Title of the invention: Cold-rolled steel sheet excellent in high temperature characteristics and room temperature workability, and its manufacturing method
Technical field
[One]
The present invention relates to a cold-rolled steel sheet that can be used in a can for a battery case for a secondary battery, and a method of manufacturing the same, and more particularly, to a cold-rolled steel sheet having excellent high-temperature characteristics and room temperature workability, and a manufacturing method thereof.
[2]
Background
[3]
Steel round cans used in battery cases for primary batteries are plated with nickel (Ni) or copper (Cu) on the steel plate as the fundamental corrosion resistance is required to withstand the properties of alkalis added to the battery contents. . Recently, a material for a battery battery case has been widely used not only as a primary battery, but also as a battery case material for a secondary battery that is charged/discharged, such as in the field of electric vehicles, including mobile devices such as mobile phones, power tools, and energy storage devices.
[4]
[5]
In this way, as the use environment of the materials for the battery case using the steel plate is diversified, the demand for improving the characteristics of the battery case and improving the lifespan is increasing. In addition, in order to increase the battery capacity by increasing the capacity of the charging body, the development of a technology to secure safety at the same time as well as thinner the thickness of the case body is being actively promoted.
[6]
[7]
As the application use of the battery case using steel is expanded from mobile to automobile, there is an increasing demand for improvement of characteristics to ensure the safety of the case, especially high temperature properties. As a material for battery case cells that used conventional materials such as stainless steel or aluminum in the field of electric vehicles and hybrid vehicles, activities for the application of circular battery case cells using steel in terms of cost reduction and productivity improvement are actively reviewed. , In progress. In the case of automobile batteries, which are emerging as a new application for battery cases, the battery is exposed to a high temperature of several hundred degrees Celsius (℃) instantaneously in the use environment, so it is necessary to secure heat-resistant characteristics applicable to high-temperature environments in terms of driving safety as well. .
[8]
[9]
Heat resistance can be evaluated in a variety of ways. For example, a maker leading the electric vehicle market evaluates the stability of a battery by filling the battery case cell with a battery and then rapidly heating it to a temperature of around 600℃ to investigate the breaking characteristics of the cell. Although the evaluation methods for each manufacturer are different, such as applying the method to do so, the material makers' efforts to satisfy them are concentrated.
[10]
[11]
In addition, if the battery case is deteriorated and deformed due to a local increase in temperature during driving of an electric vehicle, it may affect the driving of the vehicle. Therefore, in order to prevent this, deformation characteristics at high temperatures are also strictly managed. From this perspective, sag resistance can also be considered an important management factor. Sag (Sag) refers to a phenomenon in which a component or material exposed to high temperature is deteriorated due to repetitive heat history, resulting in sagging of a material or component. If a part sag occurs at a high temperature, it becomes difficult to maintain the shape of the molded part.If the thermal stress is concentrated in a specific part, the high-temperature proof strength decreases and the case shape is deformed or severe, the case part breaks, causing a cascading case. In order to secure the stability of the part by securing the shape freezeability of the processed product as it may lead to fracture, the high-temperature strength at 600°C should be satisfied at a temperature of 110 MPa or less and 3 mm or less for the treatment of the material. do. In addition, in order to suppress processing defects, it was necessary to prevent the aging of dynamic deformation due to solid solution elements in the steel during high temperature maintenance.
[12]
[13]
In the past, stainless steel sheets have been mainly used for heat resistance purposes, but stainless steel sheets are not only expensive to manufacture as a large amount of expensive alloying elements such as chromium (Cr) and nickel (Ni) are added, and when applied at high temperatures, the grain boundary As chromium bonds with carbon (C) and precipitates in the form of chromium-carbide at the grain boundaries, intergranular corrosion occurs in the chromium depleted zone, resulting in poor corrosion resistance.
[14]
[15]
On the other hand, battery cases used in electric vehicles have a structure in which thousands of identical cell processed products are stacked in a limited space. In addition, since each circular case requires multi-stage processing such as drawing and stretch processing during molding, processability at room temperature is an important management factor in addition to the high-temperature characteristics. That is, the steel plate used as a material for alkaline manganese batteries or lithium battery battery cases is currently manufactured as a two-piece circular can in which the lower part of the can and the body part are integrally processed by press molding into a cylindrical shape. do. In this case, a process of punching an original material of 0.4mm or less with a circular blank and forming a deep-drawing cylindrical shape and passing these drawing materials through a plurality of ironing molds. It goes through an ironing process to make the can thinner and increase the height of the can. As such, it is possible to make the thickness of the two-piece battery case thinner than that of the original material as the case body is subjected to ironing in the case manufacturing process. It is known that the thickness is reduced by about 40%. As a conventional technique for manufacturing such a cylindrical battery case, the technique disclosed in Patent Document 1 can be exemplified.
[16]
[17]
In addition, Patent Document 2 discloses a technique for improving the sealing property of a battery can by increasing the strength of the material through secondary rolling by utilizing medium and low carbon steel. However, even in this case, it was not possible to solve the fundamental problem of securing sufficient strength at high temperature and 600℃ and occurrence of dynamic deformation aging required by battery cases for electric vehicles, etc. As the process is added, there is a problem that acts as a factor that increases the manufacturing cost.
[18]
[19]
[Prior technical literature]
[20]
(Patent Document 1) Japanese Unexamined Patent Application Publication No. 07-099686
[21]
(Patent Document 2) Japanese Unexamined Patent Application Publication No. Hei 11-189841
[22]
Detailed description of the invention
Technical challenge
[23]
An aspect of the present invention is to provide a cold-rolled steel sheet having excellent high temperature characteristics and room temperature workability, and a method of manufacturing the same.
[24]
Another aspect of the present invention is to provide a method of manufacturing an ultrathin cold-rolled steel sheet having excellent high-temperature characteristics and room temperature workability at low cost.
[25]
Means of solving the task
[26]
One embodiment of the present invention is by weight %, carbon (C): 0.0005 to 0.003%, manganese (Mn): 0.20 to 0.50%, aluminum (Al): 0.01 to 0.10%, phosphorus (P): 0.003 to 0.020% , Nitrogen (N): 0.0005 to 0.004%, Sulfur (S): 0.015% or less, Niobium (Nb): 0.005 to 0.040%, Chromium (Cr): 0.10 to 0.50%, Tungsten (W): 0.02 to 0.07%, The balance contains Fe and other inevitable impurities, and the C, Nb and W satisfy the following relational formula 1, and the microstructure is area%, including 95% or more polygonal ferrite and 5% or less needle-like ferrite, and average size It provides a cold-rolled steel sheet with excellent high temperature properties and room temperature workability including (Nb,W)C-based precipitates of 0.005 to 0.10 μm.
[27]
[Relational Equation 1] 0.00025 ≤ (2×Nb/93)×(W/184)/(C/12) ≤ 0.0015 (however, C, Nb and W are% by weight.)
[28]
[29]
Another embodiment of the present invention is by weight %, carbon (C): 0.0005 to 0.003%, manganese (Mn): 0.20 to 0.50%, aluminum (Al): 0.01 to 0.10%, phosphorus (P): 0.003 to 0.020% , Nitrogen (N): 0.0005 to 0.004%, Sulfur (S): 0.015% or less, Niobium (Nb): 0.005 to 0.040%, Chromium (Cr): 0.10 to 0.50%, Tungsten (W): 0.02 to 0.07%, Heating a steel slab that contains the balance Fe and other inevitable impurities, wherein C, Nb, and W satisfy the following relational formula 1; Obtaining a hot-rolled steel sheet by hot finishing rolling the heated slab at 900 to 950°C; Winding the hot-rolled steel sheet at 560 to 680°C; Cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Cracking the cold-rolled steel sheet at 730 to 850°C; And cooling the cracked cold-rolled steel sheet at a cooling rate of 30 to 80° C./s. It provides a method of manufacturing a cold-rolled steel sheet having excellent high-temperature characteristics and room temperature workability.
[30]
[Relational Equation 1] 0.00025 ≤ (2×Nb/93)×(W/184)/(C/12) ≤ 0.0015 (however, C, Nb and W are% by weight.)
[31]
Effects of the Invention
[32]
According to an aspect of the present invention, it is excellent in room temperature workability in various processing processes such as ironing and dip-drawing, and dynamic deformation aging does not occur at high temperature, as well as excellent high temperature strength and high temperature deformation characteristics. Since it is possible to secure the shape freezing properties of the products used, it is possible to manufacture cold-rolled steel sheets for processing with high heat resistance characteristics with high product safety.
[33]
According to another aspect of the present invention, it is possible to provide a cold-rolled steel sheet having excellent high temperature properties and room temperature workability that can be manufactured at low cost compared to stainless steel sheets.
[34]
Brief description of the drawing
[35]
1 is a graph showing the results of a high-temperature tensile test after maintaining Inventive Example 2 at 600° C. for 15 minutes according to an embodiment of the present invention.
[36]
2 is a graph showing the results of a high-temperature tensile test after maintaining Comparative Example 6 according to an embodiment of the present invention at 600° C. for 15 minutes.
[37]
Best mode for carrying out the invention
[38]
The present inventors satisfies various room temperature processing characteristics such as ironing, bendability, and dip-drawing resistance, aging resistance, and corrosion resistance at low cost, while simultaneously achieving high temperature properties such as high temperature strength of 110 MPa or more and sag behavior of 0.05 mm or less at 600°C. The present invention was completed by repeated studies and experiments to obtain a satisfactory steel sheet.
[39]
[40]
The main concept of the present invention is as follows.
[41]
[42]
1) Excellent room temperature workability and high temperature properties can be secured by appropriately controlling the alloy composition and composition range.
[43]
[44]
2) By appropriately controlling the effective atomic ratio of carbon (C) to niobium (Nb) and tungsten (W), a fine (Nb, W) C-based composite precipitate of 0.005 to 0.10 µm is formed, and its strength and deformation behavior at high temperature Can be controlled and secured aging resistance and processability at room temperature.
[45]
[46]
3) By controlling the microstructure of the steel sheet, excellent room temperature workability, as well as excellent high temperature strength and high temperature properties can be secured.
[47]
[48]
4) By appropriately controlling the heat treatment conditions of the cold rolled steel sheet, it is possible to secure an appropriate microstructure of the steel sheet.
[49]
[50]
5) If necessary, excellent corrosion resistance can be ensured by forming a plated layer and/or an alloyed plated layer on the surface of the steel plate. In particular, for example, it is possible to ensure excellent corrosion resistance to the alkali component of the battery contents.
[51]
[52]
Hereinafter, the present invention will be described in detail.
[53]
First, the alloy composition of the present invention will be described. The content of the alloy composition described below means% by weight.
[54]
[55]
Carbon (C): 0.0005~0.003%
[56]
Carbon (C) is an element added to improve the strength of the steel sheet, and in the present invention, it is mainly consumed by the reaction between niobium and Nb and W for forming tungsten carbide-based precipitates. As the amount of C added increases, the strength increases, but there is a problem in that the workability decreases. Therefore, the upper limit is preferably controlled to 0.003%. However, if C is less than 0.0005%, the precipitation of sufficient (Nb, W)-based composite carbide is not satisfactory, and thus grain growth at high temperatures cannot be suppressed. Since grain growth may cause processing defects such as orange peel during molding, the content of C is preferably controlled to 0.0005 to 0.003%. It is more preferable that the content of C has a range of 0.0010 to 0.0028%.
[57]
[58]
Manganese (Mn): 0.20~0.50%
[59]
Manganese (Mn) is not only a representative austenite stabilizing element, but as a solid solution strengthening element, it increases the strength of steel and prevents hot cracking of the slab by precipitating S in the form of MnS. In order to obtain such an effect, it is necessary to add 0.20% or more. . On the other hand, when a large amount of Mn is added, ductility is lowered and central segregation occurs, as well as corrosion resistance of the steel sheet and it acts as a factor of lowering the plating adhesion during Ni plating, so the upper limit is preferably controlled to 0.50%. Therefore, it is preferable that the content of Mn has a range of 0.20 to 0.50%. It is more preferable that the content of Mn has a range of 0.21 to 0.45%.
[60]
[61]
Aluminum (Al): 0.01~0.10%
[62]
Aluminum (Al) is an element added for deoxidation of molten steel, and it is necessary to contain at least 0.01% in order to improve aging characteristics by combining with solid solution nitrogen in steel. However, if it exceeds 0.10%, the effect of Al is saturated and the amount of inclusions in the steel increases, causing surface defects and deteriorating formability, so the content of Al is preferably in the range of 0.01 to 0.10%. It is more preferable that the content of Al has a range of 0.015 to 0.080%.
[63]
[64]
Phosphorus (P): 0.003~0.020%
[65]
Phosphorus (P) is a relatively inexpensive element that improves the strength and corrosion resistance of steel.In order to secure such properties, it is preferable to contain more than 0.003%, but when the P content exceeds 0.020%, it is segregated at grain boundaries, causing grain boundary embrittlement and workability. In addition to deteriorating the degree, since the adhesion is deteriorated during the plating operation, the content of P is preferably controlled to 0.003 to 0.020%.
[66]
[67]
Nitrogen (N): 0.0005~0.004%
[68]
Nitrogen (N) is an element that exists in a solid solution inside the steel and is effective for material reinforcement. If it contains less than 0.0005%, sufficient rigidity cannot be obtained and the site for formation of precipitates decreases. The content of N is preferably controlled to 0.0005 to 0.0040%, as it causes deformation aging due to hardening of the material and deteriorates formability. It is more preferable that the content of N has a range of 0.0010 to 0.0039%.
[69]
[70]
Sulfur (S): 0.015% or less
[71]
Sulfur (S) combines with Fe in steel to form non-metallic inclusions that act as a corrosion initiation point and causes red shortness, so it is desirable to reduce the content as much as possible, so the content of S is limited to 0.015% or less. . On the other hand, in order to efficiently secure such an effect, it is more preferable to manage the S content to 0.010% or less.
[72]
[73]
Niobium (Nb): 0.005~0.040%
[74]
Niobium (Nb) is an effective element in terms of securing workability of steel sheet and refining high-temperature grains. In the present invention, it is preferentially combined with C dissolved in the steel to form NbC and (Nb, W) C-based carbides to form aging resistance and molding at room temperature. It provides an effect of securing properties and suppressing crystal grain growth at high temperatures by these fine precipitates. In order to obtain such an effect, it is preferable to contain more than 0.005% of Nb, but if the content exceeds 0.040%, not only the hot workability is deteriorated, but also the recrystallization temperature rises rapidly, which significantly decreases the heat treatment operability of the ultrathin material. Therefore, it is preferable to control the content of Nb to 0.005 to 0.040%. It is more preferable that the content of Nb has a range of 0.008 to 0.035%.
[75]
[76]
Chrome (Cr): 0.10~0.50%
[77]
Chromium (Cr) is an element that not only improves the corrosion resistance of steel, but also improves high-temperature properties.In order to improve its applicability as a material for a battery case, it is necessary to add 0.10% or more, but if it exceeds 0.50%, recrystallization is delayed and steel sheet It is preferable to control the amount of Cr addition to 0.10 to 0.50% because not only the mail-order property is deteriorated, but also acts as a factor of increasing the manufacturing cost due to the increase in the use of expensive chromium. It is more preferable that the content of Cr has a range of 0.13 to 0.45%.
[78]
[79]
Tungsten (W): 0.02~0.07%
[80]
Tungsten (W) is an element added for the purpose of improving high-temperature properties and corrosion resistance, and requires 0.02% or more to secure such an effect. As well as acting as a factor of increasing manufacturing cost due to an increase in the use of expensive tungsten, it is desirable to control the W content to 0.02 to 0.07%. It is more preferable that the content of W has a range of 0.023 to 0.065%.
[81]
[82]
On the other hand, it is preferable that C, Nb and W of the alloy components satisfy the above-described composition range and at the same time satisfy the following relational formula 1. The following relational formula 1 relates to the effective atomic ratio of Nb and W to C. In the case of niobium (Nb) and tungsten (W), it is also important to manage independently, but the effective atomic ratio of Nb and W to C, that is, (2×Nb/93)×(W/184)/(C/12) ) To satisfy a certain range, by controlling the precipitation conditions of (Nb, W)-based composite carbides, it is possible to simultaneously secure high-temperature characteristics such as strength, sag and deformation behavior at high temperatures as well as room temperature characteristics of aging resistance and workability. have. When (2×Nb/93)×(W/184)/(C/12) is less than 0.00025, the amount of solid solution in steel increases, deteriorating room temperature aging resistance and workability, and (Nb, W) C-based composite precipitate As the precipitation amount of is small, it is not possible to suppress the high temperature strength and the dynamic strain aging phenomenon at high temperature. On the other hand, if it exceeds 0.0015, it causes cost increase.In addition, the recrystallization temperature of the material rises rapidly, resulting in poor plateability during heat treatment, as well as deterioration of the surface properties, which lowers the workability of the plating process. Nb/93) x (W/184)/(C/12) preferably satisfies the range of 0.00025 to 0.0015. It is more preferable that (2×Nb/93)×(W/184)/(C/12) has a range of 0.00026 to 0.00145.
[83]
[84]
[Relational Equation 1] 0.00025 ≤ (2×Nb/93)×(W/184)/(C/12) ≤ 0.0015 (however, C, Nb and W are% by weight.)
[85]
[86]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in this specification. In addition, it should be noted that the cold-rolled steel sheet of the present invention may additionally include a separate alloying element as needed in addition to the above-described alloy composition, and even if these alloying elements are not described, it is not outside the scope of the present invention.
[87]
[88]
The microstructure of the cold-rolled steel sheet of the present invention is preferably 95% or more polygonal ferrite and 5% or less needle-shaped ferrite in area%. If the fraction of polygonal ferrite is less than 95 area%, it is difficult to secure high temperature characteristics, especially high temperature strength, and if the fraction of acicular ferrite exceeds 5%, room temperature workability deteriorates due to material hardening, and thus, a prototype for a battery case having an appropriate shape. There is a problem with the manufacture of the tube.
[89]
[90]
In addition, it is preferable that the cold-rolled steel sheet of the present invention contains (Nb,W)C-based precipitates having an average size of 0.005 to 0.10 μm. If the average size of the precipitate is less than 0.005 μm, it is preferable in terms of suppressing grain growth, but increases the recrystallization temperature to significantly deteriorate the annealing plateability of the ultrathin material. On the other hand, when the average size of the precipitate exceeds 0.10 μm Since the effect of suppressing abnormal growth of crystal grains during high-temperature operation is reduced, it is not possible to secure target high-temperature properties, so the average size of the precipitates is preferably in the range of 0.005 to 0.10 μm. It is more preferable that the average size of the precipitate has a range of 0.008 to 0.09 μm.
[91]
[92]
On the other hand, it is preferable that the cold-rolled steel sheet of the present invention includes an alloyed plating layer on at least one surface. In this case, the plating layer may include a single or multiple plating layer, a single or multiple alloyed plating layer, or a multiple layer of a plating layer and an alloyed plating layer. There is no particular restriction on the type of the plating layer or the alloyed plating layer as long as it can secure corrosion resistance, and a single or multiple plating layer and/or an alloyed plating layer obtained by thermal diffusion of this plating layer is placed on at least one side, preferably both sides of the steel sheet It is advantageous to form.
[93]
[94]
Meanwhile, in the present invention, the alloyed plating layer may be an Fe-Ni alloyed plating layer, and in this case, the Fe-Ni alloyed plating layer preferably has an alloying ratio of 5 to 25 area%. For example, when the cold-rolled steel sheet of the present invention is applied to a steel sheet for a battery case, an Fe-Ni alloy plating layer may be included on both sides of the steel sheet. Through the formation of the Fe-Ni alloy plating layer, it is possible to secure excellent corrosion resistance to alkali components of the battery contents. Since the alloyed layer fraction of the Fe-Ni alloyed plating layer has a close relationship with the corrosion resistance and surface hardness of the material, it is necessary to secure an appropriate alloyed layer fraction. If the alloying of the Fe-Ni alloyed plating layer is less than 5%, the surface material of the plating material is hardened due to the low alloying degree, which may cause a problem of deteriorating the life of the machining mold.If the alloying rate exceeds 25%, the mold life is improved. Although advantageous in terms of the surface layer corrosion resistance may occur, the alloying rate of the Fe-Ni alloyed plating layer is preferably in the range of 5 to 25%. The alloying rate of the Fe-Ni alloyed plating layer is more preferably in the range of 6 to 23%. Here, the alloying rate refers to the fraction of the compound layer of Fe and Ni composed of FeNi 2 formed at the interface of Fe and Ni among the entire Ni plating layer. In the case of the Fe-Ni alloyed plating layer, it is lighter than Fe but softer than pure Ni. Characteristics.
[95]
[96]
The cold-rolled steel sheet of the present invention provided as described above may have a thickness of 0.1 to 0.5 mm, and it is more preferable to have a thickness of 0.16 to 0.4 mm.
[97]
[98]
In addition, the cold-rolled steel sheet of the present invention satisfies various room temperature processing properties such as ironing, bendability, and deep-drawing properties, aging resistance, and corrosion resistance, while high temperature strength of 110 MPa or more and sag behavior of 0.05 mm or less at 600°C. Satisfy the characteristics at the same time.
[99]
[100]
Hereinafter, a method of manufacturing a cold-rolled steel sheet according to the present invention will be described.
[101]
[102]
First, the steel slab having the above alloy composition is heated. The heating temperature of the steel slab is not particularly limited, but may be preferably performed at 1180 to 1280°C. When the heating temperature of the steel slab is less than 1180°C, there is a problem of causing material non-uniformity due to a decrease in temperature during hot rolling, and when it exceeds 1280°C, the surface scale layer increases, leading to surface defects during subsequent work. There is a drawback that causes it.
[103]
[104]
[105]
Subsequently, the heated slab is hot finish-rolled at 900 to 950°C to obtain a hot-rolled steel sheet. If the finish rolling temperature is less than 900, as the hot rolling is completed in a relatively low temperature region, there is a problem that the final formed crystal grains are mixed, resulting in a decrease in workability and rollability, and if it exceeds 950°C, uniform rolling is performed throughout the thickness. Since grain refinement is insufficient, the impact toughness due to grain coarsening decreases and surface scale increases to cause surface defects, so the hot finish rolling temperature is preferably in the range of 900 to 950°C.
[106]
[107]
Then, the hot-rolled steel sheet is wound at 560 to 680°C. The winding may be performed after being cooled to a desired condition in a run-out table (ROT). On the other hand, when the coiling temperature is less than 560°C, the material of the hot-rolled material is hardened, which increases the rolling load in the cold-rolling step, which is the secondary process, resulting in a decrease in rolling operability. As the precipitation pattern of is changed, there is a problem that acts as a factor of material deviation and deterioration of workability. On the other hand, when the coiling temperature is higher than 680°C, crystal grains of the product grow and not only degrade the high-temperature characteristics during high-temperature treatment, but also act as a factor for lowering the corrosion resistance of the material. Do. The coiling temperature is more preferably in the range of 570 ~ 670 ℃.
[108]
[109]
The wound hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The rolling reduction during the cold rolling is preferably 80 to 95%. In the cold rolling, the rolling reduction ratio is preferably 80% or more in order to secure the strength and thickness of the material, but when it exceeds 95%, it may act as a factor of equipment problems due to the roll kissing of the rolling mill, so the cold rolling It is preferable that the reduction ratio is in the range of 80 to 95%, more preferably 85 to 90%.
[110]
[111]
The cold-rolled steel sheet is subjected to a crack treatment at 730 to 850°C. When the soaking treatment temperature is less than 730°C, the fraction of the recrystallized grains is low, and the strength is high, while the ductility is low, so that the workability for making a battery case cannot be secured. On the other hand, if the cracking process exceeds 850℃, the recrystallization is completed and it is advantageous in terms of securing the transformation driving force of the acicular ferrite structure, but it causes defects such as heat-buckle by heat treatment, so that the annealing plateability of the steel sheet Since it acts as a factor of deterioration, the cracking temperature is preferably in the range of 730 ~ 850 ℃. The soaking temperature is more preferably in the range of 735 ~ 840 ℃.
[112]
[113]
The cracked cold-rolled steel sheet is cooled at a cooling rate of 30 to 80°C/s. If the cooling rate is less than 30°C/s sec, the fraction of acicular ferrite grains obtained after heat treatment is lowered, making it difficult to suppress grain growth at high temperatures, making it difficult to secure target high-temperature characteristics. If it exceeds 80°C/s, it is preferable that the cooling rate has a range of 30 to 80°C/s because it acts as a factor of poor shape and material variation due to lowering of room temperature workability due to increase in strength and uneven cooling in the width direction. . It is more preferable that the cooling rate has a range of 35 to 75°C/s.
[114]
[115]
The cooling end temperature during the cooling is preferably 450 ~ 350 ℃. If the cooling end temperature is less than 350℃, it is difficult to control the shape of the material, and it may act as a factor of increasing the load of the cooling facility, thereby reducing the workability of the material. If it exceeds 450℃, the amount of solid solution in the steel increases. As a result, the cooling end temperature is preferably 450 to 350°C, since a problem of lowering the high temperature aging resistance may occur. The cooling end temperature is more preferably in the range of 445 ~ 340 ℃.
[116]
[117]
After the cooling step is finished, it may further include forming a plating layer on the surface of the cold-rolled steel sheet. In addition, the present invention may further include the step of obtaining a cold-rolled steel sheet having an alloyed plating layer formed thereon by alloying heat treatment of the plated cold-rolled steel sheet. The plating method for forming the plating layer may include, for example, a hot dip plating method and an electroplating method, and among them, an electroplating method may be preferably applied.
[118]
[119]
The alloying heat treatment is preferably performed at 650 to 750°C. When the alloying heat treatment temperature is less than 650° C., there is a problem of deteriorating the workability of the battery case, as the desired alloying fraction cannot be secured. When the alloying heat treatment temperature exceeds 750° C., Although it is advantageous, the alloying heat treatment temperature is preferably in the range of 650 to 750° C. since it may act as a factor deteriorating workability and corrosion resistance due to abnormal growth of the surface crystal grains of the plated material. The alloying heat treatment is preferably performed for a short period of time within 3 seconds. The alloying heat treatment temperature is more preferably in the range of 660 ~ 740 ℃.
[120]
[121]
Meanwhile, the plating layer may be a Ni plating layer, the alloyed plating layer may be an Fe-Ni alloy plating layer, and the Fe-Ni alloy plating layer may be obtained by a method of heat treating the Ni plating layer. At this time, the thickness of the Ni plating layer is preferably 1 ~ 5㎛. If the thickness of the Ni plating layer is less than 1 μm, there may be a disadvantage that it is difficult to secure corrosion resistance, and if it exceeds 5 μm, it is advantageous to secure corrosion resistance, but it will not only increase the cost due to the increase in expensive Ni use. In addition, as the hard Ni layer is formed thick, there may be a disadvantage that acts as a factor of processing defects during processing. It is more preferable that the thickness of the Ni plating layer has a range of 1 to 4 μm.
[122]
Mode for carrying out the invention
[123]
Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.
[124]
[125]
(Example)
[126]
After heating the steel slab having the alloy composition of Table 1 to 1230°C, the steel slab was hot-rolled, wound, cracked, and cooled under the conditions of Table 2 to prepare a cold-rolled steel sheet having a thickness of 0.30 mm. The cold-rolled steel sheet thus manufactured was evaluated for properties at room temperature and high temperature, and the results are shown in Table 3 below. In addition, after Ni-plating the cold-rolled steel sheet, an alloying treatment was also performed by applying the alloying treatment temperature shown in Table 2 below to form an Fe-Ni alloyed plating layer on the surface of the cold-rolled steel sheet.
[127]
[128]
Among the properties listed in Table 3 below, the high-temperature aging property is "occurred" for the case where dynamic strain aging occurs when a high-temperature tensile test is performed after maintaining at 600°C for 15 minutes. "Not occurring", and the goal pursued by the present invention is to achieve "not occurring".
[129]
[130]
In addition, in the sag resistance test, a material having an overall length of 250 mm and a width of 30 mm is heated at 600° C. for 100 hours using a heat treatment facility, and then the deflection of the steel sheet is measured. If the degree of deflection is less than 3 mm, it is good ("○") and 3 mm or more. In the case, it was determined as defective ("x").
[131]
[132]
In addition, if the high-temperature strength obtained through the high-temperature tensile test at 600° C. was less than 110 MPa, it was judged as defective ("x"), and if it was 110 MPa or more, it was judged as good ("o").
[133]
[134]
The workability test is performed at room temperature drawing processing under the condition that the drawing ratio expressed as the ratio of the drawing die diameter to the blank diameter of the material is 1.85, and if the ear generation rate is more than 2.5% or a processing crack occurs, it is considered as defective ("×"). , If the ear incidence rate was less than 2.5% and no processing cracking occurred, it was marked as good ("○"). Here, the ear incidence rate was calculated by the equation {(average mountain height)-(average bone height)}/[0.5×{(average mountain height) + (average bone height)}]×100 of the drawn cup. .
[135]
[136]
On the other hand, the corrosion resistance evaluation of the plated processed product was conducted through the salt spray test (SST, Salt Spray Test), and if red rust occurs within 12 hours through this, it is defective ("×"), and if the time when red rust occurs exceeds 12 hours, It was marked as good ("○").
[137]
[138]
[Table 1]
division Alloy composition (% by weight)
C Mn Al P N S Nb Cr W Relational expression
Invention Lesson 1 0.0018 0.28 0.029 0.008 0.0028 0.005 0.026 0.18 0.034 0.00069
Invention Lesson 2 0.0024 0.41 0.043 0.007 0.0035 0.008 0.019 0.34 0.041 0.00046
Invention Lesson 3 0.0014 0.36 0.051 0.011 0.0018 0.009 0.031 0.27 0.044 0.00137
Invention Lesson 4 0.0021 0.23 0.044 0.009 0.0014 0.011 0.016 0.42 0.028 0.00030
Comparative Steel 1 0.0028 0.37 0.025 0.006 0.0071 0.006 0.003 0.12 0.011 0.00002
Comparative lecture 2 0.0057 0.42 0.019 0.012 0.0019 0.005 0.142 0.03 0.047 0.00164
Comparative lecture 3 0.0022 0.89 0.132 0.009 0.0024 0.013 - 0.34 0.248 0
Comparative lecture 4 0.0019 0.32 0.002 0.047 0.0034 0.029 0.024 0.92 - 0
Comparative lecture 5 0.0164 0.11 0.047 0.008 0.0031 0.007 0.032 - 0.006 0.00002
[Relationship] (2×Nb/93)×(W/184)/(C/12) (however, C, Nb, and W are weight percent.)
[139]
[140]
[Table 2]
division Steel grade No. Hot finish rolling temperature (℃) Winding temperature (℃) Cracking temperature (℃) Cooling rate (℃/s) Cooling end temperature (℃) Alloying heat treatment temperature (℃)
Comparative Example 1 Invention Lesson 1 790 580 740 65 425 780
Invention Example 1 920 580 760 47 430 700
Inventive Example 2 920 580 780 73 410 740
Invention Example 3 Invention Lesson 2 910 620 760 57 385 670
Invention Example 4 910 620 760 46 440 720
Comparative Example 2 910 500 760 52 480 720
Invention Example 5 Invention Lesson 3 930 660 750 64 410 690
Invention Example 6 930 660 800 62 370 740
Comparative Example 3 930 660 700 21 410 730
Invention Example 7 Invention Lesson 4 920 640 780 55 415 730
Comparative Example 4 920 640 880 104 240 720
Comparative Example 5 920 720 800 67 410 600
Comparative Example 6 Comparative Steel 1 910 620 780 43 420 730
Comparative Example 7 Comparative lecture 2 910 620 780 48 425 730
Comparative Example 8 Comparative lecture 3 910 620 780 45 425 740
Comparative Example 9 Comparative lecture 4 910 620 780 51 420 740
Comparative Example 10 Comparative lecture 5 910 620 780 54 390 740
[141]
[142]
[Table 3]
division PF fraction (area%) Average size of precipitate (㎛) Alloying rate (%) Room temperature characteristics Characteristics at 600℃
Processability Ear incidence rate (%) Processing crack Corrosion resistance Aging characteristics Sag resistance High temperature strength
Comparative Example 1 84 0.003 26.4 × - Fracture ○ × × ×
Invention Example 1 97 0.028 11.3 ○ 1.46 Good ○ ○ ○ ○
Inventive Example 2 99 0.032 17.2 ○ 1.52 Good ○ ○ ○ ○
Invention Example 3 96 0.052 13.8 ○ 1.79 Good ○ ○ ○ ○
Invention Example 4 97 0.048 15.7 ○ 2.04 Good ○ ○ ○ ○
Comparative Example 2 91 0.008 13.8 × 4.12 Good ○ ○ ○ ○
Invention Example 5 98 0.047 7.4 ○ 1.57 Good ○ ○ ○ ○
Invention Example 6 98 0.035 19.3 ○ 1.96 Good ○ ○ ○ ○
Comparative Example 3 100 0.004 10.2 × 3.65 Good ○ × × ×
Invention Example 7 97 0.065 14.3 ○ 2.19 Good ○ ○ ○ ○
Comparative Example 4 82 0.003 14.9 × - Fracture ○ × × ×
Comparative Example 5 94 0.014 3.9 × 3.42 Good × × ○ ○
Comparative Example 6 93 0.001 4.5 ○ 2.35 Good × × × ×
Comparative Example 7 92 0.074 24.5 × - Fracture ○ × ○ ○
Comparative Example 8 94 - 3.8 × - Fracture × × × ×
Comparative Example 9 92 - 4.2 × 3.67 Good × × × ×
Comparative Example 10 98 0.002 10.5 × - Fracture ○ × × ×
The PF means polygonal ferrite, and the microstructure other than PF is acicular ferrite.
[143]
[144]
As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 6 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure, the average size of precipitates, the alloying rate, etc. were satisfied to evaluate physical properties at high temperature. Not only did not the dynamic strain aging behavior occur, but also the high temperature strength was more than 110 MPa, and the sag resistance was less than 3 mm, so that good results were obtained. In addition, even in the room temperature processing test, the ear generation rate due to drawing was less than 2.5%, and at the same time, processing cracking did not occur, and thus the room temperature workability was excellent. And, even after 12 hours in the salt spray experiment, red rust did not occur, and thus good results were obtained in terms of corrosion resistance.
[145]
[146]
On the other hand, although it is an inventive steel that satisfies the alloy composition of the present invention, in the case of Comparative Examples 1 to 5 in which some of the manufacturing conditions are outside the scope of the present invention, the microstructure fraction, the average size of precipitates, or the alloying rate of the present invention are satisfied. It was confirmed that it did not have excellent high-temperature characteristics, or that the processing characteristics and aging resistance at room temperature were poor, as the problems such as that the recrystallization was not completed in the heat treatment process appeared.
[147]
[148]
In addition, the manufacturing conditions of the present invention are satisfied, but Comparative Examples 6 to 10, which do not satisfy the alloy composition of the present invention, also do not satisfy the microstructure fraction, the average size of the precipitate, or the alloying rate. It can be seen that a case of poor characteristics appeared.
[149]
[150]
Among them, in the case of Comparative Example 6, the ear generation rate was 2.35%, and the workability was good because no processing fracture occurred during drawing processing, but the corrosion resistance could not be secured due to the low alloying rate. Overall, it was poor, which is thought to be because the effect of forming (Nb, W) C-based composite precipitates decreased as the amount of addition of components such as Nb and W was lower than the range suggested by the present invention. In addition, in the case of Comparative Examples 8 to 10, the amount of solid-solution elements in the steel was large and the room temperature workability was poor, and the high-temperature characteristics were not able to obtain the target, so it was difficult to simultaneously satisfy the room-temperature workability and high-temperature characteristics.
[151]
[152]
1 is a graph showing the results of a high-temperature tensile test after maintaining Inventive Example 2 at 600° C. for 15 minutes. As can be seen from FIG. 1, in the case of Inventive Example 2, it can be seen that a uniform stress change was shown as the strain increased during a high-temperature tensile test, that is, dynamic strain aging did not occur.
[153]
[154]
2 is a graph showing the results of a high-temperature tensile test after maintaining Comparative Example 6 at 600° C. for 15 minutes. As can be seen from FIG. 2, in the case of Comparative Example 2, the stress-strain curve exhibited a serrated stress fluctuation behavior in which the stress increased and decreased as the strain increased. This deformation behavior is due to the dynamic deformation aging phenomenon of the steel, and this behavior may cause the fracture of the structure due to the increase in local deformation during a sudden thermal shock.
Claims
[Claim 1]
In% by weight, carbon (C): 0.0005 to 0.003%, manganese (Mn): 0.20 to 0.50%, aluminum (Al): 0.01 to 0.10%, phosphorus (P): 0.003 to 0.020%, nitrogen (N): 0.0005 ~0.004%, sulfur (S): less than 0.015%, niobium (Nb): 0.005 ~ 0.040%, chromium (Cr): 0.10 ~ 0.50%, tungsten (W): 0.02 ~ 0.07%, balance Fe and other inevitable impurities Including, wherein the C, Nb and W satisfy the following relational formula 1, the microstructure in area%, including 95% or more polygonal ferrite and 5% or less needle-shaped ferrite, and the average size is 0.005 ~ 0.10㎛ ( Cold-rolled steel sheet with excellent high temperature properties and room temperature workability including Nb,W)C-based precipitates. [Relational Equation 1] 0.00025 ≤ (2×Nb/93)×(W/184)/(C/12) ≤ 0.0015 (however, C, Nb and W are% by weight.)
[Claim 2]
The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet includes an alloyed plating layer on at least one surface thereof and has excellent room temperature workability.
[Claim 3]
The cold-rolled steel sheet according to claim 2, wherein the alloyed plating layer is an Fe-Ni alloyed plating layer.
[Claim 4]
The cold-rolled steel sheet according to claim 3, wherein the Fe-Ni alloyed plating layer has an alloying ratio of 5 to 25 area% and excellent high temperature properties and room temperature workability.
[Claim 5]
The cold-rolled steel sheet according to claim 1 to 4, wherein the cold-rolled steel sheet has a thickness of 0.1 to 0.5 mm and has excellent high-temperature properties and room temperature workability.
[Claim 6]
The cold-rolled steel sheet according to claim 1 to 4, wherein the cold-rolled steel sheet is for a battery case for a secondary battery and has excellent high-temperature properties and room temperature workability.
[Claim 7]
In% by weight, carbon (C): 0.0005 to 0.003%, manganese (Mn): 0.20 to 0.50%, aluminum (Al): 0.01 to 0.10%, phosphorus (P): 0.003 to 0.020%, nitrogen (N): 0.0005 ~0.004%, sulfur (S): less than 0.015%, niobium (Nb): 0.005 ~ 0.040%, chromium (Cr): 0.10 ~ 0.50%, tungsten (W): 0.02 ~ 0.07%, balance Fe and other inevitable impurities Including, wherein the C, Nb and W are heating the steel slab satisfying the following relational formula 1; Obtaining a hot-rolled steel sheet by hot finishing rolling the heated slab at 900 to 950°C; Winding the hot-rolled steel sheet at 560 to 680°C; Cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; Cracking the cold-rolled steel sheet at 730 to 850°C; And cooling the cracked cold-rolled steel sheet at a cooling rate of 30 to 80°C/s. [Relational Equation 1] 0.00025 ≤ (2×Nb/93)×(W/184)/(C/12) ≤ 0.0015 (however, C, Nb and W are% by weight.)
[Claim 8]
The method of claim 7, wherein the reheating of the steel slab is performed in a temperature range of 1180 to 1280° C. and excellent high temperature properties and room temperature workability.
[Claim 9]
The method of claim 7, wherein the cold rolling is performed at a reduction ratio of 80 to 95%.
[Claim 10]
The method of claim 7, wherein the cooling end temperature during cooling is 450 to 350°C.
[Claim 11]
The method according to claim 7, after the cooling step is finished, forming a plating layer on the surface of the cold-rolled steel sheet; And an alloying heat treatment of the plated cold-rolled steel sheet to obtain a cold-rolled steel sheet having an alloyed plating layer formed thereon.
[Claim 12]
The method of claim 11, wherein the alloying heat treatment is performed at 650 to 750° C. and has excellent high temperature properties and room temperature workability.
[Claim 13]
The method of claim 11, wherein the plating layer is a Ni plating layer, and the alloyed plating layer is an Fe-Ni alloy plating layer.
[Claim 14]
The method of claim 13, wherein the Ni plating layer has a thickness of 1 to 5 µm and excellent high temperature properties and room temperature workability.