JP4452157B2 - 600-1200 MPa class high-strength member for automobiles with excellent strength uniformity in the member and method for producing the same - Google Patents

Description

  The present invention relates to a structural member, a reinforcing member, and a suspension member for automobiles having a maximum tensile strength (TS) of 600 MPa to 1200 MPa. Particularly, impact energy absorption characteristics and fatigue characteristics are required. It is suitable for a member.

  In recent years, the use ratio of high-strength steel plates called high tension has been increasing in order to reduce the weight of automobile members. However, the higher the strength of the steel plate, the lower the cold press formability, and it becomes extremely difficult to break the plate at a portion with a high degree of processing or to form the desired size and shape due to the springback phenomenon. This decrease in press formability is apparent when cold press forming a steel sheet of 600 MPa or more. The higher the steel sheet strength, the more remarkable the decrease in formability.

  As a means to solve this problem, the steel sheet is heated to the austenite region, hot pressed in a soft and highly ductile state, and at the same time, the martensite transformation is caused by rapid cooling in the formed mold. A method for producing a member having a high strength and a complicated shape is disclosed. Moreover, the technique which concerns on this hot press steel plate or hot press galvanized steel plate is disclosed by patent document 1 and patent document 2, for example.

  However, in the conventional method, it is difficult to manufacture a member having a strength of 600 to 1200 MPa, which accounts for the majority of the amount of the member used, or even if it can be manufactured, the strength non-uniformity in the member becomes extremely high, There is a problem that characteristics such as impact energy absorption characteristics and fatigue characteristics that have a strong correlation with non-uniformity in strength of the steel deteriorate.

JP 2003-147499 A JP 2003-73774 A

  The present invention provides an automotive structural member that has a complicated shape that is difficult to manufacture with a cold press and that has a low strength non-uniformity within the member and that has a strength of 600 to 1200 MPa in TS, easily and inexpensively. The purpose is to do.

  As a result of earnestly experimenting and studying to achieve the above object, the present inventors have used a steel plate material to which an appropriate amount of an alloy element has been added, which has been a problem in the conventional technology. It has been found that a member having a tensile strength of 600 to 1200 MPa with very little strength non-uniformity can be produced by a hot pressing method.

That is, the present invention is a method for producing a high-strength member for a 600 to 1000 MPa class automobile that is excellent in strength uniformity within the member,
1st invention is the mass%,
C: 0.05% or less,
Si: 1.5% or less,
Mn: 1.5~2.5%
Containing
Mo: 1.0% or less,
Cr: 10.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
B: 0.005% or less
1 type or 2 types or more of
P: 0.2% or less,
S: 0.1% or less,
N: not more than 0.02% , in addition to the above-mentioned components, including the following group a, the balance consisting of Fe and inevitable impurities, and A value = [% Mn] + 2 × [% Mo +% Cr] +0 Heating a steel sheet that satisfies 2.3 × [% Cu +% Ni] + 300 × [% B] of 2.3 or higher to an Ac ₃ temperature or higher, and then starts hot press forming at a temperature of Ar ₃ or higher. It is what.
Group a: 0.001 to 0.2 mass% of Al alone, or 0.001 to 0.2 mass% in total of Al and one or more of Nb, Ti, V, and Ta.

The second invention is, in addition to the invention and is characterized in that it comprises a lower Symbol b group.
Group b : 0.001 to 0.01% by mass in total of one or more of Ca, Mg, Zr, and REM.

  A third invention is characterized in that in the first or second invention, a steel plate on which an aluminum-based or zinc-based plating is applied is used.

  A fourth invention is a 600 to 1000 MPa class high-strength member for automobiles excellent in strength uniformity in a member produced by any one of the manufacturing methods of the first to third inventions, wherein the minimum hardness portion in the member The maximum hardness portion is a bainite structure.

Moreover, the present invention is a method for producing a high-strength member for a 600 to 1200 MPa class automobile excellent in strength uniformity within the member,
5th invention is the mass%,
C: 0.07% or less,
Si: 1.5% or less,
Mn: 1.5~2.5%
Containing
Mo: 1.0% or less,
Cr: 10.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
B: 0.005% or less
1 type or 2 types or more of
P: 0.2% or less,
S: 0.1% or less,
N: not more than 0.02% , in addition to the above components, including the following group a, the balance consisting of Fe and inevitable impurities, and [% Mn] + 2 × [% Mo +% Cr] + 0.5 × A steel sheet satisfying [% Cu +% Ni] + 300 × [% B] of 2.3 or higher is heated to an Ac ₃ temperature or higher, and then hot press forming is started at a temperature of Ar ₃ or higher. It is.
Group a: 0.001 to 0.2 mass% of Al alone, or 0.001 to 0.2 mass% in total of Al and one or more of Nb, Ti, V, and Ta.

A sixth invention, in addition to the invention and is characterized in that it comprises a lower Symbol b group.
Group b : 0.001 to 0.01% by mass in total of one or more of Ca, Mg, Zr, and REM.

  A seventh invention is characterized in that, in the fifth or sixth invention, a steel plate on which an aluminum-based or zinc-based plating is applied is used.

  An eighth invention is a 600 to 1200 MPa class high strength member for automobiles having excellent strength uniformity in a member produced by any one of the fifth to seventh manufacturing methods, wherein the minimum hardness portion and the maximum The hardness part consists of a mixed structure of a bainite structure and a martensite structure.

  INDUSTRIAL APPLICABILITY The present invention can easily and inexpensively provide a structural member, a reinforcing member, and a suspension member for automobiles having a high strength of 600 MPa to 1200 MPa with TS having excellent strength uniformity in the member. Furthermore, since the present invention uses the hot pressing method, it is possible to manufacture a member having a complicated shape that has been difficult to form by the cold pressing method.

  The present inventors have investigated the cause of increased strength non-uniformity in a member when a member having a tensile strength of 600 to 1200 MPa is made by a hot press molding method. As a result, the cooling rate of the steel sheet in the mold varies greatly depending on the location. I found out.

  For example, in a certain component system, a portion where a cooling rate of 150 ° C./s or more is obtained by contact with the mold has a martensite structure of 1000 MPa, but is not in contact with the mold in the vicinity thereof. In addition, the portion considered that only a cooling rate of about 30 ° C./s was obtained, the ferrite phase was mixed, and the strength was 600 MPa or less.

  Therefore, the present inventors suppressed the formation of a soft ferrite phase or a pearlite phase at a cooling rate of 30 ° C./s, which was considered to be the slowest cooling rate of the steel sheet in the mold, and further increased the cooling rate. Therefore, many experiments and examinations were repeated in recognition that it was important to solve the problem by making the component difficult to form a hard martensite phase.

  As a result, as shown in FIG. 1, it was found that it is possible to reduce the decrease in the hardness or the tensile strength accompanying the change in the cooling rate by setting the components within an appropriate range, and the present invention has been achieved.

  The present invention is described in detail below.

  First, the reasons for limiting the components will be described. In addition,% means the mass%.

  C: C is used for adjusting the strength of the member. However, if it exceeds 0.07%, it becomes difficult to ensure both the strength range of 600 to 1200 MPa and the strength uniformity accompanying the change in the cooling rate in the mold. For this reason, the range was limited to 0.07% or less.

  As a guideline for the amount of C that can ensure the strength uniformity accompanying the change in the cooling rate in the mold, C: 0.05% or less for the production of a member of TS: 1000 MPa or less, C for the production of a member of TS: 800 MPa or less. : 0.04% or less is preferable.

  Although a minimum is not specifically limited, From a viewpoint of steelmaking cost, it is desirable that it is 0.0005% or more. From the viewpoint of suppressing the formation of ferrite or pearlite phase as much as possible, C is preferably 0.04% or less.

Si: Si, like C, is used to adjust the strength of the member. However, if it exceeds 1.5%, the descaling property is deteriorated, and the Ac ₃ temperature rises, resulting in an increase in production cost. Therefore, the Si content is limited to a range of 1.5% or less. Although a minimum is not specifically limited, From a viewpoint of steelmaking cost, it is desirable to contain 0.0005% or more.

  Mn: Mn is one of the most important elements in the present invention, and is used for adjusting the strength of the member and reducing the dependency of the strength on the cooling rate. Since Mn is inexpensive compared with the effect, it is preferable to use a large amount.

  However, if it is less than 1.0%, it is necessary to use a large amount of alloy elements such as Cr, Mo, Ni and Cu in order to sufficiently suppress ferrite transformation or pearlite transformation in the low cooling rate portion, which is extremely costly. Become high. Moreover, when it exceeds 5.0%, the spot weldability of a member will deteriorate.

  For this reason, the appropriate range of Mn content was limited to the range of 1.0 to 5.0%. In addition, from the viewpoint of manufacturing cost, it is more preferable to add 1.5% or more, and from the viewpoint of preventing processing cracks accompanying macrosegregation of the steel sheet, the content thereof is 2.5% or less. More desirable.

  Mo: Mo is mainly used as an alternative element for Mn in order to reduce the dependency of strength on the cooling rate. However, if it exceeds 1.0%, the cost becomes high. For this reason, the appropriate range of Mo content was limited to 1.0% or less, preferably 0.5% or less.

  Cr: Cr is mainly used as a substitute for Mn in order to reduce the dependence of strength on the cooling rate. It is also effective for improving the oxidation resistance when heating the steel sheet. However, if it exceeds 10.0%, the cost increases, so the appropriate range of Cr content is limited to 10.0% or less. In the case where the oxidation resistance is not required, it is preferably 2.0% or less from the viewpoint of cost.

  Cu: Cu is mainly used as a substitute for Mn in order to adjust the strength and reduce the cooling rate dependency. However, if it exceeds 1.0%, hot working cracks occur during the production of the steel sheet, and the quality of the steel sheet is deteriorated. For this reason, the appropriate range of Cu content was limited to 1.0% or less.

  Ni: Ni is mainly used as a substitute for Mn in order to reduce the dependence of strength on the cooling rate. However, if it exceeds 1.0%, the cost becomes high. For this reason, the appropriate range of Ni content was limited to 1.0% or less.

  B: B is mainly used as an alternative to Mn in order to reduce the dependence of strength on the cooling rate. However, if it exceeds 0.005%, cracks occur during hot press molding due to precipitation of coarse borides or carbocarbides. For this reason, the appropriate range of B content was limited to 0.005% or less.

Although a minimum is not specifically limited, 0.00005% or more shall be contained as an unavoidable impurity. B is an element that increases the A value and decreases Ar ₃ when added in a small amount, so it is desirable to add 0.0003% or more in view of cost.

  P: P is mainly used for adjusting the strength of members. If it exceeds 0.2%, secondary processing cracks become prominent, so the range of P content is set to 0.2% or less. Although a minimum is not specifically limited, 0.0003% or more shall be contained as an unavoidable impurity.

  S: S is an impurity, and if it is contained in a large amount, it causes hot working cracks at the time of steel plate production or breakage during hot pressing, so it was made 0.1% or less. Although a minimum is not specifically limited, 0.0003% or more shall be contained as an unavoidable impurity.

  N: N is mainly used for crystal grain size control and strength adjustment in the austenite region. However, if N exceeds 0.02%, it becomes difficult to reduce the dependency of strength on the cooling rate, so the range of N content is set to 0.02% or less.

  [% Mn] + 2 × [% Mo +% Cr] + 0.5 × [% Cu +% Ni] + 300 × [% B] value (A value): This value is the most important parameter in the present invention. When this value is less than 1.8, formation of a soft ferrite phase or a pearlite phase cannot be suppressed in a portion having a cooling rate of 30 ° C./s, which is considered to be the slowest cooling rate in the member. Therefore, the appropriate range is limited to 1.8 or more.

  In addition, formation of soft phases, such as a ferrite and a pearlite phase, can be suppressed, so that a cooling rate is so low that this value is large. Therefore, if the low cooling rate portion cannot be avoided due to the design of the mold, it is necessary to increase this value. Since a part where the minimum cooling rate in the mold is about 10 ° C./s is also assumed, it is preferably 2.0 or more, and more preferably 2.3 or more.

  In the present invention, in addition to the above-described components, the object of the present invention can also be achieved by containing one or two groups out of groups a and b.

  Group a: 0.001 to 0.2% of the total of one or more of Nb, Ti, V, Ta, and Al.

  Since Nb, Ti, V, Ta, and Al are used as deoxidizing elements or carbonitride-forming elements to adjust the strength of steel materials, the total of one or more types is 0.001% or more. It is preferable to contain. However, if the total exceeds 0.2%, the cost becomes high. Therefore, the range of the total amount is set to 0.001 to 0.2%.

  Group b: 0.001 to 0.01% in total of one or two of Ca, Mg, Zr, and REM.

  Ca, Mg, Zr, and REM are elements used for deoxidation, and it is preferable to contain one or two kinds in total in an amount of 0.001% or more. However, if the total content exceeds 0.01%, the moldability is deteriorated. Therefore, the total amount range is set to 0.001 to 0.01%. In the present invention, REM refers to La and lanthanoid series elements.

  In addition, 0.01% or less of O may be included as other inevitable impurities.

  Next, the reason for limitation of the hot press molding method will be described.

When the heating temperature is less than the Ac ₃ temperature, it becomes difficult to produce a member having excellent strength uniformity and a strength of 600 to 1200 MPa, so the appropriate range is limited to Ac ₃ or higher. The upper limit of the heating temperature is not particularly defined, but is preferably set to 1000 ° C. or less from the viewpoint of avoiding scale formation during heating of the steel sheet.

In addition, if the hot press molding start temperature is less than Ar ₃ , soft phases such as ferrite and pearlite phases tend to be formed unevenly in the member, and the strength non-uniformity tends to increase. The appropriate range of the press molding start temperature was limited to Ar ₃ or more. From the viewpoint of avoiding the formation of the soft phase as much as possible, Ar ₃ + 50 ° C. or higher is more desirable.

The Ac ₃ temperature and the Ar ₃ temperature can be simply calculated from the chemical composition (mass%) using the following formula.
^{_{Ac 3 = 910-203 * C 1/2 +}} 45 * Si-30 * Mn + 700 * P-15 * Ni- 20 * Cu-11 * Cr + 10 * Mo
_{Ar 3 = 900-325 * C + 30Si} + 40Al-90 * (Mn + Cu) -50Ni- 50000 * B

  The upper limit of the press molding start temperature is not particularly defined, but is preferably set to 1100 ° C. or less from the viewpoint of avoiding plate breakage during hot press molding. Here, the start of molding refers to the moment when the steel plate is placed on the mold.

  In the present invention, it is important that a place where quenched and slowly cooled in a hot press mold have a mixed structure of a bainite structure and a martensite structure. When TS is 1000 MPa or less, it is preferably a structure having a bainite structure fraction of 80% or more, and more preferably a bainite single-phase structure from the viewpoint of further improving the uniformity in the member.

  Also when TS exceeds 1000 MPa and is equal to or less than 1200 MPa, the bainite structure fraction is preferably 50% or more from the viewpoint of further improving the uniformity within the member. Here, bainite refers to lath-shaped bainite and granular bainite.

  Note that the bainite phase is identified as follows: bainite photo collection of steel 1: [continuous cooling (intermediate stage) transformation structure of low carbon steel Atlas for Bainitic Microstructures Vol. Year].

  In addition, when C exceeds 0.04%, it may be difficult to distinguish between the lath bainite and the martensite phase. In this case, the microstructure is observed by transmission electron microscopy, and some carbide precipitation occurs. If it is found, it shall be classified as lath bainite.

  The raw steel plate for hot press forming may be either a hot-rolled steel plate, a cold-rolled steel plate, or a cold-rolled annealed plate. The effect of the present invention can be achieved. However, from the viewpoint of ensuring plating quality after hot pressing, it is more desirable that the main component of plating is aluminum or zinc.

  The manufacturing method of the steel plate and member of the component which concerns on this invention is applicable also when performing hot pressing, after joining the steel plate of different intensity | strength. For example, after mold cooling, by joining a steel plate having a component of 700 MPa and a steel plate having a component of 1500 MPa according to the present invention, and then performing hot press molding under the conditions indicated in the present invention, It becomes possible to manufacture an integral part (tailored blank part) having completely different strengths.

  In the present invention, excellent strength uniformity means that the portion having the minimum tensile strength (or hardness) among the portions cooled in the mold is 80% or more of the maximum tensile strength (or hardness). It points to something. From the viewpoint of stably absorbing shock absorption energy or obtaining good deformation resistance, it is more preferably 85% or more.

  As the evaluation method, the steel sheet was heated to 920 ° C. and then cooled to room temperature at a cooling rate of 180 ° C./s, and the maximum tensile strength of the steel plate was gradually cooled in the mold. It is quantitative and preferable that the maximum tensile strength of what is cooled to room temperature at a cooling rate of 30 ° C./s is TSmin, the value of TSmin / TSmax is obtained, and this value is evaluated. is there.

Alternatively, when the Vickers hardness (load 10 kgf) of the molded member is measured, and the maximum hardness is Hv _max and the minimum hardness is Hv _min among the parts cooled in the mold, Hv _min A value of / Hv _max may be obtained and this value may be evaluated.

  As a method for heating the steel sheet, either a method of charging in a heating furnace or a method by high frequency induction heating may be used. The hot press forming method for producing this member may be any of deep drawing, overhang forming, stretch flange forming, bending deformation, or a method in which these deformation modes are combined.

  Next, the present invention will be described in detail with reference to examples.

  In order to simulate heating and cooling in a hot press die, steel plates A to H having the components shown in Table 1 were varied in heating temperature and cooling rate using a thermal history simulator. The structure and the material under each condition were measured. The material was evaluated using a JIS No. 5 test piece.

  In addition, the steel sheet was actually heated to 920 ° C., a hot press test was performed using a hat-shaped mold (length: 300 mm, length of one side of the cross section: about 50 mm), and then the strength almost the same as the member The test material which covered the remainder with the steel plate which has this was produced, the weight was dropped from the upper part, and the experiment which measured the crush shape and absorbed energy of a test material was done. A test material that was crushed into a bellows shape and sufficiently absorbed impact energy was judged as ◯.

  The impact absorption energy of the member of the present invention was 20% or more larger than the impact absorption energy of the member having the same TSmax with large non-uniform strength.

  The strength uniformity was determined by the following evaluation. That is, the steel sheet was heated to 920 ° C and then cooled to room temperature at a cooling rate of 180 ° C / s, corresponding to the rapidly cooled portion, and the maximum tensile strength was TSmax, which was gradually cooled in the mold. Correspondingly, the maximum tensile strength of what was cooled to room temperature at a cooling rate of 30 ° C./s was defined as TSmin, and the value of TSmin / TSmax was determined. The results are shown in Table 2.

  The closer this TSmin / TSmax is to 1, the higher the strength uniformity of the member is. The value of 0.80 or more indicates a bellows-like crushing form in the above crushing test, and the impact absorption energy is high. It showed that.

As a method for evaluating the strength uniformity, the Vickers hardness (load 10 kgf) of the molded member is measured, the maximum hardness of the parts cooled in the mold is Hv _max , and the minimum hardness is When the thickness is Hv _min , a value of Hv _min / Hv _max may be obtained and this value may be evaluated.

  No. 6 and no. No. 7 has a C component and [% Mn] + 2 × [% Mo +% Cr] + 0.5 × [% Cu +% Ni] + 300 × [% B] values that are outside the scope of the present invention. Is formed, the strength non-uniformity in the member is large, the crushing form is also bad, and sufficient impact energy cannot be absorbed.

No. No. 9 is an example in which the strength of 600 MPa was not obtained because the heating temperature of the steel sheet was not more than the Ac ₃ temperature even though the components were within the appropriate range. No. No. 10 is an example in which the strength of 600 MPa or more could not be obtained because the hot press start temperature was not higher than the Ar ₃ temperature even though the components were within the appropriate range.

  As described above, the present invention can easily and inexpensively provide a structural member, a reinforcing member, and a suspension member for automobiles having high strength of 600 MPa to 1200 MPa with TS having excellent strength uniformity in the member. Furthermore, since the hot press method is used, it is possible to manufacture a member having a complicated shape, which is difficult to form by the cold press method. Therefore, the present invention has extremely high industrial applicability.

It is a figure which shows the hardness of a steel plate when this invention steel (steel D) and a comparative steel (steel H) are cooled to room temperature with the cooling rate of 0.1-180 degreeC / s after heating at 920 degreeC for 5 minutes. Yes (when the hardness after cooling at 180 ° C./s and 30 ° C./s is compared, the change in hardness is greatly suppressed in the steel of the present invention compared to the comparative steel).

Claims (8)

% By mass
C: 0.05% or less,
Si: 1.5% or less,
Mn: 1.5 to 2.5%
Containing
Mo: 1.0% or less,
Cr: 10.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
B: 0.005% or less
1 type or 2 types or more of
P: 0.2% or less,
S: 0.1% or less,
N: not more than 0.02% , in addition to the above components, including the following group a, the balance consisting of Fe and inevitable impurities, and [% Mn] + 2 × [% Mo +% Cr] + 0.5 × A member characterized in that a steel sheet satisfying [% Cu +% Ni] + 300 × [% B] of 2.3 or more is heated to an Ac ₃ temperature or higher, and then hot press forming is started at a temperature of Ar ₃ or higher. The manufacturing method of the 600-1000 MPa class high strength member for motor vehicles which is excellent in the intensity | strength uniformity in the inside.
Group a: 0.001 to 0.2 mass% of Al alone, or 0.001 to 0.2 mass% in total of Al and one or more of Nb, Ti, V, and Ta. In addition to the above components, the production method according to claim 1, wherein the 600~1000MPa class automobile high-strength member excellent in strength uniformity in member, characterized in that it comprises a lower Symbol b group.
Group b : 0.001 to 0.01% by mass in total of one or more of Ca, Mg, Zr, and REM.   3. A method for producing a high strength member for a 600 to 1000 MPa class automobile having excellent strength uniformity in the member according to claim 1 or 2, wherein the steel plate is subjected to aluminum or zinc plating.   It is the member manufactured by the manufacturing method of any one of Claims 1-3, Comprising: The minimum hardness part in a member and the maximum hardness part are bainite structures, The intensity | strength uniformity in the member 600-1000 MPa class high-strength member for automobiles with excellent properties. % By mass
C: 0.07% or less,
Si: 1.5% or less,
Mn: 1.5 to 2.5%
Containing
Mo: 1.0% or less,
Cr: 10.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
B: 0.005% or less
1 type or 2 types or more of
P: 0.2% or less,
S: 0.1% or less,
N: not more than 0.02% , in addition to the above components, including the following group a, the balance consisting of Fe and inevitable impurities, and [% Mn] + 2 × [% Mo +% Cr] + 0.5 × A member characterized in that a steel sheet satisfying [% Cu +% Ni] + 300 × [% B] of 2.3 or more is heated to an Ac ₃ temperature or higher, and then hot press forming is started at a temperature of Ar ₃ or higher. The manufacturing method of the 600-1200 MPa class high strength member for motor vehicles excellent in the intensity | strength uniformity in the inside.
Group a: 0.001 to 0.2 mass% of Al alone, or 0.001 to 0.2 mass% in total of Al and one or more of Nb, Ti, V, and Ta. Wherein in addition to the ingredients, production method of 600~1200MPa class automobile high-strength member excellent in strength uniformity in the member according to claim 5, characterized in that it comprises a lower Symbol b group.
Group b : 0.001 to 0.01% by mass in total of one or more of Ca, Mg, Zr, and REM.   The method for producing a 600 to 1200 MPa class high strength member for automobiles excellent in strength uniformity in the member according to claim 5 or 6, wherein the steel plate is subjected to aluminum or zinc plating.   It is the member manufactured by the manufacturing method of any one of Claims 5-7, Comprising: The minimum hardness part and maximum hardness part in a member consist of a mixed structure of a bainite structure and a martensitic structure. A high-strength member for a 600 to 1200 MPa class automobile having excellent strength uniformity in the member.

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