JP5883350B2 - Hot press-formed product, manufacturing method thereof, and thin steel plate for hot press forming - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot press-formed product that requires strength, such as used for structural members of automobile parts, a manufacturing method thereof, and a thin steel plate for hot press forming, and in particular, a pre-heated steel plate (blank). The present invention relates to a hot press-formed product that obtains a predetermined strength by performing heat treatment at the same time as forming the shape, and a method for producing such a hot press-formed product, and a thin steel sheet for hot press forming. Is.

  As one of the measures to improve the fuel efficiency of automobiles that originated from global environmental problems, the weight of the vehicle body has been reduced, and it is necessary to increase the strength of steel plates used in automobiles as much as possible. However, when the strength of steel sheets is increased to reduce the weight of automobiles, the elongation EL and r value (Rankford value) decrease, and the press formability and shape freezeability deteriorate.

  In order to solve such a problem, the steel sheet is heated to a predetermined temperature (for example, a temperature at which it becomes an austenite phase) to reduce the strength (that is, to facilitate forming), and then at a lower temperature than the thin steel sheet ( For example, a hot press molding method that secures the strength after molding by forming a mold with a room temperature mold and performing a quenching heat treatment (quenching) using the temperature difference between the two at the same time as giving the shape. It has been adopted.

  According to such a hot press forming method, since the material is formed in a low strength state, the spring back becomes small (good shape freezing property), and a material with good hardenability to which alloy elements such as Mn and B are added. By using it, the strength of 1500 MPa class is obtained by the tensile cooling. Such a hot press forming method is called by various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method in addition to the hot press method.

  FIG. 1 is a schematic explanatory view showing a mold configuration for carrying out the above hot press molding (hereinafter may be represented by “hot stamp”). In FIG. Die, 3 is a blank holder, 4 is a steel plate (blank), BHF is a crease pressing force, rp is a punch shoulder radius, rd is a die shoulder radius, and CL is a punch / die clearance. Of these components, the punch 1 and the die 2 have passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium is allowed to pass through the passages. These members are configured to be cooled.

When hot stamping (for example, hot deep drawing) using such a mold, the steel plate (blank) 4 is set to the two-phase region temperature (Ac ₁ transformation point to Ac ₃ transformation point) or higher than the Ac ₃ transformation point. Molding is started in a state of being softened by heating to a single phase temperature. That is, in a state where the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel plate 4 is pushed into the hole of the die 2 (between 2 and 2 in FIG. 1) by the punch 1, and the outer diameter of the steel plate 4 is reduced. While shrinking, it is formed into a shape corresponding to the outer shape of the punch 1. Further, by cooling the punch 1 and the die 2 in parallel with the forming, heat is removed from the steel plate 4 to the mold (punch 1 and die 2), and the bottom dead center of the forming (the punch tip is located at the deepest part). The material is quenched by further holding and cooling in the state shown in FIG. By carrying out such a molding method, it is possible to obtain a 1500 MPa class molded product with good dimensional accuracy and to reduce the molding load compared to the case of molding parts of the same strength class in the cold. The capacity of the can be small.

  As steel plates for hot stamping that are currently widely used, steel plates made of 22MnB5 steel are known. This steel sheet has a tensile strength of 1500 MPa and an elongation of about 6 to 8%, and is applied to an impact resistant member (a member that is not deformed as much as possible and does not break). In addition, the development of increasing the C content and further increasing the strength (1500 MPa or higher, 1800 MPa class) based on 22MnB5 steel is also in progress.

  However, steel grades other than 22MnB5 steel are rarely applied, and the strength and elongation of parts are controlled (for example, low strength: 980 MPa class, high elongation: 20%, etc.) At present, there is almost no examination of the steel types and construction methods to be expanded.

  For medium-sized and larger passenger cars, considering the compatibility (function to protect the other party when a small car collides) at the time of a side collision or a rear collision, in the parts of the B pillar and rear side member, There are cases where both functions of the energy absorption site are provided. In order to produce such a member, there has been a method in which, for example, laser welding (tailored weld blank: TWB) of high strength super high tensile strength of 980 MPa class and high tensile strength of 440 MPa class is performed by cold press molding. It was mainstream. However, recently, development of a technique for separately creating strength in a part by hot stamping has been advanced.

  For example, Non-Patent Document 1 proposes a method of hot stamping 22MnB5 steel for hot stamping and a material that does not become high strength even if quenched with a mold and laser welding (tailored weld blank: TWB). The tensile strength is 1500 MPa (elongation 6 to 8%) on the high strength side (impact resistant site side), and the tensile strength is 440 MPa (elongation 12%) on the low strength side (energy absorption site side). . In addition, as a technique for creating different strengths in a part, techniques such as Non-Patent Documents 2 to 4 have been proposed.

  In the techniques of Non-Patent Documents 1 and 2 above, the tensile strength is 600 MPa or less and the elongation is about 12 to 18% on the energy absorption site side, but it is necessary to perform laser welding (tailored weld blank: TWB) in advance, As the number increases, the cost increases. Moreover, the energy absorption site | part which does not need to quench naturally is heated, and it is unpreferable also from a viewpoint of heat consumption.

  The technology of Non-Patent Document 3 is based on 22MnB5 steel, but due to the influence of boron addition, the robustness of the strength after quenching is poor with respect to the heating at the two-phase region temperature, and it is difficult to control the strength on the energy absorption site side. Further, only about 15% of elongation is obtained.

  The technique of Non-Patent Document 4 is based on 22MnB5 steel, which is not rational in terms of controlling the 22MnB5 steel with good hardenability so as not to be quenched (mold cooling control).

Klaus Lamprecht, Gunter Deinzer, Anton Stich, Jurgen Lechler, Thomas Stohr, Marion Merklein, “Thermo-Mechanical Properties of Tailor Welded Blanks in Hot Sheet Metal Forming Processes”, Proc. IDDRG2010, 2010. Usibor1500P (22MnB5) / 1500MPa ・ 8% -Ductibor500 / 550 ～ 700MPa ・ 17% [Search April 27, 2011] Internet <http://www.arcelormittal.com/tailoredblanks/pre/seifware.pl> 22MnB5 / above AC3 / 1500MPa ・ 8% -below AC3 ／ Hv190 ・ Ferrite / Cementite Rudiger Erhardt and Johannes Boke, “Industrial application of hot forming process simulation”, Proc, of 1st Int. Conf. On Hot Sheet Metal Forming of High- Performance steel, ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp83-88, 2008. Begona Casas, David Latre, Noemi Rodriguez, and Isaac Valls, “Tailor made tool materials for the present and upcoming tooling solutions in hot sheet metal forming”, Proc, of 1st Int. Conf. On Hot Sheet Metal Forming of High-Performance steel , ed.By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp23-35, 2008.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot press-molded product that can control the balance between strength and elongation within an appropriate range and has high ductility, and such hot press molding. It is an object of the present invention to provide a useful method for manufacturing a product and a thin steel sheet for hot press forming.

  The hot press-formed product of the present invention that has achieved the above object is a hot press-formed product obtained by forming a thin steel plate by a hot press forming method, and the metal structure is ferrite: 30 to 80 area%. Bainitic ferrite: less than 30 area% (excluding 0 area%), martensite: 30 area% or less (not including 0 area%), residual austenite: 3-20 area% It has a gist.

  In the hot press-formed product of the present invention, the chemical component composition is not limited, but as a typical example, C: 0.1 to 0.3% (meaning mass%. Hereinafter, the same applies to the chemical component composition). , Si: 0.5 to 3%, Mn: 0.5 to 2%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), Al : 0.01-0.1% and N: 0.001-0.01%, respectively, and the remainder consists of iron and inevitable impurities.

  In the hot press-formed product of the present invention, if necessary, as other elements, (a) B: 0.01% or less (excluding 0%) and Ti: 0.1% or less (0%) (B) one or more selected from the group consisting of Cu, Ni, Cr and Mo: 1% or less in total (not including 0%), (c) V and / or Nb: in total It is also useful to contain 0.1% or less (not including 0%) and the like, and the properties of the hot press-formed product are further improved depending on the type of element contained.

In producing the hot press-formed product of the present invention, a hot-rolled steel sheet having a metal structure of 50% by area or more of ferrite or a cold-rolled steel sheet having a cold rolling rate of 30% or more is used. In the press forming, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a temperature not lower than the Ac ₁ transformation point and not higher than (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7). During the molding, the molding may be terminated at a temperature of (bainite transformation start temperature Bs-100 ° C.) or less while securing an average cooling rate of 20 ° C./second or more in the mold. The molding end temperature is preferably (bainite transformation start temperature Bs-100 ° C.) or lower, a temperature range higher than the martensite transformation start temperature Ms point, and is preferably held for 10 seconds or longer in that temperature range.

Alternatively, as another method, when press forming a thin steel sheet using a press mold, the thin steel sheet is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1000 ° C., and then an average of not higher than 10 ° C./second. Cooling is performed at a cooling rate of 700 ° C. or lower and 500 ° C. or higher, and then molding is started. During the molding, while maintaining an average cooling rate of 20 ° C./second or higher in the mold (Bainite transformation start temperature Bs− The molding may be completed at a temperature of 100 ° C. or lower. Also in this method, it is preferable that the molding end temperature is (bainite transformation start temperature Bs-100 ° C.) or less, a temperature range of the martensite transformation start temperature Ms point or more, and is held for 10 seconds or more in the temperature range. .

  The present invention also includes a thin steel sheet for hot press forming for producing the above hot press-formed product, and the thin steel sheet is a hot-rolled steel sheet having a metal structure of ferrite of 50 area% or more, or It is a cold-rolled steel sheet with a cold rolling rate of 30% or more.

  According to the present invention, in the hot press forming method, by appropriately controlling the conditions, it is possible to adjust the metal structure in the presence of an appropriate amount of retained austenite, compared with the case of using the conventional 22MnB5 steel. However, it is possible to realize a hot press-formed product having a higher ductility (residual ductility) inherent in the formed product, and the strength and elongation can be controlled by a combination with the heat treatment conditions and the structure (initial structure) of the steel sheet before forming. Further, by adjusting the heating temperature in the two-phase region, it is possible to freely make strength and elongation.

It is a schematic explanatory drawing which shows the metal mold | die structure for implementing hot press molding.

  The inventors of the present invention, after heating a steel plate to a predetermined temperature, when producing a molded product by hot press forming, heat that shows good ductility (elongation) while securing a predetermined strength after forming. In order to realize a press-formed product, we examined it from various angles.

  As a result, when hot pressing a steel sheet using a press mold, the type of steel sheet, the heating temperature, and the conditions at the time of forming are appropriately controlled, and the structure is adjusted so as to contain 3-20% by area of retained austenite. It has been found that a hot press-molded product excellent in strength-ductility balance can be realized by controlling, and the present invention has been completed.

  The reason for setting the range of each structure (basic structure) in the hot press-formed product of the present invention is as follows.

[Ferrite: 30-80 area%]
By making the main structure fine and ferrite having high ductility, it is possible to achieve high ductility of a hot press-formed product. From such a viewpoint, the area fraction of ferrite needs to be 30 area% or more. However, when the area fraction exceeds 80 area%, the predetermined strength cannot be secured. A preferred lower limit of the ferrite fraction is 40 area% or more (more preferably 45 area% or more), and a preferred upper limit is 70 area% or less (more preferably 65 area% or less).

[Bainitic ferrite: less than 30 area% (excluding 0 area%)]
Although bainitic ferrite is effective in improving the strength, the ductility is somewhat lowered, so the upper limit of the fraction must be less than 30 area%. The preferable lower limit of the bainitic ferrite fraction is 5 area% or more (more preferably 10 area% or more), and the preferable upper limit is 25 area% or less (more preferably 20 area% or less).

[Martensite: 30 area% or less (excluding 0 area%)]
Martensite is effective in improving the strength, but the upper limit of the fraction must be 30% by area or less in order to greatly reduce the ductility. A preferred lower limit of the martensite fraction is 5 area% or more (more preferably 10 area% or more), and a preferred upper limit is 25 area% or less (more preferably 20 area% or less).

[Residual austenite: 3 to 20 area%]
Residual austenite has the effect of increasing the work hardening rate (transformation-induced plasticity) and improving the ductility of the molded product by transforming into martensite during plastic deformation. In order to exert such an effect, the fraction of retained austenite needs to be 3 area% or more. As for the ductility, the higher the retained austenite fraction, the better. However, in the composition used for the steel sheet for automobiles, the retained austenite that can be secured is limited, and the upper limit is about 20 area%. The preferable lower limit of retained austenite is 5 area% or more (more preferably 7 area% or more), and the preferable upper limit is 15 area% or less (more preferably 10 area% or less).

In producing the hot press-formed product of the present invention, a hot-rolled steel sheet having a metal structure of 50% by area or more of ferrite or a cold-rolled steel sheet having a cold rolling rate of 30% or more is used. In the press forming, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a temperature not lower than the Ac ₁ transformation point and not higher than (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7). During molding, the molding may be terminated at a temperature not higher than the average cooling rate of 20 ° C./second or more (bainite transformation start temperature Bs-100 ° C.) in the mold. The reasons for specifying each requirement in this method are as follows.

[Use a hot-rolled steel sheet having a metal structure with ferrite of 50 area% or more, or a cold-rolled steel sheet with a cold rolling rate of 30% or more]
In order to obtain a ferrite structure that greatly contributes to ductility when heated to a two-phase region temperature, it is necessary to appropriately select the type of steel plate (forming steel plate). When a hot-rolled steel sheet is used as the forming steel sheet, it is important that the ferrite fraction is high and the ferrite remains at the two-phase temperature when heated. From such a viewpoint, it is preferable that the hot-rolled steel sheet to be used has a ferrite metal structure of 50 area% or more. The preferred lower limit of this ferrite fraction is 60 area% or more (more preferably 70 area% or more), but if the ferrite fraction becomes too high, the ferrite fraction in the molded product becomes too large, so 95 area% The following is preferable. More preferably, it is 90 area% or less.

On the other hand, when using a cold-rolled steel sheet, recrystallization takes place during heating, and it is an important requirement that ferrite not containing dislocations be formed. Intermediate rolling) must be performed. In the case of a cold-rolled steel sheet, any structure may be used. From such a viewpoint, when using a cold-rolled steel sheet, it is preferable to use a cold-rolled steel sheet with a cold rolling rate of 30% or more. The cold rolling rate is preferably 40% or more, more preferably 50% or more. The “cold rolling ratio” is a value obtained by the following equation (1).
Cold rolling rate (%) = [(steel plate thickness before cold rolling−steel plate thickness after cold rolling) / steel plate thickness before cold rolling] × 100 (1)

[After the steel sheet is heated to a temperature not lower than Ac ₁ transformation point and not higher than (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7), forming is started]
In order to partially make austenite while retaining the ferrite contained in the steel sheet, it is necessary to control the heating temperature within a predetermined range. By appropriately controlling the heating temperature, it can be transformed into retained austenite or martensite in the subsequent cooling process, and can be formed into a desired structure by a final hot press-formed product. When the heating temperature of the steel sheet is less than the Ac ₁ transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be secured in the final structure (structure of the molded product). When the heating temperature of the thin steel plate exceeds (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7), the transformation amount to austenite increases too much during heating, and the final structure (structure of the molded product) The predetermined amount of ferrite cannot be secured.

[During molding, molding is terminated at a temperature of (bainite transformation start temperature Bs-100 ° C.) or less while securing an average cooling rate of 20 ° C./second or more in the mold.
In order to secure a predetermined amount of retained austenite while preventing the formation of cementite from the austenite formed in the heating step, it is necessary to appropriately control the average cooling rate during molding and the molding end temperature. is there. From such a viewpoint, the average cooling rate during molding is set to 20 ° C./second or more, and the molding end temperature is set to be equal to or lower than (bainite transformation start temperature Bs-100 ° C .: hereinafter sometimes abbreviated as “Bs-100 ° C.”). There is a need. The average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more). Moreover, although shaping | molding completion temperature may be complete | finished cooling to room temperature with the said average cooling rate, after cooling to Bs-100 degrees C or less, cooling may be stopped and shaping | molding may be complete | finished after that. . The average cooling rate during the molding is controlled by means such as (a) controlling the temperature of the molding die (cooling medium shown in FIG. 1), (b) controlling the thermal conductivity of the die. It can be achieved (same for cooling in the following method).

As another method for producing the press-formed product of the present invention, when press-molding a steel sheet using a press-molding die, the thin steel sheet is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1000 ° C. Cooling to a temperature of 700 ° C. or lower and an average cooling rate of 10 ° C./second or lower to a temperature of 700 ° C. or lower, and then starting molding, while securing an average cooling rate of 20 ° C./second or higher in the mold You may make it complete | finish shaping | molding at the temperature below (bainite transformation start temperature Bs-100 degreeC). The reason why the requirements in this method are specified is as follows (the cooling end temperature is the same as above).

[The thin steel plate is heated to a temperature not lower than Ac ₃ transformation point and not higher than 1000 ° C.]
In order to appropriately adjust the structure of the hot press-formed product, it is necessary to control the heating temperature within a predetermined range. By appropriately controlling this heating temperature, it is transformed into a structure mainly composed of ferrite while securing a predetermined amount of retained austenite in the subsequent cooling process, and the desired structure is formed with the final hot press-formed product. Can be included. When the heating temperature of the steel sheet is less than the Ac ₃ transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be secured in the final structure (structure of the molded product). On the other hand, when the heating temperature of the thin steel plate exceeds 1000 ° C., the austenite grain size increases during heating, and ferrite cannot be generated by subsequent cooling.

[Cool at an average cooling rate of 10 ° C./second or lower to a temperature of 700 ° C. or lower and 500 ° C. or higher, and then start molding]
This cooling step is an important step in forming ferrite during cooling. If the average cooling rate at this time exceeds 10 ° C./second, a predetermined amount of ferrite cannot be secured. This average cooling rate is preferably 7 ° C./second or less, more preferably 5 ° C./second or less. The cooling stop temperature in this cooling step (this temperature may be referred to as “cooling rate changing temperature”) needs to be 700 ° C. or lower and 500 ° C. or higher. When the cooling stop temperature exceeds 700 ° C., a sufficient amount of ferrite cannot be secured, and when the cooling stop temperature is less than 500 ° C., the ferrite fraction becomes too large to secure a predetermined strength. A preferable upper limit of the cooling stop temperature is 680 ° C. or lower (more preferably 660 ° C. or lower), and a preferable lower limit is 520 ° C. or higher (more preferably 550 ° C. or higher).

  In any method, the molding end temperature needs to be (Bs-100 ° C.) or less, but a temperature range above the martensite transformation start temperature Ms point (this temperature is referred to as “cooling rate change temperature”). It is preferable to hold for 10 seconds or more in that temperature range. By holding for 10 seconds or more in the above temperature range, the bainite transformation proceeds from supercooled austenite to obtain a structure mainly composed of ferrite. The holding time at this time is preferably 50 seconds or more (more preferably 100 seconds or more). However, if the holding time becomes too long, the austenite starts to decompose, and the retained austenite fraction cannot be secured. Or less (more preferably 800 seconds or less).

  The holding as described above may be any of isothermal holding, monotonous cooling, and reheating step as long as it is within the above temperature range. As for the relationship between such holding and molding, the above-described holding may be added at the stage of completion of molding, but a holding step may be added within the above temperature range in the middle of finishing molding. . After the molding is completed in this manner, it may be cooled to room temperature (25 ° C.) by cooling or at an appropriate cooling rate.

  In the method for producing a hot press-formed product of the present invention, even if any of the above methods is adopted, the case where a hot press-formed product having a simple shape as shown in FIG. That is, the present invention can also be applied to the production of a molded product having a relatively complicated shape. However, in the case of a complicated part shape, it may be difficult to create the final shape of the product by a single press molding. In such a case, a method of performing cold press forming in a pre-process of hot press forming (this method is called “indirect method”) can be employed. This method is a method in which a portion that is difficult to be molded is preliminarily molded to an approximate shape by cold working, and the other portions are hot press molded. If such a method is adopted, for example, when a part having three uneven portions (peaks) of a molded product is formed, the two parts are formed by cold press molding, and then the third part is formed. Will be hot pressed.

  In the present invention, it is made assuming a hot press-formed product made of a high-strength steel plate, and its steel type may be of a normal chemical composition as a high-strength steel plate, but C, Si, About Mn, P, S, Al, and N, it is good to adjust to an appropriate range. From such a viewpoint, the preferable ranges of these chemical components and the reasons for limiting the ranges are as follows.

[C: 0.1 to 0.3%]
C is an important element in securing retained austenite. At the time of heating at a two-phase region temperature or a single-phase region temperature equal to or higher than the Ac ₃ transformation point, the austenite is concentrated to form residual austenite after quenching. It also contributes to an increase in the amount of martensite. When the C content is less than 0.1%, a predetermined retained austenite amount cannot be secured, and good ductility cannot be obtained. On the other hand, if the C content is excessive and exceeds 0.3%, the strength becomes too high. A more preferable lower limit of the C content is 0.15% or more (more preferably 0.20% or more), and a more preferable upper limit is 0.27% or less (more preferably 0.25% or less).

[Si: 0.5-3%]
Si suppresses the formation of austenite after heating to cementite at a two-phase temperature or a single-phase temperature equal to or higher than the Ac ₃ transformation point, and exerts an effect of increasing residual austenite. In addition, the solid solution strengthening also exerts the effect of increasing the strength without significantly degrading the ductility. If the Si content is less than 0.5%, a predetermined retained austenite amount cannot be secured, and good ductility cannot be obtained. On the other hand, if the Si content is excessive and exceeds 3%, the solid solution strengthening amount becomes too large, and the ductility is greatly deteriorated. The more preferable lower limit of the Si content is 1.15% or more (more preferably 1.20% or more), and the more preferable upper limit is 2.7% or less (more preferably 2.5% or less).

[Mn: 0.5-2%]
Mn is an element that stabilizes austenite and contributes to an increase in retained austenite. In order to exhibit such an effect, it is preferable to contain 0.5% or more of Mn. However, if the Mn content is excessive, it becomes impossible to secure a predetermined amount of ferrite by hindering the formation of ferrite, so 2% or less is preferable. Further, since the strength of austenite is significantly improved, the hot rolling load becomes large and the production of the steel sheet becomes difficult. Therefore, it is not preferable to contain more than 2% from the viewpoint of productivity. A more preferable lower limit of the Mn content is 0.7% or more (more preferably 0.9% or more), and a more preferable upper limit is 1.8% or less (more preferably 1.6% or less).

[P: 0.05% or less (excluding 0%)]
P is an element inevitably contained in the steel, but it deteriorates ductility, so it is preferable to reduce P as much as possible. However, extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is preferable to make it 0.05% or less (not including 0%). A more preferable upper limit of the P content is 0.045% or less (more preferably 0.040% or less).

[S: 0.05% or less (excluding 0%)]
Similarly to P, S is an element inevitably contained in steel, and deteriorates ductility. Therefore, S is preferably reduced as much as possible. However, extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is preferable to make it 0.05% or less (not including 0%). A more preferable upper limit of the S content is 0.045% or less (more preferably 0.040% or less).

[Al: 0.01 to 0.1%]
Al is useful as a deoxidizing element, and also fixes solid solution N present in steel as AlN, which is useful for improving ductility. In order to effectively exhibit such effects, the Al content is preferably 0.01% or more. However, when the Al content is excessive and exceeds 0.1%, Al ₂ O ₃ is excessively generated, and ductility is deteriorated. A more preferable lower limit of the Al content is 0.013% or more (more preferably 0.015% or more), and a more preferable upper limit is 0.08% or less (more preferably 0.06% or less).

[N: 0.001 to 0.01%]
N is an element inevitably mixed in, and is preferably reduced. However, since there is a limit to reducing it in the actual process, 0.001% was set as the lower limit. If the N content is excessive, the ductility deteriorates due to strain aging, or when B is added, it precipitates as BN and lowers the effect of improving hardenability by solute B, so the upper limit is 0.01. %. The upper limit with more preferable N content is 0.008% or less (more preferably 0.006% or less).

  The basic chemical components in the press-formed product of the present invention are as described above, and the balance is substantially iron. In addition, “substantially iron” means a trace component that does not inhibit the properties of the steel material of the present invention other than iron (for example, Mg, Ca, Sr, Ba, REM such as La, and Zr, Hf). , Ta, W, Mo and other carbide-forming elements) are acceptable, and inevitable impurities other than P, S, N (for example, O, H, etc.) can also be included.

  In the press-formed product of the present invention, if necessary, (a) B: 0.01% or less (not including 0%) and Ti: 0.1% or less (not including 0%), (b) Cu 1 or more selected from the group consisting of Ni, Cr and Mo: 1% or less in total (excluding 0%), (c) V and / or Nb: 0.1% or less in total (0% It is also useful to contain (not contained) and the like, and the properties of the hot press-formed product are further improved depending on the type of element contained. The preferable range when these elements are contained and the reason for limiting the range are as follows.

[B: 0.01% or less (not including 0%) and Ti: 0.1% or less (not including 0%)]
B is an element that prevents formation of cementite during cooling after heating and contributes to securing retained austenite. In order to exhibit such an effect, B is preferably contained in an amount of 0.0001% or more, but the effect is saturated even if it is contained in excess of 0.01%. A more preferable lower limit of the B content is 0.0002% or more (more preferably 0.0005% or more), and a more preferable upper limit is 0.008% or less (more preferably 0.005% or less).

  On the other hand, Ti fixes N and maintains B in a solid solution state, thereby exhibiting an effect of improving hardenability. In order to exert such an effect, it is preferable to contain Ti at least four times the content of N. However, if the Ti content is excessive and exceeds 0.1%, a large amount of TiC is formed, The strength increases by precipitation strengthening, but the ductility deteriorates. A more preferable lower limit of the Ti content is 0.05% or more (more preferably 0.06% or more), and a more preferable upper limit is 0.09% or less (more preferably 0.08% or less).

[One or more selected from the group consisting of Cu, Ni, Cr and Mo: 1% or less in total (excluding 0%)]
Cu, Ni, Cr and Mo prevent the formation of cementite during cooling after heating and effectively act to secure retained austenite. In order to exhibit such an effect, it is preferable to contain 0.01% or more in total. Considering only the characteristics, it is preferable that the content is large, but since the cost of alloy addition increases, the total content is preferably 1% or less. Moreover, since it has the effect | action which raises the intensity | strength of austenite significantly, since the load of hot rolling becomes large and manufacture of a steel plate becomes difficult, it is preferable to set it as 1% or less also from a viewpoint of productivity. The more preferable lower limit of the content of these elements is 0.05% or more (more preferably 0.06% or more) in total, and the more preferable upper limit is 0.9% or less (more preferably 0.8% or less) in total. ).

[V and / or Nb: 0.1% or less in total (excluding 0%)]
V and Nb have the effect of forming fine carbides and making the structure fine by the pinning effect. In order to exhibit such an effect, it is preferable to contain 0.001% or more in total. However, if the content of these elements is excessive, coarse carbides are formed and the ductility is deteriorated by becoming the starting point of destruction, so the total content is preferably 0.1% or less. The more preferable lower limit of the content of these elements is 0.005% or more (more preferably 0.008% or more) in total, and the more preferable upper limit is 0.08% or less (more preferably 0.06% or less) in total. ).

  The hot press-formed thin steel sheet of the present invention may be either a non-plated steel sheet or a plated steel sheet. In the case of a plated steel sheet, the type of plating may be any of general zinc-based plating and aluminum-based plating. The plating method may be any one of hot dipping, electroplating, etc., and may be further subjected to alloying heat treatment after plating, or may be subjected to multilayer plating.

  According to the present invention, by appropriately adjusting the press molding conditions (heating temperature and cooling rate), properties such as strength and elongation of the molded product can be controlled, and high hotness (residual ductility) can be achieved. Since a press-molded product can be obtained, it can be applied to parts that have been difficult to apply with conventional hot-pressed products (for example, energy absorbing members), which is extremely useful in expanding the range of application of hot-pressed products. It is. In addition, the molded product obtained by the present invention has a larger residual ductility than a molded product whose structure is adjusted by performing normal annealing after cold press molding.

  Hereinafter, the effects of the present invention will be described more specifically by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

A steel material having the chemical composition shown in Table 1 below was vacuum-melted to obtain a slab for experiment, then hot rolled, and then cooled and wound up. Further, after cold rolling to obtain a thin steel sheet, a quenching process is performed so as to have a predetermined initial structure. The Ac ₁ transformation point, Ac ₃ transformation point, Ms point, and (Bs-100 ° C.) in Table 1 are determined based on the following formulas (2) to (5) (for example, “ Lesley Iron and Steel Material Science, Maruzen, (1985)). Table 1 also shows the calculated value (hereinafter referred to as “A value”) of (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7).

Ac ₁ transformation point (° C.) = 723 + 29.1 × [Si] −10.7 × [Mn] + 16.9 × [Cr] −16.9 [Ni] (2)
Ac ₃ transformation point (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V ] -11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)
Ms point (° C.) = 550−361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al] ( 4)
Bs point (° C.) = 830−270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −83 × [Mo] (5)
However, [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] are C, The contents (mass%) of Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni are shown. Moreover, when the element shown by each term of said Formula (2)-Formula (5) is not included, it calculates as the thing without the term.

The obtained steel sheet was heated under the conditions shown in Table 2 below, and then subjected to forming and cooling treatment using a rapid heat treatment test apparatus for steel (CAS series ULVAC-RIKO) that can control the average cooling rate. The steel plate size at the time of cooling was 190 mm x 70 mm (plate thickness: 1.4 mm). Test No. Nos. 1 to 14, 17 to 19, and 22 to 25 are hot-rolled steel plates as forming steel plates. Nos. 15, 16, and 20 use cold-rolled steel plates as forming steel plates. “Cooling 1” shown in Table 2 indicates cooling from the heating temperature to 700 to 500 ° C., and “Cooling 2” refers to the temperature range from [(Bs−100 ° C.) to Ms point] thereafter. (Test Nos. 19 to 20, 22, and 23 start molding at this stage). In addition, the steel plate was immersed in the molten zinc as needed, and zinc plating was made to adhere to the steel plate surface (test No. 25).

  For each steel plate subjected to the above treatment (heating, forming, cooling), the tensile strength (TS), elongation (total elongation EL), and observation of metal structure (fraction of each structure) were performed as follows.

[Tensile strength (TS) and elongation (total elongation EL)]
A tensile test was performed using a JIS No. 5 test piece, and tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate of the tensile test was set to 10 mm / second. In the present invention, either (a) the tensile strength (TS) is 780 to 979 MPa and the elongation (EL) is 25% or more, and (b) the tensile strength (TS) is 980 to 1179 MPa and the elongation (EL) is 15% or more. When it was satisfied, it was evaluated as passing.

[Observation of metal structure (fraction of each structure)]
(1) Regarding the structure of ferrite and bainitic ferrite in the steel sheet, the steel sheet is corroded with nital, and the ferrite and bainitic ferrite are distinguished by SEM (magnification: 1000 times or 2000 times) observation. The rate (area ratio) was determined.
(2) The retained austenite fraction (area ratio) in the steel sheet was measured by an X-ray diffraction method after grinding to a thickness of 1/4 of the steel sheet and then chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P.776).
(3) For the area ratio of martensite (as-quenched martensite), the steel sheet was repeller-corroded, and the area ratio was measured as a mixed structure of martensite (as-quenched martensite) and residual austenite by SEM observation Then, the fraction of retained austenite determined by X-ray diffraction was calculated, and the martensite fraction was calculated as quenched.

  These results are shown in Table 3 below together with the type of steel sheet before forming (ferrite fraction, cold rolling ratio in the case of cold rolled steel sheet).

From this result, it can be considered as follows. Test No. 1 to 10 , 13 , 15 , 19 , 20 , and 25 are examples that satisfy the requirements defined in the present invention, and it can be seen that parts having a good balance between strength and ductility are obtained.

  In contrast, test no. Nos. 11 to 12, 14, 16 to 18, and 22 to 24 are comparative examples that do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. No. 11 uses steel with insufficient C content (steel type K in Table 1), retained austenite is not secured, and only low elongation (EL) is obtained. Test No. No. 12 is a steel using a steel with insufficient Si content (steel type L in Table 1). Residual austenite is not ensured and only low elongation (EL) is obtained.

Test No. No. 14 is a conventional 22MnB5 equivalent steel (steel type N in Table 1). Although high strength is obtained, retained austenite is not ensured and only low elongation (EL) is obtained. It is not done. Test No. No. 16 is a cold-rolled steel sheet having a low cold-rolling rate, and the structure of the molded product is ferrite: 25 area%, and the elongation (EL) is low. Test No. In _No. 17, the heating temperature is lower than the Ac ₁ transformation point, the structure of the molded product is ferrite: 81 area% (remaining martensite and cementite), residual austenite is not secured, and elongation (EL ) And tensile strength is low. Test No. In No. 18, the heating temperature is higher than the value A, the martensite is generated excessively, ferrite and bainitic ferrite are not secured, and the elongation (EL) is low.

  Test No. In the case of No. 22, the average cooling rate in the cooling 1 is high, bainitic ferrite is generated, ferrite is not secured, and elongation (EL) is low. Test No. In No. 23, the average cooling rate in cooling 1 is slow and the cooling rate changing temperature is low, the structure of the molded product is ferrite: 83 area% (remainder bainitic ferrite), and retained austenite is secured. The elongation (EL) is low. Test No. In No. 24, the molding end temperature is high, pearlite is generated in the molded product structure, the retained austenite is not secured, and the elongation (EL) is low.

1 Punch 2 Die 3 Blank holder 4 Steel plate (blank)

Claims (8)

A hot press-formed product formed from a thin steel plate,
The chemical composition is
C: 0.1 to 0.3% (meaning mass%, hereinafter the same for the chemical composition)
Si: 0.5-3%,
Mn: 0.5-2%
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Al: 0.01-0.1%, and
N: 0.001 to 0.01%,
Each of which contains iron and inevitable impurities,
Metal structure: ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%), martensite: 30 area% or less (excluding 0 area%), residual austenite: 3 A hot press-molded product comprising ˜20 area%. The hot press according to claim 1 , further comprising B: 0.01% or less (not including 0%) and Ti: 0.1% or less (not including 0%) as other elements. Molding. The heat according to claim 1 or 2 , further comprising at least one element selected from the group consisting of Cu, Ni, Cr and Mo as a further element: 1% or less (excluding 0%) in total. Inter-press molded product. The hot press-formed product according to any one of claims 1 to 3 , further comprising V and / or Nb: 0.1% or less (excluding 0%) in total as other elements. In manufacturing the hot press-formed product according to any one of claims 1 to 4 , a hot-rolled steel sheet having a metal structure of ferrite of 50 area% or more, or a cold-rolled steel sheet subjected to a cold-rolling rate of 30% or more. When performing press forming using a press molding die, the hot-rolled steel sheet or cold-rolled steel sheet is heated to a temperature not lower than Ac ₁ transformation point and not higher than (Ac ₁ transformation point × 0.3 + Ac ₃ transformation point × 0.7). Then, molding is started, and during the molding, the average cooling rate of 20 ° C./second or more is secured in the mold, and the molding is finished at a temperature of (bainite transformation start temperature Bs-100 ° C.) or less. A method for producing a hot press-formed product. In producing the hot press-formed product according to any one of claims 1 to 4 , when the thin steel plate is press-formed using a press mold, the thin steel plate has an Ac ₃ transformation point or higher and 1000 ° C or lower. After heating to a temperature, it is cooled to a temperature of 700 ° C. or lower and 500 ° C. or higher at an average cooling rate of 10 ° C./second or less, and then molding is started. A method for producing a hot press-molded product, characterized in that the molding is terminated at a temperature equal to or lower than (bainite transformation start temperature Bs-100 ° C) while securing the speed. The forming finishing temperature, (bainite transformation starting temperature Bs-100 ° C.) or less, and martensite transformation start temperature Ms point or temperature range, according to claim 5 or 6 is molded and held at that temperature range for 10 seconds or more Production method. In hot press forming for thin steel sheet for making a hot press-formed article according to any one of claims 1-4, wherein the ferrite is a hot-rolled steel plate having 50 area% or more of the metal structure A thin steel sheet for hot press forming.

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