JP6296797B2 - High strength cast steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength cast steel material excellent in strength, ductility and weldability, and a method for producing the same, particularly for use in special vehicle parts, structural members such as buildings, civil engineering, and bridges, and dies.

  In recent years, axle parts and suspension parts for special vehicles such as axles and suspensions used under harsh conditions, hardware parts for building structures to create various designs and large spaces, and mold members that require a long life In manufacturing, a material having high strength, excellent ductility, and a high degree of freedom in shape is required. This is because in order to suppress the increase in the total weight accompanying the increase in size of these final products, it is necessary to make the parts and members as thin and light as possible.

  There are rolled steel materials, forged steel materials, and cast steel materials as iron-based structural materials used as materials for the above-mentioned parts and members. Generally, rolled steel materials and forged steel materials are applied to applications that have high demands. This is because cast steel has low ductility when compared with rolled steel or forged steel of the same strength, and it is difficult to satisfy the above requirements.

  On the other hand, recently, the demand for weight reduction of the above parts and members has been increasing, and in order to reduce the weight of these parts and members, more complex shapes have been required. ing. However, since rolled steel and forged steel materials are limited in the degree of freedom of the parts to be obtained and the shape of the parts due to the manufacturing process, the problem of not being able to sufficiently meet such requirements has become apparent. Yes.

  Against this background, in recent years, techniques have been proposed for increasing the strength and ductility of cast steel materials that can obtain complex shapes. For example, Patent Document 1 proposes a cast steel material whose component composition is limited by a hardenability multiple or weldability index. Patent Document 2 proposes a cast steel material whose component composition is limited by bainite parameters, high toughness multiple, carbon equivalent, and weld crack sensitivity. Patent Document 3 proposes a high strength and high toughness cast steel obtained by austempering a specific composition cast steel. Further, in Patent Document 4 by the applicant of the present invention, a cast steel material having high strength and toughness is realized by adding Ni that is effective in improving strength and hardenability and has a small adverse effect on ductility and weldability. .

Japanese Patent No. 3509634 Japanese Patent No. 3553601 JP 2003-306741 A Japanese Patent No. 4790512

  By the way, as parts and members for the above applications, there are those that still need a thickness exceeding 100 mm even if they are made as thin as possible for weight reduction, and cast steel constituting such thick parts and members. There is a demand for high strength and high ductility of a material having a tensile strength of 900 MPa or more and an elongation of 10% or more. In addition, the material cost is relatively low, and the room temperature impact absorption energy is required to be 27 J or more in order to improve the weldability that is high in toughness as a material and is indispensable for a practical material.

  However, the materials of Patent Document 1 and Patent Document 2 both have a tensile strength of around 700 MPa, and Patent Document 4 achieves a tensile strength of more than 1000 MPa. Since it was necessary to add 4%, the material cost increased, and its application was limited. Furthermore, in these prior arts, since high strength is obtained only by quenching, there is a limit to the quenching depth, and a decrease in strength inside the thick product is inevitable.

  Further, in Patent Document 3, a tensile strength of over 900 MPa and an elongation of about 10% are realized, but austempering is essential, and the size of applicable parts and members is limited to relatively small ones, It is difficult to apply to thick parts and members of 100 mm or more.

  Thus, when applied to thick parts and members exceeding 100 mm, a tensile strength of 900 MPa or more, an elongation of 10% or more, and a room temperature impact absorption energy of 27 J or more can be obtained, and it is relatively inexpensive. The technology for obtaining such cast steel material has not yet been realized.

  The present invention has been made in view of such circumstances, and has a high strength cast steel material that has both high strength and appropriate ductility, is excellent in toughness and weldability, and is relatively inexpensive, even in a thick product. It aims at providing the manufacturing method.

As a result of intensive studies to solve the above problems, the present inventors have found that it is effective to combine the following matters.
(I) An appropriate amount of Cu is added, and high strength is obtained by the strengthening action accompanying the precipitation.
(Ii) In thick products, the mass effect that reduces the internal strength compared to the surface layer that occurs during rapid cooling (corresponding to quenching) after solution treatment is mitigated by precipitation strengthening.
(Iii) The addition of V + Mo enables high temperature aging, and simultaneously obtains high strength and moderate ductility.
(Iv) Add Ca according to the amount of S to control the form of the sulfide to improve ductility.

  The present invention has been made based on these findings and provides the following (1) to (5).

(1) In mass%,
C: 0.05 to 0.2%
Si: 0.1 to 0.5%,
Mn: 0.5 to 1.5%
P: 0.015% or less,
S: 0.015% or less,
Cu: 1.5-3%,
Ni: 0.5 to 1.5%,
Mo: 0.3 to 0.5%,
V: 0.1-0.25%,
Ca: 0.01 to 0.05%,
And the content of Ca and S is mass% S × 2.5 ≦ Ca ≦ S × 4.5
Satisfied with the relationship
When the content of Mo and V is mass%, 0.3% ≦ V + 0.53 × Mo ≦ 0.4%
Satisfied with the relationship
A high-strength cast steel material characterized in that the balance consists of Fe and inevitable impurities.

(2) The high-strength cast steel material according to (1), wherein the Ca is added by a Ni—Ca alloy.

(3) In the above (1) or (2), C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14
A high-strength cast steel material having a carbon equivalent value of 0.6% or less by mass%.

  (4) In any one of the above (1) to (3), the thickness is 200 mm or less, the tensile strength is 900 MPa or more, the elongation is 10% or more, and the room temperature impact absorption energy is 27 J or more. Characteristic high strength cast steel material.

  (5) After heating the cast steel material having the component composition according to any one of (1) to (3) above at 850 to 1000 ° C., from the temperature to 150 ° C., an average cooling rate of 5 ° C./second or more. And a aging treatment is performed at 450 to 650 ° C. for 1 to 2 hours.

  According to the present invention, a relatively inexpensive alloy composition and a simple heat treatment provide a cast steel material that has excellent strength and ductility, and has good toughness and weldability, even for thick parts or members, and a method for producing the same. The effect is very remarkable.

It is a figure which shows the test material shape for evaluating the mechanical property of a cast steel product. It is a figure which shows the weld part groove shape for weldability evaluation. It is a photograph which shows the macro structure near the joint of a welding material.

  Hereinafter, the present invention will be described in detail with respect to the chemical components of steel and the reasons for limitation of the production method. In addition, unless otherwise indicated, the% display in a component is the mass%.

[Chemical composition]
C: 0.05 to 0.2%
C is an element effective for improving the strength and quenching hardness of cast steel. Moreover, it combines with V and Mo to form carbides, improves the temper softening resistance, offsets the strength decrease due to high temperature aging, and prevents the mechanical properties of the weld heat affected zone from decreasing. However, if the content is less than 0.05%, a tensile strength of 900 MPa or more cannot be obtained, and if it exceeds 0.2%, the ductility is lowered and weld cracks are likely to occur. Therefore, the C content is in the range of 0.05 to 0.2%.

Si: 0.1 to 0.5%
Si is an element added for the purpose of deoxidation and improvement of hot water flowability. However, if the content is less than 0.1%, the molten steel has low flowability, resulting in deterioration of casting quality. Further, Si moves the transition temperature to the high temperature side to reduce toughness, and if its content exceeds 0.5%, embrittlement increases. Therefore, the Si content is in the range of 0.1 to 0.5%.

Mn: 0.5 to 1.5%
Mn is an element effective for improving the strength of cast steel and has a deoxidizing effect. However, if the content is less than 0.5%, the effect is small, and if it exceeds 1.5%, the ductility is lowered and the carbon equivalent is increased, so that the weldability is greatly deteriorated. Therefore, the Mn content is in the range of 0.5 to 1.5%.

Cu: 1.5 to 3%
When Cu is added to an Fe alloy, its solubility greatly varies depending on the temperature. Therefore, after Cu is homogenized at a high temperature and then subjected to an aging treatment, there is an effect of increasing strength. However, if the content is less than 1.5%, the effect of improving the strength is small, a tensile strength of 900 MPa or more cannot be obtained, and if it exceeds 3%, the ductility decreases and an elongation of 10% or more cannot be obtained. . Therefore, the Cu content is set to a range of 1.5 to 3%.

Ni: 0.5 to 1.5%
Ni is added because it is effective in improving the strength of cast steel, has a very small adverse effect on ductility and weldability, and has a large effect of improving toughness. However, if the content is less than 0.5%, the effect is insufficient, and if it exceeds 1.5%, an increase in material cost cannot be ignored. Therefore, the Ni content is in the range of 0.5 to 1.5%.

Mo: 0.3-0.5%
Mo is an element that improves the hardenability of the cast steel material and is effective in improving the strength. Moreover, since it combines with C to form a carbide and secondarily cures to maintain strength, the aging temperature can be increased, and as a result, ductility can be improved. However, if the content is less than 0.3%, the effect is small, and if it exceeds 0.5%, ferrite precipitates in the thick wall portion where the cooling rate is slow, the strength decreases, the carbon equivalent increases, and the weldability increases. Reduce. Therefore, the Mo content is set to a range of 0.3 to 0.5%.

V: 0.1-0.25%
V has an effect similar to that of Mo and is combined with C to form a carbide and maintain strength. Therefore, the aging temperature can be increased and ductility can be improved. However, if the content is less than 0.1%, the effect is insufficient, and if it exceeds 0.25%, the ductility decreases. Therefore, the V content is in the range of 0.1 to 0.25%.

P: 0.015% or less S: 0.015% or less P and S are elements that greatly affect the toughness of the base material. If each content exceeds 0.015%, the toughness of the base material is significantly reduced. Therefore, the P and S contents are set to 0.015% or less.

Ca: 0.01 to 0.05%
Ca combines with S to form a high melting point sulfide, and has the effect of preventing low melting point FeS and MnS from forming at the grain boundaries and improving ductility. In the case where S is contained in an amount of 0.015% or less, the effect is small if the Ca content is less than 0.01%, and if it exceeds 0.05%, the S content becomes excessive and the material cost increases. . Therefore, the Ca content is in the range of 0.01 to 0.05%.

S × 2.5 ≦ Ca ≦ S × 4.5
Furthermore, it is preferable to adjust Ca content according to S amount. However, when Ca <S × 2.5, the effect of Ca tends to be small, and when Ca> S × 4.5, the effect is saturated. Furthermore, there is a risk of inclusion formation and entrainment, and the material cost is high. It tends to be higher. Therefore, the Ca content is in the range of S × 2.5 ≦ Ca ≦ S × 4.5.

  Ca is added as a Ca-containing alloy and can be added by Ca-Si or the like. However, Ca is highly reactive with oxygen, and 95% Ni-5% Ca or the like is used in order to add at a high yield. It is preferable to add the Ni-Ca alloy.

0.3% ≦ V + 0.53 × Mo ≦ 0.4%
As described above, V and Mo are combined with C to form carbides and have an effect of improving strength, and each has a single range. Preferably, V + 0.53Mo is 0.3% or more and is large. A strength improvement effect is obtained, and when V + 0.53Mo exceeds 0.4%, the ductility tends to decrease. Therefore, the range is 0.3% ≦ V + 0.53Mo ≦ 0.4%.

C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 ≦ 0.6%
The carbon equivalent represented by C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 is a value for evaluating weldability. When the carbon equivalent exceeds 0.6%, the curability increases, and special thermal management is required to prevent the occurrence of cold cracking and the deterioration of the ductility of the weld. Therefore, the carbon equivalent is preferably 0.6% or less. In addition, it is also possible to adjust to a carbon equivalent equivalent to the cast steel product for welded structure (JIS G5102) within the composition range of the present invention as necessary.

  The balance of the alloy component having the above composition is Fe and inevitable impurities.

[Production conditions]
Rapid cooling after heating at 850-1000 ° C (average cooling rate 5 ° C / sec)
If the heating temperature is less than 850 ° C., the solid solution of Cu in the matrix is not complete and sufficient strength improvement cannot be obtained. If the heating temperature exceeds 1000 ° C., the solid solution amount of Cu does not increase any more, and the structure Becomes coarse and the toughness of the material decreases. Further, in order to obtain a sufficient effect of aging treatment, Cu needs to be dissolved in a supersaturated state in the matrix before the treatment, and for this reason, the cooling after heating is rapidly cooled. Specifically, when the average cooling rate from the cooling start temperature to 150 ° C. is less than 5 ° C./second, precipitation of coarse Cu particles occurs, and a desired tensile strength cannot be obtained. Therefore, the average cooling rate from the cooling start temperature of 850 to 1000 ° C. to 150 ° C. is defined as rapid cooling of 5 ° C./second or more. The cooling conditions are preferably adjusted according to the size of the processed material. For example, air cooling or oil cooling may be used when the wall thickness is 150 mm or less, and oil cooling or water cooling may be used when the wall thickness exceeds 150 mm.

Aging treatment at 450 to 650 ° C. for 1 to 2 hours After the heating and quenching, Cu held in a supersaturated state is precipitated as fine particles in the structure by holding at an appropriate temperature to harden the material, and tensile Strength increases. When the temperature is lower than 450 ° C., the time until the completion of precipitation becomes long. When the temperature exceeds 650 ° C., the tensile strength starts to decrease. Therefore, the aging temperature is defined as 450 to 650 ° C. In addition, if the holding time is less than 1 hour, the amount of Cu fine particles precipitated is low and the desired tensile strength cannot be obtained in low temperature aging. The holding time of the treatment is defined as 1 to 2 hours.

  The high-strength cast steel material of the present invention thus obtained has a thickness of 200 mm or less, a tensile strength of 900 MPa or more, an elongation of 10% or more, and a property that satisfies room temperature shock absorption energy of 27 J or more at the same time. be able to.

Examples of the present invention will be described below.
Cast steel materials having respective chemical compositions shown in Table 1 were melted in the atmosphere in a high frequency induction furnace, and a test body 1 conforming to FIG. 1b of JIS G0307 shown in FIG. 1 was cast. Furthermore, no. For 10 composition materials, in addition to test body 1, test body 2 (width 200 mm × thickness 200 mm / 100 mm / 50 mm / 25 mm stepped test body) and test body 3 (50 mm × 150 mm × 300 mm plate-shaped test body) Casted. In either case, a CO ₂ process silica sand mold was used as a mold.

  First, no. The influence of the heat treatment conditions on the mechanical properties was examined using 7 composition materials (test body 1).

  Table 2 shows the heat treatment test conditions and the tensile test results. Here, cooling is performed from the heating temperature shown in Table 2 to 150 ° C. or lower, and the average cooling rate from the cooling start temperature to 150 ° C. is 5 ° C./second or more in water cooling and air cooling, and 5 ° C./second in furnace cooling. Was less than. In conditions C, D, G, H, J, and L in which the heat treatment conditions are within the scope of the present invention, the aging temperature is 450 ° C. or more and the elongation is 10% or more, and the aging temperature is 650 ° C. or less and 900 MPa or more. Tensile strength is obtained, and the balance between strength and ductility is good. This is because V and Mo, which were dissolved in accordance with the aging temperature, are precipitated as fine carbides to offset softening due to overaging.

  On the other hand, none of the conditions A, B, E, F, I, K, and M in which the heat treatment conditions were outside the scope of the present invention could simultaneously satisfy the tensile strength of 900 MPa or more and the elongation of 10% or more.

  Next, the specimen 1 of each composition was held at 900 ° C. for 2 hours and then cooled with water, followed by aging treatment at 650 ° C. for 2 hours to obtain a tensile test piece (diameter 10 mm, distance between grades 50 mm) and Charpy impact. A test piece (2 mm V notch) was machined and a material test was performed at room temperature to examine the influence of chemical composition on the mechanical properties of JIS standard materials.

  Table 3 shows the material test results of the test body 1. No. Nos. 1 to 11 are cast steels of the present invention, all of which were confirmed to be cast steels that simultaneously satisfy a tensile strength of 900 MPa or more, an elongation of 10% or more, and a room temperature impact absorption energy of 27 J or more. On the other hand, no. Since the comparative material shown by 12-21 has a component composition outside the scope of the present invention, as described in detail below, the desired properties were not satisfied.

No. The tensile strength of 12, 14, 16, and 20 is 900 MPa or more, but the C amount, Cu amount, V + 0.53 × Mo amount, and carbon equivalent are too much, so the elongation is less than 10%, and room temperature shock absorption The energy was less than 27J. No. 21 is C. No. 13 is Cu. No. 15 had insufficient tensile strength because V + 0.53 × Mo was lower than the range of the present invention.

  No. Since No. 17 had less Ni than the range of the present invention, room temperature impact absorption energy was less than 27J. No. 18, no. No. 19 is a composition within the scope of the present invention when the element is used alone. No. 18 does not satisfy Ca ≧ S × 2.5. No. 19 did not satisfy Ca ≦ S × 4.5 and was excessive, so that both the elongation and room temperature shock absorption energy were insufficient.

  Next, no. Ten specimens of composition 2 were held at 900 ° C. for 2 hours, then cooled with water, subsequently subjected to aging treatment at 650 ° C. for 2 hours to cut out the material from a predetermined position, and a tensile test piece (diameter 10 mm, distance between scores 50 mm) ) And a Charpy impact test piece (2 mmV notch), and a material test was performed at room temperature to examine the influence of the material thickness on the mechanical properties.

  Table 4 shows the results of examining the relationship between the wall thickness and the mechanical properties. As shown in this table, even at a wall thickness of 200 mm, the cast steel simultaneously satisfies a tensile strength of 900 MPa or more, an elongation of 10% or more, and a room temperature impact absorption energy of 27 J or more.

  No. Ten specimens of composition 3 were held at 900 ° C. for 2 hours, then cooled with water, subsequently subjected to aging treatment at 650 ° C. for 2 hours, and machined into a welded specimen having a groove shape shown in FIG. Butt joint welding was carried out using the weld specimens under the welding conditions shown in Table 5.

  FIG. 3 shows the result of observing the macro structure near the weld joint in the state of welding. As shown in this figure, no welding defects such as cracks were observed, and it was confirmed that the film had good weldability.

  The material is cut out so that the weld joint shown in FIG. 2 is in the center of the parallel part of the tensile test piece and the weld heat affected zone is at the notch position of the impact test piece, and the tensile test piece (diameter 10 mm, between the scores) Distance 50 mm) and Charpy impact test piece (2 mm V notch). The weld properties were evaluated by examining the mechanical properties of the weld at room temperature.

  As shown in Table 6, the fracture position was not the base metal but the weld metal, and no reduction in strength of the base metal was observed. Moreover, no deterioration of the impact value was observed.

Claims (5)

% By mass
C: 0.05 to 0.2%
Si: 0.1 to 0.5%,
Mn: 0.5 to 1.5%
P: 0.015% or less,
S: 0.015% or less,
Cu: 1.5-3%,
Ni: 0.5 to 1.5%,
Mo: 0.3 to 0.5%,
V: 0.1-0.25%,
Ca: 0.01 to 0.05%
And the content of Ca and S is mass% S × 2.5 ≦ Ca ≦ S × 4.5
Satisfied with the relationship
When the content of Mo and V is mass%, 0.3% ≦ V + 0.53 × Mo ≦ 0.4%
Satisfied with the relationship
A high-strength cast steel material characterized in that the balance consists of Fe and inevitable impurities. The high-strength cast steel material according to claim 1, wherein the Ca is added by a Ni-Ca alloy. C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14
The high-strength cast steel material according to claim 1, wherein a carbon equivalent value represented by the formula is 0.6% or less by mass%.   The wall thickness is 200 mm or less, the tensile strength of 900 MPa or more, the elongation of 10% or more, and the room temperature shock absorption energy of 27 J or more are satisfied at the same time. The high-strength cast steel material described.   A cast steel material having the component composition according to any one of claims 1 to 3 is heated at 850 to 1000 ° C, and then cooled from the temperature to 150 ° C at an average cooling rate of 5 ° C / second or more. And aging treatment at 450 to 650 ° C. for 1 to 2 hours.