HK1254792B - High-strength stainless steel sheet having excellent fatigue characteristics, and method for manufacturing same - Google Patents

Description

High-strength stainless steel sheet with excellent fatigue properties and method for manufacturing the same

Technical Field

The present invention relates to a steel plate in which the formation of coarse hard non-metallic inclusions is significantly suppressed in a stainless steel grade capable of achieving very high strength by forming a work-induced martensite phase, solid solution strengthening due to a high Si content, and age hardening. Furthermore, the present invention relates to a method for producing the same.

Background Art

As high-strength stainless steel, quasi-stable austenitic stainless steel, represented by SUS301, is widely used. However, in order to obtain high strength in SUS301, the cold rolling rate must be increased, which is accompanied by a corresponding decrease in toughness. As a technology to avoid this problem and achieve both high strength and high toughness at a high level, a method of achieving high strength by utilizing the formation of a processing-induced martensite phase, solid solution strengthening caused by high Si content, and age hardening is known. This method has been used in applications such as ID saw blade substrates (Patent Document 1).

The type of stainless steel disclosed in Patent Document 1 is cold-rolled to obtain a dual-phase structure of work-induced martensite and austenite, which has high strength and high toughness. It also has good fatigue resistance as a rotating component with a plate thickness of 0.1 mm or more, such as an ID saw blade. However, when processed into thin plates with a plate thickness of less than 0.1 mm, especially 20 to 70 μm, and used in spring materials subjected to repeated elastic deformation, it is desired to further improve the fatigue resistance. The presence of non-metallic inclusions can be cited as a major cause of the reduction in fatigue resistance of steel. Even for inclusions of the same size, the thinner the plate thickness, the greater the proportion of the inclusion's length in the plate thickness direction. Stress is concentrated around these inclusion particles, making it easier for them to function as crack starting points and propagation paths. The thinner the plate, the more difficult it is to improve the reduction in fatigue properties caused by non-metallic inclusions.

As a method of reducing the amount of non-metallic inclusions in steel (i.e., improving cleanliness), various methods of rationalizing the slag composition during refining have been studied. However, from the perspective of preventing machining cracks and fatigue failure, simply improving cleanliness may not be sufficient, and it is believed that controlling the composition of non-metallic inclusions is effective. For example, Patent Document 2 discloses the following method: in the smelting of general austenitic steel grades such as SUS304, a refining furnace with a dolomite-based refractory lining is used to adjust the slag basicity to 1.4 to 2.4, thereby controlling the composition of non-metallic inclusions and obtaining austenitic stainless steel without machining cracks. However, according to the inventor's investigation, in the case of steel grades with high Si content, even if the method disclosed in Patent Document 2 is tried, it is difficult to significantly improve the fatigue properties in thin plate materials.

On the other hand, Patent Document 3 discloses a technique for improving corrosion resistance in high-temperature, high-concentration nitric acid by reducing the total amount of high-melting-point _B1- type inclusions, primarily aluminum oxide, in high-Si austenitic stainless steel. The technique describes suppressing the formation of B1 _- type inclusions by not performing Cr reduction recovery with Al and adding an Fe-Si alloy with an Al content as low as approximately 0.1% (paragraphs 0052 and 0053). However, the steel targeted in Patent Document 3 is a single-phase austenitic steel with a Ni content of 10% or more (paragraph 0033), not intended for high strength achieved through processing-induced martensite formation. The document does not teach a method for improving fatigue resistance in thin sheet materials for spring applications. As will be described later, suppressing the formation of TiN-type inclusions is crucial for improving the fatigue resistance of the steel targeted in the present invention. However, the melting method disclosed in the document does not consistently reduce TiN-type inclusions.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent No. 3219117

Patent Document 2: Japanese Patent No. 3865853

Patent Document 3: Japanese Patent No. 5212581

Summary of the Invention

Problems to be solved by the invention

Among the non-metallic inclusions contained in steel, those with high melting points and hard properties remain as granular materials even after hot rolling, and remain in the form of somewhat broken hard particles arranged in the rolling direction after cold rolling. Therefore, it is believed that as long as the generation of such hard non-metallic inclusions can be significantly suppressed, the fatigue resistance of the thin plate can be improved. However, in order to obtain a very high strength level in steels that generate processing-induced martensite, a large amount of Si exceeding 2% by mass must be contained. If the Si content in the steel is increased in this way, it becomes very difficult to suppress the generation of hard non-metallic inclusions. Even if a method is adopted in which no Al addition is made and an Fe-Si alloy with a low Al content is added, as disclosed in Patent Document 3, this alone will not achieve a stable improvement in the fatigue resistance of the thin plate.

The present invention aims to realize, in mass production sites, a thin plate having a thickness of 20 to 500 μm in which a distribution of non-metallic inclusions effective in improving fatigue resistance is achieved in a high Si content work-induced martensite-forming stainless steel.

Means for solving problems

The inventors' detailed research has revealed that reducing the amount of TiN-based inclusions with an equivalent circular diameter of 6.0 μm or greater and spinel-based inclusions containing at least one of Al and Mg in the hot-rolled steel sheet is extremely effective in improving fatigue resistance when used as a thin leaf spring material in a work-induced martensite-forming stainless steel with a high Si content. Furthermore, they discovered that reducing such coarse inclusions can be achieved in mass production by strictly controlling the incorporation of Ti and Al from deposits, auxiliary materials, and slag-forming agents from the molten steel container, and by controlling the basicity of the final slag formed after Si addition to a narrow range slightly lower than normal. Furthermore, when the hot-rolled steel sheet is thinned in the cold rolling process, the inclusions within the sheet assume a form that is extremely beneficial for improving fatigue resistance. The present invention is based on this discovery.

That is, in order to achieve the above object, the present invention provides a stainless steel plate having a plate thickness of 20 to 500 μm, comprising, in mass %, C: 0.010 to 0.200%, Si: more than 2.00% and less than 4.00%, Mn: 0.01 to 3.00%, Ni: 3.00 to less than 10.00%, Cr: 11.00 to 20.00%, N: 0.010 to 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: The present invention relates to a steel having a chemical composition consisting of 0 to 0.008% Fe, the remainder being Fe and unavoidable impurities, and in a cross section (L cross section) parallel to the rolling direction and the plate thickness direction, when a group of non-metallic inclusion particles arranged with an inter-particle distance of 20 μm or less (i.e., 0 to 20 μm) in the rolling direction and an inter-particle distance of 10 μm or less (i.e., 0 to 10 μm) in the plate thickness direction is regarded as one non-metallic inclusion, the number density of non-metallic inclusions with a length of 40 μm or more in the rolling direction is 3.0 particles/ ^mm2 or less in the L cross section.

Among them, when the inter-particle distance (μm) in the rolling direction of two particles appearing in the L section does not repeat in the rolling direction range of each particle, it is defined as the rolling direction interval (μm) between the two rolling direction ranges; if there is any overlap, it is defined as 0μm. Similarly, when the inter-particle distance (μm) in the plate thickness direction of two particles appearing in the L section does not repeat in the plate thickness direction range of each particle, it is defined as the plate thickness direction interval (μm) between the two plate thickness direction ranges; if there is any overlap, it is defined as 0μm. Two particles with an inter-particle distance of less than 20μm in the rolling direction and less than 10μm in the plate thickness direction belong to the same "group".

Among the above-mentioned non-metallic inclusions having a rolling direction length of 40 μm or more, non-metallic inclusions that have a particularly large influence on fatigue resistance can be listed as non-metallic inclusions including (i) TiN-based inclusion particles and (ii) one or more spinel-based inclusion particles containing Al and Mg, and one or two of the above (i) and (ii).

The above-mentioned Ti content is the total Ti content in the steel including Ti present as inclusions. Similarly, the above-mentioned Al content is the total Al content in the steel including Al present as inclusions.

The number density of inclusions can be measured by observing a mirror-polished observation surface of an L-section using a SEM (scanning electron microscope). Whether the inclusions are TiN-based inclusions or spinel-based inclusions containing one or more of Al and Mg can be determined by elemental analysis using, for example, an EDX (energy dispersive X-ray fluorescence) analyzer attached to the SEM.

FIG1 shows an SEM photograph of inclusions observed in the L section of a conventional cold-rolled steel sheet having a thickness of 120 μm (conventional example No. 1 described later). The transverse direction of the figure corresponds to the rolling direction, and the longitudinal direction corresponds to the plate thickness direction. Groups of non-metallic inclusion particles arranged roughly along the rolling direction are observed at two locations (A) and (B). The inter-particle distance in the plate thickness direction between the closest particles of group (A) and group (B) is indicated by symbol S in FIG1 . Since the inter-particle distance S in the plate thickness direction exceeds 10 μm, when all particles of group (A) and group (B) are taken as objects, these particles do not correspond to "a group of non-metallic inclusion particles arranged with an inter-particle distance of less than 20 μm in the rolling direction and an inter-particle distance of less than 10 μm in the plate thickness direction". If only the particles of group (A) are taken as the object, these constituent particles have an inter-particle distance of 20 μm or less in the rolling direction and an inter-particle distance of 10 μm or less in the plate thickness direction in the positional relationship with at least one other constituent particle. Therefore, each particle constituting group (A) is equivalent to "a group of non-metallic inclusion particles arranged with an inter-particle distance of 20 μm or less in the rolling direction and an inter-particle distance of 10 μm or less in the plate thickness direction." Therefore, each particle constituting group (A) is considered to be one non-metallic inclusion. Similarly, each particle constituting group (B) is also considered to be one non-metallic inclusion. In Figure 1, there are two non-metallic inclusions, and their respective rolling direction lengths are indicated as _LA and _LB in Figure 1. Among them, _LA is 40 μm or more. Therefore, of these two non-metallic inclusions, the non-metallic inclusion with a rolling direction length of _LA is equivalent to "a non-metallic inclusion with a rolling direction length of 40 μm or more."

The results of EDX analysis show that these non-metallic inclusions are all TiN-based inclusions.

FIG2 shows an SEM photograph of inclusions seen in a different field of view from FIG1 in the L-section of a cold-rolled steel sheet with a thickness of 120 μm according to the present invention (Example No. 5 of the present invention described later). The transverse direction of the figure corresponds to the rolling direction, and the longitudinal direction corresponds to the thickness direction. The non-metallic inclusion particles arranged in FIG2 correspond to "a group of non-metallic inclusion particles arranged with an inter-particle distance of less than 20 μm in the rolling direction and an inter-particle distance of less than 10 μm in the thickness direction," and therefore are considered to be a single non-metallic inclusion. The rolling direction length of this non-metallic inclusion slightly exceeds 40 μm, and therefore corresponds to "a non-metallic inclusion with a rolling direction length of more than 40 μm."

The results of EDX analysis showed that the non-metallic inclusions were TiN-based inclusions.

The number density of non-metallic inclusions having a rolling direction length of 40 μm or more in the L-section of the steel plate can be determined as follows.

[Method for measuring the number density of non-metallic inclusions with a rolling direction length of 40 μm or more]

SEM observation was performed on a mirror-polished observation surface of a cross section (L cross section) parallel to the rolling direction and the thickness direction of the steel plate. A measurement area with a rolling direction length of 100 μm or more and a thickness direction length equal to the entire thickness of the plate was randomly determined. The number of "non-metallic inclusions with a rolling direction length of 40 μm or more" that were entirely or partially present within the measurement area was counted, including those inclusions entirely present within the measurement area and those inclusions with a portion extending outside the measurement area but with at least 1/2 of the rolling direction length present within the measurement area. This operation was repeated for one or two or more non-overlapping measurement areas until the total area of the measurement areas reached 10 ^mm² or more. The value obtained by dividing the sum of the counts in each measurement area by the total area of the measurement area was defined as the "number density (inclusions/ ^mm² ) of non-metallic inclusions with a rolling direction length of 40 μm or more."

Furthermore, in the hot-rolled steel sheet stage, the total number density of TiN-based inclusions having an equivalent circle diameter of 6.0 μm or more and spinel-based inclusions containing one or more of Al and Mg in the L-section is preferably 0.05/mm ² or less.

The equivalent circle diameter is the particle size converted to the diameter of a circle having an area equal to the projected area of the inclusion particle appearing on the observation surface. The equivalent circle diameter of each inclusion particle can be calculated, for example, by computer processing a SEM image of the inclusion. The number density of the inclusions in the hot-rolled steel sheet can be determined as follows.

[Method for measuring the number density of inclusions in hot-rolled steel sheets]

SEM observation is performed on a mirror-polished observation surface of a cross section (L cross section) parallel to the rolling direction and the thickness direction of the steel plate. A rectangular measurement area is defined within a randomly selected field of view. The number of inclusion particles observed within the field of view that are TiN-based inclusions or spinel-based inclusions containing one or more of Al and Mg and have an equivalent circular diameter of 6.0 μm or greater, and in which the entire particle is within the measurement area, and in which a portion of the particle protrudes outside the measurement area but in which at least 1/2 of the particle area is within the measurement area, is counted. This operation is repeated for multiple, non-repeating fields of view until the total area of the measurement area reaches 200 ^mm² or greater. The value obtained by dividing the sum of the counts in each field of view by the total area of the measurement area is defined as the "total number density (individuals/ ^mm² ) of TiN-based inclusions and spinel-based inclusions containing one or more of Al and Mg with an equivalent circular diameter of 6.0 μm or greater."

If the above-described inclusion distribution is present in a hot-rolled steel sheet, then when it is subsequently cold-rolled into a thin plate, the above-described inclusion distribution is achieved, resulting in a significant improvement in fatigue resistance. Particularly preferred examples of such thin plates include those with a tensile strength in the rolling direction of 2000 N/ ^mm² or greater. The steel sheet matrix (metal base) obtained by cold-rolling a hot-rolled steel sheet having the above-described composition has a mixed structure of a work-induced martensite phase and an austenite phase.

As a method for manufacturing the above-mentioned steel plate, a method for manufacturing a stainless steel plate is provided, which comprises:

A process for adjusting the composition of molten steel by adding auxiliary raw materials and a slag-forming agent to molten steel having a carbon content of 0.20% or less and having slag containing Cr oxides on the melt surface after a decarburization process by blowing oxygen into Cr-containing molten iron has been completed, wherein the molten steel container, auxiliary raw materials, and slag-forming agent to be used are selected so that the Ti content and the Al content in the molten steel are 0.008% by mass or less and at least an Fe-Si alloy is dissolved in the molten steel as an auxiliary raw material to carry out deoxidation, reduce and recover Cr in the slag in the molten steel, and adjust the Si content in the steel, and further add a slag-forming agent containing Ca to adjust the slag basicity (CaO/ _SiO2 mass ratio) to 1.3 to 1.5, thereby obtaining molten steel having the above-mentioned chemical composition;

a step of casting the molten steel obtained in the above steps to obtain a cast slab;

a step of subjecting the cast slab to hot working including at least hot rolling to obtain a hot-rolled steel sheet; and

A step of subjecting the hot-rolled steel sheet to annealing and cold rolling one or more times to produce a cold-rolled steel sheet having a thickness of 20 to 500 μm.

The phrase "selecting the molten steel container, auxiliary raw materials, and slag-forming agents to reduce the Ti content in the molten steel to 0.008% by mass or less and the Al content to 0.008% by mass or less" means using a molten steel container with little or no deposits and using auxiliary raw materials and slag-forming agents with low impurity levels. This ensures that the Ti content and Al content in the molten steel do not exceed 0.008% by mass due to the deposits, auxiliary raw materials, and slag-forming agents in the molten steel container. Molten steel that has completed the decarburization process by injecting oxygen into the Cr-containing molten iron can be considered to have substantially zero Ti and Al contents. Therefore, by preventing or minimizing external contamination, steel with a Ti content of 0-0.008% and an Al content of 0-0.008% can be obtained.

More preferably, the molten steel container, auxiliary raw materials, and slag-forming agent used are selected so that the Ti content in the molten steel becomes 0.006 mass % or less and the Al content becomes 0.006 mass % or less.

Specific examples of molten steel storage vessels include refractory-lined refining vessels and ladles. Ladles can also be used directly as refining vessels. For molten steel storage vessels, it is preferable to use a refractory vessel whose inner surface has not been used to store molten steel (new vessel).

As the Fe—Si alloy, it is preferable to use an Fe—Si alloy having an Al content of 0.05 mass % or less and a Ti content of 0.05 mass % or less.

Furthermore, there is no specific regulation on the content of Mg, a spinel inclusion-forming element, in steel. However, in order to reduce the Ti and Al contents as described above, it has been confirmed that the content can be reduced to a level that does not pose a problem by effectively selecting a molten steel container, auxiliary raw materials, and slag-forming agents. In this case, the total Mg content in the steel is 0.002 mass% or less.

By subjecting the cold-rolled steel sheet to aging treatment, a steel sheet can be obtained in which the matrix (metal base) is a mixed structure of a work-induced martensite phase and an austenite phase and the tensile strength in the rolling direction is, for example, 2000 N/mm ² or more.

Effects of the Invention

According to the present invention, thin sheets can be produced in mass production operations in a high-Si stainless steel grade that produces process-induced martensite formation, with a significantly reduced number of hard, non-metallic inclusions extending in the rolling direction. This high-Si steel grade exhibits the highest levels of strength among stainless steels and has been primarily used in applications such as ID saw blades. By controlling inclusions according to the present invention, fatigue resistance in thin sheet materials is improved, enabling their use as leaf spring materials. Therefore, the present invention, utilizing the unique high strength characteristics of this steel grade, can contribute to the further miniaturization of leaf spring components used in electronic devices and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM photograph of non-metallic inclusions observed in the L-section of the cold-rolled steel sheet of Conventional Example No. 1. FIG.

FIG. 2 is a SEM photograph of non-metallic inclusions observed in the L-section of the cold-rolled steel sheet of Example No. 5 of the present invention.

FIG. 3 is a SEM photograph of typical TiN-based inclusions observed in the L-section of the hot-rolled steel sheet of Conventional Example No. 4.

FIG. 4 is a SEM photograph of typical TiN-based inclusions observed in the L-section of the hot-rolled steel sheet of Example No. 5 of the present invention.

DETAILED DESCRIPTION

[Chemical composition]

Unless otherwise specified, "%" in chemical composition means "mass %".

The present invention targets steel having the chemical composition shown in the following (A).

(A) In terms of mass%, C: 0.010-0.200%, Si: more than 2.00% and less than 4.00%, Mn: 0.01-3.00%, Ni: more than 3.00% and less than 10.00%, Cr: 11.00-20.00%, N: 0.010-0.200%, Mo: 0-3.00%, Cu: 0-1.00%, Ti: 0-0.008%, Al: 0-0.008%, and the remainder is Fe and inevitable impurities.

Steels with this composition form work-induced martensite during cold rolling, increasing strength. Furthermore, during the subsequent aging treatment, solute atoms such as C and N primarily form a cottrell atmosphere within the martensite phase, pinning dislocations and enhancing strength (deformation aging). Furthermore, the large amount of Si present in the steel leads to solid solution strengthening of the martensite and retained austenite phases, contributing to increased strength.

In the present invention, in order to fully benefit from the aforementioned strength-enhancing effect of Si, steel containing more than 2.00% Si is targeted. However, excessively high Si content can lead to significant disadvantages such as increased susceptibility to hot working cracking. Therefore, the Si content is limited to 4.00% or less.

C is an austenite-forming element and essential for strengthening steel. However, excessive C content reduces corrosion resistance and toughness. The present invention targets steels with a C content of 0.010 to 0.200%. When aiming for high strength, it is advantageous to have a C content within the range of 0.050 to 0.100.

Nitrogen is an austenite-forming element and essential for strengthening steel. However, excessive nitrogen content can significantly promote the formation of TiN-based inclusions. The present invention targets steels with a nitrogen content of 0.010% to 0.200%. Within this range, by employing a production method that suppresses the incorporation of Ti, as described below, the particle size distribution of TiN-based inclusions can be optimized to fall within the range specified by the present invention. A more preferred range for the nitrogen content is 0.050% to 0.085%.

Mn is an element that facilitates the control of austenite stability by adjusting its content. Its content is adjusted within the range of 0.01-3.00%. Excessive Mn content makes it difficult to induce a strain-induced martensite phase. The Mn content is more preferably adjusted within the range of 1.00% or less, and can be controlled within the range of 0.50% or less.

Ni is an austenite-forming element. To form a semi-stable austenite phase at room temperature, a content of 3.00% or more is required. Excessive Ni content makes it difficult to induce a strain-induced martensite phase, so the Ni content is limited to less than 10.00%. More preferably, it is limited to 7.00-9.50%.

Cr is an essential element for ensuring corrosion resistance. The present invention targets steels with a Cr content of 11.00 to 20.00%. Cr is a ferrite-forming element, and if it is present in excessive amounts exceeding the above, a single-phase austenite structure may no longer be achieved at high temperatures. A more preferred range for the Cr content is 12.00 to 15.00%.

In addition to improving corrosion resistance, Mo also contributes to strengthening by forming Mo-based precipitates through aging treatment, and since the structure hardened by cold rolling is difficult to soften through aging treatment, it can be contained as needed. In order to fully enjoy these effects, it is preferable to ensure a Mo content of 1.0% or more. In addition, when aiming to achieve a strength level of 2000N/ ^mm2 or more in the rolling direction, containing 2.00% or more Mo is extremely effective. However, the presence of a large amount of Mo leads to the formation of δ-ferrite phase at high temperatures, so when Mo is contained, the content range is specified to be 3.00% or less. It can be managed to a range of 2.50% or less.

Cu has the effect of increasing strength by interacting with Si during aging treatment, so it can be contained as needed. In this case, it is more preferable to set the Cu content to 0.01% or more. The presence of a large amount of Cu is a major factor in reducing hot workability. If Cu is contained, it is set to a content range of 1.00% or less.

Ti is an element that forms TiN-based inclusions. TiN is particularly prone to formation in steels with high Si content, so the Ti content must be kept low. Various studies have shown that the Ti content must be kept to 0.008% or less, and more preferably to 0.006% by mass or less. While a lower Ti content is preferred, a Ti content of 0.001% or more is reasonable for mass production operations, considering cost.

Al forms Al ₂ O ₃ and is the main cause of the formation of spinel inclusions. In particular, Al ₂ O ₃ is easily generated in molten steel in steels containing high Si content, so the Al content must be controlled to be low. In the present invention, the lower the Al content, the better. As a result of various studies, the Al content must be specified to be 0.008% or less, and more preferably 0.006% by mass or less. The lower the Al content, the better. Considering the cost in mass production operations, it is reasonable to specify the Al content in the range of 0.001% or more. However, even if the Al content is within the above range, if the slag basicity after Si addition is not rationalized as described later, it is difficult to stably make the particle size distribution of the spinel inclusions fall within the range specified in the present invention.

As inevitable impurities, the P content is preferably set to 0.040% or less, the S content is preferably set to 0.002% or less, and the Mg content is preferably set to 0 to 0.002%.

In order to adjust the easiness of forming the strain-induced martensite phase during cold rolling, the Md ₃₀ value defined by the following formula (1) is preferably in the range of -50 to 0.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo…(1)

Here, the value of the content of the element expressed in mass % is substituted into the element symbol portion of formula (1).

[Non-metallic inclusions]

Non-metallic inclusions in steel are broadly categorized into two types: those with low melting points and soft properties, and those with high melting points and hard properties. In the steel subject to the present invention, the former, softer types are primarily CaO- _SiO2 -based non-metallic inclusions. These soft inclusions are liquid at hot rolling temperatures, stretching them in the rolling direction during hot rolling. They collapse during subsequent cold rolling, dispersing them into finer particles. These soft inclusions have little adverse effect on the fatigue resistance of sheet materials.

The non-metallic inclusions that are problematic are the latter hard inclusions. This type of inclusion remains as granular matter even after hot rolling, and remains in the form of hard particles that are broken to a certain extent and arranged in the rolling direction after cold rolling. The thinner the plate thickness becomes, the greater the proportion of the plate thickness length of the inclusions in the plate thickness increases, and stress concentrates around these inclusion particles, making it easier for them to function as the starting point and propagation path of cracks. It can be seen that the hard inclusions that are problematic in the target steel of the present invention are TiN-based inclusions and one or more spinel-based inclusions containing Al and Mg. In particular, TiN-based inclusions have a tendency to grow as the solubility of Ti decreases during the process of lowering the temperature of the molten steel during casting, and are prone to becoming a problem.

According to the inventors' research, by reducing the number ratio of TiN-based inclusions and spinel-based inclusions with an equivalent circular diameter of 6.0 μm or greater during hot-rolled steel sheet production, a distribution of inclusions advantageous for improving fatigue resistance under repeated elastic deformation can be achieved when producing thin sheet materials, such as those with a thickness of 20 to 500 μm. Specifically, during hot-rolled steel sheet production, it is extremely effective to achieve a microstructure in which the total number density of TiN-based inclusions with an equivalent circular diameter of 6.0 μm or greater and spinel-based inclusions containing at least one of Al and Mg is 0.05 inclusions/ ^mm² or less in a cross section (L cross section) parallel to the rolling direction and the thickness direction.

A more preferred microstructure of the hot-rolled steel sheet, in addition to the aforementioned number density requirements, is a metal structure in which the maximum particle size of the TiN-based inclusions and the spinel-based inclusions containing one or more of Al and Mg in the L-section is 10.0 μm or less based on the equivalent circle diameter. In this case, the L-section measurement area used to determine the maximum particle size can be set to 200 ^mm² or greater.

It was found that in order to improve fatigue resistance when subjected to repeated elastic deformation, it was extremely effective to set the number density of non-metallic inclusions with a rolling direction length of 40 μm or more in a thin plate with a thickness of 20 to 500 μm to 3.0/ ^mm2 or less in the L-section. As described above, a group of non-metallic inclusion particles arranged with an inter-particle distance of 20 μm or less in the rolling direction and an inter-particle distance of 10 μm or less in the plate thickness direction was considered to be one non-metallic inclusion.

Regarding fatigue resistance, adjacent inclusion particles arranged in close proximity to each other can function as crack initiation points, similar to when they exist as a single continuous particle. Various studies have shown that a particle group consisting of multiple non-metallic inclusion particles arranged to maintain an inter-particle distance of less than 20 μm in the rolling direction and less than 10 μm in the plate thickness direction (considered as a single non-metallic inclusion), particularly non-metallic inclusions with a rolling direction length of 40 μm or greater, are likely to serve as crack initiation points when subjected to repeated elastic deformation in the high-strength steel targeted by the present invention. However, even with such non-metallic inclusions, fatigue resistance improves when the number density in the L-section is reduced to less than 3.0 inclusions/ ^mm² . This is presumably because sufficiently reducing the density of non-metallic inclusions with a rolling direction length of 40 μm or greater makes it difficult for these inclusions to function as crack propagation paths.

The lower the number density of non-metallic inclusions with a rolling direction length of 40 μm or more in the L-section, the more beneficial it is for improving the fatigue resistance of thin plates. It is believed that by using only high-purity raw materials without scrap, for example by melting steel in a laboratory melting furnace, thin plates with extremely few non-metallic inclusions can be produced. However, when manufacturing steel plates with a thickness of 20 to 500 μm in a mass production site, completely preventing the formation of non-metallic inclusions with a rolling direction length of 40 μm or more increases the load on the steelmaking process and leads to increased costs. Therefore, it is reasonable to set the number density of non-metallic inclusions with a rolling direction length of 40 μm or more in the L-section of thin plates with a thickness of 20 to 500 μm to 0.1 to 3.0 inclusions/ ^mm² .

[Manufacturing method]

Stainless steel sheets with optimized particle size distribution of hard non-metallic inclusions can be produced using standard stainless steel melting equipment. Representative examples include the VOD and AOD methods. Both methods first produce molten steel with a carbon content of 0.20% or less, with slag containing Cr oxides on the melt surface, after a decarburization process involving oxygen injection into Cr-containing molten iron. The molten steel container used is selected to contain little or no Ti or Al from deposits, etc., until this stage. Otherwise, the steelmaking process can be carried out according to standard methods.

Since the molten steel at this stage has completed decarburization by injecting oxygen, nearly all of the easily oxidizable elements, Ti, Al, Mg, and Si, have been oxidized and removed from the molten steel. In other words, Ti, Al, Mg, and Si are virtually absent from the molten steel. Furthermore, a portion of the Cr, which is present in large quantities in the molten steel, is also oxidized, forming slag as Cr oxides on the surface of the molten steel. In this slag, which is primarily composed of Cr oxides, the Ti, Al, Mg, and Si removed from the molten steel also exist as oxides. Meanwhile, a large amount of oxygen injected into the molten steel for decarburization is dissolved in the molten steel. Therefore, deoxidation is essential before casting. Furthermore, in order to produce high-Si steel with a Si content exceeding 2.00% in the present invention, Si must be incorporated into the steel. It is preferable to further perform a process (Cr reduction recovery) in which the Cr removed from the molten steel during decarburization is returned from the slag to the steel. Therefore, in the present invention, the aforementioned "deoxidation," "Si content adjustment," and "Cr reduction recovery" are performed simultaneously by adding an Fe-Si alloy to the molten steel. Furthermore, other auxiliary materials are added as needed to adjust the composition.

When Fe-Si alloy is added to molten steel and the Si content is adjusted to exceed 2.00%, the steel is deoxidized by the Si source added in large quantities. The oxygen concentration in the steel caused by this deoxidation is determined by the chemical equilibrium in the chemical reaction of the following formula (2).

Si (in metal) + 2O (in metal) = SiO ₂ (in slag)…(2)

The equilibrium constant K is represented by the following formula (3).

K＝A(SiO ₂ )/A(Si)/A(O) ² ...(3)

Here, A(X) is the activity of component X. As can be seen from equation (3), the higher the Si activity (i.e., Si concentration) in the molten steel, the lower the oxygen activity (i.e., oxygen concentration) in the molten steel. Therefore, in the molten steel of the present invention, to which a large amount of Si source is added, the oxygen concentration in the molten steel is lower than in the case of steel containing low Si content (e.g., general steel such as SUS304).

On the other hand, a chemical equilibrium based on the following formula (4) also exists between the Al oxides in the slag and the oxygen in the molten steel.

2Al (in metal) + 3O (in metal) = Al ₂ O ₃ (in slag)…(4)

This chemical equilibrium maintains equilibrium when the oxygen concentration in the molten steel is low, as the Al concentration in the molten steel increases. This relationship also applies to Ti and Mg. That is, when the oxygen concentration in the molten steel is low, the Al, Ti, and Mg concentrations in the molten steel increase.

The higher the Al and Mg concentrations in the molten steel, the easier it is for spinel-based inclusions to form and grow. The higher the Ti concentration in the molten steel, the easier it is for TiN-based inclusions to form and grow. Therefore, in order to suppress the formation and growth of these inclusions, the reduction in oxygen concentration in the molten steel that accompanies the increase in Si concentration in the molten steel must be suppressed as much as possible. In order to suppress the reduction in oxygen concentration in the molten steel, the higher the _SiO2 concentration in the slag, the more advantageous it becomes. Therefore, the present invention adopts a method of controlling the slag basicity (CaO/ _SiO2 mass ratio) to be slightly lower. Specifically, the amount of Ca-containing material added as a slag-forming agent is adjusted. As a slag-forming agent, quicklime CaO can be used. In addition, Ca is also supplied to the slag from _CaF2 added as a flux component as needed. The Ca supplied from _CaF2 is also converted into the CaO amount and added to the CaO value when calculating the basicity. The results of various studies have shown that it is effective to put the slag present at the surface of the melt after the Fe-Si alloy is completed so that its slag basicity is within the range of 1.3 or more and 1.5 or less. It is more preferably set to 1.3 or higher and 1.45 or lower, and even more preferably set to 1.3 or higher and 1.4 or lower. The lower the slag basicity, the more the oxygen concentration in the molten steel is suppressed, making it more difficult for Ti, Al, and Mg to enter the molten steel from the slag. However, if the slag basicity is excessively low, other types of inclusions such as _{Cr₂O₃ will} form in large quantities. Furthermore, the desulfurization capacity will also be reduced. Therefore, it is extremely effective to control the final slag basicity within a narrow range that does not fall below 1.3.

As mentioned above, molten steel that has completed oxygen decarburization contains almost no Ti, Al, or Mg. These elements exist as oxides in the slag, which is primarily composed of Cr oxides. The Ti, Al, and Mg in this slag are mixed in from raw materials and refractory materials, as well as from slag and metal adhering to forehearth charges from equipment such as electric furnaces and converters. In order to optimize the particle size distribution of hard inclusions in the steel plate by controlling the slag alkalinity as described above, it is necessary to prevent the re-incorporation of Ti, Al, and Mg as much as possible after the completion of oxygen decarburization, i.e., after the Fe-Si alloy is added. In particular, if slag and other materials adhering to the forehearth charge remain in the molten steel container, coarse hard inclusions are likely to form due to the small amount of Ti and Al adhering to these materials. To prevent incorporation from the molten steel container, it is most preferable to use a molten steel container (new pot) whose refractory components, forming the inner surface of the container, have not yet been used to contain molten steel. Furthermore, Fe-Si alloys commonly used in stainless steel manufacturing contain impurities such as Al and Ti. The Al and Ti introduced by these impurities have been shown to be a major cause of the formation of coarse hard inclusions. Therefore, the present invention requires the use of high-purity Fe-Si alloys. Specifically, Fe-Si alloys with an Al content of 0.05% by mass or less and a Ti content of 0.05% by mass or less are preferably used. Care should be taken to minimize the incorporation of Ti and Al from other auxiliary materials or slag-forming agents.

It is important to select the molten steel container, auxiliary raw materials, and slag-forming agents used so that the Ti content in the final molten steel is 0.008 mass% or less, and the Al content is 0.008 mass% or less. If the Ti and Al contents in the final steel exceed these limits, even if the slag basicity is controlled as described above, it will be difficult to stably achieve the desired hard inclusion particle size distribution described above. Furthermore, it is preferable to control the Mg content in the final molten steel to 0.002 mass% or less. It has been confirmed that if the molten steel container, auxiliary raw materials, and slag-forming agents used are selected so that the Ti and Al contents fall within these limits, even without specifying the Mg content in the steel, the spinel inclusion particle size distribution achieves the desired state described above, without causing any problems.

More preferably, the Ti content in the molten steel is set to 0.006 mass % or less, and the Al content is set to 0.006 mass % or less.

Casting can be carried out using conventional methods. Continuous casting is generally used to produce cast slabs. In this specification, steel materials (steel materials with a solidified structure) obtained by casting are referred to as cast slabs. Therefore, for convenience, steel blocks (ingots) obtained by agglomeration are also included in the term "cast slabs" herein.

The obtained ingot is subjected to hot working including at least hot rolling to obtain a hot-rolled steel plate. In the case of the block making method, hot rolling is carried out after block rolling and hot forging. The heating temperature of hot rolling can be specified as 1100-1250°C, and the thickness of the hot-rolled steel plate can be specified as 2.5-6.0 mm, for example. In this way, a stainless steel hot-rolled steel plate is obtained in which the total number density of TiN inclusions with a circle equivalent diameter of 6.0 μm or more and spinel inclusions containing one or more of Al and Mg in a cross section (L cross section) parallel to the rolling direction and the plate thickness direction is less than 0.05 pieces/ ^mm2 .

Next, by annealing, cold rolling, and aging the hot-rolled steel sheet, a high-strength stainless steel sheet can be obtained. The cold rolling process can be performed multiple times, including an intermediate annealing process. Pickling is performed as needed after each heat treatment process. The conditions for annealing (hot-rolled sheet annealing) applied to the hot-rolled steel sheet can be set, for example, to 1000-1100°C for 40-120 seconds, the final cold rolling rate (the final cold rolling rate after intermediate annealing when intermediate annealing is performed) can be set, for example, to 40-70%, and the aging treatment conditions can be set, for example, to 400-600°C for 10-60 minutes. In the use of thin leaf spring materials, it is preferred that the final sheet thickness be less than 150 μm, and more preferably less than 100 μm. Thin sheets with a thickness of, for example, 20-70 μm can also be produced. In this way, a high-strength stainless steel sheet with a matrix (metal base) having a mixed structure of a processing-induced martensite phase and austenite phase can be obtained. The ratio of the area fraction M of the work-induced martensite phase to the area fraction A of the austenite phase is generally in the range of 30:70 to 50:50. For example, if Mo is contained at least 2.00%, a high tensile strength of 2000 N/ ^mm² or greater in the rolling direction can be achieved. In a thin plate, when a group of non-metallic inclusion particles arranged with an inter-particle distance of 20 μm or less in the rolling direction and 10 μm or less in the plate thickness direction is considered to be a single non-metallic inclusion, a microstructure in which the number density of non-metallic inclusions with a rolling direction length of 40 μm or greater is 3.0/ ^mm² or less in the L-section is achieved. This thin steel plate exhibits excellent fatigue resistance in applications such as spring materials subjected to repeated elastic stresses.

Example

The steels listed in Table 1 were melted using the VOD process. VOD equipment was used to complete the final decarburization process by blowing oxygen into the Cr-containing molten iron. This yielded molten steel with a carbon content of 0.10% or less, with slag containing Cr oxides on the melt surface. The carbon content at this stage was approximately equal to the final carbon content shown in Table 1.

[Table 1]

Table 1

In the final decarburization using the VOD equipment, a ladle (a pot) was used as a molten steel storage container. The same ladle was then used for the entire process up to the casting stage. Conventional Examples Nos. 1 and 2 used a forehearth charge used in the melting of Ti-containing stainless steel. Conventional Examples Nos. 3 and 4 used a forehearth charge used in the melting of stainless steel without Ti addition. In Example No. 5 of the present invention, a ladle (a new pot) was used, in which the refractory material constituting the inner surface of the ladle had not yet been used to store molten steel.

An Fe-Si alloy was added to the molten steel after the final decarburization, and the Si content in the molten steel was adjusted to the target value. Deoxidation and Cr reduction recovery in the slag were also performed. The Si content in the molten steel at this stage was approximately equal to the final Si content shown in Table 1. Conventional examples Nos. 1 to 4 used Fe-Si alloys corresponding to Ferrosilicon No. 2 specified in JIS G2302:1998. Analysis results showed that this Ferrosilicon No. 2 product contained approximately 1.0 mass% Al, approximately 0.07 mass% Mg, and approximately 0.08 mass% Ti, although the content varied slightly depending on the batch. Meanwhile, in the inventive example No. 5, a high-grade Fe-Si alloy with a significantly reduced Al content was used. Analysis results showed that the Al, Mg, and Ti contents of this high-grade Fe-Si alloy were 0.009 mass% Al, less than 0.001 mass% Mg, and 0.012 mass% Ti.

Following the addition of the Fe-Si alloy, industrial quicklime (CaO) was added to the slag as a slag-forming agent. The slag was then sampled and analyzed for composition. The results showed that the slag basicity ranged from 1.60 to 1.65 in conventional samples No. 1 to 4, and was 1.33 in the inventive sample No. 5.

In each example, the molten steel obtained as described above was continuously cast and hot-rolled to obtain a hot-rolled steel plate with a thickness of 3.8 mm. The heating temperature during hot rolling was set to 1230°C. The L-section of the obtained hot-rolled steel plate was observed using SEM, and the total number density of TiN-based inclusions with an equivalent circular diameter of 6.0 μm or greater and spinel-based inclusions containing one or more types of Al and Mg was measured according to the "Method for Determining the Number Density of Inclusions in Hot-Rolled Steel Plates" described above. The results showed that the total number density was 0.20 to 0.45 inclusions/ ^mm2 in the conventional examples No. 1 to 4 and 0.02 inclusions/ ^mm2 in the inventive example No. 5. It can be seen that the formation of coarse hard inclusions can be significantly suppressed by the smelting method according to the present invention.

Figure 3 shows an SEM photograph of typical TiN-based inclusions observed in the L-section of the hot-rolled steel sheet of Conventional Example No. 4, and Figure 4 shows an SEM photograph of typical TiN-based inclusions observed in the L-section of the hot-rolled steel sheet of Inventive Example No. 5. In both photographs, the horizontal direction corresponds to the rolling direction. The crosshairs shown in the photographs indicate the beam irradiation position for EDX analysis.

Next, samples taken from hot-rolled steel sheets were subjected to hot-rolled annealing at 1050°C for 60 seconds, cold rolling, intermediate annealing at 1050°C for 60 seconds, cold rolling, and aging at 500°C for 30 minutes. This resulted in 120-μm-thick thin sheets with a matrix (metal base) composed of a mixed structure of work-induced martensite and austenite. The resulting thin sheets exhibited tensile strength exceeding 2000 N/ ^mm² in the rolling direction.

The number density of non-metallic inclusions with a rolling direction length of 40 μm or longer was measured in the L-section of these thin plates according to the previously described "Method for Measuring the Number Density of Non-Metallic Inclusions with a Rolling Direction Length of 40 μm or Longer." However, as mentioned above, a group of non-metallic inclusion particles arranged with an inter-particle distance of 20 μm or less in the rolling direction and 10 μm or less in the plate thickness direction was considered a single non-metallic inclusion. The measurement results showed that the number density of non-metallic inclusions with a rolling direction length of 40 μm or longer in the L-section ranged from 8.2 to 33.2 particles/ ^mm² in conventional examples No. 1 to 4 and 2.4 particles/ ^mm² in example No. 5 of the present invention. EDX analysis showed that the non-metallic inclusions counted were composed of TiN-based inclusion particles or spinel-based inclusion particles containing at least one of Al and Mg.

In the examples of the present invention, the number of hard non-metallic inclusions having a rolling direction length of 40 μm or more, which is a major cause of degradation of fatigue resistance, is significantly reduced compared to the conventional examples.

For reference, a manufacturing example is shown in which SUS304 (Si content 0.55%) was deoxidized using an Fe-Si alloy. The ladle used for melting Ti-containing stainless steel in the pre-charge was used as a molten steel container. A decarburization process was carried out by blowing oxygen into the Cr-containing molten iron using a VOD device, resulting in molten steel with a C content of approximately 0.05% and slag containing Cr oxides on the melt surface. An Fe-Si alloy equivalent to the above-mentioned ferrosilicon No. 2 was added to the molten steel to adjust the Si content. Furthermore, industrial quicklime (CaO) was added as a slag-forming agent. Chromium nitride was then added to complete the composition adjustment. The final slag was analyzed and found to have a slag basicity of 1.65. The molten steel was continuously cast and hot-rolled using conventional methods to obtain a hot-rolled steel plate with a thickness of 3.5 mm. This hot-rolled steel plate was examined for the presence of non-metallic inclusions in the same manner as in the above-mentioned cases Nos. 1 to 5. The results showed that no TiN inclusions with an equivalent circular diameter of 6.0 μm or more and no spinel inclusions containing one or more of Al and Mg were found. Comparing this SUS304 example with the conventional examples No. 1 to 4 above, it can be seen that it is very difficult to suppress the formation of hard inclusions in stainless steels with high Si content.

Claims (8)

1. Stainless steel plates with a thickness of 20–500 μm, comprising, by mass%, C: 0.010–0.200%, Si: more than 2.00% and less than 4.00%, Mn: 0.01–3.00%, Ni: more than 3.00% and less than 10.00%, Cr: 11.00–20.00%, N: 0.010–0.200%, Mo: 0–3.00%, Cu: 0–1.00%, Ti: 0–0.008%, Al: 0–0.008%, with the remainder being Fe and non-renewable minerals. The chemical composition of the impurities to be avoided, in a cross section parallel to the rolling direction and the plate thickness direction, i.e., cross section L, when a group of non-metallic inclusion particles arranged with an interparticle distance of less than 20 μm in the rolling direction and an interparticle distance of less than 10 μm in the plate thickness direction are regarded as one non-metallic inclusion, the number density of one or two types of non-metallic inclusions with a length of more than 40 μm in the rolling direction and containing TiN-based inclusion particles and spinel-based inclusion particles containing one or more of Al and Mg is less than 3.0 particles/ ^mm² in cross section L. 2. The stainless steel plate according to claim 1, wherein the tensile strength in the rolling direction is 2000 N/ ^mm² or higher. 3. The stainless steel plate according to claim 1, wherein the matrix, i.e. the metal substrate, is a mixed structure of processing-induced martensite phase and austenite phase. 4. A method for manufacturing stainless steel plates, which includes: When adding auxiliary raw materials and slag-forming agents to molten steel with a C content of less than 0.20% and slag-forming agents to adjust the composition after the decarburization process of blowing oxygen into Cr-containing molten iron has been completed and the slag containing Cr oxides on the molten surface has been formed, the steel container, auxiliary raw materials and slag-forming agents are selected to make the Ti content in the molten steel less than 0.008% by mass and the Al content less than 0.008% by mass. The auxiliary raw materials are used to dissolve at least the Fe-Si alloy in the molten steel, to perform deoxidation, reduction and recovery of Cr in the slag in the molten steel and adjustment of the Si content in the steel. Furthermore, a slag-forming agent containing Ca is added to adjust the slag basicity, i.e., the CaO/ _SiO2 mass ratio, to 1.3 to 1.5, to obtain molten steel with the chemical composition shown in (A) below. The process of casting the molten steel obtained in the above process to obtain a casting sheet; The above-mentioned castings are subjected to a process including at least hot rolling to obtain hot-rolled steel sheets; and The process of producing cold-rolled steel sheets with a thickness of 20 to 500 μm by subjecting the above-mentioned hot-rolled steel sheets to annealing and cold rolling at least once. (A) By mass%, C: 0.010–0.200%, Si: more than 2.00% and less than 4.00%, Mn: 0.01–3.00%, Ni: more than 3.00% and less than 10.00%, Cr: 11.00–20.00%, N: 0.010–0.200%, Mo: 0–3.00%, Cu: 0–1.00%, Ti: 0–0.008%, Al: 0–0.008%, with the remainder being Fe and unavoidable impurities. 5. The method for manufacturing stainless steel plate according to claim 4, wherein, as a molten steel container, a new pot that has not yet been used in the containment of molten steel is used as the refractory material constituting the inner surface of the container. 6. The method for manufacturing a stainless steel plate according to claim 4, wherein, as the above-mentioned Fe-Si alloy, an Fe-Si alloy with an Al content of less than 0.05% by mass and a Ti content of less than 0.05% by mass is used. 7. The method for manufacturing stainless steel sheet according to claim 4, further comprising a step of performing an aging treatment on the above-mentioned cold-rolled steel sheet. 8. The method for manufacturing stainless steel plate according to claim 7, wherein a steel plate is obtained with a matrix, i.e., a metal substrate, having a mixed microstructure of processing-induced martensite and austenite phases, and a tensile strength in the rolling direction of 2000 N/ ^mm² or higher.