JP3546287B2 - High-strength hot-rolled steel sheet excellent in workability and method for producing the same - Google Patents

Description

【００２６】
【表２】

【００２７】
【発明の効果】
以上説明したように、本発明によれば、鋼板の化学組成、熱間圧延、その後の冷却および巻き取りの条件を適正化することによって、張出し成形性、形状凍結性、伸びフランジ成形性をともに具えた、従来よりも格段に優れた成形性を有する高張力熱延鋼板を提供することができる。
また、本発明によれば、製造条件の厳しい制御の必要性がなくなり、材質が安定した鋼板を、高生産性を維持しながら製造することが可能になる。
【図面の簡単な説明】
【図１】フェライト体積率に及ぼすTi量および加熱温度の影響を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thin steel sheet mainly used as a part for an automobile, which is subjected to processing such as press forming, and is particularly suitable for use as a member of an automobile body, a structural member such as a frame, or a suspension part such as a suspension. The present invention proposes a hot-rolled steel sheet and a method for producing the same.
[0002]
[Prior art]
Application of a high-tensile steel plate is one of the most effective methods for reducing the weight of an automobile body. Of these, hot rolled steel sheets, which are economical, are often used for parts that do not require very good surface quality, such as body strength members or suspensions. Under such circumstances, the use of hot-rolled high-tensile steel sheets is being expanded more and more.
Conventionally, as a method for strengthening the material of a high-tensile hot-rolled steel sheet with a tensile strength of about 490 to 780 MPa, 1) a transformation structure strengthening method in which a martensite phase, a pearlite phase, or a bainite phase is precipitated in a ferrite phase , 2) Precipitation strengthening methods using Ti, Nb, and V carbonitrides have been used, and these methods have been selected according to the formability of steel sheets or the properties required for parts.
For example, inexpensive general-purpose high-strength hot-rolled steel sheets include a precipitation-strengthened ferrite phase and a pearlite phase or bainite phase structure steel sheet (HSLA). When ductility is required, the dual phase of ferrite phase and martensite phase is required. If the steel requires stretch flangeability, a precipitation-hardened Dual Phase steel is selected.
Further, as a manufacturing technique of these steel sheets, for example, Japanese Patent Publication No. 62-37089, Japanese Patent Publication No. 58-27328, Japanese Patent Publication No. 62-39230, etc. Methods for improving flange formability have been proposed.
[0003]
[Problems to be solved by the invention]
However, all of the above-mentioned conventional methods mainly aim at improving only a single processing characteristic, and furthermore, in the hot rolling process, an operation for controlling the temperature in an extremely narrow temperature range is required. And the productivity was inferior.
[0004]
Accordingly, an object of the present invention is to provide a high-strength hot-rolled steel sheet having both stretch formability (ductility), shape freezing property (yield ratio), and stretch flange formability (hole expanding property) and excellent in multiple press formability. To provide.
Another object of the present invention is to set the slab heating temperature, finish rolling temperature, winding temperature, etc. in a wide range, except for performing cooling at an extremely high speed after the completion of rolling in the hot rolling step, and the material is stable. It is an object of the present invention to provide a method of manufacturing a high-tensile hot-rolled steel sheet which can be obtained by the above method and can drastically improve productivity.
The specific material goals of the present invention are to provide a hot-rolled steel sheet having a tensile strength of 540 to 590 MPa, a proof stress of 400 to 450 MPa, an elongation of 30 to 38%, and a hole expansion ratio of 100% or more, and a method for producing the same. Is to do.
[0005]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present inventors have made intensive studies on Si, Mn-containing low-carbon steel as a base, mainly focusing on the component composition, metal structure, and hot rolling process. As a result, they found that by adding a trace amount of carbonitride forming element and properly controlling the hot rolling and the cooling process after hot rolling, the ferrite transformation after finish rolling was activated, and the present invention It was completed. That is, the gist configuration of the present invention is as follows.
[0006]
(1) C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.00 wt%, P: 0.01 to 0.15 wt%, S: 0.005 wt% or less, N: 0.001 to 0.005 wt %, Ti : 0.005 to 0.03 wt%, the balance is composed of Fe and unavoidable impurities, and the low temperature is mainly composed of a ferrite phase having a volume fraction of 80 to 97% and an average particle size of 10 μm or less, and the rest being a bainite phase. High tensile strength hot rolled steel sheet with excellent workability consisting of transformation phase.
(2) The high-strength hot-rolled steel sheet according to the above (1) , further comprising V: 0.005 to 0.03 wt % in addition to the above component composition .
[0007]
(3) C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.00 wt%, P: 0.01 to 0.15 wt%, S: 0.005 wt% or less, N: 0.001 to 0.005 wt %, Ti : A steel material containing 0.005 to 0.03 wt% , the balance being Fe and unavoidable impurity components. After keeping the steel material in a temperature range of 900 to 1300 ° C. for a period of 3600 seconds or less, the finish rolling end temperature is set to 780 to Continuous hot rolling at 980 ° C., cooling within 1 second after the completion of rolling at a cooling rate of 50 to 200 ° C./sec, and subsequently winding the coil in a temperature range of 300 to 650 ° C. For producing high-strength hot-rolled steel sheets with excellent heat resistance.
(4) The method for producing a high-tensile hot-rolled steel sheet according to the above (3) , further comprising V: 0.005 to 0.03 wt % in addition to the above component composition .
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an example of a research result on which the present invention is based. That is, a slab in which the amount of Ti is changed to 0.08 wt% C, 0.7 wt% Si, 1.2 wt% Mn, 0.05 wt% P, 0.0015 wt% S, and 0.0025 wt% N is held at 1050 to 1250 ° C. for 1800 seconds. After heating, finish rolling was completed at 880 ° C., and within 1 second after the completion of rolling, forcibly cooled at 70 ° C./sec, wound around a coil at 470 ° C. It shows the relationship with temperature.
[0009]
From FIG. 1, the following can be said. In the cooling process after finish rolling, the transformation behavior from austenite to ferrite phase is closely related to the grain size before rolling, and the transformation occurs as a nucleation site at the grain boundaries. Is activated. From this, by adding Ti , V, etc., which form carbon nitrides which are relatively stable even at high temperatures, and by lowering the heating temperature of the steel material, austenite grain growth is suppressed and ferrite transformation is promoted. However, according to FIG. 1, when the addition amount of the carbonitride forming element is increased beyond a certain amount, the ferrite transformation is delayed due to the coarsening of the carbonitride and the effect of the solid solution carbonitride forming element. You can see that. As described above, the inventors have found that Ti and other carbonitride forming elements have an appropriate addition range.
[0010]
Hereinafter, the reason why the constituent elements in the present invention are limited to the above range will be described.
C: 0.010 to 0.10 wt%
If the content of C is less than 0.010%, not only the precipitation of the low-temperature transformation phase is reduced and sufficient strength is not obtained, but also the carbide is reduced during heating of the steel material, the structure becomes coarse, and Is reduced. On the other hand, when the content exceeds 0.10 wt%, carbides are coarsened during heating of the steel material, and the structure is coarsened, thereby reducing workability. Therefore, the C content is in the range of 0.010 to 0.10 wt%, preferably 0.05 to 0.09 wt%.
[0011]
Si: 0.50-1.50wt%
If the content of Si is less than 0.50 wt%, the volume ratio of the ferrite phase decreases and the formability decreases, whereas if it exceeds 1.50 wt%, the ferrite phase becomes hard and the ductility decreases. Therefore, the Si content is in the range of 0.50 to 1.50 wt%, preferably 0.6 to 1.0 wt%.
[0012]
Mn: 0.50-2.00wt%
If the content of Mn is less than 0.50 wt%, the volume fraction of the second phase decreases and sufficient strength cannot be obtained, while if it exceeds 2.00 wt%, on the contrary, the volume fraction of the second phase increases. As a result, ductility decreases. Therefore, the Mn content is in the range of 0.50 to 2.00 wt%, preferably 0.9 to 1.3 wt%.
[0013]
P: 0.01-0.15wt%
If the content of P is less than 0.01 wt%, the amount of precipitation of the ferrite phase becomes small and sufficient ductility cannot be obtained. On the other hand, if the content exceeds 0.15 wt%, the spot weldability deteriorates. Therefore, the P content is in the range of 0.01 to 0.15 wt%, preferably 0.02 to 0.05 wt%.
[0014]
S: 0.005 wt% or less S is an element harmful to workability, and if its amount is reduced, precipitates in steel are reduced and workability is improved. Such effects can be obtained when the S content is 0.005 wt% or less.
[0015]
Ti: 0.005 to 0.03wt%
Ti is an element forming carbonitride, and if its content is less than 0.005 wt%, the amount of carbonitride deposited is insufficient, and the slab heating temperature dependence of the material increases. On the other hand, if the content exceeds 0.03 wt%, the precipitates become coarse, and the amount of solid solution Ti during slab heating increases, and similarly, the slab heating temperature dependence of the material increases. Therefore, the Ti content is in the range of 0.005 to 0.03 wt%, preferably 0.01 to 0.02 wt%.
[0016]
V: 0.005 to 0.03 wt%
V is an element forming carbonitrides. If the content is less than 0.005 wt%, the amount of carbonitrides deposited is insufficient, and the slab heating temperature dependence of the material increases. On the other hand, if it exceeds 0.03 wt%, the precipitates become coarse, and the solid solution V during slab heating increases, and similarly, the slab heating temperature dependence of the material increases. Therefore, the V content is in the range of 0.005 to 0.03 wt%, preferably 0.01 to 0.02 wt%.
[0018]
Incidentally, Ti, Nb, carbonitride of V has the both grain growth inhibiting effect, among others, Ti is in the case of solid solution, influence on the ferrite transformation delays, is smaller than Nb and V, Among these carbonitride forming elements, it is most preferable to use Ti.
[0019]
If the volume fraction of the ferrite phase, the grain size and the volume fraction of the remaining microstructure ferrite phase are less than 80%, sufficient ductility cannot be obtained. On the other hand, if it exceeds 97%, the second phase becomes hard and stretch flange forming. Is reduced. For this reason, the volume ratio of the ferrite phase is set in the range of 80 to 97%.
If the average particle size of the ferrite phase exceeds 10 μm, the stretch flangeability decreases, so the average particle size is set to 10 μm or less.
Further, the remaining structure needs to be a low-temperature transformation phase mainly composed of a bainite phase. This is because, if the main phase of the second phase is pearlite or martensite other than bainite, the stretch flange formability decreases.
[0020]
Heating of the steel material The heating of the steel material is carried out at a temperature of 900 to 1300 ° C for a period of 3600 seconds or less. This is because if the temperature is lower than 900 ° C., the surface quality of the steel sheet is degraded, while if it exceeds 1300 ° C., the carbonitride is dissolved or condensed and the ferrite phase is coarsened. If the retention time exceeds 3600 seconds, the carbonitride will be dissolved or condensed and the ferrite phase will be coarsened. In addition, a preferable heating temperature is 1000 to 1250 ° C.
[0021]
After the rough finish rolling in the continuous hot rolling, the finish rolling is performed at a rolling end temperature of 780 to 980 ° C. If the finish temperature of finish rolling is lower than 780 ° C, the work structure remains in the steel sheet and the ductility deteriorates.On the other hand, if the temperature exceeds 980 ° C, the structure becomes coarse and ferrite transformation is delayed, resulting in lower formability. Because you do.
[0022]
Cooling after completion of hot rolling After the completion of the hot rolling, cooling is performed at a cooling rate of 50 to 200 ° C / sec within 1 second. If the elapsed time from the hot rolling to the cooling exceeds 1 sec, the structure becomes coarse, and the ferrite transformation is delayed, thereby reducing the formability. Therefore, it is necessary to start forced cooling within 1 sec after the finish rolling.
If the cooling rate of the forced cooling is less than 50 ° C / sec, the volume ratio of the ferrite phase exceeds 97%, while if it exceeds 200 ° C / sec, the volume ratio of the ferrite phase becomes less than 80%. 50 to 200 ° C / sec. The preferred cooling rate is 60 to 120 ° C / sec.
[0023]
Winding temperature The winding temperature of the coil is in the temperature range of 300 to 650 ° C. If the winding temperature is less than 300 ° C., the volume fraction of the ferrite phase is less than 80%, and if it exceeds 650 ° C., the second phase becomes a pearlite-based structure and the formability is reduced. The preferred winding temperature is 400 to 500 ° C.
[0024]
【Example】
Steels having various chemical compositions shown in Table 1 were melted in a converter to obtain continuous cast slabs. The slab was heated and hot-rolled under the respective manufacturing conditions shown in Table 2, cooled, and then wound around a coil to produce a hot-rolled steel sheet having a thickness of 2 mm.
A JIS No. 5 test piece was sampled from the center position of the obtained hot-rolled steel sheet in the coil longitudinal direction, a tensile test was performed, elongation, a yield ratio, and the like were obtained, and a hole expansion test was performed to evaluate formability.
Further, the test material was sampled from the center position in the longitudinal direction of the coil, and the cross-sectional structure in the rolling direction was observed with an optical microscope, and the volume ratio of the ferrite phase at the center in the thickness direction was obtained.
The volume fraction of the ferrite phase is obtained by calculating the number and average diameter of the ferrite phase and the remaining phase by image processing, converting the average diameter into a three-dimensional diameter by the following equation, The volume ratio was determined from the diameter. The structure of the remaining phase was examined with an electron microscope.
D = 1.128 L
Here, D: average diameter (two-dimensional), L: average three-dimensional diameter, and the results are shown in Table 2.
From these results, it can be seen that according to the method of the present invention, a high-tensile hot-rolled steel sheet having sufficient formability, stretch flangeability and shape freezing property and excellent workability can be manufactured.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
【The invention's effect】
As described above, according to the present invention, by optimizing the chemical composition of a steel sheet, hot rolling, and subsequent cooling and winding conditions, both stretch formability, shape freezing property, and stretch flange formability can be improved. It is possible to provide a high-strength hot-rolled steel sheet having much better formability than before.
Further, according to the present invention, it is no longer necessary to strictly control the production conditions, and it is possible to produce a steel plate having a stable material while maintaining high productivity.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of the amount of Ti and the heating temperature on the volume fraction of ferrite.

Claims (4)

C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.00 wt%, P: 0.01 to 0.15 wt%, S: 0.005 wt% or less, N: 0.001 to 0.005 wt %, Ti : 0.005 About 0.03wt % , the balance being composed of Fe and unavoidable impurities, with a ferrite phase having a volume fraction of 80-97% and an average grain size of 10μm or less, and a balance consisting of a low-temperature transformation phase mainly composed of a bainite phase. High tensile hot rolled steel sheet with excellent workability. In addition to the above component composition, V: 0.005 ~ 0.03wt %. The hot-rolled high-strength steel sheet according to claim 1, wherein C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.00 wt%, P: 0.01 to 0.15 wt%, S: 0.005 wt% or less, N: 0.001 to 0.005 wt %, Ti : 0.005 After the steel material containing 0.03wt % , the balance being Fe and unavoidable impurities, the steel material was kept in the temperature range of 900-1300 ℃ for 3600sec or less, and the finish rolling finish temperature was 780-980 ℃. Excellent hot workability, characterized in that it is cooled at a cooling rate of 50 to 200 ° C / sec within 1 sec after the end of rolling, and then wound around a coil in a temperature range of 300 to 650 ° C. A method for manufacturing high-strength hot-rolled steel sheets. In addition to the above component composition, V: 0.005 ~ 0.03wt The method for producing a high-tensile hot-rolled steel sheet according to claim 3, wherein

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