Cracks Are a Carbide Problem: Continuous Casting of D2 Cold-Work Die Steel

Cracks Are a Carbide Problem: Continuous Casting of Cr12MoV Cold-Work Die Steel

Cr12MoV is a classic high-carbon high-chromium cold-work die steel with high hardness, high wear resistance, and good hardenability. It is widely used for blanking dies, shearing dies, and cold-forming dies. A note on designations before we begin: Cr12MoV (GB/T 1299) belongs to the AISI D2 family — the common international counterparts are AISI D2 / UNS T30402, JIS SKD11, DIN X155CrVMo12-1 (1.2379), ISO 4957 X160CrMoV12-1, and their commercial grades such as K110 and XW41.

As special-steel production moves toward short-flow routes, CC is increasingly used for die steels for its high productivity and yield. But the high carbon and chromium of Cr12MoV widen its solidification range: in the final stage of solidification, interdendritic segregation is pronounced and Cr-rich eutectic carbides form, making the slab inhomogeneous and crack-prone. Carbide control and crack suppression are therefore the two decisive problems in Cr12MoV continuous casting — and, as this case shows, they are in fact one problem.

Previous research confirms the link: Man et al. found marked solidification segregation in D2 CC slabs, with eutectic carbides coarsening from edge to center; Feng et al. showed that segregation and eutectic carbide evolution occur together, and that chain-like or continuous carbide distributions weaken ductility; Wu et al. linked cooling rate and strain to corner cracks in Cr12MoV; Liu et al. showed that carbides break and evolve during hot working. Mold flux and mold cooling matter equally: Mills et al. tie flux lubrication and heat transfer to surface quality, and Won et al. identify thermal stress concentration as a major cause of CC cracks.

At a Chinese special-steel producer, Cr12MoV CC slabs suffered surface longitudinal cracks, corner cracks, and local fractures, while final products showed eutectic carbide clustering and structural inhomogeneity. This article, adapted from the study published in the journal Special Steel, describes how the plant linked the cracks to carbide enrichment, optimized the flux and mold cooling, and verified the results industrially. The standards referenced below are paired with their international counterparts.

1. Process Route and Chemical Composition

To lower cost, the plant uses an inter-mixed route: molten alloy from an alloy-melting furnace is mixed with converter steel, refined through LF, RH/VD, and cast on a continuous slab caster at 200 mm × 630–830 mm × 5800 mm (thickness × width × cut length). Table 1 gives the main composition control requirements, with internal control ranges tightened well inside the standard ranges.

2. Defect Characterization: Cracks Meet Carbides

2.1 Macro features of surface cracks and fractures

Surface longitudinal cracks in CC slabs are linked to composition, mold heat transfer, secondary cooling, mold-level fluctuation, and straightening. At the plant, the defects were mainly surface longitudinal cracks, corner cracks, and local fractures at the slab corners and edges, extending further during secondary cooling, straightening, and hot-charge transfer. Part of the cracks ran along the casting direction with openings and local peeling — a signature of local stress concentration.

For a high-carbon high-chromium steel such as Cr12MoV, the high C and Cr contents promote interdendritic segregation at the end of solidification and the formation of Cr-rich eutectic carbides. This structural inhomogeneity lowers local ductility, so the slab cracks under cooling shrinkage, straightening deformation, and transfer disturbance. In addition, uneven mold heat transfer and fluctuating initial shell thickness reduce local load capacity and increase the tendency for surface and corner cracking.

2.2 Microstructure of the crack zones

Metallographic and SEM examination of the surface crack, corner crack, and adjacent uncracked zones shows bright-white carbides near the cracks, in places distributed in chains and continuous networks, with the crack path coinciding almost exactly with the carbide accumulation areas and clear clustering at crack tips. Given the composition, these bright phases are Cr-rich eutectic carbides formed by solute enrichment in interdendritic regions at the end of solidification.

Where carbides are coarse or locally continuous, they lower the local ductility of the structure and become crack sources during subsequent cooling and straightening. The finished flat bars cracked mainly at 30–50 mm from the corner, with diagonal cracks along the rolling direction. SEM results show stronger carbide clustering in the crack zones than in the uncracked zones, confirming that carbide segregation correlates with crack formation. The conclusion is direct: the cracks are a carbide problem, not just a surface-formation problem.

3. Process Optimization: Flux and Cooling

3.1 Dedicated mold flux optimization

Mold flux directly governs lubrication, interfacial heat transfer, and initial shell growth. For Cr12MoV, with its wide solidification range and high surface-crack sensitivity, mismatched flux parameters cause unstable flux film, insufficient local lubrication, and uneven heat flux, provoking surface longitudinal cracks and corner cracks. The plant adjusted the key flux parameters as shown in Table 2.

Lowering the basicity from 1.02 to 0.85, the viscosity at 1300 °C from 0.10 to 0.05 Pa·s, and the melting point from 1000 to 990 °C improved flux fluidity and spreading, stabilized the flux film in the mold, and made heat transfer more even. The number of surface cracks decreased. Yet local longitudinal cracks persisted, showing that mold cooling conditions also matter.

3.2 Mold cooling homogenization

Mold cooling affects initial shell thickness and surface temperature distribution. Because the high-temperature ductility of Cr12MoV is low, large differences in cooling intensity between mold regions create uneven shell thickness, which cracks under straightening and cooling stresses. The plant changed the cooling pattern as shown in Table 3.

Reducing the wide-face flow from 130 to 115 m³/h and the narrow-face flow from 30 to 20 m³/h, and cutting the water temperature difference from 2.8–3.0 °C to 2.2–2.4 °C, narrowed the cooling difference between regions and improved shell uniformity. Surface longitudinal cracks and corner cracks decreased. Moderately lowering local cooling intensity and homogenizing the mold cooling field reduce local thermal stress concentration and crack tendency.

4. Industrial Verification and Quality Evaluation

4.1 Quality indicators

Three indicators were tracked after optimization: UT acceptance rate, center porosity grade, and eutectic carbide grade. Table 4 shows the results.

The UT acceptance rate reached 95–97%, center porosity was held at 1.0–2.0, and the eutectic carbide grade at 2.0–3.5. Per GB/T 14979, carbide heterogeneity was rated on samples from one quarter of the flat-bar width and thickness. Internal cracks and porosity were controlled and carbide clustering reduced — the CC product approached the same-specification ingot-cast quality.

4.2 Ingot-cast versus CC carbide morphology

Before optimization, the eutectic carbides in the CC product were distributed as banded and flow-line structures with local chains, dominated by elongated, short-rod, and blocky morphologies with obvious directionality and local continuity — the visible footprint of interdendritic segregation. After optimization, carbide size decreased, continuous distribution and local clustering were reduced, and structure homogeneity improved. Samples of both routes were taken from the same grade with sampling location and subsequent state kept as consistent as possible.

4.3 Product appearance and hot-charging behavior

After optimization, the slabs showed no visible longitudinal cracks, corner cracks, or local fractures; corners and edges were intact, and the slabs ran normally through hot charging without crack propagation, local opening, or corner loss. The improved shell-growth conditions and reduced thermal stress concentration satisfied the hot-charging and subsequent rolling requirements.

5. Conclusions

The study draws four conclusions, all with direct industrial implications:

• Crack zones in Cr12MoV CC slabs show clear Cr-rich eutectic carbide clustering. Carbides near cracks are coarse and in places chain-like or continuous, and the crack path coincides with the carbide accumulation areas. Interdendritic segregation and eutectic carbide accumulation at the end of solidification are key causes of crack formation.
• Flux optimization — basicity from 1.02 to 0.85, viscosity at 1300 °C from 0.10 to 0.05 Pa·s, melting point from 1000 to 990 °C — improved mold lubrication and heat transfer and reduced surface cracks.
• Mold cooling adjustment — wide-face flow 130 to 115 m³/h, narrow-face flow 30 to 20 m³/h, water temperature difference 2.8–3.0 °C to 2.2–2.4 °C — reduced the cooling difference between regions, improved shell uniformity, and cut surface longitudinal and corner cracks.
• Industrial verification shows a UT acceptance rate of 95–97%, center porosity grade 1.0–2.0, and eutectic carbide grade 2.0–3.5, with product quality approaching that of the same-specification ingot-cast product.

There is a long-standing assumption that high-carbon die steels such as Cr12MoV must be ingot-cast. This case challenges it: when carbide discipline — a dedicated flux and a homogenized mold cooling field — is enforced, CC quality approaches ingot-cast quality at a fraction of the cost and yield loss. For buyers of cold-work die steel, the practical question is no longer whether the material was continuously cast, but whether the caster controlled its carbides and cooling as tightly as this producer did.