KR100722391B1 - Carbon steel sheet superior in stretch flanging properties and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a high carbon steel sheet having excellent elongation flangeability and a method for manufacturing the same, and more particularly, the steel sheet may be C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr : 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91/14% by weight [N]% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less Negative Fe and other unavoidable impurities, wherein B and N satisfy a condition of B (atomic%) / N (atomic%)> 1.

The steel sheet according to the present invention has excellent elongation flangeability and shows excellent formability. In addition, it is possible to shorten the spheroidizing annealing time using a batch annealing furnace (BAF) that requires a long time in the manufacture of the steel sheet.

Steel sheet, manufacturing method, formability, ferrite, pearlite, bainite, martensite.

Description

High carbon steel sheet with excellent elongation flange and manufacturing method thereof {CARBON STEEL SHEET SUPERIOR IN STRETCH FLANGING PROPERTIES AND MANUFACTURING METHOD THEREOF}

Figure 1 is a schematic diagram showing the control of the phase transformation by the addition of boron, Figure 1a shows the phase transformation of the ordinary steel without the addition of boron, Figure 1b shows the phase transformation of the ordinary steel added with boron.

[Technical Field]

The present invention relates to a high carbon steel sheet having excellent elongation flangeability and a method of manufacturing the same, and more particularly, has excellent elongation flangeability and can exhibit excellent moldability, and requires BAF (Batch Annealing), which requires a long time in manufacturing a steel sheet. The present invention relates to a high carbon steel sheet having excellent elongation flangeability capable of shortening the spheroidizing annealing time using a furnace and a manufacturing method thereof.

[Private Technology]

In general, high carbon steel used for processing is produced by hot rolled steel sheet, and then subjected to spheroidization annealing to make the pearlite structure into spheroidized cementite. In this case, in order to complete spheroidization, annealing is required for a long time, and thus there is a problem in that manufacturing cost increases and productivity decreases.

In addition, the high-carbon steel for processing, which has undergone hot rolling and spheroidizing annealing for the production of hot rolled steel sheet, is additionally subjected to representative processing modes such as drawing molding, elongation molding, extension flange molding, and bending molding. However, in the case of high carbon steel composed of two phases of ferrite and cementite, the shape, size, and distribution of ferrite and cementite have a great influence on the formability. That is, in the case of high carbon steel containing a large amount of saltpeter ferrite structure, carbides are not included inside because of the property of saltpeter ferrite, but the ductility is excellent, but the elongation flange property which is evaluated as the hole expandability is not necessarily excellent. In addition, a high carbon steel having a structure composed of a ferrite containing a cornerstone ferrite and a spheroidized carbide has a larger carbide size than a structure of a high carbon steel composed only of a ferrite containing carbide. Therefore, as the hole is expanded during processing, a strain difference occurs between the cornerstone ferrite and the ferrite containing spheroidized carbide, and the deformation is concentrated at the interface between the coarse carbide and the ferrite to ensure the deformation continuity of the material. This concentration of deformation leads to void generation at the interface, which eventually grows into cracks and degrades the stretch flangeability.

In addition, in the case of spheroidizing annealing steel composed of a structure of ferrite and pearlite, a method of shortening the spheroidization annealing time by performing cold rolling after hot rolling in order to shorten the spheroidizing time is widely used. In addition, the smaller the thickness of the layered layer of carbide in the pearlite structure, that is, the finer the structure, the faster the spheroidization, and thus the time required to complete the spheroidization is relatively short, but a long time BAF (Batch Annealing Furnace) heat treatment is still required.

In addition, in the case of high carbon steel for processing, after the austenitization heat treatment after processing, the process of increasing the hardness through a subsequent cooling process of quenching, if the thickness or size of the sample is thin or small, the hardness is uniform throughout the sample, If the sample is thick or large, the hardness distribution will be heterogeneous. However, it is very important to obtain a homogeneous material distribution after heat treatment because it leads to a deviation in durability when hardness variation exists in precision parts such as automobile parts.

Methods for solving such heterogeneous material distribution problem are Japanese Patent Laid-Open No. 11-269552, Japanese Patent Laid-Open No. 11-269553, US Patent No. 6,589,369, Japanese Patent Laid-Open No. 2003-13144, and Japanese Patent Laid-Open. No. 2003-13145.

First, Japanese Patent Laid-Open No. 11-269552 and Japanese Patent Laid-Open No. 11-269553 use a steel having a carbon content of 0.1 to 0.8% by weight, and the metal structure substantially ferrite and pearlite structure is used to form a cornerstone ferrite area ratio of 0.4ⅹ (1). -[C]% / 0.8) ⅹ100 or more, and a hot rolled steel sheet comprising a pearlite layer spacing of 0.1㎛ or more, after cold rolling 15% or more, using a heating pattern of two steps, and then cooled By applying a heating pattern of a total of three steps to maintain at a specific temperature is disclosed a method for producing a medium, high carbon steel sheet excellent in stretch flange formability. However, this method has a disadvantage in that manufacturing costs are increased by applying cold rolling before spheroidizing annealing.

In addition, U.S. Patent No. 6,589,369 contains 0.01 to 0.3 wt% C, 0.01 to 2 wt% Si, 0.05 to 3 wt% Mn, 0.1 wt% or less P, 0.01 wt% or less S, and 0.005 to 1 wt% Al. , Ferrite as the first phase, martensite or residual austenite as the second phase, and the share of the volume fraction of the second phase divided by the average grain size is 3 to 12, and the average hardness value of the second phase is the average of the ferrite. A method for producing a steel sheet excellent in elongation flangeability having a quotient of 1.5 to 7 divided by the hardness value is disclosed. However, this method cannot provide a high hardness value obtained upon cooling after austenitizing heat treatment, which is an important factor in conventional high carbon steels. In addition, even when the spheroidization heat treatment is applied, a uniform carbide distribution cannot be obtained, resulting in a deterioration of the hole expandability after the final spheroidization.

In Japanese Patent Application Laid-Open No. 2003-13144 and Japanese Patent Laid-Open No. 2003-13145, 0.2 to 0.7% by weight of C steel is hot rolled at a temperature of Ar3 -20 ° C or higher, and then cooled at a cooling rate exceeding 120 ° C / sec. The cooling was stopped at 650 ° C. or higher, followed by winding at 600 ° C. or lower, followed by pickling, followed by annealing between 640 ° C. and Ac1 temperature to have a carbide average particle diameter of 0.1 to 1.2 μm and a volume of ferrite free of carbide. By producing a hot rolled high carbon steel sheet having excellent elongation flangeability by controlling the structure to have a structure with a rate of 10% or less, or applying a cold rolling of 30% or more after pickling a hot rolled steel sheet in the manufacturing method, which is 600 ° C. to Ac1 temperature. By performing annealing therebetween by controlling to have a structure having a carbide average particle diameter of 0.1 to 2.0 占 퐉 and a carbide-free ferrite having a volume ratio of 15% or less. Chapter flangeability is disclosed a method for manufacturing high carbon cold-rolled steel sheet excellent. However, the above method is also impossible in a typical hot rolling mill to perform cooling at a cooling rate exceeding 120 ° C./sec after hot rolling. For this purpose, a specially designed cooling device is required and high cost is required for its installation. This takes a disadvantage.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a high-carbon steel sheet excellent in stretch flangeability and a method of manufacturing the same.

In order to achieve the above object, the present invention is C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91/14% [N]% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, balance Fe and other unavoidable impurities, wherein B and N are B Provided is a high carbon steel sheet having excellent elongation flangeability that satisfies the condition (atomic%) / N (atomic%)> 1. In this case, the steel sheet may further include Ti of 0.5 to 48/14 K [N]% by weight or less.

The present invention also provides C: 0.2 to 0.5 wt%, Mn: 0.1 to 1.2 wt%, Si: 0.4 wt% or less, Cr: 0.5 wt% or less, Al: 0.01 to 0.1 wt%, S: 0.012 wt% or less, Zr : 0.5 to 91/14 '[N] wt% to 0.03 wt%, B: 0.0005 to 0.0080 wt%, N: 0.006 wt% or less, providing a high carbon steel sheet having excellent elongation flangeability including the balance of Fe and other unavoidable impurities do. In this case, the steel sheet may further include 0.5 to 48% by weight [N] of 0.03% by weight of Ti.

The steel sheet includes carbide having an average particle size of 1 μm or less and ferrite having an average grain size of 5 μm or less.

In the present invention, the slab is C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less Zr: 0.5 to 91/14% [N] wt% or less, B: 0.0005 to 0.0080 wt%, N: 0.006 wt% or less, balance Fe and other unavoidable impurities, wherein B and N are B (atomic%) / N (atomic%)> 1, or C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 To 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91/14% by weight [N] to 0.03% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, balance of Fe and other unavoidable impurities Casting a slab, reheating the cast slab, hot rolling the reheated slab to finish at a temperature of at least Ar3, cooling at a cooling rate of at least 20 ° C / sec, and attaining a temperature of at most 530 ° C. Issues taken in manufacturing a hot-rolled steel sheet, and the stretch-flange formability, which comprises the step of spheroidizing annealing the hot-rolled sheet at 600 ℃ temperature Ac1 to excellent and provides a process for the production of carbon steel.

Hereinafter, the present invention will be described in more detail.

The presence of the cornerstone ferrite in the steel sheet causes an uneven distribution of cementite, thereby lowering the flangeability of the steel sheet. In addition, since carbides are hardly present in the cornerstone ferrite, and the characteristics of these microstructures persist to the final spheroidized annealed steel sheet. In addition, the ferrite intensively generates carbides in the localized region, thereby increasing the average diameter of the carbides. As a result, deterioration of hole expandability and coarsening of ferrite grains occur, resulting in deterioration of formability of the steel sheet.

The presence or absence of the cornerstone ferrite depends on the finishing temperature of the hot rolling, the cooling rate after the hot rolling (ROT cooling rate) and the winding temperature, in particular the Ar3 temperature mainly depends on the cooling rate after starting cooling in the austenite zone. Accordingly, there is a concern that the cornerstone ferrite is generated when the finishing temperature is performed at an Ar3 temperature or lower during hot rolling, and the resulting cornerstone ferrite causes an uneven distribution of cementite. In addition, as the ROT cooling rate is slow, ferrite and pearlite transformation is promoted, and as the cooling rate is faster, bainite and martensite transformation occurs. The lower the winding temperature at which the hot roll transformation is finished, the lower the probability of the cornerstone ferrite is present. This is because even in the same composition and cooling conditions, the higher the winding temperature, the more the cornerstone ferrite is generated.

Accordingly, in the present invention, by optimizing the composition of the steel and by adjusting the finishing temperature of the hot rolling, the cooling rate after the hot rolling and the coiling temperature during the production of the steel sheet, the content of the cornerstone ferrite and pearlite in the steel sheet is reduced to high carbon with excellent elongation flange properties Steel sheet can be produced. In addition, it is possible to shorten the spheroidizing annealing time using a batch annealing furnace (BAF) that requires a long time in the manufacture of the steel sheet.

That is, the high carbon steel sheet according to an embodiment of the present invention, C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1 Weight%, S: 0.012% by weight or less, Zr: 0.5 to 91/14% by weight [N]% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, balance Fe and other unavoidable impurities B and N satisfy the condition of B (atomic%) / N (atomic%)> 1. At this time, the steel sheet may further include Ti of 0.5ⅹ48 / 14ⅹ [N]% by weight or less.

High carbon steel sheet according to another embodiment of the present invention is C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight %, S: 0.012% by weight or less, Zr: 0.5 to 91/14% by weight [N] to 0.03% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, balance Fe and other unavoidable impurities . In this case, the steel sheet may further include Ti in an amount of 0.5 to 48/14 kPa [N] to 0.03 wt%.

Hereinafter, the components of the high carbon steel sheet according to the present invention will be described in detail.

Carbon (C) is preferably included in 0.2 to 0.5% by weight. One of the advantages of high carbon steel is the increase in hardness due to quenching, i.e. ensuring good durability. Therefore, when the content of C is less than 0.2% by weight, there is a fear that durability is lowered, which is not preferable. In addition, when the content of C exceeds 0.5% by weight, the increase in absolute amount of cementite, which is the second phase, may cause deterioration of moldability such as elongation flangeability after spheroidization annealing.

Manganese (Mn) serves to prevent the red light brittleness due to the formation of FeS combined with S and Fe inevitably contained in the steel manufacturing process. The Mn is preferably included in 0.1 to 1.2% by weight relative to the total weight of the steel. If the content of Mn is less than 0.1% by weight, red brittleness may occur, and if the content is more than 1.2% by weight, segregation, such as central segregation or micro segregation, will be severe.In this case, since Mn is a cementite element, It is not preferable because the density and size of the carbides may increase and the moldability may be impaired.

Silicon (Si) serves to improve the strength of the ferrite by solid solution strengthening, it is preferably included in 0.4% by weight or less. If the content of Si exceeds 0.4% by weight, scale defects increase, which may lower the surface quality.

Chromium (Cr), as well as boron (B) serves to improve the hardenability of the steel, thereby exhibiting the effect of controlling the phase transformation when added in combination with B. The Cr is preferably contained in 0.5% by weight or less. If the content of Cr exceeds 0.5% by weight, there is a fear of delaying the spheroidization rate, which is not preferable.

Aluminum (Al) serves to prevent the formation of non-metallic inclusions during solidification by removing oxygen present in the steel, and also has a small effect when B is added in a complex, but the grain size is fixed by fixing nitrogen in the steel to AlN. Serves to refine. Accordingly, the Al is preferably contained in 0.01 to 0.1% by weight. If the content of Al is less than 0.01% by weight, the effect of preventing the formation of non-metallic inclusions and miniaturization of grain size is insignificant, and if the content of Al is more than 0.1% by weight, the strength of the steel is lowered and thus the strength increase is lowered. It is not preferable because there is a problem of rising of the steel making unit for.

Sulfur (S) is an impurity that increases the amount of precipitates by being precipitated in the form of MnS, and is preferable in view of improving moldability including as low a content as possible. Accordingly, the S is preferably included in 0.012% by weight or less. If the content of S exceeds 0.012% by weight, it is not preferable because the moldability may be lowered.

Titanium (Ti) reacts with N before B to precipitate N in the form of TiN, thereby suppressing the precipitation of BN. Ti, which plays such a role, may be added at 0.5 wt% / 48 wt% [N] wt% or less. However, in this case, the amount of Ti added is small, so the effect of scavenging N in the matrix due to the addition of Ti is small, and the resultant formation of BN cannot be effectively prevented. Therefore, when Ti is added in an amount of 0.5 x 48/14 kPa [N] or less, it is preferable that B and N satisfy the condition of B (atomic%) / N (atomic%)> 1.

On the other hand, when Ti is added in the range of 0.5ⅹ48 / 14ⅹ [N]% by weight to 0.03% by weight, N and B can be efficiently removed for formation and precipitation of TiN by the added Ti. There is no need to satisfy the condition. In this case, when a large amount of Ti is added in excess of 0.03% by weight, the heat treatment property, which is an advantage of high carbon steel, may be reduced due to the decrease of the C content due to the formation of TiC. .

Zirconium (Zr) also serves to suppress BN precipitation by depositing N. When Zr is added at 0.5 k91 / 14 k [N]% by weight or less, the effect of scavenging N from the matrix is rather small, and thus the formation of BN cannot be effectively prevented. Accordingly, when Zr is added at 0.5 k91 / 14 kPa [N] weight% or less, it is preferable that B and N satisfy the condition of B (atomic%) / N (atomic%)> 1. On the other hand, when Zr is included in the range of 0.5ⅹ91 / 14ⅹ [N]% by weight to 0.03% by weight, it is possible to efficiently remove by ZrN precipitation of N, so B and N do not have to satisfy the above formula. However, when the content of Zr exceeds 0.03% by weight, the steelmaking unit increases, which is not preferable.

Nitrogen (N) forms BN when only B is added without addition of Ti to suppress the effect of B. Accordingly, the lower the content of N is preferable, but because it is an impurity remaining inevitably, it is preferable to be included in 0.006% by weight or less in a range satisfying the condition of B (atomic%) / N (atomic%)> 1. If it exceeds 0.006% by weight, the number of precipitates increases, which cancels the effect of B, which is not preferable. However, since N does not form BN when Ti or Zr is added to the steel in a predetermined amount or more, the condition of B (atomic%) / N (atomic%)> 1 does not have to be applied.

Boron (B) suppresses the transformation of austenite into ferrite or bainite by lowering the grain boundary energy due to segregation at grain boundaries or by lowering the grain boundary area due to segregation of fine precipitates of Fe ₂₃ (C, B) ₆ at grain boundaries. Do it.

Figure 1 is a schematic diagram showing the control of the phase transformation by the addition of boron, Figure 1a shows the phase transformation of the ordinary steel without the addition of boron, Figure 1b shows the phase transformation of the ordinary steel with the addition of boron.

Referring to FIG. 1, as shown in FIG. 1A, ordinary steel without boron may be cooled from a high temperature (eg, finishing finishing temperature) to a different cooling rate (cooling rate 1> 2> 3) at room temperature. In this case, martensite single phase is obtained when cooling at a cooling rate of 1, ferrite + bainite + martensite structure is obtained when cooling at a cooling rate of 2, and ferrite + pearlite + bay when cooling at a cooling rate of 3. The tissue of the knight is obtained. However, as shown in Figure 1b, the ferrite and pearlite bainite transformation curve is a phase transformation diagram of the general steel as the boron-added ordinary steel is cooled from a high temperature to a different cooling rate (cooling rate 1> 2> 3) at room temperature Compared to the position at, it moves to the right along the time axis, resulting in the delay of transformation. The effect of B is to obtain different microstructures for the same cooling rate than for ordinary steel, that is, martensite is obtained at cooling rate of 1, and martensite is also obtained at cooling rate of 2. The microstructure of bainite and martensite is obtained. In other words, there is an advantage that the cooling rate is enhanced without the enhancement of the cooling rate.

Accordingly, by controlling the phase transformation and the cooling rate after hot rolling, B has a hardenability during bainite or bainite and martensite as the main phase after hot rolling, and during heat treatment performed after processing in the manufacture of CRT. Even plays a very important role.

Accordingly, in the case where Ti is not added, and in consideration of the reduction of the B effect due to bonding with N, the B is preferably included in 0.0005 to 0.0080% by weight. If the content of B is less than 0.0005% by weight, the effect of B is insignificant, and if the content is more than 0.0080% by weight, the toughness deterioration and quenchability due to grain boundary precipitation of B precipitates may occur, which is not preferable.

In addition to the above, the high carbon steel sheet of the present invention contains the balance of Fe and other unavoidable impurities.

The high carbon steel sheet according to the present invention having the composition as described above includes carbide having an average particle size of 1 μm or less and ferrite having an average grain size of 5 μm or less. If the size of the carbide and ferrite included in the steel sheet exceeds the size of the carbide and ferrite out of the above range, the toughness may be reduced and the extension flange may deteriorate. Can be represented.

Hereinafter, a method of manufacturing a high carbon steel sheet according to the present invention will be described in detail.

In a conventional manner, a slab having a composition according to the invention is cast and the cast slab is reheated. At this time, the slab reheating is preferably carried out at a temperature of 1200 ℃ or less.

Thereafter, the hot rolled slabs are cooled and wound to prepare hot rolled steel sheets.

It is preferable that the hot rolling finish temperature at the time of hot rolling shall be Ar3 (transformation temperature) or more. When the hot rolling finish temperature is above Ar3, it is possible to prevent the occurrence of two-phase rolling, but below Ar3, two-phase rolling is performed. Accordingly, a large amount of cornerstone ferrite free of carbides is generated, resulting in uniform uniformity over the entire structure of the steel sheet. The distribution of carbides cannot be obtained.

The faster the cooling rate after cooling after hot rolling, the better. This is because the faster the cooling rate, the easier the bainite and martensite structure can be obtained. Accordingly, in the present invention, the cooling is performed at a cooling rate of 20 ° C / sec or more. If the cooling rate is less than 20 ° C / sec, ferrite and pearlite are precipitated in a large amount, which is not preferable because it is difficult to obtain a hot rolled bainite, martensite, or a mixed structure of bainite and martensite as intended in the present invention.

The winding of the hot rolled steel sheet is preferably carried out at a temperature of 530 ° C. or lower. However, when the temperature at the time of winding exceeds 530 ° C., pearlite transformation is caused, and thus the low temperature structure intended by the present invention cannot be obtained. In addition, since the formation of the microstructure in the steel sheet in the present invention is substantially dependent on the performance of the winder, if the winding machine is capable of excellent performance of the actual winder, the fine bainite, martensite, or Since these mixed tissue structures can be obtained easily, it is not necessary to limit the lower limit of the heat treatment temperature at the time of odor.

The hot rolled steel sheet includes a salt-free ferrite without carbides and a pearlite having a layered carbide structure of 10 wt% or less, respectively. When the content of the cornerstone ferrite and pearlite in the hot rolled steel sheet is present in a large amount in excess of 10%, there is a fear that the carbide distribution in the steel sheet is uneven and the elongation flange property is lowered. This is because there is almost no carbide in the cornerstone ferrite, and in the process of the present invention, since the spheroidization annealing is carried out without hot rolling after hot rolling, the microstructure characteristic of the hot rolled steel sheet is maintained up to the final high carbon steel sheet. to be.

Further, the hot rolled steel sheet has bainite, martensite or a mixed structure thereof. Thus, by obtaining bainite or martensite structure from the hot rolled steel sheet, it can be spheroidized even by applying a very short annealing time as compared with changing ordinary pearlite structure into spheroidized cementite.

Accordingly, the hot rolled steel sheet thus produced can be spheroidized and annealed at a temperature of 600 ° C. to Ac1 without the application of ordinary cold rolling to produce a high carbon steel sheet.

When the annealing temperature during the spheroidizing annealing is less than 600 ° C., it is difficult to substantially eliminate the potential inherent in the hot-rolled low-temperature tissue and form the spheroidized carbides. When the annealing exceeds the Ac1 temperature, it causes reverse transformation and then perlite during cooling. It is preferable to perform spheroidization annealing in the temperature range of 600 degreeC-Ac1, since a transformation may be made and moldability may deteriorate by this.

After the final spheroidization annealing, the ferrite diameter is related to the size of the hot rolled microstructure and the carbide. In the presence of the cornerstone ferrite or pearlite in the hot rolled microstructure, the diameter of the ferrite becomes large. In addition, when carbides are present locally, the size of the carbides becomes relatively large, and as a result, the grain size of the final ferrite also increases. In addition, as the final ferrite grains grow larger, the toughness decreases. However, the steel sheet obtained by the above-described spheroidizing annealing treatment can suppress the formation of cornerstone ferrite and pearlite, and evenly distribute fine carbide and ferrite grains, thereby exhibiting excellent toughness and elongation flangeability. As described above, the high carbon steel sheet obtained by spheroidizing annealing without the application of cold rolling after the production of the hot rolled steel sheet includes carbide having an average particle size of 1 µm or less and ferrite having an average grain size of 5 µm or less, and they are uniform It is a high carbon fine spheroidized steel plate distributed well.

The high carbon steel sheet of the present invention produced according to the above production method has excellent elongation flangeability and shows excellent moldability. In addition, the spheroidizing annealing time using a batch annealing furnace (BAF), which requires a long time in manufacturing the steel sheet, can be shortened.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

Production Example  1-9. Manufacture of ingots.

A steel ingot having a composition of A to I shown in Table 1 was prepared by vacuum induction melting to a thickness of 60 mm and a width of 175 mm (unit: wt%).

Steel grade C Mn Si Cr Al S B N Ti Zr Etc A 0.25 0.61 0.19 0.14 0.04 0.0033 0.0055 0.0015 - - Balance Fe and impurities B 0.34 0.73 0.21 0.09 0.03 0.0027 0.0058 0.0010 - - C 0.44 0.71 0.22 0.13 0.036 0.0026 0.0058 0.0014 - - D 0.35 0.65 0.19 0.09 0.039 0.0036 0.0020 0.0014 0.013 0.012 E 0.44 0.69 0.22 0.11 0.042 0.0038 0.0025 0.0018 0.015 0.013 F 0.32 0.76 0.20 0.09 0.030 0.0026 - 0.0014 - - G 0.35 0.65 0.19 0.13 0.040 0.0031 0.0003 0.0049 - - H 0.45 0.72 0.21 0.12 0.046 0.0025 - 0.0011 - - I 0.61 0.43 0.18 0.14 0.050 0.0051 0.0041 0.0020 - -

In Table 1, steel grades A to E are included in the composition range of the present invention, but steel grades F to I are outside the composition of the present invention.

Example  1-11 and Comparative example  1 to 22: Manufacture of steel sheet

After reheating the ingot obtained in Production Examples 1 to 9 at 1200 ° C. for 1 hour, hot rolling was performed to make a hot rolled thickness of 4.3 mm. At this time, the hot rolling finish temperature was at least Ar3 temperature. The hot rolled winding was then simulated by cooling to the target coiling temperature and then holding in a furnace previously heated to 300-600 ° C. for 1 hour and then cooling. A steel sheet was produced by performing a nodular annealing heat treatment.

At this time, the type of steel, the ROT cooling rate, the coiling temperature and the nodular annealing heat treatment temperature were changed as shown in Tables 2 and 3 below.

Thereafter, the prepared steel sheets were evaluated for the presence or absence of cornerstone ferrites according to the cooling rate (ROT cooling rate) and winding temperature after hot rolling, the microstructure characteristics, and the hole expandability of the final spheroidized annealing steel sheet, respectively. The results are shown in Tables 2 and 3 below.

The hole expandability is when a circular hole is punched into the test piece and then expanded using a conical punch, and the hole enlargement amount until the crack generated at the edge of the hole penetrates in the thickness direction from at least one place to the initial hole. It is known as an index for evaluating the elongation flangeability, expressed as a ratio with respect to, and is represented by the following equation (1).

λ = (Dh-Do) / Do ⅹ 100 (%)

(Equation 1, λ is the hole expandability (%), Do is the initial hole diameter (10mm in the present invention), Dh means the hole diameter (mm) after fracture.

In addition, when evaluating the hole expandability, it is necessary to define the clearance when punching the initial hole. The clearance is expressed as a ratio of the thickness of the die and the punch to the thickness of the test piece, is defined by the following equation (2), in the present invention used a clearance of 10%.

C = 0.5 ⅹ (dd-dp) / t ⅹ 100 (%)

(In Equation 2, c is the clearance (%), d _d is the inner diameter of the punching die (mm), d _p is the diameter of the punch punch (dp = 10mm), t means the thickness of the test piece.

In addition, the criterion of salt-free ferrite and no-bit is expressed as a flow path when the amount of the salt-free ferrite is more than 10%, and is expressed as no if it is less than 10%.

division Steel grade ROT cooling rate (℃ / sec) Coiling temperature (℃) Cornerstone ferrite with / without Nominal Temperature (℃) / Hr (hr) Ferrite Average Diameter (㎛) Carbide Average Diameter (㎛) Hole expandability (λ,%) Comparative Example 1 A 10 450 U 680/30 17.8 0.68 67.2 Example 1 30 450 radish 680/30 4.3 0.21 120.4 Comparative Example 2 B 10 500 U 640/40 7.5 0.69 48.0 Comparative Example 3 U 680/30 7.6 0.71 49.7 Comparative Example 4 U 710/10 7.8 0.73 50.4 Example 2 30 300 radish 680/30 2.3 0.50 72.1 Example 3 radish 710/10 2.3 0.51 75.0 Example 4 30 500 radish 640/40 2.4 0.48 57.1 Example 5 radish 680/30 2.5 0.55 59.3 Example 6 radish 710/10 2.5 0.52 67.1 Comparative Example 5 30 600 U 680/30 15.2 1.03 52.5 Comparative Example 6 C 10 500 U 680/30 7.1 1.41 39.3 Example 7 30 500 radish 680/30 2.3 0.88 51.7 Comparative Example 7 30 600 U 680/30 10.0 1.17 40.3 Comparative Example 8 D 10 500 U 680/30 7.6 0.75 46.6 Comparative Example 9 U 710/10 7.8 0.75 48.9 Example 8 30 300 radish 680/30 2.3 0.52 72.3 Example 9 30 500 radish 680/30 2.5 0.53 57.4 Example 10 radish 710/10 2.5 0.52 62.3 Comparative Example 10 30 600 U 680/30 12.5 1.03 47.7

division Steel grade ROT cooling rate (℃ / sec) Coiling temperature (℃) Cornerstone ferrite with / without Nominal Temperature (℃) / Hr (hr) Ferrite Average Diameter (㎛) Carbide Average Diameter (㎛) Hole expandability (λ,%) Comparative Example 11 E 10 450 U 680/30 7.2 1.26 39.9 Example 11 30 450 radish 680/30 2.4 0.76 48.4 Comparative Example 12 F 10 500 U 680/30 - No visualization 40.0 Comparative Example 13 30 500 U 680/30 7.8 0.74 49.6 Comparative Example 14 30 600 U 680/30 - No visualization 44.0 Comparative Example 15 G 30 500 U 680/30 8.1 0.73 48.7 Comparative Example 16 U 710/10 8.3 0.77 49.9 Comparative Example 17 30 600 U 680/30 - No visualization 41.3 Comparative Example 18 U 710/10 - No visualization 42.7 Comparative Example 19 H 10 450 U 680/30 - No visualization 28.3 Comparative Example 20 30 450 U 680/30 7.2 1.37 36.4 Comparative Example 21 I 30 500 radish 680/30 5.5 0.82 34.4 Comparative Example 22 30 600 U 680/30 - No visualization 23.6

As shown in Tables 2 and 3, the cornerstone ferrite content in the hot rolled steel sheet obtained after the hot rolled up during the manufacturing process of the high carbon steel sheet of Examples 1 to 11 was "no" to 10% or less, except for Comparative Example 21. The content of the cornerstone ferrite in the hot rolled steel sheet obtained during the steel sheet manufacturing process of Comparative Examples 1 to 22 all exceeded 10%. From this it can be predicted that the steel sheets of Examples 1 to 11 having the composition of the steel according to the present invention and manufactured according to the production method have excellent moldability.

The present invention is intended to have a uniform and fine distribution of carbides in the final high-carbon steel sheet produced by spheroidizing annealing without the application of cold rolling after hot rolled sheet production. The uniform distribution of fine carbide in such a high carbon steel sheet is possible by suppressing the formation of cornerstone ferrite and pearlite in the hot rolled steel sheet and obtaining bainite or martensite structure. That is, when the cornerstone ferrite is present in the hot rolled steel sheet, the carbide distribution of the final spheroidized annealing plate also becomes inhomogeneous, which means that almost no carbide is present in the cornerstone ferrite, and in the process of the present invention, even the fine high carbon steel sheet is used. Because organizational characteristics persist. In addition, since a bainite or martensite structure is obtained from the hot rolled steel sheet, it can be spheroidized even with a very short annealing time as compared with changing the normal pearlite structure into spheroidized cementite. This can also be confirmed from the annealing time at 710 ° C. of Examples 3, 6 and 10.

In addition, with respect to the ferrite diameter in the high carbon steel sheet obtained after the final spheroidization annealing, the ferrite in the high carbon steel sheet of Examples 1 to 11 had a fine average grain size of 5 μm or less, but the comparison with the cornerstone ferrite is present. The steel sheets of Examples 1 to 22 had an average grain size of more than 5 µm and were very coarse than the examples. However, as the final ferrite grains were smaller, the toughness was improved. Accordingly, it was confirmed from the experimental results that the high carbon steel sheets of Examples 1 to 11 had excellent toughness.

In addition, the average diameter of the carbides was also smaller on average in the steel sheets of Examples 1 to 11 than in Comparative Examples 1 to 22. The average diameter of the carbides is due to the intensive production of carbides in localized regions, which in turn leads to an inhomogeneous distribution of carbides as a whole. This also causes deterioration of the hole expandability and causes coarsening of the ferrite grains. From this, it can be predicted that the steel sheets of Examples 1 to 11 have excellent elongation flange characteristics compared to Comparative Examples 1 to 22, and as a result, have excellent moldability.

The steel sheet according to the present invention has excellent elongation flangeability and shows excellent formability. In addition, the spheroidizing annealing time using a batch annealing furnace (BAF), which requires a long time in manufacturing the steel sheet, can be shortened.

Claims (9)

C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91 / 14 k [N]% by weight or less, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, the balance includes Fe and other unavoidable impurities, wherein B and N are B (atomic%) / N (atomic%) High carbon steel sheet with excellent elongation flangeability that satisfies the condition of> 1. The method of claim 1, The high carbon steel sheet is a high carbon steel sheet excellent in elongation flangeability further comprises Ti of 0.5ⅹ48 / 14ⅹ [N]% by weight or less. C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91 / A high carbon steel sheet having excellent elongation flangeability including 14 Pa [N] wt% to 0.03 wt%, B: 0.0005 to 0.0080 wt%, N: 0.006 wt% or less and the balance of Fe and other unavoidable impurities. The method of claim 3, The high carbon steel sheet is a high carbon steel sheet having excellent elongation flangeability further comprises 0.5 to 48 / 14kW [N]% by weight of 0.03% by weight of Ti. The method according to any one of claims 1 to 4, The high carbon steel sheet is a high carbon steel sheet having excellent elongation flangeability comprising a carbide having an average particle size of less than 1㎛ and ferrite having an average grain size of 5㎛ or less. C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S: 0.012% by weight or less, Zr: 0.5 to 91 / 14 k [N]% by weight or less, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, the balance includes Fe and other unavoidable impurities, wherein B and N are B (atomic%) / N (atomic%) Satisfying the condition of> 1, or C: 0.2 to 0.5% by weight, Mn: 0.1 to 1.2% by weight, Si: 0.4% by weight or less, Cr: 0.5% by weight or less, Al: 0.01 to 0.1% by weight, S : 0.012% by weight or less, Zr: 0.5 to 91/14% by weight of [N] to 0.03% by weight, B: 0.0005 to 0.0080% by weight, N: 0.006% by weight or less, cast slab containing the balance of Fe and other unavoidable impurities Making; Reheating the cast slab at a temperature of 1200 ° C. or less; Hot rolling the reheated slab to finish at a temperature of at least Ar3, cooling at a cooling rate of at least 20 ° C / sec, and winding at a temperature of at least 530 ° C to produce a hot rolled steel sheet; And A method of manufacturing a high carbon steel sheet having excellent stretch flangeability, including the step of spheroidizing annealing the hot rolled steel sheet at a temperature of 600 ° C. to Ac1. The method of claim 6, The slab is a method of manufacturing a high carbon steel sheet excellent in stretch flange further comprising Ti. The method of claim 6, The hot rolled steel sheet is a manufacturing method of high carbon steel sheet having excellent elongation flangeability having bainite, martensite or a mixed structure thereof. The method of claim 6, The hot rolled steel sheet is a method of producing a high carbon steel sheet having excellent elongation flangeability, each containing 10% or less of salt-free ferrite and pearlite having a layered carbide structure.

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