1]The present invention relates to an excellent formability and fatigue properties and a method for manufacturing the hot-rolled steel sheet. And more particularly, to a high manganese steel having superior formability and fatigue properties, which can be used such as the chassis of the vehicle structural member by press forming.
[2]
BACKGROUND
[3]Recently according to the regulation of carbon dioxide to reduce global warming and the weight of the vehicle is required strongly, the second increase in strength of automotive steel sheets are constantly being made simultaneously in order to improve the stability of the car crash.
[4]
To prevent the chassis components generally are applied to the hot rolled steel pickling and ointment, because it produced by a cold press forming destruction by, and at the same time fatigue driving moldability be solid, such as Lower arm, the wheel disc from the car parts in order to be excellent fatigue properties. The chassis part, which serves for supporting a vehicle is a very high partial effect of the fuel efficiency by weight reduction part because at the bottom of the vehicle center of gravity. On the other hand, the fatigue of the chassis part destruction must conservatively apply a safety factor so, and is difficult to check the progress or not, may adversely affect greatly to safety at the time of driving damage passenger during use, and applicable to automobile structural members ideally designed for less fatigue limit in the fatigue cycle mode. Therefore, the fatigue limit of the material is improved if you could lightweight chassis components and can expect excellent fuel economy savings.
[5]
[6]
Generally, in order to produce hot-rolled steel sheet for automotive chassis parts and utilizing the most low-temperature transformation organization. But use it is difficult to apply to the parts of a high strength in the case of utilizing the low-temperature transformation tissue to secure characteristic fatigue, the tensile strength is 600MPa in or above complex to obtain 40% or more elongation is to a hard, cold-press forming a desired shape the free part was designed for the difficult problems.
[7]
[8]
On the other hand, Patent Document 1, the addition of austenite stabilizing elements such as carbon (C) and manganese (Mn) much to maintain the steel structure to the austenite phase, and using a twin crystal that occurs during deformation to secure the strength and formability at the same time this method was proposed. However, as shown in the prior Considering only the high manganese steel, the strength and elongation, did not comment on the improvement of fatigue properties to ensure the nature, of the car safety of automobile members for a long time that this stress concentration.
[9]
Therefore, the situation while the strength and moldability and is excellent as well as the necessary development of a steel sheet for automobiles which can secure a high fatigue strength.
[10]
(Prior art document)
[11]
[Patent Document 1] Korea Patent Laid-Open Publication No. 2007-0023831 No.
[12]
Detailed Description of the Invention
SUMMARY
[13]
One aspect of the invention provides high tensile strength and at the same time having an excellent elongation and fatigue characteristics are excellent in molding can be preferably applied to the automobile chassis structure member properties and fatigue excellent hot-rolled steel sheet properties and a method of manufacturing the same.
[14]
[15]
On the other hand, object of the present invention is not limited to the above-described information. An object of the present invention will be understood from the overall context of the present specification, Those having ordinary skill in the art will not have any difficulty to understanding an additional object of the present invention.
[16]
Problem solving means
[17]
One aspect of the present invention in weight%, C: 0.3 ~ 0.8%, Mn: 13 ~ 25%, V: 0.1 ~ 1.0%, Si: 0.005 ~ 2.0%, Al: 0.01 ~ 2.5%, P: 0.03% or less , S: 0.04% or less, and include (excluding 0%), the rest Fe and unavoidable impurities, 0.03% or less, N
[18]
The thickness direction cross-section observed during formability and fatigue characteristics comprising a non-recrystallized structure with an area fraction 20-70%, the recrystallized structure 30-80% relates to hot-rolled steel sheet excellent.
[19]
[20]
Further, another aspect of the present invention in weight%, C: 0.3 ~ 0.8%, Mn: 13 ~ 25%, V: 0.1 ~ 1.0%, Si: 0.005 ~ 2.0%, Al: 0.01 ~ 2.5%, P: 0.03% or less, S: 0.03% or less, N: 0.04% or less of the steps of preparing a slab containing (excluding 0%), the rest Fe and unavoidable impurities;
[21]
Heating the slab to 1050 ~ 1250 ℃;
[22]
Obtaining a hot-rolled steel sheet by finish rolling in a recrystallization temperature or less of the heated above the recrystallization temperature of slab region having an average density V, having twice the average concentration of V region; And
[23]
Step for winding the hot-rolled steel sheet at 50 ~ 700 ℃; formability and fatigue characteristics including a method of producing hot-rolled steel sheet excellent.
[24]
[25]
In addition, solving means of the above-described problems is, it is not enumerate all features of the present invention. The various features and advantages and effects thereof of the present invention can be understood in more detail with reference to specific embodiments below.
[26]
Effects of the Invention
[27]
According to the present invention, there is an effect capable of providing a high tensile strength and at the same time, having excellent elongation, excellent in hot-rolled steel sheet and a method of producing the durability is excellent fatigue characteristics.
[28]
Brief Description of the Drawings
[29]
1 is a schematic view showing the microstructure of an austenitic high-manganese steel to be implemented in the present invention.
[30]
(A) of Fig. 2 is a graph showing a coagulation when the liquid within the vanadium thickening of the vanadium addition amount, (b) the liquid phase within the (V thickening station) according to the vanadium addition amount of the temperature at which the liquid phase is the remaining 20% ​​in the coagulation process V represents the concentration in the solid phase (minong hwayeok) V concentrations.
[31]
Figure 3 (a) is a graph showing the recrystallization behavior and Mn Steels according to the amount of vanadium, rolling end temperature, (b) is a graph showing the finish rolling temperature range of the recrystallization temperature of the V and thickening station minong hwayeok.
[32]
(A) and (b) of Figure 4 is the comparative example 1, (c) and (d) in Comparative Example 2, (e) and (f) is a scanning electron micrograph showing the microstructure of Inventive Example 1.
[33]
Figure 5 (a) and (b) the microstructure of the invention example 2, (c) is a scanning electron micrograph showing a vanadium component distribution of the invention example 2.
[34]
Figure 6 is a graph showing the fatigue test results of Examples 1 and Comparative Example 1.
[35]
Best Mode for Carrying Out the Invention
[36]
Hereinafter, the preferred embodiments of the present invention. However, embodiments of the present invention can be modified in many different forms and is not limited to the embodiments and the scope of the present invention described below. In addition, embodiments of the present invention is provided in order to explain more fully the present invention to those having ordinary skill in the art.
[37]
[38]
The present inventors that the addition of a large amount of manganese and carbon in the manganese spectrometer hot-rolled steel sheet to obtain a microstructure lecture at room temperature in the austenite, and in the case of maintaining the particle size of the spherical type are dynamic and static recrystallization during hot rolling is completed strength and molding the castle is possible to secure one, which is a problem for low fatigue crack propagation resistance to thermal fatigue performance,
[39]
Production of the components by cold forming to moldability heat resistance to generation and propagation of fatigue cracks when controlling the hot rolling during recrystallization inverse temperature of finish rolling to the microstructure of high dislocation density non-recrystallized structure at least the higher, but recognize that this is a problem that it is impossible, and in-depth research in order to solve them.
[40]
[41]
As a result, also properly control the matter content to perform the stabilization of austenite during lectures component and at the same time, as the microstructure and 1 is excellent in resistance to the recrystallized structure and the fatigue crack propagation of high concrete-type moldability stretching by controlling such a dual non-recrystallized structure of the type that was confirmed that formability is excellent and at the same time greatly improve the fatigue properties it can provide a manganese steel, and completed the present invention.
[42]
[43]
Formability and fatigue properties are excellent hot-rolled steel sheet
[44]
Will now be described in detail with respect to formability and fatigue properties are excellent hot-rolled steel sheet according to an aspect of the invention.
[45]
[46]
Formability and fatigue properties are excellent hot-rolled steel sheet according to an aspect of the present invention in weight%, C: 0.3 ~ 0.8%, Mn: 13 ~ 25%, V: 0.1 ~ 1.0%, Si: 0.005 ~ 2.0%, Al : 0.01 ~ 2.5%, P: 0.03% or less, S: 0.03% or less, N: 0.04% or less (0% excluded), the rest Fe and unavoidable contain impurities and non-recrystallized structure in a thickness direction cross-section observed when an area fraction a it comprises 20-70%, the recrystallized structure 30 to 80%.
[47]
[48]
First, a detailed description of the alloy composition of the present invention. Units or less for each element content refers to a weight% unless otherwise specified.
[49]
[50]
Carbon (C): 0.3 ~ 0.8%
[51]
Carbon is an element contributing to stabilizing austenite phase, the more the content is increased there is a favorable aspect to secure the austenite phase. The carbon serves to increase the tensile strength and the elongation by increasing the stacking fault energy lecture at the same time. This is the content of carbon is less than 0.3% in the surface layer by the hot working during the decarburization of the steel sheet α '(alpha-back) is formed martensite phase, and the problem of weak delayed fracture and fatigue performance, and the tensile strength and elongation it is difficult to ensure the problem. On the other hand, there is a possibility that when the content exceeds 0.8%, the weldability decreases and electrical resistivity increases. Therefore, in the present invention it is preferred to limit the content of carbon 0.3 to 0.8%.
[52]
The lower limit of the carbon content may be more preferably 0.4%, and may be still more preferably 0.5%. In addition, the upper limit of the carbon content may be more preferably 0.75%.
[53]
[54]
Manganese (Mn): 13 ~ 25%
[55]
Exceeds the martensite phase is formed it is difficult to obtain a stable austenite phase, while 25% - Manganese is an element for stabilizing the austenite phase together with the carbon, of when the content is less than 13% strain α '(alpha-back) When a further improvement with respect to the increase in the interests of the present invention the strength does not occur substantially, there is a problem in that manufacturing cost is increased. Therefore, the content of Mn in the present invention is preferably limited to 13 to 25%.
[56]
The lower limit of the manganese content may be more preferably 14%, and may be still more preferably 15%. In addition, the upper limit of the manganese content may be more preferably 23%, and may be still more preferably 21%.
[57]
[58]
Vanadium (V): 0.1 ~ 1.0%
[59]
Vanadium is the most important role in the present invention as an element raising the recrystallization during hot rolling temperature. Vanadium tends to be concentrated in the liquid phase during solidification, the solid phase because the diffusion rate is slow in the rear via the re-heating step for rolling, even, and the steel distribution of the solidification structure maintained largely, Rolling vanadium concentrations a high portion and a low of since the recrystallization behavior in part differently to implement a dual microstructure of the recrystallized structure and the non-recrystallized structure.
[60]
If the content of V is less than 0.1%, the compliance of the rolling conditions for implementing the dual microstructure can result in variations in the tissue harder steel. On the other hand, if the content of V exceeds 1.0%, even though the mounting is generated upon re-heating step coagulation bath to precipitate it is likely to remain in the steel sheet cause cracking during rolling. In addition, it can also be difficult to comply with the rolling conditions for implementing the dual microstructure when the content of V excessive.
[61]
Therefore, the content of the vanadium in the present invention is preferably from 0.1% to 1.0%. In order to make it easier to comply with the rolling conditions for implementing the microstructure of the dual may be a more preferable lower limit of the vanadium content was 0.15%, more preferred lower than the vanadium content may be 0.2%, than the vanadium content A preferred upper limit may be 0.9%, more than the preferred upper limit may be 0.8%.
[62]
[63]
Silicon (Si): 0.005 ~ 2.0%
[64]
Silicon is a component which can be added in order to improve the yield strength and tensile strength of the river due to solid solution strengthening. Silicone may be included in 0.005% or more steel in the usual because it is used as a deoxidizer, When the content of silicon exceeds 2.0%, the silicon oxide formed in a large amount at the time of hot rolling surface and decrease the pickling performance, by increasing the resistivity there is a problem that it is inferior weldability. Therefore, the content of silicon is preferably limited to 0.005 to 2.0%.
[65]
[66]
Aluminum (Al): 0.01 ~ 2.5%
[67]
Although aluminum is an element that is added for deoxidation conventional teaching, the present invention increases ε (entrance run), the stacking fault energy - serves to enhance the ductility and resistance to delayed fracture properties lecture by suppressing the generation of martensite. When the case where the aluminum content is less than 0.01% has a problem that it is rather Steel ductility by rapid work hardening phenomenon deteriorates in delayed fracture characteristics inferior, on the other hand, that the aluminum content exceeds 2.5 wt%, Steel Tensile Strength the decrease is, becomes the main composition is to heat the steel surface is oxidized during hot rolling has intensified the problem of the surface degradation. Therefore, in the present invention, it is preferable to limit the aluminum content of 0.01 to 2.5%.
[68]
[69]
Of (P): 0.03% or less
[70]
The phosphorus is an impurity that is inevitably contained, so that this element sikineunde leading cause decreased workability lecture by the segregation, it is preferable to a low controllable its content. Theoretical phosphorus content of a glass is limited to 0%, the manufacturing process can not but be contained inevitably as normal. Therefore, it is important to manage the upper limit, in the present invention, the upper limit of the content is managed by 0.03%.
[71]
[72]
Sulfur (S): 0.03% or less
[73]
Sulfur as inevitably contained impurities, it is preferred that the tank to form the manganese sulfide (MnS) generates defects such as cracks flange, significantly reduced because the hole expandability of the steel sheet, a controllable low the content. In theory, the sulfur content of a glass is limited to 0%, the manufacturing process can not but be contained inevitably as normal. Therefore, it is important to manage the upper limit, in the present invention, the upper limit of the sulfur content will be managed by 0.03%.
[74]
[75]
(Excluding 0%), 0.04% or less Nitrogen (N)
[76]
Since action and Al in the solidification process in a nitrogen (N) to precipitate a fine austenitic grain nitride by promoting the twin crystal (Twin) occurs, thereby improving the strength and ductility of the steel sheet during forming. However, When the content exceeds 0.04%, the nitrides were precipitated excessively can be a hot workability and elongation decrease. Therefore, it is desirable to limit the content of nitrogen is less than 0.04% in the present invention.
[77]
[78]
The remaining components of the invention are iron (Fe). However, in the conventional manufacturing process it is not intended from the raw material or the environment, because impurities may be unavoidably incorporated, it is impossible to exclude them. These impurities are, because you know if anyone skilled in the normal manufacturing process does not specifically mention that all the contents herein.
[79]
[80]
Further, in weight percent in addition to the above composition, Ti: 0.01 to 0.5%, Nb: 0.05 ~ 0.5%, Mo: it may further comprise at least one selected from 0.0005 ~ 0.005%, 0.01 to 0.5% and B.
[81]
[82]
Tina lithium (Ti): 0.01 ~ 0.5%
[83]
Titanium (Ti) is 0.01 to 0.5% are preferred. Titanium nitride is precipitated by reaction with nitrogen within the steel to improve the formability of the hot-rolling. In addition, the titanium serves to react with carbon to increase the strength by forming a precipitation phase in the steel part. For this, titanium deteriorates the fatigue property of the desirable, but is formed in the precipitate is excessive when it exceeds 0.5% of components contained 0.01% or more. Therefore, the content of the titanium is preferably from 0.01 to 0.5%.
[84]
[85]
Niobium (Nb): 0.05 ~ 0.5%
[86]
Niobium is a component which can be added to increase the yield strength by reinforcing an element forming a carbonitride to react with carbon or nitrogen, grain size refinement and precipitation. In order to obtain these effects, it is preferred that the amount of niobium less than 0.05%. On the other hand, if the content of the niobium exceeds 0.5%, the coarse carbonitride at a high temperature are formed, there is a problem that the hot workability decrease. Accordingly, the content of vanadium is preferably 0.05% to 0.5%.
[87]
[88]
Molybdenum (Mo): 0.01 ~ 0.5% or less
[89]
Molybdenum also to finely maintain the size of the carbonitride-forming elements and the composite addition of the precipitate, such as titanium, vanadium as an element which forms carbide serves to increase the yield strength. When in order to obtain these effects, a preferred content of molybdenum of 0.01% or more, the content of the molybdenum exceeds 0.5%, the effect is saturated and that, resulting in increase in manufacturing cost. Therefore, the content of the denyum molybdate is preferably from 0.01 to 0.5%.
[90]
[91]
Boron (B): 0.0005 ~ 0.005%
[92]
When boron is added a very small amount to enhance the cast grain boundaries to improve the hot-rolling properties. However, if the content of boron is less than 0.0005%, the effect of the do not appear enough, if the content of boron in excess of 0.005%, resulting in an increase in can not be expected further improvement of the performance cost. Accordingly, it is the content of boron is preferably 0.0005 ~ 0.005%.
[93]
[94]
Hot-rolled steel sheet of the present invention comprises a non-recrystallized structure in a thickness direction cross-section observed when an area fraction 20-70%, the recrystallized structure 30 to 80%.
[95]
Fatigue crack growth should be advanced by the movement of the electric potential in the crack tip nearby tissue. Thus crack propagation rate in the non-recrystallized structure with a dislocation density is already formed in a higher level is to considerably slow compared to the rate in the recrystallized structure. If the non-recrystallized structure, if this is less than 20% has to be the effect of suppressing the fatigue crack propagation characteristics are inferior to insufficient blood, greater than 70%, it can not be sufficient recrystallized structure to secure the moldability.
[96]
Recrystallized structure plays a role of improving the formability of the steel sheet, when the recrystallized structure is less than 30% there may be moldability inferior not secure the elongation of the steel sheet, in the case of greater than 80% is sufficient to ensure the non-recrystallized structure You can not.
[97]
[98]
At this time, the non-recrystallized structure is stretched in the rolling direction of the form, and an aspect ratio of 2 or more, and the recrystallized structure may be of concrete. US higher recrystallization temperature V thickening station by the concentration of V is and remains in the steel sheet in a stretched in the rolling direction by rolling type, V minong hwayeok is in the steel sheet in the same rolling temperature to particle size of the spherical-shaped by the dynamic and static recrystallization It is retained.
[99]
[100]
In addition, there may be a layer made of a layer made of a non-recrystallized structure and the recrystallized structure in the thickness direction when observed cross-section is formed alternately.
[101]
According to this form than the non-recrystallized structure formed between the layer made of a recrystallized structure it is possible to easily suppress the crack propagation.
[102]
[103]
In addition, the microstructure of the hot rolled steel sheet according to the invention may comprise less than 95% austenite. It is to ensure the strength and elongation at the same time. And more preferably may be a single-phase austenite. Austenite single phase is both a microstructure other than the carbide, saying that consisting of austenite, may be included in some unavoidable impurities tissue.
[104]
[105]
On the other hand, an austenitic high manganese steel is at least 40% elongation according to the present invention, the fatigue limit (Number of cycles to failure, Nf) may be at least 300MPa. It is possible to secure such excellent elongation and fatigue characteristics, it can be preferably applied to a part for automotive chassis structure member.
[106]
[107]
Method of manufacturing a hot rolled steel sheet having excellent formability and fatigue characteristics
[108]
Will now be described in detail with respect to the production method of the present invention, another aspect of formability and fatigue properties of the hot rolled steel sheet is excellent.
[109]
[110]
Production process of the other one side of the hot-rolled steel sheet excellent yield strength and fatigue properties of the invention may include the steps of preparing a slab which satisfies the above-described alloy composition; Heating the slab to 1050 ~ 1250 ℃; Obtaining a hot-rolled steel sheet by finish rolling in a recrystallization temperature or less of the heated above the recrystallization temperature of slab region having an average density V, having twice the average concentration of V region; It includes; and a step for winding the hot-rolled steel sheet at 50 ~ 700 ℃.
[111]
[112]
Slab preparation step
[113]
Prepare the above-mentioned alloy composition satisfies the slab.
[114]
At this time, the molten steel can be performed so that the difference between V concentration in the slab caused by casting at a cooling rate of less than 50 ℃ / s.
[115]
[116]
Of Figure 2 (a) shows the solidification when the liquid within the vanadium enriched according to the amount of vanadium. Decreasing a fraction of the liquid phase and increased the fraction of the solid phase and progress the concentration of vanadium in the liquid phase, solidified immediately before the end concentration of the vanadium liquid phase can be confirmed to be elevated to 3 times that of the added amount.
[117]
In Fig. 2 (b) represents the liquid phase within the (V thickening station) V concentration and a solid phase within the (minong hwayeok) V concentration at the temperature at which the liquid phase is the remaining 20%. V in solid concentration in the liquid phase point of 20% indicates a very similar V concentration and amount, is more than twice the amount it can be seen that the thickening 20% ​​liquid which is the last to solidified.
[118]
[119]
By solid-phase and liquid partition coefficient difference occurs at the time of solidification and the density distribution of the dual vanadium in the steel, which allows to implement the tissue influences the recrystallization behavior of the hot-rolled and finally dual. When the cooling rate of the molten steel exceeds 50 ℃ / s does not facilitate the diffusion between the solid phase and liquid phase it can not obtain a concentration distribution intended. On the other hand, when the cooling rate is slow, since the phase distribution element phenomenon proceeds smoothly it is not particularly limited to the lower limit of the cooling rate.
[120]
[121]
Slab heating step
[122]
And heating the slabs to 1050 ~ 1250 ℃.
[123]
When the slab heating temperature is less than 1050 ℃ it is difficult to secure the finish rolling temperature during hot rolling, the rolling load increases due to the decrease in temperature is difficult to sufficiently rolled to a predetermined thickness problem. On the other hand, the increase in the crystal grain size if the slab heating temperature exceeds 1250 ℃, and the surface oxidation caused by the reduced strength or is not preferable because there is a tendency that the surface is inferior. In addition, there is a fear of cracks during hot rolling, so that subsequent liquid film is created in the columnar grain boundaries of the performance slab occurs.
[124]
[125]
Hot rolling step
[126]
The heated slab above the recrystallization temperature of the region having an average density V, finishing recrystallization temperature or less of the area having the twice the average concentration of V-rolled to obtain a hot-rolled steel sheet.
[127]
Vanadium concentrated layer through the finish rolling temperature is controlled to obtain a non-recrystallized rolled tissue, minong hwacheung is to obtain a tissue is complete, re-crystallization of the spherical type. Further, the reason for limiting to the recrystallization temperature of the area having the twice the upper limit of the finish rolling temperature average V concentration is 2 times that of the V concentration of the coagulation end liquid 20% point average V concentration in the tissue of the steel sheet 20 because it can ensure non-recrystallized structure% or more.
[128]
[129]
Figure 3 (a) shows the re-crystallization behavior of the addition of V and Mn steel producing rolling end temperature in the laboratory. In this case, the V concentration deviation is not generated slab steel cooling rate of the casting in the slab using a copper mold with a thickness of 40mm, width of 160mm so that they are at 60 ℃ / s was an ingot casting, insert pipes for cooling in a copper mold and then it cooled to room temperature.
[130]
Notice that to the recrystallization temperature rises rapidly by the addition of vanadium and more than 1.0wt% in the region can be confirmed that the increase rate to be lowered. 3 (b) is re-crystallization temperature, having an average V concentration obtaining the recrystallization temperature (dotted line) having twice the average V concentration to obtain the rolling end temperature of the vanadium addition amount for implementing a dual-structure region domain ( It was displayed, as shown in solid line).
[131]
[132]
For example, the recrystallization temperature of the area having the average V levels if the river by the addition of vanadium of 0.25% by weight is 920 ℃, more than 0.5% by weight of vanadium, which accounts for 20% by area (area fraction having double the average V concentration Since the recrystallization temperature of the concentrated station) containing the 960 ℃, if the rolling finishing between 960 ℃ at 920 ℃, dual tissue consisting of non-recrystallized structure of the microstructure and 20% of the 80% recrystallization is completed in an area fraction the can be secured. Thus, the microstructure of intent by setting the amount of vanadium and finish rolling temperature can be easily secured.
[133]
[134]
Winding steps
[135]
And a step for winding the hot-rolled steel sheet at 50 ~ 700 ℃.
[136]
If the coiling temperature is less than 50 ℃ are to require cooling by the cooling water spray, so causing an increase in the unnecessary non-process to reduce the temperature of the steel sheet. On the other hand, if the coiling temperature exceeds 700 ℃, there is a problem in that the dislocation density in the non-recrystallized structure by reducing the yield strength of the steel plate fall by recovery. Therefore, the winding temperature is preferably limited to 50 ~ 700 ℃.
[137]
[138]
At this time, it is possible to add the step of pickling the said take-up the hot-rolled steel sheet. This is to remove the oxidation layer.
[139]
Mode for the Invention
[140]
The present invention the following examples will be described in detail. However, the following examples are to be noted in that not intended to limit the may make the scope of the present invention to be described in more detail by illustrating the invention. The scope of the present invention is because it is determined by what is a reasonable inference from this information and set forth in the claims.
[141]
[142]
After heating to a slab having the composition components shown in Table 1 to 1200 ℃, and to finish rolling in the rolling end temperature shown in Table 2, to prepare a hot-rolled steel sheet by taking volume from 450 ℃.
[143]
[144]
To observe the microstructure of the hot rolled steel sheet, measuring the yield strength, tensile strength, elongation and fatigue limit are shown in Table 2.
[145]
Microstructure was measured by observing the cross section in the thickness direction with a scanning electron microscope, the mechanical property was measured using a universal tensile testing machine.
[146]
Fatigue limit is Comparative Example 1, Comparative Example 2, to stress ratio -1 was measured on the condition to bending fatigue testing machine for the example 1, the fatigue limit is a result set to 10,000,000.
[147]
[148]
[149]
TABLE 1
division Grades C And Mn P S Al you V Ti N
Compare River A 0.65 0.01 17.5 0.01 0.002 1.8 0 0 0 0.0003
River invention B 0.60 0.01 16.5 0.01 0.002 1.3 0 0.25 0 0.0003
River invention C 0.72 0.70 17.0 0.01 0.002 1.2 0.3 0.3 0.06 0.0003

[150]
The units of each element content in Table 1 is% by weight.
[151]
[152]
TABLE 2
division Grades Rolling end temperature Microstructure (area%) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Fatigue limit (MPa)
Temperature (℃) Are happy Recrystallized structure US recrystallized structure
Comparative Example 1 A 941 X 98 2 442 892 72 262
Comparative Example 2 B 881 X 18 82 681 1058 38 405
Examples 1 933 O 54 46 612 1043 51 360
Comparative Example 3 972 X 97 3 502 986 42 -
To honor two C 945 O 52 48 647 1048 48 -
Comparative Example 4 980 X 85 15 492 973 71 -
Comparative Example 5 1019 X 96 4 446 952 75 -

[153]
Whether or not satisfied in the Table 2 show whether a finish rolling performed below the recrystallization temperature of the area having the second V-fold of the average concentration of more than the recrystallization temperature of the region V having an average concentration of each type of steel, each steel grade. If content was represented by O, when not satisfied is indicated by X.
[154]
[155]
Examples 1 and 2, which satisfy the composition and manufacturing conditions of the present invention can be seen that an area fraction of non-recrystallized structure is 20% or more, and satisfies the product with at least 40% elongation.
[156]
On the other hand, Comparative Example 1 is not possible to secure the non-recrystallized structure of 20% or more do not satisfy the composition of the present invention to an area fraction, and thus the performance was inferior fatigue.
[157]
Comparative Example 2 but the composition of the present invention is satisfied, the failure to satisfy the manufacturing conditions, it is not possible to secure the area fraction as a spherical-shaped recrystallized microstructure of the 30% excess, and therefore could not ensure more than 40% elongation.
[158]
[159]
Comparative Example 3 to Comparative Example 5 was not able to secure a non-recrystallized structure of 20% or more, but in compositions of the present invention is satisfied, the failure to satisfy the manufacturing condition area fraction.
[160]
[161]
1 is a schematic view of the microstructure to be implemented in the present invention. Parallel to the surface 30 and the non-recrystallized structure (20) extending in the rolling direction has a structure located within the spherical-type recrystallized structure 10, a fatigue crack 40 is difficult to spread in the non-recrystallized structure the fatigue crack It shows an excellent structure to propagate.
[162]
[163]
4 is a Comparative Example 1, Comparative Example 2 and to a scanning electron microscope photograph showing the microstructure of Example 1, Fig. 4 (a) is a measure of the comparative example 1 of the Kernal Average Misorientation (KAM), 4 ( b) shows the shape of each tissue in Image Quality (IQ) Map of the same area. Figure 4 (c) is a measure of the KAM of Comparative Example 2, Fig. 4 (d) is an IQ Map of the same area. Figure 4 (e) is a measure of the KAM of Examples 1 value, is a 4 (f) are IQ Map of the same area. KAM is represented by a color, the portion represented in blue on the KAM is tissue re-crystallization is completed, the area is a non-recrystallized structure with high dislocation density, represented in noksaeng, yellow, orange, and red. Figure 4 (a), 4 (c), 4 when converting the KAM as in (e) in black and white, blue is the so shown in a darker color and the tissue is represented by the dark area is complete the recrystallization, a relatively light this region can be described as a color dislocation density is high non-recrystallized structure.
[164]
As can be seen in the 4 (a) and (b), the microstructure of Comparative Example 1, as most has maintained a particulate of spherical type recrystallization is complete, to check in (c) and (d) of FIG. 4 , the microstructure of Comparative example 2 is mostly composed of non-recrystallized structure with high dislocation density. As can be found in (e) and (f) of 4, the microstructure of the invention example 1 and the non-recrystallized structure of a stretched form in the rolling direction between the recrystallized structure of the spherical type exists 46% in area fraction.
[165]
[166]
Figure 5 is a scanning electron micrograph showing the microstructure of Inventive 2.
[167]
Figure 5 (a) is a measure of the Kernal Average Misorientation (KAM). KAM is represented by a color, the portion represented in blue on the KAM is tissue re-crystallization is completed, the area is a non-recrystallized structure with high dislocation density, represented in noksaeng, yellow, orange, and red. When converting the KAM as shown in Fig. 5 (a) in black and white, blue is the so shown in a darker color the most, and tissue regions is complete, the recrystallization represented in a darker color, a relatively represented by light-colored region with high dislocation density a non-recrystallized structure.
[168]
Figure 5 (b) shows the shape of each tissue as a Map Image Quality (IQ) of the same area. Recrystallized structure is a concrete type of aspect ratio below 2, the non-recrystallized structure is a stretched form in the rolling direction by the aspect ratio of two. Figure 5 (c) was confirmed that a high vanadium concentration of the non-recrystallized areas compared to spherical type denotes a vanadium tissue distribution of the same area, the recrystallization is completed.
[169]
[170]
Figure 6 is a high-cycle fatigue property measurement results of Comparative Example 1 and Inventive Example 1. US For Recrystallization area fraction is high and a high yield strength of Inventive Example 1, as compared to Comparative Example 1, and to secure excellent fatigue properties in the stress (Stress Amplitude) In the same fatigue, fatigue limit (Number of cycles to failure, Nf) in a steel sheet There is 100Mpa can see the risen about, because even if the micro-cracks were generated, some did not develop into a destructive and excellent resistance to fatigue propagation.
[171]
[172]
Above embodiment has been with reference to describe, understand that without departing from the spirit and scope of the invention defined in the claims below are those skilled in the art can make various modifications and variations to the present invention It will be.

Claims

[Claim 1]By weight%, C: 0.3 ~ 0.8%, Mn: 13 ~ 25%, V: 0.1 ~ 1.0%, Si: 0.005 ~ 2.0%, Al: 0.01 ~ 2.5%, P: 0.03% or less, S: 0.03% or less , N: (excluding 0%) 0.04%, remainder Fe and unavoidable including impurities, and 20 to 70% of non-recrystallized structure in a thickness direction cross-section observed when an area fraction, the molding including a recrystallized structure 30-80% castle and fatigue properties are excellent hot-rolled steel sheet.
[Claim 2]
The method of claim 1, wherein the hot rolled steel sheet in weight%, Ti: additionally comprise at least one selected from 0.0005 ~ 0.005%: 0.01 ~ 0.5%, Nb: 0.05 ~ 0.5%, Mo: 0.01 ~ 0.5% and B formability and fatigue properties are excellent hot-rolled steel sheet.
[Claim 3]
The method of claim 1 wherein the non-recrystallized structure is stretched in the rolling direction of the form, and an aspect ratio of 2 or more, and the recrystallized structure is spherical type formability and fatigue properties are excellent hot-rolled steel sheet.
[Claim 4]
The method of claim 1, wherein the hot rolled steel sheet thickness direction cross-section observed during the molding are formed from the alternating layers consisting of a layer made of a non-recrystallized structure and the recrystallized structure properties and fatigue properties are excellent hot-rolled steel sheet.
[Claim 5]
The method of claim 1, wherein the hot-rolled steel sheet has formability and fatigue properties are excellent hot-rolled steel sheet containing austenite 95% or more.
[Claim 6]
The method of claim 1, wherein the hot-rolled steel sheet is the elongation is more than 40%, the fatigue limit (Number of cycles to failure, Nf) is 300MPa or more formability and fatigue properties are excellent hot-rolled steel sheet.
[Claim 7]
By weight%, C: 0.3 ~ 0.8%, Mn: 13 ~ 25%, V: 0.1 ~ 1.0%, Si: 0.005 ~ 2.0%, Al: 0.01 ~ 2.5%, P: 0.03% or less, S: 0.03% or less , N: 0.04% or less (0% is excluded), the method comprising: preparing a balance of Fe and unavoidable slab which contains impurities; Heating the slab to 1050 ~ 1250 ℃; Obtaining a hot-rolled steel sheet by finish rolling in a recrystallization temperature or less of the heated above the recrystallization temperature of slab region having an average density V, having twice the average concentration of V region; Formability and fatigue properties are excellent method of producing a hot rolled steel sheet including a; and a step for winding the hot-rolled steel sheet at 50 ~ 700 ℃.
[Claim 8]
The method of claim 7, wherein the slab is in weight%, Ti: 0.01 ~ 0.5%, Nb: 0.05 ~ 0.5%, Mo: 0.01 ~ 0.5% and B: 0.0005 ~ 0.005%, including a first adding at least one selected from method of producing a hot-rolled steel sheet excellent formability and fatigue properties.
[Claim 9]
The method of claim 7, wherein the step of preparing the slab is produced in formability and fatigue excellent hot-rolled steel sheet properties, characterized in that performed to generate the molten steel V concentration differences in the slab by casting at a cooling rate of less than 50 ℃ / s Way.
[Claim 10]
The method of claim 7, wherein the step of pickling the rolled hot-rolled steel sheet; method for manufacturing a hot-rolled steel sheet excellent formability and fatigue characteristics, further comprising a.