Cracks on the surface of an S355J2-NbTi steel

Cracks on the surface of an S355J2-NbTi steel - Slab defects

Figure 2: Cracks on the surface of an S355J2-NbTi steel continuous cast billet.

Table 1: Chemical composition of S355J2-NbTi steel grade.

Figure 3: Arrangement of cracks in the corner area; the sample after etching (with Nital).

Figure 4: Corner position of the transverse crack chosen for analysis

Figure 5: Positions of the areas (marked with Arabic numerals) selected for analysis, in which chemical composition determination was made.

Table 2: Chemical composition of the phases present in cracks on the S355J2-NbTi steel billet.

Figure 6: Distribution of Non-metallic phases inside the transverse crack situated near the corners of a 160 x 160 mm billet.

Figure 7: Locations of temperature measurements on the surface of a continuous cast slab (1-3) of steel S355J2 – NbTi (cracks at the corners are marked with the arrows).

Table 3: Chemical composition of phases present within a crack on a continuous cast billet of steel S355J2-NbTi (analysis at point 1, Fig. 6).

Defect name: Cracks
Record No.: 2170
Type of defect (Internal/Surface): Internal, surface
Defect classification: Slab defects
Steel name: S355J2-NbTi steel
Steel composition in weight %: See the table 1.
Note: An analysis of the causes of transverse corner and surface crack formation during continuous casting of low carbon niobium and titanium-containing steel into square billets is presented in this paper. Using X – ray analysis method, the chemical composition of non- metallic phases inside corner cracks was determined. The temperature ranges of the occurrence of nitrides and carbonitrides of Nb, Ti and Al were established using FactSage 6.2., a commercial computer program. The need for the precise selection of cooling conditions during casting of billets both in the mould and in spray cooling regions was pointed out with the aim of obtaining the optimum surface, and particularly corner temperature of the billet before its entrance to the straightening stand.

The subject of investigation was the S355J2-NbTi grade steel melted in an electric arc furnace with the use of steel scrap with the addition of pig iron up to 8.0% of metallic charge mass in a solid state. After secondary metallurgy processes carried out in a ladle furnace, the steel was cast using an curved-type Continuous Casting Machine to form 160 mm square cross-section billets. Liquid metal temperature, as measured in the tundish during casting ranged from 1560 to 1565 C, while the average casting rate was 359 kg/min. per strand. Chemical composition of the continuous cast billet, on which transverse cracks were found to have occurred at the corners and in their immediate vicinity, is presented in Table 1.

As can be seen from the values given in Table 1, low concentrations of detrimental admixtures, i.e. phosphorus and sulphur, have been obtained; similar is also true for nonferrous metals, copper and tin. The relatively high nitrogen content, 110 ppm, is characteristic of steels melted in electric arc furnaces. On the surface of several billets, transverse cracks were found, which extended from the corners to the surface centre, as well as discontinuities along large grain boundaries, stretching over a considerable area along all continuous cast billet planes. A view of the surface is shown in Fig. 2, while a fragment of the corner area with a patterns of cracks visible after etching of the sample is presented in Fig. 3.
It should also be noted that the oscillation marks deflect distinctly with increasing distance from the edge, assuming a curvilinear shape. Such a shape of these traces is indicative of clear disturbances of the steel level position in the mould. Metal turbulences at the meniscus level and a variable metal level in the mould, which cause the state as seen in Fig. 2, are an impermissible irregularity in the continuous casting technology. Some of the cracks visible in Fig. 3 are up to 5 millimetres deep, so their removal by grinding of the billet surface is not fully possible.
To identify the phase composition immediately around and inside the cracks, observations of metallographic sections were made using a light microscope and a scanning electron microscope. The contents of elements in selected micro-areas were determined by the X-ray microanalysis method using an energy dispersion spectrometer (EDS) on an Inspekt F scanning electron microscope with an EDAX microanalysis attachment. Examinations of the scanning microscope were made at an accelerating voltage of 15 kV. The position of a randomly selected cracks at the billet edge is shown in Fig. 4, while Fig. 5 shows the magnification of the lower crack part, with the locations, for which the determinations of chemical composition of the phases present there were made, being marked with numerals.
The chemical composition of phases, as determined at particular points, is given in Table 2.

When analyzing the chemical composition of the material filling the crack and its neighbourhood it can be noticed that at point ”2” there is an agglomeration of aluminium oxide, which can either be the product of steel deoxidation with aluminium or be a component of the protective flux added to the mould. Point “3” corresponds to the mixture of the oxides of iron, silicon and manganese, whereas point ”4”, the oxides of manganese and iron. At points “3”, ”4” and “5”, considerable concentrations of oxygen and iron occur, which suggests the oxidation of the inner surface of the cracks. It can also state with a high probability that, in this region of the crack, the oxides filling it up come to some extent from the flux material, which contained among other 1.0.2.5% Fe2O3, 2.5.4.0% MnO and up to 5.5% Al2O3. Since it is hard to explain their origin by the incomplete modification of the composition of oxide Non-metallic inclusions in the liquid steel at the ladle furnace process stage, considering the 0.017% Al content of the steel and its modification with a Ca-Si alloy (with a finished steel Ca content of 26 ppm). The confirmation of this observation is the chemical composition of the Non-metallic phase represented by point “5”. The presence of carbon, sodium and chlorine in this zone indicates that the source of these elements are chemical compounds being components of the granular flux added to the mould. The relatively high concentrations of chromium and nickel, found in the region where point “5” lies, also require explanation. These elements come from the protective coat applied on the inner plane of the mould’s copper tube walls. This coat consists of chromium and nickel, among others. This means that this coat might have been partially damaged as the continuous cast billet skin moved along the mould walls.
To make sure that the composition of the Non-metallic phases present inside transverse cracks, as determined by X-ray microanalysis, has not the nature of an isolated case, a crack situated in another area (Fig. 6) was randomly chosen and subjected to similar analysis, whose results are given in Table 3.

By comparing the chemical composition of Non-metallic phases, determined at point ”1” (Fig. 6), with the composition determined for cracks in another area (Table 2), great similarity can be found to exist between them. The differences between the contents of particular elements not being large enough to allow the observation that their source is the mould protective flux, what it was earlier affirmed. It is also characteristic that, in the case analyzed, chromium and nickel also occur inside the crack, whose source is an internal Cr+Ni alloy protective coat that is applied on the mould’s copper surface. On the basis of the obtained analysis results, a decision was made on the need for changing the chemical composition of the mould flux so as to achieve its lower viscosity in the temperature range of 1200 - 1500 C.

To obtain additional information on the location of occurring transverse cracks, using an optical pyrometer, a measurement of the temperature of their surface and corners was taken immediately after the material had exited the straightener (about 0.6 m beyond the stand). The areas in the billet surface, where the temperature Points 1 and 3 correspond to the temperatures of the upper and lower corners, whereas point 2 was situated approximately in the middle of the lateral plane. The cracks at the billet corners, visible to the naked eye, situated on the upped plane side (smaller radius of CC machine), are marked with the arrows. The obtained temperature values are given in Table 4. As the temperature measurement was also taken at the exit from the straightening stand, it can be assumed that the measured temperature of the surface and the corners corresponds to the temperature, at which the cast billet deformation caused by straightening took place. As indicated by the values shown in Table 4, the temperature of the upper corner (3) assumed values from the range, in which steels with a niobium content of 0.028% exhibit a distinct loss of plastic properties, as follows from Fig. 1. The clearly lower temperature of corner (3) compared to corner (1) was indicative of a higher intensity of cooling this area of the continuous cast billet. As turned out, the cause of this situation was an excess of water, which, in an uncontrolled manner, dripped down one of the spray cooling zone segments onto that fragment of the billet surface. The material deformation (tensile stresses on the upper billet plane) caused by straightening of the material at too low a temperature was probably one of the causes of transverse cracks occurring at the corners.
The material weakening state could have been additionally aggravated by precipitates of nitrides (e.g. of niobium, titanium) or carbides and carbonitrides of niobium at primary austenite grain boundaries. The relatively low concentrations of copper and tin in the examined steel (Table 1) may, in this case, not to be taken into account as the additional cause of the low plasticity of material during deformation.

The high prevalence of transverse cracks in the corner area of 160 mm-square cross-section continuous cast billets from S355J2 – NbTi grade steel being classified into the group of subperitectic steels with the micro-additions of niobium and titanium, justified the need for undertaking the analysis of the causes of the occurrence of this defect that had a substantial effect of reducing the yield of the full-value material. The metallographic analysis of the crack zone on samples taken from the commercial melt with the use optical microscopy and X-ray microanalysis methods using an energy dispersion spectrometer (EDS) on an Inspekt F scanning electron microscope showed that the crack interior was filled with no-metallic phases of diverse chemical composition. In the composition of those phases, compounds were identified, which might be either deoxidation products (e.g. Al2O3, SiO2 and MnO) or elements typical of the flux added to the mould, such as carbon, sodium and chlorine. Moreover, chromium and nickel were also found to be present within the cracks, which might have come from the internal mould wall protective coat. These observations indicated the need for changing the flux properties (viscosity at a temperature of 1300.C), and thus the change of the conditions of the flux layers interacting between the mould wall and the forming billet skin.
The performed measurements of continuous cast billet surface temperature immediately after the straightener, using an optical pyrometer, showed that the temperature in the upper corner zone reached values in the range of 785 - 890 C. According to the results of a literature review, in the temperature range of 800 - 900 C steels with 0.028% of niobium are characterized the lowest values of the reduction of area. It was found that billet straightening was conducted at too low temperatures. It should also be noted that distinct differences in temperature between individual corners were found. With the aim of correcting this irregularity, efforts were undertaken to eliminate the uncontrolled water streams in the spray cooling zone.
The negative influence of the total amount of copper and tin in this type of steels in the form of precipitates at the austenite grain boundaries, as confirmed by the results of numerous studies, could be omitted in the case of the examined chemical composition of the steel, due to the fact that the calculated value of the so called equivalent copper content factor was Cuequ: = 0.17, against the recommended value of 0.20.
The possibility of precipitation of nitrides, carbides and carbonitrides of alloying elements (Nb, Ti) and Al in the steel of the analyzed chemical composition, as confirmed by calculations, while considering the potential for their precipitation at grain boundaries, indicates that the proposition to limit the concentration of nitrogen in steel melted in an electric arc furnace is legitimate. The amount of precipitated nitrides will be in that case proportionally smaller.
Observations of the surface of cast billets revealed irregularities in formation of oscillation marks, which, as has been found, make the location for transverse crack initiation. The correction made to the main mould operation parameters associated with the oscillatory mould movement showed that the optimal casting speed values for these steels were contained in the range of 2.0.2.2 m.min..1, with stroke length of mould of 7.0 mm and an oscillation frequency value in the range of 172 +/- 189 cycles/minute, depending on the casting speed.
Reference: Not shown in this demonstration version.