JP2007308745A - High strength member and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength member having improved toughness and ductility and also to provide its manufacturing method. <P>SOLUTION: The high strength member can be obtained by heating a high strength steel sheet up to a temperature in an austenite region and then applying press forming and cooling in a die, and the high strength steel sheet has a composition containing 0.1 to 0.4% C, 0.1 to 2.0% Mo and 0.1 to 2.0% Cr and also has a structure containing precipitates composed of metal carbides of 0.01 to 5μm grain size in the steel. In the method for manufacturing the high strength member, the high strength steel sheet is heated up to a temperature in an austenite region by sandwiching both faces of the high strength steel sheet between induction heated steel materials and is then press formed and cooled in a die to obtain the desired high strength member. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength member and a manufacturing method thereof, and more particularly to a high-strength member having improved toughness and a manufacturing method thereof.

Up to now, in order to achieve both environmental problems and vehicle collision safety and to improve the characteristics thereof, the structural members of the vehicle body have been increased in strength and weight. As a measure for increasing the strength of such a vehicle body structural member, a higher strength steel plate has been applied.
Car body structural members are press formable from the viewpoint of manufacturability, spot weldability from the viewpoint of strength reliability, and further tough ductility (toughness and ductility in the member state in order to be stably deformed at the time of vehicle collision. (Compatibility) has been demanded. And when toughness is low, it has been limited to application in a portion where there is no direct large input during impact loading.

On the other hand, as a high-strength member, the steel sheet is heated, the heated steel sheet is press-molded with a mold, and the mold is cooled by contact between the mold and the steel sheet at the time of molding. Hot press-formed members have been developed, and steel plates and members for hot pressing have been proposed (see Patent Documents 1 and 2).
JP 2005-126733 A JP 2006-9116 A

  However, the steel sheets and members for hot pressing described in Patent Documents 1 and 2 described above have poor tensile toughness and stable deformation at the time of vehicle collision, although the material after hot press forming exhibits a tensile strength of about 1200 MPa to 1600 MPa. There was a problem that was difficult.

  This invention is made | formed in view of the subject which such a prior art has, and the place made into the objective is to provide the high strength member which improved toughness, and its manufacturing method.

  As a result of intensive studies to achieve the above object, the present inventors have found that carbon (C): 0.1 to 0.4%, molybdenum (Mo): 0.1 to 2.0%, and chromium (Cr ): A high-tensile steel sheet having a structure containing 0.1 to 2.0% and containing precipitates made of metal carbide having a particle diameter of 0.01 to 5 μm in the steel is heated to the austenite region, The inventors have found that the above object can be achieved by press molding and cooling, and have completed the present invention.

  That is, the high-strength member of the present invention is formed by heating a high-tensile steel plate to an austenite region, press-molding and cooling in a mold, and the high-strength steel plate is C: 0.1 to 0.4%, Mo : 0.1 to 2.0% and Cr: 0.1 to 2.0%, and having a structure containing precipitates made of metal carbide having a particle size of 0.01 to 5 μm in steel. Features.

  Further, in the production method of the high-strength member of the present invention, the high-strength member of the present invention is heated in the austenite region by sandwiching both surfaces of the high-tensile steel plate with the induction-heated steel material. And press-molding and cooling to obtain a desired high-strength member.

  According to the present invention, it contains C: 0.1 to 0.4%, Mo: 0.1 to 2.0% and Cr: 0.1 to 2.0%, and 0.01 to 5 μm in the steel. A high-strength member with improved toughness, such as heating a high-tensile steel sheet having a structure containing precipitates of metal carbide having a particle size of in the austenite region, press forming and cooling in the mold, And a manufacturing method thereof.

Hereinafter, the high strength member of the present invention will be described in detail. In the present specification and claims, “%” for concentration, content, etc. represents mass percentage unless otherwise specified.
As described above, the high-strength member of the present invention is obtained by heating a high-tensile steel plate to the austenite region, press-forming and cooling in a mold.
And this high-tensile steel plate contains C: 0.1-0.4%, Mo: 0.1-2.0% and Cr: 0.1-2.0%, 0.01 in steel It has a structure containing precipitates made of metal carbide having a particle diameter of ˜5 μm.
By setting it as such a structure, it becomes a high strength member which improved toughness.

Here, when heating to the austenite region, the heating method is not particularly limited as long as a desired high-strength member is obtained, but it is preferable to heat to about Ac3 point temperature to about Ac3 point temperature + 200 ° C.
When the heating temperature is lower than the Ac3 point temperature, reverse transformation cannot be performed, and it is difficult to obtain a high-strength member having excellent toughness. On the other hand, when the heating temperature is higher than the Ac3 point temperature + 200 ° C., austenite becomes coarse, which is not preferable.

  In addition, a high strength member having excellent tough ductility is obtained by refining the prior austenite grain size from the starting structure in the finely precipitated state and performing phase transformation by press molding and cooling in the mold after heating. Can be obtained. Below, the component elements etc. in a high-tensile steel plate are demonstrated.

C: C is the most effective element for increasing the strength. In order to obtain a strength of 980 MPa or more, it is preferable to contain 0.1% or more, but if it exceeds 0.4%, it tends to cause toughness deterioration, so that it contains 0.10 to 0.40%. did.

Mo: Mo is an important element in the high-tensile steel sheet to be used, and is effective in stably generating martensite by cooling after heating the steel sheet. Moreover, it is effective for atomization by forming alloy carbide. Such an effect appears at 0.10% or more. On the other hand, Mo is an expensive alloy element. Therefore, the content is 0.1 to 2.0%. And what makes 0.5 to 2.0% is preferable.

Cr: Cr is an element effective for improving the hardenability, and is an element effective for increasing the strength of the steel sheet by solid solution in cementite. Therefore, in order to ensure hardenability and strength, the content is 0.1% or more. On the other hand, since the effect will be saturated and toughness will fall when it adds excessively, the upper limit was made 2.0%.

By making the metal (alloy) carbide finely dispersed in the structure of the high-tensile steel plate before hot pressing, the alloy carbide in the high-strength member obtained after reheating and cooling is It becomes uniform and fine and can further improve toughness.
However, if the particle size of the metal carbide in the high-tensile steel plate is less than 0.01 μm, the effect cannot be expected, and if it exceeds 5 μm, it is too coarse and lowers the toughness.

Moreover, in this invention, it is preferable that the prior austenite particle size in the structure | tissue of the said high strength member is 1-10 micrometers.
Thereby, the toughness can be further improved. When the prior austenite particle size is less than 1 μm, the mechanical properties are likely to deteriorate, and the production tends to be difficult. On the other hand, when the prior austenite particle size exceeds 10 μm, the effect of improving moldability such as deep drawability, stretchability, and shape freezeability becomes small.

Furthermore, in the present invention, it is desirable that the high-strength member has a structure containing precipitates made of metal carbide having a particle size of 2 μm or less.
Thereby, the toughness can be further improved. If the particle size of the metal carbide exceeds 2 μm, the aperture may be lowered. On the other hand, when the particle size of the metal carbide is less than 0.01 μm, it is difficult to obtain a remarkable effect.

  Moreover, in this invention, it is preferable that the tensile strength of the said high strength member is 1400 Mpa or more. When such a member is used, there is an advantage that when applied to a vehicle body structure of an automobile, the weight can be reduced while maintaining high strength.

Furthermore, in the present invention, a tailored blank steel plate can be used as the high-tensile steel plate.
For example, the strength of the member can be increased more accurately by matching the thickness distribution of the tailored blank steel sheet with the strength distribution required for the member.
The thickness of the high-tensile steel plate to be used may be the same as that of a conventional high-tensile steel plate and may be 0.6 to 4 mm. However, since the strength is higher, it is possible to obtain the same strength with a thin plate thickness. it can.

  Moreover, in this invention, the said high strength member may have a resistance welding part. That is, the high-strength members of the present invention or the high-strength members of the present invention and other members can be assembled by resistance welding, so that it can be easily applied as a vehicle body structural member of an automobile. it can.

Furthermore, in the present invention, the high-strength member may have a structure tempered at 150 to 700 ° C, and preferably has a structure tempered at 400 to 700 ° C.
In such a high-strength member, Mo is secondarily cured and precipitated, and the toughness can be further improved. In particular, when it has a structure tempered at 400 to 700 ° C., the delayed fracture resistance due to metal deposition of Mo or the like can be improved.

  In addition to the above-described component elements, various elements can be added to the above-described high-tensile steel plate as long as the desired effects are not hindered. Hereinafter, the additive element will be described.

Silicon (Si): Si is an element effective for deoxidation and strength increase. Therefore, it is preferable that the content is 0.2% or more including those added as a deoxidizer and remaining in the steel. However, since excessive addition may cause toughness deterioration, the upper limit is preferably 2.5%.

Manganese (Mn): Mn is an element effective for increasing the strength of the steel sheet. If it is less than 0.1%, it is difficult to obtain a desired effect. On the other hand, when the content is too large, not only co-segregation of P and S is promoted, but also toughness deterioration may be caused.

Phosphorus (P): P is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably made 0.02%.

Sulfur (S): S is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably set to 0.01%.

Copper (Cu): Cu is effective for strengthening, and its fine precipitation contributes to the improvement of delayed fracture, so it is preferable to contain 0.1% or more. Moreover, since excessive addition causes deterioration of workability, the upper limit is preferably set to 3.0%.

Nickel (Ni): Ni is an element effective in improving the corrosion resistance while ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet. If it is less than 0.1%, the desired effect cannot be obtained. On the other hand, if it exceeds 3.0%, the workability deteriorates, so 0.1 to 3.0% is preferably contained.

  Furthermore, it is preferable to contain vanadium (V), titanium (Ti), niobium (Nb), etc., in order to ensure good delayed fracture resistance while ensuring moldability.

Aluminum (Al): Al is added for deoxidation, but if the amount added is too large, inclusions increase and workability deteriorates, so 0.001 to 0.1% is preferably contained.

  As a suitable example of the high-tensile steel plate as described above, C: 0.10 to 0.40%, Mo: 0.1 to 2.0% and Cr: 0.1 to 2.0%, For tungsten (W), vanadium (V), titanium (Ti) or niobium (Nb), and any combination thereof, W: 0.20-1.5%, V: 0.002-1. 0%, Ti: 0.002 to 1.0%, and Nb: 0.005 to 1.0%. Further, phosphorus (P) and sulfur (S), which are impurities, are contained in P: ≦ 0. The content is 02%, S: ≦ 0.01%, and the balance may be substantially iron (Fe) and inevitable impurities, but is not necessarily limited thereto.

Next, the manufacturing method of the high strength member of this invention is demonstrated in detail.
As described above, the manufacturing method of the high-strength member of the present invention, in manufacturing the high-strength member of the present invention, is heated to the austenite region by sandwiching both surfaces of the high-tensile steel plate with the steel material induction-heated, This is a method of press molding and cooling in a mold.
By adopting such a configuration, the steel sheet can be heated uniformly, in a short time with high temperature accuracy, and further in a state in which oxidation of the steel sheet surface is suppressed, but the high-strength member of the present invention is manufactured in this way. It is not limited to what was produced by the method.
In addition, the stable heating can be performed by using a steel material having a sufficiently large heat capacity as compared with the heat capacity of the steel sheet.
Further, the heat capacity of the steel material can be increased, for example, by increasing the thickness of the steel material.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

(Examples 1-9 and Comparative Examples 1-3)
Using the high-tensile steel plate having the chemical composition shown in Table 1, high-strength members of each example were produced under the heating conditions of the steel plate shown in Table 2.
Specifically, the steel plate is inserted into an oven maintained at the heating and holding temperature shown in Table 2, and then held for 120 seconds after reaching the predetermined heating and holding temperature shown in Table 2. After that, with a mold It was press-molded into a hat shape and cooled in the mold as it was.
The obtained high-strength member has a rectangular shape with a length of 50 mm and a width of 70 mm and a hat shape with a flange width of 30 mm and a length of 900 mm.

[Performance evaluation]
The high-strength member of each example was subjected to mechanical property evaluation and structural investigation. The obtained results are shown in Table 3. Each performance evaluation was performed as follows.

(1) Tensile strength The tensile strength (TS) of the part corresponding to the bottom of the press punch was evaluated by conducting a tensile test in accordance with JIS Z2241, using No. 5 test piece of JIS Z2201.

(2) Degree of stress reduction FIG. 1 is a schematic diagram showing a stress-strain diagram by a tensile test using a plate-like test piece (for example, a No. 5 test piece or a No. 13 test piece defined in JIS Z2201). . The difference between the tensile strength (TS) and the breaking stress is defined as the degree of stress reduction (SD).
In the above stress-strain diagram, those exhibiting uniform elongation and having a value of the stress reduction (SD) of 180 MPa or more until rupture had good toughness.

(3) Presence / absence of cracks during the member bending test A flat plate having a width of 130 mm and a length of 900 mm was spot-welded to the obtained high-strength member, and a member having a closed cross section was subjected to an impact three-point bending test. In the impact three-point bending test, the span was 700 mm, the pusher was a rectangle having a width of 80 mm, and the displacement was pushed to 100 mm at a displacement speed of 5 m / s.

In addition, it was confirmed by electron microscope observation that the high-tensile steel plates A to C had Mo alloy carbide precipitated in the range of 0.01 to 5 μm before heating. On the other hand, in the high-strength steel plates D and E, the presence of alloy carbides was not confirmed by electron microscope observation.
Further, the prior austenite particle size in the high-strength member was measured in accordance with JIS G0551.
Furthermore, in the high-strength members of Examples 1 to 3 and Examples 5 to 9, a structure containing precipitates made of metal carbide having a particle diameter of 0.01 to 2 μm was observed, but other examples ( In Example 4) and Comparative Examples 1 to 3, no precipitate was observed.

From Table 3, it can be seen that Examples 1 to 9 belonging to the scope of the present invention have improved toughness compared to Comparative Examples 1 to 3 which are outside the present invention.
Specifically, in Examples 7 to 9, the initially prepared steel sheet is composed of martensite and bainite as main phases, and also includes residual austenite and alloy precipitates, and is an Ac3 point 780 ° C. material. In Examples 8 and 9 in which the temperature during heating was maintained within a certain low temperature range from directly above the Ac3 point, the reverse-transformed austenite was not coarsened so much, and the alloy precipitates were not completely re-solidified, and were fine. Since it is dispersed, the high-strength member quenched from this state has a fine-grained and fine alloy-deposited structure with excellent toughness, and is difficult to break even in the bending test of the member. Thus, when the alloy precipitate remains finely as a carbide, the amount of solid solution carbon in the parent phase is substantially reduced, which contributes to improvement of toughness. In the case of Example 7 heated to the Ac3 point or less, the alloy precipitates are coarsened, so the toughness is not improved so much. The other structures also show the same tendency, and the heating temperature is preferably just above the Ac3 point from the viewpoint of refining the prior austenite grain size and finely dispersing the alloy precipitate, and at least if it is in the range of Ac3 point to Ac3 point + 200 ° C. It is considered that the above effects are sufficiently exhibited.

It is a schematic diagram which shows the stress-strain diagram by the tension test using a plate-shaped test piece (For example, No. 5 test piece and No. 13 test piece prescribed | regulated to JISZ2201).

Claims (8)

A high-strength member formed by heating a high-tensile steel plate to an austenite region, press-forming and cooling in a mold,
The high-tensile steel plate contains C: 0.1 to 0.4%, Mo: 0.1 to 2.0% and Cr: 0.1 to 2.0%, and 0.01 to 5 μm in the steel. A high-strength member having a structure containing a precipitate made of a metal carbide having a particle size of.   The high-strength member according to claim 1, wherein the prior austenite grain size in the structure of the high-strength member is 1 to 10 µm.   The high-strength member according to claim 1 or 2, wherein the high-strength member has a structure containing a precipitate made of a metal carbide having a particle size of 2 µm or less.   The high-strength member according to any one of claims 1 to 3, wherein the high-strength member has a tensile strength of 1400 MPa or more.   The high-strength member according to any one of claims 1 to 4, wherein the high-tensile steel plate is a tailored blank steel plate.   The high-strength member according to any one of claims 1 to 5, wherein the high-strength member has a resistance weld.   The high-strength member according to any one of claims 1 to 6, wherein the high-strength member has a structure tempered at 150 to 700 ° C.   In manufacturing the high-strength member according to any one of claims 1 to 7, the steel is heated in an austenite region by sandwiching both surfaces of the high-tensile steel plate with induction-heated steel, and press-molded in a mold. And cooling the high strength member.

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