JP7758165B2 - Hot-pressed parts - Google Patents

Description

The present invention relates to hot-pressed components, and in particular to hot-pressed components that have excellent external corrosion resistance on painted end surfaces.

In recent years, the automotive industry has seen an increase in the use of high-strength steel sheets, as the demand for lighter weight has increased while improving the performance of steel sheets. However, as the strength of steel sheets increases, press formability generally decreases, making it difficult to produce complex part shapes.

Against this backdrop, the use of hot pressing technology, which forms steel hot rather than cold, is increasingly being applied. Hot pressing is a forming method in which steel plate is heated to the austenite single-phase temperature range (around 900°C), then press-formed while still at a high temperature, and simultaneously rapidly cooled (quenched) by contact with a mold. Press-forming is performed in the heated, softened state, and then the strength is increased by quenching. Hot pressing therefore makes it possible to manufacture components with high strength while maintaining press formability.

On the other hand, automotive components are also required to have high corrosion resistance. The corrosion resistance required of automotive components can be broadly divided into resistance to perforation corrosion (perforation corrosion resistance) and resistance to cosmetic corrosion (cosmetic corrosion resistance). Perforation corrosion is, as the name suggests, corrosion that forms through holes in the steel that makes up the component. On the other hand, cosmetic corrosion is corrosion that damages the appearance, such as the formation of red rust and paint blistering due to corrosion.

In the field of hot pressing, it is common to use aluminum-plated steel sheets to provide rust resistance to hot-pressed components. Using aluminum-plated steel sheets as the material significantly improves the puncture corrosion resistance of hot-pressed components.

However, while hot-pressed components obtained using conventional Al-plated steel sheets have excellent perforation corrosion resistance, they suffer from the problem of poor external corrosion resistance. Specifically, hot-pressed components obtained by hot-pressing Al-plated steel sheets have a surface intermetallic compound or diffusion layer composed primarily of Al, a constituent of the plating, and Fe diffused from the base steel sheet. However, because these layers have a small potential difference with the base material (steel), they have little anodic protection against the base material. As a result, areas such as cut edges where there is no plating layer and the upper coating is thin can quickly corrode the base material, resulting in red rust. In addition, because the intermetallic compounds themselves contain a high concentration of Fe, red rust can sometimes form during anodic protection, regardless of whether the base material is corroded or not.

As such, while conventional hot-pressed components made from general Al-based plated steel sheets have excellent perforation corrosion resistance, their external corrosion resistance is insufficient compared to press components manufactured by cold pressing zinc-based plated steel sheets.

One of the reasons for the insufficient external corrosion resistance is poor chemical conversion treatability. Specifically, in the manufacture of typical automotive components, hot-pressed components are subjected to a zinc phosphate-based chemical conversion treatment as a surface treatment, followed by painting. During the chemical conversion treatment, a chemically stable aluminum oxide film is formed on the outermost surface of the aluminum-plated steel sheet after hot pressing, so almost no zinc phosphate-based chemical conversion treatment film is formed.

Against this background, various technologies have been proposed to improve various properties, such as the appearance and corrosion resistance, of hot-pressed components made from Al-based plated steel sheets.

For example, Patent Document 1 proposes a technology for using a plated steel sheet having an Al plating layer and a surface coating layer containing ZnO formed on the Al plating layer as a steel sheet for hot pressing.

In addition, Patent Document 2 proposes a method in which a plated steel sheet having an Al-based plating layer is heated in an atmosphere in which the hydrogen concentration and dew point are controlled, and then hot-pressed.

WO 2009/131233 Japanese Patent Application Laid-Open No. 2006-051543

However, as a result of the inventors' investigations, it was found that the conventional technologies proposed in Patent Documents 1 and 2 above still provide insufficient external corrosion resistance on the painted end faces of hot-pressed parts.

For example, Patent Document 1 reports that forming a coating containing ZnO, a wurtzite-type compound, improves post-painting corrosion resistance. However, Patent Document 1 evaluates post-painting corrosion resistance based solely on the width of paint blistering, and does not consider the occurrence of red rust. In actual automotive components, it is necessary to suppress not only paint blistering but also the occurrence of red rust, which has a significant impact on appearance. While the technology in Patent Document 1 improves chemical conversion treatability and thereby suppresses paint blistering, it cannot be said that red rust resistance is sufficient.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a hot-pressed component with excellent external corrosion resistance on the painted end surface. More specifically, the present invention aims to provide a hot-pressed component in which paint blistering and the occurrence of red rust from the painted end surface are suppressed.

The present invention has been made to solve the above problems, and its gist is as follows:

1. A steel material and a coating layer on at least one surface of the steel material,
the coating layer includes an FeAl alloy phase and a Zn phase,
A hot-pressed member having a natural immersion potential in an air-saturated 0.5 mass % NaCl aqueous solution at 25°C of -1100 to -900 mV relative to a silver-silver chloride-saturated potassium chloride electrode.

2. The hot-pressed member according to 1 above, wherein the amount of the Zn phase in the coating layer is 5 to 60 g/ ^m2 per side of the steel material.

3. A hot-pressed member described in 1 or 2 above, wherein the average grain size of the FeAl alloy phase is 3 to 20 μm.

The present invention provides hot-pressed components with excellent corrosion resistance and appearance at the painted end surface. More specifically, the present invention can suppress the occurrence of paint blistering and red rust from the painted end surface. Furthermore, as a result of suppressing corrosion, delayed fracture caused by hydrogen generated by corrosion can also be prevented.

The following provides a detailed description of the embodiments of the present invention. Note that the following description illustrates one preferred embodiment of the present invention, and the present invention is not limited to the following description. Furthermore, "%" used as a unit of content represents "% by mass" unless otherwise specified.

[Hot-pressed members]
A hot-pressed member according to one embodiment of the present invention includes a steel material and a coating layer on at least one surface of the steel material. The coating layer includes an FeAl alloy phase and a Zn phase. The hot-pressed member according to this embodiment has a natural immersion potential of −1100 to −900 mV in an air-saturated 0.5 mass % NaCl aqueous solution at 25°C, relative to a silver-silver chloride-saturated potassium chloride electrode.

(Steel)
In the present invention, the above-mentioned problems are solved by controlling the composition of the coating layer and the natural immersion potential, as will be described later. Therefore, the steel material to be used as the base material is not particularly limited, and any steel material can be used.

However, from the perspective of use as automotive frame components, etc., it is desirable for the hot-pressed components to have high strength. In particular, to obtain hot-pressed components with a tensile strength exceeding 1000 MPa, it is preferable to use steel material with the following chemical composition:

In mass%,
C: 0.1 to 0.5%,
Si: 0.1-2.0%,
Mn: 0.1 to 5.0%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.1% or less, and N: 0.01% or less,
The balance is Fe and unavoidable impurities.

The composition may further optionally contain Nb: 0.05% or less,
Ti: 0.05% or less,
B: 0.0050% or less,
At least one selected from the group consisting of Cr: 1.0% or less, and Sb: 0.03% or less may be contained.

Below, we will explain the effects and suitable contents of each element in the above preferred component composition.

C: 0.1-0.5%
C is an element that improves strength by forming a structure such as martensite. From the viewpoint of obtaining a strength exceeding 1000 MPa, the C content is preferably 0.1% or more. On the other hand, if the C content exceeds 0.5%, the toughness of the spot weld will deteriorate. Therefore, the C content is preferably 0.5% or less.

Si: 0.1-2.0%
Si is an effective element for strengthening steel to obtain good material properties. To obtain this effect, the Si content is preferably 0.1% or more. On the other hand, if the Si content exceeds 2.0%, ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 2.0% or less.

Mn: 0.1-5.0%
Mn is an element effective in increasing the strength of steel. From the viewpoint of ensuring excellent mechanical properties and strength, it is preferable that the Mn content be 0.1% or more. On the other hand, if the Mn content is excessive, surface segregation during annealing increases, which affects the adhesion of the coating layer to the steel material. Therefore, from the viewpoint of improving the adhesion of the coating layer, it is preferable that the Mn content be 5.0% or less.

P: 0.02% or less If the P content is excessive, local ductility deteriorates due to grain boundary embrittlement caused by P segregation to austenite grain boundaries during casting. As a result, the balance between strength and ductility of the steel material is reduced. Therefore, from the viewpoint of improving the balance between strength and ductility of the steel material, it is preferable that the P content be 0.02% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, since an excessive reduction leads to an increase in manufacturing costs, it is preferable that the P content be 0.001% or more.

S: 0.01% or less S becomes inclusions such as MnS, which can cause deterioration in impact resistance and cracking along the metal flow of welds. Therefore, it is desirable to reduce the S content as much as possible, and specifically, it is preferable to set it to 0.01% or less. Furthermore, from the viewpoint of ensuring good stretch flangeability, it is more preferable to set it to 0.005% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, since excessive reduction leads to an increase in manufacturing costs, it is preferable that the S content be 0.0001% or more.

Al: 0.1% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.1%, hardenability decreases. Therefore, the Al content is preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Al content, from the viewpoint of enhancing the effect as a deoxidizer, the Al content is preferably 0.01% or more.

N: 0.01% or less If the N content exceeds 0.01%, AlN is generated during heating before hot pressing, resulting in reduced hardenability. Therefore, the N content is preferably 0.01% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, since excessive reduction leads to increased manufacturing costs, the N content is preferably 0.001% or more.

Nb: 0.05% or less Nb is an effective component for strengthening steel, but excessive Nb content reduces shape fixability. Therefore, when Nb is added, the Nb content is preferably 0.05% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%.

Ti: 0.05% or less Like Nb, Ti is an effective component for strengthening steel, but if it is contained in excess, shape fixability decreases. Therefore, when Ti is added, the Ti content is preferably 0.05% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%.

B: 0.0050% or less B is an element that has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. However, the addition of excessive B significantly impairs formability. Therefore, when B is added, from the viewpoint of improving formability, the B content is preferably 0.0050% or less. On the other hand, although there is no lower limit for the B content, from the viewpoint of enhancing the effect of adding B, it is preferably 0.0002% or more.

Cr: 1.0% or less Cr is a useful element for strengthening steel and improving hardenability. However, since Cr is an expensive element, when Cr is added, the Cr content is preferably 1.0% or less to reduce alloy costs. On the other hand, although there is no particular lower limit for the Cr content, it is preferably 0.1% or more from the viewpoint of enhancing the effect of adding Cr.

Sb: 0.03% or less Sb is an element that has the effect of preventing decarburization of the surface layer of a steel sheet during hot pressing. However, excessive Sb increases the rolling load, thereby reducing productivity. Therefore, when Sb is added, the Sb content is preferably 0.03% or less from the viewpoint of further improving productivity. On the other hand, although there is no particular restriction on the lower limit of the Sb content, it is preferably 0.003% or more from the viewpoint of enhancing the effect of adding Sb.

As described below, the hot-pressed member of the present invention can be manufactured by hot-pressing a hot-press steel sheet having a plating layer under specified conditions. Therefore, the steel material constituting the hot-pressed member of the present invention can also be said to be a hot-pressed steel sheet.

(covering layer)
The hot-pressed member of the present invention has a coating layer containing an FeAl alloy phase and a Zn phase on the surface of a steel sheet. The coating layer may be provided on at least one surface of the steel sheet, or may be provided on both surfaces. The Zn phase contributes to improving appearance corrosion resistance, particularly red rust resistance. Furthermore, the FeAl alloy phase contributes to pitting corrosion resistance and has the effect of reducing the corrosion rate of the Zn phase.

The thickness of the coating layer is not particularly limited, but to ensure corrosion resistance, it is preferably 7 μm or more, more preferably 10 μm or more, and even more preferably 13 μm or more. On the other hand, from the perspective of adhesion of the coating layer, the thickness of the coating layer is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.

FeAl alloy phase As described above, the FeAl alloy phase contributes to pitting corrosion resistance and has the effect of reducing the corrosion rate of the Zn phase. In the present invention, the FeAl alloy phase is defined as a phase containing Fe and Al in a total amount of 80 atomic % or more. The FeAl alloy phase can be identified based on the chemical composition measured by energy dispersive X-ray analysis (EDS). More specifically, the presence or absence of the FeAl alloy phase can be determined by the method described in the examples.

The form of existence of Fe and Al contained in the FeAl alloy phase is not limited. Fe and Al may form an alloy (solid solution) or an intermetallic compound. An example of the solid solution is an α-Fe phase in which Al is dissolved. The intermetallic compound is also not particularly limited, and examples thereof include FeAl intermetallic compounds such as Fe ₂ Al ₅ , Fe ₄ Al ₁₃ , and FeAl.

The FeAl alloy phase may contain elements other than Fe and Al in a total amount of 20 atomic % or less. In one embodiment of the present invention, the elements other than Fe and Al may include, for example, at least one selected from the group consisting of Mg, Ca, Si, Cr, Mn, and Zn. The elements other than Fe and Al are not particularly limited and may be present in the FeAl alloy phase in any form. For example, the elements other than Fe and Al may form an intermetallic compound, a solid solution, or a compound such as an oxide.

The particle size of the FeAl alloy phase is not particularly limited. However, if the particle size is excessively small, the interface area between the Zn phase and the FeAl alloy phase increases, and the corrosion rate of the Zn phase due to galvanic corrosion increases. As a result, corrosion resistance decreases. Therefore, from the perspective of further improving corrosion resistance, the average particle size of the FeAl alloy phase is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 7 μm or more. On the other hand, from the perspective of adhesion of the FeAl alloy phase to the steel material, the average particle size of the FeAl alloy phase is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less.

The average grain size of the FeAl alloy phase can be measured by observing the cross section of the hot-pressed member with a scanning electron microscope (SEM). Specifically, the cross section of the mirror-finished hot-pressed member is observed with an SEM, and a backscattered electron image is obtained at an accelerating voltage of 5 kV and a magnification of 500x. The FeAl alloy phase grains are identified from the backscattered electron image based on the crystal orientation contrast. The major and minor axes of each identified grain are measured, and the average value is taken as the grain size of each FeAl alloy phase. The average value of the grain sizes of 20 randomly selected FeAl alloy phases is taken as the average grain size of the FeAl alloy phase in that hot-pressed member.

Zn phase: To achieve the natural immersion potential within the above range, the coating layer must contain a Zn phase. The presence of the Zn phase reduces the rate of corrosion (appearance corrosion) from areas where the anti-corrosion function of the coating has decreased, such as scratches in the coating or the edges of the coating, thereby maintaining good appearance quality. It also reduces the risk of delayed fracture caused by hydrogen generated by corrosion.

Furthermore, the coating layer contains both an FeAl alloy phase and a Zn phase, providing excellent corrosion resistance over a long period of time. In other words, the FeAl alloy phase has a low anodic dissolution rate, providing excellent corrosion resistance, and also has a low rate of oxygen reduction reaction, a cathodic reaction in atmospheric corrosive environments. By including the FeAl alloy phase in addition to the Zn phase, corrosion of the Zn phase is suppressed, resulting in stable resistance to red rust formation over a long period of time. The internal structure of the coating layer is not particularly limited, but generally, it may have a structure in which the Zn phase exists between the crystal grains of the FeAl alloy phase.

The presence or absence of Zn phase in the coating layer can be determined by observing the cross section of the hot-pressed part using SEM-EBSD (electron backscatter diffraction). Specifically, the cross section of a mirror-finished hot-pressed part is observed using SEM-EBSD at an accelerating voltage of 15 kV and a magnification of 2000x to obtain a backscattered electron image. The Zn phase is identified in the backscattered electron image as having a bright contrast compared to the FeAl alloy phase and a structure with a hexagonal crystal structure.

The Zn phase may contain other metal components to the extent that the crystal structure is not changed or an intermetallic compound is not formed. The Zn phase may exist as a single phase or as a mixture with other metals or intermetallic compounds.

The amount of the Zn phase deposited is not particularly limited, but if it is less than 5 g/ ^m2 per side, the period during which the red rust formation resistance effect is obtained will be shortened. Therefore, the amount of the Zn phase deposited is preferably 5 g/ ^m2 or more per side, more preferably 10 g/m2 or ^more , and even more preferably 20 g/ ^m2 or more. On the other hand, if the amount of the Zn phase deposited exceeds 60 g/ ^m2 per side, the red rust formation resistance effect will saturate and there is a risk of LME (liquid metal embrittlement) cracking occurring during welding. Therefore, the amount of the Zn phase deposited is preferably 60 g/ ^m2 or less per side.

The amount of Zn phase deposition can be determined by subjecting the hot-pressed member to anodic electrolysis to dissolve the Zn phase in the coating layer in an aqueous solution, and then quantitatively analyzing the resulting aqueous solution using ICP-MS (inductively coupled plasma-mass spectrometry). Specifically, first, using the hot-pressed member as the working electrode and a platinum mesh electrode as the counter electrode, constant-current anodic electrolysis is performed at 4 mA/ ^cm2 in a 3% sodium hydroxide-1% aluminum chloride aqueous solution. Electrolysis is stopped when the potential becomes abruptly nobler, and the amount of Zn in the solution is quantitatively analyzed using ICP-MS to measure the amount of dissolved Zn. The amount of Zn can be quantified by dividing the amount by the surface area of the hot-pressed member.

The coating layer may further contain other phases in addition to the FeAl alloy phase and the Zn phase. The other phases are not particularly limited and may be any phase. For example, the other phases may be one or both of a metal phase and an intermetallic compound phase. Examples of the other phases include intermetallic compound phases such as MgZn ₂ and Mg ₂ Zn ₁₁ .

The proportion of the other phases contained in the coating layer is not particularly limited. However, from the perspective of further enhancing the effect of the FeAl alloy phase and Zn phase in improving the appearance and corrosion resistance, it is desirable that the proportion of the other phases be low. Specifically, the area ratio of the other phases in the cross section of the coating layer is preferably 30% or less, and more preferably 15% or less. The lower limit of the area ratio is not particularly limited and may be 0%.

The hot-pressed member of the present invention may further have an oxide layer on the coating layer. Examples of oxides contained in the oxide layer include Zn oxide, Al oxide, Mn oxide, and composite oxides thereof.

If the oxide layer is excessively thick, the adhesion of the coating may decrease. Therefore, it is preferable that the thickness of the oxide layer be 5 μm or less. The oxide layer is not essential in the present invention, and it may be removed by shot blasting or other methods after hot pressing and before painting. In other words, the lower limit of the thickness of the oxide layer may be 0 μm. However, since this would increase the number of processes and increase costs, it is also possible to paint the hot-pressed steel as long as it is within a range that does not affect adhesion.

(natural immersion potential)
In the present invention, it is important that the natural immersion potential of the hot-pressed member is -1100 to -900 mV based on a silver-silver chloride-saturated potassium chloride electrode (SSE). By setting the natural immersion potential within this range, the cathodic protection performance of the steel material constituting the hot-pressed member is maximized, resulting in excellent appearance corrosion resistance. In addition, delayed fracture due to hydrogen generated during corrosion can be reduced.

If the natural immersion potential is more noble (positive) than -900 mV, the potential difference between the steel material and the coating layer is small, resulting in insufficient cathodic protection performance and inferior appearance corrosion resistance. Therefore, the natural immersion potential is set to -900 mV or less, preferably -925 mV or less, and more preferably -950 mV or less. On the other hand, if the natural immersion potential is more noble (negative) than -1100 mV, the effect of improving appearance corrosion resistance saturates. In addition, the amount of hydrogen generated due to corrosion becomes excessive, increasing the risk of delayed fracture. Therefore, the natural immersion potential is set to -1100 mV or more, preferably -1075 mV or more, and more preferably -1050 mV or more.

In this invention, the natural immersion potential refers to the natural immersion potential in an air-saturated 0.5% by mass NaCl aqueous solution at 25°C, expressed relative to a silver-silver chloride-saturated potassium chloride electrode. In measuring the natural immersion potential, a 10 mm diameter area on any flat portion of the hot-pressed part having an area of 10 mm diameter or more is used as the working electrode. The average value of the immersion potential measured between 60 seconds and 600 seconds after the working electrode is immersed in the NaCl aqueous solution is taken as the natural immersion potential of the hot-pressed part. In the measurement, the temperature of the NaCl aqueous solution is adjusted to 25±5°C.

While there are no particular limitations on the mechanical properties of the hot-pressed member of the present invention, it is preferable that the residual stress measured using X-ray diffraction at any location of the hot-pressed member is less than 600 MPa. If trimming or piercing is performed by cold working after hot press forming, residual stress will be generated at that location, increasing the risk of delayed fracture. To reduce the risk of delayed fracture, it is preferable not to perform cold working after hot press forming. Furthermore, if trimming or piercing is performed, it is preferable to use a laser processing device.

[Manufacturing method]
Next, a preferred method for producing the hot-pressed member of the present invention will be described.

The hot-pressed member of the present invention can be manufactured by plating a base steel sheet to form a plated steel sheet, and then hot pressing the plated steel sheet.

The base steel sheet is not particularly limited and any steel sheet can be used. The suitable chemical composition of the steel sheet is the same as the preferred chemical composition of the steel material for the hot-pressed member described above. The base steel sheet is preferably a hot-rolled steel sheet or a cold-rolled steel sheet.

The plating of the base steel sheet can be carried out by any method, but it is preferably carried out by hot-dip plating. Below, we will explain how to produce plated steel sheets using the hot-dip plating method.

First, prior to hot-dip galvanizing, the base steel sheet is annealed. Next, the annealed base steel sheet is immersed in a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet with a hot-dip galvanized layer on the surface of the base steel sheet. The hot-dip galvanizing bath preferably contains one or both of Ca and Sr in addition to Al, Zn, and Fe that flows out from the base material and bath-immersed equipment. Furthermore, the hot-dip galvanizing bath may optionally further contain Si. More preferably, the hot-dip galvanizing bath has a chemical composition containing 30-60% Zn, 0.1-13% Si, 0-10% Mg, 0.05-3% Ca + Sr, and 0-5% Fe, with the balance consisting of Al and unavoidable impurities.

The coating weight of the hot-dip coated layer is not particularly limited, but is preferably 20 g/ ^m2 or more per side of the steel sheet, more preferably 30 g/m2 or ^more , even more preferably 35 g/m2 or ^more , and particularly preferably 50 g/m2 or ^more . The coating weight of the hot-dip coated layer is preferably 300 g/m2 or ^less per side of the steel sheet, more preferably 250 g/m2 or ^less , and even more preferably 200 g/m2 or ^less . As mentioned above, when hot pressing is performed, the coating weight of the coated layer increases due to the diffusion of Fe from the base steel sheet. Therefore, by setting the coating weight of the hot-dip coated layer on the hot-dip coated steel sheet before hot pressing within the above-mentioned range, the coating weight of the coated layer on the hot-pressed member can be set within the above-mentioned preferred range.

The coating weight per side of the hot-dip galvanized layer is determined using the following method. First, the hot-dip galvanized steel sheet to be evaluated is punched to obtain three 48 mm diameter samples. One side of each sample (the side opposite to the side on which the coating weight is measured) is then masked. Each sample is immersed for 20 minutes in a 17% hydrochloric acid solution containing 1 mL of hexamethylenetetramine as an inhibitor to dissolve the hot-dip galvanized layer, and the weight of each sample is then measured again. The difference in mass before and after dissolution of the hot-dip galvanized layer is divided by the area of the sample to calculate the coating weight per unit area for each sample. The average coating weight of the three samples is then taken as the coating weight per side of the hot-dip galvanized layer for that hot-dip galvanized steel sheet.

Next, the hot-dip plated steel sheet is hot-pressed to produce a hot-pressed member. The hot pressing includes a heating step of heating a steel sheet for hot pressing, and a hot-pressing step of hot-pressing the steel sheet for hot pressing heated in the heating step. In the heating step, it is preferable to raise the temperature from room temperature to a heating temperature between the _Ac3 transformation point of the base steel sheet and 1000°C in an atmosphere with an oxygen concentration of 21 to 35% by volume over a temperature-raising time of 60 to 600 seconds. The steel sheet for hot pressing after the temperature-raising step may further be held at the heating temperature in the atmosphere for a holding time of 300 seconds or less.

Oxygen Concentration: Air or dry air with a reduced dew point is typically used as the atmospheric gas for the heating step in hot pressing, as this is economical and common. When a plated steel sheet containing Al and Zn is heated under these common atmospheric gases, evaporation or oxidation results in almost no metallic Zn phase remaining on the surface of the hot-pressed steel sheet. The inventors have extensively investigated the effect of the atmosphere in the heating step on the state of Zn on the surface after heating. As a result, they have found that by supplying pure oxygen in addition to air as the atmospheric gas to enhance oxidization, a large amount of metallic Zn, i.e., the Zn phase, can be retained in the coating layer of the final hot-pressed steel sheet. This is because a dense zinc oxide coating is formed on the surface of the steel sheet for hot pressing during the heating step, suppressing further oxidation and evaporation.

To achieve this effect, it is preferable to set the oxygen concentration in the atmosphere during the heating process to 21% by volume or higher. However, if the oxygen concentration is less than 22% by volume, the above effect is not necessarily sufficient. Therefore, it is more preferable to set the oxygen concentration to 22% by volume or higher, and even more preferable to set it to 25% by volume or higher. If the oxygen concentration is less than 22% by volume, in order to prevent the Zn phase from disappearing, it is necessary to lower the heating temperature and include at least one of Ca and Sr in the plating film. On the other hand, if the oxygen concentration exceeds 35% by volume, the effect saturates and costs increase significantly. Therefore, it is preferable to set the oxygen concentration to 35% by volume or lower.

Heating temperature: If the heating temperature in the heating step is lower than the _Ac3 transformation point, the strength required for a hot-pressed member may not be obtained. Therefore, the heating temperature is preferably set to the _Ac3 transformation point or higher. On the other hand, if the heating temperature exceeds 1000°C, the operating cost increases. Therefore, the heating temperature is preferably set to 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower. In particular, when the oxygen concentration is less than 22% by volume, the heating temperature is preferably set to 850°C or lower.

The _Ac3 transformation point can be determined by the following formula (1).
_Ac3 transformation point (°C) = 881 - 206C + 53Si - 15Mn - 1Cr... (1)
In the formula (1), the element symbols represent the content (mass%) of each element. The content of elements that are not contained is calculated as 0.

- Temperature Rise Time In the heating step, if the temperature rise time from the start of heating to the heating temperature is short, the alloying reaction between Fe and Al will not proceed sufficiently. Therefore, the temperature rise time is preferably 60 seconds or longer. In particular, from the viewpoint of achieving an average particle size of the FeAl alloy phase of 3 μm or more, the temperature rise time is more preferably 120 seconds or longer. On the other hand, if the temperature rise time exceeds 600 seconds, Zn will dissolve in the FeAl alloy phase, resulting in a decrease in the amount of Zn phase. Therefore, from the viewpoint of ensuring the amount of Zn phase, the temperature rise time is preferably 600 seconds or shorter, and more preferably 240 seconds or shorter.

Holding Time In the heating step, after the heating temperature is reached, the heating temperature may be further held. However, if the time for holding at the heating temperature (holding time) exceeds 300 seconds, the solid solution of Zn into the FeAl alloy phase progresses, and the amount of Zn phase decreases. Therefore, from the viewpoint of ensuring the amount of Zn phase, the holding time is preferably 300 seconds or less. On the other hand, since holding is not essential, the lower limit of the holding time is 0 seconds. However, from the viewpoint of operational stability of the hot press, holding for 5 seconds or more is preferable.

The following describes the functions and effects of the present invention based on examples. Note that the present invention is not limited to the following examples.

Hot-dip galvanized steel sheets were prepared using the following procedure, and the hot-dip galvanized steel sheets were hot-pressed to form hot-pressed components.

The base steel sheet used was a cold-rolled steel sheet with a thickness of 1.4 mm, containing, by mass, C: 0.34%, Si: 0.25%, Mn: 1.2%, Cr: 0.2%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the remainder consisting of Fe and unavoidable impurities.

After annealing the above-mentioned base steel sheet, hot-dip plating was sequentially performed using a hot-dip plating bath having the bath temperature and chemical composition shown in Tables 1 and 2 to obtain hot-dip plated steel sheets. The coating weight per side of the obtained hot-dip plated steel sheets was measured using the method described above. The measurement results are shown in Tables 1 and 2. For comparison, in some examples (Comparative Example No. 58), the above-mentioned base steel sheet was annealed and then electroplated to obtain plated steel sheets. In some examples (Comparative Example No. 59), no plating was performed.

Next, 200 x 300 mm test pieces were taken from the obtained steel plates and heated under the conditions shown in Tables 1 and 2. An electric furnace was used for the heating. During the heating, gas was supplied so that the oxygen concentration in the furnace reached the values shown in Tables 1 and 2.

After the specified holding time had elapsed, the test specimen was removed from the electric furnace and immediately hot pressed using a hat-shaped die at a forming start temperature of 700°C to obtain a high-strength steel component. The shape of the obtained high-strength steel component measured 100 mm in length for the flat portion of the top surface, 50 mm in length for the flat portion of the side surface, and 50 mm in length for the flat portion of the bottom surface. The bending radius of the die was 7R for both shoulders of the top surface and both shoulders of the bottom surface.

(natural immersion potential)
The natural immersion potential of the obtained hot-pressed member was measured using the following procedure. First, three 16 mm diameter samples were punched from the flat portion of the top surface of the hat-shaped hot-pressed member. Using a 10 mm diameter region in the center of the sample as the working electrode and a silver-silver chloride-saturated potassium chloride electrode (SSE) as the reference electrode, the immersion potential was measured in an air-saturated 0.5 mass% NaCl aqueous solution at 25 ± 5 °C. The time average of the immersion potential from 60 seconds to 600 seconds after the working electrode was immersed in the NaCl aqueous solution was taken as the natural immersion potential of the sample. The average of the natural immersion potentials of three different samples was taken as the natural immersion potential of the hot-pressed member being evaluated. The measurement results are shown in Tables 3 and 4.

Furthermore, for each of the obtained high-strength steel members, the presence or absence of an FeAl alloy phase, the average grain size of the FeAl alloy phase, the presence or absence of a Zn phase, and the amount of attached Zn phase were measured using the following methods. The measurement results are shown in Tables 3 and 4.

(Presence or absence of FeAl alloy phase)
A test piece was taken from the flat portion of the upper surface of the hot-pressed member, and the presence or absence of an FeAl alloy phase was determined by observing the cross section of the test piece. The cross section was observed using a SEM after the surface to be observed was mirror-finished, and a backscattered electron image was obtained at an accelerating voltage of 15 kV and a magnification of 1000 times. In the backscattered electron image, a point analysis was performed using EDS in a region with a dark contrast, i.e., a low electron density, compared to the base material. In the chemical composition obtained by the analysis, a region where the total content of Fe and Al was 80 atomic % or more was considered to be an FeAl alloy phase.

(Average particle size of FeAl alloy phase)
A test piece was taken from the flat portion of the upper surface of the hot-pressed member, and the cross section of the test piece was observed to measure the average grain size of the FeAl alloy phase. The cross section was observed using a SEM after the surface to be observed was mirror-finished, and a backscattered electron image was obtained at an accelerating voltage of 5 kV and a magnification of 500 times. Next, crystal grains of the FeAl alloy phase were identified from the backscattered electron image based on the crystal orientation contrast, and the major and minor axes of each identified crystal grain were measured. The average of the obtained major and minor axes was taken as the grain size of each FeAl alloy phase. The average of the grain sizes of 20 randomly selected FeAl alloy phases was taken as the average grain size of the FeAl alloy phase in that hot-pressed member.

(Presence or absence of Zn phase)
The presence or absence of a Zn phase in the coating layer was determined by observing the cross section of the hot-pressed member using SEM-EBSD. Specifically, the cross section of the mirror-finished hot-pressed member was observed using SEM-EBSD at an acceleration voltage of 15 kV and a magnification of 2000 times to obtain a backscattered electron image. The Zn phase is distinguished in the backscattered electron image as having a bright contrast compared to the FeAl intermetallic compound phase and a structure with a hexagonal crystal structure.

(Zn phase adhesion amount)
The deposition amount of the Zn phase was determined by anodic electrolysis of a sample taken from the hot-pressed member, dissolving the Zn phase in the coating layer in an aqueous solution, and quantitatively analyzing the resulting solution using ICP-MS (inductively coupled plasma mass spectrometry). Specifically, three 48 mm diameter samples were first punched from the flat top surface of a hat-shaped hot-pressed member. The surfaces of the samples were masked except for the surface to be measured. Next, the samples were used as the working electrode and a platinum mesh electrode as the counter electrode, and constant-current anodic electrolysis was performed at 4 mA/ ^cm2 in a 3% sodium hydroxide-1% aluminum chloride aqueous solution. The electrolysis was stopped when the potential became abruptly nobler, and the amount of Zn in the solution was quantitatively analyzed using ICP-MS to measure the amount of dissolved Zn. The deposition amount of the Zn phase was determined by dividing the amount of Zn by the surface area of the hot-pressed member.

(appearance corrosion resistance)
Next, in order to evaluate the appearance and corrosion resistance of the obtained hot-pressed parts, the occurrence of paint film blisters from the painted edge and the occurrence of red rust from the painted edge were tested according to the following procedures.

First, a 70 mm wide region was cut out of the resulting hat-shaped high-strength steel member using a laser cutter, and the test piece was then subjected to a zinc phosphate conversion treatment and electrodeposition coating to prepare a corrosion resistance test piece. The zinc phosphate conversion treatment was carried out under standard conditions using a PB-SX35 manufactured by Nihon Parkerizing Co., Ltd. The electrodeposition coating was carried out using an Electron GT-100 manufactured by Kansai Paint Co., Ltd., so that the coating film thickness was 5 μm. The baking conditions for the electrodeposition coating were to reach 170°C and then hold for 20 minutes.

The obtained corrosion resistance test pieces were subjected to a combined cycle corrosion test (SAE-J2334) without masking, and the corrosion condition after 40 cycles was evaluated. The appearance corrosion resistance of the painted edge was judged based on the width of the paint film blister from the edge and the occurrence of red rust on the cut edge, based on the following criteria. A rating of 3 or higher for both the paint film blister width and the area ratio of red rust on the edge was considered to be pass. The evaluation results are shown in Tables 3 and 4.
Paint film blister width from the painted edge 1: Paint film blister width > 5 mm
2: 3 mm < coating blister width ≦ 5 mm
3: 2 mm < coating blister width ≦ 3 mm
4: 1 mm < coating blister width ≦ 2 mm
5: Paint blister width ≦1 mm

Red rust appears on the painted edge 1: Red rust area ratio on the edge > 50%
2: 30% < red rust area rate on end surface ≦ 50%
3: 20% < red rust area rate on end surface ≦ 30%
4: 10% < red rust area rate on end surface ≦ 20%
5: Red rust area rate on end surface ≦10%

As can be seen from the results shown in Tables 3 and 4, hot-pressed parts meeting the conditions of the present invention had good corrosion resistance against both paint film swelling from the painted edge and the occurrence of red rust, and were excellent in overall appearance corrosion resistance.

In contrast, the hot-pressed components of the comparative examples, which did not meet the conditions of the present invention, exhibited poor results in at least one of paint blistering and red rust formation from the painted edge. For example, in comparative examples Nos. 1, 2, 40, 41, 48, and 49, the coating weight of the hot-pressed steel sheet was low, and metallic Zn was lost through alloying or oxidation during the heating process, resulting in a decrease in the Zn phase in the coating layer of the hot-pressed components. As a result, the immersion potential became nobler, and good post-painting corrosion resistance could not be achieved.

In Comparative Example No. 55, a steel sheet for hot pressing containing a large amount of Mg in the plating layer and with a large plating coating weight was subjected to hot pressing. In this comparative example, an excessive amount of MgZn-based intermetallic compound phase was formed in the surface layer of the hot-pressed part. As a result, the immersion potential became excessively noble, and the corrosion rate of the plating layer increased, making it impossible to achieve good post-painting corrosion resistance.

Claims (3)

a steel material and a coating layer on at least one surface of the steel material;
the coating layer includes an FeAl alloy phase and a Zn phase,
A hot-pressed member having a natural immersion potential in an air-saturated 0.5 mass % NaCl aqueous solution at 25°C of -1100 to -900 mV relative to a silver-silver chloride-saturated potassium chloride electrode. The hot-pressed member according to claim 1, wherein the amount of the Zn phase in the coating layer is 5 to 60 g/m ² per side of the steel material. A hot-pressed member as described in claim 1 or 2, wherein the average grain size of the FeAl alloy phase is 3 to 20 μm.