JP5503476B2 - Steel - Google Patents

Description

  The present invention relates to a steel material used as a large thick cast steel product such as a crank throw of a large diesel engine for ships, and a component design method for the steel material.

  Large thick cast steel products such as crank throws of large diesel engines for ships are manufactured by casting molten steel into molds. Steel materials used as these large thick cast steel products have strength, weldability, castability, etc. Characteristics are required.

  Patent Document 1 discloses a heat treatment method for improving the strength, ductility, toughness, and weldability of a cast steel product used as a large structural member. In this heat treatment method, the steel is kept at 900 to 960 ° C., then air-cooled or oil-cooled, then kept at 620 to 680 ° C., and then cooled in the furnace. However, for example, it is difficult in terms of equipment and operation to cool a large thick cast steel product having a diameter of 1 m or more with water or oil.

  On the other hand, in Patent Document 2, in order to solve the problem of Patent Document 1, the strength is high even when the cooling rate during the austenitizing treatment is slow, such as air cooling, and the weldability is excellent, and it is closed during casting. A low alloy cast steel material in which no nest is generated is disclosed. In this Patent Document 2, a hardenability factor α, a weldability index β, and a castability index γ are set based on the content of the alloy element to improve strength, weldability, and castability.

  By the way, at the time of casting when manufacturing a large-sized thick cast steel product, it is required that no shrinkage defect occurs as pointed out in Patent Document 2, but in order to further improve the quality of the cast steel product, the reverse V It is also required to reduce segregation. When manufacturing large thick cast steel products, macro segregation is likely to occur because the solidification rate is low, and the composition of the components in the cast steel product becomes non-uniform, especially when a local segregation zone called reverse V segregation occurs. This is because shrinkage defects may occur inside the reverse V segregation, and the strength of the center axis position and the outer periphery of the cast steel product may vary, and weldability may deteriorate.

  The technique of Patent Document 3 is known as a technique for reducing reverse V segregation. In this patent document 3, generation | occurrence | production of reverse V segregation is prevented by reducing Si, Mn, P, and S among inevitable impurities. However, if Mn is reduced too much, the strength cannot be secured. Further, in Patent Document 3, 2% or more of Cr is positively added, but if excessive Cr is contained, the density of the molten steel becomes small, so the concentrated molten steel and the bulk molten steel in the solid-liquid coexistence region during solidification. And the density difference between the two increases and reverse V segregation is likely to occur.

  Patent Document 4 has been proposed as a technique for suppressing streak-like macro and micro segregation by considering the segregation form due to the density difference in the molten steel. However, in this patent document 4, Ni is suppressed to 0.30% or less and Cr is contained in an amount of 1.0% or more. When the present inventor examined, the density difference in the molten steel is large, and reverse V segregation occurs. It turned out that generation | occurrence | production of is not fully suppressed.

Non-Patent Document 1 discloses the relationship between the reverse V segregation line density of the killed steel ingot and the composition of the steel material. This document describes the reverse V segregation line density of the killed steel ingot in the solid-liquid coexisting phase. It is described that the value increases as the value of the index φ representing the average buoyancy acting on the inner molten steel increases. V (vanadium) is an element that reduces head segregation and V segregation, but it is described that the coefficient of V in the index φ considering only the density difference of molten steel is zero.
φ = 0.01 × [% C] + 0.63 × [% Si] + 0.10 × [% Mn] + 3.64 × [% P] + 4.24 × [% S] −0.19 × [% Mo ] + 0.01 × [% Cr] −0.01 × [% Ni] (a)

JP 59-136451 A JP 2000-345285 A JP-A-5-239593 JP 2003-268485 A

Osamu Haida, “Estimation of reverse V segregation line density in steel ingots by molten steel composition”, Iron and Steel, 61st (1981) No. 7, p. 114-118

  In the said nonpatent literature 1, attention is paid about the relationship between reverse V segregation and the component composition of steel materials. However, no attention has been paid to casting defects such as shrinkage defects.

  This invention is made | formed in view of such a condition, The objective is to provide the steel materials which reduced casting defects, such as a shrinkage nest, and the component design method of this steel materials.

The steel materials according to the present invention that have been able to solve the above problems are in mass%, C: 0.15 to 0.27%, Si: 0.10% or less (not including 0%), Mn: 0.5 -1.2%, Ni: 0.35-1.60%, Cr: 0.1-0.7%, Mo: 0.1-0.4%, V: 0.3% or less (0% Al: 0.03% or less (not including 0%), Cu: 0.03% or less (not including 0%), P: 0.01% or less (not including 0%), S : A steel material containing 0.01% or less (not including 0%), the balance being iron and inevitable impurities. The value of the index φ1 calculated by the following equation (1) is −0.5 to 0 and / or the value of the index φ2 calculated by the following equation (2) is −0.008 to 0.00. The point is that it is 008.
φ1 = 0.01 × [C] + 0.63 × [Si] + 0.10 × [Mn] −0.4 × [Ni] + 0.01 × [Cr] −0.19 × [Mo] −0.20 * [V] + 1.0 * [Al] + 0.05 * [Cu] + 3.64 * [P] + 4.24 * [S] (1)
φ2 = (φ1 + 0.2) / (100−ΔT) (2)

However, in the above formula (2), φ1 is the value obtained by the above formula (1), ΔT is a value obtained by the following formula (3), and in the following formula (3), T _L is the following formula (4), and T _S is It is a value obtained by the following equation (5).
ΔT = T _L −T _S (3)
T _L = 1536− (0.1 + 83.9 × [C] + 10 × [C] ² + 12.6 × [Si] + 5.4 × [Mn] −30 × [P] −37 × [S] +4.6 × [Cu] + 5.1 × [Ni] + 1.5 × [Cr] −3.3 × [Mo]) (4)
T _S = 1536− (415.5 × [C] + 12.3 × [Si] + 6.8 × [Mn] + 124.5 × [P] + 183.9 × [S] + 4.3 × [Ni] +1. 4 × [Cr] + 4.1 × [Al]) (5)
In the above formulas (1) and (3) to (5), [] means the content (% by mass) of each element.

  The present invention also includes cast steel products (for example, crank throws) manufactured from the above steel materials.

The steel materials according to the present invention that have been able to solve the above problems are in mass%, C: 0.15 to 0.27%, Si: 0.10% or less (not including 0%), Mn: 0.5 -1.2%, Ni: 0.35-1.60%, Cr: 0.1-0.7%, Mo: 0.1-0.4%, V: 0.3% or less (0% Al: 0.03% or less (not including 0%), Cu: 0.03% or less (not including 0%), P: 0.01% or less (not including 0%), S : 0.01% or less (excluding 0%) in steel materials with the balance being iron and inevitable impurities, the value of the index φ1 calculated by the following formula (1) satisfies −0.5 to 0 By adjusting the components so that the value of the index φ2 calculated by the following equation (2) satisfies −0.008 to 0.008. Can be obtained.
φ1 = 0.01 × [C] + 0.63 × [Si] + 0.10 × [Mn] −0.4 × [Ni] + 0.01 × [Cr] −0.19 × [Mo] −0.20 * [V] + 1.0 * [Al] + 0.05 * [Cu] + 3.64 * [P] + 4.24 * [S] (1)
φ2 = (φ1 + 0.2) / (100−ΔT) (2)

However, in the above formula (2), φ1 is the value obtained by the above formula (1), ΔT is a value obtained by the following formula (3), and in the following formula (3), T _L is the following formula (4), and T _S is It is a value obtained by the following equation (5). In the following formulas (1) and (3) to (5), [] means the content (% by mass) of each element.
ΔT = T _L −T _S (3)
T _L = 1536− (0.1 + 83.9 × [C] + 10 × [C] ² + 12.6 × [Si] + 5.4 × [Mn] −30 × [P] −37 × [S] +4.6 × [Cu] + 5.1 × [Ni] + 1.5 × [Cr] −3.3 × [Mo]) (4)
T _S = 1536− (415.5 × [C] + 12.3 × [Si] + 6.8 × [Mn] + 124.5 × [P] + 183.9 × [S] + 4.3 × [Ni] +1. 4 × [Cr] + 4.1 × [Al]) (5)
In the above formulas (1) and (3) to (5), [] means the content (% by mass) of each element.

  According to the present invention, the composition of the steel material is designed based on the above equation (1) and / or the above equation (2), and the addition balance of the alloy elements contained in the steel material is appropriately controlled. Not only can the occurrence of V segregation be suppressed, but also the occurrence of casting defects such as shrinkage can be prevented. Therefore, it is possible to provide a steel material that can maintain high strength even when a large-sized thick cast steel product is produced, and that the difference in strength between the central axis position and the outer peripheral portion of the cast steel product is reduced and weldability can be prevented from being deteriorated.

FIG. 1 is a graph showing the relationship between the value of the index φ1 and the number of casting defects. FIG. 2 is a graph showing the relationship between the value of the index φ2 and the number of casting defects. FIG. 3 is a graph showing the relationship between the value of the index φ and the number of casting defects.

  When the present inventors cast a molten steel to produce a large thick cast steel product, the strength difference between the central axis and the outer surface side is small while maintaining high strength, and the weldability is good. In order to do so, we have conducted intensive studies aimed at preventing the occurrence of casting defects such as shrinkage cavities. As a result, if the component composition of the steel material satisfies the index φ1 calculated by the following equation (1) and / or the component design so as to satisfy the index φ2 calculated by the following equation (2), The present invention was completed by finding out that casting defects are less likely to occur.

  First, the component composition of the steel material of the present invention will be described. The steel materials of the present invention are: C: 0.15 to 0.27%, Si: 0.10% or less (not including 0%), Mn: 0.5 to 1.2%, Ni: 0.35 to 1 .60%, Cr: 0.1 to 0.7%, Mo: 0.1 to 0.4%, V: 0.3% or less (excluding 0%), Al: 0.03% or less (0 %), Cu: 0.03% or less (not including 0%), P: 0.01% or less (not including 0%), S: 0.01% or less (not including 0%) It contains. The reason for specifying such a range is as follows.

  C is an element that improves the strength of the steel material and improves the hardenability, and needs to be contained by 0.15% or more. Preferably it is 0.17% or more. However, if excessively contained, the sensitivity to weld cracking increases, so the content is made 0.27% or less. Preferably it is 0.25% or less, more preferably 0.24% or less.

  Si acts as a deoxidizer and is an element that acts to improve hardenability. In order to exhibit such an action effectively, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more. However, if excessively contained, casting defects occur during casting, so the content is made 0.10% or less. Preferably it is 0.08% or less, More preferably, it is 0.06% or less.

  Mn is an element that improves the strength of the steel material and improves the hardenability, and it is necessary to contain 0.5% or more. Preferably it is 0.55% or more. However, if excessively contained, casting defects occur during casting. Preferably it is 1.15% or less, More preferably, it is 1.0% or less.

  Ni is an element that improves the strength of the steel material and improves the hardenability without degrading the weldability, and is an element recommended to be positively contained. Therefore, in the present invention, it is necessary to contain 0.35% or more of Ni. Preferably it is 0.4% or more, More preferably, it is 0.5% or more. However, since Ni is an expensive element, the cost of containing excessively becomes high. Therefore, Ni is 1.60% or less, preferably 1.5% or less, more preferably 1.4% or less.

  Cr is an element that improves the strength of the steel material, improves the hardenability, and also acts to increase the temper softening resistance. In order to exert such effects, it is necessary to contain 0.1% or more. Preferably it is 0.2% or more, More preferably, it is 0.3% or more. However, if excessively contained, the weldability is lowered, so the content is made 0.7% or less. Preferably it is 0.65% or less, More preferably, it is 0.6% or less.

  Mo, like Cr, is an element that improves the hardenability in addition to improving the strength of the steel material, and also acts to increase the temper softening resistance. In order to exert such effects, it is necessary to contain 0.1% or more. Preferably it is 0.2% or more, More preferably, it is 0.3% or more. However, if excessively contained, the weldability is lowered, so the content is made 0.4% or less. Preferably it is 0.33% or less.

  V is an element that complicates the dendrite form and suppresses the occurrence of reverse V segregation by suppressing the flow of the concentrated molten steel in which the alloy element is concentrated when coexisting with solid and liquid. V is an element that suppresses the occurrence of casting defects such as shrinkage by complicating the dendrite form. V is an element that increases the temper softening resistance and contributes to the refinement of the solidified structure. In order to exhibit such an action effectively, it is preferable to contain 0.01% or more. More preferably, it is 0.03% or more, More preferably, it is 0.06% or more. However, even if contained excessively, the effect of increasing the temper softening resistance is saturated, and in order to reduce the weldability, the content is made 0.3% or less. Preferably it is 0.2% or less, More preferably, it is 0.1% or less.

In addition to acting as a deoxidizer, Al is an element that effectively acts to increase the toughness by refining the crystal grains of the steel material. However, if it is excessively contained, a large amount of Al ₂ O ₃ inclusions are produced, the cleanliness of the molten steel is lowered, and the strength is lowered. Preferably it is 0.028% or less, More preferably, it is 0.025% or less.

  Cu is an element that improves toughness and is also an element that effectively acts to increase the yield stress of cast steel. In order to exhibit such an effect effectively, it is preferable to make it contain 0.01% or more. More preferably, it is 0.015% or more. However, if it is contained excessively, the surface properties deteriorate as hot cracking is likely to occur, so the content is made 0.03% or less. Preferably it is 0.025% or less, more preferably 0.02% or less.

  P is an element contained as an unavoidable impurity, and if contained excessively, P is an element that concentrates in the segregated portion and reduces the difference in molten steel density in the concentrated portion, and easily causes reverse V segregation. Further, it is an element that lowers the solidus temperature of molten steel and increases ΔT to easily cause casting defects. Further, excessive P causes a decrease in toughness. Therefore, P must be suppressed to 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.006% or less.

  S is an element contained as an unavoidable impurity as in the case of P, and if contained excessively, S is an element that concentrates in the segregation part and reduces the difference in the density of molten steel in the enriched part to easily generate reverse V segregation. is there. Further, it is an element that lowers the solidus temperature of molten steel and increases ΔT to easily cause casting defects. In addition, excessive S causes a decrease in toughness. Therefore, S must be suppressed to 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.006% or less.

  The component composition of the steel material of the present invention is as described above, and the balance is iron and inevitable impurities.

In the steel material of the present invention, after satisfying the above-described component composition, the value of the index φ1 calculated by the following formula (1) is −0.5 to 0 and / or the following formula (2): It is important that the calculated value of the index φ2 is −0.008 to 0.008.
φ1 = 0.01 × [C] + 0.63 × [Si] + 0.10 × [Mn] −0.4 × [Ni] + 0.01 × [Cr] −0.19 × [Mo] −0.20 * [V] + 1.0 * [Al] + 0.05 * [Cu] + 3.64 * [P] + 4.24 * [S] (1)
φ2 = (φ1 + 0.2) / (100−ΔT) (2)

However, ΔT is a value obtained by the following equation (3), and in the following equation (3), T _L is a value obtained by the following equation (4), and T _S is a value obtained by the following equation (5).
ΔT = T _L −T _S (3)
T _L = 1536− (0.1 + 83.9 × [C] + 10 × [C] ² + 12.6 × [Si] + 5.4 × [Mn] −30 × [P] −37 × [S] +4.6 × [Cu] + 5.1 × [Ni] + 1.5 × [Cr] −3.3 × [Mo]) (4)
T _S = 1536− (415.5 × [C] + 12.3 × [Si] + 6.8 × [Mn] + 124.5 × [P] + 183.9 × [S] + 4.3 × [Ni] +1. 4 × [Cr] + 4.1 × [Al]) (5)
In the above formula (1) and formulas (3) to (5), [] means the content (% by mass) of each element.

<< About index φ1 >>
The above equation (1) is an equation showing the balance of alloy element content that affects the occurrence of casting defects, and each coefficient indicates the degree of influence on the occurrence of casting defects. That is, C, Si, Mn, Cr, Al, Cu, P, and S having a positive coefficient have an action of promoting the occurrence of casting defects. These elements reduce the density of the concentrated molten steel in the solid-liquid coexistence region at the time of solidification, increase the density difference from the bulk molten steel, and float the concentrated molten steel, making it easy to generate reverse V segregation. Further, these elements easily cause casting defects such as shrinkage cavities. On the other hand, Ni, Mo, and V having negative coefficients have an effect of suppressing the occurrence of casting defects such as shrinkage cavities. These elements have the effect of increasing the density of the concentrated molten steel, preventing the concentrated molten steel from rising, and preventing the occurrence of casting defects. In addition, Mo and V have the effect of complicating the form of dendrites formed immediately after the start of solidification of the molten steel, and the concentrated molten steel flows during the coexistence of solid and liquid to generate casting defects. It is an element to prevent.

  In addition, in order to improve casting defects such as shrinkage cavities, the content balance of V, Al, and Cu must be appropriately adjusted as defined by the above formula (1). In Non-Patent Document 1, no consideration is given. Further, it is clear that Ni has a great influence on the occurrence of casting defects such as shrinkage cavities, and must be larger than the coefficient set by the equation (a) described in Non-Patent Document 1 above. became.

  If the index φ1 calculated from the above equation (1) exceeds 0, the strength of the steel material is increased, but the amount of alloying elements becomes excessive, so that casting defects are likely to occur. Therefore, the value of the index φ1 is set to 0 or less. Preferably it is -0.10 or less, More preferably, it is set to -0.15 or less.

  On the other hand, it is generally considered that casting defects are less likely to occur when the amount of alloying elements is smaller. However, it has been found that when the index φ1 is less than −0.5, casting defects are generated as shown in the examples described later. The reduction of the index φ1 means that the density of the concentrated molten steel in the solid-liquid coexistence region during solidification is increased. That is, when the index φ1 is decreased, the density difference from the bulk molten steel is increased, and the concentrated molten steel is settled without floating. Accordingly, segregation bands are likely to occur. As a result, casting defects such as shrinkage cavities tend to occur inside the segregation zone. On the other hand, when the index φ1 is less than −0.5, the amount of alloy elements is insufficient, and the strength of the steel material is lowered. Therefore, the value of the index φ1 is set to −0.5 or more. Preferably it is -0.45 or more, More preferably, it is set to -0.4 or more.

<About index φ2>
The above equation (2) shows the degree of influence of the alloy element content balance (index φ1) and the solid-liquid coexistence temperature range (ΔT) that influence the occurrence of casting defects on the occurrence of casting defects. The solid-liquid coexistence temperature range (ΔT) is obtained as a temperature difference between the liquidus temperature [T _L (° C.)] and the solidus temperature [T _S (° C.)] as is apparent from the above equation (3). It is done. When this ΔT is increased, the time until the molten steel is solidified becomes longer, so that the concentrated molten steel enriched with the alloy elements is likely to move through the steel, and reverse V segregation is likely to occur. In addition, casting defects such as shrinkage cavities are likely to occur. Therefore, in the above equation (2), the index φ2 is calculated in consideration of both the content balance of alloy elements contained in the steel material and the solid-liquid coexistence temperature range. The index φ1 described above can be used as a simple method, but the occurrence of casting defects can be more accurately suppressed by adjusting the component composition of the steel material using the index φ2.

  As shown in the above equation (2), the reason for adding 0.2 to the value of φ1 is that when the index φ1 is −0.2, the evaluation index for the shrinkage defect defined by the above equation (1) Therefore, when the index φ2 is calculated by incorporating ΔT into the evaluation index in the above equation (2), the minimum number of shrinkage defects is obtained. It is for preparing to become a value.

  Further, the reason why ΔT is subtracted from 100 is that ΔT is 100 or less in the general composition of steel materials, and when ΔT is increased, reverse V segregation is likely to occur and shrinkage defects are likely to occur. Therefore, in the present invention, a value obtained by subtracting ΔT from 100 is used.

  When the index φ2 calculated from the above equation (2) exceeds 0.008, the strength of the steel material is increased, but the amount of alloying elements becomes excessive, and the temperature range of solid-liquid coexistence is widened so that the concentrated molten steel is easily moved. Therefore, casting defects are likely to occur. Therefore, the value of the index φ2 is set to 0.008 or less. Preferably it is 0.005 or less, More preferably, it is 0.001 or less.

  On the other hand, it is considered that casting defects are less likely to occur when the amount of alloying elements is smaller and the solid-liquid coexistence temperature range is narrower. However, when the index φ2 is less than −0.008, as shown in the examples described later, It was found that casting defects occurred. The fact that the index φ2 becomes smaller means that the density of the concentrated molten steel in the solid-liquid coexistence region at the time of solidification becomes larger as when the index φ1 becomes smaller. That is, when the index φ2 is decreased, the density difference from the bulk molten steel is increased, and the concentrated molten steel is settled without floating. Accordingly, segregation bands are likely to occur. As a result, casting defects such as shrinkage cavities tend to occur inside the segregation zone. On the other hand, when the index φ2 is less than −0.008, the amount of alloying elements is insufficient and the strength of the steel material is lowered. Therefore, the value of the index φ2 is set to −0.008 or more. Preferably it is -0.007 or more, More preferably, it is set to -0.006 or more.

  The steel material of the present invention may be cast by adjusting the components according to a conventional method so that the component composition satisfies the above range and the value of the index φ1 or the index φ2 satisfies the above range. The index φ1 and the index φ2 may be satisfied simultaneously.

  The ingot obtained by casting may be quenched or normalized. Although the temperature at the time of quenching or normalizing is not particularly limited, it may be, for example, 900 to 950 ° C. Cooling after quenching or normalizing may be air cooling. In the air cooling, the average cooling rate in the temperature range of 900 ° C. to 400 ° C. may be reduced. It is recommended that the average cooling rate be controlled so that the cooling rate at the center position of the material having a diameter equivalent to 1000 mm is 1 ° C./min or less.

  Further, after quenching or normalizing, tempering may be performed. By tempering, the tensile strength can be increased (specifically, 450 MPa or more). Tempering is preferably performed at a temperature of 600 to 700 ° C. and a time of 10 to 30 hours, for example.

  In the present invention, by appropriately controlling the component composition of the steel material, it is possible to prevent the occurrence of reverse V segregation during casting, and further to prevent the occurrence of casting defects such as shrinkage cavities. Even when a large thick cast steel product of about 1200 mm is manufactured, it is possible to provide a steel material having high strength, little difference in strength between the shaft position and the outer peripheral portion, and excellent weldability.

  The steel material of the present invention can be used as a raw material for a cast steel product, for example, a crank throw of a large diesel engine for ships.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steel having the composition shown in Table 1 below (the balance being iron and inevitable impurities) was melted in a high-frequency induction melting furnace, and an ingot of φ1200 mm × height 2000 mm and 25 tons was cast using a sand mold. Of the height of 2000 mm, the length of the main body portion is 1200 mm, and the length of the feeder portion is 800 mm. The casting temperature was 1545 ° C.

Based on the component composition shown in Table 1 below, the values of index φ1, index φ2, ΔT, T _L , T _S are calculated from the above formulas (1) to (5), and the calculation results are shown in Table 2 below. . Further, as reference data, the value of the index φ calculated from the equation (a) described in Non-Patent Document 1 is similarly calculated, and the calculation result is shown in Table 2 below.

  In addition, for the obtained ingot, the number of casting defects (in the shrinkage cavity) on two cross-sections: a cross section under the feeder (diameter: φ1200 mm) and a central longitudinal section of the main body (diameter 1200 mm × main body length 1200 mm). The number of resulting casting defects) was measured and totaled. The number of casting defects was measured by a penetrant inspection (PT; color check) which is a nondestructive inspection. The measurement results are shown in Table 2 below. In this example, the case where the number of defects detected by the penetrant flaw detection test (PT) was 150 or less was regarded as acceptable, and the case where the number of defects exceeded 150 was regarded as unacceptable. Note that the criterion for passing a defect number of 150 or less is a stricter condition than before, and conventionally, even if the total number of defects is about 300, it is allowed as a pass.

  FIG. 1 shows the relationship between the value of the index φ1 and the number of defects detected by the penetration flaw detection test (PT), and the relationship between the value of the index φ2 and the number of defects detected by the penetration flaw detection test (PT). It is shown in 2. As a reference diagram, FIG. 3 shows the relationship between the value of the index φ calculated from the above equation (a) and the number of defects detected by the penetration flaw detection test (PT). 1 to 3, No. 1 shown in Table 2 below. The results of 1 to 11 are indicated by ■, No. The results of 12 to 16 are indicated by ▲.

  Next, the obtained ingot was cut into 90 mm × 90 mm × 190 mm to prepare a test material. The specimen was heated to 920 ° C. and held, and then cooled at an average cooling rate of 1 ° C./min. This average cooling rate is a relatively slow side of the rate at which the thick cast metal is cooled, and simulates air cooling. After cooling, tempering was performed at 620 ° C. for 20 hours.

  A test piece for tensile test (JIS 14A test piece) was cut out from the tempered test material, a tensile test was performed, and a tensile strength (TS) was measured. The measurement results are shown in Table 2 below. In the present embodiment, TS is considered to be acceptable if it is 450 MPa or more. In addition, “−” means that the tensile strength is not measured.

  It can be considered as follows from Table 2 below. No. 1 to 11 are examples that satisfy the requirements defined in the present invention, the composition of the steel material satisfies the range defined in the present invention, and the values of the index φ1 and the index φ2 are defined in the present invention. I am satisfied with the range. Therefore, the generation of shrinkage cavities is suppressed, and the number of casting defects can be reduced. Also, the tensile strength can be secured.

  On the other hand, no. Nos. 12 to 16 are examples that do not satisfy the requirements defined in the present invention. Of these, No. No. 12 is an example in which the component composition of the steel material satisfies the range defined by the present invention, but the values of the index φ1 and the index φ2 are outside the range defined by the present invention. Therefore, shrinkage cavities occur and the number of casting defects exceeds 150. No. Examples 13 to 16 are examples in which the component composition of the steel material does not satisfy the requirements defined by the present invention, and the values of the index φ1 and the index φ2 do not satisfy the range defined by the present invention. Therefore, shrinkage cavities occur and the number of casting defects exceeds 150.

  Next, it can consider as follows from FIGS. 1-3. 1 and 2 showing the results of component design using the index φ1 or index φ2 defined in the present invention, the present invention example in which the number of defects detected by the penetration flaw detection test (PT) is 150 or less ( {Circle around (2)}: No. 1 to 11) in Table 2 and over 150 comparative examples (（: No. 12 to 16 in Table 2) are well separated. On the other hand, in FIG. 3 showing the result when the component is designed using the index φ calculated from the above equation (a), the present invention example (■) and the comparative example (▲) are not completely separated. Therefore, it can be seen that a steel material with a reduced number of casting defects such as shrinkage can be provided by designing the components using the index φ1 or the index φ2.

Claims (3)

% By mass
C: 0.15-0.27%,
Si: 0.10% or less (excluding 0%),
Mn: 0.5 to 1.2%
Ni: 0.35 to 1.60%,
Cr: 0.1 to 0.7%,
Mo: 0.1 to 0.4%,
V: 0.3% or less (excluding 0%),
Al: 0.03% or less (excluding 0%),
Cu: 0.03% or less (excluding 0%),
P: 0.01% or less (excluding 0%),
S: 0.01% or less (not including 0%),
The balance is steel consisting of iron and inevitable impurities,
A steel material characterized in that the value of the index φ1 calculated by the following formula (1) is −0.5 to 0.
φ1 = 0.01 × [C] + 0.63 × [Si] + 0.10 × [Mn] −0.4 × [Ni] + 0.01 × [Cr] −0.19 × [Mo] −0.20 * [V] + 1.0 * [Al] + 0.05 * [Cu] + 3.64 * [P] + 4.24 * [S] (1)
[However, in the above formula (1), [] means the content (% by mass) of each element. ] % By mass
C: 0.15-0.27%,
Si: 0.10% or less (excluding 0%),
Mn: 0.5 to 1.2%
Ni: 0.35 to 1.60%,
Cr: 0.1 to 0.7%,
Mo: 0.1 to 0.4%,
V: 0.3% or less (excluding 0%),
Al: 0.03% or less (excluding 0%),
Cu: 0.03% or less (excluding 0%),
P: 0.01% or less (excluding 0%),
S: 0.01% or less (not including 0%),
The balance is steel consisting of iron and inevitable impurities,
A steel material characterized in that the value of the index φ2 calculated by the following equation (2) is −0.008 to 0.008.
φ2 = (φ1 + 0.2) / (100−ΔT) (2)
[However, in the above formula (2), φ1 is a value calculated by the following formula (1) , and the value of φ1 is −0.5 to 0. ΔT is a value obtained by the following equation (3), and in the following equation (3), T _L is a value obtained by the following equation (4), and T _S is a value obtained by the following equation (5). In the following formulas (1) and (3) to (5), [] means the content (% by mass) of each element.
φ1 = 0.01 × [C] + 0.63 × [Si] + 0.10 × [Mn] −0.4 × [Ni] + 0.01 × [Cr] −0.19 × [Mo] −0.20 * [V] + 1.0 * [Al] + 0.05 * [Cu] + 3.64 * [P] + 4.24 * [S] (1)
ΔT = T _L −T _S (3)
T _L = 1536− (0.1 + 83.9 × [C] + 10 × [C] ² + 12.6 × [Si] + 5.4 × [Mn] −30 × [P] −37 × [S] +4.6 × [Cu] + 5.1 × [Ni] + 1.5 × [Cr] −3.3 × [Mo]) (4)
T _S = 1536− (415.5 × [C] + 12.3 × [Si] + 6.8 × [Mn] + 124.5 × [P] + 183.9 × [S] + 4.3 × [Ni] +1. 4 × [Cr] + 4.1 × [Al]) (5)]   A cast steel product manufactured from the steel material according to claim 1 or 2.

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