JP5359925B2 - Manufacturing method of energy absorbing member having strength difference in member - Google Patents

Description

  The present invention relates to a method for manufacturing an energy absorbing member having a strength difference within the member.

  Conventionally, as a method for producing a member having regions having different strengths in one member, for example, a method of press forming after joining steel plates having different strengths with a tailored blank is known. However, the strength of the joint becomes too high due to the steel component, and there is a concern about the deterioration of formability due to a decrease in deformability.

  On the other hand, Patent Document 1 discloses a method of manufacturing a part having regions having different strengths in one member by hot pressing. That is, when heating the steel sheet by heat radiation, by installing a heat insulating material having a smaller thermal conductivity than the steel sheet, by partially entering the temperature range lower than the Ac3 point, the subsequent press work and The strength obtained by cooling is changed. Therefore, the aim is completely different from the present invention in which the entire member is heated to a temperature range higher than the Ac3 point.

JP 2009-61473 A

  Parts for automobiles, such as front side members, are divided into a part that efficiently absorbs energy at the time of collision and a part that ensures proof strength and transmits energy at the time of collision without deformation. Therefore, when forming integrally, press molding is implemented, for example, after welding the steel plate of the required steel plate intensity | strength with a tailored blank before shaping | molding.

  However, in order to further reduce the weight of the vehicle body and improve the energy absorption characteristics, it is necessary to increase the strength of the steel sheet, so not only press forming becomes more difficult, but especially with tailored blanks, the strength of the welded part Therefore, not only is there a risk of fracture at the weld during molding, but there is also a concern that the deformation characteristics at the time of collision may deteriorate due to the same cause.

  Therefore, there is a need for a method for manufacturing a part having different strength levels without including a welded part in one part.

Therefore, in view of such circumstances, the present inventors have completed the present invention as a technique for making a part for efficiently absorbing energy and a part for ensuring proof stress during press molding in one collision member.
The summary is as follows.

(1) By mass%, C: 0.19 to 0.35%, Si: 0.1 to 0.5%, Mn: 0.1 to 1%, P: 0.015% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.001 In the range of Ti: Nb, V, and Mo, Ti: 0.005-0.1%, Nb: 0.005-0.02%, V: 0.01-0.1%, Mo: 0.01-0.1% In total: 0.005 to 0.15%, and after heating the slab composed of the balance Fe and inevitable impurities, it is obtained by sequentially performing any one of the three manufacturing steps of hot rolling, cold rolling and continuous annealing. Steel plate,
When cooling after heating to a temperature range of Ac3 or higher, a quenching part that cools the temperature range of Ar3 to 300 ° C at a cooling rate of 200 ° C / s or more, and a slow cooling part that cools at a cooling rate of less than 150 ° C / s An energy absorbing member having a difference in strength within the member, wherein a difference in strength (ΔTS) between a relatively high strength quenching portion and a relatively low strength slow cooling portion is 490 MPa or more Manufacturing method.

  (2) In the above (1), after completion of any one of the above three steps, further plating is performed, and then the above-described treatment of cooling to a temperature range higher than the Ac3 point is performed. The manufacturing method of the energy absorption member which has an intensity | strength difference within a member.

  That is, in the present invention, a slab having a predetermined chemical composition is heated up to or above the Ac3 transformation point after completion of any one of the three steps of hot rolling → cold rolling → continuous annealing, and then the predetermined temperature range is set to the member region. This is a method in which a microstructure is formed separately by cooling at different predetermined speeds to form regions having different strengths in one member. Alternatively, after completion of any one of the above three steps, the above heating / cooling may be performed after further plating.

  That is, in the part forming process, after heating the steel sheet to a temperature range above the Ac3 transformation point, finish forming at a temperature range above the Ar3 transformation point and immediately cool it to form parts with different strengths. Is a method of manufacturing an energy absorbing member characterized by

  By adjusting the cooling conditions to be performed after hot stamping according to the present invention, so-called firing is performed to create different microstructures in one member, and the energy absorption performance at the time of collision is reduced, thereby reducing the weight of the vehicle body It contributes to.

It is a figure which shows the difference in the hardenability by the difference in cooling rate. It is a figure which shows a delayed fracture characteristic test piece.

First, the experiment that led to the completion of the present invention will be described.
The present inventors used a cold-rolled / annealed plate having a thickness of 1.6 mm having the components shown in Table 1, and investigated the relationship between the cooling rate and the Vickers hardness (load: 1 kgf). The obtained results are shown in FIG.

In other words, the strength is affected by the cooling rate by reducing or omitting elements that have been added to improve the hardenability, and steel A (conventional steel) is not slowly cooled to a cooling rate of less than 25 ° C / s. On the other hand, it was found that the difference in strength appears in the steel B (the steel of the present invention) at a cooling rate of less than 150 ° C./s, and the strength can be adjusted by the cooling rate.
The present invention has been completed based on these experimental facts.
The reason for limitation of the present invention will be described below.
First, the reason for limiting the content of component elements will be described.
In the following, a component element having a lower limit in content is an essential component, and a component element having no lower limit is an impurity.

(C: 0.19-0.35%)
C is an element that plays an important role in the present invention, and has a great influence on the strength after quenching. Therefore, 0.19% or more of addition is necessary to obtain a strength of 1470 MPa or more. On the other hand, if it exceeds 0.35%, breakage tends to occur at the time of impact deformation, and weldability deteriorates and the strength of the welded portion decreases, so this is the upper limit.

(Si: 0.1-0.5%)
Si is not only a solid solution strengthening element but also an element that suppresses precipitation of cementite, so 0.1% or more is added. On the other hand, if excessively added, as described later, when plating is performed, the plating property deteriorates, so 0.5% is made the upper limit.

[Mn: 0.1-1%]
Mn is one of the important elements for ensuring hardenability, and it is necessary to add 0.1% or more even at the cooling rate in the present invention. On the other hand, if over 1% is added, the hardenability increases and the strength cannot be kept low, so this is the upper limit.

[P: 0.015% or less]
P is a solid solution strengthening element and can raise the strength of the steel sheet relatively inexpensively, but segregates at the grain boundary, and if the strength is high, low temperature embrittlement becomes a problem, so 0.015% is the upper limit. And On the other hand, lowering the content of less than 0.001% is not preferable because the cost for removing P is extremely increased, so this is preferably set as the lower limit.

[S: 0.01% or less]
S is an element that affects the hot brittleness of steel, and is not only an element that deteriorates the workability particularly in the hot state, but also decreases the workability in the cold state. Therefore, the upper limit is 0.01%. However, when the content is less than 0.001%, the desulfurization cost is extremely increased, so this is preferably set as the lower limit.
Note that P and S are both inevitable impurities in the present invention.

[Al: 0.005-0.05%]
Al is added for deoxidation. If it is less than 0.005%, deoxidation becomes insufficient, and a large amount of oxide remains in the steel. In particular, the local deformability deteriorates and the characteristic variation also increases. On the other hand, if the content exceeds 0.05%, a large amount of oxide mainly composed of alumina remains in the steel, which also causes deterioration of local deformability, which is not preferable.

[N: 0.001 to 0.003%]
Lowering N too much is not preferable because it increases costs, so 0.001% is set as the lower limit. On the other hand, if the content exceeds 0.003%, inclusions are formed and the toughness after quenching deteriorates, so this is the upper limit.

[Total of Ti, Nb, V and Mo: 0.005 to 0.15%]
Ti, Nb, V, and Mo are elements added to refine the structure from the viewpoint of securing toughness. That is, if the total is less than 0.005%, the effect cannot be obtained. On the other hand, even if added over 0.15%, the effect is saturated and the cost increases, so this is the upper limit.

[Ti: 0.005-0.1%]
Ti has the effect of improving toughness when the steel sheet is heated to the Ac3 point or higher, because fine carbides are formed to suppress recrystallization and grain growth to make austenite grains fine. Therefore, the lower limit is 0.005%. On the other hand, adding over 0.1% not only saturates the effect but also increases the cost, so this is the upper limit.

(Nb: 0.005-0.02%)
Nb, as well as Ti, has the effect of improving toughness when the steel sheet is heated to the Ac3 point or higher, because the formation of fine carbides suppresses recrystallization and grain growth and makes austenite grains fine. Therefore, the lower limit is 0.005%. On the other hand, adding over 0.02% not only saturates the effect but also increases costs, so this is the upper limit.

[V: 0.01 to 0.1%]
V, like Ti and Nb, has the effect of improving toughness, because when the steel sheet is heated to the Ac3 point or higher, the formation of fine carbides suppresses recrystallization and grain growth and makes austenite grains fine. There is. Therefore, 0.01% is made the lower limit. On the other hand, adding over 0.1% not only saturates the effect but also increases the cost, so this is the upper limit.

[Mo: 0.01-0.1%]
Similar to V, Ti, and Nb, Mo also improves toughness by heating the steel plate to Ac3 point or higher to suppress the recrystallization and grain growth and make the austenite grains fine by forming fine carbides. There is an effect to. Therefore, 0.01% is made the lower limit. On the other hand, adding over 0.1% not only saturates the effect but also increases the cost, so this is the upper limit.

[Inevitable impurities]
B is an element that enhances hardenability, so it is basically not added, but it needs to be less than 0.0002% even when it is mixed from scraps.

  In addition, the steel components shown above used REM containing Ca, Ce, etc. as a deoxidizing element when elements such as Cu, Cr, Sn, Ni, Mo by using scraps in the steelmaking stage are contained. Even in the case, the effect of the present invention does not change at all. Further, the continuous casting method is not particularly specified, and the effect in the present invention is not changed by the normal continuous casting method or the thin slab method having a slab thickness of 100 mm or less.

Next, the manufacturing conditions of the present invention will be described.
The hot rolling conditions in the present invention may be within the range that is usually carried out. That is, the heating temperature should just be the conditions from which the deformation resistance which enables the rolling in the subsequent hot rolling process is obtained. Further, the finishing temperature may be carried out in the temperature range of Ar3 or higher, and the subsequent cooling conditions need not be specified, and the winding is carried out in a temperature range of 750 ° C. or lower. However, if the winding is performed at a temperature lower than 400 ° C., the hot-rolled sheet strength becomes too high, so this is the lower limit.

  The cold rolling conditions following the hot rolling, the annealing conditions, and the plating conditions are not particularly defined in the present invention, and may be carried out in a normal range. That is, the cold rolling is performed in the range of the cold rolling reduction that is normally performed, and specifically, it is performed at 40 to 80%. Plating is performed after pickling after hot rolling, or after cold rolling or after recrystallization annealing, but heating conditions and cooling conditions are not particularly specified. Furthermore, Zn or Al is also plated as the plating type, but the presence or absence of alloying is not particularly limited for Zn plating. As for Al plating, the inclusion of Si in the plating does not affect the present invention.

  The temper rolling after annealing or plating is not particularly specified, but is performed to adjust the shape appropriately. In that case, YP increases if implemented excessively, so the upper limit is 1%.

  For steel plates obtained under any conditions, in order to obtain a strength difference of 490 MPa or more at ΔTS in the steel plate, the portion between Ar3 point and 300 ° C to 200 ° C / s and the same temperature range 150 Must be less than ° C / s. That is, a structure having only martensite is obtained at a portion set to 200 ° C./s or more, but a structure containing no martensite is obtained at less than 150 ° C./s, and thus a strength difference of 490 MPa or more is obtained in ΔTS.

  After the steels with the components shown in Table 2 are produced in a converter and made into slabs, hot rolling is performed under the desired hot rolling conditions (heating temperature: 1220 ° C, finishing temperature: 870 ° C, winding temperature: 600 ° C). The result was a 3 mm hot rolled steel sheet. After making this hot-rolled steel sheet into a 1.4 mm cold-rolled steel sheet, continuous annealing was performed under the conditions shown in Table 3, or plating was performed after annealing. The plating treatment at that time was hot dip galvanization (GI (no alloying treatment) / GA (with alloying treatment)) or hot dip aluminum plating (Al) containing 10% of Si.

  After heating these steel sheets to 900 ° C in a laboratory heating furnace, they are sandwiched in a mold having a water supply outlet from which water is spouted and a drain outlet for sucking the water, and cooled to room temperature by injecting water. It was. At that time, a part where water ejection was partially stopped was made and cooled to room temperature while being sandwiched between molds.

  The cooling rate in the temperature range of Ar3 to 300 ° C. was 300 ° C./s in the water-cooled part where water was injected, and 100 ° C./s in the non-water-cooled part where water injection was stopped. The measuring method is as follows.

<Measurement method of cooling rate>
The cooling rate was obtained from the forming time (5 s) by measuring the surface temperature of the steel sheet before and after press forming using a radiation thermometer.

  Evaluation of strength after heat treatment was performed by cutting a sample from a part with different cooling conditions, processing it into a No. 5 test piece described in JIS Z 2201, and conducting a tensile test performed according to the test method described in JIS Z 2241. . The obtained results are shown in the same table.

  For the water-cooled part (high-strength part), delayed fracture characteristics and low-temperature toughness were also evaluated.

  For delayed fracture characteristics, use specimens with V-notches as shown in Fig. 2 and immerse them in an aqueous solution of 3 g / l of ammonium thiocyanate in 3% saline at room temperature for 24 hours. (No breakage: ○, with breakage: ×).

  On the other hand, for low-temperature brittleness, a Charpy test was conducted at -40 ° C., and a case where a ductile fracture surface ratio of 50% or more was obtained was determined to be acceptable (◯), and if it was less than 50%, it was determined to be unacceptable (x).

In the steel (A steel to S steel) according to the present invention, the portion where water was injected (water cooling portion), TS: strength of 1470 to 1960 MPa was obtained, and the portion where water ejection was stopped (non-water cooling portion) ), The strength is less than 980 MPa (100 kgf / mm ² ), and the strength difference (ΔTS) within one member is 490 MPa or more.

  On the other hand, T steel with a low C content has only obtained a strength of less than 1490 MPa even in the area where water was injected.

  In addition, in U steel with a high C content, ΔTS is 490 MPa or more, but the strength is 2450 MPa, and even in the part where water ejection is stopped, it exceeds 1180 MPa. Therefore, there are concerns about delayed fracture characteristics and low temperature brittleness degradation.

  In V and W steels with Si exceeding 1%, plating performance is poor (×).

  In addition, the X and Y steels with high Mn and the Z steel mixed with B have a strength of 1470 MPa even at the part where the water ejection is stopped, and ΔTS is less than 490 MPa. Outside.

  For steel C, steel K and steel O shown in Table 2, a hot-rolled steel sheet having a thickness of 2 mm under the desired hot-rolling conditions (heating temperature: 1250 ° C., finishing temperature: 880 ° C., coiling temperature: 550 ° C.) is used. After washing and heating to 930 ° C in a laboratory heating furnace as it is, it is sandwiched in a mold that has a water supply port from which water spouts and a drain port that sucks in the water, and cools to room temperature by injecting water. It was. At that time, similarly to Example 1, a part where water ejection was partially stopped was made and cooled to room temperature while being sandwiched between molds. Further, after pickling, hot-cooling treatment similar to that described above was performed after hot-dip aluminum plating containing 10% of zinc (GI, GA) or Si was performed. On the other hand, after forming a hot rolled steel sheet having a thickness of 3.2 mm under desirable hot rolling conditions (heating temperature: 1250 ° C., finishing temperature: 890 ° C., coiling temperature: 500 ° C.), 50% cold rolling after similar pickling 1.6mm cold rolled steel sheet, heated to 900 ° C in a laboratory heating furnace, and then cooled by a mold similar to the above, a part where water was partially stopped was created, It was cooled to room temperature while sandwiched between molds. About the obtained steel plate, the material characteristic similar to Example 1 was evaluated, and the obtained result is shown in Table 4. Since both are steels according to the present invention, there are no problems with delayed fracture characteristics and low temperature toughness, and the characteristics according to the present invention are obtained.

  According to the present invention, in the cooling step when cooling after heating to a temperature range of Ac3 point or higher, by changing the strength within the member by partially changing the cooling rate, the member having excellent energy absorption characteristics Manufacture is possible.

Claims (2)

In mass%, C: 0.19 to 0.35%, Si: 0.1 to 0.5%, Mn: 0.1 to 1%, P: 0.015% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.001 to 0.003% And one or more of Ti, Nb, V, and Mo are totaled in the ranges of Ti: 0.005-0.1%, Nb: 0.005-0.02%, V: 0.01-0.1%, Mo: 0.01-0.1%, respectively: A steel plate obtained by performing up to one of the three manufacturing steps of hot rolling, cold rolling and continuous annealing after heating a slab containing 0.005 to 0.15% and the balance Fe and inevitable impurities.
When cooling after heating to a temperature range of Ac3 or higher, a quenching part that cools the temperature range of Ar3 to 300 ° C at a cooling rate of 200 ° C / s or more, and a slow cooling part that cools at a cooling rate of less than 150 ° C / s An energy absorbing member having a difference in strength within the member, wherein a difference in strength (ΔTS) between a relatively high strength quenching portion and a relatively low strength slow cooling portion is 490 MPa or more Manufacturing method.   2. The process according to claim 1, wherein after the process up to any one of the above three steps is completed, the plating is further performed, and then the above-described treatment of heating and cooling to a temperature range of the Ac3 point or higher is performed. The manufacturing method of the energy absorption member which has an intensity | strength difference.

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