Patent Publication Number: US-11655515-B2

Title: Far-infrared radiation heating furnace for steel sheet for hot stamping

Description:
TECHNICAL FIELD 
     The present invention relates to far-infrared radiation heating furnace for steel sheets for hot stamping, and in particular to a far-infrared radiation heating furnace to heat the steel sheets for hot stamping to a temperature for example ranging from the Ac 3  temperature to 950° C. 
     BACKGROUND ART 
     High strength steel sheets are widely used as a blank for making components of an automobile body in order to achieve both a further improvement in the strength, stiffness, and collision safety of the automobile body and an improvement in the fuel economy resulting from the reduced weight of the body. However, the press-formability of steel sheets decreases with increasing strength. As a result, high strength press-formed articles having a desired shape may not be produced. 
     In recent years, hot press-forming methods (also referred to as hot stamping methods) have been utilized as methods for press-forming components of an automobile body. In hot press-forming methods, a steel sheet (blank) for hot stamping to be press-formed is heated to a temperature equal to or greater than the Ac 3  temperature, and immediately after that, is subjected to forming and rapid cooling by a pressing die to be quenched (also referred to as die quenching). In this manner, high strength press-formed articles having a desired shape are produced. 
     Production of high strength hot press-formed articles in large volumes by a hot press-forming method requires use of a heating furnace for heating steel sheets for hot stamping. Inventions relating to such heating furnaces have been proposed heretofore. 
     Patent Document 1 discloses a multi-stage heating furnace. The multi-stage heating furnace includes a plurality of accommodation spaces for accommodating a plurality of steel sheets for hot stamping. The plurality of accommodation spaces are aligned in a vertical direction so as to be horizontal to each other. Means for transferring the steel sheets for hot stamping during heating are provided in the plurality of accommodation spaces. 
     Patent Document 2 discloses a multi-stage heating furnace that includes a box-shaped body and a heat source. Heating chambers are formed within the body. The heat source heats the insides of the chambers to about 900° C. This multi-stage heating furnace is capable of heating a plurality of steel sheets for hot stamping simultaneously and discharging the heated steel sheets for hot stamping separately. 
     Patent Document 3 discloses a multi-stage heating furnace that includes a body. Heating chambers to be heated by heat sources are provided within the body. Multiple-staged openings arranged in a vertical direction are provided in the front wall of the body. An opening and closing door is provided for each opening at each stage. 
     Furthermore, Patent Document 4 discloses a heat treatment method. The heat treatment method includes a first step and a second step. In the first step, a steel sheet for hot stamping is heated to an alloying temperature. In the second step, a first region of the steel sheet for hot stamping is held at a temperature equal to or greater than the A 3  transformation temperature utilizing thermal energy imparted in the first step while depriving a second region of the steel sheet for hot stamping of thermal energy. As a result, the second region of the steel sheet for hot stamping cools to a temperature equal to or less than the A 1  transformation temperature. This heat treatment method can effectively utilize thermal energy imparted in the alloying process and shorten the time for heat treatment. 
     The heating furnaces disclosed by Patent Documents 1 to 4 use a gas burner, an electric coil heater, a radiant tube, an electromagnetic heater, or another type of heater as the heat source for steel sheets for hot stamping. 
     These heating furnaces need to meet the following requirements: rapid and uniform heating of the steel sheet for hot stamping over all regions to a high temperature range of equal to or greater than the Ac 3  temperature (e.g., from 850 to 950° C.); an improvement in the ability for mass production; and minimization of the area for installation. In recent years, heating furnaces utilizing a far-infrared radiation heater as its heat source have been increasingly used. Heating furnaces of this type have the characteristics a to c listed below: 
     (a) capable of uniformly heating a steel sheet for hot stamping; 
     (b) capable of being compact by virtue of the vertically extending multi-stage configuration; and 
     (c) having a thin planar shape and being capable of heating a steel sheet for hot stamping at both sides thereof. 
     Patent Document 5 discloses a multi-stage heating furnace using a flexible far-infrared radiation heater as its heat source. The flexible far-infrared radiation heater is constructed of numerous insulators arranged in rows and knitted together to form a flexible panel. The numerous insulators have slits for receiving a resistive heating conductor. A heating conductor that emits far-infrared radiation is inserted and provided in the slits. 
     LIST OF PRIOR ART DOCUMENTS 
     Patent Document 
     Patent Document 1: JP2007-298270A 
     Patent Document 2: JP2008-291284A 
     Patent Document 3: JP2008-296237A 
     Patent Document 4: JP5197859B 
     Patent Document 5: JP2014-34689A 
     SUMMARY OF INVENTION 
     Technical Problem 
     As illustrated in FIG. 2 of Patent Document 4, it is desirable that transport of the steel sheet for hot stamping, from the multi-stage type heating furnace to a stamping machine, be carried out by a PTP-controlled industrial robot gripping the steel sheet. The industrial robot is provided between the multi-stage type heating furnace and the stamping machine. This enables smooth and rapid transport of the steel sheet for hot stamping from the multi-stage type heating furnace to the stamping machine and therefore increases the productivity for mass production of hot-stamped articles by hot stamping. 
     However, the temperature within the multi-stage type heating furnace during operation reaches 850 to 950° C. If the thermal insulation properties of the furnace body of the multi-stage type heating furnace are insufficient, the outer wall of the furnace body and the internal structural parts (structural parts made of metal in particular) mounted within the furnace significantly deform as a result of thermal expansion. 
     Even if the thermal insulation properties of the furnace body are enhanced, hot spots and cold spots inevitably occur in the furnace body and the internal structural parts. The difference in thermal expansion between the hot spots and the cold spots causes repeated thermal stress to the furnace body. As a result, plastic deformation accumulates in the furnace body and the plastic deformation of the furnace body progresses. The following problems exist: 
     (a) When the furnace body thermally expands or irreversibly plastically deforms during operation, the height position of the exit for discharging the steel sheet for hot stamping out of the furnace changes. As a result, the industrial robot, which moves along a constant trajectory under PTP control, becomes unable to insert the gripping tool (end effector) attached to the end of its manipulator from the exit to grip the steel sheet for hot stamping, and therefore stable operation becomes impossible.
 
(b) The service lives of the refractories and internal structural parts mounted within the furnace decrease with thermal deformation of the furnace body, and damage such as cracking occurs in the furnace body, which results in increased maintenance costs of the multi-stage type heating furnace.
 
     Accordingly, it is necessary to reliably minimize thermal expansions and plastic deformations of the outer wall of the furnace body and internal structural parts, and an irreversible deformation of the furnace body. However, neither Patent Document 4 nor Patent Document 5 discloses any means for reliably minimizing them. 
     The present invention is intended to provide a far-infrared radiation heating furnace for steel sheets for hot stamping capable of solving the problems of the conventional art. 
     Solution to Problem 
     The present invention is as set forth below. 
     (1) A far-infrared radiation heating furnace for a steel sheet for hot stamping, the far-infrared radiation heating furnace including: a heating unit and a metallic furnace body frame disposed around the heating unit, the heating unit including: blocks made of a thermal insulation material, the blocks being disposed around a horizontal plane of a space for accommodating the steel sheet for hot stamping; and far-infrared radiation heaters positioned above and below the steel sheet for hot stamping to heat the steel sheet for hot stamping to a temperature ranging from the Ac 3  transformation temperature to 950° C. for example, the furnace body frame including spacers that space the heating unit apart from the furnace body frame and support (receive) the heating unit.
 
(2) The far-infrared radiation heating furnace according to item 1 for a steel sheet for hot stamping, wherein each of the far-infrared radiation heaters is a planar structure formed of a plurality of insulator elements arranged in rows, the insulator elements being made of sintered form of far-infrared radiation emitting ceramics, and wherein the plurality of insulator elements are coupled together by a heating wire so as to be capable of being displaced from each other so that the far-infrared radiation heater has flexibility, the heating wire being inserted in heating wire through holes formed in the respective insulator elements.
 
(3) The far-infrared radiation heating furnace according to item 1 or 2 for a steel sheet for hot stamping, wherein the space has an approximately rectangular outer shape in a horizontal plane and wherein the blocks include fixed blocks fixedly disposed at respective four sides of the rectangular outer shape and cover blocks disposed at respective two opposing sides of the four sides so as to be openable and closable.
 
(4) The far-infrared radiation heating furnace according to any one of items 1 to 3 for a steel sheet for hot stamping, wherein the heating unit includes a metallic furnace shell (iron shell) that encloses outer peripheries of the fixed blocks to retain the fixed blocks.
 
(5) The far-infrared radiation heating furnace according to any one of items 1 to 4 for a steel sheet for hot stamping, wherein the heating unit is a plurality of heating units arranged in a vertical direction.
 
(6) The far-infrared radiation heating furnace according to item 5 for a steel sheet for hot stamping, wherein the plurality of heating units are spaced apart from each other.
 
     Advantageous Effects of the Invention 
     In the present invention, the heating unit, which has a space in which the ambient temperature reaches 850 to 950° C. during operation, is spaced apart from the furnace body frame and supported by spacers mounted to the furnace body frame. This can prevent the heating unit from contacting the frame. As a result, the furnace body frame is free of thermal expansion or thermal stress, which prevents deformation of the furnace body frame due to thermal expansion or thermal contraction, repetitive thermal stress loading, unstable operation, shortened service life of the blocks (refractories) that are made of a thermal insulation material, and further, damages such as cracking of the furnace body frame. Consequently, the maintenance cost of the far-infrared radiation heating furnace can be significantly reduced and the capacity utilization of the far-infrared radiation heating furnace can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 ( a )  is a plan view of an insulator element used in a flexible far-infrared radiation heater;  FIG.  1 ( b )  is a front view of the insulator element;  FIG.  1 ( c )  is a plan view of the flexible far-infrared radiation heater;  FIG.  1 ( d )  is a front view illustrating an array of insulators knitted together to look like a bamboo blind with a heating wire passed therethrough;  FIG.  1 ( e )  is a side view of  FIG.  1 ( c ) ; and  FIG.  1 ( f )  is an illustration of the insulator elements arranged such that adjacent rows are offset by half the length of the preceding row. 
         FIG.  2    is an overall view of a far-infrared radiation multi-stage type heating furnace according to the present invention. 
         FIG.  3    presents illustrations of the far-infrared radiation multi-stage type heating furnace according to the present invention:  FIG.  3 ( a )  is an illustration of the exterior of the far-infrared radiation multi-stage type heating furnace;  FIG.  3 ( b )  is an illustration of a heating unit;  FIG.  3 ( c )  is a cross-sectional view taken along the line A-A of  FIG.  3 ( b ) ;  FIG.  3 ( d )  is an illustration of the heating unit with its cover block removed;  FIG.  3 ( e )  is a cross-sectional view taken along the line B-B of  FIG.  3 ( b ) ; and  FIG.  3 ( f )  is a perspective view of a steel sheet support member. 
         FIG.  4    is an illustration of the far-infrared radiation multi-stage type heating furnace. 
         FIG.  5    is a front view of the far-infrared radiation multi-stage type heating furnace with a ceiling unit illustrated therein. 
         FIG.  6 ( a )  is an illustration of a heater support member in a heating unit;  FIG.  6 ( b )  is a top view of the heating unit;  FIG.  6 ( c )  is an illustration depicting a positional relationship between the heater and the steel sheet for hot stamping; and  FIG.  6 ( d )  is an illustration of an alternative heater support member in a heating unit. 
         FIG.  7 ( a )  is an illustration of an exemplary steel sheet support member;  FIG.  7 ( b )  is a cross-sectional view of the steel sheet support member; and  FIGS.  7 ( c ) to  7 ( f )  are each an illustration of an alternative example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention will be described with reference to the accompanying drawings. 
     1. Configuration of Furnace Body Frame  12   
       FIG.  2    is an overall view of a far-infrared radiation multi-stage type heating furnace  10  according to the present invention, illustrating exterior panels  11   a ,  11   b ,  11   c  and a furnace body frame  12 . 
       FIG.  3    presents illustrations of the far-infrared radiation multi-stage type heating furnace  10  according to the present invention.  FIG.  3 ( a )  is an illustration of the exterior of the far-infrared radiation multi-stage type heating furnace  10 ,  FIG.  3 ( b )  is an illustration of heating units  13 - 1  to  13 - 6 ,  FIG.  3 ( c )  is a cross-sectional view taken along the line A-A of  FIG.  3 ( b ) ,  FIG.  3 ( d )  is an illustration of the heating units  13 - 1  to  13 - 6  with the cover blocks  16   c ,  16   d  removed,  FIG.  3 ( e )  is a cross-sectional view taken along the line B-B of  FIG.  3 ( b ) , and  FIG.  3 ( f )  is a perspective view of a steel sheet support member  32 . 
       FIG.  4    is an illustration of the far-infrared radiation multi-stage type heating furnace  10  with only the heating units  13 - 1 ,  13 - 2  illustrated therein.  FIG.  5    is a front view of the far-infrared radiation multi-stage type heating furnace  10  with a ceiling unit  19  illustrated therein. 
     As illustrated in  FIGS.  2  to  5   , the far-infrared radiation multi-stage type heating furnace  10  includes heating units  13 - 1  to  13 - 6 , the ceiling unit  19 , and the furnace body frame  12 . 
     The heating units  13 - 1  to  13 - 6  each have a space for accommodating steel sheets for hot stamping  15 - 1  to  15 - 6 , respectively. The space is formed by blocks  16   a ,  16   b ,  16   c ,  16   d ,  16   e ,  16   f  made of a thermal insulation material that are disposed around the space. The heating units  13 - 1  to  13 - 6  respectively accommodate steel sheets for hot stamping  15 - 1  to  15 - 6  supported approximately horizontally within the spaces. 
     The heating units  13 - 1  to  13 - 6  are a plurality of (six in the case of the far-infrared radiation multi-stage type heating furnace  10  illustrated in  FIGS.  2  to  5   ) heating units that are stacked in a vertical direction. 
     The heating units  13 - 1  to  13 - 6  include far-infrared radiation heaters  14 - 1  to  14 - 6 , respectively, and the ceiling unit  19  includes a far-infrared radiation heater  14 - 7 . The far-infrared radiation heaters  14 - 1  to  14 - 7  are positioned above and below the steel sheets for hot stamping  15 - 1  to  15 - 6  accommodated in the spaces. Specifically, the far-infrared radiation heaters  14 - 1 ,  14 - 2  are respectively positioned above and below the steel sheet for hot stamping  15 - 1 , the far-infrared radiation heaters  14 - 2 ,  14 - 3  are respectively positioned above and below the steel sheet for hot stamping  15 - 2 , the far-infrared radiation heaters  14 - 3 ,  14 - 4  are respectively positioned above and below the steel sheet for hot stamping  15 - 3 , the far-infrared radiation heaters  14 - 4 ,  14 - 5  are respectively positioned above and below the steel sheet for hot stamping  15 - 4 , the far-infrared radiation heaters  14 - 5 ,  14 - 6  are respectively positioned above and below the steel sheet for hot stamping  15 - 5 , and the far-infrared radiation heaters  14 - 6 ,  14 - 7  are respectively positioned above and below the steel sheet for hot stamping  15 - 6 . 
     Thus, the far-infrared radiation heaters  14 - 1  to  14 - 7  heat corresponding ones of the steel sheets for hot stamping  15 - 1  to  15 - 6  from above and below to a temperature ranging from the Ac 3  transformation temperature to 950° C. for example. 
     The far-infrared radiation heaters  14 - 1  to  14 - 7  are flexible planar far-infrared radiation heaters (hereinafter also referred to as “flexible far-infrared radiation heater”) as disclosed in Japanese Registered Utility Model Publication No. 3056522. 
     The far-infrared radiation heaters  14 - 1  to  14 - 7  includes insulator elements  1  as illustrated in  FIGS.  1 ( a ) to  1 ( f ) . The insulator elements  1  are made of sintered form of far-infrared radiation emitting ceramics such as for example Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , SiC, CoO, Si 3 N 4 . The far-infrared radiation heaters  14 - 1  to  14 - 7  are each a planar structure formed of a plurality of insulator elements  1  arranged in rows. The plurality of insulator elements  1  are coupled together so as to be capable of being displaced from each other by a heating wire  4  inserted in heating wire through holes  2  formed in the respective insulator elements  1 . The far-infrared radiation heaters  14 - 1  to  14 - 7  are flexible far-infrared radiation heaters having flexibility. 
     The far-infrared radiation heaters  14 - 1  to  14 - 7  generate heat from the inside of the insulator elements  1  upon application of current through the heating wire provided within the insulator elements  1 . As a result, a high rate of temperature increase is achieved in the far-infrared radiation heaters  14 - 1  to  14 - 7 . The far-infrared radiation heaters  14 - 1  to  14 - 7  are capable of performing heating at both sides thereof and therefore achieve reduced heat loss. The far-infrared radiation heaters  14 - 1  to  14 - 7  emit high-density far-infrared radiation energy and therefore provide for enhanced heating efficiency. The far-infrared radiation heaters  14 - 1  to  14 - 7  are flexible, and therefore are less likely to have cracks or deformation at high temperatures and the size thereof can be easily set ranging from a small size to a large size. In addition, the far-infrared radiation heaters  14 - 1  to  14 - 7  are thin, and further, capable of heating both sides of the steel sheets for hot stamping  15 - 1  to  15 - 6 . 
     Hence, the far-infrared radiation heaters  14 - 1  to  14 - 7  are preferable as heaters that are respectively provided in the heating units  13 - 1  to  13 - 6  and ceiling unit  19  of the multi-stage heating furnace and required to exhibit high heating efficiency and excellent furnace temperature controllability. 
     The furnace body frame  12  is a frame made of metal (carbon steel for example) disposed so as to surround the heating units  13 - 1  to  13 - 6  and the ceiling unit  19 . 
     As illustrated in  FIG.  3 ( b ) , the spaces of the heating units  13 - 1  to  13 - 6  each have an approximately rectangular outer shape in a horizontal plane. The heating units  13 - 1  to  13 - 6  each include blocks  16   a ,  16   b ,  16   c ,  16   d ,  16   e ,  16   f  made of a thermal insulation material that surround the periphery of each space in a horizontal plane. 
     The heating units  13 - 1  to  13 - 6  are each constituted by fixed blocks  16   a ,  16   b , fixed blocks  16   e ,  16   f , and cover blocks  16   c ,  16   d . The fixed blocks  16   a ,  16   b  are fixedly placed at two opposing sides of the rectangular shape. The fixed blocks  16   a ,  16   b  have an approximately rectangular solid outer shape. The fixed blocks  16   e ,  16   f  are fixedly placed at the remaining two opposing sides. The fixed blocks  16   e ,  16   f  have an approximately rectangular solid outer shape. The cover blocks  16   c ,  16   d  are disposed to engage with the fixed blocks  16   e ,  16   f  so as to be openable and closable. 
     Opening and closing of the cover blocks  16   c ,  16   d  is actuated by a suitable opening and closing mechanism (not illustrated). In a closed state the cover blocks  16   c ,  16   d  are in contact with the front faces, upper faces, and lower faces of the fixed blocks  16   e ,  16   f  and end faces in the longitudinal direction of the fixed blocks  16   a ,  16   b . In this manner, the cover blocks  16   c ,  16   d , together with the fixed blocks  16   a ,  16   b  and the fixed blocks  16   e ,  16   f  thermally insulate the internal spaces of the heating units  13 - 1  to  13 - 6  from the outside. 
     The heating units  13 - 1  to  13 - 6  each include metal (steel for example) furnace shells (iron shells)  18 , which surround peripheries of the fixed blocks  16   a ,  16   b  and fixed blocks  16   e ,  16   f  and retain the fixed blocks  16   a ,  16   b  and fixed blocks  16   e ,  16   f.    
     Spacers  17 - 1  to  17 - 7  made from steel for example are mounted at heights that conform to the placement heights of the heating units  13 - 1  to  13 - 6  and ceiling unit  19  in the furnace body frame  12  by suitable means such as for example welding or fastening. It suffices if the spacers  17 - 1  to  17 - 7  exhibit heat resistance to a degree sufficient to avoid deformation that may be caused by heat transmitted from the fixed blocks  16   a ,  16   b , and thus the spacers may be formed from a metal material other than steel. 
     The fixed blocks  16   a ,  16   b  of the heating units  13 - 1  to  13 - 6  and ceiling unit  19  are supported (received) by the spacers  17 - 1  to  17 - 7  interposed between them and the furnace body frame  12 . The fixed blocks  16   a ,  16   b  are in contact with the spacers  17 - 1  to  17 - 7  but not in contact with the furnace body frame  12 . 
     As described above, the heating units  13 - 1  to  13 - 6  and ceiling unit  19 , which have the spaces in which the ambient temperature reaches 850 to 950° C. during operation, contact the spacers  17 - 1  to  17 - 7  but do not contact the furnace body frame  12 . As a result, the heat of the heating units  13 - 1  to  13 - 6  and ceiling unit  19  does not transfer to the furnace body frame  12 . Consequently, thermal expansion of the furnace body frame  12  is prevented. 
     For example, the amount of displacement of the furnace body frame  12  at the height at the center in the height direction of the uppermost heating unit  13 - 6  during operation of the far-infrared radiation multi-stage type heating furnace  10  is approximately 0.4 to 0.5 mm. Thus, deformation of the furnace body frame  12  due to thermal expansion is substantially eliminated. 
     As a result, the furnace body frame  12  is free of thermal stress, and deformation of the furnace body frame  12  due to thermal expansion or thermal contraction, repetitive thermal stress loading, unstable operation, shortened life of the refractories that are the thermal insulation materials  16  and also damages such as cracking of the furnace body frame  12  are prevented. This results in a significant reduction in the maintenance cost and an improvement in capacity utilization of the far-infrared radiation multi-stage type heating furnace  10 . 
     2. Support Members  24 - 1 ,  24 - 2  for Far-Infrared Radiation Heater  14 - 1   
       FIG.  6 ( a )  is an illustration of a heater support member (hereinafter simply referred to as “support member”)  24 - 1  for the far-infrared radiation heater  14 - 1  in the heating unit  13 - 1 ;  FIG.  6 ( b )  is a top view of the heating unit  13 - 1 ;  FIG.  6 ( c )  is an illustration depicting a positional relationship between the far-infrared radiation heater  14 - 1  and the steel sheet for hot stamping  15 - 1 ; and  FIG.  6 ( d )  is an illustration of an alternative support member  24 - 2  for the far-infrared radiation heater  14 - 1  in the heating unit  13 - 1 . 
     As illustrated in  FIGS.  6 ( a ) to  6 ( c ) , the far-infrared radiation heater  14 - 1  is supported by the support member  24 - 1  horizontally in a manner to prevent deflection. The support member  24 - 1  is made up of first metal strips  26  and support pieces  27 . The first metal strip  26  is formed from a nickel-based heat resistant alloy for example. A plurality of (four in  FIGS.  6 ( a ) to  6 ( d ) ) the first metal strips  26  are provided in alignment in a first direction. The support pieces  27  support the first metal strips  26 . The support pieces  27  are sheets formed of a stainless steel for example. 
     As illustrated in  FIG.  6 ( b ) , the far-infrared radiation heater  14 - 1  is received by the four first metal strips  26  to be disposed approximately horizontally. The far-infrared radiation heater  14 - 1  is disposed within the region surrounded by the fixed blocks  16   a ,  16   b ,  16   e ,  16   f  in a horizontal plane. 
     The four first metal strips  26  are all provided such that their strong axis direction (direction in which the flexural rigidity (area moment of inertia and section modulus) is greater) approximately corresponds to the direction of gravity. This minimizes deflection of the first metal strips  26 . 
     The first metal strips  26  are fitted into respective slits or holes  27   a  (slits are illustrated in the figure) formed in the support pieces  27  so as to provide clearance in the slits or holes, and are supported. This configuration allows the first metal strips  26  to be supported by the support pieces  27  so as to be expandable and contractible in a longitudinal direction by thermal expansion or thermal contraction. As a result, the first metal strips  26  are free of thermal stress caused by temperature changes. 
     Preferably, the first metal strips  26  receive the far-infrared radiation heater  14 - 1  via an insulating member (made of Al 2 O 3  for example) having thermally insulating properties and insulating properties. An example of such insulating member is one having a cross sectional shape with a groove and which is attached to the first metal strip  26  by being fitted into the upper end of the first metal strip  26 . 
       FIG.  6 ( d )  illustrates an alternative support member  24 - 2 , which may be constituted by a plurality of (two in  FIG.  6 ( d ) ) second metal strips  28  together with the first metal strips  26 . The plurality of second metal strips  28  are provided in alignment in a second direction intersecting (orthogonal in the illustrated example) the first direction in which the first metal strips  26  are oriented. The second metal strips  28  are formed of a stainless steel for example. 
     Similarly to the first metal strips  26 , the second metal strips  28  are provided such that their strong axis direction approximately corresponds to the direction of gravity. The second metal strips  28  are fitted into respective slits  28   a  formed in the first metal strips  26  so as to provide clearance in the slits, and are supported. This configuration allows the second metal strips  28  to be supported by the first metal strips  26  so as to be expandable and contractible in a longitudinal direction by thermal expansion or thermal contraction. As a result, the second metal strips  28  are free of thermal stress caused by temperature changes. 
     As illustrated in  FIG.  6 ( b ) , through holes  29  are formed in the thermal insulation materials  16   e ,  16   f . The first metal strips  26  pass through the through holes  29  of the thermal insulation materials  16   e ,  16   f  and are supported by the support pieces  27 . The support pieces  27  are located outside the steel sheet accommodating regions surrounded by the fixed blocks  16   a ,  16   b ,  16   e ,  16   f , which are the thermal insulation materials. The outer portions of the first metal strips  26  protruding from the thermal insulation materials  16   e ,  16   f  become hot and therefore preferably a thermal insulation process is applied to the outer portions of the first metal strips  26  by enclosing them with thermal insulation materials or covers for example. 
     As described above, outside the thermal insulation materials  16   a ,  16   b ,  16   e ,  16   f , the support pieces  27  support the plurality of first metal strips  26  or the plurality of first metal strips  26  and plurality of second metal strips  28 . 
     The first metal strips  26  (1000 mm in overall length) formed from Inconel (registered trademark) were placed at predetermined locations in the heating unit  13 - 1  of the far-infrared radiation multi-stage type heating furnace  10  in the manner described above, and the far-infrared radiation multi-stage type heating furnace  10  was used 24 hours a day for one month. The result was that the amount of vertically downward deflection at the longitudinal center of the first metal strips  26  was less than 0.1 mm. This demonstrates that the first metal strips  26  are able to support the far-infrared radiation heater  14 - 1  sufficiently flatly without causing deflection. 
     As described above, the support members  24 - 1 ,  24 - 2  are capable of supporting the far-infrared radiation heater  14 - 1  without causing deflection despite their small projected areas, by means of the first metal strips  26  or by means of the first metal strips  26  and the second metal strips  28 , even during heating at 850° C. or above. 
     Thus, the present invention reduces the frequency or number of times of maintenance of the far-infrared radiation heater  14 - 1  having flexibility, and thereby achieves all of the following: a significant reduction in the maintenance cost of the far-infrared radiation multi-stage type heating furnace  10 ; an improvement in capacity utilization of the far-infrared radiation multi-stage type heating furnace  10 ; retention and improvement of heating uniformity of steel sheets for hot stamping  15 - 1 ; and size reduction of the far-infrared radiation multi-stage type heating furnace  10  due to its multi-stage configuration. 
     In the exemplary embodiment illustrated in  FIG.  6 ( c ) , the steel sheet for hot stamping  15 - 1  is supported by round tubes  35  in line contact. However, the present invention is not limited to this embodiment. For example, the steel sheet for hot stamping  15 - 1  may be supported by a variety of below-described steel sheet support members  31  to  34  illustrated in  FIGS.  7 ( a ) to  7 ( f ) . 
     3. Steel Sheet Support Members  30  to  34  for Steel Sheet for Hot Stamping  15 - 1   
       FIG.  7 ( a )  is an illustration of an exemplary steel sheet support member  30 ;  FIG.  7 ( b )  is a cross-sectional view of the steel sheet support member  30 ; and  FIGS.  7 ( c ) to  7 ( f )  are illustrations of alternative exemplary steel sheet support members  31  to  34 . 
     For example, any of the steel sheet support members  30  to  34  each made of a heat resistant alloy can be mounted to the heating unit  13 - 1  of the far-infrared radiation multi-stage type heating furnace  10 . The steel sheet support members  30  to  34  support the steel sheet for hot stamping  15 - 1  by point contact or by line contact with the steel sheet for hot stamping  15 - 1 . 
     In the present invention, “point contact” refers to contact by a contact surface, for example of a pin, formed on its front edge and having an outside diameter of approximately 6 mm or less, or contact by the outer circumferential surface for example of a ring having a cross-sectional diameter of approximately 7 mm or less, and “line contact” refers to contact by a contact surface, for example of a sheet, formed on its edge by beveling or other means and having a width of approximately 3 mm or less, contact by the outer circumferential surface of a steel bar having an outside diameter of approximately 6 mm or less, or contact by the outer circumferential surface for example of a thin-wall round tube having an outside diameter of approximately of 20 mm or less. By virtue of the point contact or line contact, dispersion of a coating at the contact region is prevented in the case where the steel sheet for hot stamping is a zinc-coated steel sheet. 
     Examples of steel sheet support members that provide a point contact with the steel sheet for hot stamping  15 - 1  include: a rectangular tube  30  in a laterally vertical position having upright pins  30   a  provided on its surface (see  FIGS.  7 ( a ) and  7 ( b ) ); a rectangular bar  34  in a laterally vertical position having upright pins  34   a  provided on its surface (see  FIG.  7 ( f ) ); or a round tube  32  having, on its outer circumferential surface, a wire  32   a  of a circular cross section wound therearound (see  FIG.  7 ( d ) ). In these instances, it is preferred that the bodies of the rectangular tube  30  and the rectangular bar  34  are made of a super heat resistant alloy such as Inconel for example and that the pins  30   a ,  34   a  provided on the bodies of the rectangular tube  30  and the rectangular bar  34 , respectively, are made of ceramics (e.g., Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , SiC, CoO, Si 3 N 4 ), which are non-metallic materials, in order to ensure the quality of the steel sheet for hot stamping. 
     Examples of steel sheet support members that provide a line contact with the steel sheet for hot stamping  15 - 1  include: a triangular tube  31  having an equilateral triangular cross section (see  FIG.  7 ( c ) ); and a sheet member  33  in a laterally vertical position having an acute angle portion  33   a  disposed on its surface (see  FIG.  7 ( e ) ). 
     Similarly to the first metal strips  26  and the second metal strips  28 , it is preferred that the steel sheet support members  30  to  34  are supported by the support pieces so as to be expandable and contractible in a longitudinal direction by thermal expansion or thermal contraction in order to prevent thermal stress caused by temperature change. For example, the steel sheet support members  30  to  34  are supported by support pieces mounted to the upper surfaces of the thermal insulation materials  16   e ,  16   f  so as to be expandable and contractible in a longitudinal direction by thermal expansion or thermal contraction. 
     If the steel sheet support members  30  to  34  have been deflected in use, they may be turned upside down and relocated so as to project upwardly. 
     The rectangular tubes  30  formed from Inconel having a cross-sectional shape as illustrated in  FIG.  7 ( b )  (800 mm in overall length) were placed as steel sheet support members at predetermined locations in the heating unit  13 - 1  of the far-infrared radiation multi-stage type heating furnace  10  in the manner described above, and the far-infrared radiation multi-stage type heating furnace  10  was used 24 hours a day for one month. The result was that the amount of vertically downward deflection at the longitudinal center of the rectangular tubes  30  was less than 0.2 mm. This demonstrates that the steel sheet for hot stamping  15 - 1  can be supported at substantially constant positions. 
     In addition, the difference between the maximum temperature and the minimum temperature between regions of the steel sheet for hot stamping  15 - 1 , which was heated to 900° C., was approximately 7° C. Thus, sufficiently uniform heating of the steel sheet for hot stamping  15 - 1  is achieved. 
     Other steel sheet support members than the steel sheet support members  30  to  34  illustrated in  FIGS.  7 ( a ) to  7 ( f )  may be used. Examples of other steel sheet support members that may be used include: a rectangular tube formed by integrating the pins with the rectangular tube  30  in a laterally vertical position or a rectangular bar formed by integrating the pins with the rectangular bar  34  in a laterally vertical position; a rectangular tube having, on its upper surface and lower surface, alternating recesses and projections that are formed by providing cutouts in parts of the upper surface and lower surface of the rectangular tube  30  in a laterally vertical position; a member having, on its upper surface, alternating recesses and projections that are formed by providing cutouts in parts of the upper surface of a member having a channel-shaped cross section in a laterally vertical position; and a rectangular tube having, on its upper surface and lower surface, successive round holes that are formed by providing round holes in the upper surface and lower surface of the rectangular tube  30  in a laterally vertical position. 
     The present invention significantly minimizes thermal deformation and other damage to the steel sheet support members  30  to  34 . As a result, the present invention achieves a significant reduction in the maintenance cost of the far-infrared radiation multi-stage type heating furnace  10 , an improvement in capacity utilization of the far-infrared radiation multi-stage type heating furnace  10  and heating uniformity therein; and size reduction of the far-infrared radiation multi-stage type heating furnace  10  by virtue of the multi-stage configuration. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  far-infrared radiation heating furnace 
               12  furnace body frame 
               13 - 1  to  13 - 6  heating unit 
               14 - 1  to  14 - 7  far-infrared radiation heater 
               16   a  to  16   f  block made of a thermal insulation material 
               17 - 1  to  17 - 7  spacer 
               19  ceiling unit