JP7812779B2 - Wear-resistant cast steel and its manufacturing method - Google Patents

Description

This invention relates to wear-resistant cast steel used for crusher parts, etc.

Crushers are known for crushing scrap metal, crushed stone, bulky trash, and other materials. Crushers use breakers that rotate at high speed inside a shell to roughly crush the material, and then a rotating grinder to finely crush it. These components require wear resistance and impact resistance, so high-hardness, high-toughness materials are used.

As one such material, Patent Document 1 discloses a wear-resistant cast steel containing, by mass%, 0.30% to 0.35% C, 0.30% to 0.60% Si, 0.90% to 1.50% Mn, 0.91% to 1.50% Cr, 1.60% to 1.90% Ni, 0.20% to 0.30% Mo, 0.05% or less P, 0.05% or less S, and the balance being Fe and impurities. This wear-resistant cast steel has a product thickness of 1 inch or more, an HRC of 45 to 53 (HBW 421 to 525), and a Charpy impact value (U-notch) of 20 to 40 J/ ^cm2 .

The wear-resistant cast steel of Patent Document 1 requires three essential processes to achieve the desired hardness and Charpy impact value: a homogenization process in which the prepared cast steel ingot is heated and held at 1000°C to 1100°C and then furnace cooled; a quenching process in which the ingot is heated and held at 850°C to 950°C and then water cooled; and a tempering process in which the ingot is heated and held at 150°C to 280°C and then furnace cooled to room temperature.

JP 2012-246564 A

High-temperature homogenization and tempering processes are costly. Furthermore, the tempering process is carried out to restore the toughness lost during quenching, but tempering also reduces hardness. Therefore, there is a need to develop cast steel materials with excellent toughness without reducing hardness.

The object of the present invention is to provide a wear-resistant cast steel that can maintain toughness and hardness without the need for a tempering process, and a method for manufacturing the same.

The wear-resistant cast steel of the present invention is
The alloy contains, in mass %, C: 0.27% to 0.36%, Si: 0.10% to 0.50%, Mn: 0.30% to 0.70%, P: 0.04% or less, S: 0.04% or less, Ni: 1.4% to 1.9%, Cr: 0.4% to 1.0%, Mo: 0.2% to 0.3%, Al: 0.1% or less, with the balance being Fe and impurities;
The product has a portion with a wall thickness of 1 inch or more, an HB hardness of 421 to 560, and a Charpy impact value of 20 to 40 J/ ^cm2 .

Si + Mn: Desirably 1.0% or less.

Si: 0.10% to 0.30%, Mn: 0.50% to 0.65%, Cr: 0.7% or more are more desirable.

Average grain size of martensite: 50 μm to 300 μm is desirable.

Further, the method for producing wear-resistant cast steel of the present invention comprises the steps of:
a cast steel ingot containing, by mass%, C: 0.27% to 0.36%, Si: 0.10% to 0.50%, Mn: 0.30% to 0.70%, P: 0.04% or less, S: 0.04% or less, Ni: 1.4% to 1.9%, Cr: 0.4% to 1.0%, Mo: 0.2% to 0.3%, Al: 0.1% or less, with the balance being Fe and impurities;
a homogenization treatment step of heating and holding the material at 900°C to 960°C and then furnace cooling;
A quenching step is carried out in which the material is heated and held at 800°C to 950°C, and then water quenched.

After the quenching process, the tempering process may not be performed.

After the quenching step,
A tempering step can also be carried out in which the material is heated and held at 150°C to 250°C and then air- cooled to room temperature.

The wear-resistant cast steel of the present invention ensures toughness by reducing the Si and Mn content, so the tempering process for restoring toughness can be omitted. By omitting the tempering process, there is no decrease in hardness due to tempering, so wear-resistant cast steel with high toughness and high hardness can be obtained.

FIG. 1 is an explanatory diagram of the test grinder used in the examples.

<Reason for limiting ingredients>
The reasons for limiting the components of the wear-resistant cast steel of the present invention are as follows: In the following, unless otherwise specified, "%" means "mass %".

C: 0.27% to 0.36%
C is a major element contributing to hardenability, and is contained in an amount of 0.27% or more to transform the structure into martensite and ensure hardness. Hardenability includes both the hardenability, which increases the hardness of the hardened portion, and the hardening depth. Since the wear-resistant cast steel of the present invention has a thickness of 1 inch or more, C alone is insufficient to ensure the hardness inside the thick wall, and the addition of elements such as Cr is necessary. On the other hand, a high C content can lead to a decrease in toughness and cracking during hardening (quench cracking), so the upper limit is set to 0.36%.

Si: 0.10% to 0.50%
Silicon is an element necessary for deoxidization and ensuring melt flow, suppressing gas defects and poor melt flow, and thereby contributing to the soundness of cast steel. Therefore, the content is set to 0.10% or more. On the other hand, a high Si content leads to a decrease in toughness, and the decreased toughness is difficult to recover even by tempering. Therefore, the upper limit is set to 0.50%, preferably 0.30%, and more preferably less than 0.30%. Since Si is consumed as a deoxidizer during casting, it is preferable to add about 1.5 to 2 times the final composition during casting.

Mn: 0.30% to 0.70%
Mn is also an element necessary for deoxidation, suppressing gas defects and thereby contributing to the soundness of cast steel, so it is contained in an amount of 0.30% or more, preferably 0.50% or more. On the other hand, a high Mn content leads to a decrease in toughness, so the upper limit is set to 0.70%, preferably 0.65%. Note that Mn is also consumed as a deoxidizer during casting, so it is preferable to add about 1.5 to 2 times the final composition during casting.

P: 0.04% or less, S: 0.04% or less P and S are impurities that cause embrittlement and casting cracks, so the upper limit is set at 0.04%.

Ni: 1.4% to 1.9%
Ni is an element necessary for ensuring the toughness and strength of cast steel, and is contained in an amount of 1.4% or more. In particular, since the wear-resistant cast steel of the present invention has a thickness of 1 inch or more, Ni is essential for ensuring the hardness and toughness of the thick wall interior. However, if the Ni content is too high, these effects will saturate and costs will increase, so the upper limit is set to 1.9%.

Cr: 0.4% to 1.0%
Cr, along with C, is an important element for ensuring hardenability. In particular, since the wear-resistant cast steel of the present invention has a thickness of 1 inch or more, the thick-wall interior cannot be rapidly cooled and is difficult to harden, so it is necessary to ensure the hardness of the thick-wall interior with Cr. For this reason, Cr is contained in an amount of 0.4% or more, preferably 0.7% or more. On the other hand, a high Cr content not only increases costs but also reduces manufacturability, such as formability and weld repairability, so the upper limit is set to 1.0%, preferably 0.9%.

Mo: 0.2% to 0.3%
Like Cr, Mo contributes to ensuring hardenability and hardness. It also contributes to improving mechanical properties and toughness. For this reason, Mo is contained in an amount of 0.2% or more. However, since a high Mo content increases costs, the upper limit is set to 0.3%.

Al: 0.1% or less Al is an element necessary for deoxidation, suppresses gas defects, and as a result contributes to the soundness of cast steel, so it is contained. On the other hand, a high Al content leads to a decrease in toughness, so the upper limit is set to 0.1%.

The balance is Fe and impurities that are mixed in due to various factors during the melting process, raw materials such as ore and scrap, and the manufacturing process. Examples of impurities include, but are not limited to, O, S, N, and H. The total amount of impurities is 0.5% or less, preferably 0.1% or less.

A cast steel ingot with the above composition is produced by casting. Casting is performed using sand mold casting, with the casting temperature being approximately 100°C above the melting point (for example, 1500°C to 1620°C). A low casting temperature can result in poor molten metal flow, while a high casting temperature can lead to oxidation and casting cracks. Cast steel ingots are used, for example, for crusher parts. Crusher parts have portions that are over 1 inch (2.54 cm) thick, so the area near the center of the wall thickness cannot be rapidly cooled and is difficult to harden. For this reason, cast steel ingots are heat treated.

The heat treatment consists of the homogenization process and quenching process described below, and can optionally include a tempering process.

<Homogenization treatment step>
The homogenization process is performed to remove inhomogeneous structures, such as the segregation of coarse dendrite structures, present in the cast steel ingot, to relieve casting stress, and to obtain an austenitic structure. The homogenization temperature can be less than 1000°C. Homogenization temperatures above 1000°C increase costs and promote oxidation, resulting in severe surface irregularities and reduced dimensional accuracy. The homogenization temperature is preferably 900°C to 960°C. Homogenization temperatures below 900°C prevent carbides from dissolving in the matrix, making feeder cutting and grinder maintenance difficult. The holding time for the homogenization process varies depending on the thickness of the cast steel ingot, but is typically 2 to 8 hours, preferably 3 to 6 hours. After holding at the specified temperature for the specified time, furnace cooling is desirable to prevent residual stress. Furnace cooling transforms the austenitic structure into a ferrite structure. This homogenization treatment homogenizes the structure in the cast ingot and removes casting stress, thereby improving the mechanical properties.

After the homogenization process, the cast steel ingot is subjected to machining, fusion cutting, etc. as needed.

<Quenching process>
After the homogenization treatment step, the cast steel ingot is quenched. Quenching is performed to transform the austenite state into a martensite structure by rapidly cooling it, thereby increasing hardness. Quenching is preferably performed so that the average grain size of the martensite is 50 μm to 300 μm, and more preferably 100 μm to 200 μm. The average grain size of the martensite can be the average grain size of the prior austenite. Because the cast steel ingot has thick portions with a thickness of 1 inch or more, the average grain size of the martensite in the thick portions is relatively coarse.

Specifically, the quenching temperature can be between 800°C and 950°C. If the quenching temperature is below 800°C, the structure will become non-uniform, resulting in reduced hardness and wear resistance. If the quenching temperature exceeds 950°C, the crystal grains will become coarse, resulting in reduced mechanical properties. The quenching holding time varies depending on the thickness of the cast steel ingot, but is usually between 2 and 6 hours, preferably between 3 and 4 hours.

When the thickness of a cast steel ingot exceeds 60 mm, the hardening may not reach the center of the wall. However, the composition of the present invention contains 0.4% to 1.0% Cr, which allows for a greater hardening depth and hardening all the way to the center of the wall.

If the tempering process described below is not carried out after the quenching process, the cast steel ingot after the quenching process is polished as necessary to produce the finished product.

<Tempering process (optional)>
The tempering process after the quenching process is carried out to restore toughness, even at the expense of some hardness. In the present invention, the cast steel ingot has low Si and Mn, which reduce toughness, so that the cast steel ingot has high toughness even after quenching. Therefore, since there is no need to restore toughness, the tempering process can be omitted, and the reduction in hardness due to the tempering process can also be avoided. Therefore, the cast steel after the quenching process can have high toughness and hardness.

Although it will increase costs, a tempering process may be carried out if necessary. Tempering reduces hardness, but it can restore the toughness lost by quenching. If tempering is carried out, a low-temperature tempering temperature of 150°C to 280°C is sufficient, and the holding time varies depending on the thickness of the cast steel ingot, but should be 1 to 3.5 hours, preferably 1.5 to 3 hours. To obtain the desired tempering effect, the tempering temperature x time value, calculated as the Larson-Miller parameter [LogT * (20 + log(t), where T is the tempering temperature (K) and t is the tempering time (h)], must be 9460 or greater, preferably 9543 or greater. As the tempering temperature x time value increases, toughness improves but hardness decreases significantly. Therefore, it is preferable to keep the tempering temperature x time value at 9717 or less, and preferably 9686 or less. Table 1 shows the relationship between the tempering time t and the Larson-Miller parameter when the tempering temperature T is 200°C (473K).

The resulting cast steel product has high hardness and toughness, making it suitable for use as, for example, crusher parts. Specifically, the cast steel product has a portion with a product thickness of 1 inch or more, an HB hardness of 421 to 560 (tungsten carbide ball) (hardness HRC: 45 to 55, hardness HS: 60 to 74), and a Charpy impact value (U-notch) of 20 to 40 J/ ^cm2 . Desirably, the HB hardness is 450 to 480, and the Charpy impact value (U-notch) is 30 to 40 J/ ^cm2 .

Two test grinders 10, which are crusher components shown in Figure 1, were prepared from the cast steel ingots of the invention example, reference example 1, and comparative example, each with the compositions shown in Table 2, and various tests were conducted. The test grinder 10 had a diameter of 340 mm, a thickness of 65 mm (2.56 inches), and a mass of approximately 29 kg.

Inventive Examples 1 and 2 and Reference Example 1 all have compositions within the range of the present invention. Furthermore, Comparative Example 1 is an example in which the Mn and Si + Mn contents are higher than the range of the present invention, and Comparative Example 2 is an example in which the Si, Mn, Si + Mn, and Ni contents are higher than the range of the present invention.

The test grinder 10 was produced by obtaining a cast steel ingot by sand casting, and then subjecting the ingot to the homogenization process, machining, quenching process, and optional tempering process shown in Table 3. The heat treatment conditions are shown in Table 3, with Inventive Example 1 and Comparative Examples 1 and 2 being examples without tempering, and Inventive Example 2 and Reference Example 1 being examples with tempering.

The homogenization treatment involved heating to 930°C followed by furnace cooling, and the quenching treatment involved heating to 880°C followed by water cooling. Tempering for Inventive Example 2 and Reference Example 1 involved holding at 200°C for two hours followed by air cooling for Inventive Example 2, and holding at 200°C for four hours followed by air cooling for Reference Example 1, as in Inventive Example 2.

As shown in Figure 1, test pieces 11, 12, and 13 were cut out from one of the two test grinders 10 produced. Reference numeral 11 denotes two tensile test pieces for tensile testing. Reference numeral 12 denotes three impact test pieces for impact testing, with the outer periphery of the grinder 10 serving as the notch side. Reference numeral 13 denotes one hardness test piece for hardness testing.

The other test grinder, 10, was used for wear tests using the actual machine.

Various tests were conducted on test pieces 11, 12, and 13 as follows:

In the tensile test, the tensile strength (N/mm ² ), 0.2% yield strength (N/mm ² ), elongation (%) and reduction in area (%) of the tensile test piece 11 were measured in accordance with JIS Z2241.

The impact test was conducted in accordance with JIS Z2242 on impact test specimens 12 with a 2mm U-notch according to JIS X2202, and the Charpy impact value was measured. The Charpy impact value was measured at three locations on each test specimen 12, and the average value was calculated.

The hardness test involved measuring the Brinell hardness (HB hardness) of hardness test piece 13 in accordance with JIS Z2243.

The results of each of the above tests are shown in Table 4.

First, comparing untempered Invention Example 1 with Comparative Examples 1 and 2, Invention Example 1 has the same tensile strength and 0.2% proof stress as Comparative Examples 1 and 2, but exhibits higher elongation and reduction of area. Its Charpy impact value is also higher than the comparative examples. This is the result of Invention Example 1 suppressing Si, Mn, and Si + Mn, and it demonstrates high toughness. Comparative Examples 1 and 2 have high Si, Mn, or Si + Mn, resulting in poor elongation, reduction of area, and Charpy impact value, and insufficient toughness. Furthermore, while the hardness of Invention Example 1 is slightly lower than Comparative Examples 1 and 2, there is no significant difference between Invention Example 1 and Comparative Examples 1 and 2, and they are equivalent.

Comparing the invention examples, invention example 1 and invention example 2 have the same composition, but have higher elongation, reduction of area, and Charpy impact value than invention example 1, demonstrating superior toughness. This is because tempering restored the toughness that had been reduced by quenching. On the other hand, invention example 2 showed a decrease in hardness due to tempering.

Comparing Example 2 and Reference Example 1, both of which were tempered, Reference Example 1 has the same toughness as Example 2, but the drop in hardness is even greater. This is because the tempering time was long and excessive tempering was performed.

For reference, the average grain size of the martensite in the test grinder 10 from which the test specimens were taken was measured, and as shown in Table 4, the size was approximately the same for the invention example, comparative example, and reference example 1.

Next, the test grinder 10 was attached to a crusher at an actual plant, and the wear loss during operation was measured. The crusher used was a KE-600 manufactured by Kubota Environmental Engineering Co., Ltd. The test grinder 10 rotated at 300 rpm, and the materials to be crushed were printed circuit boards and scrap metal. The wear test involved operating the crusher for a total of 891 hours, and evaluating the weight loss of the test grinder 10 before and after the test. The results are shown in Table 4.

Referring to Table 4, Invention Examples 1 and 2 have reduced wear loss and superior wear resistance compared to Comparative Examples 1 and 2. This is because Invention Examples 1 and 2 have the same or slightly lower hardness than Comparative Examples 1 and 2, but have higher toughness. Reducing wear loss allows for a longer grinder life.

Comparing Inventive Examples 1 and 2 with Reference Example 1, Reference Example 1 exhibits greater wear loss. This is because Reference Example 1's hardness was significantly reduced due to excessive tempering.

In crushers, the grinders can become hydrogen embrittled and peel off due to moisture contained in the material being crushed and in the air. This peeling begins with white tissue that forms on the surface of the grinder. For this reason, the test grinders 10 were examined for the presence of white tissue after the abrasion test. As a result, as shown in Table 4, white tissue was observed in all examples. The amount of white tissue in Invention Examples 1 and 2 and Comparative Examples 1 and 2 was relatively light, but the amount of white tissue in Reference Example 1 was severe. In Reference Example 1, a large amount of white tissue was produced, which led to tissue peeling during the abrasion test, and this is thought to be one of the reasons for the large abrasion loss.

The above description is intended to explain the present invention and should not be construed as limiting the invention described in the claims or narrowing its scope. Furthermore, the configuration of each part of the present invention is not limited to the above-described embodiment, and various modifications are possible within the technical scope described in the claims.

For example, in the above description, the wear-resistant cast steel of the present invention is applied to crusher parts, but this is not a limitation, and the wear-resistant cast steel of the present invention can also be applied to crushers, etc.

Claims (7)

The alloy contains, in mass %, C: 0.27% to 0.36%, Si: 0.10% to 0.50%, Mn: 0.30% to 0.70%, P: 0.04% or less, S: 0.04% or less, Ni: 1.4% to 1.9%, Cr: 0.4% to 1.0%, Mo: 0.2% to 0.3%, Al: 0.1% or less, with the balance being Fe and impurities;
The product has a portion with a wall thickness of 25.4 mm ( 1 inch ) or more, an HB hardness of 421 to 560, and a Charpy impact value of 20 to 40 J/ ^cm2 .
Wear-resistant cast steel. Si + Mn: 1.0% or less,
The wear-resistant cast steel according to claim 1 . Si: 0.10% to 0.30%, Mn: 0.50% to 0.65%, Cr: 0.7% or more;
The wear-resistant cast steel according to claim 2. Average grain size of prior austenite : 50 μm to 300 μm;
The wear-resistant cast steel according to claim 1. A method for producing the wear-resistant cast steel according to claim 1 ,
a cast steel ingot containing, by mass%, C: 0.27% to 0.36%, Si: 0.10% to 0.50%, Mn: 0.30% to 0.70%, P: 0.04% or less, S: 0.04% or less, Ni: 1.4% to 1.9%, Cr: 0.4% to 1.0%, Mo: 0.2% to 0.3%, Al: 0.1% or less, with the balance being Fe and impurities;
a homogenization treatment step of heating and holding the material at 900°C to 960°C and then furnace cooling;
A quenching process is carried out in which the material is heated and held at 800°C to 950°C, and then water quenched.
Manufacturing method for wear-resistant cast steel. After the quenching step, no tempering step is performed.
A method for producing the wear-resistant cast steel according to claim 5. After the quenching step,
A tempering process is carried out by heating and holding at 150°C to 250°C for 1 to 3.5 hours, and then air-cooling to room temperature.
A method for producing the wear-resistant cast steel according to claim 5.