Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
  • B21D22/20—Deep-drawing

Definitions

• the present invention relates to hot-pressed components, and in particular to hot-pressed components that have excellent external corrosion resistance on the painted end faces.
• Hot pressing is a forming method in which steel plate is heated to the temperature range of the austenite single phase (around 900°C), then press-formed while still at a high temperature, and simultaneously quenched (hardened) by contact with a die. Press forming is carried out in the heated and softened state, and then the strength is increased by quenching, so with hot pressing it is possible to manufacture parts with high strength while maintaining press formability.
• automotive components are also required to have high corrosion resistance.
• the corrosion resistance required for 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 material that makes up the component.
• cosmetic corrosion is corrosion that mars the appearance, such as the formation of red rust and paint blistering due to corrosion.
• hot-pressed members obtained using general Al-plated steel sheets have a problem that, although they have excellent perforation corrosion resistance, they have poor appearance corrosion resistance.
• hot-pressed members obtained by hot-pressing Al-plated steel sheets have an intermetallic compound layer or diffusion layer on the surface, mainly consisting of Al, a constituent component of the plating, and Fe diffused from the base steel sheet.
• the base material steel material
• the anodic corrosion protection effect on the base material is small. Therefore, in places such as cut edges where there is no plating layer and the upper coating film is thin, the base material corrodes early and red rust occurs.
• the intermetallic compound itself contains a high concentration of Fe, red rust may occur even during the anodic corrosion protection process regardless of whether the base material is corroded or not.
• hot-pressed components made from conventional aluminum-based plated steel sheets have excellent corrosion resistance against holes, their external corrosion resistance is insufficient compared to pressed components manufactured by cold pressing zinc-based plated steel sheets.
• Patent Document 1 proposes a technology in which a plated steel sheet having an Al plating layer and a surface coating layer containing ZnO formed on the Al plating layer is used as a steel sheet for hot pressing.
• Patent Document 2 also 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.
• Patent Document 1 reports that forming a film containing ZnO, a wurtzite compound, improves post-painting corrosion resistance.
• post-painting corrosion resistance is evaluated based only on the width of paint blistering, and the occurrence of red rust is not taken into consideration.
• it is required to suppress not only paint blistering, but also the occurrence of red rust, which has a significant impact on appearance.
• paint blistering is suppressed as a result of improved chemical conversion treatability, red rust resistance was not sufficient.
• the present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a hot-pressed member with excellent external corrosion resistance on the painted end surface. More specifically, the present invention aims to provide a hot-pressed member in which paint blistering and the occurrence of red rust from the painted end surface are suppressed.
• the present invention was made to solve the above problems, and its gist is as follows:
• 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 based on a silver-silver chloride-saturated potassium chloride electrode.
• the present invention can provide hot-pressed members with excellent corrosion resistance in appearance at the painted end faces. More specifically, the present invention can suppress the occurrence of paint blistering and red rust from the painted end faces. Furthermore, as a result of suppressing corrosion, delayed fracture caused by hydrogen generated by corrosion can also be prevented.
• a hot-pressed member 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., based on a silver-silver chloride-saturated potassium chloride electrode.
• the steel material to be the base material is not particularly limited and any steel material can be used.
• the hot-pressed member it is desirable for the hot-pressed member to have high strength.
• a hot-pressed member with a tensile strength exceeding 1000 MPa it is preferable to use a steel material with the following composition.
• 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
• N 0.01% or less
• the balance is Fe and unavoidable impurities.
• the composition may further include Nb: 0.05% or less.
• C 0.1-0.5% C is an element that has the effect of improving strength by forming a structure such as martensite. From the viewpoint of obtaining a strength exceeding 1000 MPa class, it is preferable that the C content is 0.1% or more. On the other hand, if the C content exceeds 0.5%, the toughness of the spot welds deteriorates. Therefore, the C content is preferably 0.5% or less.
• Si 0.1-2.0% Silicon is an effective element for strengthening steel and obtaining good material properties.
• the silicon content is preferably 0.1% or more. If the Si content exceeds 0.0%, the ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably set to 2.0% or less.
• Mn 0.1-5.0%
• Mn is an element that is effective in increasing the strength of steel. From the viewpoint of ensuring excellent mechanical properties and strength, the Mn content is preferably 0.1% or more. If the Mn content is excessive, the 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, the Mn content is set to 5.0% or less. It is preferable to set the above.
• the P content is 0.02% or less.
• 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 is 0.001% or more.
• S 0.01% or less S becomes an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld. 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. In addition, from the viewpoint of ensuring good stretch flangeability, it is more preferable to set it to 0.005% or less.
• 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 is 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%, the hardenability decreases. Therefore, the Al content is preferably 0.1% or less.
• the Al content is preferably 0.01% or more.
• the N content is preferably 0.01% or less.
• the lower limit of the N content is not particularly limited and may be 0%. However, since excessive reduction leads to an increase in 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 if it is contained in excess, the shape fixability decreases. Therefore, when Nb is added, the Nb content is preferably 0.05% or less.
• the lower limit of the Nb content is not particularly limited and may be 0%.
• Ti 0.05% or less Ti is an effective component for strengthening steel, similar to Nb, 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 the austenite grain boundaries.
• the addition of excessive B significantly impairs formability. Therefore, when B is added, from the viewpoint of improving formability, it is preferable that the B content be 0.0050% or less.
• the lower limit of the B content is not limited, but from the viewpoint of enhancing the effect of adding B, it is preferable that it be 0.0002% or more.
• Cr 1.0% or less Cr is a useful element for strengthening steel and improving hardenability.
• the Cr content is preferably 1.0% or less in order to reduce alloy costs.
• the lower limit of the Cr content is not particularly limited, but from the viewpoint of enhancing the effect of adding Cr, it is preferably 0.1% or more.
• Sb 0.03% or less
• Sb is an element that has the effect of preventing decarburization of the surface layer of the steel sheet during hot pressing.
• excessive Sb leads to an increase in rolling load, which reduces productivity. Therefore, when Sb is added, it is preferable that the Sb content is 0.03% or less from the viewpoint of further improving productivity.
• the lower limit of the Sb content is not particularly limited, but it is preferable that it is 0.003% or more from the viewpoint of enhancing the effect of adding Sb.
• the hot-pressed member of the present invention can be manufactured by hot-pressing a hot-press steel plate 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 plate.
• the hot press member of the present invention has a coating layer containing an FeAl alloy phase and a Zn phase on the surface layer of a steel sheet.
• the coating layer may be provided on at least one side of the steel sheet, and may be provided on both sides.
• the Zn phase contributes to improving the appearance corrosion resistance, particularly the red rust resistance.
• the FeAl alloy phase also contributes to the 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 in order to ensure corrosion resistance, it is preferably 7 ⁇ m or more, more preferably 10 ⁇ m or more, and even more preferably 13 ⁇ m or more. On the other hand, from the viewpoint of adhesion of the coating layer, the thickness of the coating layer is preferably 30 ⁇ m or less, more preferably 25 ⁇ m or less, and even more preferably 20 ⁇ m or less.
• the FeAl alloy phase contributes to pitting corrosion resistance and has the effect of reducing the corrosion rate of the Zn phase.
• 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.
• 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 ⁇ -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.
• 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.
• the elements other than Fe and Al may form an intermetallic compound, may be in solid solution, or may form a compound such as an oxide.
• the particle size of the FeAl alloy phase is not particularly limited. However, if the particle size is too small, the interface area between the Zn phase and the FeAl alloy phase becomes large, and the corrosion rate of the Zn phase due to galvanic corrosion increases. As a result, the corrosion resistance decreases. Therefore, from the viewpoint of further improving the corrosion resistance, the average particle size of the FeAl alloy phase is preferably 3 ⁇ m or more, more preferably 5 ⁇ m or more, and even more preferably 7 ⁇ m or more.
• the average particle size of the FeAl alloy phase is preferably 20 ⁇ m or less, more preferably 18 ⁇ m or less, and even more preferably 15 ⁇ m 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 hot-pressed member, which has been mirror-finished, is observed with an SEM, and a backscattered electron image is obtained at an accelerating voltage of 5 kV and a magnification of 500 times.
• the crystal grains of the FeAl alloy phase are identified from the backscattered electron image based on the crystal orientation contrast. The long and short diameters of each identified crystal 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 the hot-pressed member.
• the coating layer In order to set the natural immersion potential within the above range, the coating layer needs to contain a Zn phase.
• the presence of the Zn phase reduces the rate of corrosion (appearance corrosion) from areas where the rust prevention function of the coating film has decreased, such as scratches in the coating film or painted edges, and allows good appearance quality to be maintained.
• the risk of delayed fracture caused by hydrogen generated by corrosion can be reduced.
• the coating layer contains both the FeAl alloy phase and the Zn phase, excellent corrosion resistance in appearance can be obtained for a long period of time. That is, the FeAl alloy phase has a low anodic dissolution rate and excellent corrosion resistance, and also has a low rate of oxygen reduction reaction, which is a cathodic reaction in an atmospheric corrosive environment. Since the coating layer contains the FeAl alloy phase together with the Zn phase, corrosion of the Zn phase is suppressed, and as a result, a stable red rust generation resistance effect can be obtained for a long period of time.
• the internal structure of the coating layer is not particularly limited, but generally, it may be a structure in which the Zn phase exists between the crystal grains of the FeAl alloy phase.
• the presence or absence of a Zn phase in the coating layer can be determined by observing the cross section of the hot-pressed part with SEM-EBSD (electron backscatter diffraction). Specifically, the cross section of the hot-pressed part, which has been finished to a mirror surface, is observed with SEM-EBSD at an accelerating 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 alloy phase and as a structure having 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 is not particularly limited, but if it is less than 5 g/ m2 per side, the period during which the red rust generation resistance effect can be obtained is shortened. Therefore, the amount of the Zn phase 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 exceeds 60 g/ m2 per side, the red rust generation resistance effect is saturated and there is a risk of LME (liquid metal embrittlement) cracking occurring during welding. Therefore, the amount of the Zn phase is preferably 60 g/ m2 or less per side.
• the amount of the Zn phase attached can be determined by dissolving the Zn phase in the coating layer in an aqueous solution by subjecting the hot-pressed member to anodic electrolysis, and quantitatively analyzing the resulting aqueous solution by ICP-MS (inductively coupled plasma-mass spectrometry). Specifically, first, the hot-pressed member is used as the working electrode and a platinum mesh electrode is used as the counter electrode, and constant current anodic electrolysis is performed at 4 mA/ cm2 in a 3% sodium hydroxide-1% aluminum chloride aqueous solution. The electrolysis is stopped at the point where the potential becomes steeply noble, and the amount of Zn in the solution is quantitatively analyzed by 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.
• 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 viewpoint of further enhancing the effect of improving the appearance and corrosion resistance by the FeAl alloy phase and the Zn phase, it is desirable that the proportion of the other phases is 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 natural immersion potential of the hot-pressed member is ⁇ 1100 to ⁇ 900 mV based on a silver-silver chloride-saturated potassium chloride electrode (SSE).
• SSE silver-silver chloride-saturated potassium chloride electrode
• the natural immersion potential is set to -900mV or less, preferably -925mV or less, and more preferably -950mV or less.
• the natural immersion potential is set to -1100mV or more, preferably -1075mV or more, and more preferably -1050mV or more.
• the natural immersion potential in the present invention refers to the natural immersion potential in an air-saturated 0.5 mass% NaCl aqueous solution at 25°C, expressed based on a silver-silver chloride-saturated potassium chloride electrode.
• a 10 mm ⁇ area on any flat part of the hot-pressed member having an area of 10 mm ⁇ or more is used as the working electrode.
• the average value of the immersion potential from 60 seconds to 600 seconds after the working electrode is immersed in the NaCl aqueous solution is taken as the natural immersion potential of the hot-pressed member.
• the temperature of the NaCl aqueous solution is adjusted to 25 ⁇ 5°C.
• the mechanical properties of the hot-pressed member of the present invention are not particularly limited, but it is preferable that the residual stress measured using X-ray diffraction at any point 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 point, increasing the risk of delayed fracture. In order 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.
• the hot-pressed member of the present invention can be manufactured by plating a base steel sheet to produce a plated steel sheet, and then hot pressing the plated steel sheet.
• any steel plate can be used without any particular limitation.
• the suitable composition of the steel plate is the same as the preferred composition of the steel material of the hot-pressed member described above.
• the base steel plate is preferably a hot-rolled steel plate or a cold-rolled steel plate.
• the plating of the base steel sheet can be carried out by any method, but it is preferably carried out by hot-dip plating.
• the base steel sheet is annealed.
• the annealed base steel sheet is immersed in a hot-dip plating bath to produce a hot-dip plated steel sheet having a hot-dip plating layer on the surface of the base steel sheet.
• the hot-dip plating 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.
• the hot-dip plating bath may further contain Si as an option.
• the adhesion weight of the hot-dip coating layer is not particularly limited, but is preferably 20 g/m 2 or more per one side of the steel sheet, more preferably 30 g/m 2 or more, even more preferably 35 g/m 2 or more, and particularly preferably 50 g/m 2 or more.
• the adhesion weight of the hot-dip coating layer is preferably 300 g/m 2 or less per one side of the steel sheet, more preferably 250 g/m 2 or less, and even more preferably 200 g/m 2 or less.
• the adhesion weight of the coating layer increases due to the diffusion of Fe from the base steel sheet. Therefore, by setting the adhesion weight of the hot-dip coating layer on the hot-dip coating steel sheet before hot pressing to the above range, the adhesion weight of the coating layer on the hot-pressed member can be set to the above-mentioned preferred range.
• the coating weight per side of the hot-dip galvanized layer is determined by the following method. First, the hot-dip galvanized steel sheet to be evaluated is punched to obtain three samples with a diameter of 48 mm. Then, one side of each sample (the side opposite to the side on which the coating weight is to be measured) is 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 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 of 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 of the hot-dip galvanized steel sheet.
• the hot press includes a heating step of heating a steel sheet for hot press, and a hot-pressing step of hot-pressing the steel sheet for hot press heated in the heating step.
• 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 having an oxygen concentration of 21 to 35% by volume for a temperature-raising time of 60 seconds to 600 seconds.
• the steel sheet for hot press after the temperature-raising may be further held at the heating temperature for a holding time of 300 seconds or less in the atmosphere.
• Oxygen concentration It is economical and common to use air or dry air with a lowered dew point as the atmospheric gas in the heating step in hot pressing.
• air or dry air with a lowered dew point as the atmospheric gas in the heating step in hot pressing.
• the Zn phase in a metallic state hardly remains on the surface of the hot pressed member due to evaporation or oxidation.
• the inventors have intensively studied 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 increase the oxidizing property, a large amount of Zn in a metallic state, i.e., the Zn phase, can be left in the coating layer of the finally obtained hot pressed member. This is because a dense zinc oxide film is formed on the surface layer of the steel sheet for hot pressing in the heating step, which suppresses further oxidation and evaporation.
• the oxygen concentration in the atmosphere during the heating process is 21% by volume or more. However, if the oxygen concentration is less than 22% by volume, the above effect is not necessarily sufficient. Therefore, it is more preferable that the oxygen concentration is 22% by volume or more, and even more preferable that it is 25% by volume or more. If the oxygen concentration is less than 22% by volume, in order to prevent the disappearance of the Zn phase, it is necessary to lower the heating temperature and to 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 the cost increases significantly. Therefore, it is preferable that the oxygen concentration is 35% by volume or less.
• the heating temperature in the heating step is lower than the Ac 3 transformation point, the strength required for the hot press member may not be obtained. Therefore, the heating temperature is preferably set to the Ac 3 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 set to 950°C or lower, and even more preferably set to 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 calculated by the following formula (1).
• Ac3 transformation point (°C) 881-206C + 53Si-15Mn-1Cr... (1)
• the element symbols represent the content (mass%) of each element. The content of elements that are not contained is calculated as 0.
• the temperature rise time is preferably 60 seconds or more. In particular, from the viewpoint of making the average grain size of the FeAl alloy phase 3 ⁇ m or more, it is more preferable to set the temperature rise time to 120 seconds or more. On the other hand, if the temperature rise time exceeds 600 seconds, the solid solution of Zn into the FeAl alloy phase will proceed, and the amount of Zn phase will decrease. Therefore, from the viewpoint of ensuring the amount of Zn phase, it is preferable to set the temperature rise time to 600 seconds or less, and more preferably to 240 seconds or less.
• the heating temperature may be further held after reaching the heating temperature.
• the time (holding time) for holding the heating temperature 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, it is preferable to set the holding time to 300 seconds or less.
• the lower limit of the holding time is 0 seconds. However, from the viewpoint of operational stability of hot pressing, it is preferable to hold for 5 seconds or more.
• a hot-dip galvanized steel sheet was prepared according to the following procedure, and the hot-dip galvanized steel sheet was hot-pressed to produce a hot-pressed component.
• 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 being Fe and unavoidable impurities.
• hot-dip plating was carried out in 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.
• the above-mentioned base steel sheet was annealed and then electroplated to obtain plated steel sheets. In some examples (Comparative Example No. 59), plating was not carried out.
• test pieces measuring 200 x 300 mm were taken from the obtained steel plate 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.
• the test piece 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 had a flat portion on the top surface that was 100 mm long, a flat portion on the side that was 50 mm long, and a flat portion on the bottom that was 50 mm long.
• the bending radius of the die was 7R on both shoulders on the top surface and both shoulders on the bottom surface.
• the natural immersion potential of the obtained hot press member was measured by the following procedure. First, three samples with a diameter of 16 mm were taken from the flat part of the upper surface of the hot press member formed into a hat shape by punching. The immersion potential was measured in an air-saturated 0.5 mass% NaCl aqueous solution at 25 ⁇ 5 ° C., using a 10 mm diameter area 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 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 value of the natural immersion potential of the three different samples was taken as the natural immersion potential of the hot press member to be evaluated. The measurement results are shown in Tables 3 and 4.
• a test piece was taken from the flat part of the upper surface of the hot press member, and the presence or absence of the 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 acceleration voltage of 15 kV and a magnification of 1000 times.
• a point analysis was performed by EDS in an area with a dark contrast, i.e., a low electron density, compared to the base material.
• an area in which the total content of Fe and Al was 80 atomic % or more was regarded as an FeAl alloy phase.
• a test piece was taken from the flat part of the upper surface of the hot press 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 acceleration voltage of 5 kV and a magnification of 500 times.
• the crystal grains of the FeAl alloy phase were identified from the backscattered electron image based on the crystal orientation contrast, and the major axis and minor axis of each identified crystal grain were measured.
• the average value of the major axis and minor axis obtained was taken as the grain size of each FeAl alloy phase.
• the average value of the grain sizes of 20 randomly selected FeAl alloy phases was taken as the average grain size of the FeAl alloy phase in the hot press member.
• the presence or absence of a Zn phase in the coating layer was determined by observing the cross section of the hot-pressed member with SEM-EBSD. Specifically, the cross section of the hot-pressed member, which had been mirror-finished, was observed with 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 having a hexagonal crystal structure.
• Zn phase adhesion amount The amount of the Zn phase was determined by dissolving the Zn phase in the coating layer in an aqueous solution by subjecting a sample taken from the hot-pressed member to anodic electrolysis, and quantitatively analyzing the resulting aqueous solution by ICP-MS (inductively coupled plasma mass spectrometry). Specifically, three samples with a diameter of 48 mm were taken from the flat portion of the upper surface of the hot-pressed member formed into a hat shape by punching. The surfaces of the samples were masked except for the surface to be measured.
• 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/cm 2 in a 3% sodium hydroxide-1% aluminum chloride aqueous solution.
• the electrolysis was stopped at the point where the potential became steeply noble, and the amount of Zn in the solution was quantitatively analyzed by ICP-MS to measure the amount of dissolved Zn.
• the amount of Zn phase was determined by dividing the amount of Zn by the surface area of the hot-pressed member.
• a 70 mm wide area was cut out of the resulting hat-shaped high-strength steel member using a laser cutter, and the test piece was subjected to a zinc phosphate-based conversion treatment and electrocoating to produce a corrosion resistance test piece.
• the zinc phosphate-based conversion treatment was carried out under standard conditions using a PB-SX35 made by Nihon Parkerizing Co., Ltd.
• the electrocoating was carried out using an Electron GT-100 made by Kansai Paint Co., Ltd. so that the coating thickness was 5 ⁇ m.
• the baking conditions for the electrocoating were to reach 170°C and then hold for 20 minutes.
• the obtained corrosion resistance test pieces were subjected to a cyclic 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 blister from the edge and the occurrence of red rust on the cut edge, according to the following criteria. A score of 3 or more for both the paint blister width and the area ratio of red rust on the edge was considered to be acceptable.
• the evaluation results are shown in Tables 3 and 4. Paint film blister width from painted edge 1: Paint film blister width > 5 mm 2: 3mm ⁇ paint film blister width ⁇ 5mm 3: 2mm ⁇ paint blister width ⁇ 3mm 4: 1mm ⁇ paint blister width ⁇ 2mm 5: Paint blister width ⁇ 1mm
• Red rust occurs from the painted edge 1: Red rust area rate on edge > 50% 2: 30% ⁇ end surface red rust area ratio ⁇ 50% 3: 20% ⁇ end surface red rust area ratio ⁇ 30% 4: 10% ⁇ end surface red rust area ratio ⁇ 20% 5: Red rust area rate on end surface ⁇ 10%
• the hot-pressed parts that met the conditions of the present invention had good corrosion resistance to both paint film blistering from the painted edge and the occurrence of red rust, and were excellent in overall appearance corrosion resistance.
• the hot-pressed members of the comparative examples that did not meet the conditions of the present invention were inferior in at least one of the following: paint film blistering from the painted edge and red rust generation.
• the coating weight of the hot-pressed steel sheet was small, and the Zn in the metallic state disappeared due to alloying or oxidation during the heating process, resulting in less Zn phase in the coating layer of the hot-pressed member.
• the immersion potential became nobler, and good post-painting corrosion resistance could not be obtained.
• Comparative Example No. 55 a hot press steel sheet containing a large amount of Mg in the plating layer and with a large plating adhesion was subjected to hot pressing.
• an excessive amount of MgZn-based intermetallic compound phase was generated in the surface layer of the hot press member.
• the immersion potential became excessively noble and the corrosion rate of the plating layer increased, making it impossible to obtain good post-painting corrosion resistance.

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• 2023
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