Heating method for hot press forming

Abstract

The present invention relates to a method for heating a steel sheet or a blank for hot press forming, in which the heating method comprises: a step of preparing a steel sheet or a blank; and a heating step in which the steel sheet or the blank is passed through a heating furnace including two or more zones, wherein, in the heating step, the temperature and residence time of each zone are controlled such that Hsum, which is represented by relational expression 1, is 1.00 or more.

Description

Heating method for hot press forming

[0001] The present invention relates to a heating method that can be applied during hot press forming.

[0002] Hot Press Forming (HPF) is a process for manufacturing high-strength metal parts, primarily used to create complex shapes using high-strength steel sheets. This technology plays a significant role, particularly in the automotive industry, and is widely utilized in the production of lightweight yet high-strength vehicle body components.

[0003] The hot press forming process first heats a metal sheet to a high temperature of over 900°C to increase the ductility of the metal and improve formability. Subsequently, the metal sheet is placed into a mold and pressed to form it into a desired shape, after which it is rapidly cooled to form a high-strength martensite structure. Parts produced through this process can achieve lightweighting while maintaining high strength.

[0004] This hot press forming process not only enables the high-precision forming of parts with complex shapes but is also essential for manufacturing high-strength components that enhance vehicle crash safety. Consequently, as it allows for the simultaneous achievement of the dual benefits of high strength and lightweighting, it has established itself as a critical technology in various metal forming fields as well as the automotive industry.

[0005] According to one embodiment of the present invention, a heating method capable of improving productivity and stability during hot press forming can be provided.

[0006] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0007] A heating method according to one embodiment of the present invention is a heating method for a steel plate or blank for hot press forming, comprising: a step of preparing said steel plate or blank; and a heating step of passing said steel plate or blank through a heating furnace comprising two or more zones, wherein in the heating step H represented by the following relational expression 1 sum The temperature and residence time of each zone can be adjusted so that this is 1 or more.

[0008] [Relationship 1]

[0009]

[0010] Here, n represents the number of zones included in the furnace, j represents the sequence number of the corresponding zone counted sequentially from the furnace inlet side, and H j It can be defined by the following relationship 2.

[0011] [Relationship 2]

[0012]

[0013] Here, when j is 1, H j_own is given by the following relationship 3, where H when j is 2 or greater j_own is given by the following relationship 4, H j_p&f is defined by the following relation 5.

[0014] [Relationship 3]

[0015]

[0016] [Relationship 4]

[0017]

[0018] [Relationship 5]

[0019]

[0020] Here, t j_own represents the actual time (in minutes) that the above steel plate or blank stays in the relevant area, and t j_p&frepresents the sum of the time (in minutes) spent in the preceding and succeeding zones when a steel plate staying in the corresponding zone temporarily moves (oscillates) to the zones before and after it. Also, t j_o can be defined by the following relationship 6, and t j_o_p&f It can be defined by the following relational expressions 7 and 8.

[0021] [Relationship 6]

[0022]

[0023] [Relationship 7]

[0024]

[0025] [Relationship 8]

[0026]

[0027] Also, a j By the following relationships 9 and 10, b j is defined by the following relational expressions 11 and 12.

[0028] [Relationship 9]

[0029]

[0030] [Relationship 10]

[0031]

[0032] [Relationship 11]

[0033]

[0034] [Relationship 12]

[0035]

[0036] And, a j_p&f wa b j_p&f is calculated as the average temperature of the preceding and succeeding zones when the steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the zones before and after it, and a calculated from this j and b j It means.

[0037] Also, t min_j is defined by the following relation 13.

[0038] [Relationship 13]

[0039]

[0040] Here, t min_j_ref represents the minimum heating time for a standard steel plate or standard blank, which can be obtained by the following relationship 14.

[0041] [Relationship 14]

[0042]

[0043] Temp here j represents the set temperature of zone j.

[0044] Also, t min_j_thk is defined by the following relation 15.

[0045] [Relationship 15]

[0046]

[0047] Here, thk means the thickness (mm) of the steel plate or blank.

[0048] Also, f_t min_j_cw is defined by the following relation 16.

[0049] [Relation Equation 16]

[0050]

[0051] Here, CW is the plating amount on one side of the steel sheet or blank (g / m²) 2 It means ).

[0052] Also, f_t min_j_wi is defined by the following relation 17.

[0053] [Relation Equation 17]

[0054]

[0055] Here, WI represents the whiteness of the material surface measured by a colorimeter and has a value between 0 and 100.

[0056] According to a heating method according to another embodiment of the present invention, the above-described a j_p&f By the following equations 18 and 19, b j_p&f is defined by the following relational expressions 20 and 21.

[0057] [Relation Equation 18]

[0058]

[0059] [Relation Equation 19]

[0060]

[0061] [Relationship 20]

[0062]

[0063] [Relation Equation 21]

[0064]

[0065] Here, t min_j_p&f is defined by the following relation 22.

[0066] [Relation Equation 22]

[0067]

[0068] Here, tmin_j_ref_p&f is defined by the following relation 23, and f_tmin_j_cw_p&f is defined by the following relation 24.

[0069] [Relation Equation 23]

[0070]

[0071] [Relationship 24]

[0072]

[0073] Temp here j_p&fIt is defined as the average temperature of the preceding and succeeding zones when the steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the preceding and succeeding zones.

[0074] Meanwhile, according to a heating method according to another embodiment of the present invention, in the heating step described above, Hratio represented by the following relational formula 25 sum The temperature and residence time of each zone can be adjusted so that this is 1 or less.

[0075] [Relationship 25]

[0076]

[0077] Here, Hmax j_own is by the following relationship 26, and Hmax j_p&f is defined by the following relationship 27.

[0078] [Relation Equation 26]

[0079]

[0080] [Relation Equation 27]

[0081]

[0082] Here, t max_j By the following relationship 28, t max_j_p&f is defined by the following relation 29.

[0083] [Relation Equation 28]

[0084]

[0085] [Relation Equation 29]

[0086]

[0087] Here, tmax_j_ref_cw is given by the following relationships 30 and 31, t max_j_thk f_t by the following relationships 32 and 33. max_j_wiis defined by the following relation 34, tmax_j_ref_cw_p&f by the following relation 35, and tmax_j_thk_p&f by the following relation 36 and 37.

[0088] [Relationship 30]

[0089]

[0090] [Relationship 31]

[0091]

[0092] [Relationship 32]

[0093]

[0094] [Relationship 33]

[0095]

[0096] [Relationship 34]

[0097]

[0098] [Relationship 35]

[0099]

[0100] [Relation Equation 36]

[0101]

[0102] [Relation Equation 37]

[0103]

[0104] In addition, the above A is defined by the following relation 38, and the above B is defined by the following relation 39.

[0105] [Relation Equation 38]

[0106]

[0107] [Relation Equation 39]

[0108]

[0109] The aforementioned Temp j_p&f can be defined by the following relationship 40.

[0110] [Relationship 40]

[0111]

[0112] Here, x and y are natural numbers, and represent the x and y values ​​when the zones that are furthest from the zone are called zone (jx) and zone (j+y) among the zones that the steel plate moves to when it moves to the zone before and after the zone (j zone).

[0113] The aforementioned H sum It may be 1.64 or less.

[0114] The aforementioned steel plate or blank has different thickness, different plating amount, or different whiteness depending on the location, and at each location all H sum : Can satisfy 1 or more. In addition, each of the aforementioned locations is all Hratio sum : Can additionally satisfy conditions of 1 or less.

[0115] The steel plates or blanks described above are a plurality of steel plates or blanks having different thicknesses (thk), and each steel plate or blank is all H sum : Can satisfy conditions of 1 or more. In addition, each of the aforementioned steel plates or blanks is all Hratio sum : Can additionally satisfy conditions of 1 or less.

[0116] The steel plates or blanks described above are a plurality of steel plates or blanks having different plating amounts (CW), and each of the steel plates or blanks is H sum : Can satisfy conditions of 1 or more. In addition, each of the aforementioned steel plates or blanks is all Hratio sum : Can additionally satisfy conditions of 1 or less.

[0117] The steel plates or blanks described above are a plurality of steel plates or blanks having different whiteness (WI), and each steel plate or blank is H sum: Can satisfy conditions of 1 or more. In addition, each of the aforementioned steel plates or blanks is all Hratio sum : Can additionally satisfy conditions of 1 or less.

[0118] According to one aspect of the present invention, the above-described steel sheet or blank may comprise, in weight percent, C: 0.05 to 0.40, Si: 0.10 to 1.00 and Mn: 0.40 to 1.80, and the remainder being Fe and unavoidable impurities.

[0119] According to another aspect of the present invention, the above-described steel sheet or blank may comprise, in weight percent, C: 0.18 to 0.25, Si: 0.10 to 0.50 and Mn: 0.90 to 1.50, and the remainder being Fe and unavoidable impurities.

[0120] The single-sided plating weight (CW) of the aforementioned steel sheet or blank is 15 to 80 g / m² 2 It could be.

[0121] The thickness (thk) of the steel plate or blank described above may be 0.7 to 4.0 mm.

[0122] The thickness (thk) of the steel plate or blank described above may be 0.7 to 2.6 mm.

[0123] The whiteness (WI) of the steel plate or blank described above may be 35 to 90.

[0124] The heating method described above allows the residence time of the steel plate or blank in the first zone to be less than or equal to {1 / n+1 / (2n)} times the total residence time in the heating furnace.

[0125] The heating method described above defines the ratio of the cumulative residence time from the first zone to the corresponding zone relative to the total residence time in the heating furnace as a j When saying a j H at ≥0.3 sum_j ≥ a j It can satisfy.

[0126] The heating method described above defines the ratio of the cumulative residence time from the first zone to the corresponding zone relative to the total residence time in the heating furnace as a j When saying a j 0.25 < a compared to the average temperature of the zones corresponding to ≤0.25 j The average temperature of the zones corresponding to ≤0.5 may be greater than or equal to 0.5.

[0127] The heating furnace described above may include four or more zones.

[0128] The heating furnace described above may be a roller hearth furnace.

[0129] The heating method described above may have a set temperature of 865℃ or lower in the first zone.

[0130] The heating method described above allows six or more different set temperatures to be set in one or more zones within the heating furnace.

[0131] In the heating method described above, the set temperature of the last zone within the furnace may be lower than the maximum set temperature within the furnace.

[0132] The present invention can provide a heating method that appropriately takes into account the thermal history up to the previous zone and oscillation phenomena during hot press forming.

[0133] In particular, the present invention can provide a method for stably heat treating a steel plate or blank by considering material characteristics such as thickness, plating amount, and whiteness during hot press forming.

[0134] The present invention can improve productivity and stability during hot press forming and can provide a heating method that is widely applicable without direct limitations on the number of temperature setting steps, the temperature range of each step, and the holding time range of each step.

[0135] FIG. 1 illustrates an example of the present invention in which the number of zones included in the heating furnace is 5, wherein H per zone sum This is a graph representing .

[0136] FIG. 2 is a schematic diagram briefly illustrating the case where a steel plate or blank stays in the corresponding zone (zone j) and temporarily moves (oscillates) to the zones before and after the corresponding zone.

[0137] Figure 3 is H j_own , H j_p&f and H j This is a graph briefly showing the relationship between them.

[0138] Figure 4 is t j_0 and H j_own This is a graph briefly showing the relationship between them.

[0139] FIG. 5 shows an example of the present invention in which the number of zones included in the furnace is 5 and there is no oscillation between the front and rear zones, wherein the Hratio per zone sum This is a graph representing .

[0140] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0141] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0142] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0143] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0144] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0145] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0146] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0147] The present invention can provide a heating method that appropriately takes into account the thermal history up to the previous zone and oscillation phenomena during hot press forming.

[0148] In particular, a heating method according to one example of the present invention can provide a heating method for stable hot press forming that takes into account the thickness, plating amount, and whiteness of the steel plate or blank.

[0149] Accordingly, the present invention can improve productivity and stability during hot press forming and can provide a heating method that is widely applicable without direct limitations on the number of temperature setting steps, the temperature range of each step, and the holding time range of each step.

[0150] A heating method according to one embodiment of the present invention is a heating method for a steel plate or blank for hot press forming, comprising: a step of preparing said steel plate or blank; and a heating step of passing said steel plate or blank through a heating furnace comprising two or more zones, wherein in the heating step H represented by the following relational formula 1 sum The temperature and residence time of each zone can be adjusted so that this is 1 or more. At this time, the zone may be defined as a part capable of maintaining the temperature at a specific set temperature, and is not separately limited according to the length of other zones or the purpose of heating.

[0151] [Relationship 1]

[0152]

[0153] Here, n represents the number of zones included in the furnace, and j represents the zone number assigned sequentially starting from the furnace inlet.

[0154] At this time, the above H sum H is the heat treatment index obtained at temperature and time in zone j (1≤j≤n). j It refers to the sum of all values, more specifically H1+H2+H3+ … +H n It means.

[0155] That is, through repeated experiments, the inventors [identified] the cumulative heat treatment index H sum The present invention was derived from the understanding that if the temperature and residence time of each zone are adjusted to be greater than 1, sufficient heat treatment suitable for hot press forming can be performed.

[0156] In addition, the above H sum If this is 1 or more, there is no problem in achieving the objective of the present invention; however, for the purpose of economical heat treatment, the present invention [invents] the above H sum The upper limit of can be set to 1.64.

[0157] FIG. 1 is an example of the present invention in which the number of zones included in the heating furnace is 5, wherein H for each zone sum This is a graph representing. Looking at Fig. 1, as the heat treatment index values ​​in each zone are 0.37, 0.32, 0.2, 0.15, and 0.13, respectively, H sum It can be seen that the value can be secured to be 1 or higher starting from the 4th zone. Through this, it can be seen that the zones where sufficient heat treatment is performed to be suitable for hot press forming are the 4th and 5th zones.

[0158] FIG. 2 is a schematic diagram briefly illustrating the case where a steel plate or blank residing in a corresponding zone (zone j) temporarily moves (oscillates) to the zones before and after that zone. As shown in FIG. 2, when the steel plate or blank reciprocates to the zones before and after zone j, the above H j It can be calculated by taking these oscillation phenomena into account.

[0159] More specifically, the above H j It can be defined by the following relationship 2.

[0160] [Relationship 2]

[0161]

[0162] Here, H j_ownAs described above, represents the heat treatment index secured in zone j itself when the steel plate or blank oscillates in zone j, and H j_p&f represents the heat treatment index secured in the front and rear zones of the j zone, where the steel plate or blank has been temporarily moved.

[0163] That is, H j H is the heat treatment index secured in zone j itself. j_own H, the heat treatment index secured in the pre- and post-zone j. j_p&f It can be expressed as the sum of, and such H j_own , H j_p&f and H j The relationship between them is briefly shown in Figure 3.

[0164] H below j_own First, regarding the explanation, H j_p&f This will be discussed later.

[0165] When j is 1, the above H j_own It can be defined by the following relation 3.

[0166] [Relationship 3]

[0167]

[0168] Here, t j_own means the actual time (in minutes) that the above steel plate or blank stays in the area.

[0169] That is, the inventors know that as the heat treatment index value accumulated up to the previous step increases, the rate of increase of the heat treatment index decreases even when heat treatment is performed under the same time and temperature conditions, and H showing the shape of a graph that becomes gentler over time. 1_own Equation 3 regarding was derived.

[0170] Also, if j is 2 or greater, the above H j_own It can be defined by the following relationship 4.

[0171] [Relationship 4]

[0172]

[0173] The inventors [have] H over time as the set temperature in each zone changes. sum The inventors designed the equation knowing that the shape of the graph representing it changes. More specifically, the inventors defined H, the cumulative heat treatment index up to zone (j-1), as shown in the following equation. sum_j-1 t is the theoretical time designed so that the value becomes equal to the initial stage heat treatment index in zone j. j_0 Introduced.

[0174]

[0175] Next, H according to time (x-axis) in zone j sum_j In a graph representing values ​​(y-axis), H j_own The value is the x-axis value t j_own +t j_0 From the value substituted, t j_0 It can be obtained by subtracting the value substituted.

[0176]

[0177] These t j_0 and H j_own A graph showing the approximate relationship between them is shown in Figure 4.

[0178] And, rearranging the above equation (a), t j_0 An expression regarding can be obtained, which is equal to the following relationship 6.

[0179] [Relationship 6]

[0180]

[0181] And, in the equations mentioned above, a j and b j The value is t, which is the minimum heat treatment time (minutes) required when considering heat treatment is performed solely based on the ambient temperature of zone j. min_j It can be expressed as an equation by.

[0182] The above a j According to the following relational expressions 9 and 10, the above bj It can be defined by the following relational expressions 11 and 12.

[0183] [Relationship 9]

[0184]

[0185] [Relationship 10]

[0186]

[0187] [Relationship 11]

[0188]

[0189] [Relationship 12]

[0190]

[0191] And, the above t min_j is a variable value that depends on the set temperature of zone j, the thickness of the steel plate or blank, the amount of plating, and the whiteness, and can be defined by the following relationship 13.

[0192] [Relationship 13]

[0193]

[0194] Here, t min_j_ref Thickness 1.2mm, single-sided plating weight 70 g / m² 2 , which refers to the minimum heating time (minutes) required for a standard steel sheet or standard blank with a whiteness of 80.6, and can be calculated by the following formula.

[0195] [Relationship 14]

[0196]

[0197] Temp here j represents the set temperature of zone j.

[0198] Looking at the above relationship 14, it can be seen that as the set temperature of zone j increases, the minimum heat treatment time for standard steel plates or standard blanks decreases.

[0199] In addition, t in the above relationship 13min_j_thk is defined by the following formula.

[0200] [Relationship 15]

[0201]

[0202] Here, thk means the thickness (mm) of the steel plate or blank.

[0203] Looking at the above relationship 15, as the thickness of the steel plate or blank increases, t min_j It can be seen that this is increasing.

[0204] And, f_t in the above relationship 13 min_j_cw is defined by the following formula.

[0205] [Relation Equation 16]

[0206]

[0207] Here, CW is the plating amount on one side of the steel sheet or blank (g / m²) 2 It means ).

[0208] When the amount of single-sided plating of the steel sheet or blank increases through the above relationship 16, the above t min_j This increases, and this is Temp, the set temperature of zone j. j It can be seen that it also relies on.

[0209] Finally, f_t in the above relationship 13 min_j_wi is defined by the following relation 17.

[0210] [Relation Equation 17]

[0211]

[0212] Here, WI represents the whiteness of the material surface measured by a colorimeter and has a value between 0 and 100.

[0213] According to the above Equation 17, when the whiteness of the steel plate or blank surface increases, the heating rate of the material slows down, so the minimum required heat treatment time t min _ j It can be seen that this is increasing.

[0214] As described above, the present invention relates to a heat treatment index H in zone j itself, which reflects the thickness of the steel plate or blank, the amount of plating on one side, the whiteness, and the set temperature and residence time in zone j. j_own You can obtain.

[0215] Below, H j_p&f This will be discussed later.

[0216] The above H j_p&f represents the additionally accumulated heat treatment index as the steel plate remaining in zone j temporarily moves (oscillates) to the zones before and after that zone. The above H j_p&f H described above j_own Apart from that, the above H j By considering it during calculation, the present invention can reflect the accumulation effect of the heat treatment index caused by the oscillation phenomenon.

[0217] Such H j_p&f It can be calculated by the following relationship 5.

[0218] [Relationship 5]

[0219]

[0220] Here, t j_p&f represents the sum of the time spent in the preceding and succeeding zones (in minutes) when a steel plate staying in zone j temporarily moves (oscillates) to the preceding and succeeding zones.

[0221] Also, the above t j_0_p&f Similar to the aforementioned tj_0, it can be derived by the following equations. That is, H sum_j-1 Value and H j_own So that the sum of the values ​​becomes equal to the initial value of the heat treatment index accumulated in the zones before and after zone j, the theoretical time t j_0_p&f It can introduce.

[0222]

[0223] Next, H j_p&f The value is the x-axis value tj_p&f +t j_0_ p&f From the value substituted, t j_0_p&f You can obtain it by subtracting the value obtained by subtracting.

[0224]

[0225] Rearranging the above equation (c), t j_0_p&f An equation regarding can be obtained, which is equal to Equation 7 below. When j is 1, the heat treatment index (H accumulated up to the previous zone) sum_j-1 Since ) is not present, relational equation 8 can also be derived by taking this into account.

[0226] [Relationship 7]

[0227]

[0228] [Relationship 8]

[0229]

[0230] a in the above-mentioned formulas j_p&f and b j_p&f The value is the aforementioned a j and b j It can be calculated similarly to the value. However, in the calculation process, Temp j Instead of Temp j_p&f The value is applied, and the above Temp j_p&f It is defined as the average temperature of the preceding and succeeding zones when the steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the preceding and succeeding zones.

[0231] As a more specific example, when a steel plate staying in zone j temporarily moves (oscillates) to zones (j-1) and (j+1), the above Temp j_p&f It can be obtained by the following formula.

[0232]

[0233] Here, Temp j-1 and Temp j+1 represents the set temperature of zone (j-1) and zone (j+1), respectively.

[0234] Meanwhile, this oscillation phenomenon may extend to a zone preceding zone (j-1) or a zone following zone (j+1), and in this case, the above Temp j_p&f It can be obtained by the following relationship 40.

[0235] [Relationship 40]

[0236]

[0237] Here, x and y are natural numbers representing the x and y values ​​when the areas moved to the front and rear of the area (j-zone) are called the areas (jx) and (j+y), respectively, and the areas farthest from the area are called the areas (jx) and (j+y). In this case, area (jx) is the area before the area (j-zone), and area (j+y) is the area after the area (j-zone).

[0238] More specifically, a j_p&f and b j_p&f To explain the method for calculating, a j_p&f and b j_p&f It can be obtained by the following formulas.

[0239] [Relation Equation 18]

[0240]

[0241] [Relation Equation 19]

[0242]

[0243] [Relationship 20]

[0244]

[0245] [Relation Equation 21]

[0246]

[0247] Here, t min_j_p&f can be defined by the following relation 22.

[0248] [Relation Equation 22]

[0249]

[0250] Here, tmin_j_ref_p&f can be defined by the following relation 23, and f_tmin_j_cw_p&f can be defined by the following relation 24.

[0251] [Relation Equation 23]

[0252]

[0253] [Relationship 24]

[0254]

[0255] As described above, in the above-described equations, Temp j_p&f is defined as the average value of the preceding and succeeding zones when a steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the preceding and succeeding zones of that zone.

[0256] Meanwhile, in order to present more stable heat treatment conditions, the inventors [provided] the cumulative value of the maximum heat treatment index ratio (Hratio sum ) was introduced.

[0257] That is, a heating method according to another embodiment of the present invention comprises, in the heating step described above, Hratio represented by the following equation 25 sum The temperature and residence time of each zone can be adjusted so that this is 1 or less.

[0258] [Relationship 25]

[0259]

[0260] As can be seen from the aforementioned relationship Equation 25, the present invention relates to a heat treatment index (H) secured in the j-zone itself when a steel plate or blank is oscillated in the j-zone. j_own ) its maximum allowable heat treatment index (Hmax j_own The value divided by ) and the heat treatment index (H) secured in the pre- and post-zones of zone j j_p&f ) its maximum allowable heat treatment index (Hmax j_p&fBy making the cumulative value of the maximum heat treatment index ratio, which is the sum of the values ​​divided by ), 1 or less, heat treatment can be performed more stably.

[0261] FIG. 5 shows an example of the present invention in which the number of zones included in the furnace is 5 and there is no oscillation between the front and rear zones, wherein the Hratio per zone sum This is a graph showing [the result]. As shown in Fig. 5, when the total sum of the Hratio1 to Hratio5 values ​​in each zone is 1 or less, more stable heat treatment is possible during hot press forming.

[0262] The above Hmax j_own I will first explain about Hmax j_p&f This will be discussed later.

[0263] The above Hmax j_own Considering that heat treatment is performed solely at the ambient temperature of zone j, the maximum allowable heat treatment time (t max_j The heat treatment index when heat treatment is performed for a period of time (unit: minutes) can be defined by the following relationship 26.

[0264] [Relation Equation 26]

[0265]

[0266] Also, the above t max_j The value is a variable that depends on the set temperature of zone j, the thickness of the steel plate or blank, the amount of plating, and the whiteness, and can be calculated by the following formula.

[0267] [Relation Equation 28]

[0268]

[0269] The above tmax_j_ref_cw is the maximum allowable heat treatment time (minutes) when considering that heat treatment is performed only at the ambient temperature of zone j on a standard steel sheet or standard blank with a thickness of 1.2 mm and a whiteness of 80.6, and is a value that takes into account the plating amount on one side of the standard steel sheet or standard blank. The above tmax_j_ref_cw can be calculated by the following formulas.

[0270] [Relationship 30]

[0271]

[0272] [Relationship 31]

[0273]

[0274] At this time, the above A and B can be defined by the following formula.

[0275] [Relation Equation 38]

[0276]

[0277] [Relation Equation 39]

[0278]

[0279] With the formulas described above, the above t max_j It can be seen that the value decreases as the set temperature increases and increases as the amount of single-sided plating increases.

[0280] In addition, if the tmax_j_ref_cw calculated by Equation 30 is 15 or greater, applying it as 15 is because it is undesirable for the standard specimen to remain in the furnace for a long time in terms of productivity.

[0281] The above t, a value dependent on the thickness of the specimen max_j_thk can be defined by the following relationships 32 and 33.

[0282] [Relationship 32]

[0283]

[0284] [Relationship 33]

[0285]

[0286] Thus, the above t max_j It can be seen that it increases as the thickness increases, but the degree of dependence varies depending on the temperature range.

[0287] The above f_t, a value dependent on whiteness max_j_wi can be defined by the following relationship 34.

[0288] [Relationship 34]

[0289]

[0290] If whiteness increases through the above equation, the above t max_j It can be seen that it increases.

[0291] Below, Hmax j_p&f Describes...

[0292] As described above, the heating method of the present invention may be characterized by additionally considering the oscillation phenomenon.

[0293] Accordingly, the present invention relates to the above-described Hmax j Hmax when calculating j_own When a steel plate staying in zone j temporarily moves (oscillates) to the zones before or after that zone, H, which is the maximum heat treatment index ratio due to such oscillation j_p&f / Hmax j_p&f Hratio considering together sum It can design.

[0294] At this time, the above Hmax j_p&f Hmax j_own Similarly, it can be defined by the following relationship 27.

[0295] [Relation Equation 27]

[0296]

[0297] The above t max_j_p&frepresents the maximum allowable heat treatment time (in minutes) when a steel plate or blank remaining in zone j oscillates into the preceding or succeeding zones of zone j due to oscillation, and t max_j It is similar to the method of calculating . The above t max_j_p&f However, t max_j Unlike, Temp j Instead of Temp j_p&f It can be obtained by substituting.

[0298] The above Temp j_p&f The method for calculating has been described above, so it will be omitted.

[0299] More specifically, t max_j_p&f It can be defined in the following formula.

[0300] [Relation Equation 29]

[0301]

[0302] In addition, the tmax_j_ref_cw_p&f and tmax_j_thk_p&f, which depend on the set temperature in the above equation, can be defined by the following equations.

[0303] [Relationship 35]

[0304]

[0305] [Relation Equation 36]

[0306]

[0307] [Relation Equation 37]

[0308]

[0309] Meanwhile, H j_own, H j_p&f , Hmax j_own and Hmax j_p&f It can be calculated as described above, but the calculation method described above can be applied at a general heat treatment temperature level. In this case, as an example, the general heat treatment temperature level is the set temperature Temp of each zone. jIt may mean the case where it is 700℃ or higher. If the set temperature in a specific zone is significantly lower, H j_own or H j_p&f may be less than 0, in which case the above H j_own, H j_p&f For the slash, 0 can be applied. Similarly, if the set temperature in a specific zone is very low, Hmax j_own and Hmax j_p&f The value may be less than 1, in which case Hmax j_own and Hmax j_p&f You can use it by changing the value to 1.

[0310] Meanwhile, the alloy composition of the steel plate or blank to be heated in the present invention is not particularly limited, but as an example, the steel plate or blank may comprise, in weight percent, C: 0.05~0.40, Si: 0.10~1.00 and Mn: 0.40~1.80, and the remainder being Fe and unavoidable impurities.

[0311] In addition, as another embodiment, the steel plate or blank may be a high-strength steel plate with a tensile strength of 1500 MPa or more. In this case, the steel plate or blank may comprise, in weight percent, C: 0.18~0.25, Si: 0.10~0.50 and Mn: 0.90~1.50, and the remainder being Fe and unavoidable impurities.

[0312] In addition, as an example, the steel plate or blank may have different thicknesses, different plating amounts, or different whiteness depending on the location, and each location may satisfy the condition of Hsum: 1 or higher. Furthermore, each location may additionally satisfy the condition of Hratiosum: 1 or lower.

[0313] As another example, the steel plate or blank may be a plurality of steel plates or blanks having different thicknesses (thk), different plating amounts (CW), or different whiteness (WI), and each steel plate or blank may satisfy the condition Hsum: 1 or higher. In addition, each steel plate or blank may additionally satisfy the condition Hratiosum: 1 or lower.

[0314] In addition, the above steel plate or blank is not particularly limited in thickness, plating composition, amount of plating on one side, or whiteness. However, as an example, the above steel plate or blank may have a thickness (thk) of 0.7 to 4.0 mm or 0.7 to 2.6 mm, and a whiteness (WI) of 35 to 90.

[0315] In addition, the plating layer of the above steel plate or blank may be selected from either an aluminum-based plating layer or an alloyed aluminum-based plating layer. Regarding the composition of each plating layer, the range conventionally applied in the relevant technical field may be applied in the same way.

[0316] As an example, the single-sided plating weight (CW) of the plating layer is 15 to 80 g / m² 2 It could be.

[0317] Meanwhile, the heating furnace according to one example of the present invention may include four or more zones, and is not necessarily limited thereto, but the heating furnace may be a roller hearth furnace.

[0318] In addition, the heating method according to one embodiment of the present invention may be a multi-stage heating process in which six or more different set temperatures are each set in one or more zones within the heating furnace.

[0319] The inventors have found that in this multi-stage heating process, appropriately controlling the residence time and set temperature of the steel plate or blank for each zone within the heating furnace is advantageous for securing more stable and efficient heat treatment conditions during hot press forming.

[0320] More specifically, in a heating method according to a non-limiting example of the present invention, the residence time of a steel plate or blank in the first zone may be less than or equal to {1 / n+1 / (2n)} times the total residence time in the furnace. Additionally, according to a heating method according to a non-limiting example of the present invention, the set temperature of the first zone may be 865°C or lower. In this way, the present invention can minimize heat energy loss to the outside by lowering the residence time and set temperature of the first zone, which has a lot of contact with the external atmosphere.

[0321] As another example, if the steel plate or blank moves to zone j within a furnace containing n zones, the total residence time in the furnace ( Cumulative time of stay from the first zone to the corresponding zone compared to ) ( The ratio of ) is a j When saying a j H at ≥0.3 sum_j ≥ a j It can satisfy.

[0322] As another example, the aforementioned a j Regarding a j 0.25 < a compared to the average temperature of the zones corresponding to ≤0.25 j The average temperature of the zones corresponding to ≤0.5 may be greater than or equal to 0.5.

[0323] In this way, by gradually increasing the heating atmosphere temperature from the front zone to the middle zone, it is possible to prevent a decrease in the heating rate that may slow down as the blank is heated, thereby shortening the heating time.

[0324] Finally, in a heating method according to a non-limiting example of the present invention, the set temperature of the last zone within the furnace may be lower than the maximum set temperature within the furnace. By doing so, the present invention can minimize heat energy loss to the outside by maintaining the temperature of the last zone, which has a lot of contact with the external atmosphere, lower than the highest value among the set temperatures of the zones within the furnace.

[0325] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0326] (Example 1)

[0327] First, steel sheets were prepared comprising, in weight percent, C: 0.18–0.25, Si: 0.10–0.50, and Mn: 0.90–1.50, with the remainder being Fe and unavoidable impurities. The whiteness, plating amount, and thickness of the steel sheets used in each experiment are listed in Table 1 below. Next, as shown in Table 1, the steel sheets were heated by passing them through a heating furnace containing four zones. At this time, the set temperature (Temp) of each zone j ), time of stay (t j_own ...is listed in Table 1 below. Since no separate oscillation phenomenon occurred before or after each zone, as shown in Table 1 below, t, which is the sum of the residence times in the zones before and after when the steel plate residing in the zone temporarily moves (oscillates) to the zones before and after it, is... j_p&f All of them were written as "O".

[0328] From the steel plate characteristics and zone-specific set temperature and residence time values ​​in Table 1, the cumulative heat treatment index (H sum) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was calculated and listed together in Table 1 below.

[0329] Next, each steel plate that passed through the heating furnace was listed together in Table 1 below according to the evaluation criteria below.

[0330] (metewand)

[0331] Strength: Tensile specimens were taken, and the tensile strength was measured by performing a tensile test using the ASTM E8 / E8M test method. In this case, if the tensile strength was 1300 MPa or higher, it was marked as “O,” and if it was less than 1300 MPa, it was marked as “X.” If it is difficult to measure the tensile strength directly, it may be calculated by converting it from the Vickers hardness of the base material. One case where it is difficult to measure the tensile strength directly is when the specimen being measured is too small. In such cases, if it is impossible to take a tensile specimen, the Vickers hardness is determined, and the tensile strength can be calculated by substituting the Vickers hardness into the following formula. In this case, the Vickers hardness was measured at a point 1 / 4T thick based on the thickness of each steel plate with an indentation load of 500g.

[0332] Formula) Tensile Strength (MPa) = 3.3762 × Vickers Hardness (Hv) - 35.004

[0333] Weldability: Weldability was evaluated by considering the tendency for weldability to deteriorate as the thickness of the diffusion layer becomes excessively thick. In this context, the diffusion layer refers to an intermediate layer formed by atomic diffusion between the plating metal Al and the substrate metal Fe. Specifically, it was marked “O” if the diffusion layer thickness was 15 μm or less, and “X” if it exceeded 15 μm. Since strength must fundamentally be ensured, weldability results were not separately indicated if strength was not secured.

[0334]

[0335]

[0336] As shown in Table 1 above, Example 1-1 has a shorter residence time in each zone compared to Example 1-2, so H sum The value was less than 1. As a result, Example 1-1 could not secure high strength characteristics after heat treatment, whereas Example 1-2 could secure high strength characteristics after heat treatment.

[0337] Examples 1-3 and 1-5 had thicknesses of 2.3 mm and 0.9 mm, respectively. Examples 1-3 and 1-5 had relatively shorter residence times in each zone compared to Examples 1-4 and 1-6 of the same thickness, resulting in H of less than 1. sum The values ​​were shown. In addition, through the above Examples 1-4 and 1-6, it can be seen that as the thickness of the steel plate increases, the residence time in each zone needs to increase.

[0338] Examples 1-7 and 1-9 have a single-sided plating amount of 20 g / mm² each. 2 and 40g / mm 2 Examples 1-7 and 1-9 had relatively shorter residence times in each zone compared to Examples 1-8 and 1-10, which had the same single-sided plating amount, so H was less than 1. sum As the value was expressed, high strength characteristics could not be secured after heat treatment. Through the above Examples 1-8 and 1-10, it can be seen that when the plating amount of the steel plate is reduced, the required characteristics can be secured even if the residence time in each zone is reduced.

[0339] Examples 1-11 had a relatively shorter residence time in each zone compared to Examples 1-12 with the same whiteness, and H sum The value was less than 1.

[0340] In Examples 1-13, 1-15, and 1-17, the temperatures of each zone were 900°C, 870°C, and 960°C, respectively. Compared to Examples 1-14, 1-16, and 1-18, which had the same temperature conditions, Examples 1-13, 1-15, and 1-17 had relatively shorter residence times in each zone, so H sumThe value was less than 1, so the required characteristics could not be satisfied. Also, through the above Examples 1-14, 1-16, and 1-18, it can be seen that as the set temperature within each zone increases, the residence time required to secure high strength characteristics becomes shorter.

[0341] Meanwhile, compared to Example 1-19 under the same conditions, the residence time in each zone of Example 1-20 was excessive, so H sum Although the value satisfied 1 or greater, Hratio sum The value exceeded 1. As a result, while high strength characteristics could be secured in Examples 1-20, the thickness of the diffusion layer increased due to excessive alloying, and weldability was inferior.

[0342] Examples 1-22 and 1-24 were 2.3 mm and 0.9 mm, respectively. Unlike Examples 1-21 and 1-23, which had the same thickness, Examples 1-22 and 1-24 had Hratio sum As the value exceeded 1, excellent weldability could not be secured.

[0343] The single-sided plating amount of Examples 1-26 and 1-28 is 20 g / mm 2 and 40g / mm 2 It was. Examples 1-26 and 1-28 had a longer stay in each zone compared to Examples 1-25 and 1-27 under the same plating amount conditions, as the Hratio sum The value exceeded 1. As a result, the weldability of Examples 1-26 and 1-28 was poor.

[0344] Example 1-30 has a relatively longer residence time compared to Example 1-29 of the same whiteness, so the Hratio sum The value exceeded 1.

[0345] Example 1-32 was under the same temperature conditions as Example 1-31, but due to the excessive residence time, Hratio sum As the value exceeded 1, it was difficult to secure an excellent level of weldability.

[0346] Looking at the above-described Examples 1-19 to 1-32, 1 or more H sum Hratio of 1 or less along with the value sum It can be seen that if the value is satisfied, excellent weldability can be secured along with high strength characteristics.

[0347] (Example 2)

[0348] A steel plate having the same alloy composition range as the steel plates of the above-described (Example 1) was prepared and passed through a heating furnace. At this time, the whiteness, plating amount, and thickness of the steel plate were 80.6 and 70 g / m², respectively. 2 and was 1.2 mm. Table 2 below shows the set temperatures (Temp for each zone inside the furnace). j ) and time of stay (t j_own, t j_p&f Along with ), the calculated cumulative heat treatment index (H sum ) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was recorded. Then, whether the required characteristics were secured according to the same criteria as (Example 1) was determined and indicated together in Table 2 below.

[0349]

[0350] From the experimental results of the above-described (Example 2), it can be seen that it is easier to secure the required physical properties as the number of zones within the furnace increases. That is, as can be seen from Table 2 above, under the assumption that the steel plate moves at a constant speed, compared to the case where the number of zones is 3 as in Example 2-1, in the case where the number of zones is 4 or more as in Example 2-2, although the sum of the residence times in each zone is the same, H sum It can be seen that it increases. As in the above assumption, when the steel plate moves at a constant speed, there is an advantage that multiple steel plates can be heated continuously.

[0351] Therefore, Example 2-2 can perform heat treatment more stably than Example 2-1 and can also shorten the heating time.

[0352] (Example 3)

[0353] A steel plate having the same alloy composition range as the steel plates of the above-described (Example 1) was prepared, and the conditions for passing through a heating furnace were examined. At this time, the whiteness, plating amount, and thickness of the steel plate were 80.6 and 70 g / m², respectively. 2 and it was 1.2 mm. Table 3 below shows the set temperatures (Temp for each zone inside the furnace). j ) and time of stay (t j_own, t j_p&f Along with ), the calculated cumulative heat treatment index (H sum ) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was recorded. Then, whether the required characteristics were secured according to the criteria below was determined and indicated together in Table 3 below.

[0354] In Example 3, the determination of whether strength was secured was made using an evaluation method different from that of Example 1. Specifically, the inventors found that whether the austenite phase transformation, which is an important requirement for securing high strength characteristics, is completed upon heating can be indirectly evaluated through the thickness of the diffusion layer. That is, through repeated experiments, the inventors found that if the thickness of the diffusion layer is 2 μm or more, the steel plate has been heated at a temperature and for a sufficient time for the austenite phase transformation, and high strength can be secured upon subsequent cooling. In this way, the inventors discovered the relationship between heat treatment conditions and the thickness of the diffusion layer and utilized this relationship to determine the conditions for securing strength. Accordingly, in Example 3, strength was indicated as “O” when the thickness of the diffusion layer was 2 μm or more, and strength was indicated as “X” when it was less than 2 μm, as shown in Table 3 below.

[0355] Meanwhile, the same evaluation method as described above (Example 1) was used to determine whether weldability was secured.

[0356]

[0357] Although the residence time and set temperature of the first and last zones in Examples 3-2 and 3-3 are lower than those of Example 3-1, Examples 3-2 and 3-3 have the same or similar level of H as Example 3-1. sum Value and Hratio sum The values ​​were shown. Through this, it can be seen that energy consumption can be saved by minimizing heat energy loss to the outside through lowering the residence time and set temperature of the first and last zones, which have a lot of contact with the external atmosphere.

[0358] (Example 4)

[0359] A steel plate having the same alloy composition range as the steel plates of the above-described (Example 1) was prepared, and the conditions for passing through a heating furnace were examined. At this time, the whiteness, plating amount, and thickness of the steel plate were 80.6 and 70 g / m², respectively. 2 and it was 1.2 mm. Table 4 below shows the set temperatures (Temp for each zone inside the furnace). j ) and time of stay (t j_own Along with ), the calculated cumulative heat treatment index (H sum ) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was recorded. Then, whether the required characteristics were secured according to the same criteria as (Example 3) was determined and indicated together in Table 4 below.

[0360]

[0361] Looking at Table 4 above, Example 4-1 with 4 temperature conditions and Example 4-2 with 5 temperature conditions are H sum Value and Hratio sum The values ​​were similar.

[0362] On the other hand, Example 4-4, which has 6 temperature conditions, compared to Example 4-3, which has 5 temperature conditions, H sum Value and Hratio sum You can see that the value has increased.

[0363] Through this, it can be seen that for roller furnaces generally used in the industry, setting the temperature for each zone to at least six different temperatures is more desirable for stable heating and reducing heating time.

[0364] (Example 5)

[0365] Steel plates were prepared having the same alloy composition range as the steel plates of the above-described (Example 1) but with different thicknesses. The whiteness and plating weight of each steel plate were 80.6 and 70 g / m², respectively. 2 ..., and the thicknesses were 1.2 mm and 2.3 mm, respectively. A review was conducted on the conditions under which two steel plates of different thicknesses were butt-joined and passed through a heating furnace. Table 5 below shows the set temperatures (Temp for each zone inside the heating furnace). j ) and time of stay (t j_own, t j_p&f Along with ), the calculated cumulative heat treatment index (H sum ) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was recorded. Then, whether the required characteristics were secured according to the same criteria as (Example 3) was determined and indicated together in Table 5 below.

[0366]

[0367] As can be seen in Table 5 above, unlike Example 5-2, Example 5-1 did not satisfy the condition of Hsum: 1 or higher at any location, and thus could not secure high strength characteristics at that location.

[0368] On the other hand, the above Example 5-2 has Hsum: 1 or more and Hratio at each location. sum By satisfying the condition of : 1 or less, high strength characteristics and excellent weldability could be secured at all locations.

[0369] Thus, when heating steel plates of different thicknesses, plating amounts, or whitenesses at each location, at each location, Hsum: 1 or more conditions, along with optionally Hratio sum : It can be seen that it is necessary to satisfy the condition of 1 or less.

[0370] (Example 6)

[0371] A steel plate having the same alloy composition range as the steel plates of the above-described (Example 1) was prepared, and the conditions for passing through a heating furnace were examined. At this time, the whiteness, plating amount, and thickness of the steel plate were 80.6 and 70 g / m², respectively. 2 and was 1.2 mm. Table 6 below shows the set temperatures (Temp for each zone inside the furnace). j ) and time of stay (t j_own, t j_p&f Along with ), the calculated cumulative heat treatment index (H sum ) and cumulative value of maximum heat treatment index ratio (Hratio sum ) was recorded. Then, whether the required characteristics were secured according to the same criteria as (Example 3) was determined and indicated together in Table 6 below.

[0372]

[0373] Unlike Example 6-1, Example 6-2 exhibited an oscillation phenomenon; however, as the set temperature was the same across the entire region, the H obtained from each example sum Value and Hratio sum The values ​​were identical. On the other hand, considering the oscillation phenomenon, Examples 6-4 and 6-5 yielded H compared to Examples 6-3 and 6-6. sum Value and Hratio sum The values ​​were different.

[0374] In particular, looking at Examples 6-5 and 6-6 above, it can be seen that when considering oscillation, the required characteristics are not satisfied, but when considering oscillation, the required characteristics are satisfied.

[0375] Therefore, in order to more accurately evaluate whether material properties are secured after heat treatment in the event of oscillation, the heat treatment index (H) at each zone itself j_own In addition to ), the heat treatment index (H) due to oscillation j_p&f It can be seen that it is necessary to consider ) together.

Claims

1. A method for heating a steel plate or blank for hot press forming, Step of preparing the above steel plate or blank; The method includes a heating step of passing the above steel plate or blank through a heating furnace comprising two or more zones, and H represented by the following Equation 1 in the above heating step sum A heating method in which the temperature and residence time of each zone are controlled so that this is 1.00 or higher. [Relationship 1] Here, n represents the number of zones included in the furnace, j represents the zone number assigned in order from the furnace inlet side, and Hj can be defined by the following relationship 2. [Relationship 2] Here, when j is 1, H j_own is given by the following relationship 3, where H when j is 2 or greater j_own is given by the following relationship 4, H j_p&f is defined by the following relation 5. [Relationship 3] [Relationship 4] [Relationship 5] Here, t j_own represents the actual time (in minutes) that the above steel plate or blank stays in the relevant area, and t j_p&f represents the sum of the time (in minutes) spent in the preceding and succeeding zones when a steel plate staying in the corresponding zone temporarily moves (oscillates) to the zones before and after it. Also, t j_o can be defined by the following relationship 6, and t j_o_p&f It can be defined by the following relational expressions 7 and 8. [Relationship 6] [Relationship 7] [Relationship 8] Also, a j By the following relationships 9 and 10, b j is defined by the following relational expressions 11 and 12. [Relationship 9] [Relationship 10] [Relationship 11] [Relationship 12] And, a j_p&f wa b j_p&f is calculated as the average temperature of the preceding and succeeding zones when the steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the zones before and after it, and a calculated from this j and b j It means. Also, t min_j is defined by the following relation 13. [Relationship 13] Here, t min_j_ref It can be obtained by the following relationship 14. [Relationship 14] Temp here j represents the set temperature of zone j. Also, t min_j_thk is defined by the following relation 15. [Relationship 15] Here, thk means the thickness (mm) of the steel plate or blank. Also, f_t min_j_cw is defined by the following relation 16. [Relation Equation 16] Here, CW is the plating amount on one side of the steel sheet or blank (g / m²) 2 It means ). Also, f_t min_j_wi is defined by the following relation 17. [Relation Equation 17] Here, WI represents the whiteness of the material surface measured by a colorimeter and has a value between 0 and 100.

2. A method for heating a steel plate or blank for hot press forming, Step of preparing the above steel plate or blank; The method includes a heating step of passing the above steel plate or blank through a heating furnace comprising two or more zones, and H represented by the following Equation 1 in the above heating step sum A heating method in which the temperature and residence time of each zone are controlled so that this is 1.00 or higher. [Relationship 1] Here, n represents the number of zones included in the furnace, j represents the zone number assigned sequentially starting from the furnace inlet side, and H j It can be defined by the following relationship 2. [Relationship 2] Here, when j is 1, H j_own is given by the following relationship 3, H j_p&f H when j is 2 or greater j_own is given by the following relationship 4, H j_p&f is defined by the following relationship 5. [Relationship 3] [Relationship 4] [Relationship 5] Here, t j_own represents the actual time (in minutes) that the above steel plate or blank stays in the relevant area, and t j_p&f represents the sum of the time (in minutes) spent in the preceding and succeeding zones when a steel plate staying in the corresponding zone temporarily moves (oscillates) to the zones before and after it. Also, t j_o can be defined by the following relationship 6, and t j_o_p&f It can be defined by the following relational expressions 7 and 8. [Relationship 6] [Relationship 7] [Relationship 8] Also, a j By the following relationships 9 and 10, b j is defined by the following relational expressions 11 and 12. [Relationship 9] [Relationship 10] [Relationship 11] [Relationship 12] Also, t min_j is defined by the following relation 13. [Relationship 13] Here, t min_j_ref It can be obtained by the following relationship 14. [Relationship 14] Temp here j represents the set temperature of zone j. Also, t min_j_thk is defined by the following relation 15. [Relationship 15] Here, thk means the thickness (mm) of the steel plate or blank. Also, f_t min_j_cw is defined by the following relation 16. [Relation Equation 16] Here, CW is the plating amount on one side of the steel sheet or blank (g / m²) 2 It means ). Also, f_t min_j_wi is defined by the following relation 17. [Relation Equation 17] Here, WI represents the whiteness of the material surface measured by a colorimeter and has a value between 0 and 100. Also, a j_p&f By the following equations 18 and 19, b j_p&f is defined by the following relational expressions 20 and 21. [Relation Equation 18] [Relation Equation 19] [Relationship 20] [Relation Equation 21] Here, t min_j_p&f is defined by the following relation 22. [Relationship 22] Here, tmin_j_ref_p&f is defined by the following relation 23, and f_tmin_j_cw_p&f is defined by the following relation 24. [Relationship 23] [Relationship 24] Temp here j_p&f It is defined as the average temperature of the preceding and succeeding zones when the steel plate staying in the corresponding zone (zone j) temporarily moves (oscillates) to the preceding and succeeding zones.

3. In Paragraph 1, In the above heating step, the Hratio represented by the following relationship 25 sum A heating method in which the temperature and residence time of each zone are controlled so that this becomes 1.00 or less. [Relationship 25] Here, Hmax j_own is by the following relationship 26, and Hmax j_p&f is defined by the following relation 27. [Relation Equation 26] [Relation Equation 27] Here, t max_j By the following relationship 28, t max_j_p&f is defined by the following relation 29. [Relation Equation 28] [Relation Equation 29] Here, tmax_j_ref_cw is given by the following relationships 30 and 31, t max_j_thk f_t by the following relationships 32 and 33. max_j_wi is defined by the following relation 34, tmax_j_ref_cw_p&f by the following relation 35, and tmax_j_thk_p&f by the following relation 36 and 37. [Relationship 30] [Relationship 31] [Relationship 32] [Relationship 33] [Relationship 34] [Relationship 35] [Relation Equation 36] [Relation Equation 37] In addition, the above A is defined by the following relation 38, and the above B is defined by the following relation 39. [Relation Equation 38] [Relation Equation 39] 4. In Paragraph 2, In the above heating step, the Hratio represented by the following relationship 25 sum A heating method in which the temperature and residence time of each zone are controlled so that this becomes 1.00 or less. [Relationship 25] Here, Hmax j_own is by the following relationship 26, and Hmax j_p&f is defined by the following relation 27. [Relation Equation 26] [Relation Equation 27] Here, t max_j By the following relationship 28, t max_j_p&f is defined by the following relation 29. [Relation Equation 28] [Relation Equation 29] Here, tmax_j_ref_cw is given by the following relationships 30 and 31, t max_j_thk f_t by the following relationships 32 and 33. max_j_wi is defined by the following relation 34, tmax_j_ref_cw_p&f by the following relation 35, and tmax_j_thk_p&f by the following relation 36 and 37. [Relationship 30] [Relationship 31] [Relationship 32] [Relationship 33] [Relationship 34] [Relationship 35] [Relation Equation 36] [Relation Equation 37] In addition, the above A is defined by the following relation 38, and the above B is defined by the following relation 39. [Relation Equation 38] [Relation Equation 39] 5. In any one of paragraphs 1 to 4, The above Temp j_p&f A heating method defined by the following relationship 40. [Relationship 40] Here, x and y are natural numbers representing the x and y values ​​when the area furthest from the area is called area (jx) and area (j+y) among the areas moved when the steel plate moves to the area before or after the area (j-zone). In this case, area (jx) is the area before the area (j-zone), and area (j+y) is the area after the area (j-zone).

6. In any one of paragraphs 1 to 5, The above H sum A heating method with a value of 1.64 or less.

7. In any one of paragraphs 1 through 6, The above steel plate or blank has different thickness, different plating amount, or different whiteness depending on the location, and at each location all H sum : A heating method satisfying conditions of 1.00 or higher.

8. In any one of paragraphs 1 through 7, Each location is all Hratio sum A heating method satisfying the condition of : 1.00 or less.

9. In any one of paragraphs 1 through 8, The above steel plates or blanks are a plurality of steel plates or blanks having different thicknesses (thk), and each steel plate or blank is all H sum Heating method satisfying : 1.00 or higher.

10. In any one of paragraphs 1 through 9, Each steel plate or blank is all Hratio sum A heating method satisfying the condition of : 1.00 or less.

11. In any one of paragraphs 1 through 10, The above steel plates or blanks are a plurality of steel plates or blanks having different plating amounts (CW), and each steel plate or blank is all H sum Heating method satisfying : 1.00 or higher.

12. In any one of paragraphs 1 to 11, Each steel plate or blank is all Hratio sum A heating method satisfying the condition of : 1.00 or less.

13. In any one of paragraphs 1 through 12, The above steel plates or blanks are a plurality of steel plates or blanks having different whiteness (WI), and each steel plate or blank is all H sum Heating method satisfying : 1.00 or higher.

14. In any one of paragraphs 1 through 13, Each steel plate or blank is all Hratio sum A heating method satisfying the condition of : 1.00 or less.

15. In any one of paragraphs 1 through 14, A heating method in which the above steel plate or blank comprises, in weight percent, C:0.05~0.40, Si:0.10~1.00 and Mn:0.40~1.80, and the remainder being Fe and unavoidable impurities.

16. In any one of paragraphs 1 through 15, A heating method in which the above steel plate or blank comprises, in weight percent, C: 0.18~0.25, Si: 0.10~0.50 and Mn: 0.90~1.50, and the remainder being Fe and unavoidable impurities.

17. In any one of paragraphs 1 through 16, The above single-sided plating amount (CW) is 15 to 80 g / m² 2 Phosphoric heating method.

18. In any one of paragraphs 1 through 17, A heating method in which the thickness (thk) of the above steel plate or blank is 0.7 to 4.0 mm.

19. In any one of paragraphs 1 through 18, A heating method in which the thickness (thk) of the above steel plate or blank is 0.7 to 2.6 mm.

20. In any one of paragraphs 1 through 19, A heating method in which the whiteness (WI) of the above steel plate or blank is 35 to 90.

21. In any one of paragraphs 1 through 20, A heating method in which the residence time of a steel plate or blank in the zone is less than or equal to {1 / n+1 / (2n)} times the total residence time in the heating furnace.

22. In any one of paragraphs 1 through 21, a j When saying a j H at ≥0.30 sum_j ≥ a j A heating method that satisfies [the condition].

23. In any one of paragraphs 1 through 22, a j When saying a j 0.25 < a compared to the average temperature of the zones corresponding to ≤0.25 j A heating method in which the average temperature of zones corresponding to ≤0.50 is greater than or equal to the average temperature.

24. In any one of paragraphs 1 through 23, The above heating furnace is a heating method comprising four or more zones.

25. In any one of paragraphs 1 through 24, The heating method described above is a roller hearth furnace.

26. In any one of paragraphs 1 through 25, A heating method in which the set temperature of the first zone is 865℃ or lower.

27. In any one of paragraphs 1 through 26, A heating method in which six or more different set temperatures are each set in one or more zones within the above-mentioned heating furnace.

28. In any one of paragraphs 1 through 27, A heating method in which the set temperature of the last zone within the furnace is lower than the maximum set temperature within the furnace.

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