COLD-ROLLING STRIP STEEL WITH STRENGTH AND HARDNESS THEREOF VARYING IN THICKNESS DIRECTION AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is a method for manufacturing a cold-rolling strip steel (1) with the strength and hardness thereof varying in a thickness direction, the method comprising the steps: smelting, continuous casting, hot rolling, cold rolling, and continuous annealing. When quenching is performed in the continuous annealing step, an asymmetric quenching and cooling process is performed on both surfaces of the strip steel. In addition, also disclosed is a cold-rolling strip steel (1) with the strength and hardness thereof varying in a thickness direction, which is prepared by the above manufacturing method. The manufacturing method realizes asymmetric mechanical property distribution of the strip steel by performing an asymmetric quenching and cooling process on the strip steel, thereby obtaining a gradual hardness gradient in a thickness direction, so as to obtain the combined properties of high hardness and high strength, and also excellent toughness, plasticity and formability, which can effectively deal with the contradiction between the strength, plasticity and toughness of ultra high-strength steel.

Claims

1. A cold-rolled strip steel with varying strength/hardness in a thickness direction, comprising chemical elements in the following mass percentages: C 0.06-0.3 wt %, Si 0.01-2.5 wt %, Mn 0.5-3 wt %, Al 0.02-0.08 wt % and a balance of Fe and other unavoidable impurities, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction has a yield strength of 420 MPa, a tensile strength of ≥800 MPa, an elongation of ≥11%, and a hardness difference between two surfaces of at least 30 HV.

2. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 1, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction further comprises at least one of Cr, Mo and B, wherein a Cr content is ≤0.2%, a Mo content is ≤0.2%, and a B content is ≤0.0035%.

3. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 2, wherein the B content of the cold-rolled strip steel with varying strength/hardness in a thickness direction is <0.0005 wt %, and Cr+Mn+Mo≤3.5 wt %; or the B content of the cold-rolled strip steel with varying strength/hardness in a thickness direction is in the range of 0.0005-0.0035 wt %, and Cr+Mn+Mo≤2.5 wt %.

4. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 1, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction further comprises at least one of V, Ti, Nb and W, wherein their contents satisfy V+Ti+Nb+W≤0.2 wt %.

5. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 1, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction comprises chemical elements in the following mass percentages: C 0.09-0.2 wt %, Si 0.3-1.2 wt %, Mn 1.5-2.5 wt %, Al 0.02-0.08 wt %, and a balance of Fe and other unavoidable impurities.

6. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 1, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction has a yield strength of 435-900 MPa, a tensile strength of 820-1260 MPa, an elongation of 11-20%, and a hardness difference between the two surfaces of 35-80 HV.

7. A manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction, comprising the following steps: smelting, continuous casting, hot rolling, cold rolling and continuous annealing, wherein when quenching is performed in the continuous annealing step, an asymmetric quenching cooling process is performed on two surfaces of the strip steel.

8. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 7, wherein the asymmetric quenching cooling process comprises at least one of the following: start temperatures for cooling the two surfaces of the strip steel being asymmetric; end temperatures for cooling the two surfaces of the strip steel being asymmetric; and cooling rates of the two surfaces of the strip steel being asymmetric.

9. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 8, wherein when the start temperatures for cooling the two surfaces of the strip steel are asymmetric, a difference between the start temperatures for cooling the two surfaces of the strip steel is 20-100° C.

10. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 9, wherein when the start temperatures for cooling the two surfaces of the strip steel are asymmetric, the difference between the start temperatures for cooling the two surfaces of the strip steel is 25-100° C.

11. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 8, wherein when the end temperatures for cooling the two surfaces of the strip steel are asymmetric, a difference between the end temperatures for cooling the two surfaces of the strip steel is 40-200° C.

12. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 11, wherein when the end temperatures for cooling the two surfaces of the strip steel are asymmetric, the difference between the end temperatures for cooling the two surfaces of the strip steel is 50-180° C.

13. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 8, wherein when the cooling rates of the two surfaces of the strip steel are asymmetric, a difference between the cooling rates of the two surfaces of the strip steel is 25-200° C./s.

14. The manufacturing method for a cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 13, wherein when the cooling rates of the two surfaces of the strip steel are asymmetric, the difference between the cooling rates of the two surfaces of the strip steel is 40-200° C./s.

15. A cold-rolled strip steel with varying strength/hardness in a thickness direction obtained by the manufacturing method according to claim 7.

16. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 15, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction has a thickness of 1.0 mm or more.

17. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 16, wherein the cold-rolled strip steel with varying strength/hardness in a thickness direction has a thickness of 1.4-2.5 mm.

18. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 15, comprising chemical elements in the following mass percentages: C 0.06-0.3 wt %, Si 0.01-2.5 wt %, Mn 0.5-3 wt %, Al 0.02-0.08 wt %, and a balance of Fe and other unavoidable impurities.

19. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 18, further comprising at least one of Cr, Mo and B, wherein when B is <0.0005 wt %, Cr+Mn+Mo is ≤3.5 wt %; and when a B content is in the range of 0.0005-0.0035 wt %, Cr+Mn+Mo is ≤2.5 wt %.

20. The cold-rolled strip steel with varying strength/hardness in a thickness direction according to claim 18, further comprising at least one of V, Ti, Nb and W, wherein their contents satisfy V+Ti+Nb+W≤0.2 wt %.

Description

DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 schematically shows a cooling process in some embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

[0055] FIG. 2 schematically shows a cooling process in some other embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

[0056] FIG. 3 schematically shows a cooling process in still some other embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

DETAILED DESCRIPTION

[0057] The cold-rolled strip steel with varying strength/hardness in a thickness direction according to the disclosure and the method for manufacturing the same will be further explained and illustrated with reference to the accompanying drawings and the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

Examples 1-6

[0058] The cold-rolled strip steel with varying strength/hardness in a thickness direction in Examples 1-6 was prepared according to the following steps:

[0059] (1) Subjecting the chemical compositions shown in Table 1 to smelting and casting;

[0060] (2) Continuous casting;

[0061] (3) Hot rolling: the temperature for heating the slab was 1170-1230° C.; the final rolling temperature was 850-910° C.; the coiling temperature was 570-630° C.; then acid washing was carried out to remove the oxide skin;

[0062] (4) Continuous annealing: firstly, the strip steel was heated to a holding temperature and held for 40-120 s; then, the strip steel was cooled at a cooling rate of 2-10° C./s; subsequently, an asymmetric quenching cooling process was carried out; tempering was performed after the quenching cooling process was finished; after tempering, the strip steel was cooled to room temperature with water; the strip steel was dried and then flattened.

[0063] In some other embodiments, after the hot rolling, the strip steel may also be cold rolled with the cold rolling reduction being controlled at 30-65%, and then the above continuous annealing in the step (4) may be carried out.

[0064] Table 1 lists the mass percentages of the various chemical elements in the cold-rolled strip steel with varying strength/hardness in a thickness direction in Examples 1-6.

TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other unavoidable impurities except for P, S and N) Amounts of Cr, Mo, B, No. C Si Mn P S Al N Nb, Ti, V Ex. 1 0.09 0.45 2.3 0.012 0.002 0.05 0.025 Nb: 0.03 Ex. 2 0.09 0.45 2.3 0.012 0.002 0.08 0.025 Ti: 0.02 Nb: 0.015 B: 0.001 Ex. 3 0.16 1 2 0.01 0.0008 0.025 0.002 Cr: 0.15 V: 0.05 Ex. 4 0.16 1 2 0.01 0.0008 0.025 0.002 Mo: 0.1 W: 0.1 Ex. 5 0.17 0.4 1.6 0.01 0.0003 0.03 0.0018 Mo: 0.1 B: 0.0015 Ti: 0.02 Ex. 6 0.17 0.4 1.6 0.01 0.0003 0.02 0.0018 Ti: 0.02 Nb: 0.015

[0065] Table 2 lists the specific process parameters used in the continuous annealing step for the cold-rolled strip steel with varying strength/hardness in a thickness direction in Examples 1-6.

TABLE-US-00002 TABLE 2 Start Start End End cooling cooling cooling cooling temperature Cooling temperature Cooling temperature temperature Holding for rate for for rate for for for Tempering Tempering Flattening temperature side I side I side II side II side I side II Cooling temperature time elongation No. (° C.) (° C.) (° C./s) (° C.) (° C./s) (° C.) (° C.) medium (° C.) (s) (%) Ex. 1 800 670 70 630 70 270 350 60% H.sub.2 270 200 0.1 Ex. 2 800 670 70 670 70 270 330 60% H.sub.2 270 200 0.1 Ex. 3 800 670 70 670 30 270 350 60% H.sub.2 270 300 0.2 Ex. 4 800 670 70 630 30 270 300 60% H.sub.2 270 300 0.2 Ex. 5 800 700 500 700 400 50 50 Water 200 400 0.3 mist Ex. 6 800 700 500 650 300 50 50 Water 200 400 0.3 mist Ex. 7 800 670 30 600 70 320 250 60%H.sub.2 300 240 0.2 Ex. 8 800 670 70 650 70 270 270 60%H.sub.2 270 200 0.1 Comp. 800 670 70 670 70 270 270 60% H.sub.2 270 200 0.1 Ex. 1 Comp. 800 670 70 670 70 270 270 60% H.sub.2 270 300 0.2 Ex. 2 Comp. 800 700 500 700 500 50 50 Water 200 400 0.3 Ex. 3 mist

[0066] It should be noted that the same mass percentages of the chemical elements as shown for Example 1 were used in Comparative Example 1 for smelting; the same mass percentages of the chemical elements as shown for Example 3 were used in Comparative Example 2 for smelting; and the same mass percentages of the chemical elements as shown for Example 5 were used in Comparative Example 3 for smelting. The same mass percentages of the chemical elements as shown for Example 1 in Table 1 were used in Example 7 for smelting. The same mass percentages of the chemical elements as shown for Example 2 in Table 1 were used in Example 8 for smelting.

[0067] In addition, in order to distinguish the two surfaces of the strip steel in the thickness direction, one of the surfaces was referred to as side I, and the other surface opposite to side I was referred to as side II.

[0068] Table 3 lists the measured results of the properties of the cold-rolled strip steel with varying strength/hardness in a thickness direction in Examples 1-8 according to the present disclosure.

TABLE-US-00003 TABLE 3 Hardness Hardness of the Hardness Yield Tensile of middle of Thickness strength strength σ.sub.b Elongation side I part side II No. (mm) (MPa) (MPa) (%) (HV) (HV) (HV) Ex. 1 1 440 820 20 240 215 200 Ex. 2 1.4 435 827 19 240 220 205 Ex. 3 1.8 630 1020 15 320 295 260 Ex. 4 2.0 550 1000 16 320 288 240 Ex. 5 2.2 900 1260 11 380 350 330 Ex. 6 2.3 860 1220 11 380 345 310 Ex. 7 2.5 850 1225 12 375 340 315 Ex. 8 1 445 826 20 238 218 203 Comp. 1.6 500 850 15 247 250 246 Ex. 1 Comp. 1.6 680 1060 13 330 337 335 Ex. 2 Comp. 1.6 940 1290 9 395 390 385 Ex. 3 Note: By preparing a metallographic sample, the hardness of the two sides and the middle part is measured in the thickness direction with a microhardness tester.

[0069] As can be seen from Tables 2 and 3, the prior art technique is adopted for the strip steel of Comparative Examples 1-3, so the cooling on both sides of the strip steel is completely identical and symmetric, and the mechanical properties of the quenched steel plate obtained are also completely symmetric and uniform. In contrast, an asymmetric quenching cooling process is adopted for the cold-rolled strip steel with varying strength/hardness in a thick direction according to Examples 1-8 in the present disclosure, so an asymmetric distribution of the mechanical properties of the strip steel is achieved. As a result, gradients of hardness/strength changing gradually in the thickness direction are obtained. Thus, combined properties of high hardness, high strength, and excellent toughness, ductility and formability are obtained at the same time.

[0070] FIGS. 1 to 3 schematically show different asymmetric quenching cooling processes used for different Examples.

[0071] FIG. 1 schematically shows a cooling process in some embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

[0072] As shown by FIG. 1, after the cold-rolled steel strip 1 enters the continuous annealing stage along the forward direction F1, the two sides of the strip steel are cooled from different start temperatures. Side I is cooled by the spray from the nozzle of the cooling module 2 before side II is cooled by the spray from the cooling nozzle. Therefore, different cooling routes can be developed on the two sides of the strip steel. For the rapid cooling at different surfaces, the start temperatures are different, and the cooling lengths are different, so that the end temperatures of the rapid cooling are also different. As a result, the contents of ferrite and martensite/bainite are different in different surfaces. Ultimately, the strength of the strip steel varies in the thickness direction.

[0073] With the use of the asymmetric cooling process shown by FIG. 1, side I of the strip steel has a higher hardness, a lower ferrite content, a higher martensite content, and a lower bainite content. In contrast, side II has a lower hardness, a lower ferrite content, a lower martensite content, and a higher bainite content.

[0074] FIG. 2 schematically shows a cooling process in some other embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

[0075] As shown by FIG. 2, after the cold-rolled strip steel 1 enters the continuous annealing stage along the forward direction F1, the two sides of the strip steel are cooled from the same start temperature, but the end temperatures are different. When the operation of the cooling nozzle of the cooling module 2 corresponding to side II of the strip steel stops, the cooling nozzle corresponding to side I continues working to cool side I to a lower temperature. Therefore, different cooling routes are developed on the two sides of the strip steel. As a result, the end temperatures for cooling sides I and II of the strip steel are different, which in turn leads to different contents of ferrite and martensite/bainite. Ultimately, the strength of the strip steel varies in the thickness direction.

[0076] With the use of this asymmetric cooling process, side I of the strip steel has a higher hardness and a higher martensite content, while side II has a lower hardness, a lower martensite content and a higher bainite content.

[0077] FIG. 3 schematically shows a cooling process in still some other embodiments of the cold-rolled strip steel with varying strength/hardness in a thickness direction according to the present disclosure.

[0078] As shown by FIG. 3, after the cold-rolled strip steel 1 enters the continuous annealing stage along the forward direction F1, the two sides of the strip steel are cooled from the same start temperature, and the end time is also the same. However, due to the different cooling abilities of the cooling nozzles of the cooling modules 2 positioned on the two sides of the strip steel, the nozzle corresponding to side I provides a faster cooling rate, while the nozzle corresponding to side II provides a relatively lower cooling rate. Therefore, different cooling routes are developed on the two sides of the strip steel. Put another way, different cooling rates are resulted, thereby leading to different contents of ferrite and martensite/bainite, and eventually variation in the strength of the strip steel in the thickness direction.

[0079] With the use of this asymmetric cooling process, side I of the strip steel has a higher hardness and a higher martensite content, while side II has a lower hardness, a higher ferrite content, a lower martensite content and a higher bainite content.

[0080] It should be noted that the difference in cooling rate can be resulted from different cooling media sprayed through the nozzles, or adjustment of the spraying speed or flow rate of the cooling medium, so that the cooling rates on sides I and II are different. For example, a medium with a higher heat exchange ability, or a higher spray speed, or a higher flow rate may be used for side I, so as to achieve a faster cooling rate.

[0081] In addition, in some other embodiments, the cooling processes illustrated in FIG. 1, FIG. 2 or FIG. 3 described above may also be combined to realize an asymmetric quenching cooling process.

[0082] In summary, it can be seen that, by utilizing a thickness-wise asymmetric cooling technique, the manufacturing method according to the present disclosure provides a phase-change strengthened strip steel with a thickness-wise asymmetric strength (hardness) distribution, so that it has the advantages of high strength and hardness on one side, and good plasticity and toughness on the other side. The hardness varies gradually along the thickness direction. Hence, the resulting cold-rolled strip steel with varying strength/hardness in the thickness direction is very suitable for applications requiring high hardness and good friction and indentation resistance at a single side as well as good overall toughness.

[0083] It's to be noted that the prior art portions in the protection scope of the present disclosure are not limited to the examples set forth in the present application file. All the prior art contents not contradictory to the technical solution of the present disclosure, including but not limited to prior patent literature, prior publications, prior public uses and the like, may all be incorporated into the protection scope of the present disclosure.

[0084] In addition, the ways in which the various technical features of the present disclosure are combined are not limited to the ways recited in the claims of the present disclosure or the ways described in the specific examples. All the technical features recited in the present disclosure may be combined or integrated freely in any manner, unless contradictions are resulted.

[0085] It should also be noted that the Examples set forth above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar variations or modifications made thereto can be directly derived or easily contemplated from the present disclosure by those skilled in the art. They all fall in the protection scope of the present disclosure.