CPC technology class · C21

C21D - Modifying The Physical Structure Of Ferrous Metals

Modifying the physical structure of ferrous metals; general devices for heat treatment of ferrous or non-ferrous metals or alloys; making metal malleable, e.g. by decarburisation or tempering. 6,851 US utility patents granted 2015–2025, with the leading assignees and year-by-year grant trajectory.

Compiled by PlainPatent · May 15, 2026

6,851
US patents granted: #1/3
in C21: 82.7%
C21 class share: +21%
5-yr velocity

The verdict

According to USPTO PatentsView, Modifying The Physical Structure Of Ferrous Metals ranks #211 of 670 CPC technology areas - 6,851 US patents granted since 2015, with recorded grant volume up 21% over five years.

#211
of 670 CPC technology areas: 6,851
US patents granted, 2015–2025: Jfe Steel
leading assignee (865): +21%
grant velocity, last 5 yrs

How C21D's grant cadence moved

Between 2020–2024, C21D recorded 3,222 grants versus 2,656 in 2015–2019 (+21% equal-window velocity). Across complete years, output peaked in 2019 at 725 grants and bottomed in 2015 at 366 (+98% peak-over-trough). Lead assignee JFE Steel Corporation holds 12.6% of the class ; the top three share 27.5%.

Leaderboard read

C21D concentration, lead at 12.6%

JFE Steel Corporation leads this historical grant record at 12.6% of C21D - foreign corporation assignees hold 90% of class grants (4,192 of 4,638). Figures describe granted US utility patents in this CPC subclass, not current investment or legal strength.

Leader's share: 12.6%
Top three share: 27.5%
Lead over #2: 249

Where C21D sits inside C21

Among 3 subclasses in C21 (Metallurgy OF Iron), C21D ranks #1 and holds 82.7% of the parent class's 8,280 recorded grants. Sibling chips below are the other subclasses in the same parent class, not the nationwide volume/lead-share peers further down.

C21C 716 C21B 713

Rising assignees active in C21D

Company-wide grant multiples (2022–2024 vs 2019–2021) among assignees with ≥5 patents in this subclass. Growth is company-wide; class patent counts are subclass-specific.

#1 Proterial, Ltd. 139x 0 → 139 company-wide · 25 in C21D
#2 Ebner Industrieofenbau GmbH 10x 1 → 10 company-wide · 9 in C21D
#3 Alloy Enterprises Inc. 9.00x 0 → 9 company-wide · 8 in C21D
#4 Tata Steel Nederland Technology B.V. 5.33x 3 → 16 company-wide · 7 in C21D
#5 Nippon Steel Stainless Steel Corporation 4.75x 4 → 19 company-wide · 22 in C21D

C21D grants by year, peak 2019 (725)

US utility-patent grants per year in C21D, 2015–2025 - recent five years are up 21% versus 2015–2019.

Jfe Steel

865
Nippon Steel

616
Posco

400
Nippon Steel &…

357
Arcelormittal

197
Baoshan Iron & Steel

116
Kobe Steel

109
Thyssenkrupp Steel…

76
Ntn

52
Hyundai Steel

44
Nhk Spring

39
thyssenkrupp

38

What this shows Jfe Steel is the most active filer in C21D, holding 865 of the 6,851 patents in this class. Concentration at the top signals how contestable this technology area is for new entrants.

View data table

Top patent holders in CPC subclass C21D
Rank	Company	Patents in C21D
#1	JFE Steel Corporation	865
#2	Nippon Steel Corporation	616
#3	Posco Co., Ltd	400
#4	Nippon Steel & Sumitomo Metal Corporation	357
#5	ArcelorMittal	197
#6	Baoshan Iron & Steel Co., Ltd.	116
#7	Kobe Steel Ltd.	109
#8	ThyssenKrupp Steel Europe AG	76
#9	NTN Corporation	52
#10	Hyundai Steel Company	44
#11	NHK Spring Co., Ltd.	39
#12	thyssenkrupp AG	38
#13	Nippon Steel & Sumikin Stainless Steel Corporation	37
#14	ATI Properties LLC	34
#15	Daido Steel Co., Ltd.	34
#16	Aktiebolaget SKF	33
#17	Voestalpine Stahl GmbH	31
#18	Nucor Corporation	29
#19	Nisshin Steel Co., Ltd.	29
#20	SMS group GmbH	27

Footprint here vs. claim strategy

Each dot is a leading C21D assignee, placed by how many patents it holds in this class (horizontal) against its company-wide average claims per patent (vertical) - a read on whether the class is led by broad-but-shallow filers or focused deep-claim players.

Top 14 C21D assignees by class footprint. Bubble size = patents in C21D. Source: USPTO PatentsView (claim depth is company-wide across all classes).

Classes near C21D's 6,851-grant footprint

Nationwide nearest neighbors for C21D (#211 of 670 by volume ; lead share 12.6%) - not the C21 parent-class siblings above.

Nearest by grant volume

A61J 6,855 C01G 6,857 B63B 6,790 B60J 6,966

Nearest by lead-assignee share

H03B 12.7% B25F 12.7% A41C 12.6% A46B 12.7%

Questions about C21D

Which company holds the most patents in C21D?

JFE Steel Corporation leads C21D with 865 patents (12.6% of the subclass). By PatentsView assignee type, foreign corporations account for 90% of class grants.

Which C21D assignees grew fastest recently?

Among assignees with ≥5 grants in C21D, Proterial, Ltd. shows the highest company-wide grant multiple (139x: 0 grants in 2019–2021 → 139 in 2022–2024) while holding 25 patents in this subclass. Growth is company-wide, not class-only.

How does C21D compare inside parent class C21?

Inside C21 (Metallurgy OF Iron), C21D ranks #1 of 3 subclasses by grant volume and holds 82.7% of the parent class total. The largest sibling is C21C (716 grants).

What to do with this data

C21D in context - here's where to go next.

JFE Steel Corporation leads C21D with 865 grants in this class. See full portfolio
C21D ranks #211 of 670 by grant volume among all tracked CPC technology areas. See the ranking
Inside parent class C21, the largest sibling subclass is C21C with 716 grants. Explore C21C

Figures reflect USPTO PatentsView grant data through 2026-05-15; pending applications not yet granted are not counted.

Methodology - C21D figures are USPTO PatentsView granted-utility totals for this subclass (6,851 grants in the tracked window).

Every figure on PlainPatent is rendered directly from USPTO source data, no number is typed in by an editor. This page draws directly on USPTO source data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.