A grain boundary engineering processing method for improving corrosion resistance of qn1803 nitrogen-containing austenitic stainless steel

Abstract

The application discloses a grain boundary engineering processing method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel. The processing method is to control the original structure and small deformation cold processing to make the proportion of Sigma 3 grain boundaries in the QN1803 nitrogen-containing austenitic stainless steel not less than 60%, and the proportion of coherent Sigma 3 grain boundaries in the Sigma 3 grain boundaries not less than 70%. The method not only improves the proportion of Sigma 3 grain boundaries in the QN1803 nitrogen-containing austenitic stainless steel, but also improves the proportion of coherent Sigma 3 grain boundaries, effectively improves the corrosion resistance of the material, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of metal material deformation and heat treatment processes, in particular to a grain boundary engineering processing method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel. BACKGROUND

[0002] Austenitic stainless steel is widely used in the fields of petroleum, chemical industry, spaceflight, energy and construction due to its good toughness, uniform corrosion resistance and local corrosion resistance. The chromium-nickel austenitic stainless steel represented by 304 becomes the largest global production and the most widely used variety in the world due to its excellent comprehensive performance, and its production accounts for about 50% of the total global production of austenitic stainless steel. The noble metal nickel content of these traditional stainless steels is generally above 8%, and the price of metal nickel is relatively high, so the development of nickel-saving or nickel-free stainless steel is one of the important research directions in the world. The nitrogen-containing low-nickel austenitic stainless steel QN1803 is one of the typical materials of this new type of stainless steel, but QN1803 also has the grain boundary corrosion problem of traditional austenitic stainless steel, which needs to be solved urgently.

[0003] Grain boundary engineering (GBE) is also commonly known as grain boundary design and control. In the past 30 years, this method has been extensively studied in medium and low stacking fault energy face-centered cubic metals, including austenitic stainless steel, nickel-based high-temperature alloy, copper alloy and lead-calcium-based alloy. The key to grain boundary control is to regulate the grain boundary characteristic distribution, and to make the grain boundary characteristic distribution (GBCD) contain a high proportion of twin grain boundaries (also known as Σ3 grain boundaries) through appropriate thermal mechanical processing.

[0004] Twin grain boundaries are divided into coherent twin grain boundaries (coherent Σ3 grain boundaries) and incoherent twin grain boundaries (also known as incoherent Σ3 grain boundaries), among which the coherent twin grain boundaries have the lowest energy and the best corrosion resistance among all grain boundary types. However, the traditional grain boundary engineering is characterized by the misorientation three-parameter method, which cannot distinguish between coherent twin grain boundaries and incoherent twin grain boundaries, and cannot accurately determine the proportion of coherent twin grain boundaries in the material, which makes the traditional grain boundary engineering research unable to optimize the process parameters to obtain a high proportion of coherent twin grain boundaries.

[0005] In recent years, Wang Weiguo et al. innovatively proposed a new method for measuring GBCD based on grain boundary interface matching, which realizes the measurement of the proportion of coherent twin grain boundaries in the material. Based on the grain boundary interface matching measurement technology, the application provides a grain boundary engineering processing method for improving the proportion of coherent twin grain boundaries in QN1803 nitrogen-containing austenitic stainless steel, which effectively improves the corrosion resistance of the material. SUMMARY

[0006] The purpose of the present invention is to provide a grain boundary engineering processing method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel. Without changing the raw materials or increasing the factory production equipment, the proportion of cosine twin boundaries in QN1803 nitrogen-containing austenitic stainless steel is increased, the distribution of grain boundary characteristics in the material is optimized, and the problem of insufficient corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel is solved. The method provides processing optimization parameters for engineering services and has engineering application value.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A grain boundary engineering method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel is disclosed. The method comprises: controlling the original microstructure and performing small deformation cold working to ensure that the proportion of Σ3 grain boundaries in the QN1803 nitrogen-containing austenitic stainless steel is not less than 60%, and the proportion of coherent Σ3 grain boundaries in the Σ3 grain boundaries is not less than 70%. The method comprises the following steps:

[0009] (1) Original structure control: First, the original structure of the QN1803 nitrogen-containing austenitic stainless steel plate is subjected to solid solution treatment at a solid solution temperature of 1100-1150°C and a solid solution time of not less than 30 minutes. Then, the QN1803 nitrogen-containing austenitic stainless steel after solid solution is subjected to large deformation rolling at room temperature. The deformation in the thickness direction is 50-80%. During the rolling process, it is necessary to ensure that the rolled surface of the material is uniformly stressed. Then, high-temperature short-time annealing is performed at an annealing temperature of 1010-1130°C and an annealing time of not less than 1 minute to finally obtain the pretreated material.

[0010] (2) Small deformation cold working: The above-mentioned pretreated materials are subjected to small deformation rolling at room temperature, with the deformation in the thickness direction being 3% to 10%. During the rolling process, it is necessary to ensure that the rolled surface of the material is uniformly stressed. The material is then annealed at a temperature of 1050°C to 1130°C for a time of not less than 2 minutes to obtain QN1803 nitrogen-containing austenitic stainless steel with improved corrosion resistance.

[0011] Furthermore, the large deformation rolling in step (1) is single-pass or multi-pass rolling, and the rolling direction is any one of the following: a) perpendicular to the original rolling direction; b) parallel to the original rolling direction; c) cross rolling using the method a) or b).

[0012] Furthermore, in step (2), when performing 3% to 10% small deformation rolling, the rolling direction should be consistent with the rolling direction in step (1), and the number of rolling passes should not exceed 5 times.

[0013] Furthermore, in steps (1) and (2), the surface of the material is cleaned before solution treatment, cold rolling treatment and annealing treatment to ensure that the surface of the material is free of contamination before treatment.

[0014] In step (2), when small deformation amount room temperature rolling is performed, the rolling force on the rolling surface of the pretreated material should be uniform to make the deformation energy storage of the material consistent and the proportion of the special grain boundary on the rolling surface of the material consistent; meanwhile, because the recrystallization nucleation and growth in the subsequent annealing process after cross-rolling is random orientation, the grains of different orientations compete in the growth process, which will affect the formation of the high proportion of Σ3 grain boundaries; when 3% to 10% small deformation amount room temperature rolling is performed, the rolling direction should be consistent with the room temperature rolling direction in step (1).

[0015] After step (2), the proportion of Σ3 grain boundaries in the QN1803 nitrogen-containing austenitic stainless steel is not less than 60%, and the proportion of coherent Σ3 grain boundaries in the Σ3 grain boundaries is not less than 70%.

[0016] The present application is mainly used for the production of QN1803 nitrogen-containing austenitic stainless steel, has the advantages of simple process method and low cost, and combines the grain boundary interface matching method to realize accurate determination of the proportion of coherent twin grain boundaries in the material, so that the grain boundary engineering processing parameters when the proportion of coherent twin grain boundaries in the material reaches the highest are obtained.

[0017] The present application not only improves the proportion of Σ3 grain boundaries in the QN1803 nitrogen-containing austenitic stainless steel, but also improves the proportion of coherent Σ3 grain boundaries, effectively improves the corrosion resistance of the material, and has a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 Different types of grain boundary reconstruction maps and orientation difference distribution maps of QN1803 high-nitrogen low-nickel austenitic stainless steel amorphous grain boundary engineering treatment samples (numbered N-GBE) and grain boundary engineering treatment samples (numbered GBE) obtained by backscattering electron diffraction (EBSD); wherein a and c are the grain boundary reconstruction map and the orientation difference distribution map of the N-GBE sample, b and d are the grain boundary reconstruction map and the orientation difference distribution map of the GBE sample, and the GBE sample is obtained by using the method of example 1 of the present application.

[0019] Figure 2 The grain boundary surface distribution map of the QN1803 high-nitrogen low-nickel austenitic stainless steel sample with an orientation difference of <1 1 1> / 60° (Σ3 grain boundary); wherein a is the N-GBE sample, and b is the GBE sample.

[0020] Figure 3 The schematic diagram for half-width W is obtained; wherein a is the N-GBE sample, and b is the GBE sample.

[0021] Figure 4 The comparison diagram of the proportion of Σ3 grain boundaries and coherent twin grain boundaries in the N-GBE and GBE samples.

[0022] Figure 5 The relationship between the weight loss per unit area and the exposure time of N-GBE and GBE samples.

[0023] Figure 6 The surface morphology of the samples after corrosion in nitric acid-hydrofluoric acid corrosion solution for 2h. Wherein, a is the N-GBE sample, and b is the GBE sample.

[0024] Figure 7 The surface morphology of the samples after corrosion in nitric acid-hydrofluoric acid corrosion solution for 6h. Wherein, a is the N-GBE sample, and b is the GBE sample.

[0025] Figure 8 The surface morphology of the samples after corrosion in nitric acid-hydrofluoric acid corrosion solution for 24h. Wherein, a is the N-GBE sample, and b is the GBE sample. DETAILED DESCRIPTION

[0026] The application will be further described in detail below in combination with the drawings and specific examples.

[0027] Example 1

[0028] A grain boundary engineering processing method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel, comprising the following steps:

[0029] Step 1: original structure regulation

[0030] Cut the high-nitrogen low-nickel austenitic stainless steel QN1803 into a size of 25mm×10mm×4.8mm strip by wire cutting, and use an annealing furnace with a model of OTF-1200X for solid solution treatment at 1100℃ for 30min, and then air cool to room temperature. Send the solid solution treated sample into the roller in multiple passes along one direction, and roll to 1.92mm thick, and use the annealing furnace for annealing at 1060℃ for 3.5min; air cool to room temperature to obtain a pretreated sample.

[0031] Step 2: small deformation cold working

[0032] The pretreated sample is subjected to small deformation treatment, and its thickness is rolled from 1.92mm to 1.84mm (rolling deformation amount is 4%), and annealed at 1100℃ for 3.5min, and air cooled to room temperature, and the product is marked as GBE.

[0033] The GBCD was characterized as follows: the above-mentioned GBE sample was ground using a metallographic sandpaper (800#-5000#), and after the sample surface to be tested was ground to be bright and scratch-free, the sample was electrolytically polished, and after polishing, deionized water and alcohol were used for cleaning. The above-mentioned sample was subjected to EBSD data collection using a FEI-Nova Nano SEM450 thermal field emission scanning electron microscope equipped with a HKL-NordlysMax2 electron backscatter diffraction accessory. The electric field acceleration voltage was 20 kv, the beam spot size was 5.5, and the step size was 2 μm.

[0034] The grain boundary data obtained by the above operation was filtered with a fixed rotation axis and rotation angle. First, <111> was selected as the rotation axis, and the axis angle deviation was ±8°. Then, the rotation angle was filtered, and the angle deviation was ±2.5°. The angle deviation is more stringent than the angle of the Palumbo-Aust criterion, ensuring that the data obtained is more reliable.

[0035] Then, the five-parameter analysis method (FPA) was used to analyze and process the grain boundary plane distribution (GBPD) of the above-mentioned filtered grain boundaries with fixed misorientation one by one, and a grain boundary plane intensity distribution map was given, which is generally projected in (0 0 1). Single-section five-parameter analysis method is based on stereology principles proposed by Rohrer et al. in 2004, which considers the influence of preferred distribution in the processing. The true length of all grain boundaries on any crystal plane (h k l) is corrected, and the area distribution intensity of the grain boundary, i.e. the grain boundary plane intensity distribution, is characterized by the multiples of random distribution (MRD) of the corrected true length and the grain boundary length randomly distributed on the crystal plane.

[0036] After obtaining the grain boundary plane distribution of the grain boundaries with fixed misorientation, the "grain boundary plane matching" (GBIC) method proposed by Wang Weiguo was used to determine the interface matching type of the material grain boundaries.

[0037] Grain boundary plane matching is usually determined according to the grain boundary plane distribution map, and whether there is a {h1 k1 l1} / {h2 k2 l2} interface matching characteristic grain boundary is usually determined according to formulas (1) and (2)

[0038]

[0039]

[0040] m = cos θ, n = sin θ, and u, v, w in formula (2) are the misorientation rotation axis indices, which are normalized results, i.e. u 2 + v 2 + w2 = 1, θ is the misorientation rotation angle. The following equation (3) is used.

[0041] F = AP + B(PM / W) (3)

[0042] The proportion of grain boundaries with {h1k1l1} / {h2k2l2} interface matching characteristics in specific misorientation grain boundaries in QN1803 high-nitrogen low-nickel austenitic stainless steel (the orientation difference of coherent twin grain boundaries is <1 1 1> / 60°, and has {11 1} / {1 11} interface matching characteristics) is calculated. Wherein, A=0.4888, B=0.0393 are fitting parameters, P is the proportion of fixed misorientation grain boundaries in total grain boundaries, W is the half width of the weighted average misorientation Gaussian distribution curve of any one group of fixed misorientation grain boundaries, M is the distribution intensity of {h1k1l1} and {h2k2l2} in FPA distribution map; F represents the proportion of grain boundaries with {h1k1l1} / {h2k2l2} interface matching characteristics in fixed misorientation grain boundaries.

[0043] P value calculation: for Σ3 grain boundaries <1 1 1> / 60°, the percentage of <1 1 1> axis filtering length is 62.201%, the percentage of <1 11> / 60° angle filtering length is 95.705%, and the P value is the proportion of Σ3 grain boundaries (the orientation difference is <1 1 1> / 57.5° to <1 1 1> / 62.5°), P=62.201%×95.705%=59.53%.

[0044] W value calculation: the W value is determined by 11 points of misorientation distribution near <1 1 1> / 60° at intervals of 0.1°, and the half width W can be obtained by Gauss function fitting.

[0045] Result analysis:

[0046] P=59.53%; it can be known from Figure 2 b that the maximum MRD is 5.56: W=0.5825.

[0047] By formula (3), the obtained results are substituted into, F=51.429% can be obtained, which is the proportion of coherent Σ3 grain boundaries with {1 11} / {11 1} matching. The proportion of Σ3 grain boundaries is 62.5% (the value obtained by using Brandon criterion), and the proportion of coherent twin grain boundaries in Σ3 grain boundaries is 86.3%.

[0048] Example 2

[0049] Comparison of intergranular corrosion resistance by surface corrosion

[0050] The experiment adopts nitric acid-hydrofluoric acid corrosion test method, and the corrosion liquid is 10% nitric acid and 3% hydrofluoric acid (weight ratio). The test corrosion solution is prepared according to the proportion. After preparation, 30ml of corrosion liquid is poured into the sample corrosion container bottle each time, then the sample is bundled with polytetrafluoroethylene fine wire, and the sample is suspended in the middle position of the solution for corrosion test. The experimental sample is corroded in batches for 2h, 4h, 6h, …, 30h (a total of fifteen groups) with time gradient. First, the N-GBE sample and the GBE sample are ground in turn using 400#, 600#, 800#, 1000#, 1500#, 2000#, 3000#, 5000# and 7000# metallographic sandpaper to remove the oxide layer, and then mechanically polished using diamond polishing paste. The sample surface is first scrubbed with a cotton swab under running water, then ultrasonically cleaned with alcohol, and dried with an electric hair dryer. To facilitate subsequent SEM observation, the sample is electrolytically polished. After electrolytic polishing, the total surface area (mm 2 ) and total weight (mg) of each sample before corrosion are measured. The fifteen groups of samples after electrolytic polishing are classified and placed in the corrosion liquid respectively. After taking out, they are cleaned with deionized water and then ultrasonically cleaned with alcohol. After corrosion, they are measured again. The reduced weight (mg) of the sample after corrosion is calculated. The weight loss per unit area of the N-GBE sample and the GBE sample is calculated by dividing the reduced weight by the surface area of the sample. The relationship between the weight loss per unit area and the exposure time is shown in Figure 5 . Then the surface morphology is observed by SEM, and the results are shown in Figure 6 、 Figure 7 、 Figure 8 . It can be seen from the comparison that under the same conditions, the GBE sample treated by grain boundary engineering has significantly enhanced resistance to grain boundary corrosion.

Claims

1. A grain boundary engineering method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel, characterized in that: By controlling the original structure and performing small deformation cold working, the proportion of Σ3 grain boundaries in QN1803 nitrogen-containing austenitic stainless steel is made to be not less than 60%, and the proportion of coherent Σ3 grain boundaries in Σ3 grain boundaries is not less than 70%. The processing method is as follows: (1) Original structure control: First, the original structure of QN1803 nitrogen-containing austenitic stainless steel plate is subjected to solid solution treatment at a solid solution temperature of 1100~1150℃ and a solid solution time of not less than 30min. Then, the solid solution QN1803 nitrogen-containing austenitic stainless steel is subjected to large deformation rolling at room temperature with a deformation of 50% to 80% in the thickness direction. Then, high temperature short time annealing is performed at an annealing temperature of 1010~1130℃ and an annealing time of not less than 1min to finally obtain the pretreated material. (2) Small deformation cold working: The above-mentioned pretreated materials are subjected to small deformation rolling at room temperature, with the deformation in the thickness direction being 3% to 10%. During the rolling process, the material is ensured to be uniformly stressed on the rolled surface. The material is then annealed at a temperature of 1050°C to 1130°C for a time of not less than 2 minutes to obtain QN1803 nitrogen-containing austenitic stainless steel with improved corrosion resistance.

2. A grain boundary engineering method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel according to claim 1, characterized in that: The large deformation rolling in step (1) is a single-pass or multi-pass rolling, and the rolling direction is any of the following: a) perpendicular to the original rolling direction; b) parallel to the original rolling direction; c) Cross rolling is carried out using the methods a) and b).

3. A grain boundary engineering method for improving the corrosion resistance of QN1803 nitrogen-containing austenitic stainless steel according to claim 2, characterized in that: In step (2), when performing small deformation rolling with a deformation amount of 3% to 10%, the rolling direction should be consistent with the rolling direction in step (1), and the number of rolling passes should not exceed 5 times.

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