JPH02149618A - Manufacture of hot rolled high strength steel plate excellent in workability - Google Patents

Abstract

PURPOSE:To manufacture a hot rolled steel plate having high strength and excellent in workability by hot-rolling a steel with a specific composition under specific conditions, cooling the resulting hot rolled plate down to a winding temp. while specifying cooling conditions, and then winding the above plate. CONSTITUTION:A steel having a composition consisting of, by weight, 0.10-0.35% C, 0.5-3.0% Si, 0.5-2.5% Mn, and the balance iron with inevitable impurities is hot-rolled at >= about 80% finish draft so that finish rolling-finishing temp. is regulated to (Ar3 - 50 deg.C) to 950 deg.C. Subsequently, slow cooling is applied to the hot rolled plate down to a temp. right above the pearlite transformation point at 1-30 deg.C/sec cooling rate to allow ferrite transformation to proceed sufficiently, and then, the above plate is cooled down to a winding temp. (300-500 deg.C) at a cooling rate (preferably >= about 30 deg.C/sec) capable of inhibiting pearlite transformation, followed by winding. By this method, the hot rolled steel plate having high strength as to have >= about 60kgf/mm<2> tensile strength and excellent in workability can be obtained.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a manufacturing technology for high-strength steel sheets, particularly tensile strength 6
The present invention relates to a method for manufacturing hot rolled steel sheets with high strength of 0 jcgf/arm" or higher and excellent workability. (Prior technology) High strength steel sheets have been used for automobiles, industrial machinery, etc. In particular, automotive steel sheets are used to reduce the weight of automobiles,
There is an increasing demand for higher strength steel plates in order to ensure safety in the event of a collision. but. In addition to simply increasing the strength of steel plates, it is also required that they have better workability and weldability. To meet this type of demand, there has conventionally been a dual-phase steel made of hot-rolled steel sheet made of ferrite and martensite. However, this duplex steel has a better strength-ductility balance (TSXEQ) than solid solution strengthened or precipitation strengthened high strength steel sheets.
), but TSXEQ'': 200o had the disadvantage of not being able to withstand stricter workability requirements. (Problem to be solved by the invention) Therefore, in order to solve this disadvantage, TSXEQ
2000, a hot-rolled steel sheet with a structure containing retained austenite was developed. As an example, one example is performing large reduction rolling with a finishing temperature of 850°C or higher, a total reduction of 80% or more, a total reduction of 60% or more in the final three passes, and a final bath reduction of 20% or more, and then There is a method (Japanese Unexamined Patent Publication No. 165320/1983) of producing a hot rolled steel sheet containing retained austenite by cooling to 300°C or less at a cooling rate of 50°C/S or more. However, manufacturing such hot-rolled steel sheets requires large reduction rolling, so the purpose of Toshite, ■
C: 0.15-0.40%, 5ilo. 5 to 2.0% and Mn: 0.5 to 2.0%, with the remainder consisting of iron and unavoidable impurities. °C), hot rolling is carried out at a total reduction rate of 80% or more, followed by 350-5
00°C at a cooling rate of 40°C/s or more, or 2) To further improve ductility and improve the strength-ductility balance, the steel plate is heated at 30'C/hr after winding.
A method for producing hot-rolled steel sheets containing retained austenite by cooling to 200°C or less at a cooling rate of
4017) etc. were proposed. However, hot-rolled steel sheets obtained by these methods do not have good workability, and in method (2), TsxEQ
Although a strength-ductility balance of >2400 can be obtained, in order to obtain high ductility, it is necessary to perform mist cooling from the side of the coil or cooling the entire coil by immersing it in water, etc., to obtain high ductility. There are problems such as making the material material in the width direction of the plate significantly non-uniform. An object of the present invention is to provide a method for manufacturing a high-strength hot-rolled steel sheet that has a high strength-ductility balance of TSXEQ>2000 or more, has a tensile strength of 60 kgf/mm2 or more, and has excellent workability. It is. (Means for Solving the Problems) In order to achieve the above object, the present inventors have developed a composition. As a result of comprehensive research on manufacturing conditions, we found that we hot rolled steel with a specific composition, slowly cooled it during cooling after hot rolling to allow ferrite transformation to proceed sufficiently, and then set a cooling rate that could prevent pearlite transformation. We have discovered that this is possible by cooling and winding to a winding temperature of 300 to 500°C, and have hereby accomplished the present invention. That is, 1. The method for manufacturing a high-strength hot-rolled steel sheet with excellent workability according to the present invention includes C: 0.10 to 0.35%, Si: 0.
5 to 3.0% and Mn: 0.5 to 2.5%, with the balance consisting of iron and inevitable impurities,
Hot rolling is carried out at a finish rolling finish temperature (Ar, -50°C) to 950"C, and after the completion of hot rolling, it is cooled to a temperature just above pearlite transformation at a cooling rate of 1 to 30'C/s, and then The present invention is characterized in that it is cooled to a winding temperature of 300 to 500°C at a cooling rate that can prevent pearlite transformation, and then wound.The present invention will be explained in more detail below.(Function) First, the present invention The reasons for limiting the chemical composition of steel used for steel are explained below.C2C is an essential element for strengthening steel, and in order to obtain a sufficient amount of retained austenite that improves ductility, it must be at least 0.1
A C content of 0% is required. An increase in the amount of C increases the hardness difference between ferrite and a martensite phase in which the second phase of bainite or retained austenite undergoes deformation-induced transformation. When the difference in hardness between the matrix and the second phase is large, deformation is difficult to transmit at the interface between the matrix and the second phase, and this interface becomes the starting point for cracks, resulting in cracking during processing. This effect becomes remarkable in the second phase generated when the amount of C exceeds 0.35%. Moreover, if it exceeds 0.35%, weldability will be significantly deteriorated, which is not preferable. Therefore, the amount of C is 0°10~0.
The range is 35%. Si: Increasing the content of SL is advantageous for the generation and purification of ferrite that contributes to improving ductility, and is also advantageous for concentrating C in untransformed austenite to obtain retained austenite. Furthermore, Si suppresses carbide formation during bainite transformation after winding, and makes C more concentrated in untransformed austenite, which is more advantageous for obtaining retained austenite. Furthermore, increasing the SL content causes the ferrite phase to harden as a solid solution, which has the effect of reducing the difference in hardness between the ferrite phase and the second phase, suppressing the occurrence of cracks to a high degree of workability, and as a result, improving the workability of the ferrite phase. Improves sex. Such an effect is not sufficiently exhibited when the content is less than 0.5%. Also, 3.0
%, the effects of ferrite generation, purification, and securing retained austenite are saturated, and on the contrary, scale properties and weldability are deteriorated, and ordered phase (B2) is formed, which impairs workability, so it is preferable. do not have. Therefore, the amount of Si is set in the range of 0.5 to 3.0%. Mn: Mn contributes to the retention of austenite as an austenite stabilizing element. This effect cannot be sufficiently obtained below 0.5%, and when it exceeds 2.5%, the effect is saturated.
This is not preferable because it actually worsens weldability. Therefore, the amount of Mn is set in the range of 0.5 to 3.0%. Note that the balance of the above steel is iron and inevitable impurities, and there is no need to add other alloying elements. Next, conditions for the method of the present invention will be explained. In order to improve the ductility of hot-rolled steel sheets with the above composition, it is necessary to generate retained austenite in the two-phase matrix of ferrite and bainite, preferably at a volume percentage of 5% or more.
It is stabilized by concentration of elements such as. Therefore, in the present invention, after the hot rolling is completed, the 1-b or 5-b ellirite transformation is allowed to proceed sufficiently until just above the pearlite transformation, promoting the concentration of elements such as C in the austenite, and preventing the residual austenite from remaining. Make a contribution. Of course, such an effect cannot be obtained if the cooling rate is outside the above range. At this time, if the finish rolling end temperature is less than Ar, -50°C, a worked ferrite structure that impairs ductility is formed, so the finish rolling end temperature needs to be Ar, -50°C or higher. In addition, if the finish rolling end temperature exceeds 950T, it takes too long to cool down to the temperature just before pearlite transformation starts, which is impractical. Therefore, the finish rolling end temperature is (Ar, -50℃) ~ 950T The range is ℃. Note that the finish rolling rate is preferably 80% or more. Further, when the ferrite transformation ends and the pearlite transformation starts, C, which is effective for remaining austenite, is consumed, and the amount of retained austenite decreases. Furthermore, when pearlite is formed, cracks are likely to occur during processing from the interface between cementite and ferrite in pearlite or from layered cementite, which significantly impairs workability. Therefore, after the above cooling, it is necessary to cool down to the coiling temperature at a cooling rate (preferably 30° C./S or higher) that can prevent pearlite transformation. If the winding temperature exceeds 500°C, pearlite will be produced after winding or bainite transformation will proceed excessively, making it impossible to obtain sufficient residual austenite. When pearlite is formed, workability deteriorates as described above. Also, 30
At a coiling temperature of less than 0°C, the hardness of the bainite that is formed increases, and hard martensite is formed in some parts, resulting in a large difference in hardness from the ferrite phase, and cracks occur during processing at these interfaces. As a result, workability (hole expandability) deteriorates. Therefore, the winding temperature is 3oO~500
The range is ℃. The resulting hot rolled steel sheet has a structure in which 5% or more of retained austenite is uniformly dispersed in a two-phase matrix of ferrite and bainite. Next, examples of the present invention will be shown. (Example) Steels A to D having the chemical components shown in Table 1 were hot rolled under the conditions shown in Table 2, wound, and air cooled. In addition, hot rolling is 3011II → 16Illl → 8m+m → 3m
I went according to the bus schedule of m. The mechanical properties and structure of the obtained hot-rolled steel sheets were investigated, as well as the workability (hole expansion rate). The results are also listed in Table 2. From Table 2, the following considerations can be made. Invention example &1, rod 4 and &8 are TSXFII>240
(See Figure 1, which shows an extremely high strength-ductility balance of 0.0).Furthermore, the yield strength is low and the workability is excellent. On the other hand, Nn2 of the comparative example has a low Si content, and Nn3 has a low C content, so almost no retained austenite is generated, the ductility is low, the strength-ductility balance is low, and the workability is also poor. In Comparative Example Ha 5, the finish rolling temperature was too low and a processed ferrite structure was formed, resulting in low ductility. Comparative example 6 has a pearlite transformation start temperature (in this case, 65
Even when the temperature is below 0°C, pearlite transformation progresses because the cooling rate is slow, no retained austenite is formed, and ductility is low. Processability is also poor. In Comparative Example &7, the coiling temperature was high, no retained austenite was formed, and the ductility was insufficient. Processability is also poor. In Comparative Example &9, the winding temperature was too low and the hole expandability was poor. In Comparative Example &10, the cooling rate after hot rolling was too high and ferrite transformation did not progress sufficiently, resulting in low ductility and poor workability.

[Blank below] (Effects of the invention) As detailed above, according to the present invention, the tensile strength is 60
kgf/mm2 or more, it is possible to obtain a high-strength hot-rolled steel sheet that has a high strength-ductility balance and is excellent in workability. Also, since it can be manufactured without adding special alloying elements,
The economical and other industrial effects are very large.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between TS and EQ. Patent applicant: Kobe Steel, Ltd. Representative Patent Attorney Takashi Nakamura

Claims (1)

[Claims] In weight% (the same applies hereinafter), C: 0.10 to 0.35%,
A steel having a composition containing Si: 0.5 to 3.0% and Mn: 0.5 to 2.5%, with the balance consisting of iron and unavoidable impurities, was heated to a finishing rolling temperature (Ar_3-50°C). ~9
Hot rolling is carried out at 50°C, and after the hot rolling is completed, the rolling speed is 1 to 30°C/s.
High strength with excellent workability, characterized by cooling to a temperature just above pearlite transformation at a cooling rate of A method for producing hot rolled steel sheets.