JPS6367524B2 - - Google Patents

Description

[Detailed description of the invention]

This invention has an elongation of about 30%, an impact fracture transition temperature of about -40℃, and a tensile strength of 50Kgf/mm ²
The above-mentioned hot-rolled high-strength steel sheet with excellent workability, toughness, and strength is related to a method of manufacturing with low energy consumption by effectively utilizing the heat retained in a hot slab obtained by continuous casting. Conventionally, non-heat-treated hot-rolled steel sheets with a tensile strength of 50 Kgf/ ^mm2 or higher are mainly known as those that are hot-rolled by adding elements such as Ti, Nb, and V, and then coiled at 550 to 600°C. Although this steel sheet has been put into use, it requires expensive additive elements, making it expensive, and it also lacks versatility in terms of workability, making it unable to meet a wide range of requests. It was the current situation. In addition, it is also known that non-tempered hot-rolled high-strength steel sheets are produced by hot-rolling ordinary carbon steel, then rapidly cooling it to a temperature below the Ms point, and then coiling it to create a duplex steel. In addition to high manufacturing costs, there were problems in that the yield ratio was low, which also limited its use. By the way, in recent years, from the viewpoint of energy saving, the direct rolling method, in which hot rolling of hot slabs after continuous casting is directly rolled without cooling or after passing through a light heating or heat retention process, has attracted attention, and many research reports have been published. It has become common practice to do so. Therefore, attempts have been made to apply the above-mentioned direct rolling method to the production of non-thermal hot rolled steel sheets in order to reduce costs, but if the hot slab is hot rolled without lowering the temperature to Ar ₃ or less, the rolling Because the previous austenite grains were ultra-coarse grains, the structure after rolling also became coarse grains, resulting in a significant deterioration of workability and toughness.
The situation continued in which the steel had no choice but to cool down to below the Ar ₃ point, then reheat and roll. From the above-mentioned viewpoints, the present inventors have developed a method for producing high-strength steel sheets with sufficient strength and excellent workability using a direct rolling method with low energy consumption. As a result of repeated research and repeated various tests and investigations on both sides of rolling processing methods, we have determined the finishing temperature, cooling rate after rolling, and coiling temperature when hot rolling carbon steel with a specific component ratio. By controlling the value _of They discovered that it is possible to obtain a hot-rolled steel sheet that has high strength and toughness. This invention was made based on the above findings, and contains C: 0.12 to 0.25%, Si: 0.50% or less, Mn: 0.50 to 1.2%, S: 0.015% or less, Al: 0.005 to 0.080%. or, in addition, contains one or more of the following: P: 0.10% or less, N: 0.0300% or less, Ca: 0.010% or less, Fe and unavoidable impurities: the remainder; After continuous casting of steel into a slab (% representing the ratio is weight %), hot rolling is performed without lowering the temperature of the slab below Ar ₃ points (Ar ₃ points + 50 ° C).
After finishing rolling at the temperature of ~Ar ₃ points, accelerated cooling at a cooling rate of 60°C/sec or more to the temperature range of 500°C to Ms point, and coiling, it has excellent workability, toughness, and strength. It is characterized by the realization of a hot-rolled high-strength steel plate for processing. Next, in the method for manufacturing a hot-rolled high-strength steel sheet for processing according to the present invention, C, Si, Mn, which are components of the raw steel,
The reason why the component compositions of S, Al, P, N, and Ca, as well as the rolling conditions and cooling conditions are limited as described above will be explained. A Composition of steel (a) C C component has the effect of strengthening steel, and 50
In order to obtain a high tensile strength steel sheet with a tensile strength of Kgf/mm ² or more, a content of 0.12% or more is required, but if the content exceeds 0.25%, it will become indispensable as a high tensile strength steel sheet for automobiles, for example. Since weldability deteriorates, the C content was set at 0.12 to 0.25%. (b) Si The Si component has the effect of strengthening steel through solid solution strengthening, and is a desirable element for obtaining high-strength steel with excellent ductility.
If it exceeds 0.50%, the surface quality of the steel sheet will deteriorate due to the formation of so-called island scales, so the Si content was set at 0.50% or less. (c) Mn The Mn component has the effect of strengthening the steel through solid solution strengthening, improving the hardenability of the steel, and improving local ductility through the dispersion of pearlite grains. is 0.50%
or more content is required, but the content is
When it exceeds 1.2%, low temperature transformed structure is mixed2.
The Mn content was set at 0.50 to 1.2% because this would result in a low yield ratio and increase cost. (d) S S produces sulfides and deteriorates workability, so it is desirable to have as little as possible, but if the content is 0.015% or less, the desired workability can be ensured, so S content upper limit of amount
It was set at 0.015%. (e) Al Al (sol.Al) is effective as a deoxidizing agent for steel, but if its content is less than 0.005%, the deoxidizing effect cannot be expected;
The Al content was limited to 0.005 to 0.080% because the deoxidizing effect would be saturated and no further effect could be expected if the Al content exceeded 0.005% to 0.080%. (f) P The P component has the effect of solid solution strengthening of steel at low cost, and is actively added when it is necessary to further improve the strength of steel. Since P content exceeding 0.10% causes embrittlement of grain boundaries, the P content was set at 0.10% or less. (g) N Similar to P, the N component also has the effect of solid solution strengthening of steel at low cost.
Since N content exceeding 0.0300% causes blistering during casting and deteriorates workability, the N content was determined to be 0.0300% or less. (h) Ca Ca component has the effect of improving the workability of steel by adjusting the shape of inclusions.
It can be added when it is necessary to further improve processability, but
If Ca content exceeds 0.010%, the amount of inclusions in the steel will increase, which is contrary to the purpose of addition, and will deteriorate workability, so the Ca content should be 0.010%.
It was determined as follows. B Reasons for not lowering the temperature of the hot slab below Ar ₃ points The temperature of the hot slab obtained by continuous casting
The reason for continuing hot rolling without lowering the Ar level to below ₃ points is to eliminate the fuel consumption of the heating furnace required for reheating, and also because if the temperature falls below the Ar level of ₃ points, N will precipitate as AlN. Therefore, solid solution N
This is because the reinforcing effect of C Rolling Finish Temperature (Rolling Finishing Temperature) Figure 1 is a diagram showing the influence of final finishing temperature on the elongation, toughness, and yield point of a steel plate, which is as follows: C: 0.16 to 0.20%, Si: 0.1 to 0.2 %, Mn: 0.8 to 1.0%, S: 0.002 to 0.010%, Al: 0.01 to 0.05%, N: 0.0060 to 0.0150%, P: 0.020 to 0.080%, Ca: 0 to 0.005%, and the remainder is real. 2.6, which was obtained by continuous casting of Fe steel into a 200 mm thick slab, and then directly rolling it without lowering the temperature below the Ar ₃ point.
These are the results of an investigation conducted on a mm-thick hot-rolled sheet. From Figure 1, when the final finishing temperature (rolling end temperature) is lower than the Ar ₃ point, the yield point becomes significantly higher because rolling is included in the ferrite region and pro-eutectoid ferrite is processed. It can be seen that workability (elongation) also deteriorates. On the other hand, when the final finishing temperature is higher than (Ar ₃ points + 50℃), the grain refinement effect of austenite by rolling is insufficient, and the structure becomes coarser, resulting in deterioration of ductility and toughness. It is clear that this is the case. Therefore, the rolling end temperature was determined to be between (Ar ₃ points + 50°C) and Ar ₃ points. D Cooling rate after completion of rolling If the cooling rate after completion of rolling is less than 60℃/sec, the ferrite grains will become coarse, resulting in deterioration of workability and toughness.
sec or more. Figure 2 is a diagram showing the influence of the cooling rate after rolling on the ferrite grain size number, elongation, and toughness of the steel plate. This study was conducted on a 2.6 mm thick steel plate with the final finishing temperature ranging from (Ar ₃ points + 50°C) to Ar ₃ points.
From Figure 2, the cooling rate after rolling is 60℃/
It is clear that below sec, the above-mentioned characteristics deteriorate. There is no need to impose any particular upper limit on the cooling rate, but if the cooling rate becomes extremely fast, bainite will be mixed in.
A cooling rate of 60 to 150°C/sec can be recommended as a preferred range. In other words, if the cooling rate after rolling is slow,
As mentioned above, primary ferrite precipitates and grows from the coarse γ grains after rolling, forming coarse ferrite grains, leading to extreme toughness deterioration, and the pearlite part, which is the second phase, also becomes larger. In particular, the lower limit of 60
It is necessary to sufficiently control the cooling rate so that it does not fall below ℃/sec. E. Accelerated cooling end temperature (winding temperature) The reason why the winding temperature was set at 500 to Ms point is that if the temperature exceeds 500℃, AlN will precipitate and strengthening by solid solution N cannot be expected. This is because if the steel is wound at a temperature lower than that, the steel becomes a so-called two-phase steel, the yield point of which becomes too low, and its uses are limited. Furthermore, by setting the winding temperature to 500°C or less, embrittlement due to grain boundary segregation of P can be prevented, and therefore a large amount of P can be added. It is advantageous to do so. The steel to be used in the method of this invention is melted, deoxidized, and compositionally adjusted in a normal melting furnace (of course degassing treatment may be performed as necessary), and the steel is continuously melted and deoxidized in a normal melting furnace. After being cast, it is hot rolled, and although there is no particular restriction on the rolling start temperature, it is important to directly roll the hot slab obtained by continuous casting without lowering the temperature below Ar ₃ points. is as described above. Here, "direct rolling" means that the hot slab is directly sent to a hot rolling facility and rolled, or the hot slab is once charged into a heating furnace and then rolled after being lightly heated or lightly heat-retained. . Next, the present invention will be explained using examples and comparing comparative examples. Example 1 First, C: 0.15%, Si: 0.1%,
Mn: 0.82%, P: 0.015%, S: 0.005%, Al:
Steel containing 0.04% and N: 0.0062% is made into a 250mm thick slab by continuous casting, and then rolled under the conditions shown in Table 1 to produce a 3.5mm thick hot rolled plate. did. In addition, this steel has ₃ Ar points.

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.
The temperature was 780℃, and the Ms point was 405℃. In addition, the "normal heating method" in Table 1 is a method in which the continuously cast slab is once cooled to room temperature and then heated again to a high temperature (1200°C) for hot rolling. , is a method in which a continuously cast slab is cooled to an Ar of ₃ points or less (600℃) and then heated again to a high temperature for hot rolling. "Direct rolling" refers to a method in which a continuously cast hot slab is cooled to an Ar of ₃ or less (600℃), and then heated to a high temperature again for hot rolling. This refers to a method of directly hot rolling without lowering the temperature. And the properties of the steel sheet obtained in this way,
In addition, the fuel consumption of the heating furnace was measured, and the results are also shown in Table 1. As is clear from the results shown in Table 1,
According to the method of the present invention, a high-strength steel plate with excellent workability and toughness can be obtained as well as achieving significant energy savings, whereas Comparative Method 1, which is a normal heating method, achieves the target of 50Kgf/ Comparative method 2, which is a reheating method, not only cannot achieve a tensile strength of mm ² or more, but also requires a large amount of heating furnace fuel.
Although this method results in some energy savings, the properties of the obtained steel sheet are only on the same level as Comparative Method 1. Comparative method 3 is a method in which continuously cast slabs are directly rolled, which can achieve significant energy savings, but
Since the finishing rolling temperature, cooling rate, and coiling temperature are all out of the range of the present invention, the properties of the obtained steel sheet are inferior to Comparative Methods 1 and 2. In addition, in Comparative Methods 6 to 10, any of the finishing temperature, cooling rate, and coiling temperature is outside the range of the present invention, and the steel sheets obtained are still compared to those obtained by the method of the present invention. The characteristics have clearly deteriorated. Example 2 Steels having the respective compositions shown in Table 2 were melted in a conventional manner, continuously cast into slabs with a thickness of 200 mm, and then directly rolled without decreasing the temperature below Ar ₃ points. . The rolling conditions at that time were rolling start temperature: 1000°C, finishing temperature: 800°C, cooling rate: 80°C/sec, and coiling temperature: 450°C, and the thickness of the obtained steel plate was 4.5 mm. .

【table】

[Table] (Note) * indicates that it is outside the scope of the present invention.
The results of measuring various properties of the hot rolled steel sheet thus obtained are also shown in Table 2. As is clear from the results shown in Table 2,
According to methods 11 to 18 of the present invention, high-strength steel sheets with excellent workability and toughness are obtained, whereas comparative method 19 uses steel whose chemical composition is outside the scope of the present invention. It can be seen that only steel sheets with poor workability or toughness can be obtained with steel sheets of 23 to 23. As described above, according to the present invention, a hot-rolled high-strength steel sheet with excellent workability, toughness, and strength is
Industrially useful effects such as low energy consumption and low cost production can be obtained.

[Brief explanation of drawings]

Figure 1 is a diagram showing the influence of final finishing temperature on the elongation, toughness, and yield point of a steel plate. Figure 2 is a diagram showing the influence of cooling rate after rolling on the ferrite grain size number, elongation, and toughness of a steel plate. be.

Claims (1)

[Scope of Claims] 1 C: 0.12 to 0.25%, Si: 0.50% or less, Mn: 0.50 to 1.2%, S: 0.015% or less, Al: 0.005 to 0.080%, Fe and inevitable impurities: the remainder. After continuously casting steel with the chemical composition (more than % by weight) to form a slab, hot rolling is performed continuously without lowering the temperature of the slab to below Ar ₃ points,
(Ar ₃ points + 50℃) ~ After finishing rolling at the temperature of Ar ₃ points,
A method for producing a hot-rolled high-strength steel sheet for processing, characterized by accelerated cooling at a cooling rate of 60°C/sec or more to a temperature range of 500°C to Ms point, and winding. 2 Contains C: 0.12 to 0.25%, Si: 0.50% or less, Mn: 0.50 to 1.2%, S: 0.015% or less, Al: 0.005 to 0.080%, and further contains: P: 0.10% or less, N: 0.0300 % or less, Ca: 0.010% or less, containing one or more of the following, Fe and unavoidable impurities: the remainder, after continuous casting steel into a slab (weight %), the slab is Continue hot rolling without lowering the temperature below Ar ₃ points,
(Ar ₃ points + 50℃) ~ After finishing rolling at the temperature of Ar ₃ points,
A method for producing a hot-rolled high-strength steel sheet for processing, characterized by accelerated cooling at a cooling rate of 60°C/sec or more to a temperature range of 500°C to Ms point, and winding.