Engineering Guide · 3 min read
Why Add Boron? ML20B, ML20MnB and ML20MnTiB in GB/T 28906
What makes ML20MnTiB different from ML20B and ML20MnB? Compare GB/T 28906 carbon, manganese, boron and titanium limits and their roles in hardening.
The short answer: ML20B, ML20MnB and ML20MnTiB are low-carbon cold-heading steels that use small boron additions to improve hardenability, with different manganese and titanium requirements. [1]
Boron-bearing cold-heading grades form a quench-and-temper group in GB/T 28906-2025. Their designations also identify additions such as manganese, titanium and vanadium. [1]
This article is part of the Chinese GB/T steel grades series.
The ML20B, ML20MnB and ML20MnTiB family
GB/T 28906-2025 lists a carbon–boron sequence including ML20B, ML23B, ML25B, ML28B, ML30B, ML33B, ML35B, ML38B and ML45B. Its manganese–boron group includes ML15MnB, ML20MnB, ML25MnB, ML30MnB, ML35MnB and ML40MnB.
The broader family also includes chromium–boron grades ML20CrB and ML25CrB, vanadium-bearing ML35VB, ML15MnVB and ML20MnVB, and manganese–titanium–boron grade ML20MnTiB.
Composition: C, Mn, B, Ti and V
| Grade | C (%) | Mn (%) | B (%) | Ti or V (%) |
|---|---|---|---|---|
| ML20B | 0.18–0.23 | 0.60–0.90 | 0.0008–0.0035 | — |
| ML20MnB | 0.18–0.23 | 0.90–1.20 | 0.0008–0.0035 | — |
| ML20MnTiB | 0.18–0.23 | 1.30–1.60 | 0.0008–0.0035 | Ti: 0.04–0.10 |
| ML15MnVB | 0.13–0.18 | 1.30–1.60 | 0.0008–0.0035 | V: 0.07–0.12 |
| ML20MnVB | 0.18–0.23 | 1.30–1.60 | 0.0008–0.0035 | V: 0.07–0.12 |
Composition values are mass percentages. Selected heat-analysis ranges, Table 4. An agreed B minimum of 0.0005% is permitted when hardenability and mechanical properties meet the requirements. A dash means no value for that selected element is shown. [1]
Boron works at a very small concentration
The listed B range, 0.0008–0.0035% by mass, equals 8–35 ppm. Convert mass percent to ppm by multiplying by 10,000.
Boron improves the ability of steel to develop a hardened structure during cooling. Its effectiveness depends on the boron remaining available for this role; nitrogen can tie it up. Titanium helps bind nitrogen and preserve the beneficial boron effect. [2] Vanadium forms stable carbides and can refine austenite grains; its effect depends on its amount and the thermal cycle. [3]
The manganese progression within the family
ML20B, ML20MnB and ML20MnTiB share C 0.18–0.23%, while their Mn intervals rise from 0.60–0.90% to 0.90–1.20% and 1.30–1.60%. Manganese also contributes to hardenability. [3] This lets the family vary hardening response while retaining that low-carbon interval.
Hardenability describes depth of hardening under a selected cooling process. Surface hardness measures the condition at one location.
What this adds to the ML35 carbon-steel route
The ML35, ML40 and ML45 family uses higher carbon intervals for quench-and-temper production. The selected ML20 boron grades combine a lower carbon interval with boron-assisted hardenability, an option when forming favors lower-carbon stock but the completed part needs hardening.
20MnTiB and 10B21 in the wider material family
20MnTiB is also a general alloy structural grade in GB/T 3077-2015. Its listed carbon interval is 0.17–0.24%, whereas ML20MnTiB in the wire-rod table above is 0.18–0.23%. The ML prefix identifies the cold-heading family used in this article. [4]
Chinese mills also supply 10B21, an SAE-style boron-steel designation. For example, Baosteel Q/BQB 517-2023 specifies 10B21 with its own supply limits. The ordered document identifies the composition to use for that material. [5]
What chromium and molybdenum add
40Cr, 35CrMo and 42CrMo under GB/T 3077 use chromium and chromium–molybdenum alloying to address hardening depth and resistance to temper softening.
Browse the HexFastener homepage or submit a drawing with the required material callout.
References
- GB/T 28906-2025 — Hot-rolled steel wire rod for cold heading. Official online text; Clause 4 and Tables 1–6.
- Digges, Irish and Carwile — Effect of Boron on the Hardenability of High-Purity Alloys and Commercial Steels. National Bureau of Standards, RP1938 (1948); boron, nitrogen and titanium.
- Nippon Steel — Special Steel Strip. Printed pages 29–30; general effects of alloying and heat treatment.
- GB/T 3077-2015 — Alloy structure steels. Table 1; chromium and chromium–molybdenum grades.
- Baosteel Q/BQB 517-2023 — Wire rods for cold heading. Table 1 and informative Annex B.