Patent Application
20240376576
This application claims the benefit of Korean Patent Application No. 10-2024-0058396 filed on May 2, 2024, which is hereby incorporated by reference herein in its entirety.
The present invention relates to a medium-manganese steel and a method of producing the same. More specifically, the present invention relates to a high-strength medium-manganese steel with excellent hydrogen embrittlement resistance and a method of producing the same.
Recently, the automobile industry has been developing processes of attaining high-strength of parts to improve passenger safety and fuel efficiency. Representative examples of these processes include a hot stamping process and a warm stamping process.
The hot stamping process may include hot-stamping boron-added carbon steel in the austenite single-phase region (generally 850 to 950° C.). In addition, this is a process that can attain high strength by obtaining a martensitic microstructure through die quenching.
The warm stamping process is characterized by warm-stamping medium-manganese steel at lower a relatively temperature (generally 700 to 850° C.) than that in the hot stamping process, followed by air cooling. The warm stamping process may include not only die quenching but also air cooling. Even with air cooling, it is possible to obtain a martensitic microstructure and to attain tensile properties equivalent to or better than those of parts produced through the hot stamping process.
However, it has been reported that ultra-high strength steel with a martensitic microstructure has low resistance to hydrogen embrittlement due to hydrogen introduced into the steel.
Therefore, to overcome this problem, attempts have been made to improve hydrogen embrittlement resistance by adding microalloying elements (Ti, Nb, V, Mo, etc.) to form “irreversible hydrogen trap sites.” However, the microalloying elements used in these attempts raised the problem of high material costs.
A high-strength medium-manganese steel with excellent hydrogen embrittlement resistance and a method of producing the same according to the present invention achieve the following objects:
A first object is to improve hydrogen embrittlement resistance by controlling the density and size of inclusions in steel by controlling the S and Ti contents without adding a new process.
A second object is to propose a heat treatment temperature range that provides steels having a tensile strength of 2,000 MPa or more and an elongation of 7.0% or more without a decrease in strength and elongation even when S is added.
Objects to be achieved by the present invention are not limited to those mentioned above, and other objects not mentioned above will be clearly understood by those skilled in the art from the following description.
The present invention provides a medium-manganese steel containing 3 to 5 wt % manganese (Mn), 0.1 to 0.3 wt % carbon (C), 0.2 to 0.7 wt % silicon (Si), 0.01 to 0.07 wt % sulfur(S), and 0.005 to 0.06 wt % titanium (Ti), with the remainder being iron (Fe) and inevitable impurities.
The medium-manganese steel according to the present invention is a high-strength medium-manganese steel with excellent hydrogen embrittlement resistance, which consists of martensite and 3 to 5 vol % retained austenite at room temperature.
In the present invention, the medium-manganese steel may have MnS inclusions and TiN—MnS complex inclusions, or have MnS inclusions and Ti(C,N)—MnS complex inclusions.
In the present invention, the density of the inclusions may be 90 to 4,000 ea/mm2.
In the present invention, the aspect ratio of each of the above inclusions may be 1 to 35.
In the present invention, the width of each of the inclusions may be 0.3 to 3.0 μm, and the length thereof may be 0.5 to 31.0 μm.
The present invention also provides a method of producing a high-strength medium-manganese steel with excellent hydrogen embrittlement resistance, the method including steps of:
In the present invention, the homogenizing in step (S2) may be performed at 1,200 to 1,300° C.
In the present invention, the hot rolling in step (S3) may be performed at 900 to 1,100° C.
In the present invention, the cold rolling in step (S4) may be performed at a reduction ratio of 45 to 55% at room temperature.
In the present invention, the annealing in step (S5) may be performed at 750° C.
In the present invention, the austenitizing in step (S6) may be performed at an austenite single phase region A3 temperature or higher.
In the present invention, the austenitizing may be performed at 750° C. to lower than 850° C.
In the present invention, the austenitizing may be performed at 850° C. to 1,000° C.
In the present invention, the austenitizing is preferably performed at 750° C.
In the present invention, the produced medium-manganese steel may consist of martensite and 3 to 5 vol % retained austenite at room temperature.
In the present invention, the produced medium-manganese steel may have MnS inclusions and TiN—MnS complex inclusions, or may have MnS inclusions and Ti(C,N)—MnS complex inclusions.
In the present invention, the density of the inclusions may be 90 to 4,000 ea/mm2.
In the present invention, the aspect ratio of each of the above inclusions may be 1 to 35.
In the present invention, the width of each of the inclusions may be 0.3 to 3.0 μm, and the length thereof may be 0.5 to 31.0 μm.
The above and other objects, features, and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art to which the present invention pertains can easily practice the present invention. As can be easily understood by those skilled in the art to which the present invention pertains, the embodiments described below may be modified into various forms without departing from the spirit and scope of the present invention. Throughout the drawings, identical or similar components are denoted by the same reference numerals.
The terminology used herein is only intended to describe specific embodiments and is not intended to limit the present invention. As used herein, singular forms include plural forms unless the context clearly indicates otherwise.
As used herein, the term “containing” or “including” specifies the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
All terms used herein, including technical and scientific terms, have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Terms defined in dictionaries are further interpreted as having meanings consistent with those in the relevant technical literature and the present disclosure, and are not to be interpreted as having ideal or strictly formal meanings unless defined otherwise.
S, which is more cost-effective than microalloying elements, may be added to the medium-manganese steel according to the present invention to form inclusions that act as irreversible hydrogen trap sites, thereby improving hydrogen embrittlement resistance.
The temperature at which inclusions are formed in the medium-manganese steel is about 1,450° C. Accordingly, the inclusions are formed in the melt, and there is no significant change in the size, shape, or distribution of the inclusions depending on the austenitizing temperature range, which will be described later.
Therefore, although the present invention uses medium-manganese steel, it is technically characterized in that it is applicable to not only to the austenitizing temperature range of the warm-stamping process, but also to the austenitizing temperature range of the hot-stamping process.
In the present specification, the contents of the components will be described using weight percent (wt %).
The present invention relates to an ultra-high-strength medium-manganese steel for warm stamping having excellent hydrogen embrittlement resistance, and specifically, to a high-strength medium-manganese steel having high hydrogen embrittlement resistance as a result of forming MnS inclusions, TiN—MnS complex inclusions, or Ti(C,N)—MnS complex inclusions, which are irreversible hydrogen trap sites, and a method of producing the same.
In the present invention, very small amounts of S and Ti were added to medium-manganese steel, which contains more manganese than ordinary carbon steel, to form MnS inclusions, TiN—MnS complex inclusions, or Ti(C,N)—MnS complex inclusions that act as irreversible hydrogen trap sites, thereby improving the hydrogen embrittlement resistance of the medium-manganese steel.
The medium-manganese steel for warm stamping according to the present invention may have a tensile strength of 1,800 MPa or more and an elongation of 7.0% or more. In addition, as shown in Table 2 below, invention steels according to some embodiments may have a tensile strength of 2,000 MPa or more.
Hereinafter, the present invention will be described with reference to the accompanying drawings. For reference, the drawings may be somewhat exaggerated in order to describe the features of the present invention. In this case, it is preferable to perform interpretation in light of the overall purpose of the present specification.
The design of the medium-manganese steel will be described below.
The present invention provides a high-strength medium-manganese steel with excellent hydrogen embrittlement resistance, which contains 3 to 5 wt % manganese (Mn), 0.1 to 0.3 wt % carbon (C), 0.2 to 0.7 wt % silicon (Si), 0.01 to 0.07 wt % sulfur(S), and 0.005 to 0.06 wt % titanium (Ti), with the remainder being iron (Fe) and inevitable impurities.
The high-strength medium-manganese steel of the present invention may consist of martensite and 3 to 5 vol % retained austenite at room temperature.
Hereinafter, the numerical range for the content of each component in the above-described steel and the reasons for numerical limitation will be described.
When the Mn content is 3 to 5 wt %, the stability of austenite at high temperatures may be improved and ferrite transformation during cooling may be suppressed, making it possible to obtain the martensite microstructure at room temperature even when cooled in air. If the Mn content is less than 3 wt %, the stability of austenite may decrease, and thus ferrite may be generated during cooling after hot rolling, reducing the martensite fraction at room temperature and increasing the ferrite fraction. In contrast, if the Mn content is more than 5 wt %, not only may the raw material cost and production cost increase, but weldability may also deteriorate. Accordingly, in the present invention, the Mn content is preferably limited to 3 to 5 wt %.
When the carbon (C) content is 0.1 to 0.3 wt %, the stability of austenite at high temperatures may be ensured and the strength of martensite at room temperature may be improved. If the C content is less than 0.1 wt %, the stability of austenite may decrease and ferrite may be generated during cooling, and the dissolved carbon (C) content in martensite may decrease, lowering the martensite strength. In contrast, if the C content is more than 0.3 wt %, cold rolling properties may be lowered and weldability may be reduced. Accordingly, in the present invention, the C content is preferably limited to 0.1 to 0.3 wt %.
Silicon (Si) has the effect of increasing strength by solid solution strengthening in the martensite base. If the Si content is less than 0.2 wt %, the effect of increasing strength is not significant. In contrast, if the Si content is more than 0.7 wt %, problems may occur in terms of plating or welding. Accordingly, in the present invention, the Si content is preferably limited to 0.2 to 0.7 wt %.
Sulfur(S) is generally considered an impurity element. However, in the present invention, S is an essential component for forming MnS inclusions, and is preferably contained in an amount of 0.01 to 0.07 wt %. If the S content is less than 0.01 wt %, a disadvantage may arise in that the fraction of MnS inclusions is excessively low. In contrast, if the S content is more than 0.07 wt %, S is likely to segregate at grain boundaries, causing embrittlement of the steel, and coarse inclusions are formed, reducing hydrogen embrittlement resistance. Accordingly, in the present invention, the S content is preferably limited to 0.01 to 0.07 wt %.
Ti combines with N and/or C in the steel to form TiN or Ti(C,N), and these Ti particles act as nucleation sites for MnS inclusions to form fine TiN—MnS or Ti(C,N)—MnS complex inclusions. If the Ti content is less than 0.005 wt %, a problem arises in that the formation of the complex inclusions becomes difficult. In contrast, if the Ti content is more than 0.06 wt %, the mechanical properties of the steel may be impaired due to coarsening of TiN. Accordingly, in the present invention, the Ti content is preferably limited to 0.005 to 0.06 wt %.
The medium-manganese steel according to the present invention may have MnS inclusions and TiN—MnS complex inclusions, or have MnS inclusions and Ti(C,N)—MnS complex inclusions.
In this case, nitrogen (N) is an inevitable impurity.
In the present invention, the density of the inclusions is preferably 90 to 4,000 ea/mm2. If the density of the inclusions is less than 90 ea/mm2, the irreversible hydrogen trap sites may be reduced and the hydrogen embrittlement resistance cannot be improved. If the density of the inclusions is more than 4,000 ea/mm2, a large amount of Mn that forms inclusions may be consumed, and the dissolved Mn content in the matrix may decrease, causing a decrease in strength.
In the present invention, the aspect ratio (the ratio between the transversal width and the longitudinal length) of each inclusion is preferably 1 to 35. Since the invention steels according to the present invention are rolled, inclusions with an aspect ratio of less than 1 cannot be formed. In contrast, if the aspect ratio is more than 35, the hydrogen embrittlement resistance may be adversely affected due to elongated inclusions.
In the present invention, each inclusion preferably has a width of 0.3 to 3.0 μm and a length of 0.5 to 31.0 μm.
Hereinafter, the method of producing a medium-manganese steel according to the present invention will be described. Since the composition of the medium-manganese steel is described in the foregoing description, the following description will be given with a focus on the differences.
The method of producing a high-strength medium-manganese steel with excellent hydrogen embrittlement resistance according to the present invention includes steps of:
The homogenizing in step (S2) may be performed at 1,200 to 1,300° C.
If the homogenizing is performed at a temperature lower than 1,200° C., sufficient homogenization may not be ensured. If the homogenizing is performed at a temperature higher than 1,300° C., a problem may arise in that delta (8) ferrite, which is an unwanted microstructure, is formed.
Meanwhile, the homogenizing time may vary depending on the size of the ingot. However, as an example, if the homogenizing time is more than 6 hours, decarburization may occur and the degree of unnecessary oxidation may increase, and if the homogenizing time is less than 6 hours, sufficient homogenization may not be ensured.
The hot rolling in step (S3) may be performed at 900 to 1, 100° C., and the hot-rolled steel may be cooled by air cooling, etc. Below 900° C., there is a problem in that rolling is difficult because the temperature is not high enough. If the hot rolling is performed above 1, 300° C., a problem may arise in that delta (δ) ferrite, which is an unwanted microstructure, is formed.
The cold rolling in stem (S4) may be performed at a reduction ratio of 45 to 55% at room temperature. The cold rolling according to the present invention is not particularly limited, may be carried out according to a conventional method, and may be performed in the reduction ratio range of 45 to 55%.
The annealing in step (S5) may be performed at 750° C. for 3 minutes.
The austenitizing in step S6 may be performed at an austenite single-phase region temperature A3 or higher.
In the present invention, if the austenitizing temperature is lower than 750° C., problems arise in that the martensite microstructure is not sufficiently ensured and a ferrite microstructure is partially formed, making it difficult to ensure a tensile strength of 1,800 MPa or more. In addition, if the austenitizing temperature is higher than 1,000° C., a problem arises in that the crystal grains become coarse, making it difficult to ensure a tensile strength of 1,800 MPa or more. Accordingly, the austenitizing temperature according to the present invention is preferably 750 to 1,000° C.
More specifically, in a first embodiment, the austenitizing temperature is preferably 750° C. to lower than 850° C. In the first embodiment, the austenitizing temperature is more preferably 750° C. Additionally, in a second embodiment, the austenitizing is preferably performed at a temperature of 850 to 950° C. The austenitizing temperature is more preferably 850° C. The austenitizing time may be 5 minutes.
The austenitizing temperature in the first embodiment is in the austenitizing temperature range that is used in a general warm stamping process. The austenitizing temperature in the second embodiment is in the austenitizing temperature range that is used in the hot stamping process.
The present invention may generally be classified as a warm stamping process because it used medium-manganese, but it is characterized by being applicable even in the austenitizing temperature range that is used in the hot stamping process.
In the austenitizing temperature according to the present invention, as shown in Table 2 below, at lower than 850° C., the initial austenite grain size was small, and thus it was possible to ensure a tensile strength of 2,000 MPa or more after cooling. Meanwhile, at 850° C. or higher, the grains became coarse and the tensile strength decreased, but at an austenitizing temperature of up to 1,000° C., it was possible to obtain a tensile strength of 1,800 to 1,900 MPa. Since this tensile strength is equivalent to or higher than the tensile strength of hot-stamped 30MnB5 steel (˜1,800 MPa), the steel of the present invention is believed to be sufficiently applicable to high-strength parts.
The produced medium-manganese steel may consist of martensite and 3 to 5 vol % retained austenite at room temperature.
The produced medium-manganese steel may have MnS inclusions and TiN—MnS composite inclusions, or have MnS inclusions and Ti(C,N)—MnS composite inclusions.
The density of the inclusions may be 90 to 4,000 ea/mm2.
The aspect ratio (length/width) of each inclusion may be 1 to 35. Each of the inclusions may have a width of 0.3 to 3.0 μm and a length of 0.5 to 31.0 μm.
Hereinafter, the present invention will be described in more detail by way of examples.
Cast ingots having the compositions shown in Table 1 below were kept at 1, 200° C. for 6 hours and then air-cooled. Thereafter, the ingots were hot-rolled in the temperature range of 1,100 to 900° C. and air-cooled. Next, the hot-rolled steel sheets were cold-rolled at a reduction ratio of about 50% at room temperature. The cold-rolled steel sheets were annealed at about 750° C. for 3 minutes and air-cooled. The annealed steel sheets were austenitized for 3 minutes at about 750° C., which is above the austenite single phase region A3 temperature, and air-cooled.
The microstructures of the austenitized steel sheets are shown in
In addition,
In Comparative Steel 2 in Table 1, coarse TiN was formed alone. TiN has a square shape. In this square-shaped TiN, a high-stress concentration area is formed and acts as a starting point for cracks, and thus the hydrogen embrittlement resistance of Comparative Steel 2 is low.
In contrast, Invention Steel 3 in Table 1 had a higher hydrogen embrittlement resistance than Comparative Steel 2 because the size of the TiN—MnS complex inclusions was small and TiN rarely existed alone.
The area fraction of inclusions increased in the order of Comparative Steel 2, Comparative Steel 1, Invention Steel 1, Invention Steel 2, and Invention Steel 3. In addition, all the invention steels had a high density of inclusions with a size of less than 1 μm, and in particular, Invention Steel 2 and Invention Steel 3 had the highest inclusion density. In addition, when the width and length of the inclusions are “a” and “b,” respectively, all the invention steels have a high aspect ratio value of 8 or more, indicating that the inclusions are elongated inclusions.
The width of the inclusions was almost constant regardless of the addition of S and Ti. However, it was found that the length of the inclusions increased as the S content increased and decreased as the Ti content increased.
In order to check martensite microstructure formation and a tensile strength of 1,800 MPa or more depending on the austenitizing temperature, a tensile test (initial strain rate (ε)=1×10−4/s) was performed using Invention Steel 2 as an example. The results are shown in
An austenitizing temperature of 700° C. corresponds to a temperature below A3. Accordingly, as a microstructure of the specimen after austenitizing, a ferrite microstructure was partially formed, and for this reason, the specimen had low tensile strength. Meanwhile, it was found that, when the specimen was austenitized at 750° C. or 800° C., it had a high tensile strength of 2,000 MPa or more, and the tensile strength decreased as the austenitizing temperature increased. In particular, when the specimen was austenitized at 1,000° C., the tensile strength of the specimen was 1, 812 MPa. This suggests that if austenitizing is performed above 1,000° C., the grain size becomes coarse, making it difficult to ensure the tensile strength of the specimen over 1,800 MPa.
To evaluate hydrogen embrittlement resistance, a slow strain rate tensile test (initial strain rate (ε)=1×10−4/s) was performed using specimens without hydrogen charging and specimens with hydrogen charging, and the results are shown in in
In this case, elongation loss (%)=(elongation before hydrogen charging−elongation after hydrogen charging)/elongation before hydrogen charging)×100.
The initial strain rate (ε) was 1×10−4/s. Hydrogen charging was performed using an electrochemical method, and the solution used in hydrogen charging was a 3% NaOH aqueous solution containing 0.3 wt % NH4SCN. Hydrogen charging was performed with a current density of 50 A/m2 for 60 minutes.
Regardless of the addition of S and Ti, the invention steels without hydrogen charging showed a tensile strength of about 2,000 MPa and an elongation of about 7%. In contrast, the invention hydrogen charging had a low elongation, which meant that hydrogen embrittlement occurred.
Comparative Steel 1 and Comparative Steel 2, to which no S was added, showed a high elongation loss. However, the elongation losses of Invention Steel 1, Invention Steel 2, and Invention Steel 3, to which S was added, decreased, and in particular, Invention Steel 2 showed the lowest elongation loss. In other words, it can be seen that the hydrogen embrittlement resistance increases as the area fraction of inclusions and the density of fine inclusions increase.
Before hydrogen charging, the tensile strengths and elongations of Comparative Steel 1 and Invention Steel 3 are similar at first glance. However, what is important from the viewpoint of hydrogen embrittlement resistance is the elongation loss.
In other words, if the elongation before hydrogen charging is the same, the hydrogen embrittlement resistance can be considered high when the elongation after hydrogen charging is high.
Before hydrogen charging, the elongations of Comparative Steel 1 and Invention Steel 3 were similar, but after hydrogen charging, the elongations were different. In terms of elongation, Invention Steel 3 increased by only 0.5% compared to Comparative Steel 1, but in terms of elongation loss, Invention Steel 3 decreased by about 7%.
Although a difference in elongation loss of 1 to 2% may not be a significant difference, the difference in elongation loss of about 7% indicates that the hydrogen embrittlement resistance of Invention Steel 3 was improved.
The high-strength medium-manganese steel with excellent hydrogen embrittlement resistance and the method of producing the same according to the present invention have the following effects:
First, there is an effect of improving hydrogen embrittlement resistance by controlling the density and size of inclusions in steel by controlling the S and Ti contents without adding a new process.
Second, there is an effect of proposing an austenitizing temperature range that provides steels having a tensile strength of 2,000 MPa or more and an elongation of 7.0% or more without a decrease in strength and elongation even when S is added.
Third, there is an effect of improving hydrogen embrittlement resistance by the degree of hydrogen dispersion, because MnS inclusions, TiN—MnS complex inclusions, or Ti(C,N)—MnS complex inclusions in the steel act as irreversible hydrogen trap sites, and the density of inclusions with a size of less than 1 μm is high.
Effects of the present invention are not limited to those mentioned above, and other effects not mentioned above will be clearly understood by those skilled in the art from the following description.
The embodiments described in the present specification and the accompanying drawings are merely illustrative of some of the technical spirit included in the present invention. Therefore, the embodiments in disclosed the present specification are intended to describe the technical spirit of the present invention rather than limiting it, and thus it is obvious that the scope of the technical spirit of the present invention is not limited by these embodiments. Modifications and specific embodiments that can be easily inferred by those skilled in the art within the scope of the technical spirit included in the specification and drawings of the present invention should be construed as being included in the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0060807 | May 2023 | KR | national |
| 10-2024-0058396 | May 2024 | KR | national |
