TechnologyTechnological
Progress of New
Materials for
Automotive Fasteners
汽車緊固件新材料技術進展
1. Status Quo of Automotive Fastener Manufacturing

China Fastener World no.67/2023

China's automotive industry continues to develop rapidly, and the production and sales volume of automobiles has ranked among thetop in the world. As the sales volume of automobiles continue to grow, people's concern about the automotive industry has changed frombrand and scale to automotive quality performance, brand and scale at the same time; the accelerated development of fasteners for newenergy cars has become a strategic area of contention. At present, the main connection methods for automobiles are welding, riveting,screwing and gluing, and so on, and screwing is still the primary connection method, so one of the quality performance of automobiles isdetermined by automotive fastener manufacturing technology [1].
Due to the lack of quality control in the manufacturing process of automotive fasteners in China and the lack of quality control
awareness from the manufacturing staff, there are obvious risks and hidden dangers. Although the scale of China's automotive market islarge, automotive fastener manufacturing enterprises are generally small in scale. The scale of manufacturing is small. Many enterprises
have poor control over the R&D and manufacturing procedures of automotive fasteners. The low R&D capability and the inability tokeep up with information on mass production cycle to meet the market demand in time lead to slow development of enterprises; there arestill some pain points of new energy automotive fasteners that need high attention.
At present, China is yet to fully achieve domestic manufacturing of materials and critical automotive fasteners and is greatlyaffected by the trade war with the United States as well as the epidemic. In this background, it is a goal and challenge for the fastener
manufacturing industry to push forward complete domestic manufacturing of critical automotive fasteners. The quality control of critical
automotive fasteners covers the whole process from raw material production to final products, and the key procedures of manufacturing
technology. This article combines collected enterprise information and accumulated experience, briefly describes thecurrent status and development of cold heading steel wires for automobiles, non-annealed cold heading steel for fasteners,
unquenched and untempered steel cold heading steel, as well as the possible problems faced by the development in thisarea, and makes an outlook on the prospect of new materials for the reference of the industry.

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2 Development of Cold Headed Steel

2.1 Cold Headed Steel Wire Rods
for Automobiles
As the automotive enterprises’ demand forcold headed steel for products is getting higher andhigher, the general technical level for the current
national standards of cold headed steel GB/T 6478
and GB/T28906, can no longer meet the demandedquality of cold headed steel for automobiles.
Furthermore, high-strength automotive fasteners
have higher material requirements and higher costs
than ordinary cold headed steel does [3]. Since there
is no national or industry standard for this kind of
special cold headed steel products in China, many
enterprises have formulated their own enterprisestandards, and the industry has aimed at improving
technology on the basis of GB/T 6478. In 2019
Special Steel Enterprises Association publishedGroup Standard T/SSEA0024-2019.
In 2021, the standard “Cold Headed steel
and Hot Rolled Wire Rods for Automobiles" waspublished. It was undertaken by the Secretariatof Steel Wire Rod and Sub-Technical Committee
in 2021, and was drafted by Jiangsu Yonggang
Group Co. and has entered the stage of waiting for
approval. The formulation of a cold headed wire rod
standard for automotive fasteners came at the righttime to upgrade the technology for cold headed
wire rods. Since cold headed steel wire rods forautomobiles have high requirements on composition,
purity, cold upsetting and surface quality, they
can meet the demand for raw materials for highstrength automotive fasteners in a targeted mannerand improve the technical quality level of Chinese
fasteners, mainly aiming to improve the technicalrequirements to meet the use of automotive fastener
manufacturing plants and facilitate the use of users
and standardization of fasteners.

2.2 Non-annealed Cold Headed Steel for Fasteners
For using medium carbon steel or medium carbon alloy steel to manufacture
high strength fasteners of grade 8.8 and above, hot rolled steel should go through"one spheroidizing and one drawing", or "two times of spheroidizing and twotimes of drawing". The process of the latter in sequence is spheroidized annealing,drawing, spheroidized annealing, drawing and finally cold heading. The purposeof annealing and softening is to obtain the structure and mechanical properties
required for subsequent processing (drawing, cold heading, etc.) [4]. The hardness
decrease caused by softening can improve the service life of the cold heading die, sospheroidizing has a great influence on the pass rate of cold heading. However, thisoff-line spheroidized annealing and softening treatment is the most time-consuming
and energy-consuming process in fastener manufacturing, with a cycle of about
18-32 hours. In light of this, a lot of research has been carried out in recent years
at home and abroad to shorten the time for spheroidized annealing and softening
or eliminate the need for annealing, so as to shorten the production cycle, reduceenergy consumption and protect the environment, and at the same time improvelabor productivity and reduce production cost.
Jiangsu Yongsteel Group has made great efforts in the research and developmentof non-annealed cold headed steel and has achieved stable sales, with the current
annual sales volume of about 130,000 tons. In addition, Jiangsu Yongsteel has
successfully taken the lead in establishing the "Non-annealed Cold Headed Steel
Wire Rods for Automotive Fasteners" for CSAE, and has recently started to prepare
the standard for approval.
For the non-annealed steel series, Jiangyin Xingcheng Special Steel Works has
successfully developed 10B30 and SWRCH35K steel wires for fasteners, which
can be drawn directly and cold-headed and skip annealing. Zenith Steel Group has
been mass-producing non-annealed cold headed steel wire rods for many years,
which can be safely applied to all kinds of grades 8 and 10.9 fasteners, covering
all kinds of bolts and nuts with large deformation, and the grades include 10B21,
10B33, SWRCH35K, 51B20, SH27, etc. Among them, SCM435 and ML35CrMo
have achieved the purpose of reducing the time for spheroidizing and annealing andsaving energy.
The fatigue strength of grade 8.8 non-annealed materials 10B33, SWRCH35K,
51B20 and the fatigue strength of the tempered parts are compared, and it is
discovered that the former has a stronger fatigue strength. This type of material
can be easily applied to long bolts to shorten processing and increase production
efficiency. Since the amount of several elements such as C, Mn and Si within nonannealed steels is as low as close to the lower limit, it increases the difficulty oftempering, results in unstable mechanical properties after heat treatment, increases
costs, and creates inconsistency between manufacturing technique and productstability. Therefore, it is still necessary to increase investment in research.

Automotive bolts and parts

China Fastener World no.67/2023The basic production process of the materials
for critical high-strength automotive fasteners
is: molten iron pretreatment (KR) → converter
steelmaking (BOF) → LF → RH/VD → billet
casting → billet opening → grinding → probing→ grinding → rolling. The focus of steelmaking
process control is impurity, segregation, grainsize, composition uniformity and consistency, andoxygen, nitrogen and hydrogen content, etc.; thefocus of rolling process control is surface quality (toavoid scratches and cracks), decarburization layer,
dimensional accuracy, oxide skin, head and tailtrimming amount, microstructure (to avoid Weissstructure), cold upsetting performance, etc. [2].
Critical fasteners also have higher requirements forhydrogen-delayed fracture resistance. Therefore, thedevelopment of automotive fasteners benefits fromthe continuous innovation of cold heading steel
technology and keeps pace with the times.

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2.3 Non-quenched and Non-tempered Cold Headed SteelThe new era demands for higher performance of automobiles are getting
higher and higher, especially with increased focus on safety, environmental
protection and highly efficient energy saving. It is required that the steel
itself should save energy, reduce manufacturing cost, and have better
comprehensive mechanical performance. Non-quenched and non-tempered
steel has the advantage of outstanding performance, high efficiency, energy
saving, low cost, and environmental protection. It has been vigorously
implemented both at home and abroad and is known as "green steel".
At present, the non-quenched and non-tempered steel for fastener manufacturing
is basically cold strengthened, and has three major types of structures: ferrite +pearlite, ferrite + bainite and ferrite + martensite duplexes. The non-quenched nontempered fastener technology in China was started in 1980s and formed 3 grades
which are MFT8, MFT9, MFT10 under GB/T3098.22-2009 "Mechanical Propertiesof Non-quenched and Non-tempered Steel Bolts, Screws, Studs with Fine Grains".
MFT8 steel wires are produced by thermomechanical rolling with controlledtemperature and cooling. The structure is ferrite and pearlite, the average grain size
is grade 12 and the size is about 5μm. In addition to high strength and plasticity, it
has excellent impact toughness and low temperature fatigue resistance, making it
particularly suitable for the manufacture of grade 8.8 high-strength bolts. When using
MFT8 steel wires to manufacture fasteners, the total compression rate during drawing
is controlled at 22% to 35%, and the optimal total compression rate is about 30%, sothat the pre-treatment annealing, quenching and tempering process can be omitted
and the stabilization of the aging process combined with the surface treatment can beadded, greatly reducing processing costs [5]. According to statistics, grade 8.8 bolts
can reduce electricity consumption of heat treatment by about 270KW/h per ton,which can reduce consumption, save energy and reduce carbon emission, benefiting
the country and the people.
In comparison, the development of applying of non-quenched and non-tempered
steel to Chinese bolts is slower, but there is also a greater breakthrough. In recent
years, the development of V-Nb and V-Nb-B-Ti alloy system grades are gaining
attention and tests are being done extensively in related areas. At present, studies
on cold strengthened, non-quenched, and non-tempered steels have shown that theaddition of trace amounts of niobium can have a positive effect without any obvious
disadvantages, but should be applied in the range of 0.020% to 0.035%; microalloyingof V-Ti-Nb can significantly reduce the susceptibility to grain coarsening and canincrease the amount of ferrite, especially the amount of intracrystalline ferrite, so as
to have the best combined mechanical properties. This results in the best combined
mechanical properties. For this reason, the addition of niobium to non-quenched andnon-tempered steels, mainly for fasteners, is necessary.

Jiangyin Xingcheng Special Steel has
successfully developed non-quenched andnon-tempered steel for fasteners with highhomogeneity grade 8.8 by adopting XDWP
(water bath), with good tensile strengthand shape (≤20Mpa), stable and reliable
performance, which is in line with lowcarbon environmental protection and green
manufacturing; Zenith Steel Group has
successfully developed non-quenched andnon-tempered cold headed wire rods, whichhave been applied in two strength grades,grade 8.8 and grade 9.8. At the same time, it
is increasing the research and development of
non-quenched and non-tempered cold headed
steel of grade 10.9. In recent years, Henan
Jiyuan Iron & Steel has developed SCN430
and SCN435 energy-saving non-quenched
and non-tempered cold headed steel, which
do not require heat treatment and have beenused in Grade 8.8 products.
Pan-Asia Technical Automotive Center,
Shanghai University and Nanjing Iron & SteelGroup jointly developed grade 8.8 M6X840
and M6X930 long bolts for electric vehicle
battery packs, which use non-quenched andnon-tempered steel MLF20MnVNb andare based on the ferrite + bainite structure,having additional stabilization aging process,a flexural strength ratio of 1:0.8 and higher
toughness. It can meet the requirement ofstraightness below 0.5mm/m, solve heat
treatment deformation in slender bolts and isworth promoting. In practice, the applicationof non-quenched and non-tempered steel tohigh-strength fasteners still comes with the
problem of worn dies.

3. Conclusion

China Fastener World no.67/2023

Technological innovation improves the performance of steel used for manufacturing bolts, the running speed ofautomotive vehicles, reduces components weight, increases drive power, and improves braking performance. It is importantto require the development of high-strength fasteners and advancement of processing technology to ensure the reliability. For
the automotive fastener research and development, on the basis of new material development and perfection, it is necessary
to develop structural design and manufacturing technology. Development of the whole vehicle and fasteners should be insync, and fastener functions should be highly integrated with the whole vehicle. Only through continuous transformation
and upgrade, steel and automotive fastener companies can join hands to win in the long term.
References[1] Zhang Xianming, Thermal processing technology and its development for automotive fasteners [J], Metal Products, 2010, 36(2): 6-9[2] Hui Weijun, Dong Han, Weng Yuqing, Steel high strength bolts [M], Beijing: Metallurgical Industry Press, 1999
[3] Zhang Xianming, Current status and development of cold headed steel in China [J], Metal Products, 2009,35(2):43-47[4] Zhang Xianming, Annealing process of cold headed wires [J], Metal Products, 2012, 38(5): 43-45[5] Shen Deshan and Zhang Xianming, Current status of production of ﬁne grain non-quenched non-tempered steel fasteners [J], Metal Products, 2010, 35(6): 1-5

Article by Hsien-Ming Chang136

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