Regular Article
Determination of Micro-alloyed Elements Containing in the Solid Solution Phase in High Tensile Steel
2014 Volume 54 Issue 4 Pages 880-884
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2014 Volume 54 Issue 4 Pages 880-884
For the steel making control with micro-alloyed elements, it is essential to accurately analyze the distribution of those elements between different metallurgical phases in steel. The micro-alloyed elements contained in precipitates have been analyzed by conventional chemical or electrochemical procedures, selectively dissolving the Fe matrix by electrolysis, and separating precipitates as insoluble particles from the matrix by filtration. But this method has become inadequate to analyze the fine precipitates because some of the fine precipitates extracted are unavoidably uncollected. Hence, we have developed a quantitative analysis for micro-alloyed elements containing in the solid solution phase of steel, named solute elements, by using of analyzing a portion of the electrolytic solution. By electrolysis, solute elements are dissolved into the electrolyte, and the precipitates remain on the surface of the sample as insoluble particles. So, the analysis of the electrolytic solution during or after electrolysis enables the determination of solute elements directly. For certified reference materials, the sum total of concentration of solute element analyzed by this method and the precipitate analyzed by conventional method suits a certified value substantially on the micro-alloyed element. However, for the samples that contain fine precipitates, they are not in agreement with the total content obtained by the spark-OES. It is estimated that the precipitates not collected by filtration in the conventional method cause the disagreement. It shows the proposed method, which analyzes solute elements directly, is useful for estimating the distribution of the micro-alloyed elements between different metallurgical phases.
Accompanying the technology trends of weight saving in automobile bodies aimed at improvement in collision safety and fuel consumption performance, the needs for high tensile strength steel are growing every year. Responding to this, steel makers are energetically developing high tensile strength steels effectively using grain and precipitate control technologies. Among methods of strengthening steel, high tensile strength can be expected with the precipitation strengthening method by addition of a trace element (micro-alloyed element), and very high tensile strength can be obtained by refining those precipitates. Outstanding high tensile strength steel sheets produced by this strengthening method have already been put to practical use to date. Since the precipitates contained in these precipitation strengthening steels are refined to the nanometer size, it is important for the development of precipitation strengthening steels to grasp the state of existence of micro-alloys in steel and to clarify the size and the amount of fine precipitates. Moreover, as micro-alloyed elements are rare metals such as titanium, niobium, and vanadium which readily combine with carbon or nitrogen, from the viewpoint of cost, the recent sharp rise in the price of these elements has heightened the necessity of maximizing the precipitates of micro-alloys in steel.
The observation and quantitative analysis of precipitates in steel have been examined in detail up to the present by electron microscopy or an electrolytic extraction method, and elucidation of the actual condition of nanometer-sized fine precipitates, including their size, composition, and other features is advancing.1) On the other hand, chemical methods, beginning with the electrolytic extraction method in a nonaqueous solvent, have been used over a long period of time as a method of analyzing the amount of the precipitates in steel.2) Electrolytic extraction in a nonaqueous solvent is considered to enable stable extraction of almost all precipitates as insoluble particles from an iron matrix in the electrolyte. However, when this method is applied to analysis of fine precipitates, it has been pointed out that the underestimated results have been provided due to chemical dissolution of precipitates to the electrolyte and collection leakage at the time of the filtration.3) To solve this problem, a technology which enables correct quantification of the amount of micro-alloyed elements in the solid solution phase or the precipitation phase has been needed.
This report suggests a newly-developed method in which micro-alloyed elements can be totally quantified both in the matrix phase (solute elements) and in the precipitates (precipitated elements), in contrast to a conventional extraction method that determines only the precipitated elements. A unique feature of this method is that a solid-liquid separation can be carried out without any filtration, by sampling a small amount of the electrolyte while almost all the precipitates remains on the sample surface. In a steel sample containing fine precipitates, this method would give an analytical result more precisely than the conventional method. It is also reported that the distribution of micro-alloyed elements between the solid solution/precipitation phases in steel sheet can be evaluated by using this new method together with the conventional extraction method.
The electrolyte was prepared by mixing commercially-available special grade reagents. The EDTA solution was prepared by dissolving ethylenediamine tetraacetic acid diammonium salt (Dojindo Laboratories) in pure water. The metal standard stock solution (Kanto Kagaku), an electrolyte in which the steel certified reference material JSS1000-1 (high purity iron) was electrolyzed as a matrix, and EDTA were used in preparing the analytical curve solution.
2.2. ApparatusA spark discharge emission spectrometer PDA-8000 (Shimadzu Corporation) was used in elementary analysis of the sample materials. An ICP emission spectrometer ICPV-1017 (Shimadzu Corporation) and ICP mass spectrometer PQ ExCell (Jerrel-Ash) were used in the elementary analysis of the solution. Table 1 shows the analytical conditions that were used.
| Spark discharge emission spectrometer | Instrument | PDA-8000 |
| Ar flow rate | 5.0 dm3/min | |
| Pre-burn time | 7.0 sec | |
| Integration time | 3.6 sec | |
| Measurement wavelength (nm) | C:193.0 Si:212.4 Mn:293.3 Cu:224.2 Ni:231.6 Cr:267.7 Ti:337.2 V:311.1 Nb:313.0 Mo:202.0 Al:394.4 | |
| ICP emission spectrometer | Instrument | ICPV-1017 |
| RF power | 1.2 kW | |
| Coolant gas flow rate | 14 dm3/min | |
| Auxiliary gas flow rate | 1.5 dm3/min | |
| Carrier gas flow rate | 1.0 dm3/min | |
| Misting chamber | Cyclone chamber | |
| Measurement wavelength (nm) | Ti:334.9 V:311.1 Nb:319.5 Mo:202.0 Fe:259.9 | |
| ICP mass spectrometer | Instrument | PQ Excell |
| RF power | 1350 W | |
| Coolant gas flow rate | 13 dm3/min | |
| Auxiliary gas flow rate | 0.92 dm3/min | |
| Nebulizer gas flow rate | 0.90 dm3/min | |
| Isotopes (m/z) | 49Ti 51V 93Nb 95Mo 57Fe |
A methanol solution containing 10% acetyl acetone and 1% tetramethylammonium chloride (hereinafter, AA based electrolyte) was used in the electrolyte, and current density 20 mAcm–2 constant-current electrolysis was performed. The electrolyzed weight of a sample was approximately 0.5 g, and the amount of electrolyte was approximately 300 ml. Nuclepore® filters (GE Healthcare Co.) with a pore size of 0.2 μm were used in filtration.
2.4. Samples and PretreatmentThe sample materials used in this research were low carbon steel (0.05%C-1.80%Mn-0.201%Ti-0.0039%N), in which nano-precipitation of carbon nitride alloys occurs, and the steel certified reference materials for instrumental analysis of composition, as shown in Table 2. The former steel was subjected to the heat treatment shown in Table 3 after hot-rolling in the austenite range. Hereinafter, the samples which received this heat treatment are called lab hot-rolled samples. The steel certified reference materials were prepared to 30 mm in diameter and 3 mm in thickness, and the lab hot-rolled samples were prepared to 30 mm × 20 mm × 2 mm3. Thin film specimens of the lab hot-rolled samples were prepared by electrolytic polishing, and the average diameter of the precipitates was measured with a transmission electron microscope (TEM). An example of a TEM photograph is shown in Fig. 1, and the average particle diameters of the precipitates are shown in Table 3.
| Unit: mass% | |||||||
| JSS No | C | Si | Mo | V | Ti | Nb | Others |
| JSS165-4 | 0.051 | (0.01) | 0.015 | 0.30 | 0.015 | 0.21 | Al0.027, As0.13, Sn0.12, B0.0009 |
| JSS1007-1 | 0.0024 | 0.006 | 0.030 | 0.030 | 0.021 | 0.030 | Cu0.049, Ni0.054, Cr0.053, Zr0.030, Co0.035, W0.030 |
| JSS1008-1 | 0.0032 | 0.009 | 0.062 | 0.060 | 0.052 | 0.060 | Cu0.099, Ni0.103, Cr0.102, Zr0.011, Co0.065, W0.058 |
| Sample | Heat treatment | Average size of particles (nm) |
|---|---|---|
| A | 450°C × 1 h | – |
| B | 550°C × 1 h | 1.7 |
| C | 600°C × 1 h | 2.6 |
| D | 650°C × 1 h | 4.5 |
TEM image of fine precipitates in steel B. The average particle diameters of the precipitates are shown in Table 3.
The outline of the sample preparation methods is shown in Fig. 2. After electrolyzing a steel sample for a definite period of time in a nonaqueous solvent system electrolyte, the electrolyte (about 0.5 ml or about 5 ml) was extracted, and it was considered as the sample for determination of solute elements. In the case of extracting the electrolyte during or after the electrolysis of a steel sample, almost all the precipitates are attached to the surface of the sample in the electrolyte thereof. Steel samples to which insoluble particles had adhered were carefully taken from the electrolyte and placed in a beaker of methanol. The insoluble particles on the steel sample were distributed in the methanol by ultrasonic vibration. The insoluble particles distributed in the methanol were collected on a filter as the sample for determination of precipitated elements, and the filtrate was considered as the sample for determination of precipitated elements which passed the filter.
Procedure of sample preparation for the determination of solute and precipitated elements. Electrolyte: AA based electrolyte; current density: 20 mAcm–2; electrolyzed weight: approximately 0.5 g; pore size of filter: 0.2 μm.
The target element i and matrix element (Fe) in the extracted electrolyte were analyzed by ICP emission spectrometer or ICP mass spectrometer, and content of solute i was calculated by substituting those contents in formula (1).
| (1) |
In formula (1), Ci and Cfe stand for the concentration of i element and iron in the analytical solution. Wfe stands for the iron content in a steel sample, and is the value which is obtained by subtracting from 100% the sum total value of the content of elements other than the iron analyzed by the spark discharge emission spectrometer. With the certified reference materials except JSS165-4, the amount of extracted electrolyte for determination of solute elements was 0.5 ml, and Ci and Cfe were analyzed as follows. After the extraction of the electrolyte, the proper quantity of EDTA solution was added, and the resulting solution was heated in hot water. After visual confirmation that the color of the solution changed from reddish-brown to yellow (because the iron (III) content changes to EDTA complex from the acetylacetone complex), heating of the solution in the hot water was continued until the methanol in the solution had evaporated. The residue was then diluted with pure water to about 20 ml and measured with the ICP mass spectrometer.
With certified reference material JSS165-4, about 5 ml of electrolyte for determination of solute elements was dried automatically, after which the residue was dissolved by heating with mixed acid (nitric acid, perchloric acid, and sulfuric acid) until white sulfuric acid smoke appeared. After cooling by ice melting, hydrochloric acid was added to the solution, which was then diluted with pure water to a fixed quantity, and Ci and Cfe were analyzed by ICP emission spectrometer. The mixed acid dissolving method was applied with JSS165-4 in order to dissolve and detect all precipitates, including the precipitates contained in the extracted electrolyte.
2.5.2. Determination of Precipitated Elements on FilterBoth insoluble particles and the filter were put into a beaker and dissolved by heating with mixed acid (nitric acid, perchloric acid, and sulfuric acid) until white sulfuric acid smoke appeared. After cooling by ice melting, hydrochloric acid was added to that solution, the solution was diluted with pure water to a fixed quantity, and content of precipitated i was analyzed by ICP emission spectrometer.
2.5.3. Determination of Precipitated Elements which Passed FilterAfter the filtrate was dried, its residue was dissolved by heating with mixed acid (nitric acid, perchloric acid, and sulfuric acid) until white sulfuric acid smoke appeared. After cooling by ice melting and addition of hydrochloric acid, the solution was diluted with pure water to a fixed quantity, and content of precipitated i was analyzed by ICP emission spectrometer. However, since i element in the filtrate consisted of the electrolyte component in addition to the precipitates which passed the filter, the amount (Mi) of precipitated i element after removing the electrolyte component was calculated by formula (2). In formula (2), Mi' and Mfe' stand for the amount of i element and iron in the filtrate.
| (2) |
When a steel sample is electrolyzed, many insoluble particles can be observed adhering on the surface of a sample. On the other hand, since solute elements dissolve into an electrolyte, if it is assumed that most precipitates have adhered to the sample surface as insoluble particles, the solute elements can be measured separately by analyzing the electrolyte from which the sample was removed after electrolysis. The analytical results for micro-alloyed elements in the steel certified reference materials obtained by this method are shown in Table 4.
| Unit: mass% | ||||||
| Micro-alloyed elements | CRMs | Analytical results | Certified values | |||
| Solute | Precipitates | Total | ||||
| On filter | In filtrate | |||||
| Ti | JSS 1007-1 | 0.002 | 0.021 | 0.000 | 0.023 | 0.021 |
| JSS 1008-1 | 0.033 | 0.019 | 0.000 | 0.052 | 0.052 | |
| V | JSS 165-4 | 0.224 | 0.076 | 0.000 | 0.300 | 0.30 |
| JSS 1007-1 | 0.030 | 0.001 | 0.000 | 0.031 | 0.030 | |
| JSS 1008-1 | 0.059 | 0.001 | 0.000 | 0.060 | 0.060 | |
| Nb | JSS 165-4 | 0.000 | 0.208 | 0.000 | 0.208 | 0.21 |
| JSS 1007-1 | 0.014 | 0.015 | 0.000 | 0.029 | 0.030 | |
| JSS 1008-1 | 0.047 | 0.014 | 0.000 | 0.061 | 0.060 | |
| Mo | JSS 1007-1 | 0.029 | 0.000 | 0.000 | 0.029 | 0.030 |
| JSS 1008-1 | 0.060 | 0.000 | 0.000 | 0.060 | 0.062 | |
In the analytical result of Nb of JSS165-4, Nb was not detected by solid solution analysis, but the analytical value of the precipitates was mostly in agreement with the certified value. This means that virtually Nb is mostly distributed in a precipitation phase in the metallurgical structure of this sample, and insoluble particles have adhered to the sample surface without being diffused into the electrolytic solution, even if the matrix dissolves by electrolysis. Therefore, it could be confirmed that only the solute element can be quantified by measuring the electrolyte.
3.2. Examination of Sample Solution Preparation MethodIt is necessary to use high sensitivity ICP mass spectrometry in order to quantify solute elements because the content and analysis volume of those elements are so small. However, if an organic solvent, for example, the methanol that is the main component of the electrolyte, is introduced directly into an ICP mass spectrometer, the plasma will become unstable and the accuracy of the spectrometer will be insufficient. In converting organic solvents to aqueous solutions, wet degradation with nitric acid, sulfuric acid, etc. is frequently used, but in this case, measurement accuracy is reduced by contamination during processing, the physical interference of the acid, ionization interference, etc. Therefore, a pretreatment method was established, in which target elements are changed from non-water soluble complexes to water-soluble complexes by adding EDTA, as EDTA has a higher ability of forming complexes with target elements than that of acetylacetone and forms stable aqueous solution complexes. This method, called the EDTA addition method, was applied to certified reference materials. As the sum total of the solid solution value analyzed by the EDTA addition method and the precipitate value analyzed by conventional method agreed with the certified value of each element, as shown in Table 3, it was possible to verify that the EDTA addition method is effective in quantification of the solid solution composition of steel samples. In addition, the EDTA addition method has the advantage of simple and short operation.
3.3. Application to Laboratory Hot Coil SampleThe results of quantification of Ti in lab hot-rolled samples are shown in Fig. 3.
Analytical results of Ti as solute and precipitated element in hot-rolled sheet samples. These samples are low carbon steel (0.05%C-1.80%Mn-0.201%Ti-0.0039%N), and heat treatment conditions after hot-rolling in the austenite range are shown in Table 3.
As with the steel certified reference materials discussed above, in all lab hot-rolled samples, the total of the solute Ti value and the filtered precipitate Ti value and precipitate Ti value in the filtrate agree with the Ti content in the solid sample. On the other hand, in the filtrates of samples B, C and D, omission precipitates which were not observed in the steel certified reference materials were detected. Omission of those precipitates results from the Ti precipitates in the samples being very small, as shown in Table 2. In other words, the reason why conventional precipitate analysis4) shows low values for samples containing fine precipitates is because the fine precipitates are lost to the filtrate. As opposed to this, it is thought that Ti was not detected in the filtrate with the steel certified reference material or sample A because these substances contained virtually no fine precipitates, but contained many large and coarse precipitates like TiN.
In comparison with sample B, with sample C and sample D, few precipitates leaked into the filtrate. In explaining this difference, in addition to the large size of the precipitates in sample C and sample D, it is considered that the captured secondary particles underwent coarsening, enabling easy capture on the filter because the total content of the precipitates in those samples was large, whereas, with sample B, the precipitates were very small and were dissolved by the electrolyte or methanol. This suggested that the analytical value of fine precipitates may be underestimated by conventional electrolytic extraction method which is now widely used. On the other hand, although the influence of chemical dissolution of the precipitates is unavoidable with the method proposed here, it can be said that it is an effective means of evaluating precipitates in specimens, including fine precipitates, and trace elements in the solid solution state.
A determination method of micro-alloyed elements in the matrix phase was established. In outline, this method consists of analyzing both the matrix element and the target elements which are contained in a small amount of extracted electrolyte of the nonaqueous solvent system electrolytic method which is now used as a conventional precipitate analysis method. Operation of the new method is very simple, requiring only addition of a complexing reagent to the electrolyte, removal of the organic solvent by heating, and analysis of a sample solution dissolved in the electrolyte with water. If the analytical value of a solute element is deducted from the total amount obtained by a spark discharge emission spectrometer, etc., the micro-alloyed element contained in precipitates can also be obtained in a simple manner. The greatest advantage of this analysis method is that it gives a more exact value of the amount of solute or precipitates of a micro-alloyed element than the conventional electrolytic extraction - filtration method, even with steel samples containing fine precipitates. It is thought that this method will be an aid in the future development of new steel materials and new processes using fine precipitates.
