New insight into the relationship between the C-Cr ratio and carbides, mechanical property of cold working die steel

An new insight into the relationship between the C-Cr Ratio and carbides, mechanical property Cr Alloyed of cold working die Steel were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), rockwell hardness and impact toughness tests. The ‘C-Cr ratio’ and ‘Cr equivalent E Cr ‘ parameters were introduced to characterize the carbide characteristics and mechanical properties. The results show that the precipitation temperature of M7C3 eutectic carbides grows linearly with E Cr when the E Cr value is less than about 22, and the precipitation temperature increases significantly when the E Cr value exceeds 22, and the growth curve takes a plateau turn. The precipitation temperature of MC carbides decreases approximately linearly with E Cr value, and the precipitation of MC eutectic carbides will be completely suppressed when the E Cr value exceeds 19. The content of eutectic carbides in the as-cast organization is more, the shape is more complex and the size is larger. The ratio of Cr and Fe content in the composition of M7C3 eutectic carbides is linearly related to the Cr-C ratio; the content of carbides in the steel after forging and the E Cr value are basically linear. The average particle size and the average length of longitudinal carbides after forging are basically proportional to the E Cr value. The average particle size of longitudinal carbides after forging is greater than 4 μm and the E Cr value are basically linear; when the Cr content is greater than 4%, the quenching peak hardness of steel and the C/Cr have a good linear relationship; as the E Cr value gradually increases, the impact toughness of steel gradually decreases. These results are important not only for understanding the strengthening mechanisms of die steel, but also for the composition design and carbide control of cold working die steel.


Introduction
The complex composition and high alloy content of high carbon and high alloy steel pose great difficulties in predicting the 'composition-structure-performance' relationship of materials, especially for high carbon and high chromium cold working die steel, which must withstand considerable compressive stress and friction during operation, and must have high hardness and wear resistance [1,2].At present, the main method for obtaining high wear resistance of materials themselves (excluding surface treatment technology) is to obtain a high hardness matrix structure and distribute high hardness carbides on the matrix structure [3,4].Therefore, sufficient carbon and carbide forming element content is usually a necessary condition for cold working die steel.Considering the economy, among the commonly used Cr, W, Mo, and V alloy elements, Cr is the cheapest carbide-forming element.Thereby, high C and high Cr alloying has become the most common alloying method for cold working die steel, and Cr12 type (C: 1.4 ∼ 2.3 wt%, Cr: 11.0 ∼ 13.0 wt%) high carbon and high chromium cold working die steel has become the most widely used steel in the world 5.This kind of steel adopts high C content to enable the matrix to obtain high quenching martensite hardness, and uses high Cr content to form a large number of carbides (mainly M 7 C 3 and M 23 C 6 ) to increase wear resistance [6][7][8].However, due to the high content of C and Cr, this kind of steel will undergo eutectic reaction and form fishbone like ledeburite carbide, resulting in extremely low toughness.In the service environment with high impact load, early failure is easy to occur because of its insufficient toughness [9][10][11][12][13][14].Low Cr and low alloy cold working die steel (C: 0.95 ∼ 1.2 wt%, Cr: 0.5 ∼ 1.6 wt%) represented by O1(9CrWMnV) steel has high toughness, but insufficient wear resistance.In order to solve this contradiction, Cr5 type cold working die steel(C: 0.5 ∼ 1.0 wt%, Cr: 5.0 wt%) has been successively developed, represented by A2(Cr5Mo1V) steel, and Cr8 type cold working die steel(C: 0.8 ∼ 1.25 wt%, Cr: 7.8 ∼ 8.7 wt%), represented by DC53 (Cr8Mo2SiV) steel [15][16][17][18][19][20].
In summary, different combinations of C-Cr ratios are the main thread in the development and application of cold working die steel with different performance requirements.However, there is no quantitative characterization of the effect of different C-Cr content ratios on the microstructure and properties of cold working die steel.It is a scientific problem worth further study to obtain the ideal distribution state and size of carbide through reasonable control of C-Cr ratio, so as to achieve a relatively ideal matching of strength and toughness.This paper introduces the 'Cr equivalent' parameter to study the influence of 'Cr equivalent' on the size and mechanical properties of carbides in steel with different C-Cr ratios, which has reference significance for the composition design and carbide control of cold working die steel.

Materials and methods
In order to study the influence of different C-Cr ratios and matching on carbides and mechanical properties, different C-Cr ratio test steels were designed, with their C content, Cr content, and Cr-C as shown in table 1. Considering the usual content of Mo, V, Si, and Mn elements in cold working die steel, each steel was designed with approximately 1.0 wt% of Mo content, 0.3 wt% of V content, 1.0 wt% of Si content, and 0.45 wt% of Mn content.The preparation process of experimental steel was as follows: vacuum induction furnace smelting (25kg ingot) →annealing→ forging into material (diameter 20mm)→steel annealing.The smelting chemical composition is shown in table 1.After cutting off the riser of the steel ingot, the as-cast microstructure was cut inward at a depth of 20mm for observation.Conduct microstructure observation and carbide analysis were on the forged and annealed steel.The metallographic structure was corroded with 4% HNO 3 -C 2 H 5 OH (volume fraction).HITACHI S-4300 SEM was used for microstructure observation and EDS analysis of carbides.Thermo-Calc thermodynamic software was used to calculate the precipitation temperature of carbides.The test steels were quenched at 900 ∼ 1150 °C to obtain the peak hardness and corresponding quenching temperature of different C-Cr ratios.Quenching was carried out at the peak quenching temperatures of the test steels, and then the impact toughness was compared after low-temperature tempering at 150 °C and high-temperature tempering at 500 °C.Quenching hardness was measured using TH300 Rockwell Hardness Tester.The impact specimen adopted U-shaped notch with the size of 10 mm × 10 mm × 55 mm.The room temperature impact test accorded to the national standard ' Metallic materials -Charpy pendulum impact test method'.

Setting of 'Cr equivalent'
The formation of coarse eutectic carbides due to the eutectic reaction between C and Cr in high carbon and high chromium steel is the main reason for deteriorating toughness, plasticity and fatigue properties 21.Therefore, it is necessary to consider and evaluate the influence of composition design factors on carbides.Research has shown that the main types of carbides formed in high C and high Cr steel are M 7 C 3 and M 23 C 6 carbides, with Cr being the main forming element [7,[22][23][24][25][26].This paper considered defining Cr as the main factor in the formation of eutectic carbides, and reflected the degree of formation of eutectic carbides in C using'Cr equivalent (Ec r )'.
Where k is the equivalent coefficient.According to literature 26, within the range of composition studied in this article, the eutectic carbides in the tested steel are mainly M 7 C3 type carbides.When Cr 7 C 3 is fully formed, the atomic ratio is Cr:C = 7:3, the mass ratio of Cr to C is 52 × 7/(3 × 12) = 10.11, and the Cr equivalent coefficient k of carbon is 10.11 when Cr 7 C 3 is formed.The expression for the degree of carbide formation can be listed as: In this paper, Cr-equivalent Ec r was introduced to characterize the effect of C-Cr ratio of alloy composition on the degree of carbides formation yield.The following focus on the study of Crequivalent Ec r on the formation and influence of carbides.According to the above methods, the Cr equivalent Ec r of the 7 kinds of test steel designed in this paper is shown in table 2.

Relationship between carbides precipitation temperature and Ec r
Thermo-Calc software was used to calculate the precipitation temperatures of various carbides in steels with different C-Cr ratios, and the calculated results are shown in figure 1.As can be seen from figure 1(a), there is a very obvious relationship between the precipitation temperature of M 7 C 3 type eutectic carbides and the Ec r value, Ec r value has a great influence on the precipitation temperature of M 7 C 3 type eutectic carbides, thus affecting the size distribution of M 7 C 3 type eutectic carbides in steel.When Ec r value is less than about 22, the precipitation temperature of M 7 C 3 type eutectic carbides increases linearly with Ec r value.When Ec r value is more than 22, the precipitation temperature of M 7 C 3 type eutectic carbides will change the linear relationship and increase significantly.With the increase of the formation temperature, eutectic carbides have a longer time to grow up in austenite, and their size will increase [27].Therefore, in order to obtain high toughness materials and avoid significant deterioration of eutectic carbides, Ec r value should be controlled less than 22 as far as possible.It can be seen from figure 1(b) that the precipitation temperature of MC eutectic carbides is also closely related to the Ec r value.The calculated results show that, when V content is 0.3%, the precipitation temperature of MC carbides and the Ec r value decrease approximately in a linear relationship, when the Ec r value exceeds 19, the precipitation of MC eutric carbides will be completely inhibited.

3.3.
Carbides in as-cast microstructure and their relationship to Ec r Figure 2 demonstrates the carbides in the as-cast microstructure of the test steel and EDS analysis by SEM.As can be seen from the figure, there is no eutectic carbides phase in the as-cast microstructure of 1# steel due to the low content of C and Cr, and the as-cast microstructure is basically composed of lamellar pearlite and granular pearlite mixed.Eutectic phase begins to appear in steel 2#, when Ec r value is greater than 13.With the increasing of Ec r value, the more eutectic carbides content, the more complex the shape and the larger the size.Eutectic  carbides have just begun to appear in 2# steel.The content of carbides is not much, and most of them exist in the as-cast structure in the form of single particles, as shown in figure 2(a).With the increase of Ec r value, the shape of carbides gradually becomes more complex, from single grain gradually transition to long strip, and closed reticulation.Especially for 7# steel, when Ec r value is higher than 25, eutectic carbides will form closed reticulation.Figure 2(h) corresponds the morphologies and corresponding energy spectra of M 7 C 3 eutectic carbides in the test steels.In order to further study the influence of different C-Cr ratios on the composition of eutectic carbides, the composition of eutectic carbides of each steel was analyzed by energy spectrum analysis.The number of analyzed carbides in each steel type was 20, the average values of different elements in the chemical composition of the eutectic carbides were taken, and the results are shown in table 3. The element composition of the eutectic carbides changes slightly in 2 ∼ 6# test steels through the analysis of energy spectrum elements.The content of original alloying elements in steel has a great influence on the composition of M 7 C 3 eutectic carbides.The main components of M 7 C 3 eutectic carbides are Cr and Fe, Cr and C are greatly affected by the content of original elements in steel.When the content of C and Cr in steel is low, the Cr element in M 7 C 3 eutectic carbides is not abundant.The main element of M 7 C 3 eutectic carbides is Fe, whose content is more than 60%.When the content of Cr and C in steel increases correspondingly, the content of Cr in M 7 C 3 eutectic carbides also increases correspondingly, and it happens with certain regularity.
As shown in figure 3, it can be seen that the content ratio of Cr and Fe in the composition of M 7 C 3 eutectic carbides is directly related to the Cr-C ratio in steel, and the original Cr-C ratio in steel directly determines the composition of M 7 C 3 eutectic carbides.With the increase of Cr-C ratio, the content of Cr in M 7 C 3 eutectic carbides also increases.The relationship is basically linear.

Relationship between carbides and Ec r in forged tissue
The cast ingot requires hot working deformation by forging or rolling.One of the important functions of hot working is to break eutectic carbides and distribute them in the steel as evenly and as fine as possible.Under the same hot working conditions, the quality of the steel after deformation is directly affected by the quality of the eutectic carbides in the as-cast structure.This feature is determined by the composition of the steel, which can be called the essential feature.In this study, all the steel ingot smelted were forged into j20 mm round bars by the same forging process, after that, metallographic statistics were carried out to determine the quality of the forged bar determined by the tempering score.The distribution of carbides after forging is displayed in figure 4.    quantity in 3# steel and 4# steel moves slightly to the right, and the largest carbides quantity appears in the range of 0.4 ∼ 0.6 μm.The maximum amount of carbides in 5# steel is in the range of 0.6 ∼ 0.8 μm.The carbides distribution in 6# steel is relatively good, and most of them are concentrated within 0 ∼ 1.6 μm.The distribution of carbides in 7# steel is the worst, except for a peak in the small size range of 0.4 ∼ 0.6 μm, there are peaks in 1.4 ∼ 1.6 μm, 2.2 ∼ 2.4 μm and larger than 3.8 μm, indicating that the proportion of large carbide particles in 7# steel is very high.Figure 6 is the relationship between eutectic carbides area percentage and Ec r value of transverse annealed test steels after forging.This figure can basically reflect the content of carbides.It can be seen that there is a linear relationship between the content of carbides and the Ec r value.With the increase of the Ec r value, the content of carbides also increases gradually.
The forged and deformed eutectic carbides usually present long strip distribution along the longitudinal direction.The dimensional characteristics of the longitudinal strip eutectic carbides are also measured and studied.Figure 7(a) presents the relationship between average particle size and Ec r value of eutectic carbides in longitudinal annealing after forging.As can be seen, the average particle size of carbides is basically in positive proportion to the Ec r value.With the increasing of Ec r value, the content of carbides also increases gradually.Figure 7(b) shows the relationship between the number of eutectic carbides with average particle size greater than 4 μm and Ec r value after longitudinal forging annealing.There is a linear relationship between the number  of carbides with average particle size greater than 4 μm and the Ec r value.With the increasing of Ec r value, the number of carbides with diameter greater than 4 μm also increases gradually.Figure 7(c) describes the relationship between the average length of eutectic carbides and Ec r value of longitudinal annealed steel.It can be seen from the figure that the average length of the carbides is basically in a positive proportional relationship with the Ec r value, and the length size of the carbides increases gradually with the increase of the Ec r value.Figure 7(d) shows the relationship between eutectic carbides area percentage and Ec r value of longitudinal annealed steel after forging.This figure can basically reflect the content of carbides.It can be seen that there is a linear relationship between the content of carbides and the Ec r value.With the increasing of Ec r value, the content of carbides also increases gradually.

Relationship between quenching peak hardness and C/Cr
Obtaining high hardness is a necessary condition for obtaining high wear resistance of die steel, so the ability to obtain high hardening hardness of cold working die steel is particularly important.Table 4 shows the maximum peak hardening hardness obtained by different test steels and their corresponding quenching temperatures.It can be seen from the table that the peak hardening hardness of steel is significantly different under the influence of different C-Cr components.When Cr content is less than 4%, the peak quenching temperature of steel hardness decreases obviously, and there is no obvious correlation between peak quenching hardness and C/Cr.When Cr content greater than 4%, peak quenching hardness of steel has a very consistent linear relationship with C/Cr, as shown in figure 8(a).C/Cr can characterize the ability of high Cr steel (Cr content > 4%) to obtain

Relationship between impact toughness and Ec r
As mentioned above, in order to avoid premature failure such as blade breakage and cracking during the use of the die, cold working die steel still needs to have good impact toughness on the premise of maintaining high wear resistance.Improving the particle size of carbide plays an important role in improving the impact toughness of cold working die steel.As shown in figure 7(b), with the increase of Ec r value, the amount of carbides with diameter greater than 4 μm in forged steel gradually increases, and the size of carbides increases significantly.
Figure 9 describes the relationship between impact toughness and Ec r value of test steel.With the increase of Ec r value, the impact toughness of steel gradually decreases, this is consistent with the change law of impact toughness deterioration of large particle carbides size with the increase of Ec r value.Ec r value can reflect the change law of impact toughness of test steel well.It can be seen from the above results that in the cold working die steel where C and Cr are the main carbide forming elements, the ratio relationship between C and Cr has obvious influence and certain regularity on the size, morphology and composition characteristics of carbides.The introduction of 'C-Cr ratio' and 'Cr equivalent (Ec r )' can better characterize the influence of the microstructure and properties of steels with different C-Cr ratios (Cr content > 4%), which has reference significance for the composition design of C and Cr steel.

Conclusions
Through the research of the characteristics and mechanical properties of eutectic carbides in cold working die steel with different C-Cr ratios, it is shown that the parameters of 'C-Cr ratio (C/Cr)' and 'Cr equivalent (Ec r )'

Figure 1 .
Figure 1.Relationship between (a) M 7 C 3 and (b) MC carbides precipitation temperature and Cr equivalent E Cr of tested steels.

Figure 3 .
Figure 3. Relationship between Cr-Fe content ratio in eutectic carbide and Cr-C content ratio of test steels.

Figure 5
Figure 5 exhibits the number of different diameters of eutectic carbides in transverse annealed steel after forging.The number of carbides in the 0 ∼ 0.2 μm range of 2# steel is the largest, and the number of large particles of carbides gradually decreases, and no carbides exceeding 2.4 μm are found.The peak of carbides

Figure 5 .
Figure 5. Number of the eutectic carbides size in different range in the transverse of tested steels after forged.

Figure 6 .
Figure 6.Relationship between area percentage of eutectic carbide and Cr equivalent E Cr of tested steels after forged in transverse.

Figure 7 .
Figure 7. Relationship between (a) average particle size, (b) number of length than 4 μm, (c) average length and (d) area percentage of eutectic carbides and Cr equivalent E Cr of tested steels after forged in lengthways.

Figure 8 .
Figure 8. Relationship between (a) quenching peak hardness and (b) corresponding quenching temperature and C/Cr of tested steels.

Figure 9 .
Figure 9. Relationship between impact toughness tempered at (a) 150 °C, (b) 500 °C and Cr equivalent E Cr of tested steels.

Table 1 .
Chemical compositions and C-Cr ratio of test steels (mass fraction /%).

Table 2 .
Cr equivalent (E Cr ) of different C-Cr ratio test steels.

Table 4 .
Quenching peak hardness and corresponding quenching temperature of test steels.
maximum quenching hardness.Meanwhile, C/Cr can also reflect the heat treatment temperature corresponding to the peak quenching hardness, as shown in figure 8(b).