The present invention relates to a plated steel sheet for hot forming excellent in impact properties after hot forming, which can be preferably applied to automobile parts requiring impact resistance properties, and a hot forming member, and a manufacturing method thereof.
background
[2]
Due to the recent 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 fuel efficiency of automobiles is to reduce the thickness of the steel plate used. However, since reducing the thickness may cause problems in the safety of the vehicle, the strength improvement of the steel plate must be supported. should be
[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 by themselves, there is a problem in that the workability is poor. That is, since the product of the strength and the elongation for each grade of the steel sheet always tends to have a constant value, when the strength of the steel sheet increases, there is a problem in that the elongation, which is an index of workability, decreases.
[4]
In order to solve this problem, a hot press forming method has been proposed. The hot press forming method is a method of increasing the strength of the final product by forming a low-temperature structure such as martensite in the steel sheet by processing the steel sheet at a high temperature suitable for processing and then rapidly cooling it to a low temperature. In this case, there is an advantage that the problem of workability can be minimized when manufacturing a member having high strength.
[5]
Patent Document 1 is a representative technique related to such a hot-formed member. In Patent Document 1, an Al-Si plated steel sheet is heated to 850° C. or higher, and then the structure of the member is formed into martensite by hot forming by a press and rapid cooling, thereby securing ultra-high strength with a tensile strength exceeding 1600 MPa. There is an advantage in that it is possible to easily achieve weight reduction of a vehicle by securing such ultra-high strength. However, according to Patent Document 1, due to the high strength, the impact properties at the time of collision are relatively inferior, and there was a problem that the phenomenon of abnormally low impact properties appeared in some depending on the hot forming conditions, etc.
[6]
Accordingly, Patent Document 2 proposes a technique of improving the impact properties after hot forming through crystal grain refinement by spheroidizing the inclusions by adjusting the Ca/S ratio in the steel sheet for hot forming, and adding an alloying element such as Nb. However, Patent Document 2 is about inclusion control and grain size control for improving the impact properties of general steel materials. is evaluated as
[7]
Accordingly, there is a demand for development of a plated steel sheet for hot forming, a hot forming member, and a manufacturing method thereof having excellent impact properties after hot forming.
[8]
(Patent Document 1) US Patent Registration Publication No. 6296805
[9]
(Patent Document 2) Korean Patent Publication No. 10-2010-0047011
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[10]
An object of the present invention is to provide a plated steel sheet for hot forming excellent in impact properties after hot forming, a hot forming member, and a manufacturing method thereof.
[11]
The object of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject matter of the present invention from the general description of the present invention.
means of solving the problem
[12]
One aspect of the present invention is in weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1 %, N: 0.001-0.02%, Cr: 0.01-0.5%, the remainder Fe and other impurities containing steel plate; and a plating layer made of zinc, aluminum, or an alloy containing them formed on the surface of the base steel sheet, wherein the ratio (CS/CB) of the C content (CB) of the base steel sheet to the C content (CS) of the surface layer part (CS/CB) is 0.6 or less and the ratio ((Mn S+Cr S)/(Mn B+) of the sum of the Mn and Cr contents of the base steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the surface layer (Mn S+Cr S) It is a plated steel sheet for hot forming with excellent impact properties after hot forming with Cr B)) of 0.8 or more. (Here, the surface layer part means a region from the surface of the base steel sheet to a depth of 15 μm excluding the plating layer.)
[13]
The base steel sheet may further include one or more of B: 0.0005 to 0.01% and Ti: 0.01 to 0.05% by weight%.
[14]
The microstructure of the base steel sheet is, in area%, 40 to 100% of ferrite in the surface layer, and the balance contains 0 to 60% of pearlite, bainite or martensite, and 30 to 90% of ferrite in the center, and the balance is pearlite, bay It may contain 10 to 70% of nite or martensite.
[15]
Another aspect of the present invention is in weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001-0.02%, Cr: 0.01-0.5%, the remainder Fe and other impurities containing steel plate; And an alloy plating layer made of an alloy containing zinc or aluminum formed on the surface of the base steel sheet; Including, the ratio of the C content (CB) of the base steel sheet to the C content (C PS) of the member surface layer portion (C PS / CB ) is 1.2 or less, and the ratio of the sum of the Mn and Cr contents of the base steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the member surface layer (Mn PS+Cr PS) ((Mn PS+Cr PS) /(Mn B+Cr B)) is a hot-formed member with excellent impact properties of 0.8 or more. (Here, the member surface layer part means an area up to a depth of 25㎛ from the surface of the base steel sheet excluding the alloy plating layer.)
[16]
The ferrite coverage at the martensite grain boundary of the member surface layer portion may be 30% or less.
[17]
Another aspect of the present invention is in weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 ~ 0.02%, Cr: 0.01 ~ 0.5%, preparing a slab containing the remainder Fe and other impurities and heating to a temperature of 1050 ~ 1300 ℃; obtaining a hot-rolled steel sheet by hot-rolling the heated slab in a finishing hot-rolling temperature range of 800 to 950°C; winding the hot-rolled steel sheet at 450 to 750° C. after finishing hot rolling; Heating the wound hot-rolled steel sheet to 740 ~ 860 ℃ and annealing for 10 ~ 600 seconds in an atmosphere having a dew point temperature of -10 ~ 30 ℃; and plating the hot-rolled steel sheet by immersing it in a plating bath made of zinc, aluminum, or an alloy containing them after annealing; It is a method of manufacturing a plated steel sheet for hot forming excellent in impact properties after hot forming comprising a.
[18]
It may further include the step of cold rolling after the hot rolling before winding to obtain a cold rolled steel sheet.
[19]
The slab may further include one or more of B: 0.00005 to 0.01% and Ti: 0.01 to 0.05% by weight%.
[20]
Another aspect of the present invention is that the plated steel sheet for hot forming prepared by the method for producing a plated steel sheet for hot forming excellent in impact properties after hot forming as described above is heat-treated in a temperature range of Ac3 to 950° C. for 1 to 15 minutes and then hot It is a method of manufacturing a hot-formed member with excellent impact properties by press-molding.
Effects of the Invention
[21]
According to the present invention, it is possible to provide a plated steel sheet for hot forming excellent in impact properties after hot forming and a method for manufacturing the same.
[22]
The hot-formed member manufactured by hot press forming with the plated steel sheet for hot forming according to the present invention has a bending angle of 60° or more measured by the VDA238-100 bending test at a tensile strength of 1500 MPa. there is
[23]
Various and advantageous advantages and effects of the present invention are not limited to the above, and will be more easily understood in the course of describing specific embodiments of the present invention.
Brief description of the drawing
[24]
1 is a result of concentration analysis of carbon (C), manganese (Mn) and chromium (Cr) in the depth direction from the surface layer using GDS before hot press forming on the plated steel sheet for hot forming of Invention Example 1.
[25]
2 is an optical micrograph showing the structure of the member surface layer after hot forming of Inventive Example 1. FIG.
[26]
3 is a result of concentration analysis of carbon (C), manganese (Mn) and chromium (Cr) in the depth direction from the surface layer using GDS before hot press forming on the plated steel sheet for hot forming of Comparative Example 1.
[27]
4 is an optical micrograph showing the structure of the member surface layer after hot forming of Comparative Example 3.
Best mode for carrying out the invention
[28]
Hereinafter, preferred embodiments of the present invention will be described. However, the embodiment of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.
[29]
The present inventors noted that, in the case of the non-plated material, the bending angle after hot forming is significantly superior to that of the plated material. As a result of further research, it was confirmed that, in the case of non-plated materials, decarburization occurred in the surface layer of the steel sheet during heating for hot forming, and as a result, a soft ferrite layer was formed in the surface layer, resulting in excellent bendability.
[30]
Accordingly, the present inventors paid attention to the idea that the bendability of the hot forming member can be improved if the C content of the surface layer portion can be lowered in the plating material to form a soft layer on the surface layer portion of the base steel sheet. However, in the case of a plated material, it is difficult to form a soft ferrite layer as in the case of a non-plated material because decarburization does not occur sufficiently during heating for hot forming like a non-plated material. Rather, it has been found that if the ferrite layer is not continuously and sufficiently formed, a problem in that the bendability is lowered occurs.
[31]
The present inventors have studied in depth to overcome this problem, and as a result, the C content of the surface layer portion of the base steel sheet is controlled to a certain level or less compared to the C content of the center through the control of the annealing conditions, and Mn and Cr of the surface layer of the base steel sheet are controlled. By controlling the total content to a certain level or more compared to the sum of Mn and Cr content in the center, it is confirmed that a plated steel sheet for hot forming, a hot forming member, and a manufacturing method thereof having excellent impact properties after hot forming can be provided, and the present invention came to complete.
[32]
Hereinafter, a plated steel sheet for hot forming and a hot forming member having excellent impact properties after hot forming according to an aspect of the present invention will be described in detail below.
[33]
Coated steel sheet for hot forming with excellent impact properties after hot forming
[34]
The plated steel sheet for hot forming with excellent impact properties after hot forming according to an aspect of the present invention, in wt%, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05 %, S: 0.0001~0.02%, Al: 0.01~0.1%, N: 0.001~0.02%, Cr: 0.01~0.5%, remainder Fe and other impurities containing steel plate; and a plating layer made of zinc, aluminum, or an alloy thereof formed on the surface of the base steel sheet, wherein the ratio (CS/CB) of the C content (CB) of the base steel sheet to the C content (CS) of the surface layer part (CS/CB) is 0.6 or less and the ratio of the sum of the Mn and Cr contents (Mn B+Cr B) of the base steel sheet to the sum of the Mn and Cr contents of the surface layer (Mn S+Cr S) ((Mn S+Cr S)/(Mn B+) Cr B)) is 0.8 or more.
[35]
First, the alloy composition of the base steel sheet of the present invention will be described in detail. In the present invention, when expressing the content of each element, it is necessary to note that unless otherwise specified, it means weight %.
[36]
C: 0.15-0.4%
[37]
C is an essential element in order to increase the strength of the hot-formed member. When the C content is less than 0.15%, it is difficult to secure sufficient strength. On the other hand, when the C content is more than 0.4%, when the hot-rolled material is cold-rolled, the strength of the hot-rolled material is too high, so that the cold-rolling property is greatly inferior, and the spot weldability can be greatly reduced. Therefore, in the present invention, the C content is preferably limited to 0.15-0.4%.
[38]
Si: 0.1~1%
[39]
Si is added as a deoxidizer in steelmaking, and as a solid solution strengthening element and a carbide generation suppressing element, it contributes to the increase in strength of hot-formed members and is an effective element for material uniformity. When the Si content is less than 0.1%, the above-described effect is insufficient. On the other hand, when the Si content exceeds 1%, the Al plating property may be greatly reduced due to the Si oxide generated on the surface of the steel sheet during annealing. Therefore, in the present invention, the Si content may be limited to 0.1 to 1%.
[40]
Mn: 0.6~8%
[41]
Mn is an element added to secure a solid solution strengthening effect and to lower a critical cooling rate for securing martensite in a hot-formed member. In order to obtain the above effect, the Mn content needs to be added in an amount of 0.6% or more. On the other hand, when the Mn content is more than 8%, there is a problem that not only the cold rolling property is deteriorated due to an increase in the strength of the steel sheet before the hot forming process, but also the cost of ferroalloy is increased and the spot weldability is inferior. Therefore, in the present invention, the Mn content may be limited to 0.6 to 8%.
[42]
P: 0.001~0.05%
[43]
P exists as an impurity in the steel, and it is advantageous as its content is as small as possible. Therefore, in the present invention, the P content may be limited to 0.05% or less, preferably limited to 0.03% or less. Since P is an impurity element which is advantageous as it is small, there is no need to specifically set a lower limit of its content. However, in order to excessively lower the P content, there is a concern that the manufacturing cost may increase, and in this case, the lower limit may be set to 0.001%.
[44]
S: 0.0001~0.02%
[45]
S is an impurity in steel, and since it is an element that impairs ductility, impact properties and weldability of members, the maximum content is limited to 0.02%, preferably 0.01% or less. However, if the minimum content is less than 0.0001%, the manufacturing cost may increase, so the lower limit of the content may be set to 0.0001%.
[46]
Al: 0.01~0.1%
[47]
Al may increase the cleanliness of steel by deoxidizing it in steel making together with Si, and may be added in an amount of 0.01% or more to obtain the above effect. However, if it exceeds 0.1%, the high temperature ductility due to excessive AlN formed during the playing process is deteriorated, and there is a problem that slab cracks are easy to occur, so the upper limit of the content can be made 0.1% or less. Therefore, in the present invention, the Al content is preferably 0.01 to 0.1%.
[48]
N: 0.001~0.02%
[49]
N is an element included as an impurity in steel, and when the N content is more than 0.02%, high-temperature ductility due to excessive AlN formed during the casting process is deteriorated, and there is a problem that slab cracks are easy to occur. Therefore, in order to reduce the sensitivity to cracking during continuous casting of the slab and to secure the impact characteristics, N may be included in 0.02% or less. There is no need to set the lower limit in particular, but considering the increase in manufacturing cost, the lower limit of the N content may be set to 0.001% or more. Therefore, it is preferable that the N content in the present invention is 0.001 to 0.02%.
[50]
Cr: 0.01~0.5%
[51]
Cr is an element added to improve the solid solution strengthening effect and hardenability during hot forming similarly to Mn, and may be added in an amount of 0.01% or more to obtain the above effect. However, in the case of more than 0.5%, hardenability can be sufficiently secured, but the properties may be saturated and the manufacturing cost of the steel sheet may increase. Therefore, it is preferable that the Cr content in the present invention is 0.01 to 0.5%.
[52]
The base steel sheet of the plated steel sheet for hot forming according to an aspect of the present invention may further include one or more of B: 0.0005 to 0.01% and Ti: 0.01 to 0.05% in addition to the above-described components.
[53]
B: 0.0005~0.01%
[54]
B is an element that can improve hardenability even with a small amount of addition, and is segregated at the prior austenite grain boundary to suppress the brittleness of the hot-formed member due to grain boundary segregation of P and/or S. 0.0005% or more may be added to obtain. However, if it exceeds 0.01%, the effect is not only saturated, but also causes brittleness in hot rolling, so the upper limit may be set to 0.01%, and preferably, the B content may be 0.005% or less. Therefore, in the present invention, the B content is preferably 0.0005 to 0.01%.
[55]
Ti: 0.01~0.05%
[56]
Ti is added to generate TiN by combining with nitrogen remaining as an impurity in the steel, thereby leaving a solid solution B essential for securing hardenability. When the Ti content is less than 0.01%, it is difficult to sufficiently expect the effect, and when it exceeds 0.05%, the properties may be saturated and the steel sheet manufacturing cost may increase. Therefore, in the present invention, the Ti content is preferably 0.01 to 0.05%.
[57]
The remainder other than the above-mentioned components is iron (Fe), and additional addition is not particularly limited as long as it is a component that can be included in the steel sheet for hot press forming. In addition, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.
[58]
The plated steel sheet for hot forming excellent in impact properties after hot forming according to an aspect of the present invention includes a plating layer made of zinc, aluminum, or an alloy thereof formed on the surface of a base steel sheet. The plating layer provides corrosion resistance of the member in the final part, and serves to suppress decarburization and scale formation of the base steel sheet during heating for hot forming.
[59]
In the present invention, the type of the plating layer is not particularly limited, and as long as it is a plating layer applied to a conventional steel sheet for hot forming, it may be applied without limitation to the present invention. As a non-limiting embodiment, the plating layer may be made of zinc, aluminum, or an alloy containing these, and more specifically, the plating layer may be a hot-dip galvanizing layer, an electrogalvanizing layer, an alloyed galvanizing layer, an aluminum plating layer, or an aluminum alloy plating layer. .
[60]
Meanwhile, according to one aspect of the present invention, the plating layer may include components that may be included during the manufacturing process within a range that does not impair the purpose of the present invention, and in particular may include other unavoidable impurities.
[61]
In addition, the thickness of the plating layer may be 5 to 100㎛. If the thickness of the plating layer is less than 5㎛, it is difficult to exhibit sufficient corrosion resistance in the hot forming member, on the other hand, when the thickness is more than 100㎛, not only the heating time for hot forming excessively increases, but also the corrosion resistance improvement effect There may be a problem that the cost is excessively increased.
[62]
On the other hand, in the plated steel sheet for hot forming according to the present invention, the ratio (CS/CB) of the C content (CB) of the base steel sheet to the C content (CS) of the surface layer (hereinafter, also referred to as 'ratio (CS/CB)') is 0.6 or less is satisfied. Here, the surface layer part means an area up to a depth of 15 µm from the surface of the base steel sheet excluding the plating layer.
[63]
In addition, according to one aspect of the present invention, in the plated steel sheet for hot forming, the ratio (CS/CB) of the C content (CB) of the surface layer portion to the C content (CS) of the plate may be preferably 0.5 or less, More preferably, it may be 0.4 or less, and most preferably, it may be 0.35 or less.
[64]
When the ratio (CS/C B) is controlled as low as 0.6 or less, it is formed in the center of the base steel sheet after hot forming.Unlike the formation of a quality martensite phase, a relatively soft martensite phase with a low C content is formed in the surface layer part. A soft martensite phase is formed in the surface layer of the plated steel sheet, and the hardness of the surface layer is decreased, thereby securing excellent bending properties. If the ratio (CS/C B) exceeds 0.6, it becomes difficult to realize the effect of improving the bendability through softening the surface layer after hot forming. The lower limit of the ratio (CS/C B) may not be specifically limited. However, if the C content in the surface layer part is too low, the lower limit of the ratio (CS/CB) may be set to 0.05 or more, but the lower limit of the ratio (CS/CB) may be set to 0.05 or more, since the strength of the member may decrease after hot forming or the fatigue characteristics may be deteriorated. it is not
[65]
In addition, the plated steel sheet for hot forming according to an aspect of the present invention, the sum of the Mn and Cr contents of the base steel sheet (Mn B + Cr B) to the sum of the Mn and Cr contents of the surface layer (Mn S + Cr S) ratio ( (Mn S+Cr S)/(Mn B+Cr B)) (hereinafter also referred to as 'ratio ((Mn S+Cr S)/(Mn B+Cr B))') may be 0.8 or more. Here, the surface layer part means an area up to a depth of 15 µm from the surface of the base steel sheet excluding the plating layer.
[66]
Meanwhile, according to an aspect of the present invention, in the plated steel sheet for hot forming, the sum of the Mn and Cr contents of the base steel sheet (Mn B + Cr B) compared to the sum of the Mn and Cr contents of the surface layer (Mn S + Cr S ) of ((Mn S+Cr S)/(Mn B+Cr B)) may be preferably 0.85 or more, and more preferably 0.87 or more.
[67]
When the ratio ((Mn S+Cr S)/(Mn B+Cr B)) is lower than 0.8, hardenability of the surface layer portion during hot forming may not be sufficient, and ferrite may be partially formed on the member surface. Since ferrite partially formed at the hard martensitic grain boundary is a factor that greatly deteriorates bendability, the ratio ((Mn S+Cr S)/(Mn B+Cr B)) preferably satisfies 0.8 or more. The upper limit of the ratio ((Mn S+Cr S)/(Mn B+Cr B)) does not need to be separately limited. However, if the Mn and Cr contents in the surface layer part are too high, the hardness of the surface layer part after hot forming will increase, so the bendability is rather This inferiority problem can arise. Accordingly, the upper limit of the ratio ((Mn S+Cr S)/(Mn B+Cr B)) may be set to 2 or less, but is not limited thereto.
[68]
On the other hand, the microstructure of the base steel sheet does not need to be particularly limited, but in terms of area fraction, 40 to 100% of ferrite in the surface layer portion, the balance contains 0 to 60% of pearlite, bainite or martensite, and 30 to 90 ferrite in the center %, the balance may include 10 to 70% of pearlite, bainite or martensite.
[69]
Hot-formed member with excellent impact properties
[70]
On the other hand, it is possible to manufacture a hot-formed member having excellent impact properties by hot-press forming the plated steel sheet for hot forming having the above-described configuration after heat treatment in a temperature range of Ac3 to 950° C. for 1 to 15 minutes.
[71]
A hot-formed member having excellent impact properties according to an aspect of the present invention includes a base steel plate having the same alloy composition as the base steel plate of the plated steel plate, and an alloy plating layer made of an alloy containing zinc or aluminum formed on the surface of the base steel plate; and , the ratio of the C content (C PS) of the member surface layer portion to the C content (CB) of the base steel sheet (C PS/CB) (hereinafter also referred to as 'ratio (C PS/CB)') is 1.2 or less, and the base material The ratio of the sum of the Mn and Cr contents of the steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the member surface layer (Mn PS+Cr PS) ((Mn PS+Cr PS)/(Mn B+Cr B) ) (hereinafter also referred to as 'ratio ((Mn PS+Cr PS)/(Mn B+Cr B))') may be 0.8 or more. Here, the member surface layer portion means a region up to a depth of 25 μm from the surface of the base steel sheet except for the alloy plating layer.
[72]
On the other hand, according to one aspect of the present invention, in the hot forming member, the ratio (C PS / CB) of the C content (CB) of the base steel sheet to the C content (C PS) of the member surface layer portion (C PS / CB) may be preferably 1.1 or less and more preferably 1.05 or less.
[73]
In addition, according to an aspect of the present invention, in the hot forming member, the sum of the Mn and Cr contents of the base steel sheet (Mn B + Cr B) compared to the sum of the Mn and Cr contents of the member surface layer (Mn PS + Cr PS) The ratio of may be preferably 0.9 or more, more preferably 0.93 or more.
[74]
In general, when a plated steel sheet is heated for hot forming, the thickness of the plated layer becomes thick as the plated layer and the base iron are alloyed. Since the plated layer has an extremely low solubility of C, C, which cannot be dissolved during the alloying process, is concentrated in the surface layer and The C content is increased, and the high C content of the surface layer portion increases the hardness of the surface layer portion and makes the bendability inferior.
[75]
On the other hand, in the case of manufacturing a hot-formed member by hot press forming with a plated steel sheet for hot forming according to an aspect of the present invention, even if C is concentrated in the member surface layer portion, C content (C) of the member surface layer compared to the C content (CB) of the base steel sheet (C PS) ratio (C PS/CB) becomes 1.2 or less, and excessive increase in hardness of the member surface layer part can be suppressed. In addition, the ratio ((Mn PS+Cr PS)/(Mn B+) of the sum of the Mn and Cr contents of the base steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the member surface layer (Mn PS+Cr PS) Cr B)) becomes 0.8 or more, and ferrite formation is suppressed due to sufficient hardenability, so that the ferrite coverage of the martensitic grain boundary in the member surface layer (when observing the cross-section, the ratio occupied by ferrite among martensite grain boundaries) is 30% or less , and as a result, it is possible to secure excellent bendability with sufficient strength.
[76]
As described above, in the hot-formed member according to an aspect of the present invention, the ratio (CS/CB) is 1.2 or less, and the ratio ((Mn PS+Cr PS)/(Mn B+Cr B)) is 0.8 or more. According to the satisfaction, the bending angle measured by the VDA238-100 bending test at the level of tensile strength of 1500 MPa becomes more than 60°, and excellent impact properties can be secured. However, if the tensile strength is increased, for example, when the tensile strength of the hot-formed member becomes 1800 MPa or more, the bending angle criterion for judging excellent impact properties may be lowered.
[77]
Next, a method for manufacturing a plated steel sheet for hot forming and a hot forming member having excellent impact properties after hot forming, which is another aspect of the present invention, will be described in detail.
[78]
Method of manufacturing plated steel sheet for hot forming with excellent impact properties after hot forming
[79]
Another aspect of the present invention, a method for manufacturing a plated steel sheet for hot forming excellent in impact properties after hot forming, includes heating a slab satisfying the above alloy composition to 1050 to 1300° C.; Finishing hot rolling the heated slab in a temperature range of 800 to 950° C. to obtain a hot-rolled steel sheet; winding the hot-rolled steel sheet at 450 to 750° C. after finishing hot rolling; heating the wound hot-rolled steel sheet to 740 to 860 °C and annealing for 10 to 600 seconds in an atmosphere having a dew point temperature of -10 to 30 °C; and immersing the annealed hot-rolled steel sheet in a plating bath made of zinc, aluminum, or an alloy containing these for plating.
[80]
Slab heating stage
[81]
First, the slab satisfying the above-described alloy composition is heated to 1050 ~ 1300 ℃. If the slab heating temperature is less than 1050 ℃, it may be difficult to homogenize the slab structure, and if it is more than 1300 ℃, there is a fear that an excessive oxide layer is formed.
[82]
hot rolling step
[83]
Finish hot rolling the heated slab in a temperature range of 800 to 950 ° C to obtain a hot rolled steel sheet. When the finish hot rolling temperature is less than 800 ° C, it is difficult to control the plate shape due to the generation of a mixed grain structure in the surface layer of the steel sheet according to the abnormal rolling.
[84]
Cooling and winding stage
[85]
After finishing hot rolling, the hot-rolled steel sheet is wound at 450 to 750°C. When the coiling temperature is less than 450℃, the material deviation in the width direction becomes large, which may cause plate breakage and shape defects during cold rolling. On the other hand, if the coiling temperature exceeds 750 ℃, there is a problem in that the carbide coarsens the bendability.
[86]
cold rolling step
[87]
If necessary, it may further include the step of cold-rolling the wound hot-rolled steel sheet before annealing to obtain a cold-rolled steel sheet. The cold rolling is performed for more precise control of the thickness of the steel sheet, and the cold rolling may be omitted and annealing and plating may be performed immediately. In this case, the cold rolling may be performed at a reduction ratio of 30 to 80%.
[88]
Annealing step
[89]
The wound hot-rolled steel sheet is heated to 740 ~ 860 ℃ and annealed for 10 ~ 600 seconds in an atmosphere having a dew point temperature of -10 ~ 30 ℃. If the annealing temperature is less than 740°C or the annealing time is less than 10 seconds, the structure may not be recrystallized sufficiently, resulting in a poor plate shape, or excessively high strength after plating, which may cause mold wear during the blanking process. Not only that, since the diffusion of C during annealing is not sufficient, it becomes difficult to secure the ratio (CS/C B) of the C content (C S) of the surface layer portion to the C content (CB) of the base steel sheet to 0.6 or less. On the other hand, when the annealing temperature exceeds 860° C. or the annealing time exceeds 600 seconds, a large amount of annealing oxide is formed on the surface of the steel sheet during annealing, which may cause non-plating or reduce plating adhesion. In addition, Mn, Cr, etc. in the base iron due to internal oxidation are formed at the interface between the plating layer and the base iron or the grain boundary of the base steel, so that the Mn and Cr content of the surface layer compared to the sum of the Mn and Cr content (Mn B + Cr B) of the base steel sheet It is difficult to ensure the ratio ((Mn S+Cr S)/(Mn B+Cr B)) of the total (Mn S+Cr S) to 0.8 or more, so that hardenability of the surface layer part may be insufficient, and accordingly, the surface layer part after hot forming Partial ferrite is formed in the ferrite, which may cause a problem of deterioration in bendability.
[90]
On the other hand, in the present invention, the base material component of the steel sheet It is very important to control the dew point temperature of the annealing atmosphere in order to control the ratio of C, Mn, and Cr contents in the surface layer. When the dew point temperature of the annealing atmosphere is less than -10 ℃, the decarburization reaction is not sufficient and the effect of improving the bendability becomes insignificant. A problem of inferior bendability may occur.
[91]
Further, according to one aspect of the present invention, the annealing may be performed for 10 to 100 seconds in an atmosphere having a dew point temperature of 10 to 30 °C by heating the wound hot-rolled steel sheet to 800 to 840 °C more preferably.
[92]
plating step
[93]
After annealing, the wound hot-rolled steel sheet is immersed in a plating bath made of zinc, aluminum, or an alloy containing these for plating. In the present invention, the components of the plating bath used when forming the plating layer may not be particularly limited. However, as a non-limiting embodiment, the plating bath used in the present invention may be made of zinc, a zinc alloy, aluminum, or an aluminum alloy. In addition, as the plating conditions may be applied without limitation to the present invention as long as the plating conditions are commonly applied to the steel sheet for hot press forming, it is not specifically mentioned herein. In addition, according to one aspect of the present invention, the plating bath may contain other unavoidable impurities, and the zinc alloy and aluminum alloy may also contain components that may be normally included within a range that does not impair the purpose of the present invention, , in particular other unavoidable impurities.
[94]
Manufacturing method of hot-formed member with excellent impact properties
[95]
It is possible to manufacture a hot-formed member having excellent impact properties by performing hot press forming on the plated steel sheet for hot forming manufactured by the manufacturing method of the present invention as described above. In this case, the hot press forming may be performed using a method generally used in the art. However, as a non-limiting example, the plated steel sheet for hot forming may be heat-treated at a temperature range of Ac3 to 950° C. for 1 to 15 minutes, and then hot formed by pressing.
Modes for carrying out the invention
[96]
Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for exemplifying and embodying the present invention, and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.
[97]
(Example)
[98]
First, a slab having an alloy composition shown in Table 1 was prepared, and the slab was heated, hot rolled, and wound under the manufacturing conditions shown in Table 2 to prepare a hot-rolled steel sheet. Thereafter, after annealing under the annealing conditions shown in Table 2, galvanized steel sheets were manufactured by immersing in a zinc plating bath and plating so that the standard coating amount on one side was 70 g/m 2 .
[99]
[Table 1]
Category C Si Mn P S Al N Cr Ti B
Steel A 0.21 0.25 1.3 0.01 0.002 0.035 0.005 0.22 0.03 0.0022
Steel B 0.2 0.1 2.5 0.009 0.001 0.03 0.004 0.1 - -
[100]
[Table 2]
Classification Steel type Slab heating temperature (℃) Finish rolling temperature (℃) Coiling temperature (℃) Annealing conditions
Heating temperature (℃) Holding time (sec) Dew point temperature (℃)
Invention Example 1 A 1250 900 560 820 42 15
Invention Example 2 B 1200 880 500 800 65 10
Comparative Example 1 A 1250 900 560 820 42 -15
Comparative Example 2 A 1250 900 560 700 45 10
Comparative Example 3 B 1200 880 500 800 65 40
Comparative Example 4 B 1200 880 500 870 620 15
[101]
For the plated steel sheets of Inventive Examples and Comparative Examples manufactured according to the above manufacturing conditions, using the GDS (Glow Discharge Spectrometer; US LECO GDS 850A) method capable of quantitative analysis of various components in the depth direction from the surface layer to the depth direction concentration analysis was conducted for carbon (C), manganese (Mn) and chromium (Cr) at a sufficient depth with is shown in Table 3 below. In the case of GDS analysis, it is difficult to specify the correct plating layer/substrate interface on the concentration profile in the depth direction because the depth direction analysis is performed on a circular area of ​​2 to 6 mm, but based on various optical and SEM analysis results, etc. In the present invention, the point where the Zn content is 1% was used as the plating layer/substrate steel plate interface.
[102]
[Table 3]
Classification Steel type C B C S ratio (C S/C B) Mn B+Cr B Mn S+Cr S ratio ((Mn S+Cr S)/(Mn B+Cr B))
Invention Example 1 A 0.21 0.03 0.14 1.52 1.32 0.87
Invention Example 2 B 0.2 0.07 0.35 2.6 2.43 0.93
Comparative Example 1 A 0.21 0.2 0.95 1.52 1.49 0.98
Comparative Example 2 A 0.21 0.19 0.90 1.52 1.5 0.99
Comparative Example 3 B 0.2 0.02 0.10 2.6 1.74 0.67
Comparative Example 4 B 0.2 0.01 0.05 2.6 1.52 0.58
[103]
In addition, hot press forming was performed under the conditions shown in Table 4 below for the plated steel sheets of each Inventive Example and Comparative Example to prepare a hot forming member. Tensile tests and bending tests (VDA238-100) were performed by taking specimens from the flat portion of the manufactured hot-formed member, and through GDS analysis, carbon (C), manganese (Mn) and chromium (Cr) Concentration analysis was performed, and the ferrite coverage at the martensite grain boundary of the member surface layer was evaluated through cross-sectional optical microscope observation, and the results are shown in Table 4.
[104]
[Table 4]
Category Hot forming conditions Ratio (C PS/C B) Ratio ((Mn PS+Cr PS)/(Mn B+Cr B)) Ferrite coverage (%) Tensile strength (MPa) Bending angle (degrees)
Heating temperature (℃) Heating time (min)
Invention Example 1 900 6 0.95 0.93 0.5 1502 72
Invention Example 2 930 5 1.05 0.97 2.7 1527 67
Comparative Example 1 930 5 1.52 0.98 0.2 1508 53
Comparative Example 2 900 6 1.29 0.99 1.3 1511 51
Comparative Example 3 900 6 0.9 0.76 36 1478 42
Comparative Example 4 930 5 0.88 0.65 48 1427 47
[105]
The plated steel sheets of Inventive Examples 1 and 2 manufactured according to the conditions of the present invention had a ratio (CS/CB) of 0.6 or less, and a ratio ((Mn S+Cr S)/(Mn B+Cr B)) of 0.8 or more. Satisfied. Accordingly, the hot-formed member manufactured by hot press forming the plated steel sheets of Inventive Examples 1 and 2 had a ratio (C PS/CB) of 1.2 or less, and a ratio ((Mn PS+Cr PS)/(Mn B+Cr B) )) satisfies 0.8 or more, the ferrite coverage at the martensitic grain boundary in the surface layer was 30% or less, and the bending angle was 60° or more at a tensile strength of 1500 MPa class, indicating good bending properties.
[106]
Comparative Example 1 is a case where the dew point temperature during annealing is less than -10 ° C., Comparative Example 2 is a case where the heating temperature during annealing is insufficient. In Comparative Examples 1 and 2, the ratio (CS/CB) of the plated steel sheet exceeds 0.6, Accordingly, the ratio (C PS/CB) in the hot-formed member also exceeded 1.2, resulting in inferior bending properties.
[107]
On the other hand, Comparative Example 3 is a case in which the dew point temperature during annealing exceeds 30° C., Comparative Example 4 is a case in which annealing is excessively performed. In Comparative Examples 3 and 4, the ratio (CS/CB) of the plated steel sheet is Although the condition was satisfied, the ratio ((Mn S+Cr S)/(Mn B+Cr B)) became less than 0.8, and the ratio of the hot formed member ((Mn PS+Cr PS)/(Mn B+Cr B)) became less than 0.8. Due to this, the ferrite coverage at the martensite grain boundary of the member surface layer exceeded 30%, and the tensile strength was relatively lowered compared to other examples, and the bendability was also very deteriorated.
[108]
Although described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. will be able
Claims
[Claim 1]
In weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02%, Cr: 0.01~0.5%, the balance Fe and other impurities containing steel plate; and a plating layer made of zinc, aluminum, or an alloy containing them formed on the surface of the base steel sheet, wherein the ratio (CS/CB) of the C content (CB) of the base steel sheet to the C content (CS) of the surface layer part (CS/CB) is 0.6 or less and the ratio ((Mn S+Cr S)/(Mn B+) of the sum of the Mn and Cr contents of the base steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the surface layer (Mn S+Cr S) A plated steel sheet for hot forming with excellent impact properties after hot forming with Cr B)) of 0.8 or more. (Here, the surface layer part means a region from the surface of the base steel sheet to a depth of 15 μm excluding the plating layer.)
[Claim 2]
The plating for hot forming with excellent impact properties after hot forming according to claim 1, wherein the base steel sheet further comprises at least one of 0.0005 to 0.01% and Ti: 0.01 to 0.05% by weight%. grater.
[Claim 3]
According to claim 1, wherein the microstructure of the base steel sheet, in area%, 40 to 100% of ferrite in the surface layer portion, the balance contains 0 to 60% of pearlite, bainite or martensite, and 30 to 90% of ferrite in the center , The balance is a plated steel sheet for hot forming with excellent impact properties after hot forming, characterized in that it contains 10 to 70% of pearlite, bainite or martensite.
[Claim 4]
In weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02 %, Cr: base steel sheet containing 0.01~0.5%, the balance Fe and other impurities; And an alloy plating layer made of an alloy containing zinc or aluminum formed on the surface of the base steel sheet; Containing, the ratio of the C content (CB) of the base steel sheet to the C content (C PS) of the member surface layer portion (C PS / CB) ) is 1.2 or less, and the ratio of the sum of the Mn and Cr contents of the base steel sheet (Mn B+Cr B) to the sum of the Mn and Cr contents of the member surface layer (Mn PS+Cr PS) ((Mn PS+Cr PS) /(Mn B+Cr B)) of 0.8 or more, a hot-formed member with excellent impact properties. (Here, the member surface layer part means an area up to a depth of 25㎛ from the surface of the base steel sheet excluding the alloy plating layer.)
[Claim 5]
[Claim 5] The hot-formed member having excellent impact properties according to claim 4, wherein the base steel sheet further comprises at least one of 0.0005 to 0.01% and Ti: 0.01 to 0.05% by weight.
[Claim 6]
[Claim 5] The hot-formed member with excellent impact properties according to claim 4, wherein the ferrite coverage at the martensite grain boundary of the member surface layer portion is 30% or less.
[Claim 7]
In weight %, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02%, Cr: 0.01 to 0.5%, preparing a slab containing the remainder Fe and other impurities and heating to a temperature of 1050 ~ 1300 ℃; obtaining a hot-rolled steel sheet by hot-rolling the heated slab in a finishing hot-rolling temperature range of 800 to 950°C; winding the hot-rolled steel sheet at 450 to 750° C. after finishing hot rolling; Heating the wound hot-rolled steel sheet to 740 ~ 860 ℃ and annealing for 10 ~ 600 seconds in an atmosphere having a dew point temperature of -10 ~ 30 ℃; and plating the hot-rolled steel sheet by immersing it in a plating bath made of zinc, aluminum, or an alloy containing them after annealing; A method of manufacturing a plated steel sheet for hot forming with excellent impact properties after hot forming, comprising:
[Claim 8]
[8] The method of claim 7, further comprising the step of cold rolling after the hot rolling to obtain a cold rolled steel sheet before winding.
[Claim 9]
The plated steel sheet for hot forming excellent in impact properties after hot forming according to claim 7, wherein the slab further comprises at least one of 0.00005 to 0.01% and Ti: 0.01 to 0.05% by weight, in weight %. manufacturing method.
[Claim 10]
10. A method for producing a hot-formed member with excellent impact properties, in which the plated steel sheet for hot forming manufactured according to any one of claims 7 to 9 is heat-treated at a temperature range of Ac3 to 950° C. for 1 to 15 minutes and then hot press-formed. .