BR102012011525B1 - low alloy cast steel and method for producing wear resistant low alloy steel castings - Google Patents

Abstract

low alloy cast steel and method for producing wear resistant low alloy steel castings. a wear-resistant low-alloy steel castings having a wall thickness of 2.54 centimeters or more is produced by subjecting a material comprising 0.30 to 0.35 mass% of c, 0.30 to 0.60 mass% of itself, 090 to 1.50 mass% of mn, 091 to 1.50 mass% of cr, 1.60 to 1.90 mass% of ni, 0.20 to 0, 30% by weight of hand, 0,05% by weight or less of powder, and 0,05% by weight or less of s, the remainder consisting of iron and unavoidable impurities, to a homogenization step of heating the material 1000 to C, keep the material at temperature, and cool the material by cooling the furnace; a tempering step of heating the material to 850 to 950 ° C, keeping the material at temperature, and cooling the material by quenching in water, and a tempering step of heating the material to 150 to 280 <198> c, keep the material at room temperature and cool the material to room temperature by cooling the oven.

Description

(54) Title: LOW ALLOY CAST STEEL AND METHOD TO PRODUCE WEAR RESISTANT LOW ALLOY CAST CAST (51) lnt.CI .: C22C 38/08 (30) Unionist Priority: 05/31/2011JP 2011-121581 ( 73) Holder (s): SINTOKOGIO, LTD.

(72) Inventor (s): MAMORU HASEGAWA; KOUJI TOKUNAGA (85) National Phase Start Date: 05/15/2012 “LOW ALLOY CAST STEEL AND METHOD TO PRODUCE WEAR RESISTANT LOW ALLOY CAST STEEL CASES”

The present invention relates to a wear-resistant, low-alloy cast steel, from which cast steel for mechanical equipment, including a variety of crushers, a conveyor line and the like in the field of mining, crushing stone, cement and similar ones can be prepared. The invention also relates to a method for producing steel castings. In particular, the invention relates to the optimization of a molten steel composition and a heat treatment process, in order to obtain both high hardness and high toughness.

BACKGROUND OF THE INVENTION

In the mining, stone crushing, cement, steel, recycling and similar industries, crushers for size reduction (spraying) various types of ores, classifiers, conveyors and other equipment are used, and in various parts of this equipment they are required to be discharged. high wear resistance and high toughness.

When looking back in the past, high manganese cast steel was first used as a material to satisfy the above mentioned requirements, and the use of high manganese steel cast has gradually spread. High manganese cast steel is characterized by a homogeneous austenite structure, so that high manganese steel castings, which can be prepared from this high manganese cast steel, are advantageously less susceptible to fracture due to their extremely toughness high. On the other hand, however, the surface hardness obtained by hardening at work is around 30 to 40 HRC (in terms of the Rockwell C hardness scale), so there is a problem of relatively rapid wear progress.

_So, a variety of low-alloy steel has been used for applications and parts required for greater wear resistance. The low-alloy cast steel used here is composed of materials prepared by adding alloying elements, such as Cr, Mo, Ni and the like, in total amounts of about 10% by weight (hereinafter abbreviated by%) or less carbon steel for improved toughness and toughness. Low alloy cast steel is standardized as high tensile strength carbon steel castings and low alloy steel castings for structural purposes by JIS and ASTM.

In particular, a typical material having high hardenability and toughness is SCNCrM2 (C: 0.25 to 0.35%, Si: 0.30 to 0.60%, Mn: 0.90 to 1.50%, Ni: 1 , 60 to 2.00%, Cr: 0.30 to 0.90%, Mo: 0.15 to 0.35%) as standardized by JIS G5111. In order to cool the material in the hardening process, oil hardening is conventionally conducted to prevent hardening cracks from occurring. Then, for structural application, the material is subjected to tempering at 550 to 650 ° C. However, for the application where high wear resistance is required, tempering is conducted at low temperatures around 200 ° C. In this case, the resulting hardness becomes about 50 HRC and the resulting impact value becomes about 30 J / cm ² , when the melt wall thickness is less than 2.54 cm.

The prior art as described above is known.

SUMMARY OF THE INVENTION

In the case where it is desired to ensure high hardness in the depth of the core portion of a part with a wall thickness of 2.54 cm or more, even the SCNCrM2 material exhibits insufficient hardenability, and the hardness obtained from the core portion is as low as HRC 45 or less. If water quenching is used to obtain greater hardness in the depth of the core portion, this will cause a problem that the occurrence of hardening cracks tends to increase.

The term hardening cracks used here includes cracks that occur in the course of hardening or hardening and cracks due to seasonal cracking when left to stand at room temperature after hardening.

Considering the aforementioned problems, an object of the invention is therefore to provide a wear-resistant low alloy cast steel, and the steel cast or steel cast product with a wall thickness of 2.54 cm or more that can be obtained from it, both which can ensure high toughness and achieve a hardness of 45 to 53 HRC to obtain improved wear resistance by optimizing an alloy composition to reduce the occurrence of hardening cracks, even when water quenching is conducted to ensure high hardness in the molten steel product with a wall thickness of 2.54 cm or more.

To solve the aforementioned problems, a wear-resistant low-alloy cast steel of the invention is characterized by comprising from 0.30 to 0.35% by weight of C, 0.30 to 0.60% by weight of Si, 0 , 90 to 1.50% by weight of Mn, 0.91 to 1.50% by weight of Cr, 1.60 to 1.90% by weight of Ni, 0.20 to 0.30% by weight of Mo , 0.05% by weight or less of P, and 0.05% by weight or less of S, the rest consisting of iron and unavoidable impurities, and having a wall thickness of 2.54 cm or more, a hardness in Rockwell C scale (HRC) from 45 to 53, and a Charpy impact value (notch U) of 20 to 40 J / cm ² .

In addition, the wear-resistant low-alloy cast steel of the invention is characterized by being produced by a heat treatment process comprising a homogenization (normalizing) step of heating a material to 1000 to 1100 ° C, keeping the material at temperature, and then cool the material by cooling the oven; a quenching step of heating the material to 850 to 950 ° C, keeping the material at temperature and then quenching the material in water, and a tempering step of heating the material to 150 to 280 ° C, keeping the material in temperature and then cool the material to room temperature by cooling the oven.

The invention also provides a method for producing wear-resistant low-alloy steel castings having a wall thickness of 2.54 cm or more, a Rockwell C scale hardness of HRC 45 to 53 and a Charpy impact value ( notch U) from 20 to 40 J / cm ² , comprising;

a homogenization step of heating a material comprising 0.30 to 0.35% by weight of C, 0.30 to 0.60% by weight of Si, 0.90 to 1.50% by weight of Mn, 0 , 91 to 1.50% by weight of Cr, 1.60 to 1.90% by weight of Ni, 0.20 to 0.30% by weight of Mo, 0.05% by weight or less of P, and 0.05% by weight or less of S, the rest consisting of iron and unavoidable impurities, at 1000 to 1100 ° C, keep the material at temperature and then cool the material by cooling the oven;

a quenching step of heating the material to 850 to 950 ° C, keeping the material at temperature, then quenching the material by quenching in water, and a tempering step of heating the material to 150 to 280 ° C, keeping the material temperature, and then cool the material to room temperature by cooling the oven.

The invention also provides a low alloy molten steel comprising 0.30 to 0.35% by weight of C, 0.30 to 0.60% by weight of Si, 0.90 to 1.50% by weight of Mn, 0.91 to 1.50% by weight of Cr, 1.60 to 1.90% by weight of Ni, 0.20 to 0.30% by weight of Mo, 0.05% by weight or less of P, and 0.05% by weight or less of S, the rest consisting of iron and unavoidable impurities.

Effects of the Invention

The invention makes it possible to provide a low wear-resistant cast steel, and steel castings that can be obtained from it, both of which have both improved wear resistance and high toughness, that is, a hardness on the Rockwell C scale 45 to 53 HRC and a Charpy impact value (U notch) of 20 to 40 J / cm ² , selecting an optimal alloy composition capable of reducing the occurrence of hardening cracks and a heat treatment process in steel castings with a wall thickness of 2.54 cm or more. The low alloy cast steel of the invention is also excellent in crack resistance.

DETAILED DESCRIPTION OF THE INVENTION

The reasons for selecting the chemical composition described in claim 1 will now be explained as follows:

1) C: 0.30 to 0.35%

Carbon is a significant element in determining the hardness of temper. To obtain a hardness on the Rockwell C scale of 45 to 53 HRC, the C content range is fixed from 0.30 to 0.35%. When the C content is less than 0.30%, the hardness of HRC 45 or more cannot be obtained. The C content of more than 0.35% leads to an increase in hardening cracks.

2) Si: 0.30 to 0.60%

Silicon (Si) has little effect on hardenability, but plays an important role in ensuring a sufficient deoxidation effect and proper fluidity of the molten metal. When the Si content is less than 0.30%, the deoxidation effect does not appear to be sufficiently sufficient to prevent the increase in gas porosity in the internal part of the molten steel or in the steel melts that can be obtained from it. The Si content of more than 0.60% increases segregation to lower the resulting toughness.

3) Mn: 0.90 to 1.50%

Manganese (Mn) is an element that has the greatest effect on improving the hardenability of steel. The range of the Mn content is fixed from 0.90 to 1.50% to stably obtain the hardness on the Rockwell C scale of HRC 45 to 53. When the Mn content is less than 0.90%, the hardness mentioned above does not_ can always be guaranteed, especially in castings with a wall thickness of more than 2.54 cm. When the Mn content exceeds 1.50%, the toughness (that is, in terms of the impact value Charpy) is lowered due to segregation. To ensure preferable hardness and toughness, the Mn content can preferably be in the range of 1.00 to 1.30%.

4) Cr: 0.91 to 1.50%

In terms of the positive effect on hardenability, chromium (Cr) is in third place after Mn and Mo. At the same time, chromium works to take the grains out of fine crystals, so that toughness is improved to increase crack resistance. hardening. Thus, the range of Cr content is fixed from 0.91 to 1.50%. When the Cr content is less than 0.91%, the effect is insufficient, thus lowering the hardness of the core portion to HRC scale or less. When the Cr content exceeds 1.50%, the increase in carbides lowers the toughness. To achieve preferable hardness and toughness, the Cr content can be in the range of 1.20 to 1.50%.

5) Ni: 1.60 to 1.90%

Nickel (Ni) is an element that has the greatest effect on improving toughness. When the Ni content is less than 1.60%, the effect is insufficient and the Charpy impact value of 20 to 40 J / cm ² cannot be obtained, especially in castings with a wall thickness of more than 2.54 cm. When the Ni content exceeds 1.90%, the impact value is saturated. To obtain a material with high toughness, the Ni content can preferably be in the range of 1.70 to 1.90%.

6) Mo: 0.20 to 0.30%

Molybdenum (Mo) is an element that is the second after manganese (Mn) in the effect on improving the hardenability, and has a promoting effect on the softening resistance by tempering. In light of this, the Mo content range is set at 0.20 to 0.30%. When the Mo content is less than 0.20%, the effect becomes insufficient. The Mo content of more than

0.30% leads to a slight decrease in toughness, because segregation occurs

7) P and S: 0.05% or less

These elements P and S work adversely to lower the toughness of the steel, so that the levels are fixed at 0.05% or less.

When the wall thickness of a cast is less than 2.54 cm, there is no risk of hardening cracks in conventional materials, such as SCNCrM2 and the like, so that the hardness on the Rockwell C scale of 45 to 53 HRC and the value Charpy impact (U notch) of 20 to 40 J / cm ² can be obtained at the same time. However, the hardness of the core portion decreases when the wall thickness exceeds 2.54 cm. Conventional material, such as SCNCrM2 or similar, is basically subjected to oil quenching. Since the rate of cooling by oil quenching is lower than by water quenching, the resulting hardness becomes less as the wall thickness increases. If water quenching is carried out to accelerate the cooling speed, the occurrence of hardening cracks increases considerably.

The heat treatment process described in claim 2 will now be explained as follows.

1) Homogenization step

In the microstructure after casting, large dendrites grow during solidification, thus making the structure remarkably heterogeneous, and the chemical composition shows considerable segregation. In addition, the casting stress is accumulated in the castings. Therefore, it is necessary to normalize the composition and structure of the material and, at the same time, release the casting stress before the tempering step by heating the material to a temperature at which the complete austenite phase can be obtained. Normally, normalization from 1050 to 1100 ° C is considered to be effective. According to the invention, the temperature range for the standardization step is set at 1000 to 1100 ° C. When the temperature of the normalization step is less than 1000 ° C, the effects obtained are insufficient, and when the temperature of the normalization exceeds 1100 ° C, the grains are growing to become coarse. The duration when the material is kept at the normalization temperature, which can be appropriately determined according to the melt wall thickness is generally in the range of 3 to 5 hours. The subsequent cooling operation can be carried out by cooling the furnace, in such a way that the residual heat treatment stress can be minimized.

2) Tempering step

Subsequent to the homogenization step, the quenching step is generally carried out in such a way that the temperature is raised once above the transformation temperature Ac3 or Acl of 30 to 50 ° C and then lowered for cooling. For rapid cooling, water quenching is used. On the other hand, oil quenching is used for alloy steel materials, or for the purpose of preventing quenching cracking and quenching distortion. In the invention, water quenching is conducted by placing the material in water after the temperature is raised to 850 to 950 ° C and maintained for some time. When the tempering temperature is less than 850 ° C, the homogeneous austenite structure cannot be obtained, which leads to a decrease in toughness. When the quench temperature exceeds 950 ° C, the grain size will become coarse and the quench hardness will decrease.

The length of time, when the material is kept at the quench temperature, which can be appropriately determined according to the melt wall thickness is preferably one hour for every 2.54 cm of wall thickness. In a complicated cast with non-uniform thickness, it is recommended to conduct the quench in water until the material is cooled to a temperature higher than the martensite start temperature (Ms) of about 50 ° C, and then take the steel material out of the water and leave it at room temperature.

3) Tempering stage

The hardening step is carried out subsequent to the 5 hardening step, in order to relieve the hardening stress and adjust the hardness and toughness. In general, the tempering temperature is selected within a range of 150 to 600 ° C to conduct the tempering step depending on the desired properties. The higher the tempering temperature is, the higher the impact value, but the lower the hardness becomes. According to the invention, the material is heated to a temperature ranging from 150 to 280 ° C, and maintained at the tempering temperature, followed by cooling the oven to room temperature. The material is generally kept at the tempering temperature for 2 to 7 hours, preferably 5 to 7 hours. When the tempering temperature is lower than 150 ° C, sufficient effect of recovering toughness can be obtained. When the tempering temperature exceeds 280 ° C, the risk of low temperature tempering fragility may increase.

The wear-resistant low-alloy cast steel of the invention is characterized by the fact that the hardness of the core portion can be ensured in the resulting product by conducting the quenching in water, even if the product wall thickness exceeds 2.54 cm, and the occurrence hardening cracks can be minimized even if the quenching step is performed at a high cooling speed by quenching in water. Therefore, it is possible to steadily obtain a Rockwell hardness of 45 to 53 and a Charpy impact value (U-notch) of 20 to 40 J / cm ² , even if the product wall thickness increases. When the steel melt is required to have a Rockwell hardness of more than 53 HRC, the increase in the C content becomes essential. The above case is not included in the present invention because the toughness is drastically decreased.

In the chemical composition of the invention, the ratio of the Cr element, which has a significant effect on the increase in temperability and toughness is defined as higher than in conventional chemical compositions of SCNCrM2 and the like. This makes it possible to obtain high toughness and high resistance to temper cracking, while the use of water hardening ensures high hardness (i.e., high wear resistance) at a depth of the core portion of the steel melt. Namely, memorable aspects can be obtained.

The invention will now be explained more specifically by reference to the following examples, which are given for purposes of illustrating the invention and are not intended to be limiting thereof.

[Examples]

Using each chemical composition shown in Table 1, Y-shaped block samples with a thickness of 2.54 cm (type B), 5.08 cm (type C) and 7.62 cm (type D) of according to JIS G5502 (ISO 1083) were prepared by casting. Five Y-blocks were prepared for each Example, or Comparative Example as shown in Table 2. The chemical composition in Comparative Example 1 was SCNCrM2.

Specimens cut from each Y block subjected to heat treatment were used to assess the hardness and impact value Charpy.

For hardness, a hardness test on the Rockwell C scale (HRC) was used to determine the hardness according to JIS Z 2245 (ISO 6508). More specifically, a specimen was fixed on a table of the testing machine, the notch (diamond cone) was allowed to press on the specimen by applying test forces in two stages (that is, a test force 98.07 N and a total test force of 1471 N (= the initial test force + additional test force). Then the additional test force was released, leaving the initial test force applied, to determine the permanent notch deformation The hardness was calculated from the permanent deformation.

The Charpy impact value was determined according to JIS Z2242 (ISO 83) using a Charpy impact testing machine. More specifically, a specimen, which was a square bar (10 mm x 10 mm x 55 mm) having a U-notch (2 mm) machined in the center of the bar, was fixed on the table of the testing machine and maintained in a secure at each end. The specimen was broken by the impact of a mallet that goes under predetermined conditions. The energy absorbed by the specimen (ie, the impact value) was determined by calculation. The specimen with U-notch (2 mm) as specified in JIS Z2202 was used.

The occurrence of cracking of temper was observed visually. The Y blocks of Examples 1 to 3 were subjected to a heat treatment so that the blocks were heated to 1070 ° C and maintained for 3 hours, followed by cooling the oven in the nonnalization step and, subsequently, the blocks were heated at 890 ° C and maintained for 3 hours, followed by quenching in water in the hardening step, and then the blocks were heated to 230 ° C and maintained for

7 hours, followed by cooling the oven in the tempering stage.

The Y blocks of Comparative Examples 1 to 5 were subjected to heat treatment in the same manner as in Examples 1 to 3.

[Table 1]

Ç Si Mn Cr Mo Ni P s Composition 1 0.33 0.48 1.21 0.98 0.26 1.74 0.024 0.011 Composition 2 0.31 0.42 1.03 0.55 0.21 1.80 0.028 0.021 Composition 3 0.31 0.45 0.94 1.43 0.25 1.86 0.038 0.007 Composition 4 0.32 0.49 1.12 1.28 0.27 1.63 0.022 0.041 Composition 5 0.33 0.48 1.25 1.48 0.25 1.83 0.015 0.009 Composition 6 0.30 0.47 0.92 0.93 0.24 1.66 0.022 0.034

[Table 2]

Composition O Chemistry Specimen Wall Thickness (cm) Hardening Stage Toughness (HRC) Impact Value Charpv (J / cni) Occurrence from Trinca de Hardening (out of 5 proof bodies) Ex. 1 1 2.54 water quenching 52.6 38.2 0 Ex. 2 1 5.08 temper in water 51.9 32.5 0 Ex. 3 1 7.62 temper in water 48.0 31.0 0 Ex. 4 3 7.62 temper in water 49.1 32.3 0 Ex. 5 4 7.62 temper in water 51.2 31.4 0 Ex. 6 5 7.62 water quenching 52.4 33.8 0 Ex. 7 6 7.62 temper in water 46.4 38.3 0 Ex. Comp. 1 2 2.54 oil quenching 49.1 23.4 1 Ex. Comp. . 2 2 5.08 oil quenching 46.3 19.5 2 Ex. Comp. 3 2 5.08 water quenching 47.8 16.2 3 Ex. Comp. 4 2 7.62 oil quenching 43.5 18.1 2 Έχ. Comp. 2 7.62 water quenching 44.9 15.0 4

As shown in Examples 1 to 3, when composition 1 according to the invention was used, the resulting melts showed little decrease in hardness, even when the thickness of the specimen wall increased from 2.54 cm to 7.62 cm , which proved to have excellent wear resistance. In addition, the Charpy impact values were high, 30 J / cm ² or more. Composition 1 was also found to be excellent in hardening crack resistance because no hardening crack occurred although water quenching was conducted.

In Comparative Example 1, a Y-block having a wall thickness of 2.54 cm was prepared from a material of composition 2 (SCNCrM2) and the oil quench was conducted. When compared to Example 1, where the wall thickness was the same, Comparative Example 1 exhibited a lower hardness and a lower Charpy impact value. In addition, quenching cracks occurred in one specimen out of five. It is assumed that more quenching cracks would have occurred if water quenching had been conducted.

In Comparative Examples 2 and 3, Y blocks with a wall thickness of 5.98 cm were prepared and then oil quenching and water quenching were conducted, respectively. When compared to Example 2, where the wall thickness was the same, each of Comparative Examples 2 and 3 exhibited a lower hardness and a lower Charpy impact value. In addition, hardening cracks occurred in two specimens out of five subjected to oil quenching, and in three specimens out of five subjected to water quenching.

In Comparative Examples 4 and 5, Y-blocks, with a wall thickness of 7.62 cm were prepared and then oil quenching and water quenching were conducted, respectively. When compared to Example 3, where the wall thickness was the same, each of Comparative Examples 4 and 5 exhibited a lower hardness and a lower Charpy impact value. In addition, hardening cracks occurred in two specimens out of five subjected to oil quenching, and in four specimens out of five subjected to water quenching.

As previously explained, the wear-resistant low-alloy steel cast of the invention has excellent resistance to hardening crack, so that no hardening crack occurs even when the heat treatment is carried out in such a way that Rockwell hardness reaches 45 at 53 HRC. At the same time, a high toughness in terms of the Charpy impact value (U-notch) from 20 to 40 J / cm ² can be ensured. In particular, it has been confirmed that castings having a wall thickness of more than 2.54 cm can take advantage of the invention.

Claims (5)

1. Low wear-resistant cast steel, characterized by the fact that it comprises 0.30 to 0.35% by weight of C, 0.30 to 0.60% by weight of Si, 0.90 to 1.50 % by mass of Mn, 0.91 to 1.50% in 5 mass of Cr, 1.60 to 1.90% by weight of Ni, 0.20 to 0.30% by weight of Mo, 0.05% by weight or less of P, and 0.05% by weight or less than S, the rest consisting of iron and unavoidable impurities, and having a wall thickness of 2.54 centimeters .or more, a Rockwell HRC hardness 45 to 53, and a Charpy impact value (notch U) of 20 to 40 J / cm ² , where steel is 10 produced by a heat treatment process comprising a homogenization step of heating a material to 1000 to 1100 ° C, keeping the material at temperature and then cooling the material by cooling the oven; a quenching step of heating the material to 850 to 950 ° C, keeping the material at temperature and then quenching the material by quenching in water, and a quenching step 15 to temper the material to 150 to 280 ° C, keep the material at temperature and then cool the material to room temperature by cooling the oven. 2. Method for producing wear-resistant low-alloy steel castings, having a wall thickness of 2.54 cm or 20 more, a Rockwell HRC hardness of 45 to 53 and a Charpy impact value (U notch) of 20 to 40 J / cm ² , characterized by the fact that it comprises: a homogenization step of heating a material comprising 0.30 to 0.35% by weight of C, 0.30 to 0.60% by weight of Si, 0.90 to 1.50% by weight of Mn, 0 , 91 to 1.50% by weight of Cr, 1.60 to 1.90% by weight 25 mass of Ni, 0.20 to 0.30% by weight of Mo, 0.05% by weight or less of P, and 0.05% by weight or less of S, the rest consisting of iron and unavoidable impurities, at 1000 to 1100 ° C, keep the material at temperature and then cool the material by cooling the oven; a quenching step of heating the material to 850 to 950 ° C, Petition 870180072154, of 08/17/2018, p. 8/9 keep the material at temperature and then cool the material by quenching with water, and a tempering step of heating the material to 150 to 280 ° C, keeping the material at temperature and then cooling the material to 5 room temperature by cooling the oven. Method according to claim 2, characterized by the fact that the material is kept at 1000 to 1100 ° C for 3 to 5 hours in the homogenization step. Method according to claim 2, characterized by the fact that the material is kept at 850 to 950 ° C for one hour by 2.54 cm in thickness of the melt wall in the tempering step. 5. Method according to claim 2, characterized in that the material is kept at 150 to 280 ° C for 2 to 7 hours in the tempering step. Petition 870180072154, of 08/17/2018, p. 9/9