Title of invention: Aluminum-iron-coated steel sheet for hot pressing with excellent mold abrasion resistance and manufacturing method thereof
technical field
[One]
The present invention relates to an aluminum-iron-based plated steel sheet for hot pressing and a method for manufacturing the same, and more particularly, to an aluminum-iron-based plated steel sheet for hot pressing with excellent mold wear resistance and a method for manufacturing the same.
background
[2]
Recently, due to the depletion of petroleum energy resources and high interest in the environment, regulations on improving fuel efficiency of automobiles are getting stronger day by day. In terms of material, one way to improve the fuel efficiency of a vehicle is to reduce the thickness of the steel sheet used. However, if the thickness is reduced, the stability of the vehicle may occur. do.
[3]
For this reason, the demand for high-strength steel sheets has been continuously generated, and various types of steel sheets have been developed. However, since these steel sheets have high strength themselves, there is a problem in that the workability is poor. That is, since the product of the strength and elongation of the steel sheet always tends to have a constant value, when the strength of the steel sheet increases, there is a problem that the elongation, which is an index of workability, decreases.
[4]
In order to solve the above problem, a hot press forming method has been proposed. The hot press forming method is a method of increasing the strength of a final product by forming a low-temperature structure such as martensite in the steel sheet by processing the steel sheet at a high temperature and then rapidly cooling it to a low temperature. This method has the advantage of being able to minimize the problem of workability when manufacturing a member having high strength.
[5]
However, the hot press forming method has a problem in that the surface of the steel sheet is oxidized because the steel sheet is heated to a high temperature.
[6]
[7]
In order to solve such a problem, in patent document 1, the steel plate which gave aluminum plating is used for hot pressing.
[8]
Since such a steel sheet has an aluminum plating layer on the surface, oxidation of the base steel sheet does not occur when heated. In addition, in order to hot press forming the aluminum plated steel sheet, a step of heating the steel sheet is performed. In this step, the temperature of the steel sheet rises, and as a result, Fe diffusion occurs from the base steel sheet to the plating layer, and alloying occurs in the plating layer.
[9]
[Prior art literature]
[10]
(Patent Document 1) US Patent Publication No. 6296805
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[11]
According to one aspect of the present invention, an object of the present invention is to provide an aluminum-iron-based plated steel sheet for hot pressing having excellent mold wear resistance and a method for manufacturing the same.
[12]
The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the general contents of the present specification.
means of solving the problem
[13]
One aspect of the present invention, the holding steel plate; and an aluminum-based plating layer provided on the surface of the base steel sheet, wherein the aluminum-based plating layer is provided on the surface of the base steel sheet, and an alloying layer made of at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 ; and an aluminum layer provided on the alloying layer, wherein the alloying layer thickness is 90% or more of the aluminum-based plating layer thickness, and the Ti surface fraction observed on the surface of the aluminum-based plating layer is 2% or less, Zn surface fraction It is possible to provide an aluminum-iron-based plated steel sheet of 1% or less.
[14]
The thickness of the aluminum-based plating layer may be 20 to 40 μm, and the double-sided plating amount may be 20 to 200 g/m 2 .
[15]
The base steel sheet in wt%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the balance may contain Fe and other unavoidable impurities.
[16]
The base steel sheet is the sum of at least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%, the sum of at least one selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, Cu + Ni: 0.005 ~ 2.0%, Sb + Sn: 0.001 ~ 1.0%, and B: may further include one or more of 0.0001 ~ 0.01%.
[17]
On the surface of the steel sheet, a fraction of Ti or greater than or equal to 4 μm in diameter per circle may be 20% or less, and a fraction of greater than or equal to or greater than 4 μm in diameter of Zn may be 20% or less.
[18]
When the steel sheet is heated at 930° C. for 6 minutes and then subjected to a high-temperature abrasion test, the reaction area of ​​the pressure-sensitive paper of the mold may be 50% or less.
[19]
[20]
Another aspect of the present invention may provide a hot press formed member obtained by hot press forming an aluminum-iron-based plated steel sheet.
[21]
On the surface of the member, the Ti surface fraction may be 5% or less, the fraction of 4 µm or more in diameter per circle may be 50% or less, the Zn surface fraction may be 5% or less, and the fraction of 4 µm or more in diameter per circle may be 50% or less.
[22]
[23]
Another aspect of the present invention, comprising the steps of preparing a steel plate; Plating by immersing the base steel sheet in an aluminum plating bath containing, by weight, Ti: 0.1% or less (including 0%), Zn: 1% or less (including 0%); cooling after plating; alloying heat treatment of the cooled steel sheet at a temperature in the range of 600 to 800° C. for 0.1 to 100 hours in an oxygen and/or nitrogen atmosphere phase annealing furnace having a dew point temperature of less than -10° C.; cooling after heat treatment; and removing oxides from the cooled steel sheet surface.
[24]
The aluminum plating bath may include, by weight, Si: 7 to 15%, the remainder Al and other unavoidable impurities.
[25]
The plating may be such that the thickness of the plating layer is 20 to 40 μm, and the plating amount on both sides is 20 to 200 g/m 2 .
[26]
The base steel sheet in wt%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the balance may contain Fe and other unavoidable impurities.
[27]
The base steel sheet is the sum of at least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%, the sum of at least one selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, Cu + Ni: 0.005 ~ 2.0%, Sb + Sn: 0.001 ~ 1.0%, and B: may further include one or more of 0.0001 ~ 0.01%.
[28]
The cooling after plating may be cooling at an average cooling rate of 3 to 20 °C/s to 250 °C or less.
[29]
The method may further include the step of temper rolling the cooled steel sheet after the plating at a temper reduction ratio of 0.1 to 1.5%.
[30]
The cooling after the heat treatment may be cooling at a cooling rate of 50°C/h or less to 400°C.
[31]
When the cooled steel sheet is cooled to less than 100°C after the heat treatment, the method may further include discharging hydrogen in the upper annealing furnace in a cooling section of 100 to 400°C.
[32]
[33]
Another aspect of the present invention may provide a method of manufacturing a hot press-formed member in which an aluminum-iron-based plated steel sheet is heated at a temperature in the range of 880 to 950° C. for 3 to 10 minutes and then hot press-formed.
Effects of the Invention
[34]
According to one aspect of the present invention, it is possible to provide an aluminum-iron-based plated steel sheet for hot pressing excellent in mold wear resistance and a method for manufacturing the same.
Brief description of the drawing
[35]
1 is a photograph of the surface of an aluminum-iron-based plated steel sheet observed with an electron probe X-ray microanalyzer (EPMA), (a), (b) are Ti and Zn of Inventive Example 2, (c), (d) are, respectively. It is a photograph of Ti and Zn of Comparative Example 2.
[36]
2 is a photograph of a cross section of an aluminum-iron-based plated steel sheet observed under an optical microscope, (a) is a photograph of Invention Example 2, (b) is a photograph of Comparative Example 1.
[37]
3 is a photograph of the surface of the steel sheet observed by EPMA after hot press forming of an aluminum-iron-based plated steel sheet, (a), (b) are Ti and Zn of Inventive Example 2, (c), (d) are respectively It is a photograph of Ti and Zn of Comparative Example 3.
[38]
4 is a photograph of an aluminum-iron-based plated steel sheet heated at 930° C. for 6 minutes, and after a high-temperature abrasion test at 700° C., the wear tool is transferred to a reduced pressure paper, (a) is Comparative Example 3, (b) is the invention example 2 It's a photo.
Best mode for carrying out the invention
[39]
Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.
[40]
[41]
In forming the aluminum plating layer on the surface of the base steel sheet in advance before heating for hot press forming, the present inventors control the content of Ti and Zn with the composition of the plating bath, and at the same time, remove the alloying heat treatment and surface oxide after plating. It was confirmed that the wear resistance of the mold of the press-molded member could be improved, and the present invention was completed.
[42]
[43]
Hereinafter, the present invention will be described in detail.
[44]
[45]
Hereinafter, the steel sheet of the present invention will be described in detail.
[46]
[47]
The aluminum-iron-based plated steel sheet according to one aspect of the present invention may include a base steel sheet and an aluminum-based plating layer, and the aluminum-based plating layer may include an alloying layer and an aluminum layer.
[48]
[49]
The aluminum-iron-based plated steel sheet of the present invention includes a base steel sheet and an aluminum-based plating layer provided on the surface of the base steel sheet, and the aluminum-based plating layer is made of at least one of Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 . It may include an alloying layer and an aluminum layer provided on the alloying layer.
[50]
When the alloying heat treatment is performed after aluminum is plated on the base steel sheet, Fe of the base steel sheet is diffused into the aluminum plating layer. At this time, since the diffusion of Fe is made from the base steel sheet to the plating layer, on the outermost surface side of the coated steel sheet There may be an aluminum layer composed of pure aluminum with a small amount of diffused Fe, and an alloying layer composed of an intermetallic compound of Al and Fe may be formed between the aluminum layer and the base steel sheet. Although not limited, the Al-Fe-based intermetallic compound alloy phase constituting the alloying layer may be Fe 3Al, FeAl(Si), Fe 2Al 5 and FeAl 3 .
[51]
[52]
Meanwhile, an aluminum layer made of pure aluminum may be formed on the outermost surface side of the aluminum-based plating layer, that is, on the alloying layer in the aluminum-based plating layer, and the thickness of the aluminum layer may be less than 10% of the thickness of the aluminum-based plating layer.
[53]
Since the interface between the aluminum layer and the alloying layer in the plated steel sheet is unstable, if the thickness of the aluminum layer exceeds 10% of the thickness of the aluminum-based plating layer, peeling of the aluminum layer may occur during the leveling process for re-scaling and/or shape correction after alloying heat treatment. can Since it is preferable that the thickness of the aluminum layer is smaller, the lower limit thereof may not be limited. Therefore, in the present invention, it may be preferably less than 5% of the thickness of the aluminum-based plating layer, and more preferably less than 1%.
[54]
[55]
The thickness of the aluminum-based plating layer may be 20-40 μm.
[56]
If the thickness of the plating layer is less than 20㎛, the corrosion resistance is inferior, whereas if the thickness exceeds 40㎛, there is a problem that the weldability is deteriorated. Accordingly, the thickness of the aluminum-based plating layer may be 20 to 40 μm, and more preferably, 25 to 35 μm.
[57]
[58]
In the case of aluminum plating, the plating amount may be 20 to 100 g/m2 based on one side.
[59]
If the plating amount is less than 20 g/m 2 , corrosion resistance is inferior, whereas when the plating amount exceeds 100 g/m 2 , there is a problem in that weldability is deteriorated. Accordingly, the plating amount may be 20 to 100 g/m 2 based on one side. More preferably, it may be 40 to 90 g/m 2 , and more preferably 60 to 80 g/m 2 .
[60]
[61]
The base steel sheet of the present invention may not be particularly limited as long as it is used for hot press forming as a steel sheet for hot pressing. In the present invention, the base steel sheet in wt%, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the balance may contain Fe and unavoidable impurities.
[62]
[63]
Carbon (C): 0.04 to 0.5%
[64]
Carbon (C) is an essential element for increasing the strength of the heat treatment member, and should be added in an amount of 0.04% or more to sufficiently secure the strength of the heat treatment member. However, when the content exceeds 0.5%, in the case of producing a cold-rolled material, there is a problem that not only the cold-rolling property is greatly inferior when the hot-rolled material is cold-rolled, but also the spot weldability is greatly reduced.
[65]
Accordingly, the content of carbon (C) may be 0.04 to 0.5%. More preferably, the lower limit may be 0.1%. In addition, more preferably, the upper limit may be 0.45%, and more preferably, it may be 0.4%.
[66]
[67]
Silicon (Si): 0.01~2%
[68]
Silicon (Si) not only serves as a deoxidizer in steelmaking, but also inhibits the formation of carbides, which have the greatest influence on the strength of a hot press-formed member. Silicon (Si) should be added in an amount of 0.01% or more in order to secure retained austenite by concentrating carbon to the martensite lath grain boundary after martensite is formed in hot press forming. On the other hand, when performing aluminum plating on the steel sheet after rolling, the upper limit may be set to 2% in order to secure sufficient plating properties.
[69]
Accordingly, the content of silicon (Si) may be 0.01 to 2%. More preferably, the upper limit may be 1.5%.
[70]
[71]
Manganese (Mn): 0.01 to 10%
[72]
Manganese (Mn) should be added in an amount of 0.01% or more in order to not only secure a solid solution strengthening effect but also to lower the critical cooling rate for securing martensite in a hot press-formed member. On the other hand, since it is possible to secure the workability of the hot press forming process, reduce the manufacturing cost and improve the spot weldability by properly maintaining the strength of the steel sheet, the upper limit is set to 10%.
[73]
Accordingly, the content of manganese (Mn) may be 0.01 to 10%. More preferably, the upper limit may be 9%, more preferably 8%.
[74]
[75]
Aluminum (Al): 0.001 to 1.0%
[76]
Aluminum (Al), together with Si, can increase the cleanliness of steel by deoxidizing it in steel making, so it should be added in an amount of 0.001% or more. On the other hand, the upper limit is set to 1.0% so that the temperature of Ac3 does not become excessively high so that the heating required for hot press molding can be performed within an appropriate temperature range.
[77]
Accordingly, the content of aluminum (Al) may be 0.001 to 1.0%.
[78]
[79]
Phosphorus (P): 0.05% or less
[80]
Phosphorus (P) is an impurity present in steel, and it is advantageous as its content is small, so it should be added in an amount of 0.05% or less.
[81]
Accordingly, the content of phosphorus (P) may be 0.05% or less, and more preferably, 0.03% or less. However, in order to excessively lower the phosphorus (P) content, the lower limit may be set to 0.001% because there is a risk of increasing the manufacturing cost.
[82]
[83]
Sulfur (S): 0.02% or less
[84]
Sulfur (S) is an impurity present in steel. It is an element that impairs ductility, impact properties and weldability of members, and should be added in an amount of 0.02% or less.
[85]
Therefore, the content of sulfur (S) may be 0.02% or less, more preferably 0.01%. However, if the content is less than 0.0001%, the manufacturing cost may increase, so the lower limit may be 0.0001%.
[86]
[87]
Nitrogen (N): 0.02% or less
[88]
Nitrogen (N) is an element included as an impurity in steel, and in order to reduce the sensitivity to cracking during continuous casting of slabs and secure impact properties, the lower the content, the better, so it is added in 0.02% or less.
[89]
Accordingly, the content of nitrogen (N) may be 0.02% or less. However, the lower limit may be set to 0.001% in consideration of the increase in manufacturing cost.
[90]
[91]
In the present invention, the base steel sheet has the above-mentioned alloy composition, the sum of one or more selected from the group consisting of Cr, Mo and W, if necessary: ​​0.01 to 4.0%, one selected from the group consisting of Ti, Nb, Zr and V Sum of more than one species: 0.001 to 0.4%, Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0%, and B: may further include one or more of 0.0001 to 0.01%.
[92]
[93]
The sum of at least one selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
[94]
Cr, Mo, and W are elements that can improve hardenability, secure strength through precipitation strengthening effect, and ensure grain refinement, and one or more of them may be added in 0.01% or more as the total content. On the other hand, in order to secure the weldability of the hot press-formed member, the content is limited to 4.0% or less. In addition, when the total content of these elements exceeds 4.0%, the above-described effect may be saturated.
[95]
Accordingly, the sum of at least one selected from the group consisting of Cr, Mo and W may be 0.01 to 4.0%.
[96]
[97]
The sum of at least one selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%
[98]
Ti, Nb, Zr, and V are effective in improving the strength of the member by forming fine precipitates and stabilizing retained austenite and improving impact toughness by refining grains. . However, when the content exceeds 0.4%, the above-described effect is not only saturated, but also may lead to an increase in cost due to excessive alloy addition.
[99]
Accordingly, the sum of one or more selected from the group consisting of Ti, Nb, Zr and V may be 0.001 to 0.4%.
[100]
[101]
Cu + Ni: 0.005~2.0%
[102]
Cu and Ni are elements that improve strength by forming fine precipitates, and may be added in an amount of 0.005% or more in the sum of one or more of them in order to obtain the above-described effect. However, if the content exceeds 2.0%, a problem of cost increase occurs.
[103]
Accordingly, the content of Cu + Ni may be 0.005 to 2.0%.
[104]
[105]
Sb + Sn: 0.001~1.0%
[106]
Sb and Sn are concentrated on the surface during annealing heat treatment for Al-Si plating to suppress the formation of Si or Mn oxide on the surface, thereby improving plating properties. In order to obtain the above-described effect, at least one of these components must be added in an amount of 0.001% or more. However, when the content exceeds 1.0%, the cost is increased due to excessive alloy addition, and it is dissolved in the slab grain boundary and causes a problem of causing coil edge cracks during hot rolling.
[107]
Accordingly, the content of Sb + Sn may be 0.001 to 1.0%.
[108]
[109]
B: 0.0001~0.01%
[110]
B is an element that can improve hardenability even with a small amount of addition, and is segregated at the grain boundary of prior austenite to suppress the brittleness of the hot press-formed member due to grain boundary segregation of P and/or S, so 0.0001% or more should be added However, when the content exceeds 0.01%, the above-described effect is not only saturated, but there is a problem of causing brittleness in hot rolling.
[111]
Accordingly, the content of B may be 0.0001 to 0.01%. More preferably, the upper limit may be 0.005%.
[112]
[113]
The steel sheet of the present invention may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art of steel manufacturing, all of them are not specifically mentioned in the present specification.
[114]
[115]
Aluminum-iron-based plated steel sheet according to an aspect of the present invention On the surface, the Ti surface fraction may be 2% or less, the fraction equal to or greater than 4 µm in diameter per circle may be 20% or less, the fraction of Zn surface fraction may be less than or equal to 1%, and the fraction equal to or greater than 4 µm in diameter per circle may be 20% or less.
[116]
Ti and Zn form oxides, so that Ti oxide and Zn oxide have inferior wear resistance. In the present invention, the target mold wear resistance can be secured by controlling the area fractions of Ti and Zn that affect the wear resistance. The Ti surface fraction observed on the surface of the aluminum-iron plated steel sheet is limited to 2% or less and the Zn surface fraction to 1% or less. In addition, the fractions of Ti and Zn with an equivalent circle diameter of 4 μm or more are limited to 20% or less, respectively. Oxides formed excessively large with a diameter of 4 μm or more may lead to inferior mold wear resistance.
[117]
[118]
A hot press-formed member may be manufactured by hot press forming the aluminum-iron-based plated steel sheet of the present invention. On the surface of the member, the Ti surface fraction may be 5% or less, the fraction of equal to or greater than 4 µm in diameter may be 50% or less, the fraction of Zn may be less than or equal to 5%, and the fraction of equal to or greater than 4 µm in diameter may be 50% or less.
[119]
In order to secure the mold abrasion resistance characteristics targeted in the present invention, the Ti surface fraction observed on the surface of the member is limited to 5% or less and the Zn surface fraction to 5% or less. In addition, the fractions of Ti and Zn with an equivalent circle diameter of 4 μm or more are limited to 50% or less, respectively. An oxide formed excessively large with a diameter of 4 μm or more may cause poor mold wear resistance.
[120]
[121]
Hereinafter, the method for manufacturing a steel sheet of the present invention will be described in detail.
[122]
[123]
The aluminum-iron-based plated steel sheet according to an aspect of the present invention may be manufactured by aluminum plating, cooling, temper rolling, alloying heat treatment, cooling, hydrogen release and oxide removal of a base steel sheet satisfying the alloy composition described above.
[124]
[125]
aluminum plating
[126]
Substrate steel sheet satisfying the above alloy composition is immersed in an aluminum plating bath containing Ti: 0.1% or less (including 0%), Zn: 1% or less (including 0%), Si: 7-15% and the remainder Al 20 Aluminum plating is possible with a thickness of ~40㎛ and a coating amount of 20~100g/m2 based on one side.
[127]
If necessary, an annealing treatment may be optionally performed on the steel sheet before plating.
[128]
The plating bath may contain Si in an amount of 7 to 15%. Si plays a role of uniformly alloying with Fe in the plating layer, and should be included in an amount of 7% or more in order to obtain the above-described effect. On the other hand, since Si also serves to suppress the diffusion of Fe, when the content exceeds 15%, Fe diffusion is excessively suppressed, so that the alloy structure targeted in the present invention may not be obtained. Accordingly, the content of Si may be 7 to 15%. More preferably, it may be 8 to 12%, and more preferably, it may be 8 to 10%.
[129]
The plating bath may include Ti: 0.1% or less (including 0%) and Zn: 1% or less (including 0%). In the present invention, Ti and Zn can be inevitably included in the plating bath, and the content must be controlled to secure mold wear resistance. When the Ti content exceeds 0.1%, Ti in the plating layer diffuses to the surface during heat treatment and hot press forming, combines with oxygen to form Ti oxide, which deteriorates mold wearability. In addition, when the Zn content exceeds 1%, Zn in the plating layer diffuses to the surface during heat treatment and hot press forming, combines with oxygen to form Zn oxide, which deteriorates mold wear.
[130]
[131]
The thickness of the plating layer may be 20-40㎛. If the thickness of the plating layer is less than 20㎛, the corrosion resistance is inferior, whereas if the thickness exceeds 40㎛, there is a problem that the weldability is deteriorated. Therefore, the thickness of the plating layer may be 20 to 40㎛, more preferably 25 to 35㎛.
[132]
[133]
In the case of aluminum plating, the plating amount may be 20 to 100 g/m2 based on one side. If the plating amount is less than 20 g/m 2 , corrosion resistance is inferior, whereas if the plating amount exceeds 100 g/m 2 , there is a problem in that weldability is deteriorated. Accordingly, the plating amount may be 20 to 100 g/m 2 based on one side. More preferably, it may be 40 to 90 g/m 2 , and more preferably 60 to 80 g/m 2 .
[134]
[135]
Cooling
[136]
After aluminum plating, it can be cooled down to 250℃ or lower at an average cooling rate of 3~20℃/s.
[137]
After aluminum plating, it can be cooled down to 250℃ or less. If the cooling end temperature exceeds 250℃, it may cause a problem of significantly reducing the life of the top roll.
[138]
The cooling rate after aluminum plating may affect the formation of Al-Si crystallization phase and diffusion suppression layer in the plating layer, which affects the aluminum layer after alloying heat treatment. If the cooling rate exceeds 20 °C/s after aluminum plating, the diffusion suppression layer is not formed uniformly, so the alloying behavior of the coil becomes non-uniform during the subsequent heat treatment, so that phases other than the phase to be obtained in the present invention can be obtained. have. On the other hand, if the rate is less than 3°C/s, the Al-Si crystallized phase is coarsely formed, and the alloying of the plating layer during the subsequent heat treatment becomes non-uniform, so that phases other than the phase to be obtained in the present invention can be obtained.
[139]
[140]
temper rolling
[141]
Before winding the cooled steel sheet, temper rolling can be performed at a reduction ratio of 0.1 to 1.5%.
[142]
In the subsequent alloying heat treatment process, the reduction ratio may be 0.1 to 1.5% in order to induce alloying of the plating layer targeted in the present invention and secure the surface quality.
[143]
[144]
alloy heat treatment
[145]
The aluminum-plated steel sheet may be subjected to an alloying heat treatment for 0.1 to 100 hours at a temperature in the range of 600 to 800 ° C in an oxygen and/or nitrogen atmosphere phase annealing furnace having a dew point temperature of less than -10 ° C.
[146]
Aluminum-coated steel sheet can be heated in a batch annealing furnace. At this time, the heating atmosphere plays a very important role in the present invention. When a steel sheet is heat treated in a hydrogen and/or nitrogen atmosphere, if the dew point temperature is -10°C or higher, Ti and Zn in the plating layer may diffuse to the surface of the steel sheet to form oxides. There is a problem with doing it.
[147]
[148]
The aluminum-coated steel sheet can be maintained at a temperature in the range of 600 to 800° C. for 0.1 to 100 hours (in the present invention, the highest temperature that the furnace atmosphere temperature reaches in the above temperature range is the heating temperature). In this case, the holding time means the time from when the ambient temperature reaches the target temperature to the start of cooling. If the heating temperature is less than 600 ° C., alloying is not made sufficiently, and for this reason, the aluminum layer that is not alloyed on the outermost layer of the plating layer exceeds 10% of the thickness of the plating layer, and there is a problem that the plating layer may be peeled off during roll leveling. On the other hand, in order to prevent excessive formation of oxides in the surface layer of the steel sheet and secure spot weldability, the upper limit may be set to 800°C.
[149]
[150]
In order to sufficiently secure the alloying layer and at the same time prevent a decrease in productivity, the holding time may be set to 0.1 to 100 hours. When the holding time exceeds 100 hours, there is a problem in that the diffusion layer is excessively grown to deteriorate the spot weldability of the hot press-formed member. More preferably, the holding time may be 0.5 to 50 hours. The temperature of the steel sheet may have a heating pattern in which the temperature continues to rise without a cooling process until the heating temperature is reached.
[151]
[152]
The difference between the ambient temperature in the upper annealing furnace and the temperature of the steel sheet can be 1~50℃. The general heating of the upper annealing furnace takes the method of heating the steel sheet through the increase of the atmospheric temperature in the annealing furnace rather than the method of directly heating the steel sheet (coil). In this case, the difference between the ambient temperature and the steel plate temperature is unavoidable, but in order to minimize the material and plating quality deviation depending on the location in the steel plate, the difference between the ambient temperature and the steel plate temperature should be 50°C or less based on the time when the heat treatment target temperature is reached. can Ideally, the temperature difference should be as small as possible, but since it may be difficult to satisfy the overall average temperature increase rate condition by slowing the temperature increase rate, the lower limit of the temperature difference may be 1°C. Here, the temperature of the steel plate means the temperature of the loaded steel plate bottom (meaning the lowest part of the coil), and the ambient temperature means the temperature measured at the center of the inner space of the heating furnace.
[153]
[154]
Cooling
[155]
After heat treatment, it can be cooled to 400°C at a cooling rate of 50°C/h or less.
[156]
After heat treatment, the aluminum plated steel sheet can be cooled by applying various methods such as furnace cooling, air cooling, and water cooling. During cooling, the average cooling rate is not particularly limited, and may be rapidly cooled to improve productivity. However, in order to prevent sticking defects and ensure material uniformity, and to sufficiently form voids, it can be cooled to 400°C at a cooling rate of 50°C/h or less. The lower limit of the cooling rate is not particularly limited, but may be 1°C/h or more in consideration of productivity.
[157]
[158]
hydrogen release
[159]
When the aluminum-iron-based plated steel sheet is cooled to less than 100℃, hydrogen in the furnace can be released in the range of 100~400℃.
[160]
Even when the aluminum-coated steel sheet is subjected to upper annealing in a hydrogen and/or nitrogen atmosphere, a small amount of moisture remains in the furnace, and hydrogen can be incorporated into the steel by surface oxidation during the upper annealing process. The hydrogen mixed in this way remains in the hot pressing process thereafter, thereby promoting delayed hydrogen destruction of the final part. In order to suppress this, when cooling the steel sheet to less than 100°C, it is important to make the furnace atmosphere almost free of hydrogen in the range of 100 to 400°C so that hydrogen in the steel is released. When the hydrogen emission start temperature exceeds 400° C., more hydrogen may be released, but workability and productivity may deteriorate due to excessive high temperature. On the other hand, if the temperature is less than 100 ℃, the thermal activation energy is small, it is impossible to make a significant contribution to the hydrogen release.
[161]
[162]
oxide removal
[163]
aluOxides formed on the surface of the aluminum-iron-based plated steel sheet can be removed.
[164]
In spite of controlling the atmosphere of the upper annealing furnace, Ti and Zn oxides are formed on the surface of the aluminum-iron-based plated steel sheet by diffusion of Ti and Zn in the plating layer, causing mold wear in the hot pressing process. In order to solve this problem, it is possible to minimize surface oxidation by controlling the content of Ti and Zn in the plating layer, controlling the upper annealing furnace temperature, time, and atmosphere, etc. can be suppressed. As a method of removing the surface oxide, various methods may be used, and there is no particular limitation thereto, but as an example, a method of removing Ti and Zn oxide from the surface of the steel sheet using a roll brush may be used.
[165]
[166]
hot press forming
[167]
The aluminum-iron-based plated steel sheet of the present invention prepared as described above may be hot press-formed to produce a hot press-formed member. At this time, the hot press forming may use a conventional method, and in the present invention, after heating at a temperature in the range of 880 to 950° C. for 3 to 10 minutes, it may be hot formed into a desired shape using a press. However, the present invention is not limited thereto.
[168]
[169]
After heating the aluminum-iron plated steel sheet of the present invention prepared as described above at 930 ° C. for 6 minutes, put it between the molds, and after a high-temperature friction/wear test, the pressure-sensitive paper reaction area of ​​the mold is 50% or less. can
[170]
[171]
Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.
Modes for carrying out the invention
[172]
(Example)
[173]
A base steel sheet having the composition shown in Table 1 was prepared, and plating was performed with the plating bath composition and plating amount shown in Table 2 below. At this time, the plating bath uses an aluminum plating bath containing 7 to 15% of Si in addition to Ti and Zn and the balance being Al. Cooling after plating satisfies the condition of cooling at an average cooling rate of 3 to 20 °C/s to 250 °C or less suggested in the present invention.
[174]
[Table 1]
[175]
[176]
[Table 2]
[177]
[178]
The prepared plated steel sheet was heat treated under the alloying heat treatment conditions shown in Table 3, and then cooled to 400° C. at a cooling rate of 50° C./h or less, and a process of releasing hydrogen was applied. Thereafter, the thickness ratio of the alloying layer in the plating layer of the cooled steel sheet was measured, and the surface fraction of Ti and Zn and the fraction having a diameter of 4 μm or more were measured and shown in Table 3.
[179]
[180]
[Table 3]
[181]
[182]
The steel sheet of Table 3 was subjected to hot press forming shown in Table 4 below to obtain a hot press formed member. The surface fraction of Ti and Zn of the member and the fraction having a diameter of 4 μm or more were measured. In addition, the reaction area of ​​the reduced pressure paper was measured to confirm the abrasion characteristics. For the pressure-sensitive paper reaction test, the aluminum-iron-based plated steel sheet is heated under hot forming heating conditions, placed between the molds, and the drawing friction test is performed 10 times in the temperature range of 700~800℃. The reaction area was measured with an image analyzer. At this time, as the reaction area of ​​the pressure-sensitive paper becomes 100%, it means that the occurrence of adhesive wear decreases and the occurrence of abrasive wear increases during the high-temperature friction/wear test.
[183]
[184]
[Table 4]
[185]
[186]
As shown in Tables 1 to 4, Inventive Examples 1 to 3 satisfies all of the plating bath composition, plating amount, and manufacturing conditions proposed in the present invention, thereby securing physical properties targeted in the present invention.
[187]
[188]
1 is a photograph of the surface of an aluminum-iron-based plated steel sheet observed with an electron probe X-ray microanalyzer (EPMA), (a), (b) are Ti and Zn of Inventive Example 2, (c), (d) are respectively It is a photograph of Ti and Zn of Comparative Example 2. In the case of (c) and (d), it can be seen that Ti and Zn are excessively observed, respectively.
[189]
2 is a photograph of an aluminum-iron-based plated steel sheet observed under an optical microscope, (a) is a photograph of Inventive Example 2, (b) is a photograph of Comparative Example 1. In (b), it can be confirmed that the unalloyed layer is excessively formed because the heating temperature does not reach the range of the present invention.
[190]
3 is a photograph of the surface of the steel sheet observed by EPMA after hot press forming of an aluminum-iron-based plated steel sheet, (a), (b) are Ti and Zn of Inventive Example 2, (c), (d) are respectively It is a photograph of Ti and Zn of Comparative Example 3. In the case of (c) and (d), it can be seen that Ti and Zn are excessively observed, respectively.
[191]
4 is a photograph of an aluminum-iron-based plated steel sheet heated at 930° C. for 6 minutes, and after a high-temperature abrasion test at 700° C., the wear tool is transferred to a reduced pressure paper, (a) is Comparative Example 3, (b) is the invention example 2 It's a photo. In the case of (a), it can be confirmed that the reaction area is observed as a wider area than the unreacted area.
[192]
[193]
In Comparative Example 1, the alloying heat treatment heating temperature was out of the range of the present invention, and the alloying did not occur sufficiently because it did not reach the temperature range of the present invention, and did not satisfy the alloying layer thickness ratio as shown in FIG. 2 .
[194]
Comparative Example 2 is a case in which surface oxides are not removed after alloying heat treatment. Oxides are present on the surface of the aluminum-iron-based plated steel sheet, and the surface fraction of Ti and Zn targeted in the present invention and the ratio of diameters of 4 μm or more are not satisfied. did. For this reason, the mold abrasion resistance was lowered, and the reaction area range of the pressure-sensitive paper of the present invention was not satisfied.
[195]
Comparative Example 3 is a case where the dew point temperature, which is an alloying heat treatment condition, does not satisfy the range of the present invention, and oxides due to Ti and Zn diffusion were excessively formed on the surface of the steel sheet, and the surface fraction of Ti and Zn targeted in the present invention And it can be seen that the mold abrasion resistance is deteriorated because the ratio of the diameter of 4 μm or more is not satisfied.
[196]
Comparative Example 4 is a case where the alloying heat treatment heating temperature is outside the range of the present invention, and the oxide is excessively formed in the surface layer by heating to a temperature higher than the temperature range of the present invention, and the surface fraction of Ti and Zn targeted in the present invention And the mold abrasion resistance was lowered because it did not satisfy the ratio of 4 μm or more in diameter.
[197]
Comparative Example 5 was a case where the alloying heat treatment time was outside the range of the present invention, and it was heated excessively for a long time, so that the surface fraction of Ti and Zn and the ratio of diameter of 4 μm or more were not satisfied. As a result, it can be confirmed that the mold wear resistance is lowered.
[198]
Comparative Example 6 was a case in which alloying heat treatment was not performed, and did not satisfy the alloying layer thickness ratio targeted in the present invention.
[199]
In Comparative Examples 7 and 8, the Ti or Zn content was outside the range of the present invention, and the oxide was excessively formed, so that the surface fraction of Ti or Zn and the ratio of 4 μm or more in diameter were not satisfied. For this reason, the reaction area range of the pressure-sensitive paper showing abrasion characteristics was not satisfied.
[200]
In Comparative Example 9, the content of Ti and Zn was outside the scope of the present invention, and the oxide was excessively formed, and thus the surface fraction of Ti and Zn and a ratio of 4 μm or more in diameter were not satisfied. For this reason, the metal mold|die wear resistance of a member fell.
[201]
[202]
Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.
Claims
[Claim 1]
Soji steel plate; and an aluminum-based plating layer provided on the surface of the steel sheet; and an aluminum layer provided on the alloying layer, wherein the alloying layer thickness is 90% or more of the aluminum-based plating layer thickness, and the Ti surface fraction observed on the surface of the aluminum-based plating layer is 2% or less, Zn surface fraction Aluminum-iron-coated steel sheet with this 1% or less.
[Claim 2]
The aluminum-iron-based plated steel sheet according to claim 1, wherein the thickness of the aluminum-based plating layer is 20 to 40 μm, and the double-sided plating amount is 20 to 200 g/m2.
[Claim 3]
According to claim 1, wherein the base steel sheet in weight%, C: 0.04 ~ 0.5%, Si: 0.01 ~ 2%, Mn: 0.01 ~ 10%, Al: 0.001 ~ 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the remainder Fe and other unavoidable impurities containing aluminum-iron-coated steel sheet.
[Claim 4]
According to claim 3, wherein the base steel sheet, by weight, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 to 4.0%, Ti, Nb, Zr, and one or more sums selected from the group consisting of V : 0.001 to 0.4%, Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01% of aluminum-iron-coated steel sheet further comprising at least one.
[Claim 5]
The aluminum-iron-based plated steel sheet according to claim 1, wherein a fraction of Ti or greater than or equal to 4 µm in diameter per circle on the surface of the steel sheet is 20% or less, and a fraction of greater than or equal to or greater than 4 µm in diameter of Zn is equal to or less than 20% on the surface of the steel sheet.
[Claim 6]
The aluminum-iron-based plated steel sheet according to claim 1, wherein the steel sheet is heated at 930° C. for 6 minutes and has a pressure-sensitive paper reaction area of ​​50% or less during a high-temperature abrasion test.
[Claim 7]
A hot press formed member obtained by hot press forming the aluminum-iron-based plated steel sheet according to any one of claims 1 to 6.
[Claim 8]
8. The method according to claim 7, wherein on the surface of the member, the Ti surface fraction is 5% or less, the fraction of 4 µm or more in diameter per circle is 50% or less, the Zn content is 5% or less, and the fraction of 4 µm or more in diameter is 50%. The following hot press-formed parts.
[Claim 9]
Preparing a base steel plate; The base steel sheet in wt%, Ti: 0 .1% or less (including 0%), Zn: plating by immersion in an aluminum plating bath containing 1% or less (including 0%); cooling after plating; alloying heat treatment of the cooled steel sheet at a temperature in the range of 600 to 800° C. for 0.1 to 100 hours in an oxygen and/or nitrogen atmosphere phase annealing furnace having a dew point temperature of less than -10° C.; cooling after heat treatment; and removing oxides from the cooled steel sheet surface.
[Claim 10]
[Claim 10] The method of claim 9, wherein the aluminum plating bath contains, by weight, Si: 7 to 15%, the balance Al and other unavoidable impurities.
[Claim 11]
The method for manufacturing an aluminum-iron-based plated steel sheet according to claim 9, wherein the plating is performed such that the thickness of the plating layer is 20 to 40 μm, and the plating amount on both sides is 20 to 200 g/m 2 .
[Claim 12]
10. The method of claim 9, wherein the base steel sheet by weight, C: 0.04 ~ 0.5%, Si: 0.01 ~ 2%, Mn: 0.01 ~ 10%, Al: 0.001 ~ 1.0%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, the remainder Fe and other unavoidable impurities containing an aluminum-iron-based method for manufacturing a plated steel sheet.
[Claim 13]
13. The method of claim 12, wherein the base steel sheet by weight, Cr, Mo, and at least one sum selected from the group consisting of W: 0.01 to 4.0%, Ti, Nb, Zr and one or more sums selected from the group consisting of V : 0.001 to 0.4%, Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01% of an aluminum-iron-based plated steel sheet manufacturing method further comprising one or more.
[Claim 14]
10. The method of claim 9, wherein the cooling after plating is cooling at an average cooling rate of 3 to 20 °C/s to 250 °C or less.
[Claim 15]
10. The method of claim 9, further comprising the step of temper rolling the cooled steel sheet after the plating at a temper reduction ratio of 0.1 to 1.5%.
[Claim 16]
The method of claim 9, wherein the cooling after the heat treatment is cooling at a cooling rate of 50°C/h or less to 400°C.
[Claim 17]
10. The method of claim 9, further comprising the step of releasing hydrogen in the upper annealing furnace in a cooling section of 100 to 400 °C when the cooled steel sheet is cooled to less than 100 °C after the heat treatment.
[Claim 18]
18. A method of manufacturing a hot press-formed member by heating the aluminum-iron-based plated steel sheet prepared according to any one of claims 9 to 17 at a temperature in the range of 880 to 950° C. for 3 to 10 minutes and then hot press forming.