JP4575799B2 - Manufacturing method of hot-pressed high-strength steel members with excellent formability - Google Patents

Description

  The present invention relates to a manufacturing method for obtaining a high-strength steel member used for, for example, an automobile underbody, a chassis, a member, a collision safety reinforcing member, and the like.

  In recent years, with the need for weight reduction of automobiles and the need to improve collision safety for global environmental protection, it is strongly desired that steel members manufactured by press molding have a tensile strength of 980 MPa or more. However, increasing the strength of the steel sheet as a material causes a decrease in press formability, making it difficult to manufacture complex shapes. Specifically, there are problems such as fracture due to reduced ductility and deterioration of dimensional accuracy due to springback. For this reason, it is not easy to manufacture complicated shapes by press working using high-strength steel sheets.

  Therefore, there is a hot press (hot press) as a technology that has been attracting attention recently. For example, as disclosed in Patent Document 1, the present technology is a method of heating and pressing a steel plate, and because the steel plate is high temperature, it becomes soft and highly ductile, and even a complicated shape can be formed with high dimensional accuracy. . In addition, if the temperature is raised to a temperature range where austenite is generated during heating, it is possible to obtain a desired material by cooling after pressing or by cooling in a mold during pressing. . That is, by increasing the cooling rate, it is possible to produce martensite that is a hardened phase and obtain a high-strength member. In Patent Document 1, a metal collision is manufactured by heating a metal material to 850 ° C. or higher and pressing it with a relatively low-temperature press die to give a desired shape in a high temperature state of 850 ° C. or higher. A method for manufacturing a reinforcing material is disclosed.

  However, in order to increase the efficiency and productivity, or to improve the moldability and manufacture complex shaped parts, it is not a single press, but a cold press, like a transfer press. Although it is required to perform the press work continuously, Patent Document 1 does not assume that the press work is performed twice or more. In the technique disclosed in Patent Document 1, since strengthening is performed using martensitic transformation from austenite at the time of quenching, the steel sheet taken out from the heating furnace can be processed at a high temperature of 850 ° C. or higher. Required. Even if it is assumed that the press working is performed twice or more by the technique disclosed in Patent Document 1, the temperature of the steel plate taken out from the heating furnace is immediately decreased, or a part of the steel plate is formed before forming. Is cooled by contact with the mold, it is difficult to carry out the pressing process at 850 ° C. or more twice or more unless a very high speed conveying machine or high speed pressing machine is used. Even if the processing is carried out twice or more, a part of the steel sheet becomes less than 850 ° C. due to contact with the mold, and there is a problem that the material varies and the shape is deformed even within the same member. In addition, in order to ensure formability, there is a method of heating again and performing a second press, but the productivity is greatly reduced.

  Further, Patent Document 2 discloses a technique for press forming twice or more in warm press forming. However, Patent Document 2 has no description about components and structures, and produces a martensite structure and a high No mention is made of a production method for obtaining a strength member.

JP 2002-102980 A JP 2001-314923 A

  The present invention solves the above-mentioned problems, and in hot pressing, a hot pressing high strength steel member manufacturing method that enables continuous pressing twice or more like cold pressing and has excellent productivity and formability. The purpose is to provide.

  As a result of various studies on the relationship between the steel plate components and the hot press forming method, the inventors have found that the amounts of Mn, Cr, Cu, and Ni contained in the steel plate are Mn + Cr / 3.1 + (Cu + Ni) / 1. Even if a steel sheet satisfying 4 ≧ 2.5% is formed in a temperature range of 300 ° C. or higher, it is possible to form a member having a required baking and a uniform strength, It was found that the shape freezing property was sufficiently good as a member. The present invention has been completed based on the above findings, and the gist thereof is as follows.

  That is, according to the present invention, by mass, C: 0.05 to 0.35%, Si: 0.005 to 1.0%, Mn: 0 to 4.0%, Cr: 0 to 3.0% , Cu: 0 to 4.0%, Ni: 0 to 3.0%, B: 0.0002 to 0.1%, Ti: 0.001 to 3.0%, P: 0.1% or less, S : 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, and Mn + Cr / 3.1 + (Cu + Ni) /1.4≧2.5% The steel sheet which is filled and the balance is Fe and inevitable impurities is heated to 750 to 1300 ° C. and maintained for 10 to 6000 seconds, and then press forming at 300 ° C. twice or more, and the area ratio is 60%. Provided is a method for producing a hot-pressed high-strength steel member excellent in formability, characterized by obtaining a member having the above martensite structure .

  The steel sheet may further contain Mo: 0.01 to 3.0% by mass. Further, the steel sheet is further in% by mass, one or more of Nb: 0.01 to 3.0%, V: 0.001 to 3.0%, W: 0.005 to 3.0%. May be contained. Moreover, the said steel plate is further mass%, REM: 0.0005-0.01%, Y: 0.0005-0.01%, Ca: 0.0005-0.01%, Mg: 0.0005-0 .01% of one kind or two or more kinds may be contained.

  Moreover, you may heat up the said steel plate to 750-1300 degreeC with the temperature increase rate of 1-100 degrees C / sec. Further, after the final press forming, the die punch may be held at the bottom dead center for 1 to 60 seconds, and the steel sheet may be cooled to 5 to 40 ° C. at a cooling rate of 10 to 500 ° C./second. Further, after the final press molding, the mold punch is moved to the top dead center without being held at the bottom dead center, the molded product is taken out from the mold, and the cooling rate of 10 to 500 ° C./second is 5 to 40. It may be cooled to ° C.

  According to the present invention, it is possible to perform press forming two or more times using a transfer press as in the cold press without requiring reheating even in the hot press. Can be improved. The obtained member is a high-strength steel member with little variation in material inside, good member shape and excellent uniformity.

The present invention is described in further detail below.
First, the reasons for limiting the chemical composition of steel in the present invention will be described.

  C: C is an essential element for increasing the strength of the steel sheet, and lowers the Ar3 point and lowers the moldable temperature, so the lower limit was made 0.05 mass%. On the other hand, if C exceeds 0.35 mass%, welding becomes difficult, so the upper limit was made 0.35 mass%.

  Si: Si is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it exceeds 1% by mass, surface scale problems will occur, so the upper limit is 1%. On the other hand, since extremely low results in an increase in manufacturing cost, it is desirable to add 0.005% or more by mass%.

  Mn: Mn is a very important element for ensuring hardenability because it suppresses ferrite transformation. In addition, since the Ar3 point is lowered, the moldable temperature can be lowered. However, excessive addition not only promotes co-segregation with P and S, but also adversely affects manufacturability during production and hot rolling, so the upper limit is 4% by mass. Preferably it is 3.5% or less. In the present invention, 0% is also included.

  Cr: Cr is a strengthening element and is important for improving hardenability. In addition, since the Ar3 point is lowered, the moldable temperature can be lowered. However, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was set to 3% by mass%. In the present invention, 0% is also included.

  Cu: Cu is effective for strengthening and is important for improving hardenability. In addition, since the Ar3 point is lowered, the moldable temperature can be lowered. However, excessive addition adversely affects manufacturability during production and hot rolling, so the upper limit was made 4.0% by mass. In the present invention, 0% is also included.

  Ni: Ni is a strengthening element and is important for improving hardenability. In addition, since the Ar3 point is lowered, the moldable temperature can be lowered. However, if the mass% exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was set to 3%. In addition, since Ni sulfide has the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, it is effective to add it to the steel sheet. In the present invention, 0% is also included.

  The above four elements of Mn, Cr, Cu, and Ni are all effective because they can lower the Ar3 point and lower the moldable temperature. Although it is not necessary to contain all these elements, if Mn + Cr / 3.1 + (Cu + Ni) /1.4 is less than 2.5%, the effect of lowering the Ar3 point cannot be obtained sufficiently, so Mn + Cr / 3. The lower limit of 1+ (Cu + Ni) /1.4 was 2.5%. Further, Ni is preferably added in an amount of Cu / 2% or more from the viewpoint of prevention of Cu shaving.

  B: B is an element effective for increasing the strength of the steel sheet. However, if the mass% is less than 0.0002%, this effect cannot be obtained, so the lower limit was made 0.0002%. On the contrary, if the content exceeds 0.1% by mass, the hot workability deteriorates, so the upper limit was set to 0.1%.

  Ti: Ti is preferably added when Ti is used to improve the hardenability of the steel sheet using B because Ti forms a compound with N and reduces the amount of dissolved N in the steel sheet. Therefore, the lower limit is set to 0.001% in mass%. On the other hand, if the content exceeds 3% by mass, the precipitation of carbonitride increases and the workability and delayed fracture resistance decrease, so the upper limit was set to 3%. Moreover, Ti has the effect of suppressing grain growth and reducing the grain size when the steel sheet is reheated, so addition of Ti is also desirable from the viewpoint of improving toughness. In addition, since precipitates and crystallized substances containing Ti serve as hydrogen trap sites, they are also important from the viewpoint of hydrogen embrittlement resistance.

  P: P is an impurity, and if it exceeds 0.1% by mass, it adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value was set to 0.1% by mass. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001% by mass.

  S: S is an impurity, which adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value is set to 0.05% by mass or less.

  Al: Al is used as a deoxidizer, so 0.005% or more is added by mass%. If it exceeds 0.1% by mass, non-metallic inclusions increase and surface flaws occur in the product. In order to facilitate, the upper limit is set to 0.1%.

  N: N is an impurity, and it is better to reduce the amount because it contributes to workability deterioration and blowhole generation during welding. If the mass% exceeds 0.01%, the workability deteriorates, so 0.01% is the upper limit.

  Mo: Mo is an arbitrary contained element in the present invention. Mo is effective for increasing the strength of the steel sheet, reducing the grain size and improving the hardenability. Since these effects cannot be obtained when the addition amount is less than 0.01% by mass, the lower limit is set to 0.01%. On the other hand, if the content exceeds 3% by mass, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%. Moreover, Mo has the effect of suppressing grain growth and reducing the grain size when the steel sheet is reheated using a heating furnace, so addition of Mo is also desirable from the viewpoint of improving toughness.

  Nb: Nb is an arbitrary contained element in the present invention. Nb is effective in increasing the strength of the steel sheet, reducing the grain size, and improving the hardenability. Since these effects cannot be obtained when the addition amount is less than 0.01% by mass, the lower limit is set to 0.01%. On the other hand, if the content exceeds 3% by mass, the precipitation of carbonitride increases and the workability and delayed fracture resistance decrease, so the upper limit was set to 3%. Further, Nb has the effect of suppressing grain growth and reducing the grain size when the steel sheet is reheated, so addition of Nb is also desirable from the viewpoint of improving toughness.

  V: V is an arbitrary contained element in the present invention. V is an extremely important element because it is effective for increasing the strength of the steel sheet and reducing the grain size, and precipitates and crystallized substances containing V become hydrogen trap sites. However, if the mass% is less than 0.001%, this effect cannot be obtained, so the lower limit was set to 0.001%. On the other hand, if the content exceeds 3% by mass, the precipitation of carbonitrides becomes remarkable, and the ductility decreases remarkably. For this reason, the upper limit is set to 3%.

  W: W is an arbitrary contained element in the present invention. W is an extremely important element because it is effective for increasing the strength of the steel sheet and precipitates and crystallized substances containing W become hydrogen trap sites. However, if the mass% is less than 0.005%, these effects cannot be obtained, so the lower limit was set to 0.005%. On the contrary, if the content exceeds 3% by mass, the workability deteriorates, so the upper limit was made 3%. Further, W has the effect of suppressing grain growth and reducing the grain size when the steel sheet is reheated, so addition of W is also desirable from the viewpoint of improving toughness.

  REM, Ca, and Y are arbitrary contained elements in the present invention. REM, Ca, and Y are effective for controlling the shape of inclusions and contribute to delayed fracture resistance. On the other hand, excessive addition deteriorates hot workability, so that the addition is 0.01% or less by mass%. Here, REM is an abbreviation for Rare Earth Metal and is a general term for lanthanoid elements starting with La.

  Mg: Mg is an arbitrary contained element in the present invention. Mg is not only effective in improving delayed fracture resistance of its own compound, but also controls the formation of composite precipitates or composite ascendants with other elements and contributes to the improvement of delayed fracture resistance. Therefore, the content is set to 0.0005% or more by mass%. However, if the mass% exceeds 0.01%, coarse oxides and sulfides are formed, which is not effective for shape control and lowers the workability, which is a basic required characteristic of thin steel sheets. 0.01%.

  Next, the reason for limiting the structure of the member after hot pressing will be described. The reason why the martensite is 60% or more in area ratio is to obtain a high tensile strength of 980 MPa or more, preferably 1180 MPa, and further 1480 MPa or more. For that purpose, martensite which is a hard phase is 60 in area ratio. % Or more, preferably 80% or more, more preferably 95% or more and 100% or less. However, the area ratio of 100% here naturally means that unavoidable impurities, carbides and inclusions are present in the steel, and strictly speaking, it is not 100%, but these unavoidable impurities are observed with an optical microscope. And 100% because inclusions exist at a level that cannot be recognized. As the remaining structure, one or more of ferrite, pearlite, bainite, and retained austenite may be contained in a total area ratio of 40% or less.

Next, the hot press molding method will be described.
The hot press molding is mass%, C: 0.05 to 0.35%, Si: 0.005 to 1.0%, Mn: 0 to 4.0%, Cr: 0 to 3.0%, Cu: 0 to 4.0%, Ni: 0 to 3.0%, B: 0.0002 to 0.1%, Ti: 0.001 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, and satisfies Mn + Cr / 3.1 + (Cu + Ni) /1.4≧2.5% Further, optionally, Mo: 0.01 to 3.0% by mass, or Nb: 0.01 to 3.0%, V: 0.001 to 3.0%, W: 0 by mass% 0.005 to 3.0%, or one or more, or% by mass, REM: 0.0005 to 0.01%, Y: 0.0005 to 0.01%, Ca: 0.0005 0.01%, Mg: A steel plate containing one or more of 0.0005 to 0.01%, the balance being Fe and unavoidable impurities is heated to 750 to 1300 ° C. and maintained for 10 to 6000 seconds. Two or more press moldings are performed at a temperature of 300 ° C. or higher.

  The heating rate of the steel sheet is preferably in the range of 1 to 100 ° C./second. If the rate of temperature increase is slower than 1 ° C./second, productivity is not preferable. On the other hand, it is difficult to increase the rate of temperature rise above 100 ° C./second with a normal furnace temperature increase. However, the effect of the present invention can be obtained even if heating is performed at a temperature rising rate exceeding 100 ° C./second by high-frequency heating or the like.

  As for the heating temperature, the press formability is improved even at about 600 ° C. However, in order to obtain a high strength of 980 MPa or more, it is necessary to obtain an austenite structure during heating and a martensite structure during cooling, so the lower limit is set to 750 ° C. . On the other hand, excessive heating causes coarsening of the steel sheet structure, an increase in scale, and an increase in heating cost, so the upper limit was set to 1300 ° C. The heating method is not particularly limited, but it is preferable to use an apparatus that can heat uniformly and quickly. Also, depending on the intended member strength, there is no problem even if the ferrite phase remains during heating.

  In addition, the heating time of 10 to 6000 seconds is that when the heating time is less than 10 seconds, a predetermined temperature is not obtained in the steel sheet, and a sufficient amount of austenite is not obtained in the steel sheet. Since predetermined strength could not be obtained after quenching, it was set to 10 seconds or more. Further, when heated for longer than 6000 seconds, the austenite grains become coarse, and a predetermined strength cannot be obtained after baking, and the heating cost is increased.

  The processing temperature during pressing is set to a temperature range of 300 ° C. or higher. The reason why the lower limit is set to 300 ° C. is that when the temperature is 300 ° C. or less, martensite starts to be generated, the strength of the steel sheet is increased, and the formability, particularly the shape freezing property is lowered. However, even if the steel plate is processed at a temperature of less than 300 ° C., the same mechanical characteristics are exhibited for the member itself.

  Even if the steel plate heated to 1300 ° C is taken out of the furnace and immediately formed, the processing temperature becomes 1100 ° C or lower due to part of the steel plate coming into contact with the mold, which is substantially higher than 1100 ° C. Molding with is difficult. Therefore, the molding temperature is substantially in the range of 300 to 1100 ° C. However, even if molding is performed in a temperature range exceeding 1100 ° C., it is possible to mold a member having a uniform material.

  Of the two or more transfer presses, the cooling rate at the time of press forming excluding the final press is not particularly limited, but in order to perform the press before the final press in a temperature range of 300 ° C or higher. , It is preferable to set the cooling rate as low as possible.

  It is also effective to hold the punch at the bottom dead center for 1 to 60 seconds after the final press forming. If the holding time at the bottom dead center is 1 second or more, baking will occur, but an excessive increase in the holding time results in a decrease in productivity and is not economically desirable, so the upper limit was 60 seconds. However, even if the holding is performed for more than 60 seconds, it is possible to produce a member having no material variation.

  The cooling rate after the final press molding is preferably 10 to 500 ° C./second. This is because if the cooling rate after the final press molding is less than 10 ° C./second, it takes an excessive amount of time for cooling, resulting in a decrease in productivity, which is economically undesirable. On the other hand, it is difficult to make the speed faster than 500 ° C./sec. In order to ensure sufficient hardenability and considering the safety of handling after press forming, a higher cooling rate is preferable. The cooling method at this time is basically held at the bottom dead center of the above-mentioned mold, but there is no problem even if cooling is performed using water or other liquid or gas in the mold after molding. Does not occur.

  It is also effective to immediately take out the molded product from the mold after the final press molding, and perform cooling using air cooling, a gas such as nitrogen gas, or a liquid such as water or an organic solvent. The reason why the cooling stop temperature is set to 5 to 40 ° C. is that, in order to keep the molded member at less than 5 ° C. or more than 40 ° C., a cooling device and a heat retaining device are separately required, which decreases productivity and economy. is there. In the present invention, the use of an Al-plated steel sheet or a Zn-plated steel sheet is effective for suppressing scale formation, but in that case, it is necessary to set the heating temperature below the boiling point of each plated alloy phase.

Next, the present invention will be described in more detail with reference to examples.
Steels having the components shown in Table 1 were melted and slab steel slabs were obtained by continuous casting according to a conventional method. Reference signs (steel Nos.) A to I are steels of components according to the present invention, and reference signs (steel Nos.) J to P are components that depart from the present invention. These steel pieces were heated to 1120 ° C. to 1280 ° C. in a heating furnace, hot rolled at a finishing temperature of 850 to 950 ° C., then cooled to 620 ° C. and wound up. The hot-rolled sheets were cold-rolled at a reduction rate of 50%, and were each made into a cold-rolled steel sheet as a material for hot pressing.

  The steel sheet thus manufactured was heated at a temperature increase rate of 1 to 100 ° C./second, heated at the temperature shown in Table 2 for 300 seconds, and then pressed twice or three times with a normal temperature mold. It was. For some parts, the bottom dead center was maintained under the conditions shown in Table 2 after the final press. The remaining members were immediately removed without holding at the bottom dead center.

  Thereafter, the microstructure of the molded members was examined and the characteristics were investigated. The material inspection was conducted according to the test method described in 2241 by processing a JIS Z 2201 No. 5 test piece from the press-formed part.

  Table 2 shows the structure observation results and tensile strength (TS) of the members thus obtained. No. Nos. 1 to 9 are examples of the present invention, and high strength after hot pressing was ensured, and a steel plate with little variation in mechanical properties was obtained without depending on the forming temperature. Moreover, there was no shape defect such as a spring back.

  On the other hand, no. 10-18 are outside the scope of the present invention for the following reasons. That is, no. 10 to 14 use JN, which is steel in which Mn + Cr / 3.1 + (Cu + Ni) /1.4 does not satisfy 2.5% or more of the present invention, so the martensite area in the structure after press forming The rate decreased and the desired high strength could not be obtained. No. In No. 15, since Ti and B were lower than the component range of the present invention, sufficient baking did not occur and the desired high strength could not be obtained. No. No. 16 had a C lower than the component range of the present invention, so that a sufficient amount of martensite structure was obtained, but the desired high strength could not be obtained. No. In No. 17, since the final pressing temperature was lower than the component range of the present invention, after the martensite was formed, that is, the pressing was performed with high strength, a large springback occurred and the desired shape could not be obtained. No. In No. 18, since the heating temperature was lower than the component range of the present invention, an austenite structure was not formed during heating. Therefore, a martensite structure was not obtained after press molding, and a desired high strength was not obtained.

  The present invention is used, for example, in the manufacture of high-strength steel members used for automobile undercarriages, chassis, members, collision safety reinforcing members, and the like.

Claims (7)

% By mass
C: 0.05 to 0.35%,
Si: 0.005 to 1.0%,
Mn: 0 to 4.0%,
Cr: 0 to 3.0%,
Cu: 0 to 4.0%,
Ni: 0 to 3.0%,
B: 0.0002 to 0.1%,
Ti: 0.001 to 3.0%,
P: 0.1% or less,
S: 0.05% or less,
Al: 0.005 to 0.1%,
N: 0.01% or less,
And a steel plate satisfying Mn + Cr / 3.1 + (Cu + Ni) /1.4≧2.5% and the balance being Fe and inevitable impurities is heated to 750 to 1300 ° C. Hot press with excellent formability, characterized in that after maintaining for 6000 seconds, press molding is performed twice or more at a temperature of 300 ° C. or more to obtain a member having a martensite structure of 60% or more in area ratio. A method for producing a high-strength steel member.   The method for producing a hot-pressed high-strength steel member with excellent formability according to claim 1, wherein the steel sheet further contains, by mass%, Mo: 0.01 to 3.0%. The steel sheet is further mass%,
Nb: 0.01 to 3.0%,
V: 0.001 to 3.0%,
W: 0.005 to 3.0%,
The method for producing a hot-pressed high-strength steel member having excellent formability according to claim 1 or 2, characterized by containing one or more of the following. The steel sheet is further mass%,
REM: 0.0005 to 0.01%
Y: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The manufacturing method of the hot press high-strength steel member excellent in the moldability in any one of Claims 1-3 characterized by containing 1 type (s) or 2 or more types of these.   The hot-pressed high-strength steel excellent in productivity according to any one of claims 1 to 4, wherein the steel sheet is heated to 750 to 1300 ° C at a heating rate of 1 to 100 ° C / sec. The manufacturing method of a member made.   The die punch is held at the bottom dead center for 1 to 60 seconds after the final press forming, and the steel sheet is cooled to 5 to 40 ° C at a cooling rate of 10 to 500 ° C / second. The manufacturing method of the hot press high-strength steel member excellent in the moldability in any one of -5.   After the final press molding, the mold punch is moved to the top dead center without being held at the bottom dead center, the molded product is taken out from the mold, and the cooling rate is 10 to 500 ° C./second to 5 to 40 ° C. The method for producing a hot-pressed high-strength steel member having excellent formability according to any one of claims 1 to 5, wherein cooling is performed.