JP2002155315A - Method for manufacturing high strength hot-rolled steel sheet having excellent uniformity of material in coil and workability, and high strength hot-rolled steel sheet produced by the method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of stably manufacturing a high strength hot-rolled steel sheet which has high uniformity in the whole longitudinal and transverse directions of a coil, and to provide the high strength hot rolled steel sheet manufactured by the method. SOLUTION: Steel having chemical components containing, as essential components, by mass, 0.07 to 0.15% C, 0.8 to 2.0% Si and 1.2 to 3.0% Mn and the balance substantially Fe is hot-rolled, is coiled at <=600 deg.C, is reheated to be in the temperature range T1 of 750 to 900 deg.C and is held at this temperature for t1=>=60 sec. Thereafter, the steel is cooled to be in the temperature range T2 of 370 to 450 deg.C at a cooling rate CR of <=15 deg.C/s, is held at this temperature for t2=>=90 sec, and is cooled to room temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a tensile strength of 440 to 440.
The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet having high strength of about 780 MPa, a small variation in the material in the coil, and excellent workability, and a high-strength hot-rolled steel sheet manufactured by the manufacturing method.

[0002]

2. Description of the Related Art Recently, steel sheets for structural members such as members that play a role of absorbing energy at the time of collision as frame members of automobiles have been developed from the viewpoint of safety improvement and to improve fuel efficiency as a measure against environmental problems. From the viewpoint of weight reduction, steel sheets are rapidly being replaced with high-strength steel sheets. On the other hand, the increase in strength leads to deterioration of workability, and makes it difficult to press the part itself. For this reason, a steel sheet having both strength and workability is strongly desired. In response to such a request, for example, JP-A-1-272720, JP-A-6-31236
As described in Japanese Unexamined Patent Application Publication No. 2000-125, a high-strength steel sheet having excellent elongation by utilizing the work-induced transformation of retained austenite has been developed.

[0003]

However, in the conventional method for producing a high-strength hot-rolled steel sheet containing retained austenite, it is necessary to control the temperature in a very narrow temperature range in the cooling process in the hot-rolling process. In many cases, it is extremely difficult to control the entire length of the coil within the control temperature range. For this reason, a part or most of the inside of the coil may cause deterioration of the elongation characteristic due to temperature deviation.

[0004] When a structure containing retained austenite is obtained only by the hot rolling step, it is necessary to continue the structure transformation even after the steel sheet winding is completed, so that the coil is cooled in the longitudinal and width directions in the coil. The possibility of material variations due to non-uniformity was high.

[0005] This type of steel sheet is used for members that are frame members of automobiles and the like, and most of them are formed by press working. It is important to stably obtain high elongation characteristics in the direction. However, as mentioned above,
The conventional high-strength hot-rolled steel sheet containing retained austenite has a problem that stable characteristics cannot be obtained over the entire length and width of the coil.

On the other hand, as a technique for solving such a material variation in the coil, a method of reheating to a two-phase temperature range of ferrite and austenite after hot rolling is disclosed in Japanese Patent Laid-Open No. 5-2.
No. 71764.

However, in this method, since the cooling rate from the two-phase temperature range is as fast as 20 ° C./s or more, the steel sheet must be cooled by mist cooling or roll cooling. For this reason, according to these methods, material variation due to cooling unevenness is likely to occur in the longitudinal direction, width direction, and plate thickness direction of the coil, and uniformity of the material in the coil is stably realized. Not in.

The present invention has been made in view of the above problems, and therefore, a manufacturing method capable of stably manufacturing a high-strength hot-rolled steel sheet having high material uniformity in the entire length direction and width direction of the coil, and a manufacturing method thereof. It is intended to provide a high-strength hot-rolled steel sheet manufactured by the method described above.

[0009]

According to the method for producing a high-strength hot-rolled steel sheet of the present invention, the chemical component is mass% and C: 0.07 to 0.07%.
0.15%, Si: 0.8 to 2.0%, Mn: 1.2 to
After hot-rolling a steel containing 3.0% and the balance substantially Fe as an essential component, it is wound at a temperature of 600 ° C. or less and then reheated to a temperature range of 750 to 900 ° C. This is a method of cooling to a temperature range of 370 to 450 ° C. at a cooling rate of 15 ° C./s or less, holding the temperature at that temperature for 90 seconds or more, and then cooling to room temperature.
Further, the high-strength hot-rolled steel sheet of the present invention is a high-strength hot-rolled steel sheet manufactured by the above-described manufacturing method, and has a main phase of ferrite and bainite and contains retained austenite in an area ratio of 3 to 10%.

[0010]

First, the reasons for limiting the chemical components of steel used in the production method of the present invention will be described.
The unit is mass%. C: 0.07 to 0.15% C is the most important element for improving the strength of steel and for generating a retained austenite phase. In order to obtain retained austenite having a tensile strength of 440 MPa or more and a volume fraction of 3% or more, 0.07% or more is required, and the lower limit is made 0.07%. On the other hand, if added in excess of 0.15%, the volume fraction of the hard second phase other than ferrite increases, and ductility deteriorates.
Preferably, it is 0.13%.

Si: 0.8-2.0% Si forms a solid solution in ferrite and has an effect of condensing austenite C element in austenite while suppressing precipitation of carbides in a cooling process from a ferrite + austenite coexisting temperature range. Thus, retained austenite can be effectively obtained. For that purpose, addition of at least 0.8% is necessary, and the lower limit is set to 0.8%, preferably 1.0%. However, even if added in excess of 2.0%, the ferrite is excessively strengthened and the ductility of the steel sheet itself is reduced, so the upper limit is made 2.0%, preferably 1.8%.

Mn: 1.2 to 3.0% Mn is an element that strengthens the steel and stabilizes austenite, and contributes to the retention of austenite while suppressing the precipitation of carbides like Si. In order to obtain the desired amount of retained austenite, it is necessary to add 1.2% or more, and this is the lower limit. In the present invention, since the cooling rate from the two-phase temperature range is set to 3 to 15 ° C./s, which is relatively slow, in order to obtain the target structure while suppressing the precipitation of carbide at the cooling rate, the amount of Mn is set to 1. It is preferable to set it to 4% or more. On the other hand, if the content exceeds 3.0%, it becomes difficult to melt the steel.
The upper limit is 3.0%, preferably 2.6%.

The chemical constituents of the steel according to the present invention include, in addition to the above constituents, the balance of Fe as an essential constituent. In addition to other unavoidable impurities, appropriate elements are added to the extent that the functions and effects of the above elements are not hindered. can do. For example, 0.01% or more and 0.10% or less of Al can be added for deoxidation.

Next, the manufacturing conditions of the present invention will be described. The steel is hot-rolled after being melted into a slab. The hot rolling may be performed by a conventional method, and is not particularly limited. Usually, the slab heating temperature is set to 1000-1
The temperature may be about 200 ° C., and the hot finishing temperature may be Ar ₃ points or more. However, the steel sheet after hot rolling must be wound at a temperature of 600 ° C. or lower. This is because a phase such as bainite, in which elements such as C and Mn are concentrated, is generated in advance before the subsequent reheating treatment, so that a sufficient amount to obtain high elongation characteristics at the product stage is obtained. This is to make it easier to obtain retained austenite. 60
If the temperature exceeds 0 ° C., carbides precipitate and the C element is consumed, so that the concentration of C in austenite becomes insufficient. Further, as described above, in the present invention, it is necessary to add a relatively large amount of elements to obtain retained austenite,
When such steel is wound at a high temperature, grain boundary oxidation may occur. For these reasons, the upper limit of the winding temperature is set to 600 ° C, preferably 560 ° C.

After the winding, if necessary, an acid washing process is performed, and then, as shown in FIG. 1, a reheating process for heating to a temperature range T1 of 750 to 900 ° C. is performed. The temperature range T1 is a temperature range in which ferrite and austenite coexist. If the temperature is less than 750 ° C., a sufficient amount of austenite cannot be obtained. If the temperature exceeds 900 ° C., a sufficient amount of ferrite cannot be obtained. For this reason, the lower limit is set to 750 ° C., preferably 770 ° C., and the upper limit is set to 900 ° C. However, in order to maintain a phase in which elements such as bainite obtained after hot rolling are concentrated, in a state of high concentration,
It is desirable that the temperature be 830 ° C. or lower. In order to sufficiently promote austenitization, at least the holding time t1 in the T1 temperature range is set to 60 s or more, preferably 9
It is desirable to keep it for 0 s or more.

After the reheating treatment, the wafer is cooled to a holding temperature T2 of 370 to 450 ° C., which will be described later, at a cooling rate CR of 15 ° C./s or less. This cooling sufficiently precipitates a soft ferrite phase, increases the C concentration in untransformed austenite,
Proper amount of retained austenite at the product stage (3 to
10%). If the cooling rate CR exceeds 15 ° C./s, the ferrite transformation is not sufficiently performed, and high elongation characteristics cannot be obtained.
s. The lower limit of the cooling rate that can be used in actual operation is desirably set to 3 ° C./s. The cooling rate can be easily realized by, for example, cooling in a furnace (including cooling, gas cooling (air cooling), and gas jet cooling (forced air cooling)). Cooling may be used in combination.

After the cooling, the bainite transformation proceeds,
In order to further increase the C concentration in the untransformed austenite, the holding temperature T2 is set to 370 to 450 ° C., and the holding time t2 is set to 9
It is held as 0 s or more. If the holding temperature is lower than 370 ° C., the elongation characteristics deteriorate. On the other hand, when the temperature exceeds 450 ° C., precipitation of carbides is recognized, and since C is consumed, there is a high possibility that C concentration in the untransformed austenite becomes insufficient, so that an appropriate amount of retained austenite cannot be obtained. Become. For this reason, the lower limit of the holding temperature T2 is 370 ° C., preferably 390 ° C., and the upper limit is 450 ° C., preferably 4 ° C.
30 ° C. If the holding time t2 is less than 90 s, bainite transformation does not sufficiently proceed, and an appropriate amount of retained austenite cannot be obtained.
0 s, preferably 120 s.

Cooling after maintaining at the T2 temperature for promoting the Payinite transformation may be performed by cooling at a suitable cooling rate to room temperature, for example, air cooling, and the cooling rate is not particularly limited. After cooling, temper rolling may be performed if necessary.

Under the above manufacturing conditions, the main phase is ferrite + bainite, and the retained austenite has an area ratio of 3 to
A 10% hot-rolled steel sheet can be obtained. At least 3% of the retained austenite phase is required to obtain high elongation characteristics by utilizing the work-induced transformation effect. on the other hand,
If it exceeds 10%, the effect saturates, so that it may be 10% or less. In addition, a small amount of pearlite and martensite may be generated in addition to the three phases.
The phase other than the phase has no practical problem as long as the area ratio is 3% or less, preferably 1% or less.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not construed as being limited to the following Examples.

[0021]

EXAMPLES Steel having the chemical components shown in Table 1 was melted to form slabs. This slab is heated to 1130 ° C. and finished at a finishing temperature of 8
Hot rolling to 2.0mm thickness at 70-930 ° C, 60 ° C
After cooling at an average cooling rate of / s, winding was performed at a winding temperature (CT) shown in Table 2. After the pickling, a predetermined heat treatment was performed by a continuous annealing line under the heating and cooling conditions shown in the same table and FIG. That is, first, it is heated to T1, held at T1 for about 90 s (t1), cooled to a holding temperature T2 at a cooling rate CR, held at T2 temperature for t2 seconds, and then cooled to room temperature by air. A temper rolling of 5% was performed.

A JIS No. 5 tensile test piece and a microstructure observation test piece were sampled from the center (width) of the middle (center) portion of the hot-rolled coil, and the amount of retained austenite was measured by a tensile test and X-ray. Table 2 shows the results. As for the invention examples, when the structure was observed with a microscope, phases other than retained austenite were almost ferrite and bainite, and the other phases were 1% or less in area ratio.

Further, in order to examine variations in the material in the length direction of the coil, a top portion (coil tip portion), a middle portion, a bottom portion (coil rear end portion),
Specimens were sampled from five positions (centers in the width direction) between the top and middle portions and between the middle and bottom portions, and the elongation was examined. Further, in the middle part, test pieces were sampled along the width direction from both edge portions, the center portion, and five positions between each of the both edge portions and the center, and the elongation was examined. Then, the homogeneity of the material in the coil was evaluated based on the difference between the measured maximum elongation and the minimum elongation (ΔL: difference in elongation in the length direction, ΔW: difference in elongation in the width direction). Table 2 also shows the values of ΔL and ΔW.

[0024]

[Table 1]

[0025]

[Table 2]

According to Table 2, in the example of the present invention, the variation in El (elongation) in both the longitudinal direction and the width direction of the hot-rolled steel sheet is as small as 2% or less, and although it has a strength exceeding 590 MPa, it is not less than 35%. High El is obtained. Also, it can be seen that an appropriate amount of retained austenite of 5 to 8% was obtained. On the other hand, in the comparative example, at least one of ΔL and ΔW was 3% or more and the homogeneity was poor, and El was 32%.
Stopped below. In the comparative examples, the variation was 2%
What stops below (No. 4, 13, 17, 21,
22, 23), but the deterioration of elongation is also remarkable.

[0027]

According to the present invention, it is possible to easily obtain a high-strength hot-rolled steel sheet having small variations in the material in the coil, excellent material uniformity, and excellent workability.

[Brief description of the drawings]

FIG. 1 shows a heating / cooling diagram after hot rolling of the present invention.

Continued on the front page F term (reference) 4K037 EA01 EA05 EA06 EA15 EA16 EA18 EA23 EA25 EA27 EA28 EB05 EB11 FA02 FC04 FD04 FE01 FE02 FE06 FF02 FF03 JA06

Claims (2)

[Claims] 1. The chemical component is mass%, C: 0.07 to
0.15%, Si: 0.8 to 2.0%, Mn: 1.2 to
After hot-rolling a steel containing 3.0% and the balance substantially Fe as an essential component, it is wound at a temperature of 600 ° C. or less and then reheated to a temperature range of 750 to 900 ° C. A method for producing a high-strength hot-rolled steel sheet, wherein the steel sheet is cooled to a temperature range of 370 to 450 ° C. at a cooling rate of 15 ° C./s or less, held at that temperature for 90 seconds or more, and then cooled to room temperature. 2. A high-strength hot-rolled steel sheet manufactured by the manufacturing method according to claim 1, wherein the high-strength hot-rolled steel sheet contains ferrite and bainite as main phases and contains retained austenite in an area ratio of 3 to 10%.