Abstract:
The invention provides a hot-working die steel for die-casting obtainable by quenching a steel comprising, in terms of % by mass, C: 0.1 to 0.3%, Si: 0.1 to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or less, Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al: 0.005 to 0.025%, N: 0.005 to 0.025%, and O: 0.005% or less, with the remainder being Fe and inevitable impurities, followed by tempering the steel at a temperature of 500° C. or lower.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a hot-working die steel for use as die-casting molds. More particularly, the invention relates to a hot-working die steel for die-casting which inhibits the cracking from a water-cooling hole, which is a major cause of serious cracks in die-casting molds, and is capable of coping with a higher cycle speed in the production of die-casting products. The hot-working die steel for die-casting of the invention can be advantageously used as a material for aluminum die-casting molds. 
       BACKGROUND OF THE INVENTION 
       [0002]    Aluminum die-casting molds have hitherto had a problem that cracks generate at the cavity surface due to thermal fatigue (i.e., heat check). This heat check is a phenomenon in which the cavity surface, when sprinkled with cooling water after mold opening, comes to have an tensile stress due to a temperature difference between the rapidly cooled cavity surface and inner parts in a heated state, and the thermal fatigue resulting from repetitions of this stress generation causes cracks at the cavity surface. 
         [0003]    It is said that it is advantageous to heighten the hardness of the mold for diminishing the heat check. 
         [0004]    On the other hand, there recently has been a desire for a reduction in cycle time (higher cycle speed) in the production of aluminum die-casting products. For the purpose of reducing a mold closing time in order to realize that desire, the water cooling of an aluminum cast in a mold tends to be enhanced. Specifically, this enhancement of water cooling is accomplished by disposing a water-cooling hole in a position closer to the cavity surface. In this case, the thermal stress generating at the surface of the water-cooling hole during the casting of an aluminum product is increased and the phenomenon in which a crack generates from the water-cooling hole becomes problematic. 
         [0005]    Such a crack generating from a water-cooling hole is not attributable only to the thermal stress repeatedly imposed during casting but is thought to be a delayed-fracture phenomenon including a combination of cracking caused by thermal stress and stress corrosion cracking caused by rust generating on the surface of the water-cooling hole. 
         [0006]    The higher the hardness of a mold, the more the cracking from the water-cooling hole is apt to occur. Consequently, it is advantageous to reduce the hardness of a mold for inhibiting such cracking from the water-cooling hole. 
         [0007]    Namely, to increase mold hardness is advantageous for diminishing the heat check but is disadvantageous for diminishing cracking from a water-cooling hole, whereas to reduce mold hardness is advantageous for diminishing cracking from a water-cooling hole but is disadvantageous for diminishing the heat check, resulting in impaired heat check resistance. 
         [0008]    From the standpoint of inhibiting the cracking from a water-cooling hole, it is desirable to regulate the mold hardness to HRC 45 to 40. 
         [0009]    Hot-working die steels of the 5Cr type represented by JIS-SKD61 have been mainly used for current aluminum die-casting molds. In recent years, the use hardness thereof has been increasing so as to inhibit the heat check generating at the cavity surface, and the risk of cracking from the water-cooling hole in the mold has been increasing with the trend toward a higher cycle speed in the production of aluminum die-casting products. 
         [0010]    In the case of the JIS-SKD61, this steel contains about 0.4% of C and the hardness of the steel in a quenched state is, for example, about HRC 53. 
         [0011]    For reducing the hardness thereof to HRC 45 or lower for the purpose of inhibiting cracking from a water-cooling hole, it is necessary to conduct annealing at a high temperature of 600° C. or above. However, when annealing at such a high temperature is conducted, the corrosion resistance of the steel decreases considerably. 
         [0012]    This material, which contains Cr in an amount of about 5%, in itself is a material having excellent corrosion resistance. However, when this steel is annealed at a temperature as high as 600° C. or above, most of the Cr contained therein separates out as a Cr carbide due to this high-temperature annealing. Accordingly, the Cr contained in the steel thus comes not to contribute to an improvement in corrosion resistance. 
         [0013]    In any event, the hot-working die steels presently in main use as aluminum die-casting molds, which are represented by JIS-SKD61, are ineffective in satisfactorily overcoming the problem concerning cracking from a water-cooling hole. 
         [0014]    It is thought that an effective measure in satisfactorily overcoming each of the problem concerning cracking from a water-cooling hole and the problem concerning heat check at the cavity surface is to prevent rusting in the water-cooling hole and to reduce the hardness of that inner part of the mold in which the water-cooling hole is present, as well as to increase the hardness of the mold cavity surface where a heat check may generate. However, no material satisfying such properties has been provided yet. 
         [0015]    Incidentally, reference document 1 shown below discloses an invention concerning a technique in which the inner circumferential surface of the water-cooling hole of a die-casting mold is regulated so as to have a lower hardness than the mold surface to thereby reconcile the prevention of water-cooling hole cracking and the heat check resistance of the mold surface. 
         [0016]    The steel disclosed in this reference document 1 is produced by regulating JIS-SKD61, which has been used hitherto, so as to have a high hardness by quenching and tempering and then regulating the surface of the water-cooling hole so as to have a low hardness by local tempering with induction heating, burner heating, laser heating, or the like. 
         [0017]    All the methods disclosed in this reference document 1 necessitate local heating, and have a problem that the shape of the water-cooling hole is limited, for example, that the diameter of the water-cooling hole should be a size which enables burner insertion. 
         [0018]    Reference Document 1: JP-A-6-315753 
       SUMMARY OF THE INVENTION 
       [0019]    The present invention has been achieved under the circumstances described above. An object of the invention is to provide a hot-working die steel for die-casting which has excellent heat check resistance and can satisfactorily inhibit cracking from a water-cooling hole. 
         [0020]    The present inventors have made eager investigation to examine the problem. As a result, it has been found that the foregoing objects can be achieved by the following hot-working die steels for die-casting. With this finding, the present invention is accomplished. 
         [0021]    The present invention is mainly directed to the following items. 
         [0022]    1. A hot-working die steel for die-casting obtainable by quenching a steel comprising, in terms of % by mass, 
         [0023]    C: 0.1 to 0.3%, 
         [0024]    Si: 0.1 to 1.5%, 
         [0025]    Mn: 0.3 to 2%, 
         [0026]    Cr: 6 to 12%, 
         [0027]    P: 0.05% or less, 
         [0028]    S: 0.01% or less, 
         [0029]    Mo: 1 to 3%, 
         [0030]    V: 0.5 to 1.5%, 
         [0031]    s-Al: 0.005 to 0.025%, 
         [0032]    N: 0.005 to 0.025%, and 
         [0033]    O: 0.005% or less, 
         [0034]    with the remainder being Fe and 
         [0035]    inevitable impurities, 
         [0000]    followed by tempering the steel at a temperature of 500° C. or lower. 
         [0036]    2. The hot-working die steel for die-casting according to item 1, which further comprises at least one member selected from the group consisting of, in terms of % by mass, 
         [0037]    Ni: 2% or less, and 
         [0038]    Cu: 1% or less. 
         [0039]    3. The hot-working die steel for die-casting according to item 1 or 2, which further comprises, in terms of % by mass, 
         [0040]    C: 5% or less. 
         [0041]    4. The hot-working die steel for die-casting according to any one of items 1 to 3, which further comprises at least one member selected from the group consisting of, in terms of % by mass, 
         [0042]    Ti: 0.2% or less, 
         [0043]    Zr: 0.2% or less, and 
         [0044]    Nb: 0.2% or less. 
         [0045]    The hot-working die steel for die-casting of the invention has a reduced C content and, on the other hand, has high and optimized Cr and Mo contents. Accordingly, the steel of the invention, when used as a die-casting mold, can effectively inhibit cracking from the water-cooling hole and can impart excellent heat check resistance to the die-casting mold. The hot-working die steel for die-casting of the invention can be advantageously used especially as a material for aluminum die-casting molds. 
         [0046]    Cr is known as an element which improves corrosion resistance. In ordinary JIS-SKD61, however, the Cr for improving corrosion resistance separates out disadvantageously as a carbide during the heat treatment for obtaining a use hardness because this steel is tempered at a temperature as high as 600° C. or above as described hereinabove. Accordingly, the effect of the Cr is almost lost. On the other hand, when the tempering temperature is lowered to such a degree that Cr carbide separation does not occur, the steel comes to have an exceedingly high hardness of 50 HRC or above. When such a steel is used as a die-casting mold, cracking from the water-cooling hole is apt to occur. 
         [0047]    A target hardness may be obtained through tempering at a low temperature of 500° C. or below by reducing the C content. In this case, however, the hardness of the cavity surface also decreases to cause a problem that heat check resistance becomes poor. 
         [0048]    Herein, in the hot-working die steel for die-casting of the invention, the C content is reduced and Mo is added in an appropriate amount. 
         [0049]    By reducing the C content, a hardness of HRC 45 or below, which is less apt to result in cracking from a water-cooling hole, can be obtained through tempering at a low temperature of 500° C. or below. 
         [0050]    Furthermore, by the addition of an appropriate amount of Mo, the mold cavity surface can be partly increased in hardness by utilizing the heat transferred from the melt (e.g., aluminum melt) during die-casting when this steel is used as a die-casting mold. 
         [0051]    Specifically, the Mo added separates out as a carbide when the mold is used for the casing of a die-casting product and the cavity surface is heated by the heat transferred from the melt (about 600-650° C. in the case of aluminum melt) to thereby serve to partly heighten the hardness of the cavity surface. 
         [0052]    Namely, the hot-working die steel for die-casting of the invention has an effect that the hardness of the cavity surface increases by means of age hardening during the use of the mold. Due to this effect, heat check in the cavity surface can be satisfactorily inhibited. 
         [0053]    Namely, in the hot-working die steel for die-casting of the invention, the phenomenon in which, when the steel is used as a die-casting mold, the cavity surface thereof undergoes age hardening due to the heat transferred from the melt can be ingeniously utilized. As a result, it is possible to obtain a mold which retains a low hardness in inner parts thereof but has a partly increased hardness in the cavity surface. In this respect, the hot-working die steel for die-casting of the invention has an excellent effect over conventional ones. 
         [0054]    Moreover, Cr as a corrosion-resistant element has been added in a larger amount in the invention than in JIS-SKD61. In the invention, annealing is conducted at a temperature as low as 500° C. or below after a quenching treatment. Accordingly, the Cr added does not separate out as a carbide but is in the state of being a solid solution in the matrix to effectively serve to improve the corrosion resistance of the steel. Namely, due to this corrosion-resistance-improving function of the Cr, when the hot-working die steel for die-casting of the invention is used as a die-casting mold, rusting in the water-cooling hole is inhibited and the cracking from the water-cooling hole, which is caused by the rusting, is satisfactorily inhibited. 
         [0055]    Furthermore, when the hot-working die steel for die-casting of the invention is used as a die-casting mold, the cavity surface of the mold undergoes secondary hardening (age hardening) due to the separation of a Mo carbide, whereby it hardens to come to have a hardness of HRC 45 or higher, at which heat check resistance can be secured. 
         [0056]    Next, reasons for the limitation of each chemical component in the invention will be described below in detail. Hereinafter, “%” means “% by mass”. 
         [0057]    C: 0.1 to 0.3% 
         [0058]    C is an element necessary for securing hardness and wearing resistance, which are important mold performances. 
         [0059]    Ordinary hot-working die steels contain C in an amount of about 0.4%. In the invention, however, the C content is lower than in the ordinary hot-working die steels so that a hardness of HRC 45 or lower can be obtained through low-temperature tempering at 500° C. or lower. The range thereof is 0.1 to 0.3%, preferably 0.15 to 0.25%. 
         [0060]    Si: 0.1 to 1.5% 
         [0061]    Si is an element necessary as a deoxidizing element in steelmaking. 
         [0062]    Furthermore, by increasing the content thereof, machinability and resistivity to temper softening can be improved. 
         [0063]    However, excessively large addition amount thereof results in reduced impact value toughness. Consequently, the range of the addition amount thereof is 0.1 to 1.5%, preferably 0.1 to 0.5%. 
         [0064]    Mn: 0.3 to 2% 
         [0065]    Mn is a component necessary for securing hardenability and hardness. The addition amount thereof id set at 0.3% or larger. 
         [0066]    On the other hand, when Mn is added excessively, hardenability becomes too high and there are some cases where quenching yields a large amount of residual γ to reduce the impact value or where annealing does not result in a reduction in hardness. Consequently, the upper limit thereof is set at 2%. The upper limit of the addition amount of Mn is preferably set at 1%. 
         [0067]    Cr: 6 to 12% 
         [0068]    Cr is an element which improves hardenability and also improves the corrosion resistance of a water-cooling hole. 
         [0069]    For obtaining the effect of improving corrosion resistance, it is necessary to add Cr in an amount of 6% or larger. It is preferred to add Cr in an amount of 8% or larger. 
         [0070]    However, addition in an excessively large amount reduces resistivity to temper softening and also reduces mold performances. Therefore, the upper limit thereof is set at 12%. Further, it is preferred that the upper limit of the content of Cr be set at 10%. 
         [0071]    P: ≦0.05% 
         [0072]    P is an element which is preferably diminished because it reduces impact value. When the steel contains it inevitably, it is preferred to diminish the content thereof to 0.05% or below. 
         [0073]    S: ≦0.01% 
         [0074]    S is an element which is preferably diminished because it forms MnS to reduce impact value. 
         [0075]    When the steel contains it inevitably, it is preferred to diminish the content thereof to 0.01% or below. 
         [0076]    Mo: 1 to 3% 
         [0077]    Mo is necessary for strengthening the matrix and improving the wearing resistance through carbide formation and also for securing hardenability. 
         [0078]    Furthermore, when the hot-working die steel for die-casting of the invention is used as a die-casting mold, this Mo carbide separates out due to the heat transferred from the melt (around 600° C. in the case of aluminum melt) to thereby heighten the hardness of the mold. 
         [0079]    Although the mold hardness after quenching and subsequent tempering has been set at HRC 45 or lower in the invention in order to prevent cracking from the water-cooling hole, the temperature of the cavity surface rises during die-casting (around 600° C. in the case of aluminum die-casting) and a hardness of HRC 45 or higher can be obtained. Thus, heat check resistance can be improved. 
         [0080]    For obtaining such an effect, it is necessary to add Mo in an amount of 1% or larger and it is preferred to add Mo in an amount of 1.5% or larger. 
         [0081]    However, even when it is added excessively, the effect is saturated and such an excessive addition is economically disadvantageous. The upper limit of the addition is therefore set at 3%. It is preferred that the upper limit of the addition amount of Mo be set at 2.5%. 
         [0082]    V: 0.5 to 1.5% 
         [0083]    V is an element which forms a carbide and separates out during tempering to thereby strengthen the matrix and improve wearing resistance. 
         [0084]    Furthermore, during heating for quenching, it forms a fine carbide and this has the effect of inhibiting crystal grain enlargement to thereby inhibit impact value decrease. 
         [0085]    For obtaining such an effect, it is necessary to add V in an amount of 0.5% or larger. 
         [0086]    On the other hand, in a case where V is added excessively, it yields coarse carbonitride crystals during solidification to reduce toughness. Consequently, the upper limit of the addition amount of V is set at 1.5%. It is preferred that the upper limit of the addition amount of V be set at 1%. 
         [0087]    s-Al: 0.005 to 0.025% 
         [0088]    Al not only functions as a deoxidizing element during steelmaking, but is an element which combines with the N in the steel and finely disperses as a nitride to inhibit crystal grain enlargement during heating for quenching. 
         [0089]    For obtaining such effects, it is necessary to add Al in an amount of 0.005% or larger. 
         [0090]    However, even when it is added in a large amount, the effect is saturated. 
         [0091]    Consequently, the upper limit of the addition amount thereof is set at 0.025%. 
         [0092]    N: 0.005 to 0.025% 
         [0093]    N is an element which combines with the Al and V in the steel to form nitrides. The nitrides finely disperse to thereby inhibit crystal grain enlargement during heating for quenching. N is hence an element effective for preventing impact value decrease. 
         [0094]    For obtaining such an effect, it is necessary to add N in an amount of 0.005% or larger. 
         [0095]    However, even when it is added in a large amount, the effect is saturated. 
         [0096]    Consequently, the upper limit of the addition amount thereof is set at 0.025%. 
         [0097]    O: ≦ 0 . 005 % 
         [0098]    O forms oxide inclusions to decrease impact value. For inhibiting impact value decrease, it is necessary to reduce the content of O to 0.005% or lower. 
         [0099]    Ni: ≦2%, Cu: ≦1% 
         [0100]    Since Ni enhances hardenability and are thus effective in toughening the matrix, it can be added according to need. 
         [0101]    However, even when these elements are added excessively, the effects are saturated and the excessive addition thereof is economically disadvantageous. The upper limits of the addition amount thereof are hence set at 2% and 1%, respectively. 
         [0102]    Co: ≦5% 
         [0103]    Co is an element which improves strength through solid-solution strengthening. It can be added according to need. 
         [0104]    However, even when it is added excessively, the effect is saturated and the excessive addition thereof is economically disadvantageous. Consequently, the upper limit of the addition amount thereof is set at 5%. 
         [0105]    Ti: ≦0.2%, Zr: ≦0.2%, Nb: ≦0.2% 
         [0106]    These are elements which form Ti(CN), Zr(CN), Nb(CN), and composite carbonitrides thereof and finely separate out to inhibit crystal grain enlargement during heating for quenching. When it is desired to form fine crystal grains to secure toughness, these elements can be added according to need. 
         [0107]    However, in case where those elements are added excessively, they separate out as coarse carbonitride crystals during solidification to reduce rather than increase impact value. Consequently, the upper limits of the addition amount thereof are set at 0.2%, respectively. 
         [0108]    Furthermore, in the case where those elements are added in combination, it is preferred that the total amount thereof be 0.5% or smaller. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0109]    Embodiments of the present invention will be described below in detail. 
         [0110]    The present invention is now illustrated in greater detail with reference to Steels of the invention and Comparative Steels, but it should be understood that the present invention is not to be construed as being limited thereto. 
         [0111]    Steels respectively having the compositions shown in Table 1 each were melted in a 150-kg vacuum high-frequency induction furnace. Each ingot thus obtained was forged at 1,200° C. into a square bar having a section of 60 mm×60 mm. 
         [0112]    This square bar was cut into a length of 500 mm, subsequently heated to 1,030° C., and then subjected to oil quenching. 
         [0113]    Thereafter, tempering was conducted twice under the conditions with a temperature of 450° C. and a period of 1 h. Each square bar which had been tempered was subjected to each of a measurement of the hardness of a ¼ H part (a part located midway between the surface and the central part), a Charpy impact test in the T direction (width direction for the square bar) using a 2-mm U-notch test piece, and a corrosion test in which a block of 10 mm×10 mm×10 mm was cut out of the ¼ H part, the surface thereof was polished with an emery paper, and this block was then wholly immersed in 20° C. industrial water for 24 h and examined for rusting. 
         [0114]    In the evaluation of corrosion resistance, ones which suffered no rusting are rated as A and ones which suffered rusting are rated as B. 
         [0115]    Furthermore, for the purpose of simulating a heat history in repetitions of the casting of an aluminum die-casting product, each of the square bars which had been tempered at 450° C. was subjected to repeated 1,000 cycles each including heating from room temperature to 650° C. by high-frequency heating, holding at this temperature for 4 seconds, and subsequent water cooling. Thereafter, the surface hardness thereof was measured. 
         [0116]    The results of these evaluations are shown in Table 2. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Composition (mass %) 
               
             
          
           
               
                   
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cr 
                 Mo 
                 V 
                 s-Al 
                 N 
                 O 
                 Others 
               
               
                   
                   
               
             
          
           
               
                 Invention 
                 1 
                 0.12 
                 0.1 
                 0.42 
                 0.008 
                 0.002 
                 8.2 
                 2.8 
                 0.6 
                 0.015 
                 0.01 
                 0.003 
                   
               
               
                 Steel 
                 2 
                 0.2 
                 0.3 
                 0.45 
                 0.012 
                 0.002 
                 9 
                 2.5 
                 0.8 
                 0.02 
                 0.012 
                 0.002 
               
               
                   
                 3 
                 0.18 
                 0.5 
                 0.8 
                 0.01 
                 0.005 
                 11.3 
                 2 
                 1.1 
                 0.005 
                 0.022 
                 0.004 
               
               
                   
                 4 
                 0.23 
                 0.3 
                 1.2 
                 0.024 
                 0.008 
                 6.5 
                 2.8 
                 1.4 
                 0.008 
                 0.008 
                 0.002 
               
               
                   
                 5 
                 0.28 
                 0.8 
                 0.6 
                 0.003 
                 0.009 
                 10.1 
                 1.6 
                 0.9 
                 0.013 
                 0.005 
                 0.003 
               
               
                   
                 6 
                 0.2 
                 1.3 
                 1.4 
                 0.018 
                 0.001 
                 8.8 
                 1.8 
                 0.8 
                 0.022 
                 0.008 
                 0.001 
                 Ni: 1% 
               
               
                   
                 7 
                 0.2 
                 0.3 
                 0.5 
                 0.009 
                 0.002 
                 9.2 
                 2.3 
                 0.6 
                 0.021 
                 0.011 
                 0.002 
                 Ni: 0.5%, Cu: 0.5% 
               
               
                   
                 8 
                 0.21 
                 0.25 
                 0.45 
                 0.003 
                 0.002 
                 9 
                 2.5 
                 0.7 
                 0.018 
                 0.009 
                 0.003 
                 Ni: 0.7%, Co: 2% 
               
               
                   
                 9 
                 0.18 
                 0.3 
                 0.6 
                 0.007 
                 0.002 
                 9.5 
                 2.2 
                 0.6 
                 0.011 
                 0.021 
                 0.002 
                 Co: 4%, Ti: 0.05% 
               
               
                   
                 10 
                 0.22 
                 0.22 
                 0.65 
                 0.031 
                 0.001 
                 10.1 
                 2.3 
                 0.55 
                 0.016 
                 0.023 
                 0.002 
                 Zr: 0.1%, Nb: 0.1% 
               
               
                   
                 11 
                 0.18 
                 0.25 
                 0.67 
                 0.021 
                 0.001 
                 8.9 
                 2.5 
                 0.61 
                 0.02 
                 0.018 
                 0.003 
                 Co: 1%, Zr: 0.2%, 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Nb: 0.05% 
               
               
                 Comparative 
                 a 
                 0.05 
                 0.2 
                 0.52 
                 0.015 
                 0.002 
                 8.3 
                 2.3 
                 0.62 
                 0.021 
                 0.011 
                 0.002 
               
               
                 Steel 
                 b 
                 0.38 
                 0.21 
                 0.48 
                 0.011 
                 0.002 
                 9.1 
                 2.3 
                 0.65 
                 0.022 
                 0.015 
                 0.002 
               
               
                   
                 c 
                 0.2 
                 2 
                 0.45 
                 0.012 
                 0.002 
                 9.2 
                 2.5 
                 0.6 
                 0.018 
                 0.012 
                 0.002 
               
               
                   
                 d 
                 0.2 
                 0.2 
                 2.5 
                 0.011 
                 0.002 
                 8.9 
                 2.4 
                 0.61 
                 0.019 
                 0.011 
                 0.003 
               
               
                   
                 e 
                 0.2 
                 0.2 
                 0.5 
                 0.08 
                 0.001 
                 9.1 
                 2.5 
                 0.6 
                 0.021 
                 0.011 
                 0.002 
               
               
                   
                 f 
                 0.2 
                 0.21 
                 0.5 
                 0.012 
                 0.05 
                 9 
                 2.5 
                 0.61 
                 0.02 
                 0.01 
                 0.002 
               
               
                   
                 g 
                 0.21 
                 0.22 
                 0.5 
                 0.011 
                 0.001 
                 5.1 
                 2.3 
                 0.6 
                 0.021 
                 0.01 
                 0.002 
               
               
                   
                 h 
                 0.19 
                 0.21 
                 0.45 
                 0.012 
                 0.002 
                 13.5 
                 2.4 
                 0.61 
                 0.019 
                 0.009 
                 0.003 
               
               
                   
                 i 
                 0.21 
                 0.25 
                 0.44 
                 0.009 
                 0.002 
                 9 
                 0.6 
                 0.6 
                 0.02 
                 0.009 
                 0.002 
               
               
                   
                 j 
                 0.2 
                 0.21 
                 0.48 
                 0.011 
                 0.002 
                 9.1 
                 2.1 
                 0.3 
                 0.015 
                 0.008 
                 0.001 
               
               
                   
                 k 
                 0.19 
                 0.22 
                 0.51 
                 0.011 
                 0.002 
                 9.3 
                 2.4 
                 0.6 
                 0.003 
                 0.007 
                 0.002 
               
               
                   
                 l 
                 0.21 
                 0.3 
                 0.52 
                 0.012 
                 0.001 
                 9 
                 2.3 
                 0.7 
                 0.012 
                 0.002 
                 0.002 
               
               
                   
                 m 
                 0.19 
                 0.29 
                 0.46 
                 0.011 
                 0.001 
                 9 
                 2.3 
                 0.6 
                 0.012 
                 0.009 
                 0.008 
               
               
                 Conventional 
                 A 
                 0.38 
                 1 
                 0.45 
                 0.011 
                 0.001 
                 5.5 
                 1.2 
                 0.85 
                 0.02 
                 0.012 
                 0.002 
               
               
                 Steel 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 450° Tempering 
                 Hardness after 
               
             
          
           
               
                   
                   
                   
                 Impact 
                   
                 repetitions 
               
               
                   
                   
                 Hardness 
                 value 
                 Corrosion 
                 of 650° 
               
               
                   
                 No. 
                 (HRC) 
                 (J/cm 2 ) 
                 resistance 
                 heating (HRC) 
               
               
                   
                   
               
             
          
           
               
                 Invention 
                 1 
                 40 
                 52 
                 A 
                 46 
               
               
                 Steel 
                 2 
                 42 
                 48 
                 A 
                 48 
               
               
                   
                 3 
                 41 
                 50 
                 A 
                 46 
               
               
                   
                 4 
                 43 
                 46 
                 A 
                 49 
               
               
                   
                 5 
                 44 
                 45 
                 A 
                 48 
               
               
                   
                 6 
                 42 
                 48 
                 A 
                 47 
               
               
                   
                 7 
                 42 
                 48 
                 A 
                 48 
               
               
                   
                 8 
                 42 
                 49 
                 A 
                 47 
               
               
                   
                 9 
                 41 
                 50 
                 A 
                 46 
               
               
                   
                 10 
                 43 
                 48 
                 A 
                 48 
               
               
                   
                 11 
                 41 
                 48 
                 A 
                 47 
               
               
                 Com- 
                 a 
                 36 
                 58 
                 A 
                 42 
               
               
                 parative 
                 b 
                 53 
                 21 
                 A 
                 48 
               
               
                 Steel 
                 c 
                 42 
                 25 
                 A 
                 48 
               
               
                   
                 d 
                 42 
                 23 
                 A 
                 48 
               
               
                   
                 e 
                 42 
                 18 
                 A 
                 47 
               
               
                   
                 f 
                 42 
                 15 
                 A 
                 48 
               
               
                   
                 g 
                 42 
                 49 
                 B 
                 48 
               
               
                   
                 h 
                 42 
                 21 
                 A 
                 48 
               
               
                   
                 i 
                 42 
                 48 
                 A 
                 44 
               
               
                   
                 j 
                 42 
                 32 
                 A 
                 47 
               
               
                   
                 k 
                 41 
                 33 
                 A 
                 47 
               
               
                   
                 l 
                 42 
                 28 
                 A 
                 48 
               
               
                   
                 m 
                 40 
                 30 
                 A 
                 48 
               
               
                 Conventional 
                 A 
                 53 
                 18 
                 B 
                 47 
               
               
                 Steel 
               
               
                   
               
               
                 * Corrosion resistance: A . . . no rusting, B . . . rusting occurred 
               
             
          
         
       
     
         [0117]    Furthermore, a steel obtained by heating Invention Steel No. 2 shown in Table 1 to 1,030° C. and subsequently subjecting it to oil quenching and then to tempering twice under the conditions with a temperature of 450° C. and a period of 1 h, one obtained by heating Conventional Steel A to 1,030° C. and subsequently subjecting it to oil quenching and then to tempering twice under the conditions with a temperature of 450° C. and a period of 1 h, and one obtained by subjecting Conventional Steel A to tempering twice under the conditions with a temperature of 630° C. and a period of 1 h were respectively evaluated for delayed-fracture resistance as an index to receptivity to cracking from a water-cooling hole. 
         [0118]    Here, the evaluation of delayed-fracture resistance was conducted in the following manner. 
         [0119]    Namely, industrial water was dropped (in order to cause rusting) onto the notched part of a test piece having a 0.1-R annular notch, and the relationship between flexural stress and fracture time was examined. 
         [0120]    The delayed-fracture resistance was evaluated by comparing in the ratio of static flexural stress (0-h rupture stress) to the stress causing rupture at 200 h. 
         [0121]    Furthermore, 10,000 cycles each including heating from room temperature to 650° C., holding at this temperature for 4 seconds, and subsequent water cooling were repeatedly conducted. Thereafter, the length of the heat crack generated at the surface was measured and evaluated as an index to heat check resistance. 
         [0122]    The results of these evaluations are shown in Table 3. 
         [0123]    In Table 3, the desired value of delayed-fracture resistance was set at 0.7 or higher. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 Heat check 
                 Delayed-fracture 
               
               
                   
                 Tempering 
                   
                 resistance 
                 resistance 
               
               
                   
                 temperature 
                 Hardness 
                 Length of largest 
                 Proportion of 200-h 
               
               
                 No. 
                 (° C.) 
                 (HRC) 
                 heat crack (μm) 
                 rupture stress 
               
               
                   
               
             
             
               
                 2 
                 450 
                 42 
                 120 
                 0.98 
               
               
                 A 
                 450 
                 53 
                 123 
                 0.65 
               
               
                   
                 630 
                 42 
                 253 
                 0.91 
               
               
                   
               
             
          
         
       
     
         [0124]    As the results given in Table 2 show, Invention Steels No. 1 to No. 11 have hardnesses of HRC 40 to 44 after the tempering at 450° C. and have hardnesses after the repetitions of heating at 650° C. of HRC 46 to 49. The hardnesses thereof have increased. 
         [0125]    Furthermore, since the tempering is low-temperature tempering at 450° C., almost no Cr carbide has separated out. Each steel shows satisfactory corrosion resistance. 
         [0126]    In contrast, Comparative Steel a has a C content of 0.05%, which is lower than the lower limit of 0.1% in the invention, and hence has a hardness after the 450° C. tempering as low as HRC 36. The hardness thereof after the repetitions of heating at 650° C. also is as low as HRC 42. It has poor heat check resistance. 
         [0127]    Comparative Steel b conversely has a C content of 0.38%, which is higher than the upper limit of 0.3% in the invention, and hence has a hardness after the 450° C. tempering as high as HRC 53. It has a low impact value. 
         [0128]    Comparative Steel c has a Si content of 2%, which is higher than the upper limit of 1.5% in the invention. It has a low impact value. 
         [0129]    Comparative Steel d has a Mn content of 2.5%, which is higher than the upper limit of 2% in the invention. It has a low impact value. 
         [0130]    Comparative Steel e has a content of P as an impurity of 0.08%, which is higher than the upper limit of 0.05% in the invention. This steel also has a low impact value. 
         [0131]    Furthermore, Comparative Steel f has a content of S also as an impurity of 0.05%, which is higher than the upper limit of 0.01% in the invention, and hence has a low impact value. 
         [0132]    Comparative Steel g has a Cr content of 5.1%, which is lower than the lower limit of 6% in the invention, and hence has low corrosion resistance. 
         [0133]    Comparative Steel h conversely has a Cr content of 13.5%, which is higher than the upper limit of 12% in the invention, and hence has a low impact value. 
         [0134]    Comparative Steel i has a Mo content of 0.6%, which is lower than the lower limit of 1% in the invention. Because of this, even through the repetitions of heating at 650° C., the hardness has not increased sufficiently. This means that heat check resistance is insufficient. 
         [0135]    Comparative Steel j has a V content of 0.3%, which is lower than the lower limit of 0.5% in the invention. Because of this, crystal grain enlargement has occurred and the steel has a low impact value. 
         [0136]    Comparative Steel k has an s-Al content of 0.003%, which is lower than the lower limit of 0.005% in the invention. Because of this, crystal grain enlargement has occurred and the steel has a low impact value. 
         [0137]    Comparative Steel l has an N content of 0.002%, which is lower than the lower limit of 0.005% in the invention. Because of this, crystal grain enlargement has occurred in this case also and the steel has a low impact value. 
         [0138]    Comparative Steel m has an O content of 0.008%, which is higher than the upper limit of 0.005% in the invention. Because of this, the steel contains a larger amount of inclusions and has a low impact value. 
         [0139]    Next, Conventional Steel A is JIS-SKD61 and has a hardness after the 450° C. tempering of HRC 53. The hardness thereof after the repetitions of heating at 650° C. has decreased to HRC 47. It is poor also in corrosion resistance. 
         [0140]    Next, in Table 3, Invention Steel No. 2 has a low hardness after the low-temperature tempering at 450° C. However, this steel is equal in heat check resistance and superior in delayed-fracture resistance to the high-hardness material obtained by tempering Conventional Steel A at 450° C. 
         [0141]    Furthermore, as compared with the steel having the same hardness obtained by the 630° C. high-temperature tempering of Conventional Steel A, Invention Steel No. 2 has higher corrosion resistance and better heat check resistance because of the low-temperature tempering. 
         [0142]    It can be seen as demonstrated above that the steels of the invention have both of the property of inhibiting cracking from a water-cooling hole and heat check resistance; these two properties have hitherto being inconsistent with each other. 
         [0143]    While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 
         [0144]    The present application is based on Japanese Patent Application No. 2005-346156 filed on Nov. 30, 2005, and the contents thereof are incorporated herein by reference.