The Influence of the Hot-Rolling Temperature on the Microstructure and Mechanical Properties of Ti-Nb Microalloyed 21%Cr Ferritic Stainless Steel

Abstract

Microalloying and heat treatment are essential processing techniques for ferritic stainless steel (FSS). Three different compositions of 21%Cr FSS with 0.28Ti, 0.21Ti + 0.05Nb, and 1.05Ti + 0.17Nb were prepared. The interaction effects of the Nb and Ti contents and hot-rolling annealing on the microstructure, mechanical properties, and precipitate phases of FSS were studied. The microstructure, crystal structure, and precipitation phase of steel at 930, 980, and 1030 °C with Ti-Nb microalloying were investigated using an optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM). The room-temperature tensile properties, surface roughness, and hardness were tested separately. This study found that the composite addition of Ti and Nb had a dual effect of fine-grain strengthening and precipitation strengthening. The 1.05Ti + 0.17Nb steel specimen had a moderate grain size and the best uniformity after hot rolling at 980 °C. The tensile strength and elongation were 454 MPa and 34.2%, which achieved an optimal balance between strength and plasticity.

1. Introduction

FSSs are nickel-saving materials that are widely used in fields such as kitchens, architectural decoration, and automobile manufacturing. However, FSSs have disadvantages, such as room-temperature brittleness and high sensitivity to intergranular corrosion caused by various factors, which restricts their further application [1]. FSS sheets tend to exhibit surface wrinkles during cold processing, such as rolling and deep drawing. This tendency is primarily attributed to the formation of grain clusters with specific plastic deformation characteristics, which are caused by the aggregation of grains with similar orientations within their structure. This defect not only affects the product’s appearance but also reduces its qualification rate and increases production costs. Enhancing the resistance to wrinkling, formability, and mechanical properties of FSS has emerged as a critical area of development focus [2,3].

Factors influencing the resistance to wrinkling of FSS encompass not only interstitial elements like C and N but also the hot-rolling process, annealing temperature, and cold-rolling reduction rate, among others [4]. Hot rolling is a critical process that determines the performance, surface quality, and smooth cold rolling of steel. Among the hot-rolling process parameters, the hot-rolling annealing temperature is an essential factor affecting the microstructure and formability of the steel [5]. Typically, low-carbon steel incorporates one or more microalloyed elements, such as Nb, V, and Ti [6,7], and is strengthened through controlled rolling and cooling [8,9] to enhance the precipitation of carbonitrides. This approach synergistically boosts the mechanical properties of structural steel via precipitation and grain-refining strengthening.

This study examined the impacts of different Nb-Ti contents on the microstructure and mechanical properties of 21%Cr FSS through alloy composition design. The FSS did not undergo phase transformation, and its only heat treatment method was recrystallization annealing. The impact of hot rolling followed by annealing on the microstructure and texture of the cold-rolled annealed sheets was significant. Three different compositions of 21%Cr FSS were hot rolled at 930, 980, and 1030 °C to investigate the effects of different hot-rolling temperatures on the microstructure and mechanical properties of the steel. Studying the effect of the regulation of the hot-rolling temperature on the formation performance is crucial.

2. Materials and Methods

2.1. Experimental Materials

This experiment utilized a 3 kg vacuum induction melting furnace to smelt Nb and Ti-stabilized 21%Cr FSS. The base steel was mass-produced Ti microalloying 00Cr21CuTi FSS ingots, to which industrial-grade pure iron, pure chromium, and pure Nb were added. After remelting in the VIF3 vacuum induction melting furnace, cast ingots were obtained. The chemical compositions of the three steels were analyzed using PDA-5500 S optical emission spectrometer (Shimadzu Corportation, Kyoto, Japan) with the specimens of different compositions designated as steels 1#, 2# and 3#, as shown in

Table 1. The chemical compositions of the tested steels (wt.%).

Steel	Cr	C	N	Ti	Nb	Si	Mn	P	S	Ni	Cu	Fe
1#	20.67	0.008	0.008	0.28	—	0.28	0.16	0.013	0.0006	0.12	0.37	Bal.
2#	20.74	0.009	0.009	0.21	0.05	0.30	0.17	0.015	0.0007	0.11	0.39	Bal.
3#	21.19	0.008	0.008	1.05	0.17	0.39	0.2	0.013	0.0009	0.12	0.43	Bal.

. Steel 2# contained a high Nb content, and steel 3# contained a low Nb content.

The surface of each stainless steel ingot obtained by vacuum melting was cut off, and electric discharge wire cutting was used to cut a steel billet with a thickness of 7 mm for hot rolling and cutting. Three 7 mm thick steel billets of uniform composition were placed into a resistance-heating furnace in separate batches. The casting billet was heated to 930, 980, and 1030 °C at a rate of °C/min and kept at that temperature for 20 min to ensure sufficient heating. Rolling was conducted with a reduction rate of 40%. The rolling speed of the rolling mill was 0.4 m/min, the rolling speed was 15 r/min, and the reduction rate of each pass was 5%. A total of eight passes of rolling were conducted.

The cooling method was furnace cooling for 5 min after each rolling pass. Water cooling was conducted after rolling. Then, each 4.2 mm thick hot-rolled plate was treated with a solution. The treatment process involved heating it in the furnace to 980 °C, holding it for 3 min, and then cooling it to room temperature before water cooling. The simulated annealing curve of the specimen steels is shown in Figure 1.

2.2. Testing Methods

Specimen sheet sizes of 10 × 10 mm were obtained from the hot-rolled plates, mechanical lapping and polishing were conducted, and ultrasonic cleaning was performed using absolute alcohol. In this way, all the specimens were prepared. Phase analysis was conducted using a D/Max-2400 XRD (Rigaku Corporation, Tokyo, Japan), and the scanning range (2θ) was 20° to 120°. The etchant was 2 mL of HNO₃ + 4 mL of HCl + 4 mL of glycerol + 2 mL of hydrogen peroxide to corrode for 90 s. The microstructure and morphology of the specimens were obtained using an Axio Scope A1 OM (Carl Zeiss AG, Jena, Germany) and Quanta FEG-450 SEM (Thermo Fisher Scientific, Vacaville, CA, USA). According to the Chinese National Standard “Method for Determination of Average Grain Size of Metals” (GB/T 6394-2002) [10], the intercept method was used to calculate the average grain size.

The tensile test was conducted using the WDW-300D electronic universal testing machine (Dongchen test instrument Co., Ltd., Jinan, China), and the elongation was measured using a contact extensometer. The displacement-loading method was adopted. The stretching rate was 0.5 mm/min, and the stretching direction was consistent with the rolling direction (RD). The shape and size of the tensile specimens are shown in Figure 2 [11]. Hardness tests were conducted using the WILSON-VH1102 microhardness tester (Wilson Instruments, Norwood, MA, USA), with a testing load of 300 g and a holding time of 12 s. Each specimen was tested at ten points, and the results were averaged. The hardness measurements were conducted [12]. The surface roughness of the test steels was measured via an LSM800 laser confocal microscope.

3. Results

3.1. Microstructure

The microstructures of the annealed hot-rolled plates of the steel 1#, 2#, and 3# specimens at the different hot-rolling temperatures of 930, 980, and 1030 °C are displayed in Figure 3. The microstructures of the three distinct steel components after hot rolling and annealing were similar, with clear grain boundaries and featuring a coarse grain in the center and a fine grain on the edge. This was due to the fast heat dissipation of the outermost layer of the billet during the cooling process, the rapid solidification rate, and the fine grain. The heat dissipation of the heart was slow, the solidification rate was slow, and grains had enough time to grow [13].

The statistical analysis of the grain size distributions for the three sheets of steel with the compositions of steels 1#, 2#, and 3# at the hot-rolling temperatures of 930, 980, and 1030 °C is presented in Figure 4. When the three component test steels were hot-rolled at 930 °C, the average grain sizes were 427.76, 421.66, and 401.34 μm; when they were hot-rolled at 980 °C, the average grain sizes were 431.62, 425.83, and 405.81 μm; and when they were hot-rolled at 1030 °C, the average grain sizes were 441.39, 435.24, and 401.34 μm, respectively. After comparison, at the same hot-rolling temperature, the average grain size was largest for steel 1#, followed by steel 2#, and smallest for steel 3#. The degree of the equiaxed grain size tended to increase gradually. This suggested that the Ti-Nb-stabilized alloy could refine the grain size, and the effect of refining the grain size was more evident after adding Nb and Ti [14,15].

The XRD diffraction patterns of the specimens subjected to diverse hot-rolling temperatures are illustrated in Figure 5. A comparative analysis of the hot-rolled test steels with varying hot-rolling temperatures was performed using MDI Jade 6.5. The spectra exhibited a complete match, with the five peaks of 434-L stainless steel, and there was no shift in the peak positions, which indicated the absence of any phase transformation. The XRD spectra revealed only the presence of the α-phase (bcc structure), with no detectable diffraction peaks corresponding to carbides. This absence could be attributed to the relatively low carbide content in the steel. This indicated that very little or no phase transformation occurred during hot rolling in the FSS, leading to quick recovery without recrystallization. This was consistent with the results of previous research [16]. With an increasing rolling temperature, the peak intensity of the (110) crystal plane diminished for all three test steels, while the (220) crystal plane exhibits a near disappearance in Figure 5. As the hot-rolling temperature increased, the peak intensity of the (110) crystal plane also weakened, while the peaks of the (211) and (220) crystal planes nearly disappeared in steel 2#. In steel 3#, the peak intensity of the (110) crystal plane also weakened, and the (210) crystal plane exhibited its highest intensity at 980 °C, followed by a decrease in the peak intensity as the temperature exceeded 980 °C.

3.2. Analysis of Precipitate Phase

The SEM morphology and EDS analysis of the hot-rolled annealing test steels are shown in Figure 6, Figure 7 and Figure 8. In Figure 7, the primary precipitated phases in the 0.28Ti steel are Ti(C,N) and TiN. A was a rectangular precipitate phase, mainly composed of Ti(C,N). B and C were composite precipitates, primarily consisting of a combination of TiN and Ti(C,N). The morphology of Ti(C,N) was predominantly rectangular, with dimensions in the range of 10–15 μm, and the eutectic temperature measured at 1032.1 °C [17,18]. The precipitation of Ti(C,N) was inevitable within the temperature range of rolling.

In steel 2#, the primary precipitates consisted of (Nb,Ti)(C,N) and Ti(C,N), as shown in Figure 7. Figure 7A,B are irregular precipitates, mainly composed of (Nb,Ti)(C,N), while C was mainly composed of Ti(C,N). (Nb,Ti)(C,N) precipitated, which consumed a portion of the stored strain energy. The onset temperature of recrystallization was delayed. Meanwhile, the pinning effect of (Nb,Ti)(C,N) on grain boundaries impeded grain coarsening during annealing [19]. Moreover, the precipitation of (Nb,Ti)(C,N) contributed to enhancing the strength, toughness, and resistance to intergranular corrosion of the FSS.

All three grades of hot-rolled annealing steel plates contained a specific quantity of carbonitride. Figure 7 and Figure 8 show that the main precipitate phase in the two 2# and 3# specimens was (Nb,Ti)(C,N), which indicated no significant difference in size and morphology with TiN. It mainly precipitated in this grain and took the form of granular particles. This was a composite precipitate phase with a lower content of NbC and a relatively higher content of Ti(C,N). For the FSS with a varying Nb/Ti content, the proportion of NbN and Ti(C,N) in the composite precipitate phase was the only variable.

3.3. Mechanical Properties

3.3.1. Tensile Properties

The tensile properties of the test steels with three different compositions at different hot-rolling temperatures are shown in Figure 9 and Figure 10. Each solid line represents the change in the tensile strength of the test steel, while each dotted line indicates the trend of elongation.

The FSS did not exhibit a distinct yield point. This was attributed to the predominance of a bcc structure, characterized by the α-phase. This microstructure possesses magnetic properties and generally cannot be hardened through heat treatment. The structural characteristics of the FSS dictated that it would not display a noticeable yield stage under stress, which was determined by its microstructural features and mechanical properties.

As the temperature increased, the tensile strength of the three types of hot-rolled annealing steel plates initially decreased and then slightly rose (or remained nearly unchanged), while the elongation first increased and then decreased, exhibiting significant variation. At rolling temperatures of 930, 980, and 1030 °C, the product of the strength and plasticity of steel 1# reached 5515, 6535, and 4738, respectively. For steel 2#, the product of the strength and plasticity were recorded as 6945, 7134, and 4257, respectively. In contrast, the product of the strength and plasticity for steel 3# showed values of 8761, 15,527, and 13,010 across the same temperature range.

3.3.2. Hardness

The hardness of the annealed plates of the three test steels at varying hot-rolling temperatures is illustrated in Figure 11. At hot-rolling temperatures of 930, 980, and 1030 °C, the hardness of steel 1# was measured at 149, 141, and 144 HV, respectively. Similarly, for steel 2#, the hardness was recorded as 154, 143, and 145 HV, while for steel 3#, it was 167, 160, and 161 HV. Overall, the high addition of Nb led to a significant increase in the steel's hardness. The relationship between the hardness and increasing the hot-rolling temperature aligned with the trend in the strength, initially exhibiting a decrease followed by a slight increase.

3.3.3. Surface Roughness

The surface roughness of each alloy sheet was typically assessed based on the average roughness (Ra), the maximum peak-to-valley height (Rt), and the determination of the surface finish grade according to the Ra value. The surface roughness curves and 3D views of the three types of hot-rolled annealing sheets at various hot-rolling temperatures are shown in Figure 12, Figure 13 and Figure 14.

Steel 1# exhibited Ra = 0.39 μm and Rt = 2.86 μm at a hot-rolling temperature of 930 °C; as the temperature increased to 980 °C, it showed Ra = 0.56 μm and Rt = 3.99 μm; and increasing the temperature to 1030 °C resulted in Ra = 0.67 μm and Rt = 5.49 μm.

Steel 2# exhibited Ra = 0.34 μm and Rt = 2.93 μm at a hot-rolling temperature of 930 °C; as the temperature increased to 980 °C, it showed Ra = 0.45 μm and Rt = 4.33 μm; and at a hot-rolling temperature of 1030 °C, the values were Ra = 0.67 μm and Rt = 5.33 μm.

Steel 3# exhibited Ra = 0.32 μm and Rt = 3.39 μm at a hot-rolling temperature of 930 °C; as the temperature increased to 980 °C, it showed Ra = 0.33 μm and Rt = 3.57 μm; and further increasing the temperature to 1030 °C resulted in Ra = 0.51 μm and Rt = 4.81 μm.

4. Analysis and Discussion

4.1. Effect of Hot-Rolling Temperature on Microstructure

The microstructure of the test steel in Figure 2 exhibited coarse grains at the core and fine grains at the edge, which are attributed to the non-uniform distribution of deformation zones and the presence of intragranular shear bands. Furthermore, due to significant recovery during the rolling process, most recrystallized grains displayed irregular shapes and a limited degree of grain equiaxiality. An increase in temperature led to a significant reduction in the number of coarse grains within the metallographic structure based on comparing the steel microstructures at different hot-rolling temperatures. Furthermore, there was enhanced clarity in the grain boundaries and an improvement in the grain equiaxiality. The grain boundaries of the OM were visible due to the low resistance of FSS to intergranular corrosion.

The results reveal that at a hot-rolling temperature of 1030 °C, the average grain size of the test steel was the largest, followed by 980 °C, and was the most fine at 930 °C, as shown via the comparison of the grain sizes of the test steel with the same composition at different hot-rolling temperatures in Figure 3. This suggests that a reduction in the hot-rolling temperature led to a slight grain size refinement, as illustrated in Figure 15. The grain size of steel 3# was consistently larger than that of steel 2#. Within a certain range, an increase in the Ti content in the steel was beneficial for promoting recrystallization but was detrimental to grain refinement. A higher concentration of Ti in the steel tended to react with the C and N at an earlier stage, forming a high−melting−point carbonitride of Ti. Due to premature precipitation (liquid precipitation) and relatively large particle sizes, these precipitates facilitated recrystallization while simultaneously reducing the activity of C and N within the steel. This reduction adversely affected the precipitation of carbonitride of Nb, which contributed to grain refinement. The grain size rating of the three test steels was grade 7 (256.1–512.0 μm) [20]. Steels with a grain size rating of 5–8 were classified as fine-grained steels in projects that tended to show small grain growth. At the same time, when the hot-rolling temperature was 1030 °C, there was a significant difference between the maximum and minimum sizes of grains. When the hot-rolling temperature exceeded 1000 °C, it led to an uneven grain size distribution characterized by abnormally large grains, resulting in an irregular microstructure. Consequently, the ability to coordinate deformation during room-temperature tensile deformation was relatively compromised.

By reducing the hot-rolling temperature, it was possible to refine the grain size of the hot-rolled annealed plates. At a hot-rolling temperature of 930 °C, the average grain size of the test steel with steel 1# was measured at 427.76 μm; it increased to 431.62 μm at 980 °C; and it further grew to 441.39 μm at 1030 °C. Therefore, lowering the hot-rolling temperature could enhance the yield strength and tensile strength of the hot-rolled annealed plates.

The thermodynamic calculations indicated that the three steel compositions remained within the ferrite single-phase region during the hot-rolled annealing processes at temperature ranges of 930, 980, and 1030 °C without undergoing the α→γ phase transformation. When temperatures exceeded 938 °C, both the LAVES phase and σ-phase were absent. Therefore, it was essential for the hot-rolling temperature to be above 940 °C. As the annealing temperature increased for hot-rolled specimens, the grain structure transitioned from a fibrous form to equiaxed grains, significantly enhancing the microstructural uniformity. A lower initial rolling temperature helps suppress recrystallization and may lead to detrimental phase precipitation. Thus, an optimal initial rolling temperature should effectively inhibit recrystallization while preventing the precipitation of the σ-phase and Laves intermediate phases in hot-rolled plates. Consequently, selecting a hot−rolling annealing temperature was advisable to ensure the complete dissolution of the precipitated phases without causing abnormal grain growth. In this study, an optimal lower initial rolling temperature of 980 °C is recommended.

Adding Nb could promote the precipitation of fine Nb(C,N) and other precipitates in FSS. Previous studies showed that precipitates containing Nb were typically quite fine, tending to ranging from tens to hundreds of nanometers. The precipitates increased the number of nucleation sites during recrystallization. These nucleation sites served to pin grain boundaries during grain growth, effectively impeding the migration of grain boundaries, delaying the onset of recrystallization, and hindering grain growth [21,22]. Furthermore, a separate portion of Nb within the steel dissolved into the matrix as a solid solution. These soluble Nb atoms also impeded grain growth [23,24,25]. The addition of Ti elements led to the precipitation of larger TiN particles and composite inclusions Ti(C,N) within the matrix. The accumulation of a substantial amount of stored energy around precipitates larger than 1 μm provided a greater driving force for recrystallization and grain growth, facilitating both processes. During the rolling process, significant deformation occurred around the precipitates due to the disparate deformation capabilities between the large precipitates and the matrix. This resulted in their transformation into nucleation sites for recrystallization during annealing following cold rolling [26,27].

4.2. Impact of Hot-Rolling Temperature on Mechanical Characteristics

4.2.1. Effect of Hot-Rolling Temperature on Tensile Properties and Hardness

The impact of the hot-rolling temperature on the mechanical properties of the tested steels was relatively minimal, primarily evidenced in the grain size. Increasing the hot-rolling temperature increased the grain size. As per the Hall–Petch relationship, the following correlations were present:

σ_s = σ₀ + Kd^−1/2

(1)

The equation consists of constants σ₀ and K, where σ₀ represents the resistance to deformation within the crystal, and K indicates the extent to which grain boundaries affect strength, primarily related to the structure of the grain boundaries; furthermore, d denotes the average grain size diameter in polycrystalline materials, measured in micrometers (μm). Equation (1) shows that in the presence of dislocation pile-ups, larger grain sizes led to higher stresses, facilitating plastic deformation in the surrounding grains compared with smaller grain sizes. For smaller grain sizes, the adjacent grains necessitated a certain magnitude of external force to undergo plastic deformation. Consequently, the tensile strength and yield strength decreased as the grain size increased.

The grain size and uniformity also served as crucial parameters for assessing the formability of the FSS. Varied grain sizes in the hot-rolled plates resulted in aggregation, which led to an increase in the r-value. Coarse grains oriented in a cubic manner presented a formability risk, resulting in wrinkling. The hardness curves of the hot-rolled steels with different temperatures and elemental contents are shown in Figure 11. Adding high amounts of Nb and Ti increased the hardness.

The primary precipitate in the matrix consisted of relatively large TiN particles. The presence of this precipitated phase significantly influenced the hardness, and larger TiN particles enhanced the hardness of the steel. After the addition of a small amount of Nb, TiN still precipitated. However, the quantity and size of the precipitates decreased. Furthermore, Nb containing the precipitate phase also formed in the matrix, reducing the hardness. With the increase in the contents of Nb and Ti, the quantity of Nb- and Ti-rich precipitates within the matrix correspondingly rose. The enhanced mechanical properties of the steel could be attributed to the combined effects of solid solution strengthening and precipitation strengthening via Nb and Ti.

4.2.2. Influence of Hot-Rolling Temperature on Surface Roughness

Revising the hot-rolling temperature downward led to a significant reduction in the surface roughness of the hot-rolled annealed plates. Lowering the hot-rolling temperature improved the wrinkle resistance of the steels, enhancing the finished plates’ surface quality. The histograms of the Ra and Rt values for the three varieties are shown in Figure 16.

At a hot-rolling temperature of 930 °C, the wrinkle resistance performance of the three test steels was notably superior, with average wrinkle height values of around 0.3 μm. When increasing the hot-rolling temperature to 980 °C, the average fold height Ra of the Nb-Ti-stabilized test steel at 0.45 μm was lower than the Ra of the Ti-stabilized steel at 0.56 μm. As the hot-rolling temperature rose to 1030 °C, the average wrinkle height of the three test steels increased, with the Ra value of 3# being the smallest. In the case of steel 1#, recrystallization occurred preferentially compared with steel 2#, and the grain size remained consistently larger than that of steel 2#. This clearly demonstrated that the inclusion of Nb led to a refinement of the grains in the steel [28], resulting in an improved surface finish and enhanced resistance to wrinkling. This phenomenon was attributed to the formation of fine precipitates as Nb combined with C and N, which effectively pinned down the grain boundaries. The process of recrystallization was inhibited, resulting in a reduction in the grain size during recrystallization.

This was attributed to the strong correlation between the surface roughness and the distribution of the microstructural grain orientation in the hot-rolled annealed plate of the 21%Cr FSS. Agglomerations of grains with similar or closely aligned orientations congregated, leading to an uneven microscopic orientation distribution, which is the primary cause of surface wrinkling during sheet metal formation. Due to the plastic deformation anisotropy in the thickness direction, differential strains occurred in the thickness direction, leading to wrinkling. During rolling and deep drawing, grains with a <111>//ND crystal orientation demonstrated an elevated plastic strain ratio. Grains with a <001>//ND crystal orientation, in particular, demonstrated a reduced plastic strain ratio. The r-values of {111}<uvw> and {001}<uvw>, which demonstrated in-plane anisotropy, differed by 20 times or more [29]. In an organizational structure with an uneven grain orientation distribution, the deformation process induces non-uniform contraction in the thickness direction of the plate, leading to wrinkles on the surface of the FSS. Conversely, in an organizational structure characterized by an uneven grain orientation distribution, the coordinated interaction between differently oriented grains serves to counterbalance non-uniform contraction during deformation, thereby mitigating the occurrence of wrinkles.

Both domestic and international scholars have analyzed wrinkle formation mechanisms from a textural perspective [30,31]. During the processing of FSS, the formation of wrinkles was attributed to the progressive increase in the ratio of the plastic strain to grain size (r-value) from the orientation of (001)//ND to (111)//ND. This led to differential contractions in various r-value grain clusters along the thickness direction, resulting in surface wrinkling on the steels [32]. FSS reduced the number of operable slip systems during deformation, thereby facilitating the formation of an anisotropic {111} texture following deformation–recrystallization [33]. The surface-wrinkling phenomenon in FSS was primarily attributed to the disparities in texture resulting from processing and formation, as well as the varied r-values exhibited by different textures during deformation.

5. Conclusions

• As the hot-rolling temperature increased, the average grain size enlarged. When the temperature reached 980 °C, the grain size was appropriately moderate, exhibiting the best degree of uniformity and overall performance. The relationship between the hardness and increasing the hot-rolling temperature aligned with the trend in the strength, initially exhibiting a decrease followed by a slight increase. Implementing medium-temperature annealing (980 °C) in hot-rolled steel is an effective approach to achieve excellent comprehensive mechanical properties.
• The primary precipitate phases in steel were Ti(C,N) and TiN. In the low-Nb steel 2#, the primary precipitated phases were (Nb,Ti)(C,N) and Ti(C,N). The formation of (Nb,Ti)(C,N) occurred during the annealing process, consuming a portion of the stored deformation energy and delaying the onset temperature of recrystallization. Additionally, the pinning effect of (Nb,Ti)(C,N) at the grain boundaries contributed to suppressing grain coarsening during annealing. The primary precipitates in the high-Nb steel 3# were (Nb,Ti)(C,N). The tested steel achieved excellent performance through the strengthening mechanisms of precipitate hardening from the Nb-Ti carbonitride phases and grain refinement.
• At an annealing temperature of 930 °C, the wrinkle resistance performance of all three types of steel was outstanding. Among them, at the same temperature, steel 3# exhibited the smallest Ra value. An increase in the Ti content in the steel was beneficial for promoting recrystallization but was detrimental to grain refinement. The addition of Nb enhanced the refinement of the grain structure, resulting in improved surface smoothness and better resistance to wrinkling.