Patent Application
20230002872
The present disclosure relates to a material appropriate for construction machinery and the like, and more particularly, to a wear-resistant steel having excellent low-temperature impact toughness and high hardness and a manufacturing method therefor.
As lightweightedness and high performance in industrial machines such as a bulldozer or a power shovel, mining equipment such as a crusher or a chute, a large dump truck, and the like are demanded, a wear-resistant steel is being used for worn areas.
In particular, in order to extend the durable years of the areas, the wear-resistant steel used herein increasingly shows a tendency of having high hardness, and due to concerns about defects such as crack occurrence by high hardness, high toughness is also required.
Meanwhile, a high hardness wear-resistant steel having excellent toughness is widely used as a bulletproof steel.
Currently, the following technologies have been suggested for a wear-resistant steel used in industrial machines, construction machinery, or the like.
Patent Document 1 discloses manufacturing of steel, in which certain amounts of Ti, B, and the like with C, Si, and Mn are contained in the steel and a cooling end temperature in the reheating and quenching of a steel sheet having a limited content of H is limited to 300° C. or lower, thereby manufacturing a steel having excellent integrity and a Brinell hardness of 450 or less.
Patent Document 2 discloses that a steel sheet to which Cr, Mo, and B are added in addition to C, Si, and Mn is reheated and quenched to manufacture a steel having a Brinell hardness grade of 500.
In addition, Patent Document 3 discloses that a steel containing limited contents of Cr, Mo, Ti, Nb, B, and the like with C, Si, and Mn therein, and if necessary, further containing Cu, Ni, V, Ca, and the like is hot rolled and then cooled to 100° C. or lower, and is continuously subjected to a tempering treatment, thereby manufacturing a steel having low-temperature toughness and a Brinell hardness grade of 500.
Besides, Patent Document 4 discloses a high-elasticity and high-strength steel fora special purposes, which has impact resistance and wear resistance secured by tempering a steel appropriately containing a relatively low content of C, a relatively low content of Si, and other elements.
However, Patent Document 1 does not satisfy a hardness level required in a real environment, Patent Document 2 satisfies the hardness level but has poor toughness, while Patent Document 3 contains a large amount of relatively expensive elements, and thus, is economically unfavorable and has limitations in application. In Patent Document 4, it is difficult to secure low-temperature toughness and manufacturing costs thereof are still high.
Accordingly, development of a wear-resistant steel having excellent low-temperature toughness together with wear resistance without including a large amount of expensive elements by an economical method, is demanded.
An aspect of the present disclosure is to provide a wear-resistant steel having high impact toughness at a low temperature together with wear resistance while having high hardness, and a manufacturing method therefor.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.
According to an aspect of the present disclosure, a high hardness wear-resistant steel having excellent low-temperature impact toughness includes, by weight: 0.25 to 0.50% of carbon (C), 1.0 to 1.6% of silicon (Si), 0.6 to 1.6% of manganese (Mn), 0.05% or less (excluding 0%) of phosphorus (P), 0.02% or less (excluding 0%) of sulfur (S), 0.07% or less (excluding 0%) of aluminum (Al), 0.5 to 1.5% of chromium (Cr), 0.0005 to 0.004% of calcium (Ca), and 0.006% or less of nitrogen (N), with a balance of Fe and other unavoidable impurities, wherein the wear-resistant steel includes a martensite and bainite composite structure as a microstructure and a retained austenite phase in an area fraction of 2.5 to 10%.
According to another aspect of the present disclosure, a manufacturing method for a high hardness wear-resistant steel having excellent low-temperature impact toughness includes: preparing a steel slab having the alloy composition described above; heating the steel slab in a temperature range of 1050 to 1250° C.; rough rolling the heated steel slab in a temperature range of 950 to 1150° C.; after the rough rolling, performing finish hot rolling in a temperature range of 850 to 950° C. to manufacture a hot rolled steel sheet; and cooling the hot rolled steel sheet to 200 to 400° C. at a cooling rate of 25° C./s or more and then performing air cooling.
According to the present disclosure, a wear-resistant steel having excellent low-temperature toughness while having high hardness may be provided.
In particular, the present disclosure may provide a wear-resistant steel having a target level of physical properties without performing a further heat treatment from optimization of an alloy composition and manufacturing conditions, and thus, is economically favorable.
The present inventors researched in depth, in order to provide a steel material having excellent physical properties such as strength and toughness while securing wear resistance which is essentially required physical properties, as a material which is appropriately applied to construction machinery and the like.
In particular, the wear resistance of a steel material was intended to be improved by an economically favorable method, and thus, the present disclosure was provided.
Hereinafter, the present disclosure will be described in detail.
A high hardness wear-resistant steel according to an aspect of the present disclosure may include, by weight: 0.25 to 0.50% of carbon (C), 1.0 to 1.6% of silicon (Si), 0.6 to 1.6% of manganese (Mn), 0.05% or less (excluding 0%) of phosphorus (P), 0.02% or less (excluding 0%) of sulfur (S), 0.07% or less (excluding 0%) of aluminum (Al), 0.5 to 1.5% of chromium (Cr), 0.0005 to 0.004% of calcium (Ca), and 0.006% or less of nitrogen (N).
Hereinafter, the reason that the alloy composition of the wear-resistant steel provided in the present disclosure is limited as described above will be described in detail.
Meanwhile, unless otherwise particularly stated in the present disclosure, the content of each element is by weight and the ratio of the structure is by area.
Carbon (C): 0.25 to 0.50%
Carbon (C) is an element which is effective for improving strength and hardness in steel having a low-temperature transformation phase such as a martensite or bainite phase and is effective for hardenability improvement. In order to sufficiently obtain the effect described above, 0.25% or more C may be included, but when the content is more than 0.50%, the weldability and the toughness of steel are deteriorated.
Therefore, C may be included in an amount of 0.25 to 0.50%.
Silicon (Si): 1.0 to 1.6%
Silicon (Si) is an element which is effective for strength improvement due to solid solution strengthening together with a deoxidation effect, and suppresses formation of carbides such as cementite in a high carbon steel containing a certain amount or more of C to promote production of retained austenite.
In particular, since retained austenite which is uniformly distributed in steel having a low-temperature transformation phase such as martensite and bainite contributes to improvement of impact toughness without strength reduction, Si is an element favorable to secure low-temperature toughness in the present disclosure.
In order to sufficiently obtain the effect described above, 1.0% or more of Si may be included, but when the content is more than 1.6%, weldability is rapidly deteriorated.
Therefore, Si may be included in an amount of 1.0 to 1.6%, and more favorably at 1.2% or more.
Manganese (Mn): 0.6 to 1.6%
Manganese (Mn) is an element favorable to suppress production of ferrite and lower an Ar3 temperature, thereby improving quenching properties of steel to increase strength and toughness.
In order to obtain a target level of hardness in the present disclosure, Mn may be included in an amount of 0.6% or more, but when the content is more than 1.6%, weldability is deteriorated and center segregation is encouraged to deteriorate the physical properties in the center part.
Therefore, Mn may be included in an amount of 0.6 to 1.6%.
Phosphorus (P): 0.05% or less (excluding 0%)
Phosphorus (P) is an element which is included unavoidably in steel and deteriorates toughness of the steel. Thus, it is preferred to lower the content of P as much as possible.
In the present disclosure, even in the case of including P up to 0.05%, the physical properties of the steel are not significantly influenced, and thus, the content of P may be limited to 0.05% or less. More favorably, the content may be limited to 0.03% or less, but 0% may be excluded considering an unavoidably contained level.
Sulfur (S): 0.02% or less (excluding 0%)
Sulfur (S) is an element which is bonded to Mn in steel to form an MnS inclusion, thereby greatly deteriorating the toughness of steel. Thus, it is preferred to lower the content of S as much as possible.
In the present disclosure, even in the case of including S up to 0.02%, the physical properties of the steel are not significantly influenced, and thus, the content of S may be limited to 0.02% or less. More favorably, the content may be limited to 0.01% or less, but 0% may be excluded considering an unavoidably contained level.
Aluminum (Al): 0.07% or less (excluding 0%)
Aluminum (Al) is an element effective for lowering an oxygen content in molten steel as a deoxidizing agent of steel. When the content of Al is more than 0.07%, cleanliness of steel is impaired.
Therefore, Al may be included in an amount of 0.07% or less. However, when the content of Al is excessively lowered, loading occurs in a steelmaking process and manufacturing costs are increased, and thus, considering the fact, 0% may be excluded.
Chromium (Cr): 0.5 to 1.5%
Chromium (Cr) increases quenching properties of steel to improve strength, and is favorable to secure hardness in a surface part and a center part of steel. Since Cr is a relatively inexpensive element, it may be included in an amount of 0.5% or more for securing high hardness and high toughness of steel, using Cr. However, when the content is more than 1.5%, the weldability of steel may be deteriorated.
Therefore, Cr may be included in an amount of 0.5 to 1.5%, and more favorably at 0.65% or more.
Calcium (Ca): 0.0005 to 0.004%
Calcium (Ca) has a good binding force with sulfur (S) and produces CaS on the periphery (around) MnS, thereby suppressing elongation of MnS to improve toughness in a direction perpendicular to a rolling direction. In addition, CaS produced by adding Ca has an effect of increasing corrosion resistance under a humid external environment.
In order to sufficiently obtain the effect described above, 0.0005% or more of Ca may be included, but when the content is more than 0.004%, defects such as nozzle clogging are caused in a steelmaking operation.
Therefore, Ca may be included in an amount of 0.0005 to 0.004%.
Nitrogen (N): 0.006% or less
Nitrogen (N) is favorable to improve strength of steel by forming precipitates in steel, but when a content thereof is more than 0.006%, toughness of steel is rather deteriorated.
In the present disclosure, there is no difficulty in securing strength even when N is not included, and thus, N may be included in an amount of 0.006% or less. However, considering the unavoidably included level, 0% may be excluded.
The wear-resistant steel of the present disclosure may further include the following elements for the purpose of favorably securing the target physical properties.
Specifically, the wear-resistant steel may further include one or more of nickel (Ni), molybdenum (Mo), titanium (Ti), boron (B), and vanadium (V).
Nickel (Ni): 0.01 to 0.5%
Nickel (Ni) is an element favorable to improving both strength and toughness of steel, and for this, Ni may be included in an amount of 0.01% or more. However, since it is an expensive element, when the content is more than 0.5%, manufacturing costs are greatly increased.
Therefore, Ni may be included in an amount of 0.01 to 0.5%, if included.
Molybdenum (Mo): 0.01 to 0.3%
Molybdenum (Mo) is an element favorable to increase quenching properties of steel, and in particular, to improve hardness of a thick material having a certain thickness or more. In order to sufficiently obtain the effect described above, 0.01% or more of Mo may be included, but when the content is more than 0.3%, manufacturing costs are increased, and weldability is deteriorated.
Therefore, Mo may be included in an amount of 0.01 to 0.3%, if included.
Titanium (Ti): 0.005 to 0.025%
Titanium (Ti) is an element favorable to maximize the effect of B which is an element favorable to improve quenching properties of steel. That is, Ti is bonded to N in steel to be precipitated into TiN to reduce the content of solid-solubilized N, while suppressing formation of BN of B therefrom to increase solid-solubilized B, thereby maximizing improvement of quenching properties.
In order to sufficiently obtain the effect described above, 0.005% or more of Ti may be included, but when the content is more than 0.025%, coarse TiN precipitates are formed to lower toughness of steel.
Therefore, Ti may be included in an amount of 0.005 to 0.025%, if included.
Boron (B): 0.0002 to 0.005%
Boron (B) is an element effective for increasing strength by effectively increasing the quenching properties of steel even with a small addition amount thereof. In order to sufficiently obtain the effect, B may be included in an amount of 0.0002% or more. However, when the content is excessive, toughness and weldability of steel are rather deteriorated, and thus, the content may be limited to 0.005% or less.
Therefore, B may be included in an amount of 0.0002 to 0.005%, if included. More favorably, B may be included in an amount of 0.0040% or less, more favorably at 0.0035% or less, and further favorably at 0.0030% or less.
Vanadium (V): 0.2% or less
Vanadium (V) is an element favorable to form a VC carbide when reheating after hot rolling, thereby suppressing growth of austenite crystal grains and improving quenching properties of steel to secure strength and toughness. Since V is a relatively expensive element, when the content thereof is more than 0.2%, manufacturing costs are greatly increased.
Therefore, V may be included in an amount of 0.2% or less, if added.
The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
In the wear-resistant steel of the present disclosure having the alloy composition described above, the microstructure may be formed of a composite structure of martensite and bainite phases.
Specifically, the wear-resistant steel of the present disclosure may include the composite structure of martensite and bainite phases in an area fraction of 90% or more, and when the phase fraction is less than 90%, it is difficult to secure strength and hardness at a target level. Here, it is revealed that the martensite and the bainite phases may include tempered martensite and tempered bainite phases, respectively.
It is preferred that the wear-resistant steel of the present disclosure has the composite structure described having an average lath size of 0.3 μm or less. When the average lath size of the composite structure is more than 0.3 μm, the toughness of steel is deteriorated.
The wear-resistant steel of the present disclosure may include a retained austenite phase in addition to the composite structure, which may be included in an area fraction of 2.5 to 10%. When the fraction of the retained austenite phase is less than 2.5%, low-temperature impact toughness is deteriorated, and when the fraction is more than 10%, hardness is deteriorated.
Meanwhile, it is revealed that the wear-resistant steel of the present disclosure has the organization structure described above over the entire thickness.
The wear-resistant steel of the present disclosure having the suggested microstructure together with the alloy composition described above may have a thickness of 5 to 40 mm, a high surface hardness of 460 to 540 HB, and an impact absorption energy at −40° C. of 17 J or more showing excellent low-temperature toughness.
Here, the surface hardness refers to a hardness value measured at a point of 2 mm to 5 mm in the thickness direction from the surface of the wear-resistant steel.
Hereinafter, a manufacturing method for a high-hardness wear-resistant steel according to another aspect of the present disclosure will be described in detail.
In brief, the wear-resistant steel may be manufactured by preparing a steel slab satisfying the alloy composition described above, and then subjecting the steel slab to the processes of [heating—rolling—cooling]. Hereinafter, each process condition will be described in detail.
[Steel Slab Heating Process]
First, a steel slab having the alloy composition suggested in the present disclosure is prepared, which may be then heated in a temperature range of 1050 to 1250° C.
When the temperature is lower than 1050° C. during heating, the deformation resistance of steel is increased, so that a subsequent rolling process may not be effectively performed, and when the temperature is higher than 1250° C., austenite crystal grains are coarsened, so that non-uniform structure may be formed.
Therefore, the steel slab may be heated in a temperature range of 1050 to 1250° C.
[Rolling Process]
The steel slab heated as described above may be rolled, and then may be subjected to rough rolling and finish hot rolling to manufacture a hot rolled steel sheet.
First, the heated steel slab is roughly rolled in a temperature range of 950 to 1150° C. to be manufactured into a bar, which may be then subjected to finish hot rolling in a temperature range of 850 to 950° C.
When the temperature is lower than 950° C. during the rough rolling, a rolling load is increased to reduce the steel slab relatively weakly, and thus, deformation is not sufficiently transferred to the center of the thickness direction of the slab, and as a result, defects such as void may not be removed.
However, when the temperature is higher than 1150° C., recrystallization granularity is coarsened, which may be harmful to toughness.
When the temperature is lower than 850° C. in the finish hot rolling, two-phase region rolling is performed, so that ferrite may be produced in the microstructure, and when the temperature is higher than 950° C., the granularity of the final structure is coarsened to deteriorate low-temperature toughness.
[Cooling Process]
The hot rolled steel sheet which undergoes the rolling process described above is water-cooled to a certain temperature, and then air-cooled.
Specifically, in the present disclosure, in cooling the hot rolled steel sheet, water cooling may be performed to a temperature range of 200 to 400° C. at a cooling rate of 25° C./s or more and then air cooling may be performed down to 150° C. or lower, and in the air cooling, self-tempering may be expressed. That is, martensite and bainite phases are tempered in the air cooling, and toughness improvement of steel may be promoted by forming a retained austenite phase at a certain fraction.
The air cooling may be performed down to room temperature.
Meanwhile, the cooling may start at Ar3 or higher. Here, Ar3 depends on an alloy component system, which is a matter recognized by any person skilled in the art.
When the cooling rate during the water cooling is less than 25° C./s, a ferrite phase is formed during cooling, or an average lath size of a hard phase (martensite+bainite) is increased, so that it is difficult to secure high hardness. The upper limit of the cooling rate during the water cooling is not particularly limited, but it is revealed that cooling may be performed at a cooling rate up to 100° C./s considering cooling equipment.
In the cooling at the cooling rate, when the cooling end temperature is lower than 200° C., a self-tempering effect is small, so that it is difficult to secure toughness at a target level, but when the temperature is higher than 400° C., an average lath size of a hard phase (martensite+bainite) is increased, so that hardness or toughness at a target level may be secured by decreased strength or toughness.
The hot rolled steel sheet obtained by the series of manufacturing processes described above is a steel material having a thickness of 5 to 40 mm, and may have high hardness and high toughness properties together with wear resistance.
In particular, according to the present disclosure, self-tempering may be realized during the cooling process, and thus, a subsequent tempering process is not required, and there is an effect of more economically manufacturing a wear-resistant steel therefrom.
Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following Examples are only for describing the present disclosure in detail by illustration, and are not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.
Steel slabs having the alloy compositions shown in the following Table 1 were prepared, and then were subjected to [heating—rolling—cooling] according to the process conditions shown in the following Table 2 to manufacture each hot rolled steel sheet. At this time, for the cooling, water cooling was performed down to a certain temperature, and then air cooling was performed down to 150° C. or lower.
Thereafter, the microstructure and the mechanical properties of each hot rolled steel sheet were measured, and the results are shown in the following Table 3.
The microstructure of each hot rolled steel sheet was cut into an optional size as a specimen to manufacture a mirror surface, a Nital etching solution was used to corrode the specimen, and then an optical microscope and a scanning electron microscope (SEM) were used to observe a ½t point which was a thickness center part. At this time, a lath size of a martensite and bainite composite structure was measured by electron back-scattered diffraction (EBSD) analysis.
In addition, the hardness and the toughness of each hot rolled steel sheet were measured using a Brinell hardness tester (load: 3000 kgf, 10 mm tungsten indentation ball) and a Charpy impact tester. At this time, in the surface hardness, an average value of three measurements after a milling process of the surface of the hot rolled sheet at 2 mm was used, and in the Charpy impact test, a specimen was collected at ¼t point in the thickness direction, and then an average value of three measurements at −40° C. was used.
(In Table 1, P*, S*, Ca*, B*, and N* are represented in ppm)
(In Table 2, the cooling start temperature of the inventive examples is Ar3 or higher.)
(In Table 3, M is a martensite phase, Bis a bainite phase, F is a ferrite phase, and r-γ is a retained austenite phase.)
As shown in Tables 1 to 3, in Inventive Examples 1 to 10 in which both the alloy composition and the manufacturing conditions suggested in the present disclosure were satisfied, it was confirmed that the microstructure included a retained austenite phase at a certain fraction together with martensite+bainite. In addition, the lath size of martensite+bainite was all formed at 0.3 μm or less. It was possible therefrom to secure excellent hardness and low-temperature impact toughness in all of Inventive Examples 1 to 10.
However, in Comparative Examples 1 to 8 in which the alloy composition suggested in the present disclosure was satisfied, but the manufacturing conditions were out of the scope of the present disclosure, a ferrite phase was formed as the microstructure, the lath size of martensite and bainite was coarse, or the fraction of the austenite phase was insignificant, and thus, it was difficult to secure both high hardness and low-temperature impact toughness well.
Meanwhile, in Comparative Examples 9 to 11 in which the C content in steel was insufficient, quenching properties were low to excessively produce a cornerstone ferrite phase, so that hardness and toughness were very poor. In addition, in Comparative Examples 12 and 13 in which the C content in steel was excessively high, the fraction of a retained austenite phase was insignificant, so that the low-temperature impact toughness was very poor.
Further, in Comparative Example 14 having insufficient contents of Si and Cr in steel, a retained austenite phase was not sufficiently produced, and production of a cementite phase having poor toughness was promoted, so that hardness was high, but toughness was poor.
In Comparative Example 15 also, the contents of Si and Cr were insufficient, so that a retained austenite phase was not sufficiently produced, the production of a cementite phase was promoted, so that toughness was poor, and hardenability was increased due to an excessive Mo content, so that toughness was very poor as compared with the specification.
Among them,
Among them,
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2019-0168076 | Dec 2019 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2020/017372 | 12/1/2020 | WO |
