SOFTENING METHOD FOR HIGH-STRENGTH Q&P STEEL HOT ROLL

Abstract

There is disclosed a softening method for a high-strength Q & P steel hot-rolled coil, comprising: after heating a Q & P steel ingot, subjecting it to rough rolling, finish rolling, laminar cooling and coiling to obtain a hot-rolled coil; after unloading the coil, covering the coil on-line with an insulating enclosure and moving it into a steel coil warehouse along with a transport chain; after a specified period of insulating time, removing the coil from the insulating enclosure, and cooling it to room temperature in air, wherein the coiling is performed at a temperature of 400-600 C.; said covering on-line with an insulating enclosure means each hot-rolled coil is individually covered with an independent, closed insulating enclosure unit within 60 minutes after unloading; the insulating time of the steel coil in the insulating enclosure is 60 minutes. The inventive method replaces the intermediate annealing step in the production process of cold-rolled Q & P steel. The inventive method has low cost and high efficiency, and it is not affected by the surrounding environment.

Claims

1. A softening method for a high-strength Q & P steel hot-rolled coil, characterized in: after heating a Q & P steel ingot, subjecting it to rough rolling, finish rolling, laminar cooling and coiling to obtain a hot-rolled coil; after unloading the coil, covering the coil on-line with an insulating enclosure and moving it into a steel coil warehouse along with a transport chain; after a specified period of insulating time, removing the coil from the insulating enclosure, and cooling it to room temperature in air, wherein the coiling is performed at a temperature of 400-600 C.; said covering on-line with an insulating enclosure means each hot-rolled coil is individually covered with an independent, closed insulating enclosure unit within 60 minutes after unloading; the insulating time of the steel coil in the insulating enclosure is 60 minutes.

2. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that the ingot is heated at a temperature of 1150 C., and a soaking time is 60 minutes.

3. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that the ingot is heated at a temperature of 1200-1300 C., and the soaking time is 1-3 hours.

4. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that the rough rolling and finish rolling are performed in a temperature zone for complete austenization, an overall hot rolling reduction rate is 90%, and a final rolling temperature is 800-1000 C.

5. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that each hot-rolled coil is individually covered with an insulating enclosure within 20 minutes after it is unloaded.

6. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that the steel coil is cooled at a cooling rate of 15 C./hour in the insulating enclosure.

7. The softening method for a high-strength Q & P steel hot-rolled coil according to claim 1, characterized in that the insulating time of the steel coil in the insulating enclosure is 1-24 hours.

Description

DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a typical metallographical photo of the test steel of Example 1 in the present disclosure.

[0031] FIG. 2 is a typical metallographical photo of the test steel of Example 2 in the present disclosure.

[0032] FIG. 3 is a typical metallographical photo of the test steel of Comparative Example 1 in the present disclosure.

[0033] FIG. 4 is a typical metallographical photo of the test steel of Comparative Example 2 in the present disclosure.

DETAILED DESCRIPTION

[0034] The disclosure will be further illustrated with reference to the following Examples and accompanying drawings.

[0035] Table 1 shows the key process parameters of the Examples in the present disclosure, Table 2 shows the key process parameters of the Comparative Examples in the present disclosure, and Table 3 shows the properties of the steel coils of the Examples and the Comparative Examples in the present disclosure.

[0036] The process flow for the Examples in the present disclosure is as follows: heating a Q & P steel ingot.fwdarw.rough rolling.fwdarw.finish rolling.fwdarw.laminar cooling.fwdarw.coiling.fwdarw.covering with an insulating enclosure on-line.fwdarw.removing from the insulating enclosure, wherein the key process parameters are shown in Table 1.

[0037] The process flow for the Comparative Examples in the present disclosure is as follows: heating a Q & P steel ingot.fwdarw.rough rolling.fwdarw.finish rolling.fwdarw.laminar cooling.fwdarw.coiling.fwdarw.slow cooling the steel coil in stack, wherein the key process parameters are shown in Table 2.

TABLE-US-00001 TABLE 1 Steel Rough Final coil Heating rolling rolling Coiling Covering Insulating thickness temperature temperature temperature Temperature time time Ex. (mm) ( C.) ( C.) ( C.) ( C.) (min) (h) 1 3.0 1261 1128 927 523 9 2 2 3.0 1265 1122 930 510 28 4 3 3.0 1259 1127 933 520 10 2 4 2.6 1267 1130 938 498 10 4 5 2.6 1263 1125 936 488 8 8

TABLE-US-00002 TABLE 2 Rough Final Heating rolling rolling Coiling Steel coil temper- temper- temper- Temper- thickness ature ature ature ature Comp. Ex. (mm) ( C.) ( C.) ( C.) ( C.) 1 3.0 1268 1129 920 522 2 3.0 1266 1130 925 530 3 3.0 1259 1125 935 529 4 2.6 1268 1125 937 481 5 2.6 1269 1129 936 486

TABLE-US-00003 TABLE 3 Yield Tensile strength strength (MPa) (MPa) Elongation/% Ex. 1 644 816 20 2 692 840 16 3 726 859 18 4 849 970 17 5 885 1056 16 Comp. Ex. 1 740 966 16 2 928 1063 14 3 1021 1184 14 4 1024 1257 15 5 970 1296 14

[0038] As can be seen from the data of the Examples and Comparative Examples in Table 3, in comparison with the method employing slow cooling of steel coils in stack, the Q & P steel hot-rolled coils produced by the method proposed by the present disclosure have a yield strength reduction of 85 MPa, a tensile strength reduction of 150 MPa, and an increase in elongation at break of >2%, indicating that the method proposed by the present disclosure can effectively soften Q & P steel hot-rolled coils, and improve the plasticity index of the material at the same time, which is beneficial to reduce the cold rolling force in the subsequent step.

[0039] FIGS. 1 and 2 show the typical metallographical photos of the test steel of Examples 1 and 2. As can be seen clearly from the photos, without the treatment using the insulating enclosure, the microstructure of the steel coils is mainly bainite+martensite.

[0040] FIGS. 3 and 4 show the typical metallographical photos of the test steel of Comparative Examples 1 and 2. As can be seen clearly from the photos, with the treatment using the insulating enclosure, the microstructure of the steel coils is mainly bainite+cementite.

[0041] The embodiments of the present disclosure are not limited to the foregoing examples. Any other changes, modifications, substitutions, combinations, and simplifications that do not depart from the spirit and principle of the present disclosure should all be equivalent alternatives, all falling in the protection scope of the present disclosure.