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Effect of calcium treatment on inclusions in 38CrMoAl high aluminum steel

  • Yang Li , Changyong Chen EMAIL logo , Cong Zhang , Shuai Ma , Hao Yang , Meng Sun , Haibo Cao and Zhouhua Jiang EMAIL logo
Published/Copyright: June 10, 2024
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High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 43 Issue 1

Abstract

Three parts of the experiment were mainly carried out in this research: the effect of Ca on inclusions in 38CrMoAl high aluminum steel under atmospheric pressure, the effect of pressure on the solubility of Ca in pure iron under pressurized conditions, and the effect of Ca on inclusions in 38CrMoAl high aluminum steel under pressurized conditions. The results indicated that Ca can significantly transform the inclusions such as Al2O3 and MgO·Al2O3 with high melting points into calcium aluminate inclusions with low melting points. It was found that the pressure condition can significantly improve the solubility of Ca in pure iron and molten steel, which provided reliable data reference and guidance for popularizing Ca treatment of steel under pressure conditions in the future. Finally, the mechanism of inclusion transformation after Ca treatment was put forward. In this study, the theoretical calculation of thermodynamics was closely compared with the experimental data. It was proposed that to modify the inclusions in high aluminum steel into calcium aluminate inclusions with low melting points, it is necessary to accurately control Ca in the target region.

Keywords: Ca treatment; 38CrMoAl steel; inclusions; high aluminum steel; pressure steelmaking

1 Introduction

38CrMoAl is one of the alloy structural steels. It is a special nitriding steel. The mass fraction of acid-soluble aluminum reaches 0.7–1.1 wt%. Aluminum nitride layer (AlN) is formed on the surface of the casting after nitriding treatment. The hardness and strength of the casting surface are improved by the dispersion strengthening of AlN [1,2]. 38CrMoAl steel could obtain high surface hardness, wear resistance, and fatigue strength and has good heat resistance and corrosion resistance after heat treatment and finishing. After treatment, it has high dimensional accuracy and is mainly used in machinery manufacturing, aviation industry, military industry, and other industries. In recent years, the market demand has increased rapidly [3].

At present, there are many problems in the production of 38CrMoAl high aluminum steel by converter continuous casting process. For example, the Al content of 38CrMoAl high aluminum steel is dozens of times higher than that of ordinary aluminum containing steel (0.02–0.08 wt%), and w [Al] in steel is easy to react with slag (SiO2), resulting in difficulty in controlling w [Si] and w [Al] composition in steel. At the same time, the generated A12O3 inclusions enter the ladle slag, tundish covering agent, and mold protective slag through the slag gold balance, which seriously worsens the performance of the slag, resulting in serious nozzle nodulation and blockage in the pouring process, and even problems such as pouring interruption, steel leakage, slag curling, and slab surface quality defects [4].

Although a few studies [5] have been reported on the modification of inclusions in 38CrMoAl steel by calcium treatment, the experiments were carried out under atmospheric pressure, and the yield of Ca was very low and unstable. On the one hand, the cost of raw material consumption is too high; on the other hand, it also makes it difficult to accurately control the Ca content in steel, and the mass fraction of N in steel is basically about 0.010 wt% under atmospheric pressure, which made a large number of high melting point hard AlN inclusions with sharp edges and corners in steel, which will seriously harm the properties of steel.

The solubility of gases (nitrogen, hydrogen, and oxygen) in molten steel can be expressed by Sivert’s law:

(1) w [ G ] = K P G .

Equation (1) shows that the solubility w [G] of gas in molten steel is directly proportional to the square root of gas partial pressure P G. The solubility increased gradually with the pressure increasing, and this law remains unchanged, although recent studies have shown that the actual solubility will deviate from Sivert’s law under high pressure. In fact, not only N, H, and O but also many elements such as Ca, Mg, K, Na, Pb, Sn, Zn, Li, and Bi mainly exist in gaseous form at steelmaking temperature (e.g., typical temperature 1,873 K). These elements are very volatile at atmospheric pressure because of their high vapor pressure. Increasing pressure can significantly improve the solubility of these elements in molten steel, so as to play a role that cannot be achieved under normal pressure. For example, increasing the pressure can significantly improve the solubility of Mg in liquid steel and greatly improve its utilization [6]. Therefore, it is a very effective and feasible method to modify the inclusions in steel with calcium under pressure. In addition, the N pollution in the air can be avoided due to the inside of the furnace cavity is high-purity inert gas when the pressurized furnace is used for steelmaking, so as to avoid a large number of high melting point AlN inclusions with sharp edges and corners in the steel.

At present, there are many studies on the inclusions of MgO·Al2O3, but the mechanism of calcium-modified MgO·Al2O3 inclusions is still controversial. Itoh and Hino [7] found that a small amount of Ca can significantly modified the phase boundary between MgO·Al2O3 and Al2O3 according to the equilibrium phase diagram of Mg–Ca–Al–O. Pistorius et al. [8] pointed out that MgO promotes to transform inclusions into liquid phase during Ca treatment. Itoh and Pistorius proposed a mechanism of Ca-modified MgO·Al2O3 inclusion: MgO in MgO·Al2O3 will be preferentially reduced by [Ca], and the reduced [Mg] will enter the molten steel to form new MgO·Al2O3 inclusions again. Yang et al. [9] found through thermodynamic calculation and combined with laboratory and factory tests that MgO·Al2O3 inclusions were difficult to denature into liquid CaO–MgO–Al2O3 or CaO–Al2O3 inclusions with uniform composition; only the size was less than 2 μm. It was also found that the inclusion size after Ca treatment was larger than the original MgO·Al2O3 inclusion size.

Therefore, this study intends to modify the inclusions in 38CrMoAl steel with Ca under atmospheric pressure and high pressure. The effect of pressure on the solubility of Ca in pure iron and molten steel was investigated, and the effect and mechanism of Ca modification on inclusions in steel were studied. It provided a valuable reference for popularizing the use of steelmaking with volatile elements under pressurized conditions in the future.

2 Methodology

2.1 Apparatus

The schematic diagram of MoSi2 furnace smelting experimental steel under atmospheric pressure is shown in Figure 1. A pressurized induction furnace was selected to smelt experimental steel; the schematic diagram is shown in Figure 2.

Figure 1 Schematic diagram of the MoSi2 resistance furnace used in the experiment.
Figure 1

Schematic diagram of the MoSi2 resistance furnace used in the experiment.

Figure 2 Schematic diagram of pressurized induction furnace. 1, pressure gauge; 2, monitoring camera; 3, feeding device; 4, temperature measuring device; 5, crucible; 6, cap mouth of mold; 7, mold.
Figure 2

Schematic diagram of pressurized induction furnace. 1, pressure gauge; 2, monitoring camera; 3, feeding device; 4, temperature measuring device; 5, crucible; 6, cap mouth of mold; 7, mold.

2.2 The way and quality of calcium adding

When steelmaking under atmospheric pressure, 70 wt% Si-30 wt% Ca alloy was selected. The Si-Ca alloy was directly poked into the molten steel with Mo wire after it was wrapped and sealed with a pure iron sheet. Five groups of steels were smelted, and the mass fractions of pure Ca were 0, 14, 21, 24, and 30 g, respectively.

In the experiment of studying the yield of Ca in pure molten iron under pressure, the selected pressures were 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5 Mpa, respectively. The addition amount of 70 wt% Si-30 wt% Ca alloy was carried out according to the standard of 550 g·t−1.

In the experiment to study the effect of Ca on inclusions in 38CrMoAl high aluminum steel under pressure, the pressure of 2 MPa was selected. At the same time, Fe-99 wt% Ca alloy wire was selected, and the amount of Ca added was 60 and 80 g, respectively.

2.3 Detection equipment and method for detecting experimental steel composition

A C/S analyzer was selected to determine the content of carbon and sulfur in steel, Lecotc500 N/O analyzer was used to determine the content of nitrogen and total oxygen in steel, and direct reading spectrometer was used to detect the content of other elements.

The forging process of the experimental steel is as follows: the steel is heated from room temperature to 1,473 K, kept warm for an appropriate time, and then forged into 18 mm bars. The starting forging temperature is 1,323–1,373 K, and the final forging temperature is higher than 1,123 K. After forging, the material is cooled to room temperature in the air.

The main element content of experimental steel was detected using an ARL-4460 direct reading spectrometer. Leco TC 500 N/O analyzer was selected to determine the content of N and T.O (total oxygen) in steel (the size of sample is 4 mm × 4 mm× 6 mm). Take 4–5 g of iron filings from the ingot and measure the mass fractions of C and S in the steel using an infrared C/S analyzer. Cutting tensile specimens from the heat-treated samples for tensile testing and the dimensions of the samples are shown in Figure 3.

Figure 3 Schematic diagram of the tensile specimen (unit, mm).
Figure 3

Schematic diagram of the tensile specimen (unit, mm).

3 Results and discussion

3.1 Ca-treated 38CrMoAl high aluminum steel under atmospheric pressure

3.1.1 Chemical composition of steel samples

The composition of 38CrMoAl high aluminum steel is shown in Table 1.

Table 1

Chemical composition of 38CrMoAl high aluminum steel (wt%)

C Si Mn Cr Mo Al P S
0.35–0.42 0.20–0.45 0.30–0.60 1.35–1.65 0.15–0.25 0.70–1.10 ≤0.035 ≤0.035

The chemical compositions of experimental steels smelted by MoSi2 furnace are shown in Table 2. The content of Ca increased gradually from 0.023 wt% in No. A-2 steel to 0.067 wt% in No. A-5 steel. The results indicated that the content of T.O (total oxygen) decreased sharply from 0.0035 to 0.0008 wt% with the content of Ca increased gradually. That means, Ca treatment has a very good deoxidation effect on molten steel.

Table 2

Chemical compositions of experiment steels (wt%)

No. C Si Mn Cr Mo Al P S T.O N Ca
A-1 0.36 0.22 0.42 1.40 0.24 0.88 0.008 0.0020 0.0035 0.0100 —
A-2 0.35 0.23 0.40 1.45 0.20 0.85 0.010 0.0021 0.0019 0.0110 0.023
A-3 0.37 0.24 0.41 1.41 0.22 0.84 0.010 0.0023 0.0018 0.0106 0.028
A-4 0.35 0.25 0.39 1.42 0.23 0.84 0.008 0.0022 0.0014 0.0095 0.050
A-5 0.38 0.23 0.40 1.43 0.24 0.87 0.008 0.0024 0.0008 0.0098 0.067

3.1.2 The size distribution of inclusions in steel

The size distribution of inclusions in steel samples treated by Ca is shown in Table 3. Obviously, the per unit area density of inclusions decreased from 466 in A-1 steel to 410 in A-4 steel before rising back to 453 in A-5 steel. The area percentage of inclusions (f A) decreased from 0.138% in A-1 steel to 0.113% in A-4 steel before rising back to 0.135% in No. 5 steel. In addition, the average diameter of inclusions increased from 1.25 μm in steel without Ca to the peak value 1.56 μm in steel with 0.050 wt% Ca before decreasing to 1.40 μm in steel with 0.067 wt% Ca. Furthermore, the size distribution of inclusions experienced a dramatic change. In detail, the percentage of inclusions with equivalent diameters smaller than 2 μm was 91.699, 86.5055, 82.631, 77.770, and 82.915%, respectively.

Table 3

The size distribution of inclusions in steel samples treated by Ca

No. Percentage distribution of inclusions with different equivalent diameter/% Number of inclusions per unit area f A/%
0 < d < 1/μm 1 ≤ d < 2/μm 2 ≤ d < 3/μm 3 ≤ d < 5/μm d ≥ 5/μm Average diameter/μm
A-1 59.716 31.983 3.644 2.024 2.631 1.25 466 0.138
A-2 54.119 32.386 8.239 4.119 1.136 1.28 450 0.133
A-3 52.951 29.680 7.757 4.553 5.190 1.52 424 0.122
A-4 23.950 53.820 16.030 5.340 2.630 1.56 410 0.113
A-5 29.146 53.769 8.945 6.633 1.508 1.40 453 0.135

The reason for the above phenomenon is that a lot of high melting point inclusions will be transformed into low melting point inclusions when the content of Ca in molten steel increases gradually. On the one hand, the low melting point inclusions were more likely to collide and grow up. on the other hand, the inclusions with low melting points were easier to float up and remove. In addition, the chemical properties of calcium are relatively active and exist in gaseous form at the steel-making temperature. Thus, a large number of inclusions were brought to the surface of the liquid steel to be removed by a large number of calcium steam gunned float up. Therefore, the number of inclusions per unit area decreased gradually while the average diameter of inclusions increased gradually with the dissolved calcium in molten steel increased. Furthermore, in A2, the increase in the inclusion size could be due to the fact that calcium aluminate is normally larger than spinel. Finally, CaS inclusions were formed in the steel when the content of Ca in molten steel was too high (0.067 wt%). This high melting point inclusion was solid at the temperature of steelmaking and was not easy to float up and remove.

3.1.3 Composition distribution of inclusions in steel

The composition distribution of inclusions in steel is shown in Figure 4. The main kind of inclusions in No. A-1 to No. A-4 steel samples was CaO–MgO–Al2O3 system. There were two inclusions system CaO–MgO–Al2O3 and CaO–Al2O3–CaS in A-5 steel samples due to it containing many CaS inclusions.

Figure 4 Inclusion composition of each steel sample: (a) without adding Ca, (b) 0.023% Ca, (c) 0.028% Ca, (d) 0.050% Ca, and (e and f) 0.067% Ca.
Figure 4

Inclusion composition of each steel sample: (a) without adding Ca, (b) 0.023% Ca, (c) 0.028% Ca, (d) 0.050% Ca, and (e and f) 0.067% Ca.

The results indicated that the main inclusions were MgO–Al2O3 with 10–60 wt% Al2O3 in No. A-1 steel without Ca. And then, the spinel inclusions transformed into CaO–MgO–Al2O3 with CaO less than 10 wt% and Al2O3 more than 70 wt% when the content of Ca in No. A-2 steel. The content of CaO in the CaO–MgO–Al2O3 system increased to 20–30 wt% in No. A-3 steel with 0.028 wt% Ca, 30–60 wt% in No. A-4 steel with 0.050 wt% Ca, and 50–80 wt% in No. A-5 steel with 0.067 wt% Ca, respectively. That means, the content of CaO in CaO–MgO·Al2O3 system increased sharply with Ca in steel increasing gradually. Moreover, the content of CaS was 10–40 wt% in CaO–Al2O3–CaS in No. A-5 steel samples.

It should be noted that most of the inclusions were located in the low melting point area with 1,300°C when the steel contained 0.050 wt% Ca. This kind of inclusion was easy to float up and remove due to it existing in molten steel at the temperature of steelmaking.

3.1.4 Typical inclusions in steel

The typical inclusions in steel samples are shown in Figure 5. There were four kinds of inclusions Al2O3, AlN, MgO·Al2O3, and MnS in No. A-1 steel. Both Al2O3 and AlN inclusions with sharp edges and corners are shown in Figure 5 A-1 and A-2. Almost all CaO–MgO–Al2O3 composite inclusions have a spherical structure and are smaller than 2 μm in size. It should be noted that AlN inclusions exited in all of the steel samples. The surface scanning results of typical inclusions in each group of samples are shown in Figure 6. It can be seen that the composite inclusions have an obvious layered structure.

Figure 5 Typical inclusions in steel samples with different contents of Ca.
Figure 5

Typical inclusions in steel samples with different contents of Ca.

Figure 6 EDS surface scanning of typical inclusions in steel sample with different Ca contents.
Figure 6

EDS surface scanning of typical inclusions in steel sample with different Ca contents.

3.2 The yield of Ca in molten iron under pressure

The yield of Ca under different pressures is shown in Table 4. Obviously, the change trend of it increased generally with the pressure increased gently. The difference between the yield value of No. B-1, No. B-2, and No. B-3 was very small, the regular of this group of data was opposite to the overall law. There are three reasons for this phenomenon: the first reason is that there was a certain measurement error; the second reason is that the pressure has little effect on the calcium yield when the pressure of the system is low, and the calcium yield will fluctuate to a certain extent; and the last reason is due to different ways of adding Ca–Si alloy. The Ca–Si alloy wrapped by a pure iron sheet was directly poked into the molten iron with molybdenum wire when smelting at atmospheric pressure, which made the Ca vapor have a long contact distance and time with the molten iron. However, in pressure smelting, Ca–Si alloy was directly put on the surface of molten iron. The contact distance and time between Ca vapor and molten steel were short, which was easy to lose due to volatilization.

Table 4

Calcium yield and content of T.O and N under different pressures (wt%)

No. P/MPa Ca Yield/% T.O N
B-1 0.1 0.00060 3.57 0.0060 0.0038
B-2 0.5 0.00045 2.75 0.0185 0.0035
B-3 1.0 0.00040 2.53 0.0104 0.0028
B-4 1.5 0.00160 9.53 0.0051 0.0027
B-5 2.0 0.00240 14.60 0.0049 0.0025
B-6 2.5 0.00220 13.21 0.0050 0.0033

The Ca yield tended to be stable when the pressure of the system was above 2 MPa. Therefore, 2 MPa was selected for further experiments.

3.3 Ca treated 38CrMoAl high aluminum steel under 2 MPa pressure

3.3.1 Chemical composition of steel samples

The chemical composition of steel smelted under 2 MPa pressure is shown in Table 5. The content of Ca was 0.052 and 0.058 wt%, respectively. The content of T.O was 0.0014 and 0.0010 wt%. It should be noted that the content of N was only 0.0030 and 0.0028 wt%; it was far less than the value of smelted under atmospheric pressure, 0.010 wt%. This is due to the fact that when using the pressurized furnace for smelting, the furnace cavity will be vacuumized in advance and then flushed with argon with high purity, which means there was very little residual air in the furnace cavity and effectively reduced the source of N in the gas. During atmospheric steelmaking, although the furnace cavity of MoSi2 furnace was also filled with high-purity argon for protection, its air tightness was much worse than that of the pressurized furnace, resulting in N in the air entering the molten steel. The lower the nitrogen content, the less AlN inclusions in the steel and the higher the purity of the steel.

Table 5

Chemical compositions of experiment steels (mass fraction/%)

No. C Si Mn Cr Mo Al P S T.O N Ca
C-1 0.35 0.25 0.42 1.44 0.20 0.78 0.011 0.0029 0.0014 0.0030 0.052
C-2 0.36 0.23 0.40 1.45 0.20 0.75 0.011 0.0032 0.0010 0.0028 0.058

3.3.2 The size distribution of inclusions in steel

The effect of Ca on the size distribution of inclusions in steel is shown in Table 6. Obviously, the percentage of inclusion with an equivalent diameter smaller than 2 μm increased from 78.87% in No. C-1 steel with 0.052 wt% Ca to 82.515% in No. C-2 steel with 0.058 wt% Ca. And the average diameter of inclusions decreased from 1.72 to 1.61 μm, simultaneously. Furthermore, the density of inclusion increased from 205 per unit area to 277. That means, Ca treatment of 38CrMoAl steel under high pressure can effectively purify the molten steel and refine the inclusions in the steel.

Table 6

Metallographic analysis results

No. Percentage of equivalent diameter distribution of inclusions/% Number of inclusions per unit area f A/%
0 < d < 1/μm 1 ≤ d < 2/μm 2 ≤ d < 3/μm 3 ≤ d < 5/μm d ≥ 5/μm Average diameter/μm
C-1 16.901 61.971 13.146 5.164 2.631 1.72 205 0.082
C-2 26.994 55.521 9.509 5.521 2.631 1.61 277 0.090

3.3.3 Composition distribution of inclusions in steel

Since the composite inclusions in the steel contained almost no MgO but a certain amount of CaS, the CaO–Al2O3–CaS ternary phase diagram was selected for component analysis, and the results are shown in Figure 7.

Figure 7 Inclusion composition of each steel sample: (a) 0.052% Ca and (b) 0.058% Ca.
Figure 7

Inclusion composition of each steel sample: (a) 0.052% Ca and (b) 0.058% Ca.

The main reason for the low content of MgO in composite inclusions in steel was that a certain amount of CaO–SiO2–Al2O3–MgO refining slag was added on the surface of molten steel during steelmaking under atmospheric pressure, and the dissolved Ca and Al in molten steel reacted with the refining slag, resulting in a high content of MgO in inclusions. Without any refining slag was added on the surface of molten steel when steelmaking under pressure, which greatly reduced the source of MgO in inclusions.

The result of the composition distribution of inclusion in steel indicated that most of inclusions in No. C-1 steel with 0.052 wt% Ca were located in the low melting point area, while only a small part of inclusions in No. C-2 steel with 0.058 wt% Ca was within the same area. Thus, the calcium content in steel should not be too high to control the composition of inclusions in steel in the low melting point area but should adopt accurate control process.

3.3.4 Typical inclusions in steel

The typical inclusions in steel smelted under 2 MPa pressure are shown in Figure 8. Most of CaO–Al2O3–CaS inclusions were in spherical structure with a diameter of less than 2 μm. Very few inclusions contain a small amount of MgO.

Figure 8 Typical inclusions in steel samples with different contents of Ca.
Figure 8

Typical inclusions in steel samples with different contents of Ca.

3.3.5 Mechanical properties

The mechanical properties of the experimental steel are shown in Table 7. It can be seen from Table 7 that the strength of group A steel shows a trend of first increasing and then decreasing as the Ca content increases, and the strength of the A4 (0.050 wt% Ca) sample is the maximum value, 735 MPa. Similarly, the plasticity of the sample first increases and then decreases as the Ca content in the steel increases.

Table 7

Mechanical properties of steel samples

No. Ca/wt% Tensile strength/MPa Yield strength/MPa Elongation/%
A-1 — 665 470 11.80
A-2 0.023 665 520 11.90
A-3 0.028 670 540 11.81
A-4 0.050 735 560 14.85
A-5 0.067 715 575 13.45
C-1 0.052 955 785 10.80
C-2 0.058 895 730 10.36

The micromorphology of the tensile fracture surface of the sample in group A is shown in Figure 9. From the macroscopic appearance, there are no obvious fiber areas in the five groups of samples, only the radiation area and the shear lip area. The A-3 sample does not even have an obvious shear lip area, indicating that the plasticity of the sample is not very good. The obvious difference between them lies in the necking phenomenon of the fracture surface. Specifically, when the Ca content in the steel is less than 0.023 wt%, there is no obvious necking in the tensile fracture surface of the sample, A-1 and A-2. When the Ca content in the steel is higher than 0.028 wt%, the tensile fracture surface of the sample shows obvious necking, and the area of the shear lip zone increases significantly, A-3, A-4, and A-5. This shows that the plasticity of the sample has been improved to a certain extent.

Figure 9 Microscopic morphology of fracture surface of steel samples.
Figure 9

Microscopic morphology of fracture surface of steel samples.

There are also certain differences in the micromorphology of the tensile fracture surfaces of the specimens. As the Ca content in the steel gradually increases, the number of secondary microcracks at the fracture surface significantly decreases, the size of the tear dimples decreases significantly, and the proportion of fine equiaxed dimples increases significantly. However, it is worth noting that when the Ca content in the steel reaches 0.058 wt%, the number of inclusions at the fracture surface increases significantly, which will harm the mechanical properties of the steel.

The strength of steel is significantly improved after forging treatment, increasing by about 200 MPa. The strength of C-2 is 60 MPa higher than that of C-1, but there is no obvious difference in plasticity between them.

The fracture micromorphology of C-1 and C-2 is shown in Figure 9. As can be seen from Figure 10, the macroscopic morphology of the fracture surfaces of C-1 and C-2 can be clearly divided into three areas, namely the central fiber area, the radial area, and the shear lip area. There is no obvious difference in the micromorphology of C-1 and C-2 fracture surfaces.

Figure 10 Microscopic morphology of fracture surface of steel samples.
Figure 10

Microscopic morphology of fracture surface of steel samples.

3.4 Mechanism of inclusion transformation

3.4.1 Mechanism of the effect of Ca on inclusions in spinal inclusions

When using Ca treatment to modify inclusions in steel, the chemical reaction is very complex, and the steps of inclusion transformation are relatively complex. Immediately after Ca treatment, Ca first reacts with MgO in spinel, reducing the MgO content in the inclusions and generating Al2O3–CaO–MgO composite inclusions, as shown in equation (2) [10]. The outer layer of these inclusions is a composite phase of CaO–Al2O3, and the inner layer is an untransformed MgO inclusion, as shown in Figure 11. In addition, a small amount of spinel inclusions with a size of about 1 μm, due to their extremely small size, [Ca] reacts with the inclusions at a relatively fast diffusion rate, resulting in the formation of Al2O3–CaO–MgO uniformly composite inclusions, as shown in Figure 12.

(2) [ Ca ] + ( MgO ) → ( CaO ) + [ Mg ] .

Figure 11 EDS surface scanning of typical inclusions CaO–MgO–Al2O3 in No. A-3 steel sample with 0.028% Ca.
Figure 11

EDS surface scanning of typical inclusions CaO–MgO–Al2O3 in No. A-3 steel sample with 0.028% Ca.

Figure 12 EDS surface scanning of typical inclusions CaO–MgO–Al2O3 in No. A-4 steel sample with 0.050% Ca.
Figure 12

EDS surface scanning of typical inclusions CaO–MgO–Al2O3 in No. A-4 steel sample with 0.050% Ca.

As the Ca content gradually increases, the amount of MgO reduced in spinel increases until it disappears. During this process, the content of MgO in Al2O3–CaO–MgO inclusions gradually decreases, while the content of CaO is exactly the opposite, gradually increasing. When the Ca content continues to increase, [Ca] reacts with [S] to form CaS inclusions, and the main type of inclusions in the steel is Al2O3–CaO–CaS–(MgO), as shown in Figures 5 and 8.

3.4.2 Mechanism of the formation of N-containing inclusions

Figure 13 presents an AlN stability diagram for the experimental steel composition of 38CrMoAl steel, obtained by FactSage 7.3 (FSstel, FToxid, and FactPS databases). From Figure 13, it can be seen that all five steels in group A smelted at atmospheric pressure will generate AlN inclusions during solidification, high Al steel should avoid direct contact with air as much as possible. This not only causes secondary oxidation of the steel liquid but also easily generates a large amount of hard AlN inclusions in the steel. On the contrary, all two steels in group C smelted under a pressure atmosphere of 2 MPa will not generate AlN inclusions during solidification. This is mainly because the cavity of the induction furnace is initially subjected to vacuum treatment, resulting in extremely low air content, and the N content in the steel liquid will not increase due to air pollution. Alba et al. [11] also found a similar phenomenon. Alba studied the influence of N content on the inclusions in Fe–5Mn–3Al steel; the results indicated that an increase in the N content to 47 and 58 ppm increased the total amount of inclusions from 13 to 64 mm−2 and from 21 to 101 mm−2, respectively. In addition, with an increase in the N content, AlN(pure) inclusions became the dominant type of inclusions as AlN was stable in the liquid steel.

Figure 13 AlN stability diagram for the experimental steel composition, obtained by using FactSage 7.3.
Figure 13

AlN stability diagram for the experimental steel composition, obtained by using FactSage 7.3.

3.4.3 The effect of Ca on mechanical properties of steel samples

Inclusions in steel are one of the important factors that harm the structure and mechanical properties of steel [12,13]. Ca can effectively purify 38CrMoAl steel, reduce the number and size of inclusions in the steel, and at the same time modify large-sized hard spinel inclusions into low melting point calcium aluminate composite inclusions. Therefore, when the Ca content is lower than 0.050 wt%, the strength of the experimental steel gradually increases as the Ca content in the steel increases. The strength of forged steel is significantly higher than that of cast steel. This is mainly due to the following reasons: (1) steel smelted under pressure avoids air pollution. The number of inclusions in the steel is smaller and the size is smaller, especially the hard AlN inclusions are significantly reduced, so the harm of inclusions to the steel is significantly reduced; (2) the grains of the steel are significantly refined after forging; and (3) the structure becomes more uniform.

4 Conclusions

In this study, Ca treatment experiments under different pressures were carried out on 38CrMoAl high aluminum steel. Combined with thermodynamic calculation, the regular inclusions’ modification of Ca-treated 38CrMoAl high aluminum steel under atmospheric pressure and 2.0 MPa was compared and analyzed, and the mechanism of inclusion modification of MgO·Al2O3 inclusions by [Ca] was deeply discussed, Thus, the influence of pressure on the dissolution characteristics of volatile element Ca and the optimal addition amount of Ca are determined, and the following conclusions were obtained: (1) the yield of Ca basically shown an increasing trend with the increased of smelting pressure. When the pressure is 2.0 MPa, the highest yield of Ca in molten iron and molten steel is 14.6 and 17.3%, respectively. The yield of Ca in molten iron and molten steel is only 3.57 and 2.23%, respectively, at atmospheric pressure. The results indicated that pressurization can improve the yield of Ca. (2) The average diameter of inclusions in steel increased gradually with the increased [Ca] content in steel, but the number of inclusions per unit area decreased gradually, showing a completely opposite regular. For the composition distribution of inclusion, when the steel contained 0.050 wt% Ca, most of the inclusions in the steel changed into liquid CaO–Al2O3 composite inclusions, which was very obvious in the ternary phase diagrams of CaO–MgO–Al2O3 and CaO–Al2O3–CaS. (3) Calcium will preferentially react with MgO in the process of modifying MgO·Al2O3 inclusions. When the [Ca] content or reaction time in molten steel increases gradually, Ca will modify the MgO·Al2O3 inclusions into homogeneous CaO–Al2O3 or CaO–MgO–Al2O3 inclusions. However, when there is too much [Ca] in molten steel, the inclusions will be transformed into CaO–CaS inclusions, which is seriously harmful to the quality of steel.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No. 51434004, U1435205) for the financial support. The authors would like to express their gratitude to EditSprings (https://www.editsprings.com/) for the expert linguistic services provided.

  1. Funding information: The authors are grateful to the National Natural Science Foundation of China (No. 51434004, U1435205) for the financial support.

  2. Author contributions: C. Y. Chen and Cong Zhang established a practical plan and prepared the raw materials for the experiment; C. Y. Chen, Cong Zhang, Shuai Ma Hao Yang and Meng Sun completed a large part of practical operation, prepared samples, tested the mechanical properties of the samples, characterized the microstructure of the samples; C. Y. Chen and Cong Zhang wrote the manuscript; Haibo Cao, Yang Li and Zhouhua Jiang revised the manuscript; Yang Li and Zhouhua Jiang provided the cost of this project.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2024-03-04
Revised: 2024-05-14
Accepted: 2024-05-14
Published Online: 2024-06-10

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/htmp-2024-0018
Keywords for this article
Ca treatment; 38CrMoAl steel; inclusions; high aluminum steel; pressure steelmaking
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. De-chlorination of poly(vinyl) chloride using Fe2O3 and the improvement of chlorine fixing ratio in FeCl2 by SiO2 addition
  3. Reductive behavior of nickel and iron metallization in magnesian siliceous nickel laterite ores under the action of sulfur-bearing natural gas
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