Keywords

Introduction

Ship plate steel (SPS) is an important material in shipbuilding industry, and its quality and performance directly affect the safety and reliability of ships. SPS steel belongs to micro-alloyed steel, which will experience a series of complex physicochemical changes such as the secondary phase precipitation and the solid phase transformation in the process of continuous casting production. When the improper cooling strategy are adopted, a large number of secondary phase particles precipitate along the austenite grain boundaries, greatly increasing the occurrence of the surface and subsurface cracks of the blank [1,2,3,4,5,6,7].

Cooling rate is a critical parameter in controlling the secondary phase precipitation behavior, which can affect the category, dimension, morphology, and distribution of the precipitates [8,9,10]. The results of Luo et al. [11] and Chen et al. [12] indicated that with the increase of cooling rate, the size of carbon-nitrides gradually decreased, and the nucleation location was changed from the grain boundary to the grain interior. Dou et al. [13, 14] and Zou et al. [15] obtained similar results in their studies of micro-alloyed steels containing the elements of Nb, V, and Ti. Increasing the cooling rate promoted diffuse precipitation of the secondary phase inside the austenite grains and strengthened the microstructure of the blank surface. Generally, the precipitation temperature of carbon-nitrides is around 800–1000 °C [16, 17]. Therefore, the secondary cooling condition is the primary factor affecting the carbon-nitrides precipitation behavior. However, to obtain the reasonable secondary cooling condition, some detailed operating parameters, such as the cooling rate and temperature range, need to be clarified in continuous casting process.

In this study, a high-temperature confocal laser scanning microscope (HTCLSM) was employed to observe the precipitation behavior of the secondary phase at different cooling rates. Besides, a field emission scanning electron microscopy (FESEM) were used to analyze the size and distribution of the secondary phase particles. Finally, the cooling rate and the corresponding temperature range for controlling the carbon-nitrides precipitation during the continuous casting process were obtained, and a new secondary cooling method was described to improve the surface quality of SPS slab. The plant trials show that the crack on the subsurface of SPS slab have virtually disappeared and the surface cracking ratio decreases by about 35%.

Materials and Methods

The present study focused on a SPS slab produced in one steelmaking plants. SPS belongs to micro-alloyed steel, which is strengthened by adding micro-alloyed elements of V, Ti, and Nb into carbon manganese steel. The main chemical composition of SPS was obtained by testing the molten steel composition in the tundish, as shown in Table 1. In this study, a HTCLSM was adopted for an in-situ observation of the solidification of SPS slab. The experimental specimens were extracted from the columnar crystal zone of the slab with a section size of 2770 mm × 320 mm, as shown in Fig. 1. The center segregation and porosity of the slab were avoided during sampling, and the as-cast microstructure of each specimen was similar. The specimen was machined into a cylinder with a size of φ5 mm × 3 mm, whose surfaces were grinded and polished before put them into alumina crucibles for experiments.

Table 1 Main chemical composition of SPS, %
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Fig. 1
A 3-D model of a Ship plate steel slab of dimension 2770 by 320 millimeters. A circular specimen of 5 by 3 millimeters is taken out from the slab. The specimen is to be put into alumina crucibles.

The schematic diagram of sampling location and specimen size

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The experiments were carried out under the thermal regime. During the experiment, the specimens were heated from room temperature to 1495 °C at a heating rate of 10 °C/s and then held isothermally for 15 min to dissolve the carbon-nitrides. After that, the specimens were cooled to experimental temperatures at five different cooling rates of 0.1, 0.5, 1, 3, and 5 °C/s to observe the effects of cooling rate on the precipitation behavior of the carbon-nitrides. During the experiments, the specimens were heated and cooled under an ultra-high purity argon atmosphere (99.999%). The thermocouple placed in the crucible was used to measure the specimen temperature. The corresponding principles and operating methods are consistent with those described by Zou et al. [15], Dou et al. [13], Griesser et al. [18], and Kimura et al. [19].

Results and Discussion

To control the carbon-nitrides precipitation behavior in continuous casting process, it is essential to clarify the detailed parameters such as precipitation temperature and reasonable cooling rate, which requires the investigations of the carbon-nitrides precipitation behavior under different cooling rates.

In-Situ Characterization of Secondary Phase Precipitation

In-situ characterization is a method for continuously observing the microstructural evolution of the specimen surface at a defined cooling rate. The precipitation temperature of the carbon-nitrides is usually obtained using thermodynamic or kinetic calculations. Unfortunately, the calculated results often deviate from the actual results due to the difficulty of accurately obtaining the phase transition parameters. In contrast, in-situ characterization of carbon-nitrides precipitation removes this problem effectively.

To simulate the secondary cooling condition during the continuous casting process of SPS slab, the carbon-nitrides precipitation behavior was investigated at a cooling rate of 0.1 °C/s, as shown in Fig. 2. The observation results illustrate that a few dark particles appear near the austenite grain boundary when the temperature decreases from 1495 to 1455 ℃ (as shown in Fig. 2a). As is found by Slater et al. [20] and other scholars [13, 21], the dark particles are the direct signs of the carbon-nitrides, and similar experimental phenomena have been reported in previous in-situ observation studies [12, 19, 22, 23]. Therefore, the present study investigates the behavior of carbon-nitrides precipitation by observing the formation and change of dark particles. Dark particles are first formed near austenite grain boundary due to the segregation of solute elements usually occurs in this area. As illustrate in Fig. 2a–c, the dark particles increase rapidly between 1455 and 1375 ℃ with the temperature decreases. After that, when the temperature decreases to 1116 ℃, a new type of dark particles with smaller sizes starts to appear near the austenite grain boundary, as shown in Fig. 2d. The new dark particles increase rapidly between 1116 and 959 °C. Furthermore, the eutectoid phase transformation occurs between 720 and 670 °C, which corresponds to the location after the straightening zone of continuous casting process.

Fig. 2
6 microscopic images of ship plate steel slab. A, presents the dark particles. B. Increase of dark particles. C. Most dark particles. D. Appearance of new dark particles. E. Increase of new dark particles. F. most new dark particles.

Experimental results of in-situ observations at 0.1 cooling rate, a 1455 ℃; b1400 ℃; c 1375 ℃; d 1116 ℃; e 1050 ℃; f 959 ℃

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Thermo-Calc software was used to calculate the diagram of SPS phase equilibrium, as shown in Fig. 3. It is found that TiN starts to precipitate at 1146 °C, and the fastest precipitation temperature is around 1350–1400 °C. The initial precipitation temperature of Nb(C, N) is about 1056 °C, and the fastest precipitation temperature is in the range of 950–1000 °C. V(C, N) precipitates with the precipitation of Nb(C, N), while the amount of V(C, N) precipitation is extremely minor (less than 1% of total precipitation). The above calculation results are consistent with the in-situ observation by HTCLSM. Furthermore, Xie et al. [17] and Parker et al. [16] investigated the carbon-nitrides precipitation in Nb-containing steel through kinetic calculation, and the results suggested that the initial precipitation temperature of carbon-nitrides was about 1100 °C. Liu et al. [22] also discovered some dark particles on Ti-containing steel surface using in-situ observation, and found that under the dark particle was Ti(C, N). Zou et al. [15] clarified the mechanism for the formation of dark particles on micro-alloyed steel surface during in-situ observation process. Therefore, in this study, the dark particles detected via in-situ observations were adopted to characterize the behaviors of carbon-nitrides precipitation. According to the experimental results, it can be inferred that the fastest precipitation temperature of TiN is around 1455–1375 °C, which is around 1116–959 °C for Nb(C, N).

Fig. 3
A phase diagram of phase fraction versus temperature. It presents the curves of micro amounts of V (C, N), T i N, N b (C, N), alpha F e, gamma F e, delta F e, and liquid. The T i N starts to precipitate at 1146 degrees Celsius, and the fastest precipitation temperature is around 1350 to 1400.

Diagram of SPS phase equilibrium calculated using Thermo-Calc software

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Effects of Cooling Rate on Secondary Phase Precipitation Behavior

In the secondary cooling zone (SCZ) of continuous casting, the cooling rate on the slab surface varies with cooling water volume. The present study simulated the solidification phase transition process of SPS slab under different cooling conditions by setting different cooling rates, which included 0.5, 1.0, 3.0, and 5.0 °C/s.

Effects of Cooling Rate on TiN Precipitation

The in-situ observation of TiN precipitation at different cooling rates is shown in Fig. 4. The results illustrate that the initial temperature of TiN precipitation gradually decreases with the increase of cooling rate. When the cooling rate is 0.5 °C/s, 1.0 °C/s, 3.0 °C/s and 5.0 °C/s, TiN starts to precipitate at the temperature of 1452 °C, 1445 °C, 1439 °C, and 1422 °C, respectively. Besides, the corresponding fastest precipitation temperature is around 1452–1373 °C, 1445–1370, 1439–1358, and 1422–1345 °C. Furthermore, as the cooling rate increases, the precipitation location of TiN gradually changes form grain boundary to grain interior, and the quantity of precipitation gradually decreases. The reason is that the diffusion coefficient of solute elements decreases exponentially with the increase of cooling rate [15]. Therefore, most of the Ti atoms are solubilized in the steel matrix at high cooling rates, rather than being enriched at grain boundaries or forming TiN precipitates. As shown in Fig. 4b–d, when the cooling rate is increased to 1 ℃/s, the TiN precipitation basically disappears from austenite grain boundaries. Moreover, when the cooling rate is enlarged to 5 ℃/s, the size of dark particles induced formation by TiN precipitation is less than 1/5 of that at 0.1 ℃/s cooling rate. Due to the high temperature of TiN precipitation, the TiN precipitation on SPS slab surface is generally completed in the mold cooling process during continuous casting, where the cooling rate on the slab surface is considerably greater than 5 ℃/s. It is indicated that TiN precipitation will be smaller in size and quantity, and have less effect on grain boundaries. Therefore, the precipitation of TiN is not the major reason for the crack formation on SPS slab surface in continuous casting process.

Fig. 4
A table has 12 microscopic images of T i N precipitation at different cooling rates, arranged in the 3 columns that represent the appearance of dark particles, the increase of dark particles, and most dark particles.

In-situ observation of TiN precipitation at different cooling rates, a 0.5 ℃/s; b 1.0 ℃/s; c 3.0 ℃/s; d 5.0 ℃/s

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Effects of Cooling Rate on Nb(C, N) Precipitation

The in-situ observation of Nb(C, N) precipitation at different cooling rates is shown in Fig. 5. Similar to the change in the precipitation distribution of TiN, the precipitation location of Nb(C, N) also changes form grain boundary to grain interior with the cooling rate increases. When the cooling rate is less than 3 ℃/s, the initial precipitation temperature and the quantity of Nb(C, N) precipitation decrease rapidly with the increase of the cooling rate, especially in increasing of the cooling rate from 0.5 to 1 ℃/s. This is because increasing the cooling rate significantly decreases the migration rate of solute elements such as Nb, which in turn reduces its enrichment at grain boundaries. Meanwhile, a large amount of Nb is solidly dissolved in the steel matrix. When the cooling rate is 0.5 °C/s, 1.0 °C/s, and 3.0 °C/s, the initial precipitation temperature of Nb(C, N) is 1094 °C, 1010 °C, and 1001 °C, with the corresponding fastest precipitation temperature in the range of 1094–954 °C, 1010–952 °C, 1001–948 °C, respectively. The effect of cooling rate on the end temperature of the fastest precipitation is minor, indicating that the fastest precipitation temperature range of Nb(C, N) gradually decreases as the cooling rate increases. When the cooling rate enlarges from 0.1 to 3 °C/s, the fastest precipitation temperature range reduces from 157 to 51 °C, which is conducive to controlling the precipitation behavior of Nb(C, N) on the SPS slab surface in the continuous casting process. However, when the cooling rate adds from 3 to 5 °C/s, the initial precipitation temperature of Nb(C, N) increases from 1001 to 1060 °C with the fastest precipitation temperature around 1060–1020 °C. The distribution of Nb(C, N) precipitation is dispersed inside the grain. Considering the high Nb content in SPS (about 510 ppm), increasing the cooling rate rapidly reduces its local solubility in the steel matrix, which in turn facilitates the precipitation of Nb(C, N). Therefore, under the cooling rate of 5 °C/s, the precipitation temperature of Nb(C, N) is increased and the precipitation is fine-sized and dispersed inside the austenite grains, as shown in Fig. 5d. In the SCZ of continuous casting, the surface temperature of SPS slab is around 900–1200 °C, which is consistent with the precipitation temperature of Nb(C, N). Hence, the precipitation behavior of Nb(C, N) has an important effect on the formation of SPS slab surface and subsurface crack. Reasonable temperature and cooling rate are key to controlling Nb(C, N) precipitation behavior in continuous casting.

Fig. 5
A table has 12 microscopic images of N b (C, N) precipitation at different cooling rates, arranged in the 3 columns that represent the appearance of dark particles, the increase of dark particles, and most dark particles.

In-situ observation of Nb(C, N) precipitation at different cooling rates, a 0.5 ℃/s; b 1.0 ℃/s; c 3.0 ℃/s; d 5.0 ℃/s

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Effects of Cooling Rate on the Morphology and Distribution of Precipitations

To clearly observe the morphology and distribution of the precipitated phase, a FESEM was applied to analyze the precipitated phase at different cooling rates. The FESEM has higher magnification and resolution. Figure 6 shows the observation results by FESEM in the secondary electrolysis mode. At a low cooling rates of 0.1 °C/s or 0.5 °C/s, chain-like carbon-nitrides precipitate at austenite grain boundaries, which greatly reduces the bonding force of austenite grain boundaries and then induces the occurrence of crack defects. When the cooling rate is between 1 and 5 °C/s, the chain-like precipitation of carbon-nitrides is basically disappeared. In addition, the precipitates are diffusely distributed on the surface of the steel matrix. The above results are consistent with the observations of HTCLSM.

Fig. 6
5 microscopic images present the morphology and distribution of the precipitated phase. A and B. Chain-like carbon nitrides precipitate at austenite grain boundaries. C, D, and E. Chain-like precipitate disappears. F. A spike graph exhibits spikes including C, O, T i, M n, A l, S i, and N b.

Morphology of precipitates at different cooling rates, a 0.1 ℃/s; b 0.5 ℃/s; c 1.0 ℃/s; d 3.0 ℃/s; e 5.0 ℃/s; f energy-dispersive spectrum analysis

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In summary, increasing the cooling rate is conducive to promoting the diffuse precipitation of carbon-nitrides, fully exerting its fine grained and precipitate strengthening, thus enhancing the strength of micro-alloyed steel. To achieve useful secondary precipitation phase on SPS slab surface, the cooling rate requires to more than 1 °C/s. The study conducted by our team on micro-alloyed steel billets has confirmed this conclusion [15]. Meanwhile, the slab surface temperature suggests to rapidly reduced to 950 °C to shorten the precipitation time of the secondary phase.

A New Cooling Method for SPS Slab

During the continuous casting process of SPS, mild cooling mode was adopted to avoid cracks in the straightening zone. However, serious cracks still existed on the slab surface and subsurface. To find the reason of the crack formation, optical microscope (OM) and scanning electron microscope (SEM) were employed to observe the micromorphology and element distribution of the cracks. Figure 7 shows the OM images of crack distribution of SPS slab. The cracks extend along the austenite grain boundaries with a length of less than 1 mm. These small cracks seriously damage the hot ductility of steel. Furthermore, Nb and Ti elements are enriched near the cracks, as shown in Fig. 8. Therefore, it can be inferred that carbon-nitrides precipitations at grain boundaries are the primary reason for subsurface crack formation.

Fig. 7
2 optical microscopic images. A. presents a dark hole on the surface of the S P S slab. B. Presents a crack on the S P S slab surface.

OM morphology of the subsurface cracks of SPS slab

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Fig. 8
5 SEM images present the damage on the surface of the S P S slab. F e, C, N b and T i elements are enriched near the cracks.

Element distribution in a surface crack

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A heat transfer model considering the actual water distribution in the SCZ is established to calculate the thermal behavior and cooling rate of SPS slab. The detailed modeling process and the calculation of cooling rate were provided in the previous works of our team [13, 24, 25]. The heat transfer model was verified by comparing the measured temperatures with the calculated ones. An infrared radiation pyrometer was used to measure the surface temperature of SPS slab, whose error range was ±1.5%. During the measurement, the pyrometer was required to remain perpendicular to the surface of the inner arc and peak values were adopted as the local temperature of the slab surface. Figure 9 shows the temperature profiles of SPS slab under different cooling patterns. Apparently, the calculated results agreed with the measured values with an error less than 1%, which confirmed the heat transfer model is generally reliable.

Fig. 9
A multiline graph of temperature versus distance to meniscus. 3 lines for the original cooling pattern at the center, new cooling pattern at the center, and the new cooling pattern at the corner follow a decreasing trend from 600 to 1600. The measured plots are distributed along the original line.

The SPS slab surface temperatures in different secondary cooling patterns

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In the SCZ of continuous casting, the surface temperature of SPS slab is between 1116 and 1032 °C under the original cooling pattern. The average cooling rates of the first to forth segments in the SCZ are 2.41, 0.88, 0.31, and 0.28 ℃/s. Obviously, the average cooling rates in other segments are less than 0.3 ℃/s. The surface temperature of SPS slab in the first segment of the SCZ is around 1116–1107 °C. Combine with the above analysis, the slab surface experienced a comparatively longer period of high temperatures in the SCZ during continuous casting, which provides sufficient conditions for Nb(C, N) to nucleate and precipitate from the matrix and the austenite grain boundaries.

To prevent the precipitation and growth of the secondary phase at grain boundaries, a new secondary cooling method for SPS slab is applied in this study. Specifically, the water flowrate in the first to third and fifth segments are increased to 1.5 times than that the original water flowrate and the other segments remains constant. The temperatures of the slab surface center and corner after conducting the new cooling pattern are shown in Fig. 9. At the end of the second segment in the SCZ, the temperature on the slab surface center is significantly decreases to 950 °C, indicating that most of carbon-nitride particles have precipitated. The cooling rate in the second segment increases from 0.88 to 1.24 °C/s, which effectively prevents the carbon-nitrides precipitation from the grain boundaries. In addition, the maximum reheating temperature on the slab surface is 96 °C/m after adopting the relatively high water flowrate in the SCZ, suggesting that no internal cracks will be induced. Due to the slab corner involves two-dimensional cooling, the solidification behavior shows relatively lower temperature and higher cooling rate [13, 26], which effectively inhibits the precipitation of carbon-nitrides. In addition, the corner temperature in the straightening zone larger than the third brittleness temperature of the SPS slab (around 666–745 °C). After the implementation of the secondary cooling scheme, the cracks on SPS slab subsurface disappeared and the surface cracking ratio decreases by about 35%.

Conclusion

The carbon-nitride precipitation behavior was characterized in-situ by HTCLSM, and the effect of cooling rate on the morphology, distribution, initial precipitation temperature, and “fast-growing region” were investigated by HTCLSM and FESEM. Based on these, a new secondary cooling method for SPS slab is proposed. The conclusions are as follows:

  1. (1)

    The precipitation process of carbon-nitrides is usually accompanied by the formation of dark particles. As the cooling rate increases, the precipitation location of carbon-nitrides gradually changes from grain boundaries to the steel matrix, and the corresponding number and size are relatively reduced.

  2. (2)

    TiN precipitates at high temperatures, and when the cooling rate increases from 0.1 to 5 ℃/s, the initial precipitation temperature decreases from 1455 to 1422 ℃. Nb(C, N) precipitates at a relatively low temperature, and its value is around 1116–1001 ℃. The lowest completion temperatures for the fastest precipitation of TiN and Nb(C, N) are 1345 and 948 ℃ at different cooling rates.

  3. (3)

    Nb(C, N) precipitated at grain boundaries is the main reason for inducing cracks on the SPS slab surface. Intensive cooling is carried out by increasing the water flowrate to 1.5 times than that the original water flowrate in the first to third and fifth segments of the SCZ. The plant trials show that the cracks disappeared and the surface cracking ratio decreases by about 35% after applying this secondary cooling scheme.