TW202214876A - Method for producing stainless steel - Google Patents

Abstract

A method of supressing embrittlement of stainless steel in a stainless steel production prDEGCess comprising the steps of • manufacturing stainless steel in a conventional stainless steel manufacturing prDEGCess • annealing the stainless steel in an annealing step • cooling the steel in a cooling step or quenching the steel in a quenching step and • applying a magnetic field to the stainless steel during the cooling or quenching step.

Description

Method of manufacturing stainless steel

The present invention relates to a method for inhibiting the embrittlement of steel. In particular, the method involves suppressing the embrittlement of duplex and ferritic steels with an external magnetic field.

The superior properties of stainless steel are due in part to the heat treatment performed during the final processing steps. In order for the stainless steel product to have the desired properties, such as good corrosion resistance, formability and strength for a long service life, it is necessary to strictly control the heat treatment performed. While heat treatment enhances these desired properties, it can also "sensitize" the part if the heat treatment is not performed adequately. As stainless steel grades become more alloyed, ie higher Cr and Mo levels, the risk of sensitization generally increases. For example, residence times in the range of 700°C to 950°C must be avoided to prevent the formation of sigma phase and chromium carbides, both of which reduce the corrosion resistance and mechanical properties of high alloyed austenitic and duplex stainless steels nature. To avoid this, cooling after heat treatment is preferably performed by water quenching in a water bath or sprinkling, depending on the product form. For duplex stainless steels, another temperature range that should be avoided as much as possible is between 250°C and 500°C. In this temperature range, a phenomenon called spinodal decomposition may occur.

Spinodal decomposition, also commonly referred to as 475°C embrittlement, is a degradation of dual-phase materials that greatly impairs toughness. For example, at 475°C, the ferrite phase in dual-phase steel and ferritic steel will decompose and become embrittled. After only 3 minutes at this temperature, the impact toughness will be reduced by 50%. At lower temperatures of 250-350°C, decomposition still occurs, but the rate of decomposition is significantly reduced, requiring up to 100,000 hours at this temperature to have the same negative effect on properties.

It has recently been recognized that for thick gauge components, such as quarter-open plates, some degree of spinodal decomposition may occur during cooling/quenching of the component after solution annealing. Cooling/quenching is most typically done batchwise in a water bath (or in-line heavy spray). However, even in a bath with a high degree of water agitation, the cooling rate of the part may be too slow, resulting in undesirable material properties. Air-cooled parts that cool slower than quenched parts are even more prone to spinodal decomposition because the material takes longer to cool to the critical temperature region. It is well known that high-alloyed dual-phase and ferritic steels are prone to precipitation of undesired phases, such as sigma, during cooling, but as mentioned above, it has been newly discovered that spinodal decomposition can also occur during the cooling/quenching step play an important role in the reduction of its properties. Lower alloy duplex and ferritic steels that do not exhibit sigma-phase precipitation may still be susceptible, albeit to a lesser extent, to spinodal decomposition when water quenched. However, some lower alloy grade products, such as bars, can be air cooled to make rolled products. Steels made in this way are prone to spinodal decomposition due to the relatively slow cooling rate when cooled in air.

JPH09217149A states that 475°C embrittlement occurs during cooling of thick gauge castings with a grain size greater than 50 μm. It teaches that an advanced cooling schedule is required to suppress sigma phase, 475°C embrittlement, and induced stress, including cooling rates from 500°C to 300°C need to be higher than 10°C/min to limit 475°C embrittlement. However, this cooling method is not feasible in in-line quenching processes, or in industrial-scale batch annealing using large quench tanks. Furthermore, for some materials, such as in thick gauge materials with grain sizes up to 50 μm, 10°C/min is still too slow to avoid spinodal decomposition. Most likely cooling rates at large plate thicknesses are still not fast enough to sufficiently suppress 475°C embrittlement due to material properties.

JP2006212674A describes how ferritic steels can become brittle due to embrittlement at 475°C in hot rolled coils without sufficient cooling during coiling. It proposes a solution to start coiling at a specific temperature so that the tip of the coil enters the mandrel just above the embrittlement temperature range, and is then rapidly cooled by the mandrel with built-in cooling during coiling. However, this method is not practically feasible on eg sheet or bar. In an in-line process, for example, pickling directly after annealing is also not feasible.

CN108315549A requests a method for eliminating aging embrittlement of duplex stainless steel, which is to perform electrical pulse treatment on aging duplex stainless steel, characterized in that the parameters of pulse processing are in the range of frequency 1 to 200 Hz, pulse width 20 μs to 1 ms, current 10 A to 2000 A, functional time 1 h to 6 h. This method is said to repair aged duplex steels after prolonged exposure to 475°C conditions, but does not prevent it from occurring during steel processing.

The inventors recently discovered that even 475°C embrittlement occurs during the cooling/quenching step. It has been determined that the properties of the material subjected to 475°C embrittlement during cooling are reduced in impact toughness, albeit to a different extent than described above for isothermal aging at 475°C. However, in combination with the sigma phase (also formed during the same cooling/quenching), the impact toughness is further reduced below acceptable levels, limiting the plate thickness or quality of the delivered material. Therefore, suppressing this embrittlement is crucial to obtain a high standard product.

The present invention is defined by what is disclosed in the independent item. Preferred specific examples are set forth in the subparagraphs.

It is an object of the present invention to overcome at least some of the disadvantages of the prior art and to provide a method of inhibiting the embrittlement of stainless steel.

According to a first aspect of the present invention, there is provided a method for suppressing the embrittlement of stainless steel in a stainless steel manufacturing process, comprising the steps of: manufacturing stainless steel in a conventional stainless steel manufacturing process, annealing the stainless steel, cooling/quenching the steel, and apply a magnetic field to the steel. The magnetic field is applied to the steel during the cooling or quenching step.

Considerable advantages are obtained by the present invention. By the present invention, it has surprisingly been found that the embrittlement of stainless steel is suppressed, thus providing a high quality stainless steel product.

Further features and advantages will become apparent from the following description.

The present invention relates to a method for suppressing the embrittlement of stainless steel in a stainless steel manufacturing process, comprising the following steps: manufacturing stainless steel in a conventional stainless steel manufacturing process, annealing the stainless steel with an annealing temperature range of 900-1250° C. in the annealing step, Quenching steel in a critical temperature range of 350°C to 550°C in a water bath or any other suitable quenching medium/process (including air cooling with or without forced air) to less than 200°C, preferably 150°C or below, suitably 100°C, and applying a magnetic field to the steel during the annealing step or the cooling/quenching step or during the annealing step and the cooling/quenching step.

Figure 1 shows that different cooling rates affect the properties of duplex stainless steels with respect to embrittlement at 475°C. Normal industrial process cooling (dashed line) gives lower impact toughness strength because some spinodal decomposition has occurred during this cooling period. The impact toughness was significantly higher in the case where the material was rapidly quenched at 550°C to 350°C (solid black line) (Table 1). This is because spinodal decomposition has not yet occurred. For air-cooled samples, the impact toughness is logically even lower than that of process-cooled samples because the residence time in the critical region is significantly longer than for normal process cooling.

The curves in Figure 2 represent different heat treatments and process conditions for super duplex materials. The material aged at 450°C for 500 hours has a large intensity peak at a Q of about 0.007 nm ^-1 , indicating a relatively large amount of spinodal decomposition and possible embrittlement. After 6 hours at this temperature, the peak intensity became smaller and shifted slightly to the higher reverse wavelength (Q 0.01 nm ^-1 ), indicating less phase separation, as expected at shorter exposure times. This trend continued in the 2-hour sample, with a smaller but still very pronounced peak. In contrast, the material showed no sign of phase separation in the presence of a magnetic field (1.5 T) at 450°C for 6 and 2 hours as no peak intensity was observed from the background level. This shows that in the presence of a magnetic field, 475°C embrittlement is completely suppressed under isothermal aging conditions for up to at least 6 hours.

Figure 3 shows the same findings as Figure 2, except that it uses duplex 2205 instead of super duplex 2507. Despite the fact that duplex steels are less susceptible to embrittlement than super duplex steels due to their less Cr. Figure 3 still shows that at 6 hours at 450°C, there is indeed a significant degree of spinodal decomposition, and even after 30 minutes some affected nanostructures can still be observed. Also, the phase separation of this steel is significantly suppressed even in the presence of an external magnetic field.

Figure 4 plots SANS data (intensity I(Q) [cm" ¹ ] versus reverse wavelength Q[nm" ¹ ]) for samples cooled under different conditions. The total exposure time in the 475°C temperature region is less than the conditions in Figure 2. The general background intensity differs between the curves of the "slow" and "fast" cooled samples due to differences in general microstructure, such as different austenite spacing. However a slight bend in intensity at about 0.01 nm ^-1 (data points showing higher intensity than the background reference line added to the graph) measures the change in nanostructure and indicates that the slowly cooled samples show initial phase separation , while the rapidly cooled samples appear to be unaffected. The process cooling rate of the slowly cooled sample in Figure 4 is similar to the conditions of the slowly cooled sample in Figure 1 (50-70°C/min and Table 1), and also has low impact toughness. This indicates that there may have been some degree of phase separation and embrittlement in the as-fabricated material. When the cooling rate is much lower than 50°C/min, the degree of phase separation and embrittlement of air-cooled components will be more serious. The "rapidly" cooled sample in Figure 4 cools faster (>500°C/min) than is possible under normal process quench conditions. Therefore, it is not possible in the actual process to cool thick plates (such as quads) fast enough to avoid the onset of some embrittlement. Applying an external magnetic field overcomes this problem and allows the material to be transported without spinodal decomposition/embrittlement. specific examples

The present invention relates to a method for inhibiting the embrittlement of stainless steel in the stainless steel manufacturing process. In one specific example, the method includes the steps of providing annealed stainless steel, cooling or quenching the steel in a cooling or quenching step, and applying a magnetic field to the stainless steel. Cooling/quenching is most typically done batchwise in a water bath (or in-line heavy spray) or air cooling. During the cooling or quenching step, a magnetic field is applied to the stainless steel. Embodiments of the present invention have been found to be applicable to all types of stainless steel. Thus, in another embodiment, the step of providing annealed stainless steel includes manufacturing stainless steel by all methods, such as, but not limited to, melting feedstock and/or scrap, casting stainless steel into ingots, slabs, blocks ( bloom) or atomized metal powder, or by rolling, pressing or forming into billets, plates, sheets, strips, coils, bars, rods, wires, profiles and shapes, Seamless and welded pipes and/or tubes, formed shapes, near-net shape powder metallurgy and profiles for further processing of stainless steel, followed by casting by ingot or continuous casting, followed by hot and/or cold rolling, or preparation for forging Final annealing is performed in the temperature range of 900°C to 1250°C. In another embodiment, stainless steel melts can be used to make metal powders for isostatic pressing or for lamination for annealing and cooling or quenching.

In one specific example, raw materials such as nickel and molybdenum are melted together with scrap metal in an electric arc furnace with or without vacuum oxygen decarburization. Electric arc furnaces are particularly effective for the purposes of embodiments of the present invention.

In one specific example, stainless steel is knocked into a mold. The mold can be made of various materials known to those skilled in the art. In one specific example, stainless steel is hammered into a copper mold. In another specific example, stainless steel is continuously cast into slabs.

As mentioned above, the stainless steel manufacturing process includes an annealing step. These processes may include one or more annealing steps in the temperature range of 900°C to 1250°C. In one embodiment, the purpose of the annealing step is to homogenize the stainless steel and soften and dissolve the second phase. The second phase includes, for example, carbides, nitrides, and intermetallic compounds, such as the sigma phase. Thus, in one embodiment, the annealing step includes raising the temperature of the steel to a temperature in excess of 900°C. In specific examples, the annealing step can be performed in a batch furnace for batch annealing or by in-line annealing.

The stainless steel manufacturing process also includes cooling or quenching steps. The number of quenching steps is generally equal to the number of annealing steps. In one embodiment, the cooling or quenching step reduces the temperature of the steel to maintain the material properties of the steel.

In one embodiment, the quenching step reduces the temperature of the steel from the annealing temperature to Below the temperature at which the second phase or embrittlement can no longer form or occur, ie 350°C or below, especially below 200°C, preferably 150°C or below, suitably 100°C or below. In some embodiments, the quenching step reduces the temperature of the steel to room temperature, or a temperature in the range of room temperature to 350°C. For the purposes of this case, room temperature refers to a temperature in the range of 20 to 25°C. There is no additional benefit to cooling below 20°C.

In another embodiment, the cooling step lowers the temperature of the steel by forced air or air cooling with natural cooling to room temperature.

In another embodiment, the field strength of the magnetic field is in the range of >0.2T, preferably >1.0, most suitably 1.5-3.0T. Since the material will reach magnetic saturation, there is no additional benefit to increasing the magnetic strength above 3.0T. Embrittlement of stainless steel is suppressed by a magnetic field.

As mentioned above, the embodiments of the present invention are applicable to all stainless steels. In particular embodiments, the stainless steel is a ferrite-containing stainless steel or a martensite-containing stainless steel.

The following non-limiting examples illustrate at least some specific embodiments of the invention. Example

Duplex steels are sensitive to embrittlement at 475°C. Impact toughness testing (Charpy impact test) is a standardized test method (for example, according to ISO 148-1:2016, ISO 17781, and as a test method to determine the presence of deleterious phases in dual-phase steels, such as ASTM A923 Method B) , which is ideal for determining if a material is sensitized in some way, such as embrittlement at 475°C. Impact toughness tests were performed on dual-phase steels at -40°C subjected to cooling conditions different from normal annealing temperatures, as shown in Figure 1. For this purpose, the GLEEBLE instrument was used to simulate the typical cooling curve of a thick plate material when it is quenched in a water bath, whereby the critical temperature range of 550-350°C is cooled at a normal rate, as shown by the dashed line in Figure 1. In contrast, other samples were cooled to 550°C under the same conditions and then rapidly quenched in the critical temperature range to completely eliminate the onset of embrittlement at 475°C, as shown by the solid line in Figure 1. The samples used for this GLEEBLE simulation and impact toughness testing are standard size 55x10x6 mm with V-notch for impact testing. The 6 mm thickness allows precise temperature control throughout the cooling step. For the cooling conditions in Fig. 1, the results in Table 1 show that the impact toughness of the normal process quenching is lower than that of the samples quenched by rapid quenching to avoid embrittlement, indicating that there is embrittlement at 475°C under the normal cooling conditions of the thick plate.

Small-angle neutron scattering (SANS) has been used to highlight nanostructural evolution due to spinodal decomposition of dual-phase steels after exposure to temperatures around 475°C during isothermal or process cooling . Also, SANS has been used to show that spinodal decomposition is inhibited after the same exposure with the application of an external magnetic field. Specifically, SANS measures spinodal decomposition in nanostructures, i.e. at the atomic level, which increases with Fe-rich and Cr-rich (layered) domains when dual-phase steels are exposed to temperatures of 250-500°C. development exists. For example, it is this phase separation that causes embrittlement and limits impact toughness. The sample size used is 10x10x1 mm. A thickness of 1 mm provides good signal strength while avoiding the multiple scattering that can occur with thicker samples. Where a heated sample is required, the 10x10 mm size is ideal to provide a uniform temperature throughout the sample.

Extensive experiments were carried out using SANS equipment to highlight the spinodal decomposition of dual phase steels under different conditions, showing how sensitive the material is to embrittlement at 475°C and how it can be suppressed in the presence of a magnetic field such as 1.5T.

Figures 2 and 3 show how the phase separation of two different austenitic-ferritic duplex stainless steels can be clearly detected using SANS. The previously annealed and quenched material has been held at 450°C for 6 hours and 500 hours. As time increases, the peak intensity increases and the position of the peak moves towards lower Q (nm ^-1 ), showing that spinodal decomposition progresses with time. This point is more pronounced in Figure 2 than in Figure 3, because the superduplex steel in Figure 2 has a higher alloy (especially chromium) content. Samples held at 475°C for up to 6 hours were also tested in the presence of an external magnetic field of strength 1.5T. These tests show that the degree of phase separation and thus the degree of embrittlement can be suppressed to a very low and insignificant degree, as indicated by the absence of intensity peaks, which would otherwise be observed in the absence of a magnetic field and embrittlement. This applies to the dual phase steels in Figures 2 and 3, which show that even in the presence of substantial spinodal decomposition (Figure 2), spinodal decomposition can be suppressed in the presence of a magnetic field.

Figure 4 shows that for the thick plate material, there is indeed some phase separation under normal process quenching with relatively slow cooling (~60°C/min) compared to the rapidly cooled material, such as a slight strength peak in the as-received condition shown. As previously mentioned, rapid cooling (>500°C/min) as in the "fast sample" in Figure 4 is not possible in many panels, so a method to suppress embrittlement at 475°C, such as the use of a magnetic field, is required.

It has been shown that with the application of an external magnetic field, the decomposition kinetics of the duplex alloys are significantly suppressed and embrittlement should also be significantly delayed.

For this purpose, an external magnetic field >0.2T should be applied to the stainless steel to be cooled or quenched during the entire cooling or quenching process of the part, preferably during cooling or quenching below 600°C. The magnetic field is applied in such a way that the entire material to be cooled or quenched is surrounded by a magnetic field >0.2T.

Process Impact toughness (J) Normal process "slow" cooling in the 550-350°C temperature range 75 Rapid quenching in the temperature range of 550-350°C 215 Table 1. Impact toughness results for different cooling curves in Figure 1.

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The present invention is described in more detail with reference to the accompanying drawings, wherein:

[Figure 1] shows the different cooling curves obtained using the GLEEBLE test equipment. GLEEBLE equipment is commonly used for thermomechanical testing of materials and to simulate physical processes such as industrial heat treatment and subsequent cooling. Therefore, the device is ideal for simulating the different cooling conditions associated with the 475°C phenomenon. The sample is heated to the desired annealing temperature, in this example, 1100°C, and then controlled to cool to simulate typical cooling conditions for industrial manufacturing of quads, bars, etc. In Figure 1, the dashed line shows a typical industrial cooling curve with an average cooling rate of 60°C/min over the entire cooling process from 1100°C to below 100°C, and the solid line shows the rapid cooling curve, simulating 60°C/min Normal industrial cooling to onset critical embrittlement temperature range (550°C), and then rapid cooling at >500°C/min in this range.

[Figure 2] is a graph of SANS data (intensity I(Q)[cm ^-1 ] versus reverse wavelength Q[nm ^-1 ]) showing evidence of phase separation in super duplex steels at 450°C (peak intensity increased and shifted to lower wavelengths at longer exposure times). The figure also shows how the external magnetic field (1.5T) suppresses the peaking (spinodal decomposition), thereby suppressing embrittlement.

[Figure 3] is a graph of SANS data (intensity I(Q)[cm ^-1 ] versus reverse wavelength Q[nm ^-1 ]) showing evidence of phase separation (increase in peak intensity) for dual phase steels at 450°C and shifted to lower wavelengths at longer exposure times). Figure 3 also shows how the external magnetic field (1.5T) suppresses the peaks, thereby suppressing embrittlement.

[Fig. 4] is a graph of SANS data (intensity I(Q)[cm ^-1 ] vs. reverse wavelength Q[nm ^-1 ]), shown with faster cooling conditions (>500° in 550-350°C) C/min) (indicated as "fast" in the figure), how the initial normal industrial cooling material at 60°C/min (indicated as "slow" in the figure) affects the nanostructures already in their as-is state.

Claims (12)

A method for inhibiting the embrittlement of stainless steel in a stainless steel manufacturing process, comprising the following steps: • Manufacture stainless steel in conventional stainless steel manufacturing processes; • annealing the stainless steel in the annealing step; • cooling the steel in the cooling step or quenching the steel in the quenching step; and • Apply a magnetic field to the stainless steel, It is characterised in that the magnetic field is applied to the stainless steel during the cooling or the quenching step. The method of claim 1, wherein the stainless steel manufacturing process comprises: • Melting raw materials/wastes for stainless steel manufacturing; • Casting stainless steel into ingots, slabs, blocks or atomized metal powder; • By rolling, pressing or forming into billets, plates, sheets, strips, coils, bars, rods, wires, profiles and shapes, seamless and welded tubes and/or Tubes, formed shapes, near-net shape powder metallurgy and profiles are used to further process the stainless steel. The method of claim 1 or 2, wherein the feedstock is melted in an electric arc furnace with or without vacuum oxygen decarburization. A method as in any preceding claim, wherein the stainless steel is hammered into a mold. A method as in any preceding claim, wherein the stainless steel is hammered into a copper mold. A method as in any preceding claim, wherein the stainless steel is continuously cast into slabs. The method of any of the preceding claims, wherein the annealing step comprises raising the temperature of the steel to a temperature in excess of 900°C for homogenizing, softening and dissolving the second phase. A method as in any preceding claim, wherein the cooling or the quenching step reduces the temperature to maintain the material properties of the steel. The method of any of the preceding claims, wherein the annealing step is performed in a batch furnace for batch annealing or in-line annealing. A method as claimed in any one of the preceding claims, wherein the cooling or the quenching step is by a quenching step (batch quenching in a water tank or in-line spray) or a cooling step (cooling by forced air or naturally cooled air) the steel The temperature is lowered from the annealing temperature to a temperature below 350°C, particularly below 200°C, preferably 150°C or below, suitably room temperature. A method as claimed in any preceding claim, wherein the field strength of the magnetic field applied during quenching is in the range >0.2T, preferably >1.0T, suitably 1.5T to 3.0T. The method of any of the preceding claims, wherein the stainless steel is a ferrite-containing stainless steel or a martensite-containing stainless steel.

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