Method for improving surface corrosion resistance of Invar alloy cold-rolled and annealed steel coil

Abstract

The invention discloses a method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed steel coil, and relates to the technical field of metal heat treatment, and the method comprises the following steps: before annealing treatment, placing a manganese sheet at the air inlet position of an annealing furnace, and placing the annealed steel coil in the annealing furnace; before annealing treatment, high-purity gas is adopted to purge the annealing furnace; and in the annealing process, high-purity gas is introduced into the annealing furnace, the dew point at a gas inlet is controlled to be smaller than or equal to-45 DEG C, the annealing temperature is controlled to be 650-700 DEG C, and the annealing time is 8-10 h. According to the method, the gas flow in the annealing furnace and the dew point of the gas inlet are controlled, and the high-purity manganese sheet is placed at the gas inlet, so that the oxygen partial pressure in the annealing atmosphere is effectively reduced, and the selective oxidation kinetics of easily oxidized elements is remarkably retarded by further optimizing the annealing process; and finally, the coverage rate of granular oxides precipitated and polymerized on the surface of the Invar alloy cold-rolled annealed steel coil is reduced, and the corrosion resistance of the surface of the Invar alloy cold-rolled annealed plate is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal treatment, and in particular to a method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed steel coil. Background Art

[0002] Invar alloy is an Fe-Ni alloy with a Ni content of about 36wt%. The thermal expansion coefficient of this alloy at room temperature is only about 1 / 10 of that of most metal materials, and the thermal expansion coefficient is approximately constant in the range of -200 to 200°C. It is widely used in actual engineering application scenarios that require high dimensional accuracy and stability within the service temperature range, such as the production, storage and transportation of liquefied natural gas, temperature control devices, etc.

[0003] Invar alloy is very easy to rust. For example, the 0.7mm Invar alloy strip used to manufacture the insulated cargo hold of the membrane type LNG carrier will produce rust spots after contact with fingers or dripping with sweat, which becomes a fatal hidden danger for LNG carriers. The production process of cold-rolled Invar alloy strip is generally: the hot-rolled pickled coil is subjected to multiple "annealing, leveling, grinding, rolling" according to the finished product specifications, and then "cleaning, annealing and straightening" are completed to obtain the finished cold-rolled strip steel coil. In production practice, it is found that when the surface of the cold-rolled Invar alloy strip is black, it is more prone to surface rust during its storage, transportation or use. Through the whole process tracing, it is found that during the hood annealing, the easily oxidized elements Mn, Si, Cr, Al, etc. (especially Si and Mn) in the Invar alloy strip are easy to precipitate and aggregate on the surface of the steel plate in the form of oxides. In other words, selective oxidation occurs under extremely low oxygen partial pressure conditions. These oxides can form corrosion micro-batteries with the Fe matrix, increase the activity of Fe atoms on the steel plate surface and accelerate the corrosion process of the steel plate by its catalytic effect, that is, Fe→Fe 2+ +2e - and 1 / 2O2+H2O+2e - →2OH - , which is manifested as easy rusting of steel plates on a macro scale. Therefore, it is urgent to find a method to inhibit the selective oxidation behavior on the surface of Invar alloy strips and thus improve their corrosion resistance. Summary of the invention

[0004] The technical problem to be solved by the present invention is: in view of the shortcomings of the prior art, a method for improving the corrosion resistance of the surface of an Invar alloy cold-rolled annealed steel coil is provided. The method effectively reduces the oxygen partial pressure in the annealing atmosphere by controlling the gas flow rate and the dew point of the air inlet in the annealing furnace and placing a high-purity manganese sheet at the air inlet. The selective oxidation kinetics of easily oxidizable elements are further slowed down by optimizing the annealing process, and finally the coverage rate of granular oxides precipitated on the surface of the Invar alloy cold-rolled annealed steel coil is reduced, thereby improving the corrosion resistance of the surface of the Invar alloy cold-rolled annealed plate.

[0005] In order to solve the above technical problems, the technical solution of the present invention is:

[0006] A method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed steel coil comprises the following steps:

[0007] Before annealing, the manganese sheet is placed at the air inlet of the annealing furnace, and the annealing steel coil is placed in the annealing furnace;

[0008] Before annealing, the annealing furnace is purged with high-purity gas;

[0009] During the annealing process, high-purity gas is introduced into the annealing furnace, and the dew point at the air inlet is controlled to be ≤-45°C, the annealing temperature is controlled to be 650-700°C, and the annealing time is 8-10h.

[0010] Preferably, the manganese flakes are high-purity manganese flakes with a purity of ≥99%.

[0011] Preferably, the weight ratio of the manganese sheet to the annealed steel coil is 1:(200-400).

[0012] Preferably, the annealing furnace is a bell-type annealing furnace.

[0013] Preferably, before annealing, when the annealing furnace is purged, the flow rate of high-purity gas is ≥100m 3 / h, purge time ≥30min.

[0014] Preferably, during the annealing process, the flow rate of the high-purity gas is ≥10m 3 / h.

[0015] Preferably, before and during the annealing process, the high-purity gas is high-purity hydrogen.

[0016] The principle of the method provided by the present invention in improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil is as follows:

[0017] 1. First, place high-purity manganese sheets at the air inlet of the bell-type annealing furnace, and use the oxidizability of the high-purity manganese sheets to preferentially consume the residual oxygen elements in the atmosphere in the furnace;

[0018] 2. Secondly, the water vapor content in the full hydrogen gas is reduced by dew point control, and the H2O, CO and CO2 gases produced by the annealing reduction reaction are blown out of the bell-type annealing furnace in time with the help of high-purity hydrogen flow, so that the oxygen partial pressure in the annealing atmosphere is at a lower level;

[0019] 3. Finally, by optimizing the annealing temperature and annealing time, the selective oxidation kinetics on the steel plate surface is minimized while ensuring sufficient recrystallization of the Invar alloy austenite.

[0020] By combining the above conditions, the easily oxidizable elements Mn, Si, Cr, Al, etc. (especially Si and Mn) in the Invar alloy cold-rolled annealed steel coil are prevented from segregating in the form of easily oxidized oxides on the surface of the steel plate, so that the coverage rate of granular oxides segregated on the surface of the Invar alloy cold-rolled annealed steel coil is greatly reduced, and the corrosion resistance is significantly improved.

[0021] Due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0022] The present invention prevents high-purity manganese flakes from forming at the air inlet of the bell-type annealing furnace, and utilizes the oxidizability of Mn to preferentially consume residual oxygen in the atmosphere in the furnace; at the same time, the flow rate of high-purity hydrogen and the dew point temperature in the annealing are effectively controlled to ensure that gases such as H2O, CO and CO2 generated by the reduction reaction during annealing are discharged from the bell-type annealing furnace in time, thereby controlling the oxygen partial pressure in the annealing atmosphere at a relatively low level; further, the present invention optimizes the annealing temperature and the annealing time, while achieving full recrystallization of the austenite of the Invar alloy, minimizing the selective oxidation kinetics on the surface of the steel coil, and finally achieving the purpose of reducing the coverage rate of granular oxides precipitated on the surface of the Invar alloy cold-rolled annealed plate and improving the corrosion resistance, thereby providing a more excellent surface quality for its storage, transportation and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0024] Figure 1 These are actual photos of the Invar alloy cold-rolled annealed steel coils of Example 1 and Comparative Example 1.

[0025] Figure 2 The SEM images of the surface morphology of the Invar alloy cold-rolled annealed steel coils of Example 1 and Comparative Example 1 are shown.

[0026] Figure 3The distribution diagram of Si% in the thickness direction in the near-surface area of ​​the Invar alloy cold-rolled annealed steel coils of Examples 1-3 and Comparative Examples 1-4.

[0027] Figure 4 The distribution diagram of Mn% along the thickness direction in the near-surface area of ​​the Invar alloy cold-rolled annealed steel coils of Examples 1-3 and Comparative Examples 1-4. DETAILED DESCRIPTION

[0028] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0029] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments. In order to further understand the present invention, the preferred embodiments of the present invention are described below in conjunction with the embodiments, but it should be understood that these descriptions are only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention.

[0030] The Invar alloy cold-rolled steel coils in the following examples and comparative examples are produced from the same batch and have a thickness of 0.7 mm.

[0031] Example 1

[0032] A method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed sheet comprises the following steps:

[0033] (1) stacking three rolls of Invar alloy cold-rolled steel coils on a bell-type annealing furnace, placing high-purity manganese sheets with a purity of 99.9% at the gas inlet, and the weight ratio of the high-purity manganese sheets to the Invar alloy cold-rolled steel coils is 1:250;

[0034] (2) Before annealing, use high-purity hydrogen at 110m 3 / h flow rate to purge the bell-type annealing furnace for 45min; during isothermal annealing, 15m 3 High-purity hydrogen was introduced into the bell-type annealing furnace at a flow rate of / h, with a dew point of -50°C at the air inlet. The steel was annealed at a temperature of 680°C for 9h to obtain Invar alloy cold-rolled annealed steel coils.

[0035] Example 2

[0036] A method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed sheet comprises the following steps:

[0037] (1) stacking three rolls of Invar alloy cold-rolled steel coils on a bell-type annealing furnace, placing high-purity manganese sheets with a purity of 99.9% at the gas inlet, and the weight ratio of the high-purity manganese sheets to the Invar alloy cold-rolled steel coils is 1:300;

[0038] (2) Before annealing, use high-purity hydrogen at 110m 3 / h flow rate to purge the bell-type annealing furnace for 45min; during isothermal annealing, 15m 3 High-purity hydrogen was introduced into the bell-type annealing furnace at a flow rate of / h, with a dew point of -50°C at the air inlet. The steel was annealed at a temperature of 680°C for 9h to obtain Invar alloy cold-rolled annealed steel coils.

[0039] Example 3

[0040] A method for improving the surface corrosion resistance of an Invar alloy cold-rolled annealed sheet comprises the following steps:

[0041] (1) stacking three rolls of Invar alloy cold-rolled steel coils on a bell-type annealing furnace, placing high-purity manganese sheets with a purity of 99.9% at the gas inlet, and the weight ratio of the high-purity manganese sheets to the Invar alloy cold-rolled steel coils is 1:250;

[0042] (2) Before annealing, use high-purity hydrogen at 120m 3 / h flow rate to purge the bell-type annealing furnace for 45min; during isothermal annealing, the flow rate is 20m 3 High-purity hydrogen was introduced into the bell-type annealing furnace at a flow rate of / h, with a dew point of -50°C at the air inlet. The steel was annealed at a temperature of 680°C for 9h to obtain Invar alloy cold-rolled annealed steel coils.

[0043] In order to further verify that the technical solution of the present invention has a better effect, the following is a detailed description with reference to Example 1 and combined with a comparative example.

[0044] Comparative Example 1

[0045] The annealing method of Invar alloy cold rolled steel coil comprises the following steps:

[0046] Three rolls of 0.7 mm thick Invar alloy cold-rolled steel coils were stacked on the bell-type annealing furnace. Before annealing, high-purity hydrogen was used at 75 m 3 / h flow rate to purge the bell-type annealing furnace for 20 minutes; during isothermal annealing, 8m 3 High-purity hydrogen is introduced into the bell-type annealing furnace at a flow rate of / h, and the dew point at the air inlet is -35°C; annealing treatment is carried out at 750°C for 15h to obtain Invar alloy cold-rolled annealed steel coils.

[0047] Comparative Example 2

[0048] Compared with Example 1, the difference is that in step (1), no high-purity manganese sheet is placed at the air inlet, and other conditions are the same as in Example 1.

[0049] Comparative Example 3

[0050] Compared with Example 1, the difference is that in step (2), high-purity hydrogen is used at 90m 3 The bell-type annealing furnace was purged at a flow rate of 1.040 / h, and other conditions were the same as those in Example 1.

[0051] Comparative Example 4

[0052] Compared with Example 1, the difference is that in step (2), the dew point at the air inlet is -35°C, and the other conditions are the same as those in Example 1.

[0053] The performance tests of the Invar alloy cold-rolled annealed steel coils prepared in the above examples and comparative examples were respectively carried out. The actual photos and surface morphology SEM images of the Invar alloy cold-rolled annealed steel coils of Example 1 and Comparative Example 1 are shown in FIG. Figure 1 , Figure 2 As shown; the distribution results of Si and Mn% in the near-surface area of ​​the Invar alloy cold-rolled annealed steel coils of Examples 1-3 and Comparative Examples 1-4 along the thickness direction are shown respectively Figure 3 , Figure 4 shown.

[0054] from Figure 1 and Figure 2 It can be seen that compared with Example 1, some areas on the surface of the Invar alloy cold-rolled annealed steel coil of Comparative Example 1 show a "blackening" or "diffuse reflection" phenomenon visible to the naked eye, which proves that the method provided in Example 1 can effectively improve the corrosion resistance of the surface of the Invar alloy cold-rolled annealed steel coil. Figure 2 It can be seen that at the microscopic level, it appears as dense granular oxides mainly distributed along the austenite grain boundaries as described in Table 1.

[0055] from Figure 3 and Figure 4 It can be seen that, compared with Comparative Examples 1-4, the Si and Mn% in the near-surface region of the Invar alloy cold-rolled annealed steel coils of Examples 1-3 are significantly reduced, which corresponds to the significant decrease in the density of surface oxide particles.

[0056] The room temperature test was carried out using an energy dispersive spectrometer (EDS) of a scanning electron microscope (SEM). The composition analysis results of the surface particles (Spect1 and 2) and the interior of the grains (Spect3) in Comparative Example 1 are shown in Table 1.

[0057] Table 1

[0058]

[0059]

[0060] From Table 1, we can see that Figure 2 The black particles in the SEM surface morphology of Comparative Example 1 are oxides of Si and Mn, and the Ni% in the matrix grains is 36.96%.

[0061] Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas, including the best mode, and also enable any technician in the field to practice the present invention, including making and using any device or system, and implementing any combined method. It should be pointed out that for ordinary technicians in this technical field, without departing from the principle of the present invention, the present invention can also be improved and modified, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. The scope of patent protection of the present invention is defined by the claims and may include other embodiments that can be thought of by those skilled in the art. If these other embodiments have structural elements similar to the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil, characterized in that: The following steps are involved: Before annealing, the manganese sheet is placed at the air inlet of the annealing furnace, and the Invar alloy cold-rolled steel coil is placed on the annealing furnace table; Before annealing, the annealing furnace is purged with high-purity gas; During the annealing process, high-purity gas is introduced into the annealing furnace, and the dew point at the air inlet is controlled to be ≤-45°C, the annealing temperature is controlled to be 650-700°C, and the annealing time is 8-10h.

2. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: The manganese flakes are high-purity manganese flakes with a purity of ≥99%.

3. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: The weight ratio of the manganese sheet to the Invar alloy cold-rolled steel coil is 1:(200-400).

4. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: The annealing furnace is a bell-type annealing furnace.

5. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: Before annealing, when purging the annealing furnace, the flow rate of high-purity gas is ≥100m 3 / h, purge time ≥30min.

6. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: During the annealing process, the flow rate of high-purity gas is ≥10m 3 / h.

7. A method for improving the surface corrosion resistance of Invar alloy cold-rolled annealed steel coil according to claim 1, characterized in that: Before and during the annealing process, the high-purity gas is high-purity hydrogen.

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