In the past 20 years, China's bearing steel has experienced rapid development along with the economic take-off, and the bearing steel in many application scenarios has entered the international advanced level. However, it is undeniable that in some high-demand application scenarios such as aviation and high-speed rail, the participation of domestic bearing steel is still low; There is a big difference in the quality level of bearing steel between steel mills, and low quality bearing steel is still being produced in large quantities. How to realize China bearing steel take-off again? Skf (SKF) in the past 20 years of deep development of China's bearing steel experience, can be used as a mirror.

      1. The development history of SKF bearing steel in China

SKF has a long history in China, having entered the country in 1912, five years after the brand was founded by a young Swedish textile technician, Sven Winkvist. SKF China began to set up bearing manufacturing facilities in China in the 1990s and has evolved into a knowledge, technology and data driven company with a mission to deliver reliable operation. In China, SKF provides products and services to more than 40 industries including automotive, wind power, railway, machine tools, medical, food and beverage, paper and so on. By combining the successful experience and knowledge gained in different industries around the world, SKF Group has achieved rapid growth in China. At present, SKF has 17 production sites in China, and actively implement the strategy of "regional resources serve the regional market".

The localized production of a large number of bearings requires a large number of high-quality bearing steel. Today, most of the bearing steel used in SKF's China plants has been locally sourced in China. Why SKF can give consideration to local procurement of bearing steel and reliable and stable bearing quality? This is because SKF used to have a special steel enterprise that could produce the world's advanced level of bearing steel, and as a bearing enterprise with a hundred years of accumulated knowledge, experience and standardized management.

As early as 1916, SKF acquired Hoforsbruk Steel Mill, the predecessor of Ovako, one of the world's leading manufacturers of bearing steel, and in nearly a century of production and operation management, it was built into a leading manufacturer of special steel industry, creating the world's leading pure steel production process, forming a group company, And expand its business globally. The SKF-MR process, which can control oxygen content and inclusions to extremely low levels, is regarded as a classic smelting method and has been included in numerous textbooks. The SKF material standards are also widely regarded as the benchmark for high-end bearing steels. Since the quality level of bearing steel largely determines the performance and life of bearings, SKF Group adopts a globally uniform and known as "rigorous" steel approval process. In terms of the certification of Chinese bearing steel, we also follow the unified certification process of the Group. The steelmaking process, physical quality and quality assurance capability must meet the global standard and must be approved by the SKF Global Materials Expert Group unanimously.

In fact, at the beginning of the 21st century, SKF could only import bearing steel from abroad. The subsequent transformation was helped by SkF's unswerving approach to local development. In the 1990s, a number of advanced ultra-high power arc furnaces were introduced by Chinese special steel enterprises, laying a foundation for the production of high quality bearing steel. However, at that time, the quality level of Chinese bearing steel was still relatively low, and there was a huge gap with SKF's requirements. This was mainly due to three reasons: 1) the lack of understanding of high quality bearing steel by steel mills; 2) the steel mill does not grasp the production process of pure bearing steel; 3) The backward quality management concept of steel enterprises leads to poor product quality stability.

Over the past two decades, SKF has invested a lot of manpower and resources in joint development with Chinese steel mills. In the 1990s, SKF sent a group of top material and testing experts to China. These experts commented on China's bearing steel industry at that time: "Chinese steel mills had not yet established the production experience of pure bearing steel as western countries did. To make Chinese steel mills produce high-quality bearing steel, experience needs to be passed on hand by hand and patiently." Since then, SKF has helped Chinese steel mills develop bearing steel in three ways.

    1.1 Help Chinese steel mills understand what is high quality bearing steel

To help Chinese steel mills understand high quality bearing steel, the first step is to guide the steel mills how to make metallographic samples. When judging the purity of steel, microscopic inclusions of several microns need to be clearly identified, but at that time, the traditional metallographic sample preparation of steel mills was of poor quality, and inspectors would often remove microscopic inclusions as pollutants. In order to promote the automation level of the sample preparation equipment in the steel plant, SKF provides training opportunities for the inspection personnel in the European laboratory of SKF, and sends experts to the on-site training of the steel plant, so as to teach the sample preparation skills to the steel plant without reservation. This is the first lesson in detail for steel mills.

However, sample preparation is only the first step to accurately evaluate microscopic inclusions. At that time, the standard of bearing steel in China was still very backward, and the evaluation criteria for micro-non-metallic inclusions were vague. Large size non-deformed micro-inclusions (DS) and non-traditional micro-inclusions such as TiCN were not included in the evaluation of micro-inclusions. In the face of this situation, SKF has carried out the training of "SKF Microscopic Non-metallic Inclusion Evaluation System" for the relevant steel mills, and repeatedly verified the consistency of the evaluation of microscopic non-metallic inclusions. In addition to inclusions, the inhomogeneity of carbide, as an index to evaluate the microstructure uniformity of steel, is also an important factor affecting the bearing life. At that time, the slab continuous casting has been popularized in China, but the evaluation criteria of carbide inhomogeneity in the international and domestic standards are not applicable to the bearing steel produced by slab continuous casting. Therefore, according to the influence of carbide non-uniformity on bearing life, SKF established a set of carbide non-uniformity evaluation system suitable for continuous casting bearing steel, and promptly spread it to Chinese steel mills.

These actions, let the steel technical personnel really understand their own production of bearing steel and SKF requirements of the gap, so as to have the direction of efforts.

    1.2 Guide the steel mill to establish the production concept of pure bearing steel

To establish the production concept of pure bearing steel is a necessary condition for producing pure bearing steel. Bearing steel, as the king of steel, has a complex production process. At that time, Chinese steel mills generally adopted the rough production mode with the composition reaching the standard and sampling inspection as the main goal. Each process will find a lot of improper operation methods that affect the purity of molten steel, for example: in order to avoid large aluminum loss in vacuum degassing process, aluminum is added after vacuum degassing; Before continuous casting, calcium silicate wire was fed to improve pouring ability. Open casting without protective atmosphere during mold casting... SKF patiently taught pure bearing steel production concept, gradually change the traditional steel mills operating habits, the refractory material, alloy, ladle, deoxidization process, refining slag, vacuum degassing, protective casting and liquid level fluctuation, high temperature diffusion, control rolling and the whole process of bearing steel production control in detail, and curing process, Ensure that all SKF brands are manufactured in the specified process path.

    1.3 Carry out the concept of zero defect to ensure the stability of quality

Pure bearing steel is produced, not checked out. Quality stability is another basic requirement of SKF for pure bearing steel. Steel mills in the 1990s had little awareness of quality. "Why complain about my quality problems when I can get the steel?" was a real case that SKF encountered in the steel mills. The idea of zero defects in steel mills seemed unthinkable at the time. But SKF did not give up, because process control was extremely important, and all steel sampling schemes were planned on the basis of stable processes. If the process is unstable, even if the inspection result is satisfactory, it is not acceptable. Through numerous on-site training and audits, the steel mill began to understand the requirement of zero defect, and took it as a daily code of conduct. No matter the technical personnel or on-site operation staff, they could memorize the technical requirements of SKF products in heart. It took more than eight years for the first companies to gain formal approval from SKF. In the development of bearing steel for Chinese steel mills, SKF has sent over one hundred expert teams to help improve the steel mills, and used many SKF laboratory resources from the United States, Germany, Sweden, the Netherlands, France, India and other countries to confirm the materials of Chinese steel mills.

The fundamental driving force of the great progress of bearing steel enterprises still comes from the determination and practical actions of the managers of steel mills. The promotion of SKF makes the steel mill take fewer detours in the development process, and this model can also be applied to promote the further development of Chinese bearing steel in the future.

      2. Status quo of bearing steel in China

    2.1 Bearing steel standards have made significant progress, and the standard cognition needs to be improved

China's bearing steel standards have made great progress in the past ten years. The new national standard GB/T 18254-2016 "high carbon chromium bearing steel" has made requirements on the control of large particle spherical inclusions (DS), titanium carbon nitridate (TiCN) inclusions and chemical elements such As Sn, As, Sb, Pb, Al, Ti, Ca, etc. At the same time, GB/T 10561-2005 "Standard Rating Chart for Determination of Nonmetallic Inclusion Content in Steel Microscopic Inspection Method" (ISO 4967) A method is taken as the evaluation criterion of microscopic inclusions, which is A great progress compared with GB/T 18254-2002 "High Carbon Chromium Bearing Steel". However, at present, there are still many domestic steel mills lack of due understanding of method A in GB/T 10561-2005 "Standard Rating Chart Microscopic Inspection Method for Determination of Non-metallic Inclusions in Steel", resulting in deviation. For example, the quality guarantee of some steel mills for continuous casting bearing steel shows that according to the A method in GB/T 10561-2005 "Standard Rating Figure Microscopic Inspection Method for the Determination of Non-metallic Inclusions in Steel", spherical non-deformation microscopic inclusions, Class D fine inclusions, Class D coarse inclusions and DS series inclusions are all zero. In fact, such a result is in the international advanced level of steel mills are difficult to achieve. Low-level steel mills can not really realize their gap, but also prompted bearing enterprises unfamiliar with bearing steel to excessively tighten the acceptance standards of microscopic inclusions.

   2.2 Strengthening the attention to macroscopic inclusion is one of the necessary conditions to improve the quality of bearing steel

When the new national standard was formulated, considering that the domestic application technology of high-frequency flaw detection was not mature at that time, the high-frequency water-immersion flaw detection was not a mandatory requirement for the detection of macro inclusions as SkF did. However, the macro inclusions are the indicators that many domestic steel mills need to pay attention to. At present, China's bearing steel enterprises still have the situation of polarization. The level of conventional bearing steel produced by some enterprises has ranked among the world's advanced ranks, while some enterprises still cannot produce high-quality bearing steel despite the use of world-class smelting equipment. One of the main reasons is that there are too many macroscopic inclusions in steel, and macroscopic inclusions are an important factor leading to early failure of bearings. In the 10MHz high frequency water immersion testing of ordinary bearing steels randomly selected from the market by SKF, it is found that there are not a few macroscopic inclusions larger than 10mm. Due to the failure of many steel mills and bearing enterprises to identify macroscopic inclusions in steel as effectively as SKF and realize the risks it brings, a large number of low-quality bearing steels are still circulating in the market.

    2.3 The development of bearing steels suitable for highly demanding application scenarios requires close cooperation between bearing enterprises and steel mills

For the vast majority of bearing steel varieties used in high-demand application scenarios, although Chinese special steel enterprises already have necessary production equipment such as protective atmosphere electroslag smelting and double vacuum smelting, Chinese steel mills lack practical opportunities because the related bearing enterprises do not purchase the high-end bearing steel in bulk in China. In addition, it is not necessary to have high quality bearing steel, so we can produce high quality bearings. For example, for the aero-engine bearings produced by SKF, in addition to the extremely high requirements for the key parts of steel, strict control is carried out on the rear process, etc., so as to finally produce bearings that can meet the harsh working environment of aero-engine. Therefore, the development of bearing steels suitable for high-demand application scenarios requires close cooperation between advanced bearing enterprises and advanced steel mills.

      3. Win-win cooperation to promote China bearing steel to take off again

Bearing with excellent performance is the combination of excellent bearing design, excellent bearing manufacturing technology and high quality bearing steel. SKF is deeply rooted in China and continues to increase its investment in China in line with the strategy of "serving regional markets with regional resources". In the future, more SKF high-end bearings will gradually move toward localization, so the localization development of the required bearing steel has been put on the agenda. SKF will continue to cooperate with Chinese steel mills in the development of high quality bearing steel, and is willing to contribute to the further improvement of the standard of bearing steel in China and the re-development of bearing steel.