JP2010024551A - Steel sheet to be hot-pressed - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet to be hot-pressed having superior toughness and a TS of 1.8 GPa or higher. <P>SOLUTION: The steel sheet has a structure after having been hot-pressed mainly formed of a self-tempered martensite composed of former austenite grains with sizes of 10 μm or smaller. Therefore, the steel sheet has a chemical composition comprising 0.25 to 0.45% C and Mn+Cr: 0.5 to 3.0%, further comprising one or more selected from ≤0.5% Si, ≤2% Ni, ≤1% Cu, ≤1% V and ≤1% Al, and, if required comprising one or more selected from ≤0.01% B, ≤1.0% Nb, ≤1.0% Mo, 3.42N+0.001≤Ti≤3.42N+0.5, and 0.001 to 0.005% Ca, and in which, regarding impurities, the content of P is controlled to ≤0.005%, S to ≤0.005% and N to ≤0.002%. The manufacturing method comprises steps of: keeping the steel sheet at a temperature between an Ac<SB>3</SB>point and (Ac<SB>3</SB>point+100°C) for 5 minutes or shorter; hot-pressing it; and subsequently cooling it to the Ms point at a cooling rate of the upper critical cooling rate or higher, and then to 150°C from the Ms point at an average cooling rate of 10 to 500°C/s, so as to perform quenching. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a high-strength hot-pressed steel sheet member excellent in toughness, suitable for machine structural parts such as automobile body structural parts and undercarriage parts, a manufacturing method thereof, and a hot-pressed steel sheet therefor About.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the weight used. In thin steel plates widely used in automobiles, press formability decreases with increasing steel plate strength, making it difficult to manufacture complex shapes. Specifically, there are problems that ductility is reduced, fracture occurs at a high degree of processing, springback and wall warp increase, and dimensional accuracy deteriorates. Therefore, it is not easy to produce a part by press molding using a steel plate having a high strength, particularly a tensile strength of 780 MPa or higher. According to roll forming instead of press forming, it is possible to process a high-strength steel sheet, but it can be applied only to parts having a uniform cross section in the longitudinal direction.

  On the other hand, as shown in Patent Document 1, in a method called hot press for press-forming a heated steel plate, the steel plate is soft and highly ductile at a high temperature, so that a complicated shape is formed with high dimensional accuracy. Is possible. Furthermore, the steel plate is heated in the austenite region and rapidly cooled (quenched) in the mold, so that the strength of the steel plate can be increased by martensitic transformation.

  Patent Document 2 discloses a pre-press quench method that simultaneously achieves high strength and formability of a steel sheet by forming into a predetermined shape at room temperature, heating to an austenite region, and quenching in a mold. ing.

British Patent Publication No. 1490535 Japanese Patent Laid-Open No. 10-96031

  Such a hot press method and a pre-press quench method are excellent molding methods that can ensure the high strength and formability of the member at the same time. However, at present, there has been a demand for higher strength. Under the conventional technical idea, when the tensile strength (hereinafter also referred to as TS) of a member becomes 1.8 GPa or more, toughness is increased. It became clear for the first time by the present inventors that a shortage problem arises. In fact, there is no practical example of a member that becomes a TS of 1.8 GPa or more after hot pressing while ensuring toughness.

  Therefore, in the conventional hot press molding, in order to produce a member having a practical TS of 1.8 GPa or more, a tempering process is performed after quenching in order to ensure toughness. However, it is not preferable to add a tempering step in the hot pressing method because of problems such as work efficiency and equipment cost increase. As described above, in the hot pressing method, there is no hot pressing steel sheet that can provide a member having a TS of 1.8 GPa or more that can be practically used without performing tempering.

  A specific subject of the present invention is a steel sheet for hot pressing, which makes it possible to relatively easily produce a hot pressed member having a TS of 1.8 GPa or more excellent in toughness after hot pressing. It is to provide a hot-pressed steel sheet member and a manufacturing method thereof.

As a result of intensive studies to improve the toughness of a hot pressed member having a TS after hot pressing of 1.8 GPa or more, the present inventors have adjusted the steel plate components and further required hot press forming as necessary. It was found that the toughness is greatly improved by optimizing the heat pattern at the time of heating and by reducing the austenite grain size during heating prior to that. The present invention completed based on the knowledge is as follows.
In mass%, C: 0.25-0.45%, Mn + Cr: 0.5-3.0% and Nb: 0.04-1.0%, Si: 0.5% or less, Ni : 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, containing one or more kinds, and having a tensile strength of 1.8 GPa having a chemical composition composed of the remaining Fe and impurities A steel sheet for hot-pressed steel sheets as described above.

The chemical composition may contain B: 0.01% or less in mass% instead of part of Fe.
The chemical composition may contain Mo: 1.0% or less in mass% instead of part of Fe.

  The present invention makes it possible for the first time to practically use a steel sheet for hot pressing that can produce a hot pressed member having a TS of 1.8 GPa or more, which is excellent in toughness without being tempered and remains hot pressed. A technically valuable effect is achieved.

It is explanatory drawing of the shape of the test piece for critical cooling rate measurement. It is a typical explanatory view of a hat forming method.

Next, the reason why the present invention is limited to each range will be described. In the following description, “%” for alloy elements represents “mass%”.
About the chemical composition of the steel plate as a base steel plate in this invention, it prescribes | regulates as follows.

C: 0.25 to 0.45%
C is a very important element that enhances the hardenability of the steel sheet and mainly determines the strength after quenching. In particular, in order to ensure TS1.8 GPa or more in the strength after quenching, the C content needs to be at least 0.25%. On the other hand, if the C content exceeds 0.45%, the strength after quenching becomes too high, and the toughness deterioration becomes significant. A more desirable C content is 0.28 to 0.33%.

Mn + Cr: 0.5 to 3.0%
Mn and Cr are very effective elements in order to increase the hardenability of the steel sheet and to ensure a stable strength after quenching. However, if the total content of Mn and Cr (hereinafter also referred to as “(Mn + Cr) content”) is less than 0.5%, the effect is not sufficient, while the (Mn + Cr) content exceeds 3.0%. And the effect is saturated, and on the contrary, it is difficult to secure a stable strength. A more desirable (Mn + Cr) content is 0.8 to 2.0%.

B: 0.01% or less B is an optional additive element, and is effective for enhancing the hardenability of the steel sheet and further enhancing the effect of ensuring the stability of the strength after quenching. It is also an important element in that it segregates at grain boundaries to increase grain boundary strength and improve toughness. Furthermore, the austenite grain growth inhibitory effect at the time of a heating is also high. However, if the B content exceeds 0.01%, the effect is saturated and the cost is increased. A more desirable B content is 0.001 to 0.0030%.

Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, Al: 1% or less These elements enhance the hardenability of the steel sheet and ensure stable strength after quenching. It is an effective element. However, since the effect is small even if it contains more than an upper limit, and it causes a cost increase unnecessarily, the content of each alloy element is set to the above range.

Nb: 1.0% or less Nb is an optional additive element that suppresses recrystallization and forms fine carbides to make austenite grains fine when the steel sheet is heated to Ac ₃ point or higher. Has the effect of greatly improving toughness. However, when the Nb content exceeds 1.0%, the effect is saturated and the cost is increased unnecessarily. The Nb content is more preferably 0.01 to 0.2%, and further preferably 0.04 to 0.15%.

Mo: 1.0% or less Mo is an optional additive element. When the steel sheet is heated to Ac ₃ point or higher, fine carbides are formed and the austenite grains are made finer, so that the toughness is greatly improved. Have However, when the Mo content exceeds 1.0%, the effect is saturated and the cost is increased unnecessarily. The more preferable Mo content is 0.01 to 0.2%, and still more preferably 0.04 to 0.15%.

3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5
Ti is an optional additive element, and has the effect of greatly improving toughness because it suppresses recrystallization and forms fine carbides to make austenite grains fine when the steel sheet is heated to Ac ₃ points or more. . In order to reliably obtain such an effect, the Ti content is preferably set to (3.42N + 0.001) or more. On the other hand, when the Ti content exceeds (3.42N + 0.5), the effect is saturated and the cost is unnecessarily increased. A more desirable Ti content is 3.42N + 0.02 ≦ Ti ≦ 3.42N + 0.08.

Ca: 0.001 to 0.005%
Ca is an optional additive element, and has the effect of reducing the inclusions in the steel and improving the toughness after quenching. In order to ensure such an effect, the Ca content is preferably 0.001% or more. On the other hand, when the Ca content exceeds 0.005%, the effect is saturated. A more desirable Ca content is 0.002 to 0.004%.

P: 0.005% or less Since P is an element that greatly deteriorates the toughness after quenching, it is preferably 0.005% or less. More desirably, it is 0.003% or less.

S: 0.005% or less Since S is an element that greatly deteriorates the toughness after quenching, it is preferably 0.005% or less. More desirably, it is 0.003% or less.

N: 0.002% or less N is an element that forms inclusions in the steel and deteriorates the toughness after quenching, and therefore is preferably 0.002% or less. More desirably, it is 0.001% or less.

  The hot pressed steel sheet member of the present invention has a prior austenite average grain size of 10 μm or less in order to ensure toughness under a strength of TS of 1.8 GPa or more. The prior austenite average particle size is desirably 8 μm or less, and more desirably 4 μm or less. The prior austenite average particle diameter changes depending on the heating conditions (holding temperature and holding time) before hot pressing, as will be described below.

According to the present invention, hot press forming is performed on a steel sheet having the above chemical composition, and the heating conditions (holding temperature and holding time) before hot pressing at that time are as follows.
In order to obtain the desired strength and toughness by quenching in the hot pressing process, the steel sheet to be subjected to hot pressing is held for a period of 5 minutes or less in a temperature range of Ac ₃ points or more and (Ac ₃ points + 100 ° C.) or less. . The lower limit of the holding temperature is to obtain the desired strength once as an austenite single phase. The upper limit of the holding temperature and the upper limit of the holding time are to suppress the prior austenite grain size after quenching to 10 μm or less, and TS is 1.8 GPa. This is to ensure toughness under the above strength. If the holding temperature exceeds (Ac ₃ point + 100 ° C.) or the holding time exceeds 5 minutes, the prior austenite grain size becomes 10 μm or more and toughness may not be ensured. A more desirable holding temperature is Ac ₃ point or more and (Ac ₃ point + 50 ° C.) or less, and a more desirable holding time is 2 minutes or less. In addition, since the older austenite particle size is so preferable that it is a fine particle, the minimum of holding | maintenance time is not prescribed | regulated in particular.

The hot press molding in the present invention is not particularly limited, including the mold used, but the cooling conditions and the method after the hot press molding are as follows.
In order to improve the toughness as much as possible under a strength of TS of 1.8 GPa or more, it is important that the quenched structure is not a complete martensite structure but an automatic tempered martensite structure. In order to obtain this automatically tempered martensite structure, cooling is performed at an upper critical cooling rate or higher so that diffusion transformation does not occur up to the Ms point, and an average cooling rate in the temperature range from the Ms point to 150 ° C. is set to 10 to 10. The cooling rate is 500 ° C./s. A preferable average cooling rate from the Ms point to 150 ° C. is 15 to 200 ° C./s.

  Thus, the average cooling rate in the temperature range from the Ms point to 150 ° C. is slower than the cooling rate to the Ms point, but since the transformation heat generated by the martensitic transformation is very large after reaching the Ms point, the Ms In some cases, a sufficient cooling rate cannot be achieved by the same cooling method as the cooling method above the point. For this reason, it is necessary to perform the cooling from the Ms point to 150 ° C. more strongly than the cooling to the Ms point. Specifically, it is preferable to perform the following.

  In the hot pressing method, cooling is usually achieved by a steel mold at room temperature or about several tens of degrees Celsius. Therefore, in order to change the cooling rate, the heat capacity may be changed by changing the die size. The cooling rate can also be changed by changing the mold material to a different metal (for example, copper). If the mold dimensions cannot be changed, the cooling rate can also be changed by changing the amount of cooling water using a water-cooled mold. You can also change the cooling rate by using a mold that has been cut into several grooves in advance and changing the cooling speed by passing water through the groove during pressing, or by raising the press machine during the press and flowing water between them. be able to. Further, the cooling rate can be changed by changing the mold clearance and changing the contact area with the steel plate. For example, the following means can be considered as means for changing the cooling rate around the Ms point.

(1) Immediately after reaching the Ms point, it is moved to a mold having a different heat capacity or a mold at room temperature to change the cooling rate;
(2) In the case of a water-cooled mold, the cooling rate is changed by changing the amount of flowing water in the mold immediately after reaching the Ms point;
(3) Immediately after reaching the Ms point, the cooling rate is changed by flowing water between the mold and the member and changing the amount of water.

  The form of molding in the hot press method of the present invention is not particularly limited as described above, but examples include bending, drawing, stretch forming, hole expansion molding, and flange molding. What is necessary is just to select suitably according to the kind of target hot press steel plate member. Representative examples of hot-pressed steel sheet members include door guard bars and bumper reinforcements that are reinforcing parts for automobiles. Further, as long as a means for cooling the steel sheet is provided at the same time as or immediately after forming, it may be applied to a forming method other than pressing, for example, roll forming.

The product according to the present invention is characterized by securing toughness, but the toughness to withstand practical use at that time is preferably 30 J / cm ² or more as the Charpy impact value at −120 ° C.

  After hot pressing, shot blasting is usually performed for scale removal purposes. This shot blasting has the effect of introducing a compressive stress on the surface, so that delayed fracture is suppressed and the fatigue strength is improved.

  In addition, during hot pressing without pre-forming, the mechanical properties at room temperature before heating are not important because the austenite is heated to the austenite temperature range during heating, and the metal structure before heating is not important. Not specified. That is, any of a hot-rolled steel plate, a cold-rolled steel plate (full hard material, an annealed material), and a plated steel plate may be used as the base steel plate, and the manufacturing method is not particularly limited. For example, examples of the plated steel sheet include an aluminum-based plated steel sheet and a zinc-based plated steel sheet.

  On the other hand, during hot pressing with pre-forming, the type and structure of the steel plate are not limited, but it is desirable that the steel plate be as soft and ductile as possible. For example, TS is preferably about 590 MPa or less. The hot rolling coiling temperature in the hot rolled steel sheet is preferably 450 ° C. or higher in order to obtain a soft steel plate and 700 ° C. or lower in order to reduce scale loss. In the cold-rolled steel sheet, it is preferable to perform annealing in order to obtain a soft steel sheet, and the annealing temperature is preferably a recrystallization temperature or higher and 900 ° C. or lower. Moreover, it is preferable that the average cooling rate to room temperature after annealing is below an upper critical cooling rate.

Examples of the present invention will be described below.
A steel plate (plate thickness: 1.6 mm) having the chemical composition shown in Table 1 was used as the base steel plate. These steel plates are steel plates manufactured by hot rolling and cold rolling of a slab melted in a laboratory. Furthermore, using a plating simulator, steel plate No. 1 has Al plating (plating adhesion amount on one side is 120 g / m ² ), and No. 2 has hot dip galvanization (plating adhesion amount on one side is 60 g / m ^2). ). Furthermore, No. 2 was alloyed (Fe content in the plating film was 15% by mass). The annealing temperature in the plating simulator was 800 ° C., and the average cooling rate from 800 ° C. to the Ms point was 5 ° C./s. Steel plates other than No. 1 and No. 2 were subjected to the following tests as they were cold-rolled (full hard).

These steel plates are cut into dimensions of 1.6 t × 100 w × 200 L (mm), heated in a heating furnace in the air atmosphere under the conditions shown in Table 1, and taken out from the heating furnace. Hot pressing was performed using a metal mold. The holding time refers to the time from when the Ac ₃ point is reached after charging into the furnace to when it is removed from the furnace. A thermocouple was attached to the steel plate, and the cooling rate was also measured.

About the obtained hot press member, it used for the prior austenite particle size measurement by a cutting method, and a tensile test (JIS No. 5 test piece).
Moreover, after stacking six 1.6-t steel plates and screwing them, V-notch test pieces were prepared and subjected to a Charpy impact test. As toughness evaluation, when the impact value at −120 ° C. is 30 J / cm ² or more, the result is “good”. If it did not reach that, it was marked “x”.

The Ac ₃ point, Ms point, and upper critical cooling rate of each steel type were measured by the following methods. That is, a cylindrical test piece (FIG. 1) having a diameter of 3.0 mm and a length of 10 mm was cut out from a hot-rolled steel sheet, heated to 900 ° C. at a heating rate of 10 ° C./s, and held at that temperature for 5 minutes. After that, it was cooled to room temperature at various cooling rates. The Ac ₃ point and Ms point were measured by measuring the thermal expansion change of the test piece during heating and cooling at that time. Moreover, the Vickers hardness measurement (load 49N, the number of measurements: 3) and structure | tissue observation of the obtained test piece were performed, and the upper critical cooling rate was estimated from those results.

These results are shown in Table 2.
It can be seen that the steel types Nos. 1 to 13 as examples of the present invention have a tensile strength of 1.8 GPa or more and a good toughness value. On the other hand, steel types Nos. 14 and 15 as comparative examples do not satisfy the scope of the present invention, and therefore have poor toughness values.

  As an example of the present invention, a steel plate of steel type No. 2 reached 900 ° C. in a heating furnace in an atmospheric atmosphere, held for 1 minute, taken out from the heating furnace, and hot-pressed with a hat type (blank size: 1.0 t) × 80 w × 320 Lmm). FIG. 2 is a schematic explanatory view of the hat forming method. The hot press molding conditions at this time were a molding height of 70 mm, Rd (die shoulder R) 8 mm, Rp (punch shoulder R) 8 mm, clearance 1.0 mm, and wrinkle holding force 12.7 kN.

  The hat molded product was subjected to a low temperature impact test. After cooling the member to −40 ° C., a weight body having a weight of 2450 N (250 kgf) from a height of 1000 mm was collided with the member, and the presence or absence of cracks was investigated. As a result, it was found that there was no occurrence of cracking and sufficient toughness.

Claims (6)

  In mass%, C: 0.25-0.45%, Mn + Cr: 0.5-3.0% and Nb: 0.04-1.0%, Si: 0.5% or less, Ni : 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, containing one or more kinds, and having a tensile strength of 1.8 GPa having a chemical composition composed of the remaining Fe and impurities A steel sheet for hot-pressed steel sheets as described above.   The steel sheet for hot press according to claim 1, wherein the chemical composition contains B: 0.01% or less in mass% instead of part of Fe.   The steel sheet for hot press according to claim 1 or 2, wherein the chemical composition contains Mo: 1.0% or less in mass% instead of part of Fe. The steel sheet for hot press according to any one of claims 1 to 3, wherein the chemical composition contains Ti in an amount satisfying the following formula (1) instead of a part of Fe.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, Ti and N in a formula show content (unit: mass%) of each element in steel.   The steel sheet for hot press according to any one of claims 1 to 4, wherein the chemical composition contains Ca: 0.001 to 0.005% in mass% instead of part of Fe.   The chemical composition is one or more of P, S and N which are impurities, in mass%, P: 0.005% or less, S: 0.005% or less and N: 0.002% or less. The steel sheet for hot pressing according to any one of claims 1 to 5, which satisfies one condition or two conditions or more.

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