JPS647132B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention uses C-Si-Mn steel or C-Mn steel,
Special elements (carbonitride forming elements such as Nb, Ti, V...)
A method for producing a high-strength hot-rolled steel sheet that does not require the addition of any of It is related to. (Prior art) In recent years, in the automobile industry, one of the measures to improve the fuel efficiency of automobiles is to reduce the weight of automobile bodies, and from the viewpoint of thinner steel plates and safety, high-strength hot-rolled steel sheets with excellent workability have been developed. Demand is increasing. Conventionally, in order to obtain high-strength hot-rolled steel sheets with excellent workability, special elements such as Nb, Ti, V, etc. are added to improve strength through solid solution hardening and precipitation strengthening through carbonitride formation. There is a manufacturing method that improves the strength-ductility balance by using dual phase steel sheets, which have recently begun to be manufactured. In the former, additive elements such as Nb, Ti, V, etc. are expensive and cause an increase in cost, and resource constraints are expected in the future. The latter DP steel plate has strength-
Although the ductility balance is good, there are problems when applying it to wheels, which are automobile parts, because the stretch flangeability is poor, so for example, when processing wheel discs, cracks may occur when forming hub holes by burring. easy. ○B) There are two points: the wheel rim has a large softening phenomenon in the weld heat affected zone, the wall thickness of that part decreases significantly during mold correction, and no improvement in fatigue properties is observed. As a way to solve these problems, C-Si-
It is possible to manufacture high-strength hot-rolled steel sheets with a composite structure of ferrite and bainite using Mn steel. for example,
As shown in Japanese Patent Application Laid-Open No. 57-145965, C-Si-
It has been demonstrated that high-strength hot-rolled steel sheets with a ferrite-bainite composite structure can be manufactured using Mn steel by controlling the cooling after the final rolling pass and controlling the coiling temperature, making it possible to process wheel discs at low cost. At the same time, improvements have been made to the cracking caused by forming hub holes through burring, but this is not sufficient. The softening phenomenon of the weld heat-affected zone is better than that of DP steel, but when performing high-level processing such as automobile suspension parts, the ferrite-bainite composite structure simply lacks ductility and cracks occur. do. The present inventors have proposed in Japanese Patent Application No. 58-2485 a method for producing polygonal ferrite and bainite, which improves the strength-ductility balance and strength through grain refinement and improves fatigue properties. Here, it is effective to increase the amount of C in order to achieve even more effective grain refinement, and moreover, it is possible to stably obtain 60 Kgf/mm ² or more, and to obtain a polygonal ferrite/bainite composite structure with good ductility. Show that. (Object of the invention) In view of the current situation, the present invention has been developed to develop general-use C-Si.
- From a high-strength hot-rolled steel sheet with a ferrite-bainite composite structure of steel with similar composition to Mn-based steel, that is, a high-strength hot-rolled steel sheet with a ferrite-bainite composite structure shown in Japanese Patent Application No. 58-2485. A tensile strength of 60Kgf/60Kgf with polygonal ferrite and bainite, which can further increase strength, improve strength-ductility balance, and improve fatigue properties.
The objective is to provide a new manufacturing method for manufacturing high-strength hot-rolled steel sheets with mm ² class or higher and excellent workability at low cost. (Structure and operation of the invention) The basic component of the present invention is C: more than 0.15 and 0.20%.
Below, Si: 1.5% or less, Mn: 0.3 to 1.5%, P:
0.02% or less, Si: limited to 0.01% or less, the remainder
A steel billet consisting of Fe and other unavoidable elements is used. C and Mn are essential elements to ensure the necessary strength and obtain a composite structure of ferrite and bainite, and C:
TS60Kgf/mm ² for 0.15% or less, Mn: less than 0.3%
It is difficult to obtain a steel plate with a strength higher than C: 0.20.
%, Mn: more than 1.5% causes a significant deterioration of ductility and impairs weldability, so C: more than 0.15 and less than 0.20%,
Mn: 0.3 to 1.5%. Preferably, when Si is added in an amount of 0.2% or more, the solid solution [C] in the ferrite grains decreases and promotes the concentration of C element in the untransformed austenite grains, so it serves to make it easier to obtain a suitable composite structure of ferrite and bainite. This improves the strength-ductility balance of steel sheets. Si: If it exceeds 1.5%, the concentration of C element in untransformed austenite grains will become saturated, which is economically disadvantageous and impairs weldability. Therefore, Si: 1.5% or less, preferably 0.2 to 1.5%.
%. Since P impairs weldability, P: 0.02
% or less. Since S forms MnS-based inclusions and reduces stretch-flangeability, in order to reduce MnS-based inclusions and improve stretch-flangeability, S: 0.01% or less. Ca has the function of controlling the shape of inclusions to form fine spheres and improves stretch flangeability, so it is preferably contained up to 0.01%. During hot rolling, the heating temperature is preferably 1100°C or lower. This is done in order to make the austenite grains as small as possible during heating, and if the heating temperature is high, the rolling speed must be reduced to ensure the final rolling pass temperature, or there is a delay time on the entry side of the finish rolling, which prevents productivity from decreasing. This is for the sake of The rolling reduction of at least the first three passes of finish rolling is 40% each.
The reason for the high rolling reduction ratio of over % is that the passing time of the steel plate between passes is considered to be sufficient time for the austenite grains to recrystallize after rolling.
The purpose is to set the rolling reduction rate as high as possible to increase the generation of recrystallized nuclei and to refine the austenite grains. Therefore, the rolling reduction rate for each of the first three passes theoretically covers the thickness of the product, but based on the current rolling mill capacity, the rolling reduction rate is determined by the angle of engagement of the rolls, rolling load, etc. Due to the limitations, the practical limit is about 60% at most. After the 4th pass, since the time between passes is short, the effect of cumulative reduction (can be evaluated as an accumulation of reductions with almost no recovery after each pass) is considered, and the total reduction ratio (before and after finishing rolling) is considered to be finer grain size. rolling reduction rate) 80
% or more. Since a higher rolling reduction is more effective for the refinement, there is theoretically no upper limit to the total rolling reduction, and the rolling reduction required to obtain the product thickness is the target. In the first invention, the temperature of the final rolling pass is set to (Ar ₃ +50°C) to (Ar ₃ -50°C) because large rolling reduction is applied and recrystallized austenite grains are refined or refined in this temperature range. This is to make the ferrite grains finer by increasing the number of deformation bands introduced into the unrecrystallized austenite grains. Further, even if the transformation point is exceeded during rolling, recrystallization of ferrite has been observed to occur due to large reduction, which is effective for grain refinement. Furthermore, the two-phase separation of ferrite and austenite is promoted, and appropriate ferrite and bainite structures can be obtained. Ar ₃ here refers to the transformation point temperature during cooling without rolling. Therefore, the transformation point varies depending on the rolling reduction rate, cooling rate, and component system, but it is between (Ar ₃ +50℃) and (Ar ₃ -50℃).
℃) can be substantially 840 to 730℃. FIG. 1 shows the influence of the temperature of the final rolling pass and the effect of the reduction rate in the finish rolling on the ferrite grains. The ferrite grains are smaller when the final rolling pass temperature is 800 to 760°C than when it is over 840°C.
Moreover, even if the temperature after the final rolling pass is the same at 800 to 760°C, the ferrite grains become even smaller by reducing the rolling reduction ratio of 40 to 50% in each of the first three passes of finish rolling. I understand. In other words, it can be inferred that the temperature of the final rolling pass and the rolling reduction rate of finish rolling have a large synergistic effect on the refinement of ferrite grains. Furthermore, as shown in Fig. 1, the particle size obtained by increasing C becomes finer even under the same rolling reduction rate and temperature conditions, and C: 0.15~
GS No. 12 or higher can be stably obtained at 0.20%. The effect of grain refinement is effective in increasing strength, improving the strength-ductility balance (ensuring elongation of 25% or more), and increasing fatigue strength. Cooling rate of 45℃/s or more from two-phase coexistence region after rolling
The purpose is to make it easier to obtain a suitable composite structure of ferrite and bainite by cooling at a rate of 100°C/s to a temperature of 300 to 550°C (including water injection cooling, air/water cooling, and gas cooling). Here, the lower limit of cooling rate
45℃/s is bainite mainly composed of bainite.
This is the minimum cooling rate necessary to obtain a ferrite composite structure. Therefore, a cooling rate of 45° C./s or higher is sufficient, but regardless of the structure theory, the upper limit of the cooling capacity of current cooling equipment for hot rolling processes is 100° C./s. If the temperature of the final rolling pass is in the two-phase coexistence range, the two-phase separation will proceed even if the product is immediately cooled after rolling, resulting in an appropriate composite structure of ferrite and bainite. Preferably, it is effective to perform air cooling for about 5 seconds to complete two-phase separation before starting cooling. However, when manufacturing thin and wide steel sheets with a composite structure of ferrite and bainite, the temperature range of the final rolling pass is (Ar ₃ +50℃) to (Ar ₃ -50℃) and the rolling load is at the limit of the rolling mill. For products that are difficult to manufacture due to exceeding the above-mentioned values, the temperature of the final pass should be set to exceed (Ar ₃ +50℃) (second invention), and the cooling rate should be set to ensure the temperature in the two-phase region after the final rolling pass. Slow cooling at less than 45℃/s, then cooling rate of 45℃/s or more to 100℃/
Cool to a temperature of 300 to 550°C around s. This is because the grain size is not sufficiently refined and the strength-ductility balance is slightly inferior. The feature of the hot-rolled steel sheet with a composite structure of ferrite and bainite produced by the method described above is that after two-phase separation is carried out, the strength is increased by the bainite structure, and the ductility is improved by making the ferrite into fine grains. Steel sheets with ferrite and pearlite structures obtained under normal manufacturing conditions and Ti, V,
It has properties with a far better strength-ductility balance than precipitation-strengthened high-strength hot-rolled steel sheets created by adding elements such as Nb... Figure 2 compares the strength-ductility balance of products manufactured under the conditions of the present invention with DP hot-rolled steel sheets and precipitation-strengthened high-strength hot-rolled steel sheets. - Although inferior in ductility balance, it is far superior to precipitation-strengthened high-strength hot-rolled steel sheets. (Examples) Examples according to the present invention are shown in Table 1. Invention example 1
Steels having predetermined components were hot rolled in accordance with the present invention. Comparative Example 10 had a component system with a low C content, was hot rolled in accordance with the present invention, and had low strength. In Comparative Example 11, the cooling rate during hot rolling was extremely low due to the predetermined components, and the steel had a ferrite/pearlite structure and low strength. Comparative Example 12 has a finish of F ₁ during hot rolling with the specified ingredients.
The rolling reduction of ~ _F3 is too low, the strength-ductility balance is low, and the drop in elongation is particularly large.
Comparative Example 13 has a high C component system, and the rolling reduction of finishing F ₁ to _{F 3} during hot rolling was low, resulting in a very large decrease in elongation.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows the temperature of final finishing rolling and F ₁ of finishing rolling.
A diagram showing the relationship between the rolling reduction ratio of ~F ₃ , the amount of C, and the grain size, and Figure 2 is a diagram comparing the balance of strength and ductility.

Claims (1)

[Claims] 1 A steel billet containing C: more than 0.15 and not more than 0.20% Si: not more than 1.5% Mn: 0.3 to 1.5% P: not more than 0.02% S: not more than 0.01%, the balance being Fe and unavoidable elements Heating, at least the first three passes of continuous hot finishing rolling with a rolling reduction of 40 to 60% in each pass, a total rolling reduction of 80% or more in all finishing rolling, and the temperature of the final rolling pass at (Ar ₃ + 50 ℃) ~ (Ar ₃ -50℃)
After finishing, it is cooled at a cooling rate of 45 to 100℃/s, and coiled at 300 to 550℃.It has a fine composite structure of ferrite and bainite, and has a good strength-ductility balance. A method for producing high-strength hot-rolled steel sheets with excellent properties. 2 A steel billet containing C: more than 0.15 but not more than 0.20% Si: not more than 1.5% Mn: 0.3 to 1.5% P: not more than 0.02% S: not more than 0.01%, with the balance consisting of Fe and unavoidable elements is heated and subjected to continuous hot heating. At least the first three passes of finish rolling have a rolling reduction of 40 to 60% in each pass, the total rolling reduction of all finishing rolling is 80% or more, and the temperature of the final rolling pass is over (Ar ₃ + 50℃). After completion, the temperature is gradually cooled from Ar ₃ to _{Ar 1} at a cooling rate of less than 45°C/s, and thereafter the cooling is performed at a cooling rate of 45 to 100°C/s.
A method for producing a high-strength hot-rolled steel sheet with excellent workability and having a composite structure of ferrite and bainite, which is characterized by winding at 300 to 550°C.