JP2019163534A - Lightweight steel excellent in corrosion resistance and specific strength and method of producing the same - Google Patents

Abstract

To provide a chemical component of a lightweight steel with improved physical properties such as corrosion resistance, tensile strength and ductility, and a method of producing the lightweight steel.SOLUTION: The lightweight steel with improved corrosion resistance and specific strength is provided, specifically lightweight steel is provided that contains carbon (C) of 1.3 to 1.5 wt.%, aluminum (Al) of 8.5 to 10 wt.%, manganese (Mn) of 20 to 25 wt.%, chromium (Cr) of 4 to 5.1 wt.%,, molybdenum (Mo) of 2 to 4.16 wt.%, and balance iron (Fe) with inevitable impurities, the lightweight steel containing aluminum and manganese can be applied to various fields and the lightweight steel with improved service life can be provided by adding chromium and molybdenum to the light steel thereby increasing the corrosion resistance of the steel, and physical properties such as tensile strength, ductility, and specific yield strength of the lightweight steel can be improved by optimizing carbon content to solve the problem of mechanical property degradation caused by the addition of chromium and molybdenum.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lightweight steel with improved corrosion resistance and specific strength, specifically, carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn) 20 to The present invention relates to a lightweight steel containing 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum (Mo) 2 to 4.16 wt%, and the balance iron (Fe) and inevitable impurities.

  Recently, research on weight reduction of steel materials has been conducted as demands for strengthening safety standards and environmental regulations and fuel efficiency increase.

  Previously, in order to achieve weight reduction of steel products, a method of using a steel material having higher strength than before and reducing its thickness or reducing the size of a part was applied. However, since there is a limit to such weight reduction methods, in order to satisfy the demand for continuously increasing weight reduction, the strength is high and the specific gravity is low, that is, the specific strength (strength / specific gravity). Therefore, it is necessary to develop steel materials that are expensive.

  Due to these demands, the industry that mainly uses structural materials replaces existing steel materials with new materials such as titanium (Ti) and composite materials, but these materials are expensive to manufacture. The development of new materials that have the above-mentioned effects while ensuring economic efficiency has emerged.

  Therefore, a new method for adding aluminum (Al), which is an element capable of achieving weight reduction, to an existing steel material was introduced. In addition to reducing the specific gravity of about 1.5% per 1% of aluminum due to the substitution effect and lattice expansion effect of aluminum as a substitutional alloy element, the lightweight steel with a large amount of aluminum added by the effects of solid solution strengthening and precipitation strengthening It is known that the specific strength has a specific strength close to that of a titanium alloy. Therefore, recently, the industry has been actively researching lightweight steels mainly made of aluminum.

  Lightweight steel containing aluminum is classified into ferritic, duplex, and austenitic, and ferritic steel has a tensile strength of 400 to 600 MPa and a stretch rate of 20 to 30%, with limited tensile strength and stretch. Duplex and austenitic steels have excellent tensile strength and stretch ratio, but the existing lightweight steels mentioned above are extremely difficult to apply in various fields due to their extremely insufficient corrosion resistance. There is.

  In order to overcome these limitations, basic research on materials that can be applied to next-generation armor plates, transportation rails, automotive steel plates, etc. is currently underway due to national policy issues. Among them, the steel type containing the most alloying elements that can contribute to corrosion resistance is the Fe-20Mn-Al-C-5Cr series, and the PREN (pitching resistance equivalent number) value is as low as 5, which has a problem that sufficient corrosion resistance cannot be secured.

  In addition, alloying elements previously added to steel to impart corrosion resistance decrease the stability of the austenite structure, impair mechanical properties, or rather decrease the corrosion resistance, thereby reducing the stability of the austenite phase. It is necessary to develop a new steel alloy that can improve the corrosion resistance while maintaining it.

  In the present invention, in order to improve the corrosion resistance and specific strength of lightweight steel containing aluminum and manganese, chemical components of lightweight steel with improved physical properties such as corrosion resistance, tensile strength and ductility by adding chromium, molybdenum and carbon. And to provide a manufacturing method thereof.

  In order to solve the above problems, a lightweight steel according to an embodiment of the present invention includes carbon (C), aluminum (Al), manganese (Mn), chromium (Cr), molybdenum (Mo), and iron (Fe). And contains inevitable impurities.

  The lightweight steel is composed of carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn) 20 to 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum (Mo) 2 to 4.16 wt%, and the balance iron (Fe) and inevitable impurities can be included.

  The lightweight steel may further include 0.55 wt% or less of vanadium (V), or may further include 5.1 wt% or less of cobalt (Co).

The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into the formula (1) is 97 or more, the value when substituted into the formula (2) is 10.6 or more, It is preferable that the value when substituted into the mathematical formula (3) is −0.1 or more.
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1)
Cr + 3.3 (Mo + 0.5 (Si + W)) + 16N Formula (2)
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)

  Moreover, it is preferable that the lightweight steel has an elongation ratio of 50% or more.

  The lightweight steel is obtained by a rolling step, and the rolling step is performed at a temperature range of 1000 to 1200 ° C. for 1 to 3 hours.

  The lightweight steel is obtained by a homogenization heat treatment step performed after the rolling step, and the homogenization heat treatment step is preferably performed in a temperature range of 1100 to 1200 ° C. for 1 to 3 hours.

  The lightweight steel is obtained by an aging heat treatment step that is additionally performed after the homogenization heat treatment step, and the aging heat treatment step has a value of 15.7 to 16.1 in HP (Holomon Jaffa parameter). It is preferably carried out at a temperature range of 500 to 550 ° C. for 2 to 6 hours.

  On the other hand, in another embodiment of the present invention, a method for producing such a lightweight steel can be mentioned, but carbon (C), aluminum (Al), manganese (Mn), chromium (Cr), molybdenum (Mo), iron It includes a rolling step for heating and forming a steel ingot containing (Fe) and inevitable impurities, and a homogenization heat treatment step.

  The steel ingot used for the rolling step is carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn) 20 to 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum (Mo) 2 to 4.16 wt%, and the balance iron (Fe) and inevitable impurities.

  The rolling step is performed in a temperature range of 1000 to 1200 ° C. for 1 to 3 hours, the homogenizing heat treatment step is performed in a temperature range of 1100 to 1200 ° C. for 1 to 3 hours, and after the homogenizing heat treatment step, An aging heat treatment step may additionally be included. At this time, the aging heat treatment step is preferably performed under conditions of a heat treatment temperature and time in which HP (Holomon Jaffle parameter) has a value of 15.7 to 16.1, specifically, 500 to 550 ° C. It is preferable to be performed in the temperature range for 2 to 6 hours.

  The steel ingot may additionally contain vanadium (V) in the range of 0.55 wt% or less, and may further contain cobalt (Co) in the range of 5.1 wt% or less.

The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into the formula (1) is 97 or more, the value when substituted into the formula (2) is 10.6 or more, It is preferable that the value when substituted into the mathematical formula (3) is −0.1 or more.
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1)
Cr + 3.3 (Mo + 0.5 (Si + W)) + 16N Formula (2)
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)

  The present invention provides a lightweight steel having an improved life while being applicable to various fields by increasing the corrosion resistance of the steel by adding chromium and molybdenum to the lightweight steel containing aluminum and manganese. Can do.

  In addition, in order to solve the problem of deterioration of mechanical properties that can be induced by the addition of chromium and molybdenum, it is possible to improve physical properties such as tensile strength, ductility and specific yield strength by optimizing the carbon content. it can.

It is a graph which shows the specific yield strength with respect to the pitting corrosion potential of the lightweight steel of an Example and a comparative example. It is a graph which shows the specific yield strength according to the draw ratio of the lightweight steel of an Example and a comparative example. It is a graph which shows the specific yield strength and extending | stretching rate according to carbon content. It is a graph which shows the value of the specific yield strength x extending | stretching rate according to carbon content. It is a graph which shows the influence of carbon, nickel, molybdenum, and chromium on an austenite fraction. It is a graph which shows the relationship between the austenite fraction calculated by Numerical formula (1), and the austenite fraction measured by the experiment example. It is a graph which shows the reaction surface analysis result with respect to the influence which austenite fraction and PREN exert on a pitting corrosion potential. It is a graph which shows the reaction surface analysis result with respect to the influence which carbon content and PREN exert on an austenite fraction. It is a graph which shows the influence which austenite fraction and carbon content have on pitting corrosion resistance. It is a graph which shows the influence of carbon, molybdenum, and chromium on pitting resistance. It is a graph which shows the relationship between the pitting corrosion calculated by Numerical formula (3), and the pitting corrosion resistance measured by the experiment example. It is the optical photograph which image | photographed the fine structure of the test piece of Example 3-Example 6. FIG. It is a graph which shows the change of the tensile strength according to process time, and a draw ratio when an aging treatment temperature is 500 degreeC. It is a graph which shows the specific yield strength and extending | stretching rate according to an austenite fraction.

  Before describing in detail through the preferred embodiments of the present invention, the terms and words used in the present specification and claims should not be construed to be limited to ordinary or lexicographic meanings. It will be clarified that it must be interpreted in the meaning and concept consistent with the technical idea of the present invention.

  Throughout this specification, when a part “includes” a component, this means that the component may further include other components, unless otherwise stated, unless otherwise stated. means.

  Throughout this specification, “%” used to indicate the concentration of a specific substance is (weight / weight)% for solid / solid, (weight / volume)% for solid / liquid, unless otherwise stated, Liquid / liquid means (volume / volume)%.

  Hereinafter, the lightweight steel with improved corrosion resistance and specific strength according to the present invention and the production method thereof will be described in more detail.

  One embodiment of the present invention relates to a lightweight steel with improved corrosion resistance and specific strength. Specifically, carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese ( The present invention relates to a lightweight steel containing 20 to 25 wt% Mn), 4 to 5.1 wt% chromium (Cr), 2 to 4.16 wt% molybdenum (Mo), and the balance iron (Fe) and inevitable impurities.

  The lightweight steel may further include 0.55 wt% or less of vanadium (V) with respect to the total weight, or may further include 5.1 wt% or less of cobalt (Co).

  Hereinafter, the elements constituting the lightweight steel according to the present invention and the composition range will be described more specifically.

Carbon (C)
Carbon is an interstitial solid solution element in steel and is essential for adjusting the strength of steel and plays a role in stabilizing austenite. In this alloy system, carbon is contained in an amount of 1.3 to 1.5 wt% with respect to the total weight.

  When carbon is contained in less than the above weight range, the stabilization effect of austenite by carbon is slight, and it cannot offset the strength reduction of the steel due to the addition of chromium and molybdenum. The specific yield strength is increased by carbon, but there is a problem that the ductility is rapidly decreased and the workability of the steel is lowered. Therefore, it is preferably included within the above weight range.

Aluminum (Al)
Aluminum is an element that has a ferrite stabilizing effect, has a high solid solubility and causes solid solution strengthening, and enables weight reduction of the alloy by an aluminum replacement effect and a lattice expansion effect in the alloy.

  It is preferable to be contained in an amount of 8.5 to 10 wt% based on the total weight. Since the austenite fraction can be lowered to lower the strength of the steel, it is preferably contained within the above weight range.

Manganese (Mn)
An element that causes solid solution strengthening and stabilizes austenite, and is useful for the formation of austenite during the heat treatment process.

  Although it is contained in an amount of 20 to 24 wt% based on the total weight, if it is contained in less than the above weight range, the strength of the steel may be lowered, and there is a problem that sufficient austenite transformation is difficult. Moreover, when it contains exceeding the said weight, since a beta-Mn phase is formed and a mechanical physical property falls, it is preferable to contain within the said weight range.

Chrome (Cr)
Chromium is an element added to improve the corrosion resistance of steel, but it lowers the stability of the austenite phase due to the ferrite stabilizing effect of chromium, and improves the mechanical properties of the alloy by the properties of forming brittle carbides. There is a feature to lower.

  It is preferable to be included in an amount of 4 to 5.1% by weight with respect to the total weight, and when included in less than the above weight range, it is difficult to obtain an effect of improving corrosion resistance due to chromium, The mechanical properties of the alloy are drastically decreased, and this is not solved even by increasing the carbon content. Therefore, it is preferably included within the above weight range.

Molybdenum (Mo)
Molybdenum, like chromium, is added to improve the corrosion resistance of steel, but has the characteristics of reducing the stability of the austenite phase, forming brittle carbides, and reducing the mechanical properties of the steel.

  Molybdenum is preferably contained in an amount of 2 to 4.16 wt% with respect to the total weight. When it is contained in an amount less than the above weight range, it is difficult to obtain an effect of improving the corrosion resistance by molybdenum. It is preferable that it is contained within the above-mentioned weight range, since the mechanical properties of the abruptly decrease.

Vanadium (V)
Vanadium is an element that has the effect of being replaced by iron to increase tensile strength, forming insoluble carbides to increase high-temperature hardness, and refining crystal grains, and is further included in the alloy system of the present invention. is there.

  When it is contained in an amount of 0 to 0.55 wt% with respect to the total weight and exceeds the above weight range, the effect of increasing the tensile strength and hardness due to the excess vanadium content is slight, and rather forms crystallized carbide. Therefore, it is preferable to be included within the above weight range.

Cobalt (Co)
Cobalt is an element that dissolves in a base to give a base strengthening effect by solid solution strengthening, improves corrosion resistance, and helps the stability of the austenite phase, and may be further included in the alloy system of the present invention. .

  When cobalt is added in an amount of 0 to 5.1% by weight based on the total weight, the above effect can be obtained, and when it is contained in a composition exceeding the above weight range, the above effect cannot be obtained or it depends on the excess amount. Since it is difficult to expect an additional effect, it is preferably included within the above weight range.

  The lightweight steel of the present invention, except for the above-mentioned components, is substantially composed of iron (Fe), which means that it contains other trace elements including inevitable impurities unless the effects of the present invention are hindered. Is also included in the scope of the present invention.

The lightweight steel satisfies the following formula (1) indicating the austenite phase fraction. This formula is derived in order to investigate the influence on the austenite fraction of each alloy element by the reaction surface method using the test pieces of Examples and Comparative Examples in the experimental examples described later.
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1)

  When the value of the formula (1) satisfies 97 or more, that is, when the austenite phase fraction of the alloy is high, not only mechanical properties such as tensile properties and impact toughness of the alloy but also corrosion properties are excellent. Therefore, a part to which a material satisfying this requirement is applied can be expected to exhibit excellent performance. For example, when applied to power generation parts, power generation efficiency can be improved, and when applied to automobile steel materials and parts, fuel consumption and stability against accidents can be improved.

  In addition, in the case of the lightweight steel according to the present invention, unlike general steel, the addition of nickel causes a decrease in tensile properties, but this reduces the austenite phase fraction due to nickel. Such features also appear in equation (1). Not only this, nickel induces the formation of ferrite and B2 phases, and the resulting difference in corrosion potential in each phase results in a decrease in corrosion properties.

  That is, when nickel is added to the alloy system of the present invention, the tensile properties, impact toughness, and corrosion properties of the alloy are lowered, so it is preferable that the lightweight steel of the present invention contain as little nickel as possible. Accordingly, the production of the composition containing the smallest possible amount as in the case where it is unavoidably included in the impurity element level can prevent the decrease in tensile properties, impact toughness and corrosion properties.

  Specifically, it is preferable that nickel is contained at 0.1 wt% or less within a range satisfying the mathematical formula (1), and the stability of the austenite phase is ensured to the maximum by adjusting the content of other elements. Although it may be contained up to 3.4 wt%, it is most preferable that it is not contained as much as described above.

On the other hand, the lightweight steel has a PREN (pitching resistance equivalent number) value of the pitting resistance index represented by the following formula (2) of 10.6 or more, and the value substituted into the formula (2) is the total alloy weight. In the case of an alloy having a high value obtained by the formula (2), it is excellent in corrosion resistance and can be applied to various fields.
Cr + 3.3 (Mo + 0.5 (Si + W)) + 16N Formula (2)

Further, the pitting corrosion resistance value obtained by substituting the composition ratio of the elements constituting the lightweight steel into the following mathematical formula (3) may be −0.1 or more, and the alloy system of the present invention forms an alloy. When the value obtained by substituting the composition of each element into the formula (3) is -0.1, it has improved corrosion resistance along with excellent tensile properties, so the value obtained by the formula (3) is -0.1 or more Is preferred.
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)

  In the alloy system of the present invention, by optimizing the content of carbon, molybdenum and chromium, excellent tensile properties and corrosion resistance properties are imparted, and the number of all cases of the composition within such a content range is expressed by the above formula. Since the minimum value when substituting for (3) is −0.1V, it is most preferable that the value of Equation (3) for satisfying all the two characteristics is −0.1V or more.

  The lightweight steel may have an elongation ratio of 50% or more. The stretch ratio is the most important physical property of the product when the material is molded into a specific shape. The stretch ratio must be 50% or more in order to form various shapes.

  The lightweight steel is obtained by a rolling step performed for 1 to 3 hours in a temperature range of 1000 to 1200 ° C and a homogenization heat treatment step performed for 1 to 3 hours in a temperature range of 1100 to 1200 ° C. There may be.

  The present invention can provide a lightweight steel having the above composition and excellent in toughness, ductility, strength and corrosion resistance, and in particular, by including chromium and molybdenum in the weight range in the lightweight steel, By imparting corrosion resistance, which is a characteristic that existing lightweight steels could not have, and by optimizing the carbon content as described above, it is possible to prevent strength reduction of lightweight steels due to the addition of chromium and molybdenum. .

  On the other hand, another embodiment of the present invention relates to a method for producing lightweight steel, specifically, carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn ) Rolling a steel ingot containing 20 to 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum (Mo) 2 to 4.16 wt%, and the balance iron (Fe) and inevitable impurities, and a homogenization heat treatment Aging heat treatment may be additionally performed after the homogenization heat treatment step.

  Since the composition and content of the elements forming the steel ingot used in the manufacturing method and the effect obtained by limiting the same are the same as those in the above-described embodiment, description thereof will be omitted.

  First, the steel ingot is made of an artificial heat source, such as an electric furnace, a vacuum induction melting furnace, or an atmospheric induction furnace, after melting metal, A vacuum induction melting furnace is preferably used in order to obtain a product of uniform quality and prevent oxidation generated during atmospheric induction melting at this time by preparing a gas such as nitrogen.

  The steel ingot thus prepared is hot-rolled and formed into a plate, a rod, a tube, a mold, etc., and the hot-rolling is performed in a temperature range of 1000 to 1200 ° C., and the soaking temperature is the same as this. It is. When hot rolling is performed under process conditions below such a temperature range, the temperature interval to the finish rolling temperature is narrow and sufficient rolling to a predetermined thickness is impossible, and hot rolling is performed at the above temperature or time range. When it is carried out under process conditions exceeding 1, high temperature brittleness may occur, and thus it is preferably carried out in the above temperature range.

  In addition, hot rolling has a soaking time of 1.5 to 2.5 hours, but if the soaking time is less than 1.5 hours, crystal grain growth is insufficient, and 2.5 hours is required. If it exceeds, the crystal grains grow excessively, and therefore it is preferable that the hot rolling step is performed so as to satisfy the condition of the soaking time.

  The finish rolling temperature in such a hot rolling process is preferably 900 ° C. or higher. However, when the finishing process is performed at 900 ° C. or lower, mixing of the ferrite structure is promoted and workability is improved. This is because the rolling load is increased, the rolling mill is forced, and the quality inside the steel sheet is adversely affected.

  After the hot rolling is completed, a step of cooling at a cooling rate of 0.5 ° C./s or more is performed. At this time, when performed at a rate lower than the cooling rate, κ-carbide and B2 phase are formed. May induce a decrease in toughness.

  Thereafter, a homogenization heat treatment step is performed. The heterogeneous structure generated during the hot working by the homogenization heat treatment step can be reduced, and the alloy element can be completely dissolved in the austenite base.

  The homogenization heat treatment step is performed in a temperature range of 1050 to 1200 ° C., but when it is performed at a temperature of 1100 ° C. or higher, it is possible to prevent a decrease in toughness due to the formation of a ferrite phase. Done in Such homogenization heat treatment is preferably performed for 30 minutes to 3 hours.

An aging heat treatment is additionally performed after the homogenization heat treatment step, and the aging heat treatment is performed at a heat treatment temperature and time at which HP (Holomon Jaffle parameter) represented by Equation (4) has a value of 15.7 to 16.1. When the heat treatment is performed under the conditions of temperature and time that satisfy this condition, the final specific yield strength of the steel material is improved to 140 MPa · cm ³ / g or more, and at the same time, the draw ratio is 20% or more. Can be secured.
HP = T (20 + log (t)) / 1000 Formula (4)

  When heat treatment is performed under conditions of temperature and time in a range where the HP value exceeds the above range, sufficient strength improvement may not be performed. When the heat treatment is performed under conditions exceeding the above range, rather than by overaging. Since the strength is lowered, it is preferably performed within the range of the process conditions. At this time, a specific temperature condition that obtains a preferable physical property improvement effect by aging treatment and the HP value satisfies the above conditions may be 500 to 550 ° C., and the time condition may be 2 to 6 hours.

  Hereinafter, the light weight steel which concerns on one Embodiment of this invention is manufactured, and it tries to demonstrate the concrete effect | action and effect of this invention by this. However, this is presented as a preferred example of the present invention, and the scope of the present invention is not limited by the examples.

[Production example]
Using a vacuum induction melting furnace, it has the composition shown in Table 1 below, and the balance is cast from a steel ingot consisting essentially of iron (Fe), heated to 1150 ° C. and heat-treated for 2 hours. It was cooled after being rolled. Furthermore, it heated up at 1150 degreeC, and it was made to cool after performing the homogenization heat processing for 2 hours, and the lightweight steel test piece of Examples 1-6 and Comparative Examples 1 and 2 was prepared.

  Then, except that it processed by setting homogenization heat processing temperature to 1050 degreeC, the remainder was performed like the method mentioned above, and the comparative examples 3-23 were prepared.

[Experimental example]
Maximum tensile strength (UTS), yield stress (YS), elongation (elongation), Charpy V-notch impact test (CVN), density, corrosion potential, and pitting corrosion potential of the test piece prepared in the above manufacturing example. The values obtained by examining the (pitting potential), the specific yield strength (SYS), and the austenite fraction are shown in Table 2 below.

  In the case of Comparative Examples 4, 6, 8 and 10, it is estimated that cracks occur in the tensile test specimen and a value close to 0 is obtained even if the tensile test is performed. Therefore, a separate tensile test is performed. There wasn't.

Comparative Analysis of Specific Yield Strength, Pitting Potential and Stretch Rate As is clear from the results in Table 2, all specimens have a density of around 6.7 g / cm ³ , which is 7.87 g / cm which is the density of general steel products. It is found that the test piece of the example which is about 15% lower than ³ and light in weight and satisfies all of the composition and manufacturing method of the present invention has a tensile strength, a yield stress, a stretch ratio, a pitting corrosion potential, a specific yield strength, etc. In the case of a comparative example that does not satisfy one or more of the composition and production method of the present invention, all of the items have excellent values, but it can be confirmed that one or more physical properties are significantly inferior to those of the examples. .

  In order to more easily confirm the physical properties of Examples and Comparative Examples, the specific yield strength with respect to the pitting corrosion potential of each test piece is shown in FIG. 1, and the specific yield strength according to ductility is shown in FIG. The physical properties of the lightweight steel of the comparative example were compared. Referring to FIG. 1, in the example, the pitting corrosion potential indicating the corrosion resistance and the specific yield strength indicating the strength both have high values, whereas in the comparative example, one or more of the two physical properties are low values. It can be seen that the lightweight steel of the present invention satisfies both the improved corrosion resistance and excellent strength properties. Moreover, when FIG. 2 is seen, while both the ductility and the specific yield strength are excellent in the examples, the comparative example has a specific yield strength when the ductility is very inferior or the ductility is excellent. We can confirm that it is inferior.

Comparative analysis of strength and stretch ratio according to carbon content The specific yield strength and stretch ratio according to carbon content are shown in Fig. 3, and in order to comprehensively judge the stretch ratio and specific yield strength, The corresponding specific yield strength × stretch ratio is shown in FIG.

FIG. 3 reveals that the specific yield strength increases as the carbon content increases, and in particular, has a high value of 100 MPa / g / cm ³ since 1.3 wt% or more. In this case, an excellent value is shown when the carbon content is 1.1 to 1.5 wt%. However, when the carbon content exceeds 1.5 wt%, it becomes clear that the ductility rapidly decreases. Also in FIG. 4, it becomes clear that when the carbon content is 1.1 to 1.5 wt%, it has an excellent specific yield strength × stretch ratio value.

Therefore, when the carbon content of steel is 1.3 to 1.5 wt%, which is the range of the present invention, it can be confirmed that the steel has a high specific yield strength of 100 MPa / g / cm ³ and excellent ductility.

Influence of alloying elements on austenite fraction In order to investigate the influence of carbon, molybdenum and chromium on austenite fraction, the average value of austenite fraction with respect to the content of each element is shown in FIG. 5 and shown in FIG. Thus, as the contents of nickel, molybdenum and chromium increase, especially when nickel and molybdenum are about 4 wt% or more, or when chromium is 5.5 wt% or more, the austenite fraction decreases rapidly. In the case of carbon, it can be confirmed that the austenite fraction increases proportionally as the carbon content increases until the carbon content reaches about 1.5 wt%.

  After that, in order to confirm the influence of the austenite fraction on the tensile properties, the energy absorbed until the steel material was damaged under tensile stress as a typical impact tensile property of steel was standardized by specific gravity (specific yield strength). + Specific tensile strength) / (2 × stretch ratio) values were obtained and shown in Table 3. As the austenite fraction increases, it can be confirmed that the characteristics increase proportionally. At this time, among the values shown in Table 3, SUTS means specific tensile strength (Specific Tensile Strength).

  From the results in Table 3, it can be confirmed that the physical properties generally have a high value when the austenite fraction is high, and the same is true in FIG. 14 showing the change in specific yield strength and stretch ratio according to the austenite fraction. You can see the result.

  Therefore, in order for Fe-Al-Mn-Cr-Mo-C-based lightweight steel to have excellent tensile properties, it is preferable that the austenite fraction is high, and the austenite fraction affects the contents of carbon, molybdenum and chromium. Therefore, it is necessary to optimize the molybdenum and chromium contents while increasing the carbon content.

Therefore, using the test pieces of Examples and Comparative Examples, a functional equation for the austenite fraction corresponding to the content of each alloy element was derived by the reaction surface method to obtain Equation (1).
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1)

  FIG. 6 shows the relationship between the austenite fraction obtained by substituting the compositions of Examples and Comparative Examples into the formula (1) and the austenite fraction measured previously, and is calculated by the formula (1). Since it can be confirmed that the austenite fraction and the actually measured austenite fraction are directly proportional to each other, it can be seen that the austenite fraction can be predicted by Equation (1).

  In the alloy system of the present invention, by optimizing the content of carbon, molybdenum and chromium, excellent tensile properties and corrosion resistance properties are imparted, and the number of all cases of the composition within such a content range is expressed by the above formula. Since the minimum value when substituted for (1) is 97%, the value of the formula (1) for satisfying all the two characteristics is most preferably 97% or more.

  At this time, even if the value of the mathematical formula (1) is 97% or more, if the content of each element forming the alloy exceeds the range of the present invention, it will appear in Comparative Example 1 that the tensile properties may be lowered. It is preferable to set the content of each element together with Equation (1) within the scope of the present invention.

Corrosion resistance analysis according to austenite fraction and PREN value (Formula (2)) In order to investigate local corrosion resistance according to austenite fraction and PREN value, the results of analysis by the reaction surface method using the results of Table 2 Each is shown in FIG. FIG. 7 shows that the pitting corrosion resistance increases as the austenite fraction and the PREN value increase. In particular, when the austenite fraction is high and the PREN value is 10.6 or more, excellent pores are obtained. It has a corrosion resistance value, and it can be seen that the corrosion resistance of the alloy is significantly improved.

  The object of the present invention is to increase the austenite fraction to improve tensile properties and at the same time impart corrosion resistance properties. Therefore, the PREN value preferably has a value of 10.6 or more.

  On the other hand, when FIG. 8 is seen, it can be confirmed that the austenite fraction increases as the carbon content increases and the PREN value decreases. Addition of chromium and molybdenum to increase corrosion resistance. When the PREN value increases, the austenite fraction decreases and the tensile properties deteriorate. By increasing the carbon content, the tensile properties deteriorate. 8 can be confirmed through FIG. 8, and it can be seen that such an effect is conspicuous when the carbon content is 1.3 wt% or more.

  Moreover, when FIG. 9 is seen, it is an austenitic steel material with an austenite fraction represented by Formula (1) of 97%, and at the same time, when the carbon content range is within the range of the present invention, excellent corrosion resistance is obtained. Can be confirmed.

Effect of Alloying Elements on Pitting Resistance Looking at FIG. 10 showing the effects of carbon, chromium and molybdenum on the pitting resistance of alloys, molybdenum is the element that has the greatest effect on pitting resistance. It has been found that when the amount is 2 to 4.16 wt%, it has the highest pitting corrosion resistance and imparts the best corrosion resistance.

Moreover, as a result of analyzing the relationship between carbon, chromium and molybdenum and pitting resistance by the reaction surface method using the test piece prepared in advance, Formula (3) was derived. FIG. 11 shows the relationship between the measured value of pitting resistance of each test piece and the calculated value of pitting resistance calculated using Equation (3). Since the measured value and the calculated value are proportional, Pitting corrosion resistance can be predicted by (3).
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)

  In the alloy system of the present invention, by optimizing the content of carbon, molybdenum and chromium, excellent tensile properties and corrosion resistance properties are imparted, and the number of all cases of the composition within such a content range is expressed by the above formula. Since the minimum value when substituting for (3) is −0.1V, it is most preferable that the value of Equation (3) for satisfying all the two characteristics is −0.1V or more.

Analysis of Influence of Vanadium Looking at Examples 3 to 6 in which only the vanadium content is significantly different, it can be confirmed that the tensile strength values of Example 4 and Example 5 in which vanadium was further added were improved. This is because the formation of stable vanadium-carbon carbide by vanadium prevents crystal grain growth during heat treatment, which can also be confirmed through FIG. Therefore, it is preferable to add vanadium additionally in order to improve tensile properties.

Composition, Evaluation of Validity of Formulas (1) to (3) The composition of the alloy elements limited in the present invention and formulas (1) to (3) satisfy 97, 10.6, −0.1 or more, respectively. Assuming the total number of cases from the case to the case where all are not satisfied, and there are test pieces corresponding to the number of cases, the specific yield strength, stretch ratio, specific yield strength x stretch ratio and hole of the test piece The results of measuring the food resistance are shown in Table 4 below. At this time, in the composition and the mathematical formulas (1) to (3), “O” was displayed when the limited range of each item was satisfied, and “X” was displayed otherwise.

  As a result of the measurement, the light weight steel of Example 1 satisfying all of the composition and the minimum values of the formulas (1) to (3) was found to have high mechanical properties and pitting corrosion resistance. It was found that when the minimum value of the mathematical formula (1) is not satisfied, the mechanical properties are low, and when the mathematical formula (2) and / or the mathematical formula (3) is not satisfied, the pitting resistance is lowered.

  At this time, the change in physical properties according to the composition was through Comparative Example 20 and Comparative Example 23, and the change in tensile properties according to the value of Formula (1) was through Formula 1 and Comparative Example 20 according to Formula (3). The change can be confirmed through Comparative Example 21 and Comparative Example 22.

  Therefore, in the case of lightweight steel containing carbon, aluminum, manganese, chromium and molybdenum as alloying elements, in order to ensure all of the high mechanical properties and corrosion resistance, while satisfying the conditions limited in the present invention, It can be seen that the values represented by 1) to (3) are preferably 97, 10.6, and −0.1 or more, respectively.

  In addition, such lightweight steel having high mechanical properties and corrosion resistance has not only improved life characteristics but also can be applied to various fields.

  The present invention is not limited to the specific embodiments and explanations described above, and anyone having ordinary knowledge in the technical field to which the invention belongs without departing from the spirit of the invention claimed in the scope of claims. However, various modifications are possible, and such modifications are within the protection scope of the present invention.

Claims (19)

  Lightweight steel containing carbon (C), aluminum (Al), manganese (Mn), chromium (Cr), molybdenum (Mo), iron (Fe) and inevitable impurities.   The lightweight steel is composed of carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn) 20 to 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum The lightweight steel according to claim 1, comprising (Mo) 2 to 4.16 wt%, and remaining iron (Fe) and inevitable impurities.   The lightweight steel according to claim 1 or 2, further comprising vanadium (V) in a range of 0.55 wt% or less.   The lightweight steel according to claim 1, further comprising cobalt (Co) in a range of 5.1 wt% or less. 5. The value according to claim 1, wherein a value when the composition ratio (wt%) of elements contained in the lightweight steel is substituted into the mathematical formula (1) is 97 or more. Light steel.
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1) The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into Formula (2) is 10.6 or more, according to any one of claims 1 to 4. Lightweight steel described.
Cr + 3.3 (Mo + 0.5 (Si + W)) + 16N Formula (2) The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into the mathematical formula (3) is -0.1 or more. Lightweight steel as described in
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)   The lightweight steel according to any one of claims 1 to 7, wherein an extension ratio of the lightweight steel is 50% or more. A rolling step of heating and forming a steel ingot containing carbon (C), aluminum (Al), manganese (Mn), chromium (Cr), molybdenum (Mo), iron (Fe) and inevitable impurities;
A method for producing lightweight steel, comprising a homogenizing heat treatment step.   The rolling step includes carbon (C) 1.3 to 1.5 wt%, aluminum (Al) 8.5 to 10 wt%, manganese (Mn) 20 to 25 wt%, chromium (Cr) 4 to 5.1 wt%, molybdenum The method for producing a lightweight steel according to claim 9, wherein a steel ingot containing (Mo) 2 to 4.16 wt% and the remaining iron (Fe) and inevitable impurities is used.   The method for producing a lightweight steel according to claim 9 or 10, wherein the rolling step is performed in a temperature range of 1000 to 1200 ° C for 1 to 3 hours.   The method for producing a lightweight steel according to any one of claims 9 to 11, wherein the homogenizing heat treatment step is performed in a temperature range of 1100 to 1200 ° C for 1 to 3 hours. The method further includes an aging heat treatment step after the homogenization heat treatment step,
The aging heat treatment step is performed under a heat treatment temperature and time condition in which a HP (Holomon Jaffle parameter) has a value of 15.7 to 16.1, according to any one of claims 9 to 12, The manufacturing method of the described lightweight steel.   The method for producing lightweight steel according to claim 13, wherein the aging heat treatment step is performed in a temperature range of 500 to 550 ° C for 2 to 6 hours.   The method for manufacturing a lightweight steel according to any one of claims 9 to 14, wherein the steel ingot further contains vanadium (V) in a range of 0.55 wt% or less.   The method for producing a lightweight steel according to any one of claims 9 to 14, wherein the steel ingot further includes cobalt (Co) in a range of 5.1 wt% or less. The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into the mathematical formula (1) is 97 or more, according to any one of claims 9 to 14, A manufacturing method for lightweight steel.
45.2 + 117.2C-3.831Ni-20.36Mo + 4.25Cr-36.6C ² -2.344Cr ² + 4.10Mo · Cr Formula (1) The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into Formula (2) is 10.6 or more, according to any one of claims 9 to 14, The manufacturing method of the described lightweight steel.
Cr + 3.3 (Mo + 0.5 (Si + W)) + 16N Formula (2) The value when the composition ratio (wt%) of the elements contained in the lightweight steel is substituted into the mathematical formula (3) is -0.1 or more, and any one of claims 9 to 14, The manufacturing method of the lightweight steel described in 2.
−0.3141 + 0.1042C−1.046Cr + 1.048Mo + 0.00714Cr ² −0.5003Mo ² + 0.5076Cr · Mo Formula (3)