JP2010174304A - Steel sheet for die quenching - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for die quenching which has excellent scale peeling resistance upon hot working. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.35 to 0.45% C, 0.15 to 0.50% Si, 1.0 to 1.8% Mn, ≤0.025% P, ≤0.001% S, 0.01 to 0.10% Al, ≤0.005% N, >1.0 to 11.0% Cr, 0.01 to 0.15% Ti and 0.0008 to 0.0030% B, and the balance Fe with inevitable impurities. Thus, the generation of scales is suppressed upon heating in die quenching, a load upon shot blast treatment can be reduced. Further, the peeling of scales is suppressed upon hot working in die quenching, the damage of a die for die quenching and the generation of linear flaws in the surface of a product are prevented, and the production of a high-strength component whose dimensional precision is remarkably improved, and having a tensile strength of ≥2,000 MPa is facilitated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet for automobile parts and the like, and in particular, for die quenching suitable for application of die quenching (also referred to as hot pressing) that increases the strength by heating the steel sheet and quenching it while pressing. It relates to steel plates.

Conventionally, many automobile body structural parts, undercarriage parts, and the like have been manufactured by pressing a steel plate having a predetermined strength. In recent years, from the viewpoint of preservation of the global environment, weight reduction of automobile bodies has been eagerly desired, and higher strength of steel plates to be used has been demanded. However, due to a decrease in workability (formability) associated with high strength of the steel sheet, it is often difficult to process the steel sheet into a desired part shape. Specifically, for example, there is a problem that the ductility is lowered and a fracture occurs at a site with a large amount of processing, or the spring back becomes large and the dimensional accuracy is lowered.

  In order to solve such problems, a technique called die quench (also referred to as hot pressing) has been put into practical use as one method for increasing the strength of automobile parts and the like. In die quenching, a material (steel plate) is heated to a temperature of about 950 ° C., and then the heated material is pressed into a desired shape and quenched at the same time using a mold cooled by water cooling or the like. Thereby, it is hardened at the same time as it is molded into a desired processed shape, and it becomes a part having both desired dimensional accuracy and desired high strength, so that both workability (formability) and high strength can be achieved.

  However, in die quenching, it is necessary to first heat the material to a high temperature of about 950 ° C., and scale generation due to oxidation of the material becomes a problem. This scale is thick and has poor adhesion to the material, so it is easy to peel off during processing, and it can be peeled and bitten between the mold and the material being molded, damaging the mold. , Generate linear wrinkles on the product (parts) surface. In addition, when a scale is generated on the surface of the material, it is impossible to perform a chemical conversion treatment or a welding operation performed on the product thereafter. Furthermore, when the thickness of the scale is increased, heat transfer at the time of cooling is deteriorated, and there is a case where the strength is lowered without burning. Methods for preventing oxidation of the material during heating include a method of applying an anti-oxidant agent to the material and a method of applying an anti-oxidation plating to the material, but these methods are all costly. It is difficult to completely prevent oxidation. Therefore, in order to remove the generated scale, shot blasting is usually applied to the product after press working.

However, if shot blasting is applied to the pressed product, the scale will be removed, and at the same time, the product will be deformed, or if the amount of scale generated is large, the reduction in the thickness of the product due to scale removal will increase, resulting in dimensional accuracy of the product. The problem is that it gets worse.
For such a problem, for example, Patent Document 1 has a composition containing Si: 0.01 to 0.5% and Cr: 0.01 to 0.5%, and the steel sheet surface having a surface average roughness of 3.0 μm or less is washed with brush water. And steel plate for hot press that is less oxidized even if it is subjected to atmospheric oxidation afterwards, by performing a treatment that performs acid pickling, further alkali cleaning, and finally brush water cleaning. The manufacturing method is described. When such a treatment is applied to the steel sheet, the scale formed on the surface layer after heating and holding the steel sheet to the AC ₃ point or higher in the atmosphere is FeO, FeCr ₂ O ₄ , Fe _{2 at} the interface with the steel plate. The scale is composed of a mixed phase of SiO ₄ and a substantially single layer of FeO as the upper layer, and the amount of scale generation is reduced.

Further, in Patent Document 2, in mass%, C: 0.10 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.025% or less, S: 0.008% or less, Al: 0.01 to 0.10 %, N: 0.005% or less, Ti: 0.01 to 0.15%, B: 0.0005 to 0.0050%, or Cr: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.05 to 1.0 % Is hot rolled at a finishing temperature of 700 to 850 ° C., starts cooling within 1 s after hot rolling, and at least 700 ° C. A method for producing a hot-rolled steel sheet for die quenching is described in which the steel sheet is cooled at an average cooling rate of 50 ° C./s or more and then wound at a winding temperature of 550 to 630 ° C. Thereby, it is said that the die-quenched hot-rolled steel sheet having a scale whose composition of the interface with the ground iron is Fe ₃ O _{4 and} whose thickness is 2.0 to 7.0 μm can be manufactured. The hot-rolled steel sheet obtained by the technique described in Patent Document 2 can suppress the growth of scale when heated by die quenching, has a small scale thickness to be generated, and is easy to remove scale by shot blasting after die quenching.

JP 2005-133180 A JP 2008-214650 A

  However, the technique described in Patent Document 1 has a problem that a complicated cleaning process is required, productivity is lowered, and manufacturing cost is increased. In addition, even with the technique described in Patent Document 2, the amount of scale generated during die quench heating is large, the adhesion of the scale during hot processing is insufficient, scale peeling occurs during hot processing, and linear wrinkles occur. The fact is that there are no cases where they are generated.

  The present invention provides a die quench steel sheet having excellent scale peel resistance, which solves the problems of the prior art and can suppress scale formation during die quench heating and suppress scale peeling during die quench processing. For the purpose. The “steel plate” here includes both hot-rolled steel plates and cold-rolled steel plates. The steel plate of the present invention is suitable for die quenching and can ensure high strength after die quenching. For example, a steel sheet in which the tensile strength of the steel sheet after die quenching is approximately 2000 MPa or higher, preferably approximately 2200 MPa or lower, after being heated and held at 950 ° C. for about 10 minutes and then pressed with a water-cooled die.

  In order to achieve the above-mentioned object, the present inventors diligently studied the influence of various alloy elements that influence the generation of scale during the heating of the die quench and the adhesion of the generated scale. As a result, if it is heating at about 950 ° C in the atmosphere, the generation of scale during heating can be suppressed and the amount of scale generated during heating can be significantly reduced by using a steel sheet composition containing more than 1.0% Cr. I found out. In addition, with the steel plate composition described above, the S content is further reduced to 0.001% or less, so that the hot scale adhesion is greatly improved and the amount of scale peeling during hot working is significantly reduced. The present inventors have newly found that the steel sheet has excellent scale peeling resistance during hot working.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.35-0.45%, Si: 0.15-0.50%, Mn: 1.0-1.8%, P: 0.025% or less, S: 0.001% or less, Al: 0.01-0.10%, N: 0.005 %, Cr: more than 1.0 to 11.0%, Ti: 0.01 to 0.15%, B: 0.0008 to 0.0030%, with the composition consisting of the balance Fe and unavoidable impurities, and resistance to scale peeling during hot working A steel plate for die quenching, which is excellent.

(2) In (1), in addition to the above composition, the composition further contains, in mass%, one or more selected from Ni: 5.0% or less, Cu: 5.0% or less, Mo: 3.0% or less A steel plate for die quenching, characterized in that it has a composition to be used.
(3) In (1) or (2), in addition to the above composition, in addition to mass, Nb: 3.0% or less, V: 3.0% or less, W: 3.0% or less A steel plate for die quenching characterized by having a composition containing at least seeds.

  According to the present invention, scale formation is suppressed during die quench heating, and scale peeling is suppressed during die quench hot processing, and die quenching mold damage and generation of linear wrinkles on the die quench processed product surface are generated. Can be prevented, and it has a remarkable industrial effect. Furthermore, according to the present invention, there is no deformation of the product after shot blasting, and there is an effect that the dimensional accuracy is remarkably improved. In addition, according to the present invention, there is an effect that a die quench processed product in which the tensile strength of the material (steel plate) is about 2000 MPa or more can be easily and inexpensively manufactured.

The reason for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.35-0.45%
C is an element having an effect of increasing the strength of steel by strengthening the structure through improvement of hardenability, and in order to obtain martensite during quenching of die quench and to obtain a die quench processed product having a desired high strength. , And contain 0.35% or more. On the other hand, if the content exceeds 0.45%, the strength after die quenching becomes too high, and ductility, spot weldability, toughness, and delayed fracture resistance decrease. For this reason, C was limited to 0.35% or more and 0.45%.

Si: 0.15-0.50%
Si is an element having an effect of improving toughness after die quenching. In order to obtain such an effect, the content of 0.15% or more is required. However, when the content exceeds 0.50%, a non-peeling scale called a red scale is generated unevenly on the surface. For this reason, Si was limited to the range of 0.15-0.50%.

Mn: 1.0-1.8%
Mn is an element that has the effect of increasing the strength of steel through the improvement of hardenability, and in order to obtain a die quench processed product having a desired high strength, the content of 1.0% or more is required. On the other hand, if the content exceeds 1.8%, segregation becomes significant, and the uniformity of the material after hot rolling or after die quenching decreases. For this reason, Mn was limited to the range of 1.0 to 1.8%.

P: 0.025% or less P is an element that has the effect of increasing the strength by dissolving in ordinary high-strength steel sheets, but such effects are not observed in die-quenched products, and it exceeds 0.025%. If it contains, it will segregate, the uniformity of the material after hot rolling or die quenching will decrease, and the toughness will also decrease. For this reason, P was limited to 0.025% or less. In addition, Preferably it is 0.015% or less.

S: 0.001% or less S is one of the important elements in the present invention that affects the scale adhesion. Adjusting S to 0.001% or less improves the scale adhesion in the hot state, that is, the resistance to scale peeling during hot working, and prevents the deterioration of the dimensional accuracy and surface defects of die-quenched products. , The quality of die-quenched products will improve. For this reason, S was limited to 0.001% or less.

Al: 0.01-0.10%
Al is an element that acts as a deoxidizing agent, and in the present invention, it is necessary to contain 0.01% or more. On the other hand, if the content exceeds 0.10%, the workability is lowered and the hardenability is also lowered. For this reason, Al was limited to the range of 0.01 to 0.10%. In addition, Preferably it is 0.03-0.08%.
N: 0.005% or less N is an element that forms a nitride such as TiN, BN, or AlN by combining with a nitride-forming element such as Ti, B, or Al. Nitride is precipitated, workability is lowered, and hardenability is lowered. For this reason, N was limited to 0.005% or less.

Cr: more than 1.0 to 11.0%
Cr is one of the important elements in the present invention that affects the scale formation. Cr has the effect of forming a sub-scale layer of Cr on the surface layer during die quench heating and suppressing the subsequent oxidation, significantly reducing the amount of scale generated during die quench heating, and significantly improving hardenability. It is an element having Such an effect becomes remarkable when the content exceeds 1.0%. On the other hand, if the content exceeds 11.0%, α → γ (austenite) transformation does not occur during die quench heating at about 950 ° C., and high strength cannot be achieved by die quench. For this reason, Cr was limited to the range of more than 1.0 to 11.0%. In addition, Preferably it is 3.0 to 6.0%.

Ti: 0.01-0.15%
Ti is a nitride-forming element and contributes to securing solid solution B effective in improving hardenability by forming nitride in preference to B and preventing the formation of BN. In order to obtain such an effect, the present invention needs to contain 0.01% or more. On the other hand, if the content exceeds 0.15%, the hot rolling load becomes excessively large, making hot rolling difficult, reducing the hardenability, and further reducing the toughness of the die quench processed product. For this reason, Ti was limited to the range of 0.01 to 0.15%. In addition, Preferably it is 0.02 to 0.12%.

B: 0.0008-0.0030%
B is contained in a small amount and is an effective element that remarkably improves the hardenability and contributes to the improvement of toughness. In order to obtain such an effect, the content of 0.0008% or more is required. On the other hand, the content exceeding 0.0030% significantly increases the rolling load during hot rolling, and martensite after hot rolling. It causes bainite and causes defects such as plate cracking. For this reason, B was limited to the range of 0.0008 to 0.0030%. In addition, Preferably it is 0.0010 to 0.0020%.

The above-described components have a basic composition. In addition to the basic composition, one or more selected from Ni: 5.0 or less, Cu: 5.0% or less, Mo: 3.0% or less, and / or Alternatively, one or more selected from Nb: 3.0% or less, V: 3.0% or less, and W: 3.0% or less can be contained.
One or more selected from Ni: 5.0 or less, Cu: 5.0% or less, Mo: 3.0% or less
Ni, Cu, and Mo are all effective elements for improving the hardenability, and can be selected and contained as necessary.

Ni is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 5.0% leads to a significant increase in cost. For this reason, when it contains, Ni is limited to 5.0% or less.
Cu, like Ni, is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 5.0% leads to a significant increase in cost. For this reason, when contained, Cu is limited to 5.0% or less.

  Mo, like Ni and Cu, is an element effective for strengthening steel and improving hardenability. Mo also has the effect of suppressing crystal grain growth and improving toughness by refining. In order to exhibit such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 3.0% leads to a significant increase in cost. For this reason, when it contains, Mo is limited to 3.0% or less.

One or more selected from Nb: 3.0% or less, V: 3.0% or less, W: 3.0% or less
Each of Nb, V, and W is an element effective for strengthening steel and improving toughness by refining, and can be selected and contained as necessary.
Nb is an element effective for strengthening steel and improving toughness by refining. In order to exhibit such an effect, it is desirable to contain 0.005% or more, but if it exceeds 3.0%, carbonitride precipitation becomes remarkable, and ductility and delayed fracture resistance deteriorate. For this reason, when it contains, Nb is limited to 3.0% or less.

  V, like Nb, is an element effective for strengthening steel and improving toughness by refining. V forms precipitates and crystallized substances and improves hydrogen embrittlement resistance as a hydrogen trap site. In order to exhibit such an effect, it is desirable to contain 0.005% or more, but if it exceeds 3.0%, the precipitation of carbonitrides becomes remarkable, and the ductility is significantly reduced. For this reason, when contained, V is limited to 3.0% or less.

W, like V, is an element effective for strengthening steel, improving toughness, and improving hydrogen embrittlement resistance. For the expression of such an effect, it is desirable to contain 0.005% or more, but if it exceeds 3.0%, the ductility is significantly reduced. For this reason, when it contains, W is limited to 3.0% or less.
The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, for example, O: 0.010% or less is acceptable.
The method for producing the steel sheet of the present invention having the above composition can be applied to any of the commonly known methods for producing hot-rolled steel plates or the commonly known methods for producing cold-rolled steel plates, and is not particularly limited. Will be described.

It is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter and is made into a slab or the like by a conventional casting method such as a continuous casting method. Subsequently, these slabs are hot-rolled to obtain hot-rolled steel sheets, or, further, pickled hot-rolled steel sheets, then cold-rolled, and annealed as necessary to obtain cold-rolled steel sheets. .
In hot rolling, the slab is preferably heated to 1100 to 1300 ° C., or when the slab is kept at a temperature at which hot rolling is possible, the finish rolling finish temperature: 850 to 920 without heating. After performing hot rolling at a temperature of ℃, if necessary, after completion of the hot rolling, preferably subjected to accelerated cooling at a cooling rate of 30 ° C./s or more, and then wound at a winding temperature of 550 to 650 ° C., It is preferable to use a hot-rolled steel sheet having a desired thickness. In addition, it is preferable to adjust suitably the cooling rate of accelerated cooling, and coiling temperature in consideration of the load etc. to a subsequent pickling process and a cold rolling process. For example, to reduce the load of the pickling process, increase the cooling speed of accelerated cooling, lower the coiling temperature, and to reduce the load of the cold rolling process, lower the cooling speed of accelerated cooling. It is preferable to set the taking temperature higher.

In the case of a cold-rolled steel sheet, the hot-rolled steel sheet that has undergone the pickling process is preferably subjected to cold rolling at a reduction rate of 60% or more to obtain a cold-rolled steel sheet having a desired thickness, and an annealing temperature: 650 to It is preferable to perform an annealing process at 850 ° C.
Preferably, the steel sheet of the present invention obtained by the manufacturing method described above is processed into a part having a predetermined size and shape by applying a die quench method. Such processing, for example, holding by heating at 950 ° C for about 10 min, and then performing die quenching such as pressing with a water-cooled mold and quenching at the same time, the tensile strength of the material steel plate that is the molding material It is possible to obtain a high-strength part (product) having a pressure of 2000 MPa or more. When the steel sheet of the present invention is a hot-rolled steel sheet, generally, the scale formed on the surface layer is removed by pickling or the like, but in the present invention, the die quench method is applied without removing the scale. There is no problem. Even if the die quench method is applied without removing the scale (black skin), the effect of the present invention is maintained.

Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (material: thickness 260 mm) by a continuous casting method. Next, the material is heated at a heating temperature of 1200 ° C. and subjected to hot rolling at a finish rolling finish temperature of 850 to 880 ° C., wound at a winding temperature of 580 to 650 ° C., and a thickness of 1.6 to 4.0 mm. The hot rolled sheet was used. The obtained hot-rolled sheet was pickled. Some hot-rolled sheets are pickled and then cold-rolled at a rolling reduction of 60 to 75% to obtain a cold-rolled sheet having a thickness of 1.0 to 1.6 mm, and an annealing temperature: Annealed at 700-800 ° C.

(1) Heat test A test plate was sampled from the obtained steel plate, and was heated and held at 950 ° C. for 10 minutes in an air atmosphere. And the weight of the scale was calculated | required with the pickling loss method, the scale thickness produced | generated on the steel plate surface was calculated by making specific gravity (density) of a scale into 5.2 g / cm < ³ >. The obtained results are shown in Table 2.
(2) Die quench test A test plate was taken from the obtained steel plate, heated and maintained at 950 ° C for 10 minutes in an air atmosphere, and then water-cooled mold was used to measure height 50 mm × width 50 mm × length 300 mm. The die quench which shape | molds in a hat shape was given, and the test article (die quench processed article) was obtained. About the obtained test article, the external appearance was observed visually and the presence or absence of the surface defect was investigated. The case where there was one or more surface defects having a width of 1 mm or more and a length of 10 mm or more was evaluated as x, and the case where there was no surface defect was evaluated as ◯. Furthermore, from the bottom of the hat of the obtained test product, a JIS No. 5 test piece was collected in accordance with the provisions of JIS Z 2201, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. The tensile strength TS of the material (steel plate) was determined. The obtained results are also shown in Table 2.

In all of the examples of the present invention, the thickness of the scale generated during heating of the die quench is thinner than that of the comparative example having a low Cr content, and it can be seen that scale generation is suppressed during heating of the die quench. On the other hand, the comparative example in which the Cr content is out of the range of the present invention has a large scale thickness, and the suppression of scale generation is insufficient.
In addition, in all of the examples of the present invention, the tensile strength TS after die quenching is high strength of 2000 MPa or more, and the occurrence of surface defects is not recognized (evaluation ○), and the examples of the present invention are resistant to hot working. It can be seen that the scale peelability is excellent. On the other hand, in the comparative example outside the scope of the present invention, the tensile strength TS after die quench was less than 2000 MPa, or the resistance to scale peeling during hot working was low, and the occurrence of surface defects was observed (Evaluation ×). .

Claims (3)

% By mass
C: 0.35-0.45%, Si: 0.15-0.50%,
Mn: 1.0 to 1.8%, P: 0.025% or less,
S: 0.001% or less, Al: 0.01 to 0.10%,
N: 0.005% or less, Cr: more than 1.0 to 11.0%,
Ti: 0.01-0.15%, B: 0.0008-0.0030%
A die quench steel sheet having a composition comprising the balance Fe and inevitable impurities and having excellent resistance to scale peeling during hot working.   In addition to the above composition, the composition further comprises, in mass%, one or more selected from Ni: 5.0% or less, Cu: 5.0% or less, and Mo: 3.0% or less. The steel plate for die quenching according to claim 1.   In addition to the above composition, the composition further comprises, in mass%, one or more selected from Nb: 3.0% or less, V: 3.0% or less, and W: 3.0% or less. The steel plate for die quenching according to claim 1 or 2.