaries can be formed between the second region B3 where the blank 101 is heated to be equal to or higher than the Ac3 transformation point and the first regions A3 where the temperature of the blank 101 is kept lower than the Ac1 transformation point.

FIG. 16 illustrates another example heating pattern of the blank 101.

The heating pattern illustrated in FIG. 16 is similar to the heating pattern of the blank 1 illustrated in FIG. 6. In this heating pattern, a first region A4 where the temperature is to be kept lower than the Ac1 transformation point is a closed central area surrounded by a second region B4 where the temperature is to be increased to the Ac3 transformation point or higher.

FIGS. 17A to 18C illustrate a method for heating the blank 101 with the heating pattern illustrated in FIG. 16.

First, as illustrated in FIG. 17A to 17C, a first electrode 102a and a second electrode 102b are placed adjacent to the relatively wide end R of the blank 101.

Then, as illustrated in FIGS. 18A to 18C, while current is caused to flow through the blank 101 between the first electrode 102a and the second electrode 102b, the first electrode 102a is moved toward the end L of the blank 101 and the distance between the first electrode 102a and the second electrode 102b is thereby increased gradually. Current flows through the region between the first electrode 102a and the second electrode 102b and this region is heated. This direct resistance heating of the blank 101 is finished after the first electrode 102a reaches the end L.

A coolant discharger 113 has an annular configuration. The cooling medium emitted from the coolant discharger 113 flows in slant directions that are inclined toward the second region B4 from the direction perpendicular to the front surface of the first region A4, and is applied to the front surface of the first region A4 such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A4 and the second region B4.

The jet of cooling medium applied to the front surface of the first region A4 flows along the front surface of the first region A4. Being directed in the slant direction that is inclined toward the second region B4 from the direction perpendicular to the front surface of the first region A4, the cooling medium flows from the circumference of the first region A4 toward its center. In other words, the cooling medium is prevented from flowing into the second region B4 from the first region A4. Thus, an area C3 of the second region B1 adjoining the first region A4 is prevented from being cooled by the cooling medium so that the entire second region B4, including the area C4, can be heated to be equal to or higher than the Ac3 transformation point. As a result, a clear boundary can be formed between the second region B4 where the blank 101 is heated to be equal to or higher than the Ac3 transformation point and the first region A4 where the temperature of the blank 101 is kept lower than the Ac1 transformation point.

In the heating method illustrated in FIGS. 11A to 11C and 12A to 12C and the heating method illustrated in FIGS. 17A to 17C and 18A to 18C, the jet of cooling medium may be applied to only the front surface of the blank 101, only the back surface of the blank 101, or both of the front and back surfaces of the blank 101. Also, the blank 101 may be supported at only the front surface of the blank 101, the back surface of the blank 101, or both the front and back surfaces of the blank 101.

The first regions A3 illustrated in FIG. 10 and the first region A4 illustrated in FIG. 16 may be formed in the single blank 101. In this case, the heating method illustrated in FIGS. 11A to 11C and 12A to 12C and the heating method illustrated in FIGS. 17A to 17C and 18A to 18C are performed in parallel.

The blank 101 described above is constant in thickness and is not rectangular in shape, that is, the width decreases gradually from the end R to the end L in the longitudinal direction. Alternatively, a blank may be used that is constant in width and whose thickness decreases gradually from the end R to the end L in the longitudinal direction. As a further alternative, a non-rectangular blank may be used whose thickness and width decrease gradually from the end R to the end L in the longitudinal direction.

The blank 101 whose temperature has been kept lower than the Ac1 transformation point in the first region A3, A4 and increased to be equal to or higher than the Ac3 transformation point in the second region B3, B4 in the above-described manner is press-formed by a press die and then cooled inside the press die so that the second region B3, B4 is quenched. A clear boundary is formed between the first region A3, A4 where the temperature has been kept lower than the Ac1 transformation point and the second region B3, B4 where the temperature has been increased to be equal to or higher than the Ac3 transformation point, that is, a clear boundary is formed between the unquenched region (first region) and the quenched region (second region).

FIGS. 19 and 20 illustrate another method for heating the blank 1 with the heating pattern illustrated in FIG. 1. Features that are different than in the heating method illustrated in FIGS. 2 and 3 will be described mainly below. Descriptions of features and advantages that are the same as or similar to features and advantages of the heating method illustrated in FIGS. 2 and 3 may not be made when appropriate to avoid redundant descriptions.

In the heating method illustrated in FIGS. 19 and 20, coolant dischargers 123, each configured to discharge cooling medium, and partition members 6 are disposed on a side of the front surface of the blank 1, and a jet of cooling medium is applied only to the front surfaces of the first regions A1. Alternatively, the coolant dischargers 3 and the partition members 6 may be disposed on a side of the back surface of the blank 1 so that the jet of cooling medium is applied only to the back surfaces of the first regions A1. As a further alternative, the coolant dischargers 3 and the partition members 6 may be disposed on both sides of the blank 1 so that the jet of cooling medium is applied to the front and back surfaces of the first regions A1. The cooling medium is not particularly limited. The cooling medium is, for example, air.

The partition members 6 extend alongside the respective edges of the blank 1. Each coolant discharger 123 is disposed adjacent to the associated partition member 6 on the side of the associated first region A1 so as to extend parallel with the associated partition member 6, and has a plurality of nozzles 124 arranged at intervals in the extending direction of the coolant discharger 123. The cooling medium emitted from the nozzles 124 is applied to the front surface of the first region A1 such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A1 and the second region B1. Instead of the nozzles 124, the coolant discharger 123 may have one or more slits extending in the extending direction of the coolant discharger 123. The nozzles 124 or the slits may be arranged in a plurality of rows.

The jet of cooling medium applied to the front surface of the first region A1 flows along the front surface of the first region A1. The partition member 6 causes the cooling medium to flow toward the side opposite to the partition member 6 and off the edge of the blank 1 in the width direction. In other words, the cooling medium is prevented from flowing into the second region B1 from the first region A1. Thus, an areas C1 of the second region B1 adjoining the first region A1 is prevented from being cooled by the cooling medium so that the entire second region B1, including the area C1, can be heated to be equal to or higher than the Ac3 transformation point. As a result, clear boundaries can be formed between the second region B1 where the blank 1 is heated to be equal to or higher than the Ac3 transformation point and the first regions A1 where the temperature of the blank 1 is kept lower than the Ac1 transformation point. The partition member 6 may be arranged such that a slight gap is provided between the partition member 6 and the blank 1. Alternatively, the partition member 6 may be arranged so as to contact the blank 1, in which case the cooling medium is further prevented from flowing into the second region B1 from the first region A.

For example, temperature variations, in the first regions A1 and the second region B1, of the blank 1 when it is heated by the heating method illustrated in FIGS. 19 and 20 are the same as or similar to the example temperature variations, in the first regions A1 and the second region B1, of the blank 1 (see FIG. 4) when it is heated by the heating method illustrated in FIGS. 2 and 3.

FIG. 21 is a modification of the heating method illustrated in FIGS. 19 and 20.

In the heating method illustrated in FIGS. 19 and 20, the blank 1 is supported in such a manner that its two end portions in the longitudinal direction are held by the respective electrodes 2. In this case, the blank 1 may be bent, for example, due to its thermal expansion in the longitudinal direction caused by the direct resistance heating or pressure produced by the application of the jet of cooling medium. If the blank 1 is bent, the positions of the portions of the blank 1 in the first regions A1 relative to the respective coolant dischargers are changed, whereby the application of the cooling medium to the first regions A1 becomes less effective.

In view of the above, in the example illustrated in FIG. 21, the jet of cooling medium is applied to the front surfaces of the first regions A1 in a state in which the back surfaces, opposite to the front surfaces, of the first regions A1 are supported by support members 5. With this configuration, the bend of the blank 1 is suppressed, whereby the jet of cooling medium can be applied to the first regions A1 in a desired manner and hence clearer boundaries can be formed between the second region B1 and the first regions A1. The support members 5 are the same as or similar to those in the example illustrated in FIG. 5.

FIGS. 22 and 23 illustrate another method for heating the blank 1 with the heating pattern illustrated in FIG. 6. Features that are different than in the heating method illustrated in FIGS. 7 and 8 will be described mainly below. Descriptions of features and advantages that are the same as or similar to features and advantages of the heating method illustrated in FIGS. 7 and 8 may not be made when appropriate to avoid redundant descriptions.

A partition member 16 has a cylindrical shape. An inner cylinder 17 is inserted in the partition member 16 approximately coaxially so as to be located over a central portion of the first region A2. A coolant discharger 133 which jets out the cooling medium is connected to the inner cylinder 17. The jet of cooling medium emitted from the coolant discharger 133 is applied to the central portion of the front surface of the first region A2. A slight gap may be formed between the partition member 16 and the blank 1. However, it is preferable that they be in contact with each other.

The jet of cooling medium applied to the central portion of the front surface of the first region A2 flows outward along the front surface of the first region A2, hits the partition member 16, and is ejected through the space between the partition member 16 and the inner cylinder 17. In other words, the cooling medium is prevented from flowing into the second region B2 from the first region A2. Thus, an area C2 of the second region B2 adjoining the first region A2 is prevented from being cooled by the cooling medium so that the entire second region B2, including the area C2, can be heated to be equal to or higher than the Ac3 transformation point. As a result, a clear boundary can be formed between the second region B2 where the blank 1 is heated to be equal to or higher than the Ac3 transformation point and the first region A2 where the temperature of the blank 1 is kept lower than the Ac1 transformation point.

FIGS. 24A to 25C illustrate another method for heating the blank 101 with the heating pattern illustrated in FIG. 10. Features that are different than in the heating method illustrated in FIGS. 11A to 12C will be described mainly below. Descriptions of features and advantages that are the same as or similar to features and advantages of the heating method illustrated in FIGS. 11A to 12C may not be made when appropriate to avoid redundant descriptions.

Coolant dischargers 143 and partition members 106 are disposed on the front surface side of the blank 101. As illustrated in FIGS. 24B and 24C, at a start of direct resistance heating, a space through which the first electrode 102a can pass exists between the front surface of the blank 101 and the coolant dischargers 143. As illustrated in FIGS. 25B and 25C, after the first electrode 102a has passed the first regions A3 of the blank 101, the interval between the front surface of the blank 101 and the coolant dischargers 143 is decreased by moving the coolant dischargers 143 and the partition members 106 toward the front surface of the blank 101 and the application of the jet of cooling medium to the front surfaces of the first regions A3 is started. As a result, the temperature of the blank 101 is increased to the Ac3 transformation point in the second regions B3 while being kept lower than the Ac1 transformation point in the first regions A3.

Though it is possible to heat the blank 101 such that the entire blank 101 is in a prescribed temperature range that is equal to or higher than the Ac3 transformation point, the temperature of the first regions A3 is kept lower than the Ac1 transformation point by the application of the jet of cooling medium to the front surfaces of the first regions A3. The partition members 106 extend alongside the respective edges of the blank 101. Each coolant discharger 143 is disposed on the first region A3 side of the associated partition member 106 so as to extend alongside the associated partition member 106 and has a plurality of nozzles 144 arranged at intervals in the extending direction of the coolant discharger 143. The cooling medium emitted from the nozzles 144 is applied to the front surface of the first region A3 such that a jet of cooling medium expands in a form of a curtain along the boundary between the first region A3 and the second region B3. A slight gap may be provided between the partition member 106 and the blank 101. However, it is preferable that the partition member 106 and the blank 101 are in contact with each other.

The jet of cooling medium applied to the front surface of the first region A3 flows along the front surface of the first region A3. The partition member 106 causes the cooling medium to flow toward the side opposite to the partition member 106 and off the edge the blank 101 in the width direction. In other words, the cooling medium is prevented from flowing in the second region B3 from the first region A3. Thus, an area C3 inside the second region B3 and adjoining the first region A3 is prevented from being cooled by the cooling medium so that the entire second region B3, including the area C3, can be heated to be equal to or higher than the Ac3 transformation point. As a result, clear boundaries can be formed between the second region B3 where the blank 101 is heated to be equal to or higher than the Ac3 transformation point and the first regions A3 where the temperature of the blank 101 is kept lower than the Ac1 transformation point.

FIGS. 26A to 27C illustrate another method for heating the blank 101 with the heating pattern illustrated in FIG. 16. Features that are different than in the heating method illustrated in FIGS. 17A to 18C will be described mainly below. Descriptions of features and advantages that are the same as or similar to features and advantages of the heating method illustrated in FIGS. 17A to 18C may not be made when appropriate to avoid redundant descriptions.

A partition member 116 has a cylindrical shape. An inner cylinder 117 is inserted in the partition member 116 approximately coaxially so as to be located over a central portion of the first region A4. A coolant discharger 153 which jets out the cooling medium is connected to the inner cylinder 117. The jet of cooling medium emitted from the coolant discharger 153 is applied to the central portion of the front surface of the first region A4. A slight gap may be formed between the partition member 116 and the blank 101. However, it is preferable that they be in contact with each other.

The jet of cooling medium applied to the central portion of the front surface of the first region A4 flows outward along the front surface of the first region A4, hits the partition member 116, and is ejected through the space between the partition member 116 and the inner cylinder 117. In other words, the cooling medium is prevented from entering into the second region B4 from the first region A4. Thus, an area C4 of the second region B4 adjoining the first region A4 is prevented from being cooled by the cooling medium so that the entire second region B4, including the area C4, can be heated to be equal to or higher than the Ac3 transformation point. As a result, a clear boundary can be formed between the second region B4 where the blank 101 is heated to be equal to or higher than the Ac3 transformation point and the first region A4 where the temperature of the blank 101 is kept lower than the Ac1 transformation point.

According to one or more illustrative aspects of the embodiments described above, a method for heating a steel plate is provided. The steel plate is a blank having a first region and a second region adjacent to the first region. The method includes heating the blank by direct resistance heating, and applying a jet of cooling medium to the first region on at least one of a front surface and a back surface of the blank during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature. The jet of cooling medium is applied along a slant direction that is inclined toward the second region from a direction perpendicular to the at least one of the front surface and the back surface in the first region such that the jet of cooling medium expands along a boundary between the first region and the second region, or a partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank.

The jet of cooling medium may be applied along the slant direction such that the jet of cooling medium expands in a form of a curtain along the boundary between the first region and the second region. The first region may include an edge of the blank, and the jet of cooling medium applied to the first region may be caused to flow off the edge of the blank.

The first region may be a closed region surrounded by the second region, and the jet of cooling medium applied to the first region may be caused to flow from a circumference of the first region toward a center of the first region.

The partition member may be provided to contact the at least one of the front surface and the back surface.

When the first region includes an edge of the blank, the partition member may extend alongside the edge of the blank to cause the jet of cooling medium applied to the first region to flow off the edge of the blank.

When the first region is a closed region surrounded by the second region, the partition member may have a cylindrical shape to cause the jet of cooling medium applied to the first region to flow from a center of the first region toward a circumference of the first region.

An inner cylinder may be inserted inside the partition member to apply the jet of cooling medium toward the center of the first region through the inner cylinder.

The jet of cooling medium may be applied in a state in which the first region is supported on at least one of the front surface and the back surface to suppress bending of the blank.

The first region may be point-supported at one or more locations on at least one of the front surface and the back surface.

When the blank is rectangular and has a constant sectional area along a longitudinal direction of the blank, the heating of the blank by the direct resistance heating may include applying electric current to pass through the blank via a pair of electrodes fixed at longitudinal ends of the blank.

When the blank is non-rectangular and has a sectional area monotonously decreasing along a longitudinal direction of the blank from a first end of the blank to a second end of the blank, the heating of the blank by the direct resistance heating may include placing a pair of electrodes on the first end of the blank, and moving one of the electrodes in the longitudinal direction toward the second end of the blank while applying electric current to pass through a portion of the blank between the pair of electrodes.

The application of the jet of cooling medium to the first region may be started after the one of the electrodes has passed the first region.

The temperature of the first region may be kept lower than an Ac1 transformation point of the blank while heating the second region to be equal to or higher than an Ac3 transformation point of the blank.

To manufacture a hot-pressed product, the blank heated in a manner described above is pressed-formed with a press die and cooled inside the press die to quench the second region.

This application claims priority to Japanese Patent Application Nos. 2018-005098 and 2018-005099 both filed on Jan. 16, 2018, the entire contents of which are incorporated herein by reference.

Claims

1. A method for heating a steel plate, the steel plate being a blank having a first region and a second region adjacent to the first region, the method comprising:

heating the blank by direct resistance heating; and

applying a jet of cooling medium to the first region on at least one of a front surface and a back surface of the blank during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature,

wherein the jet of cooling medium is applied along a slant direction that is inclined toward the second region from a direction perpendicular to the at least one of the front surface and the back surface in the first region such that the jet of cooling medium expands along a boundary between the first region and the second region; or

wherein a partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank.

2. The method according to claim 1, wherein the jet of cooling medium is applied along the slant direction such that the jet of cooling medium expands in a form of a curtain along the boundary between the first region and the second region.

3. The heating method according to claim 2, wherein the first region includes an edge of the blank, and

wherein the jet of cooling medium applied to the first region is caused to flow off the edge of the blank.

4. The heating method according to claim 2, wherein the first region is a closed region surrounded by the second region, and

wherein the jet of cooling medium applied to the first region is caused to flow from a circumference of the first region toward a center of the first region.

5. The heating method according to claim 1, wherein the partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank.

6. The heating method according to claim 5, wherein the partition member is provided to contact the at least one of the front surface and the back surface.

7. The heating method according to claim 5, wherein the first region includes an edge of the blank, and

wherein the partition member extends alongside the edge of the blank to cause the jet of cooling medium applied to the first region to flow off the edge of the blank.

8. The heating method according to claim 5, wherein the first region is a closed region surrounded by the second region, and

wherein the partition member has a cylindrical shape to cause the jet of cooling medium applied to the first region to flow from a center of the first region toward a circumference of the first region.

9. The heating method according to claim 8, wherein an inner cylinder is inserted inside the partition member to apply the jet of cooling medium toward the center of the first region through the inner cylinder.

10. The heating method according to claim 1, wherein the jet of cooling medium is applied in a state in which the first region is supported on at least one of the front surface and the back surface to suppress bending of the blank.

11. The heating method according to claim 10, wherein the first region is point-supported at one or more locations on at least one of the front surface and the back surface.

12. The heating method according to claim 1,

wherein the blank is rectangular and has a constant sectional area along a longitudinal direction of the blank, and

wherein the heating of the blank by the direct resistance heating comprises applying electric current to pass through the blank via a pair of electrodes fixed at longitudinal ends of the blank.

13. The heating method according to claim 1, wherein the blank is non-rectangular and has a sectional area monotonously decreasing along a longitudinal direction of the blank from a first end of the blank to a second end of the blank, and wherein the heating of the blank by the direct resistance heating comprises:

placing a pair of electrodes on the first end of the blank; and

moving one of the electrodes in the longitudinal direction toward the second end of the blank while applying electric current to pass through a portion of the blank between the pair of electrodes.

14. The heating method according to claim 13, wherein the application of the jet of cooling medium to the first region is started after the one of the electrodes has passed the first region.

15. The heating method according to claim 1, wherein the temperature of the first region is kept lower than an Ac1 transformation point of the blank while heating the second region to be equal to or higher than an Ac3 transformation point of the blank.

16. A method for manufacturing a hot-pressed product, the method comprising:

heating the blank by the method according to claim 1;

press-forming the heated blank by a press die; and

cooling the blank inside the press die to quench the second region.