JP3582512B2 - Steel plate for hot pressing and method for producing the same - Google Patents

Description

【００５３】
本発明例である鋼種Ｎｏ．１〜１０では、鉄系酸化物の形成状況、金型汚染、塗膜密着性および耐食性ともに良好な結果であった。一方、比較例である鋼種Ｎｏ．１１ 〜１５では、鉄系酸化物の形成状況、金型汚染、塗膜密着性および耐食性を同時に満足できるものはなかった。
【００５４】
（実施例２）
表１の鋼種Ｎｏ．６に示した組成の鋼を実験室で溶製し、スラブとした。このスラブを１２００℃にて３０分加熱した後、９００ ℃以上で熱間圧延を行い、板厚３．２ｍｍ の鋼板とした。熱間圧延後は、５５０ ℃まで水スプレー冷却したのち炉に装入し、５５０ ℃で３０分保持した後、２０℃／ 時で室温まで徐冷することにより、熱間圧延後の巻き取り工程をシミュレートした。熱延板は、酸洗によりスケールを除去した後、冷間圧延にて板厚１．０ｍｍ とした。この鋼板に、焼鈍シミュレーターを用いて、溶融亜鉛めっきラインを模擬した熱履歴を与えた。具体的な熱履歴は，図１および表２に示す。熱処理後の鋼板の断面ビッカース硬度（荷重４９Ｎ、測定数：５）を測定した結果も、表２に併せて示す。
【００５５】
また、この鋼の焼き入れ性の指標として、臨界冷却速度を測定した。熱延板から直径３．０ｍｍ 、長さ１０ｍｍの円柱試験片を切り出し、大気中で９５０ ℃まで１００ ℃／ 分の昇温速度にて加熱し、その温度で５分間保持したのち、種々の冷却速度で室温まで冷却した。その後、得られた試験片のビッカース硬度測定（荷重４９Ｎ、測定数：５）および組織観察を行った。また、加熱、冷却中の試験片の熱膨張変化を測定することにより、Ａｃ_１ 点およびＡｃ_３ 点を測定した。
【００５６】
９５０ ℃からの冷却速度が速いほど硬度は上昇し、ある冷却速度 （臨界冷却速度） 以上ではほぼ一定になった。また、臨界冷却速度以上ではほぼマルテンサイト単相組織を示した。表１の鋼種Ｎｏ．６の鋼成分の臨界冷却速度は、１７℃／ｓであった。Ａｃ_１ 点、Ａｃ_３ 点はそれぞれ、７２８ ℃、８２３ ℃であった。
【００５７】
次に、表２の結果をみると、最高加熱温度がＡｃ_１ 点未満、すなわち７２８ ℃未満の場合、温度の上昇に伴って鋼板は回復、再結晶して硬度が低下している（番号２−１ 、２−２）。最大加熱温度からめっき浴までの冷却速度の影響は小さい（番号２−９ 、２−１０）。一方、最高加熱温度がＡｃ_１ 点以上の場合、冷却速度が速いと硬度が上昇している（番号２−３ 〜２−８ 、２−１１〜１６）。さらに、合金化温度がＡｃ_１ 点より高い場合は（番号２−１８、２−２０）、硬度が上昇している。本発明範囲の条件の場合は、いずれも硬度（Ｈｖ）が２００ 以下であり、良好な通板性が確保できる。
【００５８】
【表２】

【００５９】
【発明の効果】
以上説明してきたように、本発明によれば、安価なめっき鋼板を用いて熱間プレスを行うことで、加熱炉の雰囲気制御設備が不要となるほか、、プレス成形時の鉄系酸化物の剥離処理工程も不要となり生産工程を簡素化できコスト削減を図ることができる。また犠牲防食効果のある亜鉛めっき層を有するためプレス成形製品の耐食性も向上する。
【図面の簡単な説明】
【図１】連続溶融亜鉛めっきラインを模擬する熱履歴の模式図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel sheet for hot pressing used for manufacturing mechanical structural parts such as body structural parts and undercarriage parts of automobiles.
[0002]
[Prior art]
In recent years, efforts have been made to increase the strength of steel materials and reduce the weight used to reduce the weight of automobiles. However, in the case of thin steel sheets widely used in automobiles, the press formability decreases with the increase in the strength of the steel sheets, and it becomes difficult to manufacture a complicated shape. Specifically, there arises a problem that ductility is reduced and a fracture occurs at a portion where the degree of processing is high, and that spring back and wall warpage are increased and dimensional accuracy is deteriorated. Therefore, it is not easy to manufacture parts by press molding using a high-strength steel plate, particularly a steel plate of 780 MPa class or higher. According to roll forming instead of press forming, it is possible to process a high-strength steel sheet, but it can be applied only to parts having a uniform cross section in the longitudinal direction.
[0003]
On the other hand, as disclosed in British Patent Publication No. 1490535, in a method called hot pressing in which a heated steel sheet is press-formed, since the steel sheet is soft and highly ductile at high temperatures, a complicated shape can be measured with dimensional accuracy. It is possible to mold well. Further, the steel sheet is heated to the austenite region and rapidly cooled in the mold, thereby simultaneously increasing the strength of the steel sheet by martensitic transformation.
[0004]
Although hot pressing is an excellent forming method as described above, it requires heating to a high temperature of 800 to 1000 ° C., which causes a problem that the steel sheet surface is oxidized. At this time, there is a problem that the scale formed of iron oxide is dropped at the time of pressing and adheres to the mold to reduce the productivity, or that such scale remains in the product after pressing and the appearance is poor. is there. Moreover, if such scales remain on the product, the adhesion between the steel sheet and the coating film will be poor when coating is performed in the next step, and the corrosion resistance will decrease. Therefore, after hot press forming, a scale removal treatment such as shot blasting is required, and a cost increase is unavoidable. Further, even if alloy steel or stainless steel is used in order to prevent such scale from being formed during heating, generation of scale cannot be completely prevented, and the cost will be significantly increased as compared with ordinary steel. Although it is theoretically effective to make the atmosphere at the time of heating and the entire pressing process a non-oxidizing atmosphere, it greatly increases equipment costs.
[0005]
In order to solve such a problem, Japanese Patent Application Laid-Open No. 2000-38640 proposes a steel sheet coated with aluminum so as to have oxidation resistance during hot forming. In comparison, the cost is greatly increased.
[0006]
Under these circumstances, hot presses are not fully utilized even today.
JP-A-2000-144238 and JP-A-2000-248338 disclose a technique in which a part of a part formed by a conventional method is rapidly cooled after high-frequency induction heating to transform and strengthen a steel sheet. In these, steel plates having a zinc-based coating are used for rust prevention. However, in order to suppress the dissipation of zinc by heating, there is a restriction that the heating temperature is set to 850 ° C. or less or the heating time is shortened. . Heating at 850 ° C. or lower does not result in an austenite single phase, so that the martensite volume ratio after quenching decreases and high strength cannot be obtained. Short-time heating may cause a problem that cementite cannot be completely dissolved, so that the amount of dissolved carbon decreases, and the strength after quenching becomes insufficient.
[0007]
Considering application of these techniques to hot pressing, it is difficult to heat and quench in a short time with equipment. Furthermore, it is not known whether the coating is damaged when processed at high temperatures. Therefore, it is difficult to obtain a member having high strength and excellent corrosion resistance even if these techniques are diverted to a hot press as it is.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide an inexpensive hot-pressing steel sheet that can produce high-strength components by hot pressing and can also secure corrosion resistance after forming without increasing the number of special steps and equipment for suppressing scale formation. It is an object of the present invention to provide a manufacturing method thereof.
[0009]
[Means for Solving the Problems]
The inventors of the present invention have made intensive studies to prevent scale formation by surface treatment of a steel sheet, and have found that the presence of a Zn—Fe alloy film is effective in preventing scale.
[0010]
Generally, the melting point of these alloy phases is about 800 ° C. or less, so that when heated to a temperature of 800 to 1000 ° C. necessary for hot pressing, the Zn—Fe alloy coating layer (hereinafter referred to as a plating layer) melts. It was thought that a scale (hereinafter, simply referred to as a scale) composed of an iron-based oxide harmful to the hot press by evaporating and disappearing from the surface was generated.
[0011]
However, the present inventors unexpectedly predicted that the initial Fe mass% of the plating layer, that is, the Fe content and the plating adhesion amount of the plating layer were optimized, thereby preventing evaporation and disappearance of the plating layer, and at the same time, It has been found that scale formation is suppressed. The mechanism was thought as follows.
[0012]
When a steel sheet having an optimized plating layer is heated, a ZnO 2 layer is formed on the entire surface of the plating layer during the heating step, thereby suppressing the evaporation of Zn. Further, interdiffusion of Fe and Zn occurs between the base steel sheet and the plating layer, and as a result, the Fe mass% in the plating layer increases, and an Fe—Zn alloy layer is formed. That is, when a steel sheet having an optimized plating layer is heated, a three-layer structure of a ZnO 2 layer, an Fe—Zn alloy layer, and a base steel sheet is obtained in this order from the surface, and a scale harmful to hot pressing is not formed.
[0013]
Until this ZnO 2 layer is entirely formed on the surface, the formation of the ZnO 2 layer and the evaporation of Zn compete with each other. Therefore, when the initial Fe mass% of the plating layer is low and the melting point is low, the interdiffusion of Fe and Zn is sufficiently promoted. First, because the vapor pressure of Zn increases, Zn evaporates before the ZnO 2 layer is formed on the entire surface, and scale generation cannot be suppressed. On the other hand, when the initial Fe mass% of the plating layer is high, the ZnO 2 layer is difficult to be formed on the entire surface, and the lower Fe—Zn alloy layer is oxidized, and scale is easily generated. When the amount of plating is small, the formation of scale is not suppressed because the ZnO 2 layer is not sufficiently formed. When the amount of plating is large, the ZnO 2 layer is sufficiently formed, but the interdiffusion of Fe and Zn is prevented. Does not proceed sufficiently, and the molten zinc layer remains, so that the mold is contaminated during pressing.
[0014]
As described above, by appropriately adjusting the initial Fe mass% of the plating layer and the plating adhesion amount in advance, it is possible to perform hot pressing while suppressing generation of scale.
Note that this ZnO 2 layer is thin and does not harm by peeling. It has also been confirmed that the coating adhesion is not hindered.
[0015]
The present invention completed based on the above findings is as follows.
(1) A steel sheet containing, by mass%, 0.08 to 0.45% of C: 0.5 to 3.0% in total of Mn and / or Cr; a Fe-Zn alloy having a Fe content of 5 to 80% by mass; A hot-pressing steel sheet which has a Zn plating layer having an adhesion amount of 10 to 90 g / m ² , is heated to 800 to 1000 ° C., and is pressed after forming a ZnO layer .
[0016]
(2) The steel sheet further contains, by mass%, Si: 0.5% or less, P: 0.05% or less, S: 0.05% or less, Ni: 2% or less, Cu: 1% or less, Mo 1% or less, V: 1% or less, Ti: 1% or less, Nb: 1% or less, Al: 1% or less, and N: 0.01% or less. The steel sheet for hot pressing as described in (1) above, which is contained.
[0017]
(3) The steel sheet for hot pressing as described in (1) or (2) above, wherein the steel sheet further contains B: 0.0001 to 0.004% by mass%.
(4) The steel sheet for hot pressing according to any of (1) to (3) above, wherein the steel sheet does not contain martensite .
(5) For example, in a manufacturing method of performing hot-dip galvanizing and alloying on a steel sheet obtained by performing hot rolling or cold rolling by a conventional method, in a continuous hot-dip galvanizing line, a maximum heating temperature is Ac less than ₁ point, the alloying treatment temperature is 500 ° C. or higher, above, wherein the at most _one point Ac (1) ~ (4) between the heat of any one Manufacturing method of steel sheet for press.
[0018]
(6) steel, in a continuous galvanizing line, in the manufacturing method of the galvanized steel sheet to perform galvanizing and alloying, and the maximum heating temperature in the continuous galvanizing line or Ac1 point, the Any of (1) to (3) above, wherein the average cooling rate from the maximum heating temperature to 500 ° C. is less than the critical cooling rate of steel, and the alloying treatment temperature is 500 ° C. or more and _one point of Ac or less. A method for producing a steel sheet for hot pressing as described in Crab.
(7) In a method for producing a hot-dip galvanized steel sheet, in which a hot-dip galvanizing and alloying treatment is performed on a steel sheet in a continuous hot-dip galvanizing line, the maximum heating temperature in the continuous hot-dip galvanizing line is at least _one point of Ac , the average not the cooling rate to form a martensite cooling rate from the maximum heating temperature to 500 ° C., the alloying treatment temperature is 500 ° C. or higher, above, wherein the at most _one point Ac (1) ~ (3) The method for producing a steel sheet for hot pressing according to any one of the above.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, in the present invention, the reason why the steel composition, the plating composition, and the heating conditions during production are limited to the respective ranges will be described. In this specification, “%” for an alloy element represents “% by mass”.
[0020]
1. Regarding the composition of the base steel sheet, the chemical composition of the steel sheet is specified as follows.
C: 0.08 to 0.45%
C is an important element that enhances the hardenability of the steel sheet and determines the strength after hot pressing. However, if the C content is less than 0.08%, the effect is not sufficient. On the other hand, if the C content exceeds 0.45%, toughness and weldability deteriorate. A more desirable C content is 0.1 to 0.3%.
[0021]
Mn and / or Cr (total): 0.5 to 3.0%
Mn and Cr are very effective elements for enhancing the hardenability of the steel sheet and stably securing the strength after hot pressing. However, if the total content of (Mn and / or Cr) is less than 0.5%, the effect is not sufficient, while if the total content of (Mn and / or Cr) exceeds 3.0%, the effect is saturated. On the contrary, it is difficult to secure stable strength. A more desirable total content of (Mn and / or Cr) is 0.8 to 2.0%.
[0022]
According to the present invention, it is sufficient that the hardenability by hot pressing can be ensured, and for that purpose, as described above, it is sufficient that the contents of C, Mn and Cr are specified.
[0023]
In a preferred embodiment of the present invention, in order to further increase the strength or to realize them more stably, the additional elements are defined as follows.
Si: 0.5% or less, P: 0.05% or less, S: 0.05% or less, Ni: 2% or less, Cu: 1% or less, Mo: 1% or less, V: 1% or less, Ti: 1% or less, Nb: 1% or less, Al: 1% or less, N: 0.01% or less These elements are also elements that increase the hardenability of the steel sheet and are effective in ensuring stable strength after hot pressing. . However, if the content exceeds the upper limit, the effect is small and the cost is unnecessarily increased. Therefore, the content of each alloy element is set in the above range.
[0024]
However, P and S may be inevitably present, and Si and / or Al may be added as a deoxidizer.
B: 0.0001 to 0.004%
B is an important element that enhances the hardenability of the steel sheet and further enhances the effect of ensuring the stability after hot pressing. However, if the B content is less than 0.0001%, the effect is not sufficient, while if the B content exceeds 0.004%, the effect is saturated and the cost is increased. A more desirable B content is 0.0005 to 0.002%.
[0025]
2. Fe mass%: 5 to 80% for Fe—Zn alloy layer (plating layer)
By optimizing the initial Fe content in the plating layer, it is possible to suppress the generation of scale harmful to hot pressing as described above even when heated in the air. When the initial Fe mass% of the plating layer is low and the melting point is low, the interdiffusion of Fe and Zn does not proceed sufficiently and the vapor pressure of Zn increases, so that Zn evaporates before the ZnO layer is formed on the entire surface, Scale generation cannot be suppressed. On the other hand, when the initial Fe mass% of the plating layer is high, the ZnO 2 layer is difficult to be formed on the entire surface, the lower Fe—Zn alloy layer is oxidized, and scale is easily generated. A desirable range is 10 to 30%. A more desirable range is 13 to 20%.
[0026]
Here, the term “initial” means prior to generation of ZnO 2 by heating, and more specifically, prior to heating performed in hot pressing.
[0027]
The composition of the plating layer other than Fe and Zn is not particularly limited, and is a plating layer to which alloy elements such as Al, Mn, Ni, Cr, Co, Mg, Sn, and Pb are appropriately added according to the purpose. There may be. Some of Be, B, Si, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cu, Sr, etc. which may be inevitably mixed from other raw materials, etc. There is also.
[0028]
Such a plating layer is usually provided directly on the surface of the steel sheet, but another plating layer or the like may be interposed between the plating layer and the surface of the steel sheet. The plating layer is usually provided on both sides, but as long as the other surface has a pretreatment layer that is not harmful to hot pressing, or a protective layer, the plating layer according to the present invention may be provided on only one surface. Good. Regarding the amount of plating applied: Converted to the amount of plating applied: Zn weight, 10 to 90 g / m ² per one surface of the steel sheet.
By optimizing the amount of plating applied, scale formation harmful to hot pressing can be suppressed as described above even when heated in air. When the amount of plating is small, the formation of scale is not suppressed because the ZnO layer is not sufficiently formed. When the amount of plating is large, the ZnO layer is sufficiently formed, but the mutual diffusion of Fe and Zn is suppressed. Since the molten zinc layer does not sufficiently advance and remains, the molten zinc layer is scattered at the time of pressing and the mold is contaminated. More preferable range is 20 to 80 g / ^{m 2.}
[0029]
3. Manufacturing Method The steel sheet according to the present invention is heated to an austenite region or near an austenite region during hot pressing, and is press-formed in that temperature range. Therefore, the mechanical properties at room temperature before heating are not important, and the metal structure before heating is not particularly specified. That is, any of a hot-rolled steel sheet and a cold-rolled steel sheet may be used as the base steel sheet before plating, and any steel sheet may be used, and the manufacturing method thereof is not limited. However, a preferable manufacturing method is described below from the viewpoint of productivity.
[0030]
Hot rolling:
Hot rolling may be performed by a conventional method, and is preferably performed in an austenite region from the viewpoint of rolling stability. When the winding temperature is low, the strength increases due to a martensite structure, and it becomes difficult to pass a continuous hot-dip galvanizing line and to perform cold rolling. On the other hand, if the winding temperature is high, the oxide scale becomes thicker, and the efficiency of the subsequent pickling is reduced, and when direct plating is performed without pickling, the plating adhesion is deteriorated. Therefore, the winding temperature is preferably from 500 to 600 ° C.
[0031]
Cold rolling:
Cold rolling is performed by a conventional method. Since the steel sheet of the present invention has a large amount of carbon, cold rolling at an excessive rolling reduction increases the load on the mill. Further, if the strength after cold rolling becomes too high due to work hardening, the welding strength at the time of coil connection and the line passing ability become problems in the galvanized line. Therefore, the rolling reduction is preferably 80% or less, and more preferably 70% or less.
[0032]
In addition, since performing cold rolling increases costs accordingly, for a steel sheet having a sheet thickness and a sheet width that can be manufactured by hot rolling, it is preferable to omit cold rolling and use a steel sheet as hot rolled. .
[0033]
Galvanized:
The method for forming the plating layer of the plated steel sheet according to the present invention is not limited, such as hot-dip galvanizing, electroplating, thermal spraying, and vapor deposition. Further, the steel strip may be continuously processed, or may be processed as a single cut plate. Generally, it is preferable to use a continuous hot-dip galvanizing line having excellent production efficiency. Therefore, the continuous galvanizing method will be described below.
[0034]
In a normal continuous hot-dip galvanizing line, a heating furnace, a cooling zone, a hot-dip galvanizing bath, and an alloying furnace are continuously arranged. In the present invention, since the metal structure of the base steel sheet is not particularly limited, the heat pattern in the heating furnace and the cooling zone is not particularly limited. However, since the steel sheet according to the present invention has a high carbon content and is a component that is easy to burn, there is a possibility that the steel sheet will have a very high strength in the line. Considering the ease of threading and the manufacturable range (sheet thickness, sheet width), a heat pattern in which the steel sheet does not have excessively high strength is preferable.
[0035]
Maximum heating temperature:
If the heating temperature in the heating furnace is less than _one point Ac in the heating performed before the hot-dip plating, the steel sheet recovers and recrystallizes during the heating, and the strength is reduced as compared with that before the heating. Therefore, there is no problem in the sheet passing property. From the viewpoint of saving the heating energy of the furnace, it is preferable that the heating is performed at a low temperature as long as the plating property is not hindered.
[0036]
On the other hand, when the maximum heating temperature is equal to or higher than the Ac ₁ point, recovery and recrystallization of the steel sheet occur during heating, an austenite phase appears, and a high-strength transformation phase is formed depending on the subsequent cooling conditions.
[0037]
Cooling rate:
The hot-dip plating bath is usually maintained at about 460 ° C., and the heated steel sheet is cooled to the plating bath temperature. At this time, if the heating temperature is lower than Ac ₁ point, the cooling rate does not affect the metal structure, and therefore, cooling may be performed at an arbitrary rate.
[0038]
When austenite is generated by heating to more than _one point of Ac, if the cooling rate is too high, austenite is transformed into a structure mainly composed of bainite or martensite, and the strength of the steel sheet is undesirably increased. Specifically, it is preferable that the average cooling rate from the maximum heating temperature to 500 ° C. is equal to or lower than the critical cooling rate.
[0039]
The method of measuring the critical cooling rate will be more specifically described in Examples described later.
The critical cooling rate is used as an index of the hardenability of the steel sheet, and is a cooling rate at which a martensite single phase structure is generated. Even if a small amount of bainite or martensite is contained in the steel sheet cooled under the above conditions, the effect of the production method of the present invention is not denied. However, from the viewpoint of increasing the sheet passing property by reducing the strength as much as possible, it is preferable to make the cooling rate as low as possible and not to form martensite.
[0040]
Hot dip galvanizing:
The steel sheet may be immersed in a molten zinc and zinc alloy plating bath and pulled up by an ordinary method. The plating amount is controlled by adjusting the lifting speed and the flow rate of the wiping gas blown from the nozzle.
[0041]
Alloying treatment temperature:
After performing a hot-dip galvanizing process, it is performed by reheating in a gas furnace, an induction heating furnace, or the like, metal diffusion is performed between the plating layer and the base steel sheet, and alloying proceeds.
[0042]
In order to increase the initial Fe mass% in the plating layer to 5 to 80% according to the present invention, alloying at 500 ° C. or more is desirable. If the alloying temperature is lower than 500 ° C., the alloying speed is low, so that it is necessary to reduce the line speed, which impairs productivity and requires equipment measures such as lengthening the alloying furnace. The higher the alloying temperature is, the faster the alloying speed is. However, at the alloying temperature of _one or more Ac, the steel sheet is strengthened for the same reason as the case of the maximum heating temperature described above. A preferred range is 550-650 ° C.
[0043]
Post-processing:
Post-processing such as Fe plating may be performed as needed. When plating is performed as a post-treatment, the condition defined in the present invention may be satisfied in total with the hot-dip plating layer. Usually, the amount of plating applied in the post-treatment is small, and thus does not affect the essence of the present invention.
[0044]
Temper rolling:
In order to correct the flatness of the steel sheet and adjust the surface roughness, temper rolling may be performed.
4. The hot press forming of the steel sheet according to the present invention is not particularly limited, and the usual press forming may be performed by hot working. That is, if a steel sheet heated to _three or more Ac points is formed by a method capable of cooling at a critical cooling rate or higher, the highest strength determined by the base metal component can be obtained.
[0045]
【Example】
(Example 1)
Steel having the composition shown in Table 1 was melted in a laboratory to form a slab. After heating this slab at 1200 ° C. for 30 minutes, it was hot-rolled at 900 ° C. or higher to obtain a steel sheet having a thickness of 3.2 mm. After hot rolling, the film is cooled down to 550 ° C. by water spray, charged in a furnace, kept at 550 ° C. for 30 minutes, and gradually cooled to room temperature at 20 ° C./hour, thereby taking up the winding process after hot rolling. Was simulated. After removing scale by pickling, the hot-rolled sheet was cold-rolled to a sheet thickness of 1.0 mm. The cut sheet of the base steel sheet was subjected to hot-dip galvanizing using a plating simulator, and then subjected to alloying treatment. The Fe content of the plating layer was changed by changing the alloying temperature (500 to 800 ° C.) and the time (30 minutes or less).
[0046]
The steel plate cut into strips having a width of 50 mm was heated at 850 ° C. for 3 minutes in a heating furnace in an air atmosphere, taken out of the heating furnace, and hot-pressed into a hat shape in a high temperature state as it was. At this time, the die was formed with a punch width of 50 mm, a punch shoulder R of 5 mm, a die shoulder R of 5 mm, and a molding depth of 25 mm. Further, Vickers hardness measurement (load: 9.8 N, measurement number: 10) was also performed at the center of the hat standing wall portion after pressing. In this example, the temperature of the steel plate reached 850 ° C. in about 2 minutes.
[0047]
The thus obtained hot press-formed product was evaluated for appearance after coating, coating film adhesion, and corrosion resistance after painting (hereinafter referred to as corrosion resistance) in the following manner.
[0048]
(1) Appearance after molding The presence or absence of formation of a harmful scale composed of an iron-based oxide was evaluated. In addition, the presence or absence of contamination due to scattering of the residual molten zinc layer in the press mold was evaluated. When no mold contamination was found, it was evaluated as ○, and when there was mold contamination, it was evaluated as ×.
[0049]
(2) Coating film adhesion test A test piece cut out from the single-piece hat molded product obtained in this example was treated with zinc phosphate under ordinary chemical conversion conditions with PBL-3080 manufactured by Nippon Parkerizing Co., Ltd., and then electrodeposited by Kansai Paint. The paint GT-IO was electrodeposited by applying a slope voltage of 200 V, and baked at a baking temperature of 150 ° C. for 20 minutes. The coating thickness was 20 μm. The test piece was immersed in ion-exchanged water at 50 ° C., taken out after 240 hours, cut into a 1 mm-wide grid with a cutter knife, and subjected to a peeling test with a Nichiban polyester tape to determine the number of remaining squares of the coating film. For comparison, the coating film adhesion was evaluated. The total number of cells was 100. The evaluation criteria were as follows: the number of residual cells of 90 to 100 was evaluated as good: evaluation symbol ○, and 0 to 89 was evaluated as poor: evaluation symbol x.
[0050]
(3) Corrosion resistance test after painting (corrosion resistance test)
After coating in the same manner as in the coating film adhesion test, the coating film of the test piece was scratched with a cutter knife to reach the substrate, and subjected to a salt spray test specified in JIS Z2371 for 480 hours. The swollen width or rust width of the coating film from the scratch was measured, and the corrosion resistance after coating was evaluated. The evaluation criterion was a value of 0 mm or more and less than 4 mm, whichever is larger, whichever is the rust width or the swollen width of the coating film.
[0051]
Table 1 summarizes the above results.
[0052]
[Table 1]

[0053]
Steel type No. of the present invention example. In the cases of Nos. 1 to 10, the results of formation of the iron-based oxide, mold contamination, coating film adhesion and corrosion resistance were good. On the other hand, steel type No. In Nos. 11 to 15, none of them could satisfy the formation condition of iron-based oxide, mold contamination, coating film adhesion and corrosion resistance at the same time.
[0054]
(Example 2)
In Table 1, steel type No. Steel having the composition shown in No. 6 was melted in a laboratory to form a slab. After heating this slab at 1200 ° C. for 30 minutes, it was hot-rolled at 900 ° C. or higher to obtain a steel sheet having a thickness of 3.2 mm. After hot rolling, the film is cooled by water spraying to 550 ° C., then charged into a furnace, kept at 550 ° C. for 30 minutes, and gradually cooled to room temperature at 20 ° C./hour, thereby taking up the winding process after hot rolling. Was simulated. After removing scale by pickling, the hot-rolled sheet was cold-rolled to a sheet thickness of 1.0 mm. This steel sheet was given a thermal history simulating a hot-dip galvanizing line using an annealing simulator. The specific heat history is shown in FIG. Table 2 also shows the results of measuring the cross-sectional Vickers hardness of the steel sheet after the heat treatment (load: 49 N, number of measurements: 5).
[0055]
In addition, a critical cooling rate was measured as an index of hardenability of the steel. A cylindrical test piece having a diameter of 3.0 mm and a length of 10 mm was cut out from the hot-rolled sheet, heated to 950 ° C. in the atmosphere at a heating rate of 100 ° C./min, kept at that temperature for 5 minutes, and then cooled in various ways. Cooled to room temperature at speed. Thereafter, Vickers hardness measurement (49 N of load, number of measurements: 5) and structure observation of the obtained test piece were performed. In addition, _one point Ac and _three points Ac were measured by measuring the thermal expansion change of the test piece during heating and cooling.
[0056]
The hardness increased as the cooling rate from 950 ° C. increased, and became almost constant above a certain cooling rate (critical cooling rate). Above the critical cooling rate, it showed almost a martensite single phase structure. In Table 1, steel type No. The critical cooling rate of the steel component No. 6 was 17 ° C./s. Ac ₁ point and Ac ₃ point were 728 ° C. and 823 ° C., respectively.
[0057]
Next, looking at the results in Table 2, when the maximum heating temperature is lower than the Ac ₁ point, that is, lower than 728 ° C., the steel sheet recovers and recrystallizes as the temperature increases, and the hardness decreases (No. 2). -1, 2-2). The effect of the cooling rate from the maximum heating temperature to the plating bath is small (numbers 2-9, 2-10). On the other hand, when the maximum heating temperature is equal to or higher than Ac ₁ point, the hardness increases when the cooling rate is high (numbers 2-3 to 2-8, 2-11 to 16). Further, when the alloying temperature is higher than the Ac ₁ point (Nos. 2-18, 2-20), the hardness increases. In the case of the conditions within the range of the present invention, the hardness (Hv) is 200 or less in all cases, and good threadability can be secured.
[0058]
[Table 2]

[0059]
【The invention's effect】
As described above, according to the present invention, by performing hot pressing using an inexpensive plated steel sheet, the atmosphere control equipment of the heating furnace is not required, and the iron-based oxide during press forming is not required. The peeling process is not required, so that the production process can be simplified and the cost can be reduced. Further, the corrosion resistance of the press-formed product is improved due to the presence of the galvanized layer having a sacrificial corrosion prevention effect.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a thermal history simulating a continuous galvanizing line.

Claims (7)

In a steel sheet containing 0.08 to 0.45% by mass of C and 0.5 to 3.0% in total of Mn and / or Cr in terms of mass%, an Fe-Zn alloy having a Fe content of 5 to 80% by mass and a Zn adhesion amount of A steel sheet for hot pressing, which has a Zn plating layer of 10 to 90 g / m ² and is heated to 800 to 1000 ° C. and pressed after forming a ZnO layer . The steel sheet further contains, by mass%, Si: 0.5% or less, P: 0.05% or less, S: 0.05% or less, Ni: 2% or less, Cu: 1% or less, Mo: 1% or less, V: 1%. 2. The method according to claim 1, further comprising one or more selected from the group consisting of Ti: 1% or less, Nb: 1% or less, Al: 1% or less, and N: 0.01% or less. A steel sheet for hot pressing as described. The steel sheet for hot press according to claim 1 or 2, wherein the steel sheet further contains B: 0.0001 to 0.004% by mass%. The steel sheet for hot pressing according to any one of claims 1 to 3, wherein the steel sheet does not contain martensite. In the manufacturing method of hot-dip galvanized steel sheet, in which the hot-dip galvanizing steel sheet is subjected to hot-dip galvanizing and alloying treatment in a continuous hot-dip galvanizing line, the maximum heating temperature in the continuous hot-dip galvanizing line is less than ₁ point of Ac, The method for producing a steel sheet for hot pressing according to any one of claims 1 to 4 , wherein the temperature is 500 ° C or more and ₁ point or less of Ac. In a method for producing a hot-dip galvanized steel sheet in which a steel sheet is subjected to hot-dip galvanizing and alloying treatment in a continuous hot-dip galvanizing line, the maximum heating temperature in the continuous hot-dip galvanizing line is ₁ point or more of Ac, The heat according to any one of claims 1 to 3, wherein the average cooling rate from the temperature to 500 ° C is less than the critical cooling rate of steel, and the alloying treatment temperature is 500 ° C or more and _one point of Ac or less. Manufacturing method of steel sheet for hot pressing. In the production method of hot-dip galvanized steel sheet, in which hot-dip galvanizing and alloying treatment are performed on a continuous hot-dip galvanizing line, the maximum heating temperature in the continuous hot-dip galvanizing line Ac ₁ Above the point, from the maximum heating temperature 500 The average cooling rate up to ℃ is defined as the cooling rate that does not form martensite. 500 Over ℃, Ac ₁ The method for manufacturing a steel sheet for hot pressing according to any one of claims 1 to 3, wherein

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