JP2022071515A - Hot press member, steel sheet for hot pressing, and production method of hot press member - Google Patents

Abstract

To provide a hot press member and a production method thereof with excellent corrosion resistance after painting and resistance spot weldability, and to provide a steel sheet for hot pressing suitable for the hot press member with excellent corrosion resistance after painting and resistance spot weldability.SOLUTION: A hot press member comprises a steel sheet having Zn-based plating layers on its both surfaces. One surface of the steel sheet has an amount of Zn deposition of 36 g/m2 or more, and has an average line roughness Ra of 2.5 μm or less. The other surface of the steel sheet has an average line roughness Ra of 3.5 μm or more.SELECTED DRAWING: None

Description

The present invention relates to a hot pressed member, a steel sheet for hot pressing, and a method for manufacturing the same. In particular, the present invention relates to a method for manufacturing a hot pressed member, a steel sheet for hot pressing, and a hot pressed member having excellent post-painting corrosion resistance and resistance spot weldability when a zirconium-based chemical conversion treatment is applied.

In recent years, in the field of automobiles, the performance of raw steel sheets has been improved and the weight has been reduced, and the use of high-strength hot-dip galvanized steel sheets or electrogalvanized steel sheets having rust resistance is increasing. However, in many cases, as the strength of the steel sheet increases, its press formability deteriorates, so that it becomes difficult to obtain a complicated part shape. For example, in automobile applications, rustproofing is required, and examples of difficult-to-mold parts include undercarriage members such as chassis and structural members for skeletons such as B-pillars.

Against this background, in recent years, the production of automobile parts by hot pressing, which makes it easier to achieve both press formability and high strength compared to cold pressing, has been rapidly increasing, and various hot pressing technologies have been used. Various techniques for solving the problems are disclosed.

Among them, the Zn—Ni alloy plated steel sheet is attracting attention as a hot press steel sheet because the melting point of the plated layer is high, and a hot press member using this steel sheet and a method for manufacturing the same have been proposed.

For example, Patent Document 1 discloses a hot press member having an α-Fe (Zn, Ni) mixed crystal, an intermetallic compound of Zn, Ni and Fe, and a layer containing Mn.

Further, Patent Document 2 discloses a hot pressing member having a Ni diffusion region, an intermetallic compound layer corresponding to the γ phase, and a ZnO layer.

Special Table 2013-503254A Japanese Unexamined Patent Publication No. 2011-246801 Japanese Unexamined Patent Publication No. 2004-323897

However, in recent years, zirconium-based chemical conversion treatment has begun to spread in place of the conventional zinc phosphate-based chemical conversion treatment, and post-coating corrosion resistance of members subjected to electrodeposition coating after this zirconium-based chemical conversion treatment is also required. It has become.

The hot press members disclosed in Patent Document 1 and Patent Document 2 are both hot press members manufactured by heating a Zn—Ni alloy plated steel sheet, and have corrosion resistance without coating and zinc phosphate-based chemical conversion. Although it is excellent in post-painting corrosion resistance when the treatment is applied, there is a problem that the post-painting corrosion resistance when the zinc-based chemical conversion treatment is applied is insufficient.

On the other hand, resistance spot weldability is also an important characteristic required for hot pressed members. When a zinc-based plated steel sheet is subjected to hot pressing, Zn contained in the plating layer before heating is oxidized in the hot pressing process, so that the surface is mainly composed of zinc oxide and has a thickness of several μm. The oxide film to have is formed. Zinc oxide is a semiconductor, but it has a large resistivity and reduces resistance spot weldability. Therefore, as disclosed in Patent Document 3, the oxide film may be removed by shot blasting or the like for a hot pressed member using a galvanized steel sheet. However, the shot blasting process for ensuring resistance spot weldability increases man-hours and costs, and is a problem when applying a galvanized steel sheet to hot pressing.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot pressed member having excellent post-painting corrosion resistance and resistance spot weldability, and a method for manufacturing the same. Another object of the present invention is to provide a hot pressing steel sheet suitable for a hot pressing member having excellent post-coating corrosion resistance and resistance spot weldability.

The present inventors conducted diligent research in order to achieve the above-mentioned problems, and obtained the following findings.
(1) In order to improve the corrosion resistance of the hot pressed member after painting, it is effective to set the average line roughness Ra of one surface of the hot pressed member to 2.5 μm or less. Further, in order to improve the resistance spot weldability of the hot pressed member, it is effective to set the average line roughness Ra of the other surface of the hot pressed member to 3.5 μm or more.
(2) A method for manufacturing a hot-pressed member in which a steel sheet having a Zn-based plating layer having an adhesion amount of 36 g / m ² or more on both sides of the steel sheet is heated to a temperature range of Ac ₃ transformation point to 1000 ° C. and then hot-pressed. The Zn-based plating layer has cracks that traverse the Zn-based plating layer, and the crack density per unit cross-sectional length in at least one cross section of the Zn-based plating layer on one surface is 30 points / mm or more. The first surface is coated by hot-pressing a hot-pressed steel sheet having a crack density of 20 points / mm or less per unit cross-sectional length in at least one cross section of the Zn-based plating layer on the other side. It is possible to obtain a hot pressed member having excellent post-corrosion resistance and resistance spot weldability.

The present invention is based on the above findings, and its features are as follows.
[1] A Zn-based plating layer is provided on both sides of the steel sheet.
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the average line roughness Ra is 2.5 μm or less.
A hot pressed member having an average linear roughness Ra of 3.5 μm or more on the other surface of the steel sheet.
[2] A Zn-based plating layer is provided on both sides of the steel sheet, and the Zn-based plating layer has cracks that vertically traverse the Zn-based plating layer.
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plating layer is 30 points / mm or more.
A steel sheet for hot pressing in which the crack density per unit cross-sectional length in a cross section of an arbitrary 1 mm length of the Zn-based plating layer on the other surface of the steel sheet is 20 points / mm or less.
[3] A Zn-based plating layer is provided on both sides of the steel sheet, and the Zn-based plating layer has cracks that vertically traverse the Zn-based plating layer.
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plating layer is 30 points / mm or more.
A steel sheet for hot pressing, wherein the crack density per unit cross-sectional length in a cross section of an arbitrary 1 mm length of the Zn-based plating layer on the other surface of the steel sheet is 20 points / mm or less.
The temperature is raised from room temperature to the temperature range of Ac ₃ transformation point to 1000 ° C. for 5 seconds or more and 600 seconds or less, and further, the temperature is maintained in the temperature range of Ac ₃ transformation point to 1000 ° C. for 300 seconds or less, and then hot. A method for manufacturing a hot pressed member to be pressed.

According to the present invention, it is possible to obtain a hot pressed member having excellent post-painting corrosion resistance and resistance spot weldability. Further, the steel sheet for hot pressing of the present invention is suitable for a hot pressing member having excellent post-coating corrosion resistance and resistance spot weldability.

FIG. 1 is a schematic view showing a cross section of a Zn-based plating layer before hot pressing (before heating) when cracks are formed in the Zn-based plating layer. FIG. 2 is a schematic view showing a cross section of the Zn-based plating layer after hot pressing (after heating) when cracks are formed in the Zn-based plating layer.

Hereinafter, embodiments of the present invention will be described. The following description shows a preferred embodiment of the present invention, and is not limited by the following description. In addition, the unit of the content of each element in the steel composition is "mass%", and hereinafter, it is simply indicated by "%" unless otherwise specified.

1) Hot press member The hot press member of the present invention is provided with Zn-based plating layers on both sides of the steel sheet, the amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the average line roughness Ra. Is 2.5 μm or less,
The average line roughness Ra of the other surface of the steel sheet is 3.5 μm or more. The greatest feature of the present invention is that a difference is intentionally provided in the size of the surface unevenness on the front and back surfaces of the hot pressed member.

When a steel sheet having a Zn-based plating layer is hot-pressed, Zn in the plating layer diffuses into the underlying steel sheet, and a solid solution phase containing Fe and Zn is formed in this diffusion region. When the plating layer contains other alloying elements, the alloying elements may be contained. At the same time, Zn in the Zn-based plating layer and oxygen existing in the heating atmosphere may be combined to form an oxide layer containing Zn on the surface of the Zn-based plating layer. Further, the Zn-based plating layer, which is an intermetallic compound that did not contribute to the diffusion into the base steel plate or the formation of the oxide layer, remains as an intermetallic compound phase, but Fe diffused from the base steel plate is taken in. , Zn, Fe and other alloying elements contained in the plating layer. Since both the solid solution phase and the intermetallic compound phase contain Zn having a sacrificial anticorrosion effect, they contribute to the improvement of corrosion resistance. As described above, in order to satisfy the post-painting corrosion resistance which is the subject of the present invention, a plating layer containing Zn is an indispensable constituent requirement, and this Zn-based plating layer is at least a solid solution phase and an intermetallic compound phase. It includes either.

In the present invention, the surface of the Zn-based plating layer may be provided with a layer having a thickness of 0.2 μm or more and mainly composed of a metal oxide. By setting the thickness to 0.2 μm or more, it can be expected that the corrosion resistance after coating is further improved because it is not completely dissolved and remains in the chemical conversion treatment step.

In the present invention, the amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the average line roughness Ra is 2.5 μm or less. When the amount of Zn adhered is less than 36 g / m ² , the corrosion rate of zinc under the coating film becomes remarkably high, and the corrosion resistance after painting is lowered. On the other hand, if the amount of Zn adhered exceeds 120 g / m ² , the effect of improving corrosion resistance after coating is saturated and the cost is only increased. Therefore, the upper limit is preferably 120 g / m ² . For the purpose of further improving the corrosion resistance after coating, the Zn adhesion amount is more preferably 40 g / m ² or more, and further preferably 50 g / m ² or more.

Further, in the present invention, the average line roughness Ra of the surface of the Zn-based plating layer on one surface is 2.5 μm or less. This surface is located on the outer surface of the hot pressed member and is mainly evaluated for its appearance corrosion performance. On the other hand, when a hot pressed member having large irregularities having an average line roughness exceeding 2.5 μm is subjected to zirconium-based chemical conversion treatment and electrodeposition coating to evaluate post-coating corrosion resistance, it is generally not cross-cut. The occurrence of red rust from the part becomes remarkable. It is considered that the reason for this is that the electrodeposition coating does not follow the unevenness of the surface of the hot pressed member, and the film thickness of the electrodeposition coating becomes extremely thin at the convex portion, so that red rust occurs in such a portion. However, since red rust generated from the general part progresses slowly in the plate thickness direction, it does not affect the mechanical properties of the member, but only the appearance quality. The average line roughness Ra is preferably less than 2.2 μm, more preferably less than 2.0 μm.

In the present invention, the average line roughness Ra of the surface of the Zn-based plating layer on the other surface is 3.5 μm or more. This surface is located on the opposite side of one of the above-mentioned surfaces (the surface having an average line roughness Ra of 2.5 μm or less), is located on the inner surface of the hot press member, and is used during resistance spot welding. The surface to be the mating surface (when the surface having an average line roughness Ra of the surface of the Zn-based plating layer is 2.5 μm or less is the surface of the steel sheet, the surface having the average line roughness Ra of 3.5 μm or more is the surface of the steel sheet. Corresponds to the back side). As described above, an oxide film is formed on the surface of the member after hot pressing. Since this has a large resistivity, the thicker and more uniform it is, the lower the resistance spot weldability. Specifically, when a thick oxide film is present on the surface, energization becomes unstable due to the narrowing of the energization path, and scattering (burst) due to local energization occurs at a relatively low welding current. The oxide film has a higher hardness than the metal film and the electrode metal, but has poor toughness. Therefore, it is easily broken by being pressed by the electrode or the steel plate of the mating material. At this time, by setting the average linear roughness Ra of the hot pressed member to 3.5 μm or more, the destruction of the oxide film under electrode pressure in resistance spot welding is promoted, and the energization point is secured. The occurrence of scattering is reduced. The average line roughness Ra is preferably 4.0 μm or more, more preferably 4.5 μm or more.

The amount of Zn adhered to the other surface of the steel sheet (the surface having an average line roughness Ra of the surface of the Zn-based plating layer of 3.5 μm or more) is not particularly limited, but is preferably 36 g / m ² or more. When the amount of Zn adhered is less than 36 g / m ² , the corrosion rate of zinc under the coating film becomes remarkably high, and the corrosion resistance after painting may decrease. On the other hand, if the amount of Zn adhered exceeds 120 g / m ² , the effect of improving corrosion resistance after coating is saturated and the cost is only increased. Therefore, the upper limit is preferably 120 g / m ² . For the purpose of further improving the corrosion resistance after coating, the Zn adhesion amount is more preferably 40 g / m ² or more, and further preferably 50 g / m ² or more.

2) Steel plate for hot pressing The steel sheet for hot pressing of the present invention is provided with Zn-based plating layers on both sides of the steel sheet, and the Zn-based plating layer has cracks that traverse the Zn-based plating layer, and is one of the steel sheets. The amount of Zn adhered to the surface is 36 g / m ² or more, and the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plated layer is 30 points / mm or more, and the other side of the steel sheet. The crack density per unit cross-sectional length in a cross section of an arbitrary 1 mm long surface of the Zn-based plated layer is 20 points / mm or less. In the present invention, as will be described later, it is most economical to apply cracks by utilizing the residual stress in the plating layer, so that the metal constituting the plating layer contains an alloy element. Is desirable. For example, it is preferable to contain 8 to 30% of an element selected from Cr, Fe, Co and Ni. Further, the plating layer may be one in which oxides are dispersed, and those containing 0.1 to 10% of SiO ₂ or Al ₂ O ₃ nanoparticles are exemplified.

In the present invention, the Zn adhesion amount on the surface of the Zn-based plating layer having a crack density of 30 points / mm or more per unit cross-sectional length in an arbitrary 1 mm length cross section is set to 36 g / m ² or more for coating. A hot press member having excellent post-corrosion resistance and resistance spot weldability can be obtained. Although it depends on the heating method, Zn of about 3 to 10 g / m ^{2 in a general furnace heating hot stamp and about 0 to 5 g / m 2 in} an energized heating hot stamp vaporizes and disappears, so it is left in the member after heating. It is desirable to set in anticipation of the amount of Zn. When the adhesion amount is less than 36 g / m ² , the Zn adhesion amount after heating is also less than 36 g / m ² , and as a result, red rust is remarkably generated in the coating defect portion. When the adhesion amount exceeds 120 g / m ² , the above-mentioned red rust suppressing effect is saturated. For the purpose of further improving the corrosion resistance after coating, the Zn adhesion amount is preferably 40 g / m ² or more, and more preferably 50 g / m ² or more.

The amount of Zn adhered to the other surface of the steel sheet, that is, the surface having a crack density of 20 points / mm or less per unit cross-sectional length in an arbitrary 1 mm long cross section of the Zn-based plating layer, is not particularly limited. However, it is preferably 36 g / m ² or more. When the adhesion amount is less than 36 g / m ² , the Zn adhesion amount after heating is also less than 36 g / m ² , and as a result, red rust is remarkably generated in the coating defect portion. When the adhesion amount exceeds 120 g / m ² , the above-mentioned red rust suppressing effect is saturated. For the purpose of further improving the corrosion resistance after coating, the Zn adhesion amount is preferably 40 g / m ² or more, and more preferably 50 g / m ² or more.

In the hot-pressed steel sheet of the present invention, the Zn-based plating layer has cracks that traverse the Zn-based plating layer, and the per unit cross-sectional length in any 1 mm length cross section of the Zn-based plating layer on one surface. The crack density is 30 points / mm or more, and the crack density per unit cross-sectional length in an arbitrary 1 mm length cross section of the Zn-based plating layer on the other surface is 20 points / mm or less. do. When the surface of the steel sheet has a crack density of 30 points / mm or more per unit cross-sectional length in an arbitrary 1 mm length cross section of the Zn-based plated layer, the Zn-based plated layer has an arbitrary 1 mm length. The back surface is the surface where the crack density per unit cross-sectional length in the cross section is 20 points / mm or less.

When a steel sheet for hot pressing (FIG. 1) having a crack density of 30 points / mm or more is subjected to hot pressing, an oxide layer is formed in a portion where cracks exist before hot pressing (FIG. 2). Even if the Zn-based alloy plating layer is melted and turned into a liquid by heating before hot pressing, the entire Zn-based plating layer forms large irregularities because the oxide layer divides the intermetallic compound phase. There is nothing to do. Therefore, when viewed as a whole Zn-based plating layer, a hot pressed member having a flat Zn-based plating layer can be obtained.

On the other hand, when the temperature of the steel sheet for hot pressing with a crack density of 20 points / mm or less is raised by heating before hot pressing, an oxide layer is formed on the surface of the Zn-based plating layer as the temperature rises. Is formed. When the temperature of the steel sheet eventually exceeds the melting point of the Zn-based plating layer, the plating layer located between the oxide layer and the steel sheet melts and becomes a liquid. If heating continues and the temperature of the steel sheet continues to rise, the oxide layer will continue to grow, but at this time, the oxide layer will increase its thickness in the direction perpendicular to the surface of the plating layer. It grows by increasing, and the oxide layer grows by increasing its surface surface while forming irregularities in the direction horizontal to the surface of the plating layer. This is because the plating layer located between the oxide layer and the steel plate is a fluid liquid, so that the oxide layer can change its shape. The crack density on this surface may be 0 points / mm.

From the viewpoint of ensuring resistance spot weldability, it is particularly effective to increase the surface unevenness on the mating surface side with the mating material. Therefore, it is desirable that the crack density on one surface of the hot pressed steel sheet is small. On the other hand, the surface unevenness induces a local thinning of the electrodeposition coating film and lowers the corrosion resistance after coating. Therefore, it is important to impart high-density cracks on the evaluation surface side of the corrosion resistance after painting to flatten the surface after heating.

In the hot-pressed steel sheet of the present invention, the cracks that divide the Zn-based plating layer are formed in the direction perpendicular to the surface of the Zn-based plating layer, that is, from the surface of the Zn-based plating layer toward the base steel sheet side. It shall refer to a crack. The width of the crack is 5 μm or less, more preferably 2 μm or less, from the viewpoint of corrosion resistance after painting.

The Zn-based plating layer in the hot-pressed steel sheet of the present invention may be a single-layer Zn-based plating layer, but it is a lower layer film depending on the purpose as long as it does not affect the action and effect of the present invention. Alternatively, an upper layer film may be provided. For example, as the underlayer film, a base plating layer mainly composed of Ni is exemplified.

In the present invention, in order to obtain a hot-pressed member having a temperature exceeding 1470 MPa class after hot-pressing, it is used as a base steel sheet for a Zn-based plating layer in a hot-pressed steel sheet, for example, by mass%, C: 0.20. ~ 0.50%, Si: 0.1 ~ 0.5%, Mn: 1.0 ~ 3.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.1% Hereinafter, a steel sheet containing N: 0.01% or less and having a component composition in which the balance is composed of Fe and unavoidable impurities can be used. The steel plate may be either a cold-rolled steel plate or a hot-rolled steel plate. The reasons for limiting each component are described below.

C: 0.20 to 0.50%
C improves the strength by forming martensite or the like as a steel structure. In order to obtain a strength exceeding 1470 MPa class, 0.20% or more is preferable. On the other hand, if it exceeds 0.50%, the toughness of the spot welded portion decreases. Therefore, the amount of C is preferably 0.20 to 0.50%.

Si: 0.1-0.5%
Si is an effective element for strengthening steel to obtain a good material. For that purpose, 0.1% or more is preferable. On the other hand, if it exceeds 0.5%, the ferrite is stabilized and the quenchability is lowered. Therefore, the amount of Si is preferably 0.1 to 0.5%.

Mn: 1.0-3.0%
Mn is an element effective for obtaining strength after cooling in a wide cooling rate range. In order to secure mechanical properties and strength, it is preferable to contain 1.0% or more. On the other hand, if it exceeds 3.0%, not only the cost increases but also the effect is saturated. Therefore, the amount of Mn is preferably 1.0 to 3.0%.

P: 0.02% or less When the amount of P exceeds 0.02%, the balance between strength and ductility deteriorates through deterioration of local ductility due to grain boundary embrittlement due to P segregation to austenite grain boundaries during casting. Therefore, the amount of P is preferably 0.02% or less.

S: 0.01% or less S becomes inclusions such as MnS and causes deterioration of impact resistance and cracking along the metal flow of the welded portion. Therefore, it is desirable to reduce it as much as possible, and it is preferably 0.01% or less. Further, in order to secure good stretch flangeability, it is more preferably 0.005% or less.

Al: 0.1% or less When the Al amount exceeds 0.1%, the blanking workability and hardenability of the raw steel sheet are deteriorated. Therefore, the amount of Al is preferably 0.1% or less.

N: 0.01% or less When the amount of N exceeds 0.01%, AlN nitride is formed during hot rolling or heating before hot pressing, and the blanking workability and hardenability of the raw steel sheet are improved. Decrease. Therefore, the amount of N is preferably 0.01% or less.

Further, in the present invention, in addition to the above-mentioned basic components, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.005, with the intention of further improving the characteristics of the steel sheet. %, Cr: 0.1 to 0.3%, Sb: 0.003 to 0.03%, at least one selected from these can be appropriately contained, if necessary.

Nb: 0.05% or less Nb is an effective component for strengthening steel, but if it is contained in an excessive amount, the shape freezing property is lowered. Therefore, when Nb is contained, it should be 0.05% or less.

Ti: 0.05% or less Ti is also effective for strengthening steel like Nb, but there is a problem that shape freezing property is lowered if it is contained in an excessive amount. Therefore, when Ti is contained, the content is 0.05% or less.

B: 0.0002 to 0.005%
Since B has an effect of suppressing the formation and growth of ferrite from the austenite grain boundaries, it is preferable to add 0.0002% or more. On the other hand, the addition of excess B greatly impairs moldability. Therefore, when B is contained, it is set to 0.0002 to 0.005%.

Cr: 0.1-0.3%
Cr is useful for strengthening steel and improving hardenability. In order to exhibit such an effect, addition of 0.1% or more is preferable. On the other hand, since the alloy cost is high, adding more than 0.3% causes a significant cost increase. Therefore, when Cr is contained, it is set to 0.1 to 0.3%.

Sb: 0.003 to 0.03%
Sb has the effect of suppressing decarburization of the surface layer of the steel sheet in the annealing process of the original plate for plating. In order to exhibit such an effect, it is necessary to add 0.003% or more. On the other hand, if the amount of Sb exceeds 0.03%, the rolling load is increased and the productivity is lowered. Therefore, when Sb is contained, it is set to 0.003 to 0.03%.

The rest other than the above consists of Fe and unavoidable impurities.

3) Method for manufacturing a steel sheet for hot pressing The manufacturing conditions for the steel sheet for hot pressing of the present invention are not particularly specified, but desirable manufacturing conditions will be described below. Steel with the above-mentioned components is cast, and the obtained hot piece slab is directly or heated, or the cold piece is reheated for hot rolling. At that time, almost no change in characteristics is observed between the direct rolling of the hot piece slab and the rolling after reheating. The reheating temperature is not particularly limited, but is preferably in the range of 1000 ° C to 1300 ° C in consideration of productivity. Hot rolling can be performed by either a normal hot rolling process or a continuous hot rolling process in which slabs are joined and rolled in finish rolling. It is desirable that the rolling end temperature during hot rolling be equal to or higher than the Ar ₃ transformation point in consideration of productivity and plate thickness accuracy. Cooling after hot rolling is performed by a usual method, but the take-up temperature at that time is preferably 550 ° C. or higher from the viewpoint of productivity, and if the take-up temperature is too high, pickling property It is desirable to keep the temperature below 750 ° C. because it deteriorates. Pickling and cold rolling may be carried out by conventional methods.

The subsequent method for forming a zinc-based plating film is not limited and is appropriately selected depending on the alloy system. However, as described later, when selectively cracking one surface, the electroplating method is the most suitable. It is efficient. Further, by performing an alloying treatment after plating, it is possible to efficiently obtain plating in which iron is alloyed. Regarding the atmosphere in the plating process, it is possible to perform plating under normal conditions in both continuous plating equipment with a non-oxidizing furnace and continuous plating equipment without a non-oxidizing furnace, and special control is required only for this steel sheet. Since it does not, it does not hinder productivity.

A hot-pressed steel sheet having a Zn-based plating layer on both sides of the steel sheet and having a crack in which the Zn-based plating layer traverses the Zn-based plating layer is a steel sheet having a Zn-based plating layer in an acidic aqueous solution having a pH of 4.0 or less. It is obtained by immersing it for 1.5 seconds or longer. Specifically, one surface is kept immersed in an acidic aqueous solution having a pH of 4.0 or less for 1.5 seconds or more without being energized, or electrolyzed as an anode to have an adhesion amount of 36 g / g. A surface having a Zn-based plating layer of m ² or more and having a crack density of 30 points / mm or more per unit cross-sectional length in an arbitrary 1 mm length cross section of the Zn-based plating layer can be obtained. On the other side, by electrolyzing as a cathode with a current density of 0.01 A / dm ² or more, the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plating layer is 20 points / A surface of mm or less is obtained. That is, by controlling the electrolysis conditions on the front and back surfaces, it is possible to manufacture desired hot pressing steel sheets having different crack densities on the front and back surfaces. At this time, on the surface held in the immersed state or electrolyzed as the anode, the anode dissolution of Zn in the plating layer proceeds, and as a result, the crack density of the Zn-based plating layer becomes a high numerical value. On the other hand, on the surface electrolyzed as a cathode with a current density of 0.01 A / dm ² or more, the anode dissolution of Zn in the plating layer does not proceed, and the crack density of the Zn-based plating layer becomes a low value.

When the pH of the acidic aqueous solution exceeds 4.0 for the surface on which cracks are intentionally formed, the effect of forming cracks is reduced and the desired crack density cannot be obtained. Therefore, the pH of the acidic aqueous solution is 4. It is preferably 0.0 or less. Further, even when the immersion time in the acidic aqueous solution is less than 1.5 seconds, the effect of forming cracks is reduced and the desired crack density cannot be obtained. Therefore, the immersion time in the acidic aqueous solution is 1.5. It is preferably 2 seconds or more. In particular, in the present invention, by immersing the steel sheet in an acidic aqueous solution having a pH of 4.0 or less for 1.5 seconds or longer, the crack density per unit cross section is divided by 10 for each of the two orthogonal cross sections in the Zn—Ni alloy plating layer. A steel plate for hot pressing having a location / mm or more can be obtained.

For surfaces that are not intentionally cracked, the occurrence of cracks can be prevented by performing cathode electrolysis at a current density of 0.01 A / dm ² or more. This is because the generation of cracks is caused by the slight anode dissolution of the plating layer. If the plated steel sheet is immersed in an acidic aqueous solution and electrolysis is not performed, zinc selective dissolution occurs as an anodic reaction on the plated steel sheet, and cracks occur from that point. On the other hand, when electrolysis is performed using a steel plate as a cathode, the above-mentioned anode dissolution does not occur and the occurrence of cracks is suppressed.

In the present invention, the acidic aqueous solution is preferably a plating solution that forms a Zn-based plating layer. The plating solution for forming the Zn-based plating layer is usually an acidic aqueous solution having a pH of 4.0 or less. Therefore, if the Zn-based plating layer is formed and then immersed in this plating solution, the Zn-based plating layer can be formed and the cracks can be formed using one solution, which is cost effective. It is advantageous.

In the present invention, as described above, after the Zn-based alloy plating layer is formed, the plating solution may be continuously immersed in a plating solution to perform a crack forming treatment, and then a plating treatment for further forming a Zn-based plating layer may be performed.

4) Method for manufacturing a hot-pressed member In the present invention, a Zn-based plating layer is provided on both sides of a steel sheet, the Zn-based plating layer has cracks that traverse the Zn-based plating layer, and Zn adhesion on one surface of the steel sheet is performed. The amount is 36 g / m ² or more, the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plated layer is 30 points / mm or more, and the Zn-based on the other surface of the steel sheet is Zn-based. A hot-pressed steel sheet having a crack density of 20 points / mm or less per unit cross-sectional length in a cross section of an arbitrary 1 mm length of a plated layer is placed in a temperature range of Ac ₃ transformation point to 1000 ° C. from room temperature for 5 seconds. A hot-pressed member having a desired breaking density by raising the temperature in a time of 600 seconds or less, holding the temperature in the temperature range of Ac ₃ transformation point to 1000 ° C. for 300 seconds or less, and then hot-pressing. Can be obtained.

By setting the heating temperature range of the hot-pressed steel sheet to the Ac ₃ transformation point to 1000 ° C., the Zn-based plating layer described in 1) above can be obtained. If the heating temperature is lower than the Ac ₃ transformation point, it may not be possible to obtain the strength required for the hot pressing member, and if the heating temperature exceeds 1000 ° C., Zn may disappear. The “Ac ₃ transformation point” is a value calculated from the following formula based on the component composition.
Ac ₃ transformation point (° C) = 910-203C ^1/2 + 44.7Si-4Mn + 11Cr
The element symbol in the above formula means the content (mass%) of each element, and is zero when the element is not contained.

In order to maintain the corrosion resistance after coating by leaving the intermetallic compound phase, the time required from room temperature to the above heating temperature is 600 seconds or less. The time required to reach the heating temperature from room temperature is preferably 450 seconds or less, more preferably 300 seconds or less. Further, if the heating rate is excessively high, that is, the time from room temperature to the heating temperature is too short, not only the residual amount of the intermetallic compound is saturated, but also the plating layer is melted during the heat treatment, so that the plating layer is melted. It may cause a drooping pattern and deteriorate the appearance. From this, regarding the temperature rise time, the time required from room temperature to the above heating temperature is 5 seconds or more, preferably 10 seconds or more.

Regarding the holding time at the above heating temperature, there is a viewpoint of further improving the corrosion resistance after coating by leaving as much intermetallic compound phase as possible, and a viewpoint of avoiding hydrogen intrusion by taking in water vapor in the furnace during the holding time. Therefore, the holding time is set to 300 seconds or less. The holding time is more preferably 180 seconds or less, further preferably 60 seconds or less, and most preferably no holding.

Further, the method for heating the hot pressed steel sheet is not limited in any way, and examples thereof include furnace heating by an electric furnace or a gas furnace, energization heating, induction heating, high frequency heating, and flame heating.

Following heating, hot pressing is performed, and at the same time as or immediately after the processing, cooling is performed using a refrigerant such as a die or water to manufacture a hot pressed member. In the present invention, the hot pressing conditions are not particularly limited, but pressing can be performed at 600 to 800 ° C., which is a general hot pressing temperature range.

Hereinafter, the present invention will be specifically described with reference to Examples. The following examples do not limit the present invention, and appropriate changes within the scope of the abstract structure are included in the scope of the present invention.

As a base steel sheet, in terms of mass%, C: 0.24%, Si: 0.25%, Mn: 1.28%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.02%, B: 0.002%, Cr: 0.2%, Sb: 0.008%, the balance is Fe and inevitable A cold-rolled steel sheet having a thickness of 1.4 mm and having a component composition composed of target impurities was used (Ac ₃ transformation point = 848 ° C.).

The above base steel sheet was subjected to a Zn-based plating layer on both sides (front surface and back surface) of the steel sheet by the following electroplating method to obtain a hot pressing steel sheet.
<Electroplating method>
The current density is 10 to 100 A / dm in a plating bath consisting of zinc sulfate heptahydrate 115 g / L, nickel sulfate hexahydrate 230 g / L, and sodium sulfate 55 g / L at a pH of 1.4 and a bath temperature of 50 ° C. ^2. By performing electroplating treatment with the energization time changed to 5 to 60 seconds, the level No. 1 shown in Table 1 was applied. A Zn—Ni alloy plating layer having a Ni content of 12% and a Zn adhesion amount of 1 to 19 and 26 was formed. Further, by setting the current density to 120 A / dm ² , No. A Zn—Ni alloy plating layer having a Ni content of 20% was formed. Further, by replacing the nickel sulfate hexahydrate with iron sulfate hexahydrate, cobalt sulfate hexahydrate surface, and chromium sulfate hexahydrate, No. 21 to 25 various Zn-based alloy plating layers were formed. After forming the plating layer, the dipping treatment on both sides or one side after plating was performed by changing the pattern of the current density for each surface. The surface of the Zn-based plating layer after the above plating and dipping treatment was observed, and the crack density was measured. Specifically, the surface of the Zn-based plating layer is observed at a magnification of 1000 using a scanning electron microscope (SEM), three straight lines are arbitrarily drawn, the intersections with cracks are counted, and the length of the straight line is divided. By doing so, it was converted into a crack density (location / mm) per unit length. At this time, in order to improve the measurement accuracy of the crack density, the surface observation of three fields of view was performed for one test material, and the average value was taken as the crack density. Table 1 shows the plating method, the amount of adhesion of the Zn-based plating layer, and the number density of cracks. The amount of the Zn-based plating layer adhered to the surface of the hot-pressed steel sheet was determined by the same method as the amount of Zn adhered to the hot-pressed member described later.

Next, a 100 mm × 200 mm test piece was collected from the hot pressed steel sheet obtained by the above plating treatment, and heat-treated by an electric furnace or energization heating. Table 1 shows the heat treatment conditions (heating time, heating temperature, holding time). The test piece after the heat treatment was taken out from an electric furnace or an energization heating furnace, and immediately hot pressed using a hat mold at a molding start temperature of 700 ° C. to obtain a hot pressed member. The shape of the obtained hot pressed member is a flat portion length of 100 mm on the upper surface, a flat portion length of 50 mm on the side surface, and a flat portion length of 50 mm on the lower surface. Further, the bending R of the mold is 7R for both the upper shoulders and the lower shoulders.

With respect to the obtained hot pressed member, the amount of Zn adhered and the average line roughness Ra were measured, the corrosion resistance after painting was evaluated, and the resistance spot weldability was evaluated.

<Measurement of Zn adhesion and average line roughness Ra>
With respect to the obtained hot breath member, the amount of Zn adhered to the front surface and the average line roughness Ra on both sides (front and back surfaces) were measured. The amount of Zn adhered to the hot pressed member shall be determined by the following method. The hot pressed member to be evaluated is punched, three samples having a diameter of 48 mmφ are collected, and each sample is weighed. Then, in each sample, the non-evaluation surface on the opposite side to the one surface for which the adhesion amount is evaluated is masked. Then, each sample was immersed in a solution prepared by adding 20 g of ammonium dichromate to 1 L for 60 minutes to dissolve only the oxide layer. The amount of adhesion per unit area in each sample was calculated from the mass difference before and after the dissolution of the Zn-based plating layer. Then, by immersing each sample in a solution prepared by adding 3.5 g of hexamethylenetetramine to 1 L of a 500 mL 35% hydrochloric acid aqueous solution for 10 minutes, the Zn-based plating layer is dissolved and each sample is weighed again. .. The metal component in the above-mentioned hydrochloric acid solution sample in which the plating layer was dissolved was quantified by inductively coupled plasma emission spectrometry (ICP-AES), and the amount of Zn in the plating layer of the hot press member was identified.

Arithmetic mean roughness Ra of the surface of the Zn-based plating layer is measured using Mitutoyo's Surfst SJ-2100, in accordance with JIS B 0601-2001, with a scanning speed of 0.5 mm / s, an operating distance of 4 mm, and a measured load of 0.75 mN. bottom. Measurement was performed in any 30 sections, the average value was calculated, and the average line roughness Ra of the present invention was used.

<Resistance spot weldability>
In order to evaluate the resistance spot weldability of the hot press member, a test piece of 30 mm × 50 mm was cut out from the flat portion on the upper surface of the obtained hot press member, and the resistance spot weldability was improved by using a two-plate set of the same type. gone. An AC resistance spot welder was used as the welder, and a DRφ16 type Cr-Cu electrode with a tip diameter of 6 mm was used as the electrode. The pressing force was 3.5 kN and the energizing time was 0.42 seconds. The welding current was increased from 3.0 kA in increments of 0.1 kA until dust was generated, and the maximum current value at which dust was not generated was recorded. The nugget diameter is measured by observing the cross section of the welded part of the test piece after welding, and the minimum current at which the nugget diameter is 4√t (mm) or more with respect to the plate thickness t (mm) and the maximum without dust generation. The difference in current value was taken as the appropriate current range for welding. The appropriate current range was judged according to the following criteria, and ◎ or ○ was regarded as acceptable. The evaluation results are shown in Table 1.
⊚: 1.5 kA ≤ appropriate current range ○: 0.8 kA ≤ appropriate current range <1.5 kA
×: 0.8 kA> Appropriate current range In addition, a 5 ° striking angle is provided, and welding is performed with a two-plate assembly of the same type as described above under other conditions, and the maximum length of cracks generated in the nugget is cross-sectionald. The welded portion LME crack length was determined by measuring from. The welded LME crack length was judged according to the following criteria, and ◯ was regarded as acceptable. The evaluation results are shown in Table 1.
〇: 20 μm ≧ weld LME crack length Δ: 100 μm ≧ weld LME crack length> 20 μm
×: 100 μm <appropriate current range <corrosion resistance after painting>
In order to evaluate the corrosion resistance of the hot pressed member after coating, a 70 mm × 150 mm test piece was cut out from the flat portion on the upper surface of the obtained hot pressed member, and the test piece was subjected to zirconium-based chemical conversion treatment and electrodeposition coating. Was given. The zirconium-based chemical conversion treatment is performed under standard conditions using PLM2100 manufactured by Nihon Parkerizing Co., Ltd., and the electrodeposition coating is performed using the cationic electrodeposition coating Electron GT100 manufactured by Kansai Paint Co., Ltd. so that the coating film thickness is 10 μm, and the baking conditions are as follows. It was held at 170 ° C. for 20 minutes. Next, the hot pressed member subjected to the zirconium-based chemical conversion treatment and electrodeposition coating was subjected to a corrosion test (SAE-J2334), and the corrosion state after 30 cycles was evaluated.
For general parts that have not been cross-cut, the judgment was made according to the following criteria, and ◎ or ○ was judged as acceptable. The evaluation results are shown in Table 1.
⊚: No red rust generated in general part ○: 1 place ≤ red rust generated place <3 places △: 3 places ≤ red rust generated place <10 places ×: 10 places ≤ red rust generated place Cross cut part (scratch part) The maximum swelling width on one side was measured from the above, and the judgment was made according to the following criteria, and ◎ or ○ was regarded as acceptable. The evaluation results are shown in Table 1.
⊚: Maximum swelling width on one side <1.5 mm
◯: 1.5 mm ≤ maximum swelling width on one side <3.0 mm
Δ: 3.0 mm ≤ maximum swelling width on one side <4.0 mm
×: 4.0 mm ≤ maximum swelling width on one side

From the results shown in Table 1, the hot pressed member of the present invention is excellent in corrosion resistance after painting and resistance spot weldability. Further, with the hot-pressed steel sheet of the present invention, it is possible to obtain a hot-pressed member having excellent post-coating corrosion resistance and resistance spot weldability.

Claims (3)

With Zn-based plating layers on both sides of the steel sheet,
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the average line roughness Ra is 2.5 μm or less.
A hot pressed member having an average linear roughness Ra of 3.5 μm or more on the other surface of the steel sheet. A Zn-based plating layer is provided on both sides of the steel sheet, and the Zn-based plating layer has cracks that traverse the Zn-based plating layer.
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plating layer is 30 points / mm or more.
A steel sheet for hot pressing in which the crack density per unit cross-sectional length in a cross section of an arbitrary 1 mm length of the Zn-based plating layer on the other surface of the steel sheet is 20 points / mm or less. A Zn-based plating layer is provided on both sides of the steel sheet, and the Zn-based plating layer has cracks that traverse the Zn-based plating layer.
The amount of Zn adhered to one surface of the steel sheet is 36 g / m ² or more, and the crack density per unit cross-sectional length in any 1 mm long cross section of the Zn-based plating layer is 30 points / mm or more.
A steel sheet for hot pressing, wherein the crack density per unit cross-sectional length in a cross section of an arbitrary 1 mm length of the Zn-based plating layer on the other surface of the steel sheet is 20 points / mm or less.
The temperature is raised from room temperature to the temperature range of Ac ₃ transformation point to 1000 ° C. for 5 seconds or more and 600 seconds or less, and further, the temperature is maintained in the temperature range of Ac ₃ transformation point to 1000 ° C. for 300 seconds or less, and then hot. A method for manufacturing a hot pressed member to be pressed.

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