Patent Application
20210381074
The present disclosure relates to a steel material for use in automobiles or construction steel plates, and more particularly, to a high manganese steel material having excellent anti-vibration characteristics and formability that may be used in locations in which anti-vibration characteristics for noise reduction are required, and a method of manufacturing the same.
Recently, noise reduction in materials such as automobile manufacturing or building materials is an issue that manufacturers should solve. In the case of automobile manufacturers, components such as engine parts and oil pans from which large amounts of noise occur are particularly required to have excellent mechanical properties and anti-vibration characteristics. In addition, in the case of building materials, as noise regulations for noise between floors have been strengthened, development of steel materials having excellent anti-vibration characteristics as floor plates of multi-story buildings including apartments is required.
Meanwhile, high manganese (Mn) vibration-proof steel is a steel grade that converts noise energy into thermal energy due to interfacial sliding of epsilon martensite in the case of external impacts, having high anti-vibration characteristics and excellent mechanical properties, and thus is suitable for use to reduce noise.
In general, anti-vibration properties of high manganese anti-vibration steel are secured by manufacturing a hot rolled or cold-rolled steel plate through a process of steelmaking-continuous casting-hot rolling or by adding a cold-rolling process thereto to prepare a hot-rolled or cold-rolled steel sheet and subsequently applying a post-heat treatment to form epsilon martensite and/or form a recrystallized structure.
Here, the post-heat treatment performed to secure the anti-vibration characteristics is a high-cost heat treatment to which a time exceeding 10 minutes, preferably, more than 60 minutes is applied at a temperature of usually 900° C. or higher, which has been a factor that inhibits generalization of high manganese anti-vibration steel.
Demand for noise reduction has continuously increased, and thus, there is a need to develop a steel material supporting compatibility between ensuring anti-vibration characteristics and excellent formability and omitting a high-cost post-heat treatment.
An aspect of the present disclosure may provide a method of manufacturing a high manganese steel material having excellent anti-vibration characteristics and formability at low cost, compared to the related art, while eliminating a post-heat treatment essentially performed to improve anti-vibration characteristics, and a high manganese steel material having excellent anti-vibration characteristics manufactured thereby.
The technical problem of the present disclosure is not limited to the aforementioned matters. Additional problems of the present disclosure are described in the overall contents of the disclosure, and those of ordinary skill in the art to which the present disclosure pertains will not have any difficulty in understanding the additional problems of the present disclosure from the contents described in the disclosure of the present disclosure.
According to an aspect of the present disclosure, a method of manufacturing a high manganese steel material having excellent anti-vibration characteristics and formability includes: heating a steel slab including, in percentages by weight, 0.1% or less of carbon (C), 8 to 30% of manganese (Mn), 3.0% or less of silicon (Si), 0.1% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.1% or less of nitrogen (N), 1.0% or less (excluding 0%) of titanium (Ti), 0.01% or less of boron (B), the balance iron (F) and other inevitable impurities at 1,150 to 1,350° C.; finish hot-rolling the heated steel slab to manufacture a hot-rolled steel plate; and cooling the hot-rolled steel plate to 700° C. or lower, wherein the finish hot-rolling is performed at a finishing delivery temperature (FDT) (° C.) satisfying Relational Expression 1 below:
FDT(° C.)≥928+(480×C)+(450×N)+(0.9×Mn)+(65×Ti) [Relational Expression 1]
(Here, each element represents content by weight)
According to an aspect of the present disclosure, a steel material manufactured by the aforementioned manufacturing method, having the aforementioned alloy composition, including, by area fraction, 90% or more epsilon martensite and the balance of an austenite phase as a microstructure, being a fully recrystallized structure, and having excellent, anti-vibration characteristics and formability
According to an aspect of the present disclosure, the high manganese steel material having excellent anti-vibration characteristics and formability, even if a post-heat treatment required for improving the anti-vibration characteristics of the related art high-manganese anti-vibration steel is omitted, can be provided.
In addition, the present disclosure may provide high manganese vibration-proof steel at a relatively low cost by omitting the post-heat treatment, and thus, a technical effect may be obtained in terms of economics and the present disclosure may be generally used in fields requiring anti-vibration characteristics.
The inventors of the present application found that, in order to improve anti-vibration characteristics, high-cost heat treatment (or a post-heat treatment) should be applied to the existing high manganese anti-vibration steel, which will resultantly significantly increases manufacturing costs and limits generalization.
Accordingly, the inventors studied in depth a method that may achieve both anti-vibration characteristics and excellent formability even if high-cost heat treatment is omitted. As a result, the inventors found that a fraction of epsilon martensite phase in the steel could be maximized by optimizing a manufacturing process along with the control of an alloy composition, and thus, a steel material having excellent anti-vibration characteristics and formability may be provided only with a series of hot rolling processes, and completed the present disclosure.
Hereinafter, the present disclosure will be described in detail.
According to a method of manufacturing a high manganese steel material having excellent anti-vibration characteristics and formability, a steel slab having an alloy composition described below may be prepared and hot-rolled and cooled to manufacture the high manganese steel material.
First, the reason for limiting an alloy composition to obtain a target high manganese steel material in the present disclosure will be described in detail. Here, unless otherwise specified, the content of each element means the weight content (% by weight).
Carbon (C): 0.1% or less
Carbon (C) is an element that stabilizes austenite in the steel and is advantageous to secure strength. However, if the carbon content exceeds 0.1%, the fraction of dissolved C is excessively increased, which impairs hot workability and significantly reduces anti-vibration characteristics.
Therefore, in the present disclosure, carbon (C) may be contained in an amount of 0.1% or less, and even if carbon (C) is included as 0%, it is not difficult to secure target physical properties.
Manganese (Mn): 8 to 30%
Manganese (Mn) is an essential element for stably securing austenite and epsilon martensite structures. In the present disclosure, in order to secure the epsilon martensite phase in a certain fraction or more without performing a separate heat treatment process, it is necessary to contain Mn in an amount of 8% or more. If the content exceeds 30%, manufacturing cost may increase and the content of phosphorus (P) may increase in the process of refining a large amount of Mn, causing slab cracking. In addition, as the content of Mn increases, internal grain boundary oxidation occurs excessively when the slab is heated, causing oxide defects on a steel surface and surface characteristics are also deteriorated during subsequent plating.
Therefore, in the present disclosure, Mn may be included in an amount of 8 to 30%, and more advantageously, 14 to 20%.
Silicon (Si): 3.0% or less
Silicon (Si) is an element that is solid solution strengthened and is advantageous in improving yield strength by reducing a grain size by a solid solution effect. If the content of Si increases, a silicon compound is formed on a surface of the steel sheet during hot rolling, resulting in poor pickling and surface quality of the hot-rolled steel sheet may be deteriorated. In addition, when excessively added, weldability is significantly reduced.
Accordingly, in the present disclosure, Si may be contained in an amount of 3.0% or less, and even if it is included as 0%, there is no difficulty in securing target physical properties.
Phosphorus (P): 0.1% or less and sulfur (S): 0.02% or less
Phosphorus (P) and sulfur (S) are elements that are inevitably contained in steel during production thereof, and it is advantageous for these elements to be contained as low as possible. If the content of P exceeds 0.1%, segregation may occur to reduce workability of the steel, and if the content of S exceeds 0.02%, a coarse manganese sulfide (MnS) is formed to cause a defect such as flange cracks and impair formability of the steel sheet, in particular, hole expandability of the steel sheet.
Therefore, in the present disclosure, P may be contained in an amount of 0.1% or less, and S may be contained in an amount of 0.02% or less.
Nitrogen (N): 0.1% or less
Nitrogen (N) is an element forming a nitride. If the N content exceeds 0.1%, the fraction of dissolved N may be excessively high, inhibiting hot workability and elongation and reducing anti-vibration characteristics.
Therefore, in the present disclosure, N is contained in an amount of 0.1% or less, and even if N is included as 0%, it is not difficult to secure target physical properties.
Titanium (Ti): 1.0% or less (excluding 0%)
Titanium (Ti) is an element that combines with carbon to form a carbide, and the formed carbide suppresses grain growth, which is advantageous for refining a grain size. In addition, since titanium forms a compound with C and N to obtain a scavenging effect, it is advantageous in improving anti-vibration characteristics. If the content of Ti exceeds 1.0%, excess titanium segregates to the grain boundaries to cause grain boundary embrittlement or form coarse precipitated phases to inhibit the effect of inhibiting grain growth.
Therefore, in the present disclosure, Ti may be included in an amount of 1.0% or less, and excluding 0%.
Boron (B): 0.01% or less
Boron (B) has the effect of preventing grain boundary cracking by forming a high-temperature compound at the grain boundary upon addition with Ti. However, if the content of B exceeds 0.01%, it is not preferable because it forms a boron compound and deteriorates the surface properties.
Therefore, in the present disclosure, B may be contained in an amount of 0.01% or less, and even if B is included as 0%, it is not difficult to secure target physical properties.
In the steel material containing each element with the aforementioned composition, if C and N are added in combination, the sum of their contents (C+N, wt %) is preferably 0.1% or less.
C and N are interstitial solid solution elements, and when combined with Ti and the like to form carbonitrides, vibration-proof performance may be improved, but if the sum of their contents exceeds 0.1%, the fraction of dissolved C or dissolved N may be increased to degrade hot workability and elongation and reduce anti-vibration characteristics, which are not desirable.
Therefore, when the C and N are added in combination, C and N may be contained in an amount of 0.1% or less by the sum of the contents.
Meanwhile, the steel material of the present disclosure may further include an additional element in addition to the aforementioned alloy composition to improve physical properties.
In an aspect, 0.005 to 2.0% of nickel (Ni) and 0.005 to 5.0% of chromium (Cr) may be further included.
Nickel (Ni): 0.005 to 2.0%
Nickel (Ni) is an element that effectively contributes to securing high temperature ductility. In order to obtain the aforementioned effect, Ni may be contained in an amount of 0.005% or more, and as the content increases, it is also effective in delayed fracture resistance and in preventing slab cracking. However, Ni is an expensive element and may be contained in an amount of 2.0% or less in consideration of the cost.
Chrome (Cr): 0.005 to 5.0%
Chromium (Cr) reacts with external oxygen during hot rolling or annealing to preferentially form a Cr-based oxide film (Cr2O3) with a thickness of 20 to 50 μm on the steel surface, thereby preventing Mn, Si, etc. contained in the steel from eluting to a surface layer. Accordingly, there is an effect of contributing to stabilization of the steel surface layer structure and improving plating surface characteristics. In order to obtain the aforementioned effect, Cr may be contained in an amount of 0.005% or more, but if the content exceeds 5.0%, a chromium carbide may be formed to rather workability and delayed fracture resistance characteristics, which, thus, is not desirable.
Therefore, Cr may be contained in an amount of 0.005 to 5.0% when added in the present disclosure.
In another aspect, at least one of 0.005 to 0.5% of niobium (Nb), 0.005 to 0.5% of vanadium (V), and 0.005 to 1.0% of tungsten (W) may be further included.
Niobium (Nb): 0.005 to 0.5%
Niobium (Nb) is an element that combines with carbon in the steel to form a carbide, and may obtain an effect of increasing strength or reducing a particle size. In general, since Nb forms a precipitated phase at a lower temperature than Ti, it has a large effect of precipitation strengthening due to refinement of the grain size and formation of the precipitated phase. In addition, Nb lowers the fraction of dissolved C to improve anti-vibration characteristics.
For the aforementioned effects, Nb may be contained in an amount of 0.005% or more. If the content exceeds 0.5%, excessive Nb segregates to the grain boundaries to cause grain boundary embrittlement or form coarse precipitated phases to reduce the effect of inhibiting grain growth. In addition, a rolling load may increase by delaying recrystallization during hot rolling.
Therefore, Nb may be contained in an amount of 0.005 to 0.5% when added in the present disclosure.
Vanadium (V): 0.005 to 0.5% and tungsten (W): 0.005 to 1.0%
Vanadium (V) and tungsten (W) are elements that combine with C and N to form carbonitrides. In the present disclosure, the elements forma fine precipitated phase at a low temperature, so that the precipitation strengthening effect is significant. In addition, there is an effect of improving anti-vibration characteristics by lowering the fractions of dissolved C and dissolved N.
For the aforementioned effect, each may be contained in an amount of 0.005% or more, but in the case of V exceeding 0.5% or in the case of W exceeding 1.0%, the precipitated phase may be excessively coarsened to reduce the effect of inhibiting grain growth and cause hot brittleness.
Therefore, in the present disclosure, V may be added in an amount of 0.005 to 0.5%, and W may be added in an amount of 0.005 to 1.0% when added.
The balance of the present disclosure is Fe. Since unintended impurities from raw materials or a surrounding environment may inevitably be mixed in a typical manufacturing process, the unintended impurities cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all contents are not specifically mentioned in the present specification.
After preparing a steel slab having an alloy composition as described above, the steel slab may be heated, and here, the steel slab may be subjected to heating in a temperature range of 1150 to 1,350° C.
If a temperature during heating of the steel slab is too low, a rolling load may be excessively applied during subsequent hot rolling, so it may be carried out at at least 1,150° C.
Meanwhile, in the present disclosure, as the austenite grain size increases, the fraction of the epsilon martensite phase into a final microstructure may increase, and thus, a higher temperature during heating is advantageous. In addition, as the heating temperature is higher, subsequent hot rolling may be performed more advantageously. However, since the present disclosure contains a large amount of Mn, if heating is performed at an excessively high temperature, internal oxidation may occur severely to degrade surface quality. Therefore, heating may be performed at 1,350° C. or lower, and more preferably, at 1,300° C. or lower.
As described above, the heated steel slab may be hot-rolled to be manufactured as a hot-rolled steel sheet. Here, it is preferable to perform finish hot rolling at a temperature (FDT (° C.)) that satisfies the following Relational Expression 1.
FDT(° C.)≥928+(480×C)+(450×N)+(0.9×Mn)+(65×Ti) [Relational Expression 1]
(Here, each element represents a weight content)
Relational Expression 1 is an equation derived through a number of experiments, and is an important factor in manufacturing a high manganese steel material having excellent anti-vibration characteristics and formability targeted in the present disclosure.
Specifically, in the present disclosure, the growth and recrystallization of austenite grains to a sufficient size may be induced by performing finish hot rolling at a temperature exceeding the temperature at which full recrystallization occurs, from which an epsilon martensite phase may be stably secured in a follow-up cooling and/or coiling process.
If the temperature during finish hot rolling is lower than the temperature derived by Relational Expression 1, it may be difficult to induce growth and recrystallization of austenite grains, so that the epsilon martensite phase may not be sufficiently formed into a final microstructure and a non-recrystallized structure may be formed to lower the anti-vibration characteristics.
In addition, during the finish hot rolling, a total rolling reduction ratio may be 80% or more, more preferably, 90% or more. If the total rolling reduction ratio is 80% or more during the finish hot rolling, a recrystallization driving force may be sufficiently secured.
The hot-rolled steel sheet manufactured as described above may be cooled, and here, it is preferable to perform cooling to 700° C. or lower.
If an end temperature during cooling exceeds 700° C., scale may be generated excessively and an excessive process may be required to remove the scale, post-processing is interfered along with problems such as air pollution due to dust, which is thus not desirable.
In the present disclosure, cooling may be performed to room temperature, and in this case, there is an effect of securing more excellent anti-vibration characteristics compared to a high manganese anti-vibration steel manufactured by the existing post-heat treatment (see Table 3 below).
Therefore, in the present disclosure, it is preferable to terminate the cooling in a temperature range of 700° C. or lower, more preferably 500° C. or lower, and even more preferably, at room temperature to 300° C. during the cooling. As described above, as the cooling termination temperature decreases, the amount of residual austenite decreases, so it is more advantageous in securing the epsilon martensite phase in the final microstructure.
Meanwhile, cooling may be performed through normal water cooling (e.g., a cooling rate of 10° C./s or higher), and if the cooling termination temperature is room temperature to 300° C., the cooling termination temperature may be secured through rapid cooling. A cooling rate during rapid cooling is not particularly limited, but may be performed at a cooling rate of 50° C./s or more, for example, or may be performed at 200° C./s or less in consideration of equipment specifications.
Here, room temperature is not particularly limited, but refers to about 20 to 35° C.
In the present disclosure, after cooling is completed, a coiling process may be further performed at the corresponding temperature, which may be selectively performed in consideration of a thickness of a steel.
The high manganese steel material of the present disclosure obtained by completing the aforementioned cooling process includes an epsilon martensite phase with an area fraction of 90% or more and a fully recrystallized structure, that is, does not include a non-recrystallized structure, and thus, high anti-vibration characteristics and formability may be secured.
Hereinafter, a high manganese steel material having excellent anti-vibration characteristics and formability according to another aspect of the present disclosure will be described in detail.
Since the high manganese steel material of the present disclosure may be obtained by the aforementioned manufacturing process and has the aforementioned alloy composition, an alloy composition of the steel material is replaced by the previously mentioned matters.
The high manganese steel material of the present disclosure preferably includes epsilon martensite having an area fraction of 90% or more (including 100%) and the balance austenite phase in a microstructure. In particular, the present disclosure is a fully recrystallized structure that does not contain any non-recrystallized structure, thus securing excellent anti-vibration characteristics, and more preferably, the epsilon martensite phase may be included in an amount of 95% or more.
As such, the high manganese steel material of the present disclosure contains the epsilon martensite phase in a high fraction, and a residual dislocation is effectively removed by full recrystallization, whereby a rate in which the epsilon martensite phase converts impact energy into thermal energy when an external impact is applied is increased to contribute to improvement of damping performance.
Meanwhile, the high manganese steel material of the present disclosure does not include any phase other than the aforementioned phase as a microstructure. For example, the high manganese steel material of the present disclosure does not include an (α′)-martensite phase at all.
In particular, the present disclosure may form the epsilon martensite phase in a sufficient fraction and obtain excellent formability even if the high-cost heat treatment performed to manufacture the related art high manganese anti-vibration steel is omitted. Therefore, the high manganese steel material of the present disclosure will have an economically advantageous technical effect as compared to the related art high manganese anti-vibration steel.
Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by matters described in the claims and matters reasonably inferred therefrom.
Steel slab having the alloy composition of Table 1 below was heated, hot-rolled, and cooled under the conditions shown in Table 2 to manufacture each hot-rolled steel sheet. Here, for comparison, a specific steel grade was subjected to a post-heat treatment, and the post-heat treatment was performed at 1000° C. for 30 minutes and thereafter air-cooling was performed.
Thereafter, mechanical properties and a microstructure of each hot-rolled steel sheet were measured and results thereof are shown in Table 3 below.
Here, for the measurement of mechanical properties, a tensile test piece of JIS No. 5 was manufactured, and then yield strength (YS), tensile strength (TS), and elongation (T-El and U-El) were measured. In addition, the microstructure was measured using X-ray diffraction (XRD), and a fraction of each phase was derived from a peak intensity of each phase.
As shown in
(In Table 3, α′-M represents an alpha′-martensite, γ represents austenite, and ε-M represents an epsilon martensite phase.)
As shown in Table 1 to Table 3, in Inventive Steels 1 to 6, satisfying the alloy composition and manufacturing conditions, on which finish hot rolling was performed at a temperature that satisfies Relational Expression 1 proposed in the present disclosure and the cooling was terminated at 700° C. or lower, all the epsilon martensite phases were 95% or more, securing excellent anti-vibration characteristics.
In addition, as the total elongation exceeds 40% in all of the Inventive Steels 1 to 6, it can be confirmed that formability is also excellent.
Thus, it can be seen that the Inventive Steels 1 to 6 have anti-vibration characteristics and formability equivalent to or higher than those of the high manganese anti-vibration steel (see Comparative Steel 5) subjected to a post-heat treatment in the related art.
Meanwhile, in the case of Comparative Steels 1 to 4 and 6 to 9 that do not satisfy the manufacturing conditions (Relational Expression 1, etc.) of the present disclosure, α′-martensite phase was formed, showing inferior anti-vibration characteristics, and a total elongation was secured to less than 40%, showing inferior formability.
As shown in
As shown in
As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2018-0124444 | Oct 2018 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2018/015601 | 12/10/2018 | WO | 00 |
