As a promising alloying approach, the modification of chemical composition by increasing the B content and decreasing the N content has been applied to improve the creep resistance of various 9–12% Cr heat-resistant martensitic steels. The 9–12% Cr steels have to exhibit high long-term creep strength, oxidation resistance in a high temperature steam, low cycle fatigue resistance, impact toughness, etc. The creep resistance is the main critical requirement: the minimum long-term creep rupture strength on the base of 100,000 h should be 100 MPa or higher at 650 °C.
In the fossil power plant industry, 9–12% Cr martensitic steels are widely used materials for such components as boilers, pipes, turbines, rotors, and blades, etc. 
An increase in the operation temperature to 650–720 °C and pressure of power units with ultrasupercritical (USC) and advanced ultrasupercritical (A-USC) conditions require the development of new generation steels with enhanced creep resistance. Heat-resistant materials, such as austenitic steels and nickel-based superalloys, are used only for some parts of fossil power units (approximately up to 15%) 
. Therefore, the high-chromium heat-resistant martensitic/ferritic steels remain as the main materials for fossil power units due to their excellent combination of creep resistance and fatigue resistance and smaller thermal expansion and larger thermal conductivity compared to austenitic steels and nickel-based alloys, as well as their low cost 
The creep resistance of high-chromium martensitic steels is determined by the stability of the non-equilibrium hierarchical microstructure. Prior austenite grains (PAG), packets, and blocks with martensite laths are the structural elements of the typical martensite lath structure 
. Two main dispersion strengthening phases are the nanoscale M23
(where M—Cr, Fe, W, etc.) carbides and the fine MX carbonitrides (where M—V, Nb and X—C, N). During tempering, the M23
carbides precipitated on boundaries, while the MX carbonitrides homogeneously precipitated in the lath interiors. Both M23
carbides and MX carbonitrides prevent the dislocation climb and slow down the migration of low-angle boundaries through suppressing the knitting reaction between the dislocations comprising lath boundaries and lattice dislocations 
. The transformation of the lath structure into the subgrain structure results in degradation of creep resistance.
One of the most effective ways to enhance the creep resistance was suggested by researchers at the National Institute for Materials Science (NIMS) in Japan. The method consists of increasing the B content and decreasing the N content 
. Microalloying by approximately 0.01 wt.% B can increase the long-term creep resistance of the steels 
. Boron has a positive effect on the coarsening resistance of M23
–type carbides 
. A nitrogen removal to approximately 0.01 wt.% prevents the BN formation and can also positively affect the creep resistance 
2. Obtaining and Heat Treatment of Steels
2.1. Chemical Composition of Steels
Advanced steels contain 2–3% Co, which is known to have a positive effect on the microstructure and creep strength of high-Cr steels 
. The main purpose of the Co addition is to suppress the formation of undesirable δ-ferrite during normalizing. Helis et al. studied the effect of 0–5% Co on the 9Cr-3W-0.2V-0.05Nb-0.08C-0.05N steel 
. It was found that addition of 1% Co reduced the δ-ferrite fraction from 6% to 0.4%, while 3% Co completely eliminated the δ-ferrite. This fact is due to Co being an austenite-forming element, and it extends the austenite region on the phase diagram. The absence of δ-ferrite in high-Cr steels increases the stability of the tempered martensite lath structure 
On the other hand, although precipitates do not contain Co, the addition of 3% Co provides an increasing amount of MX carbonitrides and M23
particles. It was revealed that the number of precipitates around PAG boundaries significantly increased at 3% Co from 6 (at 0% Co) to 14 per μm2 
. Co also affects the chemical composition of precipitates, increasing the V content in MX particles, and the Fe, Cr, and W content in M23
particles. Therefore, Co indirectly affects the precipitation strengthening of 9–12% Cr steels.
2.1.1. The 9% Cr Steels
The 9% Cr steels are represented by the advanced CSEF steels, such as MARBN, G115, and SAVE12AD steels, and experimental 9Cr-1.5W-3Co steel.
The MARBN steel is the Japanese 9% Cr steel developed by National Institute of Materials Science (NIMS), in co-operation with private companies in Japan, for application to thick section boiler components in USC power plant 
. MARBN is a MAR
tensitic 9Cr steel strengthened by B b
oron and N n
. Various compositions of MARBN steel are presented in literature, differing in concentrations of B and N 
The G115 steel is the 9% Cr Chinese steel developed by the China Iron and Steel Research Institute (CISRI) and Bao Steel 
. This 9Cr-3W-3Co-1CuVNbB steel is recommended for use in USC power plants at operation temperatures up to 650 °C in China owing to its excellent overall properties. It is reported that G115 with 2.8%W–3%Co is for piping and with 3% W–3% Co for tubing 
. Composition of the G115 is similar to MARBN steel, while approximately 1% Cu is added in order for additional strengthening by fine Cu-rich particles, by analogy with P122 steel 
2.1.2. The 10% Cr Steels
The 10% Cr steels are represented by the experimental 10Cr 
and 10Cr-0.2Re 
steels designed on the base of TOS 110 steel. As known, TOS 110 steel was developed in Japan at Toshiba with the main composition of 10Cr-0.7Mo-1.8W-3Co-VNb-0.01B-0.02N for turbine rotor application at 630 °C 
. The experimental 10Cr steel is a modification of TOS 110 steel by decreasing the N content to 0.003% and addition of Ti (0.002%) while maintaining the high B content of 0.008%. It was shown that this modification results in enhanced long-term creep rupture strength 
. The NF12 
and HR1200 
steels were used for comparison of creep resistance of the 10Cr steel.
2.1.3. The 11–12% Cr Steels
The 11–12% Cr steels are represented by the TAF650, SuperVM12 steels, and experimental 12% Cr steels.
The TAF650 steel is the 12% Cr Japanese steel derived from the TAF steel 
. The TAF steel was developed in 1956 by Toshio Fujita 
and has the superior high temperature strength. The TAF650 steel was developed for improvement of poor weldability and hot workability. In the TAF650 steel, the part of Mo was replaced by W; Co and Ni were added; C and B contents were reduced from 0.21% to 0.1% and from 0.03% to 0.019%, respectively 
2.2. Vacuum Induction Melting of Steels
In contrast to high-chromium steels with conventional N content (~0.05%), the method of vacuum induction melting is used for producing steels with decreased nitrogen content (<0.01%). It is attributed to the fact that the nitrogen content in alloy is determined by the gas pressure on the molten alloy.
As is known, the solubility of the diatomic nitrogen gas (N2) in metal melt can be described by the reaction:
1/2 N2 = [N],
and obeys Sievert’s law 
, according to which the solubility of nitrogen gas in metal melts is proportional to the square root of the partial pressure of the gas (under constant temperature):
where [N] is the solubility of the nitrogen gas in metal melt at a given partial pressure of gas pN2; KN is the solubility constant (Sievert’s constant), which depends on temperature and the way concentration and pressure are expressed.
2.3. Heat Treatment
Heat treatment of the steels usually consists in normalization and tempering in order to form a tempered martensite lath structure. Normalization of the considered steels is carried out in a wide temperature range of 1050 to 1170 °C (Figure 1). Heating is carried out in the austenite region for preventing the δ-ferrite formation after normalization. Duration of heating varies from 0.5 to 1 h. During cooling (by air or oil), the austenite transforms to martensite. The normalization leads to the formation of the lath martensite structure, in which the PAGs, packets, blocks, and laths with high density of lattice dislocations are distinguished. The normalization temperature affects the PAG size. The size of PAG in the steels significantly varies from 20 to 200 μm due to different normalization temperatures.