JP2014181394A - High strength cast steel material for structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cast steel material for a structure having high strength and extensibility, and excellent in low temperature toughness and heating embrittlement property.SOLUTION: There is provided high strength cast steel material for a structure containing, by mass%, C:0.15 to 0.3%, Si:0.2 to 0.7%, Mn:0.4 to 1.2%, P:0.015% or less, S:0.015% or less, Ni:2 to 3%, Cr:0.1 to 0.5%, Mo:0.3 to 0.5%, V:0.05 to 0.15%, Ca:0.01 to 0.05% and the balance Fe with inevitable impurities and having contents of Ca and S, by mass%, satisfying a relationship of S×2≤Ca≤S×4 and contents C and V, by mass%, satisfying a relationship of C×0.3≤V≤C×0.6.

Description

  The present invention is excellent in strength, ductility, and toughness suitable for various structural parts and members including civil engineering / construction machine parts (hereinafter referred to as earthworking machine parts) and civil engineering / architectural parts, and is heat brittle. The present invention relates to a structural high-strength cast steel having a small size.

  Materials such as earthen machinery parts (for example, driving wheels and crawler belts) and civil engineering / building members (for example, joints and bases) are required to have high strength and a high degree of freedom in shape. This is because there is a need to suppress the increase in the total weight accompanying the increase in size in the background of such a request. Rolled steel, forged steel, and cast steel are generally used as the steel materials that are used as these materials, but they usually tend to become brittle at low temperatures. In addition, civil engineering and building materials have a problem of increasing damage during an earthquake. Therefore, it is necessary that both have sufficient toughness at low temperatures.

  These parts are often welded on-site, and stress-relieving heat treatment may not be possible, and even at high levels of toughness, they may become brittle by heating during welding and cause the above-mentioned problems. . In addition, in the construction machinery parts that require wear resistance, in order to obtain high hardness, it is necessary to perform tempering in an embrittlement temperature range below the complete tempering temperature. There is a risk of breakage.

  Among these steel materials, cast steel materials are overwhelmingly superior to rolled steel materials and forged steel materials in the degree of freedom of shape, but have lower ductility and toughness than rolled steel materials and forged steel materials having the same strength. On the other hand, rolled steel and forged steel have no major problems in material properties other than low-temperature toughness, but there is a problem in that they cannot sufficiently meet the complex shape and thinning requirements required by designers because of the limited freedom of product shape. .

  Against this background, in recent years, cast steel materials having high strength and having both ductility and toughness are being developed. For example, Patent Document 1 proposes a cast steel material whose component composition is limited by a hardenability multiple or weldability index, and Patent Document 2 discloses a bainite parameter, a high toughness multiple, a carbon equivalent, and a weld crack sensitivity. A cast steel material having a limited component composition has been proposed. Moreover, in patent document 3 by the applicant of this invention, the influence of the cooling rate at the time of heat processing is small, the component composition used as a uniform bainite structure is found, and the highly ductile cast steel material with few intensity dispersion | variation is implement | achieved. Further, Patent Documents 4, 5, 6 and the like have been proposed as steel materials having excellent low temperature toughness.

Japanese Patent No. 3509634 Japanese Patent No. 3553601 Japanese Patent No. 4790512 Japanese Patent Laid-Open No. 11-323434 JP 2008-248382 A JP 2002-256380 A

  However, the materials of Patent Document 1 and Patent Document 2 both have a tensile strength of around 700 MPa and insufficient strength, and Patent Document 3 realizes a tensile strength of 920 MPa or more and an elongation of 12% or more. When heated at around 480 ° C., the toughness is greatly reduced, and there is a problem that it cannot be applied to applications that require on-site construction.

  Further, the materials of Patent Document 4 and Patent Document 5 have a tensile strength of about 600 to 800 MPa and insufficient strength, and Patent Document 6 has a strength of 860 MPa or more. However, the chemical composition and crystal grain size are strictly controlled. Are indispensable, and any of them is difficult to apply as a cast steel material used as cast.

  As described above, in the prior art, cast steel as a material having a high degree of freedom of shape, which simultaneously has the necessary tensile strength, ductility, low temperature toughness and heat embrittlement characteristics for structural parts and members such as civil engineering machinery parts and civil engineering / building materials. It is difficult to obtain the material.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a structural high-strength cast steel material having high strength and ductility and excellent in low-temperature toughness and heat embrittlement characteristics.

As a result of intensive studies to solve the above problems, the present inventors have focused on the following points.
(I) By defining the chemical composition of the base material within a specific range, the base material has mechanical properties having both high strength and low temperature toughness.
(Ii) The amount of Ni is specified to suppress embrittlement due to heating.
(Iii) Ca corresponding to the amount of S is added, and the sulfide is changed to Ca sulfide to obtain ductility.
(Iv) The strength reduction caused by high temperature tempering or welding is offset by the formation of V carbide.

  This invention was made | formed based on the above point, and provides the following (1)-(5).

(1) In mass%,
C: 0.15-0.3%
Si: 0.2-0.7%
Mn: 0.4 to 1.2%
P: 0.015% or less S: 0.015% or less Ni: 2-3%
Cr: 0.1 to 0.5%
Mo: 0.3-0.5%
V: 0.05 to 0.15%
Ca: 0.01 to 0.05%
Containing
And the content of Ca and S is mass% S × 2 ≦ Ca ≦ S × 4
Satisfied with the relationship
The content of C and V is mass%, and C × 0.3 ≦ V ≦ C × 0.6
Satisfied with the relationship
A structural high-strength cast steel characterized in that the balance consists of Fe and inevitable impurities.

  (2) The structural 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 for structural use, characterized in that the value of carbon equivalent represented by is 0.6% or less by mass%.

  (4) In any one of the above (1) to (3), the tensile strength ≧ 920 MPa, the elongation ≧ 12%, the −40 ° C. impact absorption energy ≧ 27 J (2 mmV notch Charpy impact test piece) is satisfied at the same time. Structural high-strength cast steel.

  (5) The structure according to any one of the above (1) to (4), wherein the 0 ° C. impact absorption energy after the embrittlement treatment at 480 ° C. for 10 hours is 50% or more of the 600 ° C. tempering value. High strength cast steel material.

  According to the present invention, by appropriately defining the alloy composition, it is possible to provide a structural high-strength cast steel material having high strength and ductility, excellent low-temperature toughness and heat embrittlement characteristics, and good weldability. And the effect is very remarkable.

It is a figure which shows the relationship between Ni content, tensile strength, and heating embrittlement rate. It is a figure which shows the test material shape for evaluating the mechanical property of cast steel materials.

  Hereinafter, the reason for limitation of the present invention will be described in detail. In addition, unless otherwise indicated, the% display in a component is the mass%.

[Chemical composition]
C: 0.15-0.3%
C is an element effective for improving the strength and hardenability of cast steel. Moreover, it combines with V to form fine carbides, improves temper softening resistance, facilitates heat treatment condition management, and prevents deterioration of the mechanical properties of the weld heat affected zone. However, if the content is less than 0.15%, a tensile strength of 920 MPa or more cannot be obtained, and if it exceeds 0.3%, ductility and toughness are lowered, and weld cracking is likely to occur. Therefore, the C content is in the range of 0.15 to 0.3%.

Si: 0.2-0.7%
Si is an element added for the purpose of deoxidation and improvement of hot water flow. However, if its content is less than 0.2%, the molten steel has low flowability, which leads to deterioration of casting quality. On the other hand, Si is a ferrite-forming element, and when it exceeds 0.7%, ferrite is precipitated in a thick portion where the cooling rate is slow, and the strength is lowered. Therefore, the Si content is in the range of 0.2 to 0.7%.

Mn: 0.4 to 1.2%
Mn is an element effective for improving the strength and hardenability of the cast steel material, and also has a deoxidizing effect. However, if the content is less than 0.4%, the effect is small, and if it exceeds 1.2%, ductility and toughness are lowered and weldability is deteriorated. Therefore, the Mn content is in the range of 0.4 to 1.2%.

Ni: 2-3%
Ni is the most important element in the present invention for simultaneously obtaining small heat embrittlement in addition to high strength, high ductility and high low temperature toughness. Ni is an element that is effective for the strength and hardenability of cast steel, but unlike other elements that have the same effect, Ni has little adverse effect on ductility, toughness, and weldability. Ni is an element that has a large austenite stabilizing effect, and by moving the ferrite transformation region to a long time side and expanding the bainite transformation region, ferrite precipitation is suppressed even in a thick portion where the cooling rate is slow, and high strength is achieved. Is obtained. However, if Ni is less than 2%, a tensile strength of 920 MPa or more cannot be obtained, and if it exceeds 3%, the heating embrittlement rate exceeds 50%, and embrittlement becomes prominent. FIG. 1 shows the effect of Ni alone while fixing elements other than Ni. The above results are clearly shown. Therefore, the Ni content is in the range of 2-3%.

Cr: 0.1 to 0.5%
Cr is an effective element for improving the strength of cast steel. However, when the content is less than 0.1%, the effect is small. When the content exceeds 0.5%, the ferrite transformation region is expanded, and ferrite is deposited on the thick wall portion where the cooling rate is slow. Reduce weldability. Therefore, the Cr content is in the range of 0.1 to 0.5%.

Mo: 0.3-0.5%
Mo is added to improve the hardenability of the cast steel and to suppress temper embrittlement. However, if less than 0.3%, the effect is small, and if it exceeds 0.5%, the effect cannot be obtained. Ferrite is precipitated in the thick part where the ferrite transformation region is expanded and the cooling rate is slow, and the strength is lowered. At the same time, the carbon equivalent increases and weldability decreases. Therefore, the Mo content is set to a range of 0.3 to 0.5%.

V: 0.05-0.15%
V is an element effective in maintaining strength during tempering heat treatment by forming carbides by combining with C and increasing temper softening resistance. It is possible to prevent deterioration of properties. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.15%, ductility and toughness are lowered. Therefore, the V content is in the range of 0.05 to 0.15%.

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 hardly obtained if the Ca content is less than 0.01%. It will only lead to an increase. Therefore, the Ca content is in the range of 0.01 to 0.05%.

S × 2 ≦ Ca ≦ S × 4
Furthermore, it is preferable to adjust Ca content according to S amount. When Ca <S × 2, the effect of Ca is small, and when Ca> S × 4, the effect is saturated and the material cost increases. Therefore, the Ca content is in the range of S × 2 ≦ Ca ≦ S × 4.

  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.

C × 0.3 ≦ V ≦ C × 0.6
As described above, V is combined with C to form a carbide to improve mechanical properties. However, if V is less than C × 0.3, sufficient strength cannot be obtained, and if it exceeds C × 0.6, the strength improvement effect is saturated and the material cost increases. Therefore, C × 0.3 ≦ V ≦ C × 0.6.

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]
In the present invention, the production conditions are not particularly limited, and the production can be carried out under the conditions employed for ordinary cast steel materials. For example, after casting molten steel having the component composition of the present invention into a product-shaped mold and solidifying it, heating at about 800 to 1000 ° C., performing rapid cooling treatment such as water cooling, and performing tempering treatment, the cast steel of the present invention Get the material.

[Characteristic]
According to the present invention, it is possible to obtain a structural high-strength cast steel material having characteristics satisfying simultaneously tensile strength ≧ 920 MPa, elongation ≧ 12%, and −40 ° C. impact absorption energy ≧ 27 J (2 mmV notch Charpy impact test piece). Moreover, the cast steel material with few embrittlement by heating that the 0 degree C impact absorption energy after 480 degreeC and the embrittlement process for 10 hours is 50% or more of 600 degreeC tempering value can be obtained.

Examples of the present invention will be described below.
Cast steel materials having respective chemical compositions shown in Table 1 were dissolved in the atmosphere in a 30 kg high-frequency induction furnace, and a test body conforming to FIG. 1b of JIS G0307 shown in FIG. 2 was cast. A CO ₂ process silica sand mold was used as a mold. No. in Table 1 1 to 10 are examples of the present invention. 11 to 17 are comparative examples.

  The specimen was held at 920 ° C. for 1 hour, then cooled with water, and tempered at 600 ° C. for 2 hours. Some specimens were subsequently referred to as “475 embrittlement” and were subjected to a heat treatment at 480 ° C. for 10 hours where embrittlement is most likely to occur.

  From these specimens, JIS-Z2241, No. 14A specimens were prepared for tensile tests, and JIS-Z2242, 2 mmV notch specimens were prepared for Charpy impact tests. The tensile test was performed at 20 ° C. and the Charpy impact test was performed at −40 ° C. The heating embrittlement rate was determined using the absorbed energy value of the Charpy impact test at 0 ° C. Moreover, the highest hardness test of the welded part was based on the highest hardness test method of the JIS Z3101 weld heat affected zone. Table 2 shows the test results.

  As shown in Table 2, no. Each of Examples 1 to 10 is a cast steel material having both a tensile strength of 920 MPa or more, an elongation of 12% or more, a −40 ° C. impact absorption energy of 27 J or more, and a heating embrittlement ratio of 50% or less.

  On the other hand, No. which is a comparative example. Since the cast steel materials indicated by 11 to 17 have a chemical composition outside the range of the present invention, the desired properties could not be obtained. No. 11 and 13 were insufficient in strength because the C and Ni contents were lower than the range of the present invention, respectively. No. In No. 12, since the C content exceeded the upper limit of the range of the present invention, the desired ductility was not obtained, and the carbon equivalent exceeded 0.6, which is a preferred value. Vickers hardness Hv350, which is the standard (according to Japan Welding Association / Welding Information Center “JWES Joining / Welding Technology Q & A1000, No.Q02 / 02/0208”) was exceeded. No. For No. 14, the Ni content exceeded the upper limit of the range of the present invention, so the desired heating embrittlement rate could not be obtained. No. Nos. 15 to 17 are compositions within the scope of the present invention when the element is used alone. No. 15 did not satisfy V ≧ C × 0.3, so the strength was insufficient. No. 16 did not satisfy V ≦ C × 0.6, and desired ductility and toughness were not obtained. No. No. 17 did not satisfy Ca ≧ S × 2, and the ductility and toughness were insufficient due to insufficient control of the form of sulfide.

Claims (5)

% By mass
C: 0.15-0.3%
Si: 0.2-0.7%
Mn: 0.4 to 1.2%
P: 0.015% or less S: 0.015% or less Ni: 2-3%
Cr: 0.1 to 0.5%
Mo: 0.3-0.5%
V: 0.05 to 0.15%
Ca: 0.01 to 0.05%
Containing
And the content of Ca and S is mass% S × 2 ≦ Ca ≦ S × 4
Satisfied with the relationship
The content of C and V is mass%, and C × 0.3 ≦ V ≦ C × 0.6
Satisfied with the relationship
A structural high-strength cast steel characterized in that the balance consists of Fe and inevitable impurities.   2. The structural high strength cast steel 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 for structural use according to claim 1 or 2, wherein a value of carbon equivalent represented by is 0.6% or less by mass%.   The tensile strength ≧ 920 MPa, the elongation ≧ 12%, and the −40 ° C. impact absorption energy ≧ 27 J (2 mmV notch Charpy impact test piece) are satisfied at the same time. 5. High-strength cast steel for structural use.   The structural absorption according to any one of claims 1 to 4, wherein the 0 ° C impact absorption energy after the embrittlement treatment at 480 ° C for 10 hours is 50% or more of the tempering value at 600 ° C. High strength cast steel.

Images