JP5653053B2 - Manufacturing method of multi-phase high Cr steel - Google Patents

Description

The present invention relates to a high Cr steel having a duplex structure of austenite phase and martensite phase, a method of manufacturing a steel material that combines with particular strength and ductility together high level.

  In applications such as automobile parts and machine parts, a high Cr steel material having high strength, good workability (ductility), and excellent corrosion resistance may be required. Examples of the high-strength, high-Cr steel having good workability include work-hardening type austenitic stainless steels such as SUS301HT and SUS304HT. The mechanical properties of this type of steel are adjusted by temper rolling. Other high-strength, high-Cr steels include precipitation hardening type and transformation strengthened type martensitic stainless steel, and ferrite + martensite dual phase high strength stainless steel. These are mechanically adjusted mainly by heat treatment.

  Work-hardening austenitic stainless steel has a problem of high material cost because it contains a large amount of Ni. On the other hand, martensitic stainless steel and ferrite + martensitic dual-phase high-strength steel are superior in strength but are inferior in workability compared to austenitic steel types when compared at the same strength level.

  On the other hand, Patent Document 1 discloses a technique for imparting weld softening resistance by applying a strain of 3% or less after quenching to austenite + martensitic dual-phase stainless steel and then performing heat treatment. However, although the material obtained by this technique exhibits relatively good ductility, it does not always exhibit a satisfactory level of strength in applications such as automobile parts and machine parts.

JP 7-216451 A

  The present invention is intended to provide a high Cr steel material that is cheaper than an austenitic steel type and has a strength-ductility balance superior to that of a martensitic steel type or a conventional high-strength stainless steel type having a multiphase structure.

  The above steel material is originally realized by devising the chemical composition and heat treatment in a high Cr steel grade (typically martensitic stainless steel) with a chemical composition that becomes nearly 100% martensite structure at room temperature by quenching treatment. can do.

That is, in the present invention, by mass%, C: 0.200% or less, Si: 5.00% or less, Mn: 1.00% or less, Ni: 0.000 to 2.00%, Cr: 10.00 15.00%, Cu: 0.000 to 1.00%, Mo: 0.000 to 1.00%, N: 0.200% or less, balance Fe and unavoidable impurities, and according to the following formula (1) There is provided a multi-phase high Cr steel material having a chemical composition having a calculated Ms point of 200 ° C. or higher, a matrix comprising an austenite phase: 8.0 to 30.0% by volume, and the balance martensite phase.
Ms = 539-423C-30.4Mn-12. 1Cr-17.7Ni-10Cu-7.5Mo (1)

  Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1). Of the above elements, Ni, Cu, and Mo are arbitrarily added elements. “Matrix” means a metal substrate, and precipitates and inclusions may exist in the matrix. In the above multiphase steel, the carbon concentration in the austenite phase is higher than the overall average carbon concentration (C content of steel). A typical form of the steel material is a steel plate.

Further, in the present invention, as a method for producing the above-mentioned multi-phase structure steel material, the steel material having the above-described chemical composition is heated to an austenite single-phase temperature range to obtain an austenite single-phase structure. A step of obtaining a quenching material in which the austenite phase remains by 10% by volume or more by quenching to a temperature Tq in a temperature range where the site generation amount is 90% by volume or less (quenching process step);
After the quenching material is maintained at a temperature of Tq or more and less than Ac ₁ point in a state where the amount of austenite phase is maintained at 10% by volume or more, C is diffused from the martensite phase to the austenite phase. A process of cooling to room temperature (carbon distribution process),
A manufacturing method is provided.

  According to the present invention, a steel material exhibiting a martensite + austenite multiphase structure at room temperature has been realized in a so-called martensitic high Cr steel type that originally has a substantially 100% martensite structure at room temperature. This steel material has significantly improved ductility compared to martensite single-phase steel material of the same steel type, and has both strength and ductility at a high level. In addition, the content of expensive elements such as Ni is greatly reduced compared to an austenitic steel material in which both strength and ductility are relatively easy to achieve, and the cost reduction effect is excellent.

The figure which showed typically the heat pattern for obtaining the multi-phase structure high Cr steel material of this invention, and the metal structure state in each step. The graph which illustrated the relationship between Tq holding time and the amount of retained austenite at the time of performing carbon distribution processing at various temperatures Tq.

  As a result of detailed studies, the inventors have provided a strength-ductility balance by subjecting a stainless steel plate adjusted to the chemical composition described later to a quenching treatment in which the austenite phase is rapidly quenched and a subsequent carbon distribution treatment by heating. It was found that a dual phase high Cr steel material excellent in the above can be obtained.

  FIG. 1 schematically shows a heat pattern for obtaining a multiphase high Cr steel material of the present invention and a metallographic state at each stage. A material (for example, a hot-rolled steel sheet or a cold-rolled steel sheet) adjusted to a predetermined shape is prepared and subjected to quenching treatment. In the quenching treatment, the steel material is first heated and held in the austenite (γ) single-phase temperature range. After the austenite single phase structure (FIG. 1A) is obtained, the structure is cooled to a temperature Tq (° C.) lower than the martensite (α ′) transformation start temperature Ms point. This cooling may be performed at a cooling rate comparable to that during normal martensitic stainless steel quenching. Since the Ms point of the steel having the chemical composition within the range specified by the present invention is accurately estimated by the above formula (1), the value determined by the formula (1) is applied to the Ms point for specifying the present invention. However, it is necessary that the steel composition is adjusted so that the Ms point according to the formula (1) is 200 ° C. or higher. In this case, the martensitic transformation end temperature Mf point is higher than room temperature (for example, 50 ° C. or higher). Therefore, the steel applied in the present invention has a 100% martensite structure by a normal quenching process.

  By varying Tq within a range that satisfies Mf <Tq <Ms, the amount of martensite generated when cooled to Tq can be controlled. In the present invention, by quenching to a temperature Tq at which the quenching martensite generation amount is 90% by volume or less, a quenching material in which the austenite phase remains at 10% by volume or more is obtained (FIG. 1 (b)). In this specification, the process of heating from room temperature to the γ single phase region and quenching to Tq is called “quenching process”, and the above Tq is called “quenching end temperature”. A material in a state after quenching is called “quenched material”. In the hardened material, the carbon concentration in the austenite phase and the martensite phase is approximately equal to the overall average carbon concentration (C content of steel). At this time, carbon is supersaturated in the martensite phase having a small C solid solubility limit.

Next, the quenched material is held at the temperature Td in a state where the austenite amount is maintained at 10% by volume or more. Td is set in a range satisfying Tq ≦ Td <Ac ₁ . By holding the steel material within this range of Td, the carbon in the martensite phase that is dissolved in supersaturation diffuses into the austenite phase with a large solid solubility limit (FIG. 1 (c)). As a result, the C concentration in the austenite phase increases, and the Ms point of the austenite phase becomes lower than the Ms point determined by the equation (1) (FIG. 1 (d)). Thereafter, when the Td is cooled to room temperature, the Ms point of the austenite phase is lowered (the Mf point is also lowered), but a new martensite phase (α ′) is generated, It does not become a 100% martensite organization. That is, a structural state in which the austenite phase remains at room temperature can be obtained (FIG. 1 (e)). In the present specification, the process of maintaining the Td and allowing the carbon to diffuse from the martensite phase to the austenite phase and then cooling to room temperature is referred to as “carbon distribution process”, and the Td is referred to as “distribution process temperature”. . The amount of retained austenite of the steel material finally obtained (FIG. 1 (e)) depends on the amount of austenite at the time of quenching to Tq, the value of temperature Td, and the holding time at Td. The amount of retained austenite of the steel material of the present invention obtained by the carbon distribution treatment is adjusted to 8 to 30%. In this way, by securing an appropriate amount of retained austenite, ductility is improved, and as a result, the strength-ductility balance represented by “tensile strength (N / mm ² ) × elongation (%)” is improved. .

[Chemical composition]
Hereinafter, “%” in the chemical composition means “% by mass” unless otherwise specified.
C is an important element having a great influence on the strength and Ms point of steel. In the present invention, the C content may be equal to or higher than that of a general martensitic stainless steel type (SUS410 or the like) that is not particularly low C. For example, it is more effective to secure a C content of 0.030% or more. However, if the C content is too high, the martensite is excessively cured, and ductility and toughness are reduced. Moreover, since C forms Cr carbide, the effect of addition is saturated even if a large amount of C is added. As a result of various studies, it is desired that the C content be suppressed to 0.200% or less, and more preferably 0.150% or less.

Si has a deoxidizing action in steel making and also has an action of suppressing consumption of C for carbide formation. Also has an effect of suppressing the formation of carbides (M ₃ C), significantly improves the amount of retained austenite and thermal stability in the carbon distribution processing step. In order to fully exhibit these actions, it is more effective to secure a Si content of 0.20% or more. However, the inclusion of a large amount of Si leads to the formation of hard inclusions mainly composed of Si oxide, which adversely affects the strength and fatigue characteristics. Accordingly, the Si content is limited to 5.00% or less. You may manage in the range of 3.50% or less or 1.00% or less.

  Mn is an element effective for controlling the Ms point and expanding the range of the appropriate heating temperature in the quenching process. Although the minimum of Mn content is not specifically limited, What is necessary is just to set optimal Mn content in the range of 0.05% or more, for example. More preferably, it is 0.20% or more. However, if Mn is contained excessively, Mn-based inclusions that become the starting point of processing cracks increase. As a result of various studies, the upper limit of the Mn content is limited to 1.00%, and it is more preferable to adjust the content within a range of 0.90% or less.

  Ni is an element effective for improving toughness. It is also effective for controlling the Ms point. For this reason, in this invention, Ni can be contained as needed. In order to sufficiently bring out the above action, it is effective to secure a Ni content of 0.05% or more. However, since excessive Ni content becomes uneconomical, when Ni is contained, the content range is 1.00% or less, and may be limited to 0.60% or less.

  In order to ensure the corrosion resistance of the steel material, Cr is made a content of 10.00% or more. It is more preferably 10.50% or more, and even more preferably 11.50% or more. However, if a large amount of Cr is contained, Cr carbide tends to be generated, and the effect of improving the strength-ductility balance by the carbon distribution treatment may not be sufficiently exhibited. As a result of various studies, the Cr content is limited to a range of 15.00% or less, and may be controlled to a range of 13.50% or less.

  Cu is an element effective for controlling the Ms point and expanding the range of the appropriate heating temperature in the quenching process. For this reason, in this invention, Cu can be contained as needed. In that case, it is more effective to secure a Cu content of 0.01% or more. However, excessive Cu content causes a decrease in corrosion resistance. Therefore, when Cu is contained, it is necessary to perform the Cu content within a range of 1.00% or less, and it may be set within a range of 0.10% or less.

  Mo, like Ni, is effective for improving toughness and also effective for improving corrosion resistance. For this reason, in this invention, Mo can be contained as needed. In that case, it is more effective to make it contain in 0.01% or more of range. However, since Mo is an expensive element, when Mo is contained, it is performed within a range of 1.00% or less. You may manage to a content range of 0.20% or less.

  N, like C, contributes to improving the strength of steel and is an important element in controlling the Ms point. For example, the N content is preferably 0.005% or more. However, if N is excessively contained, surface defects are likely to occur during hot rolling. As a result of various studies, the N content is limited to 0.200% or less.

Ms in the following equation (1) is an index representing the Ms point (° C.) of steel.
Ms = 539-423C-30.4Mn-12. 1Cr-17.7Ni-10Cu-7.5Mo (1)
For steel in the composition range defined in the present invention, the Ms point is accurately estimated by the equation (1). In the present invention, steel having a composition in which the Ms point is 200 ° C. or higher is applied. Component steels with Ms points below 200 ° C. may have a retained austenite phase at normal temperature by normal quenching, and such steel types are not so-called martensitic steel types. It is difficult to improve the balance stably. The upper limit of the Ms point is not particularly determined because it is restricted by the formula (1) and the content range of each component element (described above), but a steel type having an Ms point of 400 ° C. or less according to the formula (1) is more preferable. It becomes a target.

[Metal structure]
The austenite + martensite multi-phase structure high Cr steel material of the present invention has a metal structure in which the matrix is an austenite phase: 8.0 to 30.0% by volume and the balance is a martensite phase. When the amount of austenite is less than 8.0% by volume, the degree of ductility improvement is small with respect to a material having the same composition manufactured through a normal quenching process, and as a result, a remarkable improvement effect of strength-ductility balance is obtained. There may not be. The amount of austenite in the matrix is more preferably 10.0% by volume or more, and even more preferably 11.0% or more. On the other hand, when the amount of austenite is excessively large, the strength tends to be insufficient. In the present invention, a material whose austenite content in the matrix is 30.0% by volume or less is targeted. However, in order to secure a large amount of austenite, it is necessary to strictly control the quenching end temperature Tq to a temperature close to Ms, which makes manufacture difficult. Therefore, the amount of austenite in the matrix is more preferably 20.0% by volume or less, and the austenite amount may be controlled to be 18.0% by volume or less.

[Quenching treatment]
As a steel material to be subjected to the quenching treatment, a rolled material obtained by a normal stainless steel plate manufacturing process can be applied. For example, it is possible to use a steel plate that has been adjusted to a predetermined product thickness in a process of “continuous casting → hot rolling → annealing / pickling → cold rolling”. In the cold rolling step, cold rolling may be performed a plurality of times with intermediate annealing as necessary.

  In the quenching treatment, first, the steel material is heated and held in the austenite single-phase temperature range to completely dissolve carbon and nitrogen. For example, the conditions of 900-1200 ° C. × soaking 0-10 min can be applied. When held at an excessively high temperature or for a long time, the austenite grains become coarse, and the multiphase structure after quenching becomes coarse, leading to deterioration of characteristics. Here, the soaking of 0 min means that the material is cooled immediately after the temperature at the central portion of the material reaches a predetermined temperature. Subsequently, it cools from heating temperature to temperature Tq lower than Ms point.

  In order to control the amount of retained austenite, the setting of the quenching end temperature Tq is important. In order to finally obtain the above steel material in which the amount of austenite in the matrix is 8.0% by volume or more, the amount of austenite at the temperature Tq after the quenching treatment is preferably set to 10.0% by volume or more. Further, in order to adjust the final austenite amount in the matrix within a range of 30.0% by volume or less, the austenite amount at this stage is desirably within a range of 35.0% by volume or less. The optimum quenching end temperature Tq can be determined by determining the relationship between the quenching end temperature Tq and the amount of austenite remaining at Tq in accordance with the steel composition in a preliminary experiment.

Cooling in the quenching process can employ a technique in which a steel material heated to an austenite single phase temperature range is immersed in an oil bath or a salt bath maintained at a temperature of Tq. In addition, the steel strip in the plate is exposed to a liquid refrigerant such as water, cooled to a low temperature range from the Ms point, and then charged into the furnace, so that the minimum temperature during the cooling process is Tq. It is also possible to do. Since the steel type targeted in the present invention is a high Cr steel having a Cr content of 10.00% or more, it does not involve pearlite transformation unlike ordinary steel. Accordingly, it is not necessary to strictly control the cooling rate when passing through the point A _1, and it is sufficient if a cooling rate comparable to that of a conventional martensitic stainless steel is secured when passing through the Ms point. The quenching end temperature Tq does not necessarily coincide with the quenching end temperature. Therefore, the quenching end temperature Tq in the present invention can be regarded as the lowest temperature reached after cooling from the austenite single-phase temperature range until it is subjected to the carbon distribution treatment described later.

[Carbon distribution treatment]
In the carbon distribution treatment, as described above, heating is performed for discharging carbon that is supersaturated in the martensite phase of the quenched material into the austenite phase. The heating temperature (distribution processing temperature) Td is set to a temperature equal to or higher than Tq and lower than Ac ₁ point. However, in order to make the final amount of austenite 8.0% by volume or more, it is necessary to hold it at Td while at least maintaining the amount of austenite at 10% by volume or more. In order to promote the diffusion of C efficiently, Td is more preferably set to a temperature higher than the Ms point according to the equation (1). In particular, it is more preferable to carry out in a temperature range of “Ms point + 30 ° C. according to formula (1)” or more and less than Ac ₁ point, and a temperature range of “Ms point + 50 ° C. according to formula (1)” and below Ac ₁ point. More preferably. In addition, you may manage an upper limit at 600 degrees C or less, or 550 degrees C or less.

  The diffusion of carbon from the martensite phase to the austenite phase tends to proceed as the heating temperature increases. Further, if the heating temperature is the same, the longer the heating time (up to a certain extent), the easier it proceeds. As the C concentration in the austenite phase increases due to diffusion, the Ms point of the austenite phase decreases. Therefore, if the steel composition and the quenching treatment conditions are the same, the amount of martensite newly generated in the cooling process from Td to room temperature decreases as the progress of diffusion in the carbon distribution treatment increases. That is, the value of the temperature Td and the holding time at Td greatly affect the amount of austenite remaining finally. The amount of retained austenite largely dominates the strength-ductility balance. Since carbon diffusion can occur even in the temperature rising process until reaching Td, it is necessary to consider the “material temperature-time curve” in the heating furnace to be used. Specifically, according to the chemical composition, the austenite amount of the quenching material, and the material temperature-time curve for each thickness of the material, the heating conditions (furnace) to obtain the final retained austenite amount that achieves the target strength-ductility balance Temperature, in-furnace time) is obtained in a preliminary experiment, and heat treatment may be performed based on the data.

  There is no need to pay particular attention to the cooling rate from Td to room temperature, but an extremely slow cooling rate at which carbides precipitate should be avoided. For example, conditions for allowing the material to cool in air outside the furnace can be employed.

The steel shown in Table 1 is melted, and the slab is hot-rolled at an extraction temperature of 1200 ° C. to form a hot-rolled steel strip having a thickness of 4.0 mm, annealed at 800 ° C. × 6 h, furnace cooling, and pickled. The steel sheet was cold-rolled to a thickness of 2.0 mm, subjected to intermediate annealing and pickling at 800 ° C. for 1 minute soaking, and finish cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. The cold-rolled steel sheet was subjected to a heat treatment of 1000 ° C. × soaking for 1 min, and then subjected to a quenching treatment immersed in an oil bath at 240 to 300 ° C. to obtain a quenched material. In this case, the oil bath temperature corresponds to the quenching end temperature Tq. For comparison, a quenching material (room temperature quenching material) that was water-quenched to room temperature (25 ° C) was also prepared. In this case, the quenching end temperature Tq = 25 ° C. A quenching material having a temperature Tq is immediately charged into a furnace having a temperature higher than Tq, and subjected to a carbon distribution treatment in which heat treatment is performed at a temperature Td and a holding time shown in Table 2 and then left to cool in a normal temperature atmosphere. did. However, as for some room-temperature quenching materials, the as-quenched materials were used as test materials. In addition, all the heating temperatures of the implemented carbon distribution process are less than Ac ₁ point. In Table 1, the Ms point calculated by the equation (1) is displayed.

The following investigation was conducted for each specimen.
[Residual austenite amount]
The amount of retained austenite in the matrix (% by volume) was determined by observing the structure of the sample collected from the test material using a transmission electron microscope (TEM). In all the test materials, the phases other than the austenite phase constituting the matrix were martensite phases.

〔Hardness〕
The Vickers hardness of the steel sheet surface was measured with a load of 10 kg according to JIS Z2240.
[Tensile strength, elongation]
A JIS No. 13B tensile test piece with the rolling direction as the longitudinal direction was taken from the test material, and a tensile test was performed until it broke at a tensile speed of 1 mm / min to obtain the tensile strength and elongation at break. Further, the tensile strength and elongation values were substituted into the following equation (2) to determine the strength-ductility balance index A value.
A value = tensile strength (N / mm ² ) × elongation (%) (2)

Originally, strength by forming a multi-phase structure of martensitic high Cr steel that becomes 100% martensite structure at normal temperature-Strength for each steel type (A to C)-in order to confirm the effect of improving ductility balance- It was investigated how much the ductility balance index A value was improved with respect to a test material using a room-temperature quenching material (amount of retained austenite = 0% by volume). Specifically, the ratio of [A value of each test material] / [A value of the test material using a room-temperature quenching material] was determined. This ratio is called “strength-ductility balance improvement ratio”.
These results are shown in Table 2. In the examples of the present invention, it has been confirmed that the amount of martensite in the matrix at the stage of the quenching material is in the range of 10.0 to 35.0% by volume.

  As can be seen from Table 2, the examples of the present invention that were subjected to quenching treatment and carbon distribution treatment under appropriate conditions were the same steel type specimens prepared using a quenching treatment material (No. 1, 3, 7 respectively). ) Is a strength-ductility balance improvement ratio of 1.20 or more, and a marked improvement in the strength-ductility balance was observed. On the other hand, Comparative Example No. 6 has a quenching material with an austenite content of less than 10.0% by volume by lowering the quenching end temperature Tq. In Comparative Example No. 12, the distribution processing temperature Td is set to a temperature range below the Ms point. In all of these, the amount of austenite after the distribution treatment was small, and the improvement in the strength-ductility balance was small.

  When the lattice constant of the austenite phase (face-centered cubic lattice) before and after the carbon distribution treatment in the material of the present invention was measured by X-ray diffraction, it was confirmed that the lattice constant was increased by the carbon distribution treatment. It was. This increase in the lattice constant is considered to be caused by the increase in the amount of carbon dissolved in the interstitial position of the austenite crystal by the carbon partitioning process. Nitrogen is also considered to diffuse from the martensite phase to the austenite phase, and it is considered that the diffusion of nitrogen partially contributes to the increase in the lattice constant.

  FIG. 2 illustrates the relationship between the Tq holding time and the amount of retained austenite when carbon distribution treatment is performed at various temperatures Tq after quenching with steel C at a quenching end temperature Tq = 240 ° C. When carbon distribution treatment is performed, the amount of retained austenite decreases with respect to the hardened material (austenite amount = 18.1%), but a material that does not become a 100% martensite structure at room temperature is obtained. It can be seen that it can be controlled by the conditions (temperature, holding time).

Claims (1)

In mass%, C: 0.200% or less, Si: 5.00% or less, Mn: 1.00% or less, Ni: 0.000 to 2.00%, Cr: 10.00 to 15.00%, Cu: 0.001 to 1.00%, Mo: 0.000 to 1.00%, N: 0.200% or less, balance Fe and inevitable impurities, Ms point calculated by the following formula (1) Is heated to an austenite single-phase temperature range to an austenite single-phase structure, and then the temperature is lower than the Ms point and the quenching martensite generation amount is 90% by volume or less. By quenching to a certain temperature Tq, a step of obtaining a quenching material in which the austenite phase remains at 10% by volume or more (quenching treatment step),
After the quenching material is maintained at a temperature of Tq or more and less than Ac ₁ point in a state where the amount of austenite phase is maintained at 10% by volume or more, C is diffused from the martensite phase to the austenite phase. A process of cooling to room temperature (carbon distribution process),
The matrix has an austenite phase: 8.0 to 30.0% by volume, and a method for producing a multi-phase high Cr steel material composed of the remaining martensite phase.
Ms = 539-423C-30.4Mn-12. 1Cr-17.7Ni-10Cu-7.5Mo (1)
Here, the content of the element represented by mass% is substituted for the element symbol in the formula (1).

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