The ranked assignee list of 71 companies is led by a single steelmaker or refractory group holding 29 of the 156 records in scope, a clear margin over the fifth-ranked assignee’s 5 records and the tenth-ranked assignee’s 4. This is a leader-plus-long-tail structure rather than an evenly split field: the top 5 assignees together account for 34.0% of all records, but beyond the top ten the ranking thins quickly into single- and few-filing entrants. Anyone assessing competitive risk should focus first on the leader’s specific claim scope rather than assume broad industry-wide blocking coverage.

Core blast furnace process claims under IPC class C21B cover 64.7% of the 156 records in scope, making it by far the densest area. Furnace structural details under F27D (26.9%) and refractory ceramics/cement composition under C04B (21.8%) form the next layer, largely covering cooling stave design and brick or lining material formulations. Measurement, monitoring and data-processing classes — covering thermocouple systems, wear-thickness sensing and digital prediction — each sit at 5.1% or below, indicating comparatively lighter claim density in the instrumentation side of the field.

Filing volume rose from 7 records in 2017 to a peak of 18 in 2022, then pulled back — the last complete comparison shows 2021’s 8 filings falling to 5 by 2024, a -38% move. That pullback is real for the years it covers, but patent publication typically lags filing by around 18 months, so the 2025 and 2026 figures in any dataset are still incomplete and should not be read as confirmation of a continuing decline. A fair read is that the field cooled off its 2022 peak rather than that it has stopped moving.