1]The present invention relates to an automobile part and a method of manufacturing for having a high strength and excellent durability.
[2]
BACKGROUND
[3]As a solution which can solve the two problems of the automotive passenger protection and environmental protection at the same time, there is a concern that the vehicle body weight by increasing the rigidity of the vehicle is heightened. For example, stacking ratio riser bars (stabilizer bar), a tubular City ratio A of the vehicle chassis (Turbular Coupled Tortions Beam Axle} and the like stiffness and durability as a component supporting the weight of the vehicle body, and subjected to continuous load fatigue during driving it is necessary to ensure at the same time.
[4]
[5]
Conventionally, hot press molding method, or after in order to increase the strength of the high strength steel sheet for auto parts - has been subject to the heat treatment method, etc. it has been developed. Hot press molding is not necessarily to limited to, but a solution heat for the approximate intensity is heated to 500 ~ 800MPa, for example, a hot-rolled or cold-rolled coil in the range of Ac3 or more austenite temperature zone, and then, by this extraction from the heating to the cooling unit is It refers to a method for simultaneously forming metal mold cooling (quenching) in a press provided. Because the forming and the cooling, the resulting components, in some cases has a high strength, and may have a strength of at least 1500MPa. After-heat-treating method is part having a high strength even by such a method as a method for rapid cooling (quenching) and then after the component shape by molding a hot-rolled or cold-rolled coil at ambient temperature, solution heat for it to heat up to the austenite temperature region above Ac3 the can be prepared.
[6]
[7]
However, in the case to manufacture a steel sheet by such a method, the fine structure is formed in which the martensitic to the main tissue in the steel sheet can have high strength, but the resistance to cyclic loading due to the vulnerability that has a martensite structure, In other words it is not excellent fatigue properties can be a problem. In particular, the surface decarburization and the like and parts by the superficial scratch fatigue characteristics caused during the production process resulting from the heat treatment will be affected, in particular, the higher the strength impact of these factors is increased.
[8]
[9]
As one method of solving this problem, a hot press forming or after - there is a method for improving the fatigue characteristics and toughness were prepared, subjected to tempering heat treatment, such as components of parts by the heat treatment method developed bars.
[10]
[11]
However, if the above-described heat-treated by the process, not only to the strength of the parts decreases, there is a problem also mothada modest improvement in fatigue properties, unlike the intention.
[12]
Detailed Description of the Invention
SUMMARY
[13]
According to one aspect of the invention, it is the improvement in durability, without significantly larger reduction in strength can be provided with a part having a high strength and excellent fatigue characteristics.
[14]
[15]
According to one aspect of the invention there is only one advantageous method of manufacturing parts having high strength and excellent durability can be provided.
[16]
[17]
In addition, according to aspects of the present invention, having a high strength and excellent durability without the B be added to the component and its manufacturing method can be provided for the improvement in strength.
[18]
[19]
An object of the present invention is not limited to the above-described information. Those having ordinary skill in the art will not have any difficulty to understanding an additional object of the present invention from the overall contents of the specification.
[20]
Problem solving means
[21]
Automotive parts according to one aspect of the present invention in terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N: have more than 0.01%, the balance of the composition containing Fe and other unavoidable impurities, with the area ratio, temper de Maarten for: characterized in that with more than 90%, the retained austenite of 4% or less, and the other ferrite and and microstructure containing one or two or all selected from a bainite, in the tempered martensite which comprises the epsilon carbide to precipitate It can be made.
[22]
[23]
Method of producing parts for automobiles according to another one aspect of the present invention in terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N: 0.01% or less, the balance Fe and other preparing a material having a composition that includes unavoidable impurities step; Heating to the temperature at which the transformation of the material in the austenite; To obtain the intermediate product is cooled with shaping the heated material from the mold; And it may include the step of tempering annealing the intermediate product at a temperature of 150 ~ 250 ℃.
[24]
[25]
Method of producing parts for automobiles according to another one aspect of the present invention in terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010 % or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: preparing a having not more than 0.01%, the balance of the composition containing Fe and other unavoidable impurities material: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N the method comprising; The step of cold-forming the material; Heating to the temperature at which the transformation of the cold-forming material in the austenite; To obtain the intermediate product by cooling the heated material; And it may include the step of tempering annealing the intermediate product at a temperature of 150 ~ 250 ℃.
[26]
Effects of the Invention
[27]
As described above, the present invention is a result of appropriately controlling the internal organization of parts and limits the types on the formed precipitate, it is possible to optimize the yield strength and the elongation that affect the durability of components, excellent while maintaining the strength higher It can provide parts with durability.
[28]
Best Mode for Carrying Out the Invention
[29]
Hereinafter, the present invention will be described in detail.
[30]
[31]
The tempering heat treatment is widely used to improve such as brittleness of the substrate. Tempering heat treatment so there is generally carried out at a temperature of 500 ~ 550 ℃, also called fresh (fresh) martensitic structure to separate it from the tempered martensite tissue illustrating martensitic structure of the part since the {By such a heat treatment. Unless otherwise specified in the present invention, martensite structure is present while the carbon was contained in solid solution in the need to understand to mean fresh martensite structure} precipitated cementite is to reduce the brittleness of the martensite structure.
[32]
[33]
However, yireohal case, to the tensile strength of the part significantly reduced may be difficult to achieve a sufficiently high strength. According to the findings of the present inventors, in the case of the 1500MPa class tensile strength steel material it has, in some cases, may result in a reduction in tensile strength of about 900MPa, even, in some cases even about 400MPa, and the case of the 2200MPa class steel has.
[34]
[35]
The present inventors have found that if this depth studies to optimize the composition, tissue, the type of precipitate in one component with respect to the present invention has been reached and confirm that it is possible to simultaneously achieve high strength and excellent fatigue characteristics.
[36]
[37]
(Composition of the components)
[38]
[39]
First, a detailed description of the composition of the components of the present invention. The content of each element in the present invention is to be noted that as means a% by weight unless mandated.
[40]
[41]
Part of the present invention in weight%, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: may have more than 0.01%, the balance of the composition containing Fe and other unavoidable impurities: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N.
[42]
[43]
C: 0.20~0.50%
[44]
Wherein C is to increase the hardenability of the hot-press forming a steel sheet, cooling the mold or after - an important element to increase the strength during heat treatment. To increase the fatigue strength after tempering treatment in order to obtain a strength of at least 1500MPa may be equal to or greater than the C content is 0.20%. However, when it exceeds 0.50%, there is a problem becomes sensitive to the hot-rolled coil is to the width and length direction of the material increase in deviation of the coil difficult to cold-formability obtained from the production step, after quenching heat treatment is too high, the strength of hydrogen delayed fracture. If that not only do the welding of the steel production process or the heat-treated part manufacturing step, since the possibility is the concentration of stress around the weld zone cause a fracture, the upper limit of C content is determined to 0.45%.
[45]
[46]
Si: 0.5% or less
[47]
The Si is an important element in determining the quality and surface quality of the weld. As the amount of Si added is increased the higher the possibility that the oxide remains in the weld zone is flat (the test to evaluate the performance of the weld to the steel pipe after compression Tube Mill) or if fails to meet the verify gwansi performance. On the other hand, if the Si content is increased as Si is concentrated on the surface of the steel sheet is likely to lead to defects on the surface of the scale property. Therefore, it is necessary to strictly control the Si content in the present invention, it is necessary to control this reason, in the present invention is 0.5% or less Si. Be more strictly controlled to less than 0.5% is preferred. The Si is not added in the present invention because it is advantageous that the lower limit of the impurity element content may need to be particularly restricted. However, the content may be set to less than 0.005%, considering the load of the production process.
[48]
[49]
Mn: 1.0 ~ 2.0%
[50]
Mn improves the hardenability of the steel sheet for hot press forming and, cooling the mold after or with C - is an important element in determining the strength after heat treatment. At the same time hot press forming or after - has the effect of surface temperature of the steel sheet during the air-cooling prior to the rapid cooling (quenching) started after the heating at the time of heat treatment is delayed, the ferrite produced according to the degraded. For this reason, the lower limit of the Mn content may be determined to be 1.0%. By contrast, increasing the Mn content increases strength and transformation delay is reduced because the bending of the glass one, the heat-treated parts is restricted to the upper limit to 2.0%.
[51]
[52]
Al: 0.01~0.1%
[53]
The Al may be included in a conventional 0.01% or more for a typical element used as a deoxidizing agent to carry out this role. However, if the Al content over the AlN is precipitated by N and reaction for a continuous casting process, the surface defects and because the corner, as well as result in cracks, to the remaining excessive oxide in ERW steel pipe produced during the weld to limit the content thereof to less than 0.1% may.
[54]
[55]
P: 0.01% or less
[56]
Wherein P is a component which is inevitably contained as a kind of impurities, hot press forming or after - an element that is not of little help in improving the strength of the component after the heat treatment. In the present invention, because when in the heating step for the solution heat for austenite grain boundary segregation to deteriorate the impact energy or fatigue property limits its content to 0.01% or less. In one kinds implementation example of the present invention, the P content may be limited to 0.007% or less. As described above, the more the invention P is not added because it is advantageous impurity element that the lower limit may be set to 0%.
[57]
[58]
S: 0.003% or less
[59]
Wherein S is a mold cooled to may form a stretched emulsion in combination with Mn, In this case as an impurity element in steel (hot press molding) or the after-the content because it is a deterioration of the toughness of the parts obtained after the heat treatment elements 0.003 it is preferable that a percent or less. S content in the one kinds implementation example of the present invention may be limited to not more than 0.002%.
[60]
[61]
Ti: 0.01 ~ 0.1%
[62]
Wherein Ti is shaped by a TiN, TiC or TiMoC precipitate hot press forming or after - the effect of suppressing the austenite grain growth in the heating process of the heat treatment process. Further, if the B present in the steel to form a BN reacted with N is reduced is employed the effective B content there is the hardenability is reduced, and is thereby exhausted to N by the addition of Ti to react with N to form a TiN effective B It has the effect of increasing the amount. As a result, hot press forming or after - can contribute to reliably improve the strength after the heat treatment process. For this reason, Ti may contain more than 0.01%. The addition of Ti up to 0.1% According to one implementation example of the present invention that can sufficiently achieve the effect, because more than one content, etc. In the improved strength greater effect the Ti may be limited to not more than 0.1%.
[63]
[64]
Cr: 0.05~0.5%
[65]
The Cr improves the hardenability of the steel sheet for hot press forming, hot press forming or after - an important element contributing to the increase in strength after heat treatment. Further, in the rapid cooling process affects the critical cooling rate to easily obtain the martensite structure, an element which contributes to lowering the temperature in the hot press forming step A3. The A3 temperature can be lowered if the delay ferrite transformation. In the present invention, the addition of Cr to 0.05% or less for this reason. However, if the Cr content is excessive, according to one implementation example of the invention because the weldability decreases the Cr content may be restricted to 0.5% or less.
[66]
[67]
Mo: 0.05 to 0.3%
[68]
Wherein Mo is an element that improves the hardenability of the steel sheet for hot press forming, contributing to the quenching intensity stabilization. In addition, hot rolling, and the annealing process of the cold rolling, hot press forming or after - and expanding the austenite temperature region in the heat-up phase of the heat treatment toward a lower temperature, the present invention because it is an effective element to relieve the P segregation in the steel the addition of Mo is less than 0.05%. However, if Mo content is excessive, the strength increases by decreasing the addition amount of glass compared to a synergistic effect, so the strength in one kinds uneconomical implementation example of the present invention may limit the upper limit to 0.3%.
[69]
[70]
N: 0.01% or less
[71]
N is a component which is inevitably contained as a kind of impurity. N is causing the precipitation of AlN during the continuous casting process, such as to promote such surface defects or the cast playing the corner cracks. In addition, N is required to be minimized so that the content of these precipitates can act as a source storing the diffusible hydrogen to form TiN precipitates reacted with Ti. For this reason, in the present invention N it may limit its content to 0.01% or less.
[72]
[73]
Other components of the present invention may be further added within a range limited in the following B to increase the hardenability. However, part of the present invention it is possible to control the organization and precipitates, as will be described later to an appropriate range but not the B addition is essential, has the advantage of being able to stably secure the strength than by the addition of B.
[74]
[75]
B: 0.0005~0.005%
[76]
Wherein B is a very favorable element to increase hardenability of the steel sheet for hot press forming (hardenability). In particular, even if a very small amount is added hot press forming mold or after cooling of the - can greatly contribute to the increase of the strength during heat treatment, it may be included in addition to the further components of the composition of the present invention described above. However, the amount added increases hardenability effect compared with the increase of the addition amount is slow, it is possible to promote the corner portions defect of the continuous casting slab. The B content in the present invention In view of this point can be set to 0.0005 ~ 0.005%.
[77]
[78]
In a component according to the present invention may further include one or two or all selected from the group consisting of Cu and Ni content of the steel sheet in the following range.
[79]
[80]
Cu: 0.05 ~ 0.5%
[81]
The Cu is an element contributing to improving the corrosion resistance Steel. In addition, Cu is hot press forming or after-case of carrying out the tempered (tempering) process to increase the toughness after the heat treatment is to exert the aging hardening effect as the supersaturated copper is precipitated as the epsilon carbide. To the invention this reason, Cu is added to at least 0.05% of content is beneficial. If the Cu content is excessive, result in surface defects in the steel sheet production process, and control the addition of contrast uneconomical because the upper limit in terms of the corrosion resistance of 0.5%.
[82]
[83]
Ni: 0.05 ~ 0.5%
[84]
The Ni is advantageous in improving corrosion resistance. In addition, Ni is hot press forming or after - and contributes to not only be effective for improving strength and toughness of the parts after the heat treatment improves hardenability, is effective to reduce the hot shortening sensitivity by the addition of Cu. It is also effective to widen the variability of the effect it has the step of expanding toward the lower temperature the austenite temperature region in the annealing step, and heating step in the hot press forming process at the time of hot rolling and cold rolling. Accordingly, in one kinds implementation example of the present invention, the Ni may be added in 0.05% or transposition. However, if Ni amount is excessive, not only difficult to expect the effect of no more increase can be in the glass does not economical one kinds implementation example of the present invention limits the content of Ni to 0.5% or less.
[85]
[86]
In addition, part of the present invention may further include one or two or all selected from the group consisting of Nb and V.
[87]
[88]
Nb: 0.01~0.07%
[89]
The Nb is an element effective for refining crystal grains river. Nb will contribute not only to reduce the austenite grain growth in the heating step of hot rolling, by the non-recrystallized raise the temperature in the hot rolling step sikineunde refine the final tissue. Thus the finely divided tissue is formed or after a subsequent hot press-is effective to induce grain refinement in the heat treatment step distributed impurities such as P. Therefore, in the implementation example of one kinds of the invention it may be added to not less than 0.01% Nb. However, the addition amount is not less than 0.07%, become susceptible to slab cracking during continuous casting, so undesirable by increasing the material anisotropy of the hot-rolled or cold-rolled steel sheet The upper limit of the Nb content may be determined as 0.07%.
[90]
[91]
V: 0.05 ~ 0.3%
[92]
Wherein V is an element effective for preventing destruction Steel grain refinement and hydrogen delay. That is, not only to reduce the austenite grain growth in the heating step of hot-rolling, which contributes to miniaturization of the final tissue by raising the non-recrystallized inverse temperature in the hot rolling step. Thus the finely divided tissue is effective to induce grain refinement of the hot-forming process after the process for dispersing the impurities, such as P. Also, present in the precipitate within the quenching thermal treatment of tissue is hydrogen steel is the trap (trap) it is possible to suppress the hydrogen delayed fracture. Therefore, it is possible in the implementation example of one kinds of the invention to be added to the V to less than 0.05%. Since that year more than 0.3% When the addition amount is sensitive to the slab cracking during continuous casting, V may be limited to not more than 0.3%.
[93]
[94]
Other than the above-mentioned additive components is substantially Fe. However, the means to exclude impurities contained inevitably during the manufacturing process of the steel sheet is to be noted is not the point. In the art it will not have any difficulty to understand the type and the content range of one of ordinary skill unavoidable impurities.
[95]
[96]
In addition, the inventors of the present invention step solution heat for Reviewing various factors in order to improve the durability austenite in components for automobiles - it is important to suppress the grain boundary segregation of the (hot press forming or after the heating step of heat treatment) I learned that. That is, one in the present invention as described above, P 's only be included in the inevitable steel, the above-P are precipitated in the grain boundaries in the solution heat step for austenite, so that promotes intergranular fracture, it should be suppressed as much as possible to be segregated at the grain boundaries . According to the findings of the inventors of the present invention and the Mo contained in the steel particularly effective in suppressing the grain boundary segregation of P, in order to obtain such an effect to be added to the Mo is to satisfy the relationship Mo / P> 10 advantageously {stage, wherein Mo, P are each means the content (% by weight) of the element}. Thus, according to one implementation example of the present invention it can be defined as the Mo / P> 10.
[97]
[98]
[99]
(Microstructure of the precipitate and the control of parts)
[100]
[101]
In addition, the inventors have found that in order to secure the durability of the parts to be secured to the fatigue properties and the elongation at the same time. That is, the present inventors have found that under the condition that after applied over fatigue results, the cyclic stress yield strength carefully study the stress characteristic which is added in the endurance test produce a heat-treated components for automobiles spanning elongation significantly affect the durability, than the other hand the yield strength in conditions of low stress which is added repeatedly been able to see that the yield strength rule the service life. Therefore, it is necessary to appropriately control the yield strength, and elongation in the present invention, in order it needs to be not only properly controlling the microstructure, specifically control the type of precipitates formed in the part.
[102]
[103]
Microstructure of parts
[104]
Part of the present invention may have a microstructure, which in addition to having the above-described composition, and mainly includes a tempered martensite, containing a small amount of other bainite and ferrite. The following describes the structure of the steel material of the present invention. Ratios of the tissue means an area ratio.
[105]
[106]
Tempered martensite: 90% or more
[107]
In the present invention, as main microstructure may include a tempered martensite than the martensite. The tempered martensite is advantageous to improve the durability by improving the elongation of the steel material. In order to obtain this effect the tempered martensite is (meaning Im including 100%) may be included more than 90% in area ratio.
[108]
[109]
It retained austenite: 5% or less
[110]
Martensite is preferably as much as possible the total amount of martensite transformation to be transformed from the austenite, so it is not desirable to have large amounts of retained austenite. Therefore, in the present invention limits the ratio to less than 4%, in the cases of one kinds of implementation may be limited to less than 2%.
[111]
[112]
The aforementioned outer tissue remaining tissue is the number of days both one or two selected from the group consisting of ferrite and bainite, and may contain other impurities tissue. Among them, the ferrite may be included is less than 5% by area ratio. Hereinafter, a brief description of each tissue.
[113]
[114]
Ferrite: less than 5%
[115]
Ferrite ratio of the components in the present invention is less than 5%. Ferrite structure, so that problems such as reducing the strength of the components the ratio needs to be controlled to less than 5%.
[116]
[117]
Bainite other organizations and other impurities
[118]
In addition to the above-mentioned organizations it may contain bainite or other impurities organization. These impurities organizations can weaken the strength of the part, it is preferable that the content is limited and may be more specifically limit combined with the ferrite, the retained austenite to 10% or less.
[119]
[120]
Part of the present invention to meet the above second condition is a high-strength components can have a high strength greater than a tensile strength of 1500MPa seconds. Part of the present invention need not be so the higher the strength of glass particularly limited to the upper limit of the intensity, according to one implementation cases may have a strength of about 1500 ~ 2200MPa.
[121]
[122]
Deposit conditions
[123]
In the present invention, de-tempered martensite is precipitated as the epsilon carbide precipitates in the main site. If a conventional high-temperature tempering is applied, while the precipitates of cementite series (Fe3C) is mainly precipitated, in the present invention, the tempering de martensite within the site is the epsilon carbide tempering de martensite precipitation in area compared to the area ratio of more than 80% of the precipitate within a site do. When cementite-based precipitates are precipitates of the steel material as to reduce the tensile strength and yield strength as well, thereby further reducing the decline of tensile strength eventually to cause a problem of reducing to the durability of such low strength as well as fatigue characteristics, in the present invention When carbide precipitates are formed as epsilon decrease in the tensile strength as it can increase the yield strength with a minimum, it is effective in securing durability. According to one implementation example of the present invention, the epsilon carbide may occupy more than 70% of the precipitates by the number ratio.
[124]
[125]
(Yield ratio of components)
[126]
[127]
Device according to the present invention may be a yield ratio of 0.7 ~ 0.85. In other words, if the yield ratio is low, therefore a disadvantage to improving the fatigue characteristics and the yield strength is low, yield ratio of not less than 0.72 parts is advantageous. However, in case of the part having the conditions of the present invention becomes higher yield ratio is because the yield strength is increased by not increasing the yield ratio, the symptoms decline is increasing large, the yield ratio of the tensile strength shown, the yield ratio will be controlled to less than 0.82 desirable.
[128]
[129]
(Process for producing component)
[130]
[131]
The following describes a method of manufacturing a component according to the present invention.
[132]
[133]
Part of the present invention after post-molding or hot press molding can be prepared by a heat treatment. Not necessarily limited to this method for producing a component according to the present invention but are proposed when the method according to one implementation of cases as follows.
[134]
[135]
Component manufacturing method of the present invention is heated after the cold forming of the cooling (quenching), or material with a heated material, such as a steel plate or steel pipe having the above composition to the hot post-molding the heated material in the mold first, all the way to carry out cooling (quenching) can be used. In this case, heating conditions and cooling conditions in each method may be limited to the following.
[136]
[137]
Heating temperature: 850 ~ 960 ℃
[138]
In order to ensure the final tissue of the part obtained in more than 90% tempered martensite needs to be heated to a temperature at which the material is completely transformed to austenite. In this case the implementation of one kinds of the heating temperatures of the present invention for the same reason may be more than 850 ℃. However, if the heating temperature is excessive austenite is coarsened by nitro crystal grains coarsened crystal grains in the end part can be excessive segregation, such as P, the heating temperature can be below 960 ℃.
[139]
[140]
Holding time at the heating temperature: 100 to 1000 cho
[141]
It is advantageous to maintain at least 100 seconds in order to allow it to transformation to austenite at the heating temperature. However, if the holding time is too long, not only be the coarsened crystal grains, so, according to the energy costs required for the heating increase one kinds implementation example of the present invention, the holding time can be set to not more than 1000 seconds.
[142]
[143]
Cooling rate of martensite critical cooling rate or higher
[144]
It should be formed with a martensitic structure (fresh martensite structure) by cooling the cooling rate is required to be less than the critical cooling rate is at least the martensite is generated. As the critical cooling speed is affected by the composition of components, one of ordinary skill in a simple test is not to obtain the critical cooling rate of the part of the particular composition particular difficulties. The upper limit of the cooling rate need not be specifically determined because the glass in the faster the cooling rate martensitic structure formation. However, even when continued to increase the cooling rate as well as the effect of increased strength is not large, the cooling rate, considering the practical cooling rates, such as the cooling capacity of the cooling equipment can be set to less than 300 ℃ / sec.
[145]
[146]
Cooling stop temperature: not more than 100 ℃
[147]
In order to be able to be fully transformed into martensite temperature of the cooling stop is advantageously not more than 100 ℃. Although there is no lower limit of the cooling stop temperature need to specifically determine, it may be determined as a temperature or to room temperature of the refrigerant used.
[148]
[149]
The parts obtained by the same cooling process can be in the area ratio have a microstructure containing retained austenite of 90% or more of martensite, ferrite or less, 4%, less than 5%. However, such a microstructure as an intermediate product microstructure than the microstructure of the finished part, the present invention can provide a part that combines the strength and durability by performing an additional tempering process for the intermediate product.
[150]
[151]
Tempering treatment: Maintain at least 150 ~ 250 ℃ 10 bun
[152]
In the present invention limits the tempering heat treatment temperature below 250 ℃. The reason for and deposition of carbon was contained in solid solution in the martensite during the tempering process to form a carbide, when the higher the tempering process has been presented tissue formation, such as a carbide or sorbitan bytes, such as cementite, yield strength and a tensile strength At the same time, not only reduces Among them was significantly reduced tensile strength, because they can not get the parts with high strength and excellent durability. In the present invention, it is possible to form the epsilon carbide-based precipitates by limiting the tempering treatment temperature below 250 ℃, it is possible to obtain a high yield strength and elongation, while minimizing the tensile strength can be reduced to secure a good durability. However, the tempering treatment temperature may be more than 150 ℃ In order to obtain this effect in the same tempering treatment.
[153]
At this time, in order to obtain sufficient effect of tempering treatment wherein the tempering treatment time may be at least 10 minutes. There is no need to specifically set the upper limit of the tempering processing time. However, as well as even if a longer processing time tempering difficult to expect a rise in longer effective, because energy costs are rising the time tempering process can be set to 60 minutes.
[154]
[155]
(Method of producing material)
[156]
[157]
Hereinafter, the hot press forming or after - will be described with respect to one exemplary method for producing the material to be processed into parts by the heat treatment process. Material of the present invention can be prepared by further cold rolling process after the hot rolling or hot-rolling, and each process is as follows. However, the production method of the steel sheet to be described later is to be noted that not necessarily in this limit as a mere example.
[158]
[159]
Hot rolling
[160]
[161]
Heating the steel slab to 1150 ~ 1300 ℃
[162]
A steel slab having the above composition, it is necessary to heat at a temperature range of 1150 ~ 1300 ℃. That is, the heating temperature is advantageously less than 1150 ℃ to dissolve the segregation in the slab uniform composition and to a slab of a suitable workability in rolling. However, if the slab heating temperature is excessive, the energy cost is increased, and the crystal grains are coarse become dissolved in the slab surface occurs or because it may cause to the oxide scale over, the slab heating temperature can be limited to less than 1300 ℃ .
[163]
[164]
Comprising: hot rolling, finishing temperature above Ar3
[165]
If the ferrite is hot-rolled in a defined area is non-uniform deformation resistance rolled tongpan property is deteriorated, and when the stress is concentrated in the ferrite phase plate breakage likely higher since to rolling the hot finishing temperature of Ar3 or more that ferrite phase is formed there is a need. However, if the temperature is excessive it may cause a surface defect such as a sand-type scale in the cases of the implementation of one kinds of the hot finish rolling temperature can be limited to less than 950 ℃.
[166]
[167]
Phase winding in the 600 ~ 700 ℃
[168]
After hot rolling can be wound by cooling in run-out table. At this point, it is preferable to control the coiling temperature so that it does not include the low-temperature tissue, such as martensite in the steel sheet in order to reduce the width direction of the material variation of the hot-rolled steel sheet and to improve rolling tongpan of the subsequent cold-rolled steel sheet. That is, it is wound at a temperature of 600 ~ 700 ℃ is preferred in order to manufacture the steel sheet of the present invention. When the coiling temperature is less than 600 ℃ there is a low temperature, such as tissue martensite formed in the outer edges of the hot-rolled coil has a problem that the strength of the hot rolled steel sheet considerably increases. In particular, when the sub-cooled in a coil width direction of the rolled tongpan property is lowered in the cold rolling step that material when the deviation is increased subsequently, it is difficult to control thickness. Also, included in some implementations example of the present invention as strengthening element of excess C is 0.45%, obtained before heat treatment if the strength of a steel sheet difficult is too high cold forming, the strength of the steel sheet to the coiling temperature above 630 ℃ 800MPa It may be controlled to within. On the other hand, if it exceeds 700 ℃ There are cases that promote the internal oxidation and, if the internal oxide removed by the pickling process, the gap is formed deteriorates the flat performance of the steel pipe from the end part on the surface of the steel sheet to the upper limit value and regulations.
[169]
[170]
The steel sheet as hot press forming or after - but may be used for heat treatment, slitting the steel sheet to an appropriate size to produce an ERW steel pipe after this hot press forming or - may be used in the heat treatment.
[171]
[172]
Thus, in the present invention, hot rolled steel plate or steel pipe, such as a material for hot press forming or after - can be added directly to the heat treatment step. However, in some cases, because in addition to the hot-rolled steel sheet be used in cold rolling, it will be described below in detail even for further processing.
[173]
[174]
In the present invention, first performing cold rolling was removed by pickling the surface of the hot rolled steel sheet manufactured by hot-rolling to obtain the cold-rolled steel sheet (full hard material) the annealing and overaging the cold-rolled steel sheet. At this time, the annealing temperature in the annealing process is the 750-850 range. If the annealing temperature is less than 750 ℃ ​​and recrystallization may be insufficient, which is limited because not only the crystal grains are coarsened, if it exceeds 850 ℃ has a problem in that the annealing heat intensity increases. Overaging temperature in successively show hyodae is such that 400 to 600 to control a range ℃ end organization nitro pearlite or ferrite bay in the base composed of the tissue comprises a part. This is to obtain a tensile strength of 800MPa or less, like the strength of the cold-rolled steel sheet and hot rolled steel sheet.
[175]
[176]
The steel sheet as hot press forming or after - but may be used for heat treatment, slitting the steel sheet to an appropriate size to produce an ERW steel pipe after this hot press forming or - may be used in the heat treatment.
[177]
Mode for the Invention
[178]
It will be described the present invention to the following examples in more 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 that is determined by what is a reasonable inference from this information and set forth in the claims.
[179]
[180]
(Example)
[181]
[182]
Example 1
[183]
Using steel seulrabeueul of the compositions shown in Table 1 were subjected to hot rolling. As long as the one marked * in Table 1, components were expressed in ppm, the remaining component is expressed as weight% (even in the rest of the table).
[184]
[185]
TABLE 1
division C And Mn P* S* Al Ti Cr B* you Nb V With Ni N* Mo / P
Invention steel 1 0.35 0.15 1.3 71 27 0.029 0.029 0.16 20 0.14 - - - - 45 19.7

[186]
[187]
After the steel slab of the aforementioned composition during hot rolling was treated homogenized by heating 180 minutes at 1200 ± range 20 ℃ Thereafter, the consecutive 880 ± 20 ℃ range after a rough rolling to the target finish hot rolling 650 ± 15 by winding at a temperature of ℃ to prepare a hot-rolled steel sheet having a thickness of 3.0㎜. After pickling the hot-rolled steel sheet 930 ± 10 ℃ 6 minutes at a temperature of (360 seconds) is heated, and then, martensite a cooling water tank immersion held at the 20 ~ 30 ℃ critical cooling rate higher 60 ~ 80 ℃ / sec cooling rate than to prepare a component to tempering heat treatment for 30 minutes at a temperature described in then rapidly cooled to below 30 ℃, Table 2. Then in order to produce the part, but includes a step of forming at or before heating at high temperature the cold, the molding process does not have a particular impact on the Physical Properties of the parts, not the usual molding process, the hot press forming or after - the heat treatment it is common to test the physical properties of the components obtained by simulation. The tensile test and low-cycle fatigue life was evaluated with respect to the resulting parts. JIS5 tensile test was performed using a specimen, low cycle test is made R = -1, △ / 2 = ± 0.5% strain rate control of the ballast portion length 15 ± 0.01mm, a width of 12.5 ± 0.01mm flat portion specimen It was carried out in the conditions. The above-mentioned test results are shown in Table 2 below. In Table 2, YS is the yield strength, TS is the tensile strength, EL is the elongation (elongation), U-EL is the uniform elongation, T-El refers to elongation before. In Table 2, for example, 1-2 means that the Cordillera second embodiment of the inventive steels 1. In addition, PO represents the breed in Table 2 means that were the target of the steel sheet subjected to hot rolling and pickling.
[188]
[189]
TABLE 2
No kind Thickness (mm) The coiling temperature (℃) The physical properties of the material Tempering temperature (℃) The physical properties of the components YS × U-El ​​(MPa%) Low Cycle Fatigue Life (cycle) Remarks
YS(Mpa) TS(Mpa) T-El (%) YS(Mpa) TS(Mpa) The U (%) T-El (%)
1-1 PO 3.0 650 428 620 22 - 1250 1960 4.7 9 5875 4006 Comparative Example
1-2 PO 3.0 650 428 620 22 160 1360 1850 5.2 9.6 7022 6237 To honor
1-3 PO 3.0 650 428 620 22 220 1460 1800 5.2 10.1 7592 6445 To honor
1-4 PO 3.0 650 428 620 22 250 1470 1730 4.1 10.1 6027 5780 To honor
1-5 PO 3.0 650 428 620 22 330 1370 1500 3.3 9 4521 3300 Comparative Example
1-6 PO 3.0 650 428 620 22 500 1040 1100 4.7 9 4888 3580 Comparative Example
1-7 PO 3.0 650 428 620 22 550 960 1050 6.2 12 5952 4950 Comparative Example

[190]
[191]
If after hardening as can be seen in Table 2, tempering temperature increases the tensile strength of continuous decline in, and yield strength increase while compared to immediately after quenching tempering temperature is decreased continuously as in the tensile strength after showing the maximum value in the vicinity of 250 ℃. The uniform elongation was increased gradually again after showing a minimum at 330 ℃ while rapidly decreased after showing the maximum value in the vicinity of 220 ℃. For these tensile properties change In the yield strength × compared to the uniform elongation balance to exhibit patterns which rapidly decreased to 250 ℃ as the boundary, the result is to substantially match the low cycle fatigue life changes. In contrast, 150 ℃ tempering and compare the fatigue life of the quenching conditions we can see that the more favorable case of the smaller-state compared to 150 ℃ tempering heat treatment.

Claims

[Claim 1]In terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N: 0.01% or less, the balance Fe and other has composition containing the unavoidable impurities, in an area ratio, tempered martensite: 90% or more, the retained austenite 4% or less, and the other ferrite, and has a microstructure comprising one or two or all selected from a bainite, automobile parts for which the epsilon carbide comprising a precipitate in the tempered martensite.
[Claim 2]
The method of claim 1, wherein the composition is in a weight ratio B: Automobile parts further comprises a 0.0005 ~ 0.005%.
[Claim 3]
The method of claim 1, wherein the composition is in a weight ratio Cu: automobile parts, including both further 0.05% to 0.5% one selected from two or 0.05 to 0.5% and Ni.
[Claim 4]
Automotive parts including one or two or all selected from the group consisting of 0.05 ~ 0.3% more: according to claim 1, wherein the composition of Nb in terms of weight: 0.01 to 0.07% and V.
[Claim 5]
The method of claim 1, wherein the automobile parts for the compositions satisfy the relationship of Mo / P> 10. However, where the Mo and P are each means a content (% by weight) of the element.
[Claim 6]
Any one of claims 1 to A method according to any one of claim 5, wherein the epsilon carbide is the tempered martensite de entire precipitates of 70% or more compared to the number of components for automobiles ratio within the site.
[Claim 7]
Any one of claims 1 to A method according to any one of claim 5, wherein the automobile parts for more than the strength 1500MPa.
[Claim 8]
Any one of claims 1 to A method according to any one of claim 4, wherein the components for automobiles yield ratio is 0.7 ~ 0.85.
[Claim 9]
In terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: preparing a material having more than 0.01%, the balance of the composition containing Fe and other unavoidable impurities;: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N Heating to the temperature at which the transformation of the material in the austenite; To obtain the intermediate product is cooled with shaping the heated material from the mold; And a method of manufacturing an automotive component comprising the step of tempering annealing the intermediate product at a temperature of 150 ~ 250 ℃.
[Claim 10]
In terms of weight, C: 0.20 ~ 0.50%, Si: 0.5% or less, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, P: 0.010% or less, S: 0.003% or less, Ti: 0.01 ~ 0.1%, Cr: preparing a material having more than 0.01%, the balance of the composition containing Fe and other unavoidable impurities;: 0.05 ~ 0.5%, Mo: 0.05 ~ 0.3%, N The step of cold-forming the material; Heating to the temperature at which the transformation of the cold-forming material in the austenite; To obtain the intermediate product by cooling the heated material; And a method of manufacturing an automotive component comprising the step of tempering annealing the intermediate product at a temperature of 150 ~ 250 ℃.
[Claim 11]
Claim 9 or claim 10, wherein the composition B in a weight ratio of: The method of automobile parts for which further comprises a 0.0005 ~ 0.005%.
[Claim 12]
In the ninth or claim 10 wherein the composition is in a weight ratio Cu: 0.05% to 0.5% and Ni: 0.05 to method for producing parts for automobiles, further comprising a one or two or all selected from the group consisting of 0.5%.
[Claim 13]
Or in claim 9 wherein said composition is in weight ratio of Nb to claim 10, wherein 0.01 to 0.07% and V: The method of auto parts to 0.05 to include one or two or all selected from the group consisting of 0.3% more.
[Claim 14]
In the ninth or claim 10, wherein the method of manufacturing an automotive component wherein the composition satisfies the relationship of Mo / P> 10. However, where the Mo and P are each means a content (% by weight) of the element.
[Claim 15]
Claim 9 or claim 10, wherein in the intermediate embraces an area ratio of 90% or more of martensite, having a microstructure comprising one or two or all selected from the group consisting of residual austenite, and the other ferrite and bainite of less than 4% the method of spare parts for cars.
[Claim 16]
In the ninth or claim 10, wherein the temperature at which the transformation to the austenite is 850 ~ 960 ℃ The method of producing a part for automobile.
[Claim 17]
Claim 9 or claim 10, wherein the method of producing a 100 to automobile parts for 1000 seconds holding at the temperature at which transformation to the austenite.
[Claim 18]
In the ninth or claim 10 wherein the cooling rate in the step of obtaining the intermediate product to the cooling method for producing parts for automobiles or more martensite critical cooling rate.
[Claim 19]
19. The method of claim 18, the manufacture of the cooling to the cooling stop temperature is more than 100 parts for cars ℃ in obtaining an intermediate product.
[Claim 20]
Or in claim 9 wherein, the method for producing a tempering heat treatment holding time is longer than 10 minutes components for automobiles according to claim 10.