JPS6239230B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an inexpensive two-phase high-strength hot-rolled steel sheet. Currently, there is a strong demand for high-strength steel sheets with good workability, mainly for automotive steel materials. Common sense suggests that high tensile strength and workability are contradictory, and it is difficult to achieve both. However, recently developed duplex steels (hereinafter referred to as
In a sense, DP steel (DP steel) is an ideal material that achieves both of these requirements. However, conventional DP steel has not been widely used due to its high manufacturing cost for the reasons described below. The method currently used to obtain a two-phase structure is to allow the steel to remain in the austenite/ferrite two-phase temperature range between point A ₃ and point A ₁ for a certain period of time to fully develop ferrite. At the same time, the carbon in the austenite is concentrated and then rapidly cooled to transform the austenite into a high-hardness, low-temperature transformation-generated structure such as martensite.
The key point of this technology lies in the design of a component system that causes two-phase separation in which ferrite is easy to develop and a small amount of austenite is difficult to transform. In particular, the composition of as-hot-rolled DP steel manufactured in a continuous hot-rolling process is extremely important because the time from the end of rolling to the start of cooling is short. Si to promote ferrite transformation
A method of increasing the amount is adopted, and it is usually added up to around 1%. To suppress metamorphosis of austenite part
A method is adopted to increase the amount of Mn, which is usually added at 1.5% or more. In order to make it easier to obtain a hardened structure
Cr, Mo, B, etc. are sometimes added, but as a result the component cost becomes high. In addition to such component adjustment, cooling control is also important. The fraction of austenite at the start of quenching determines the fraction of the quenched second phase after the completion of transformation, and the fraction and strength of the second phase are shown in Figure 1, which shows the relationship between the proportion of the quenched second phase and the tensile strength. There is a linear relationship as shown in (0.1C-0.5Si-1.5Mn steel, Nα=11.5). Therefore, it is necessary to provide the steel sheet with a temperature history that accurately controls the amount of ferrite transformation over time from the end of rolling to the start of cooling. Furthermore, in order for the second phase to become a sufficiently hard structure after quenching, quenching is necessary. It is necessary to lower the temperature of the subsequent steel plate to nearly 200℃. For this reason, during cooling, delicate work is required for each rolled material, reducing productivity and contributing to increased costs. The drawbacks of the conventional hot-rolled DP steel manufacturing method described above stem from the fact that the amount of ferrite transformation must be controlled in a short period of time from the end of rolling to the start of quenching. We discovered a new method to control the amount of ferrite during rolling and to make the ferrite into ultra-fine particles, overcoming the drawbacks of the conventional method. In other words, the gist of the present invention is that C: 0.02 to 0.2%, Si: 1% or less, and Mn: 1.5% by weight.
Below, the remainder consists of Fe and unavoidable impurities, and contains less than 50 to 70% by volume of deformation-induced equiaxed ferrite crystal grains surrounded by high-angle grain boundaries with an average grain size of 5 μm or less, and the rest is martensite or bainite. It is a two-phase high-strength hot-rolled steel sheet consisting of a hardened structure. The present invention will be explained in detail below. It was previously known that the transformation point increases when processing is performed at the low temperature side of the austenite temperature range, but
Quantitative studies of temperature, amount of processing and amount of transformation have not been conducted. As a result of our research, the present inventors came to the conclusion that it is possible to control transformation by rolling. An example is shown in FIG. Figure 2 is 0.1C-
These are the results of processing 0.5Si-1.0Mn carbon steel at various reduction rates in the Ar ₃ temperature range, cooling it with water immediately after processing, and examining the amount of transformation. The larger the rolling reduction ratio and the lower the rolling temperature, the larger the amount of ferrite transformation. Ar ₃
The reason why ferrite transformation progresses at temperatures above is that there are places that are subject to high strain mainly at austenite grain boundaries due to processing, and if recovery cannot keep up, it is better to transform to ferrite than to recrystallize as austenite due to energy. This is thought to be because it may be stable. Therefore, when the temperature is high, the amount of processing to cause transformation needs to be larger.
Such transformation will occur simultaneously with processing. An important feature of this transformation is that the ferrite crystal grains after transformation are extremely small. Figure 4 a-c
is 0.12C-0.48Si-0.5Mn steel at 850℃,
Microscopic microstructure photographs obtained when immediately cooled with water after processing at various rolling reductions (rolling ratios are (a) = 80%, (b)
= 87%, (c) = 95%), but as the rolling reduction increases, the amount of transformation, that is, the ferrite fraction increases, and at the same time, the ferrite grain size decreases as the rolling reduction increases, and the minimum ferrite grain size is 3μ or less. I know what will happen. As a result of repeated experiments on steels with various composition ranges, the present inventors found that the temperature range in which fine-grained ferrite can be obtained by processing and the amount of transformation can be appropriately controlled is
It was concluded that the cumulative reduction rate within 30 seconds at Ar ₃ +100°C to _{Ar 1} +50°C needs to be 60% or more. Next, we will consider DP steel with an ultrafine-grained ferrite structure. Refinement of grains leads to an increase in yield point, as is well known as the Petch relationship. At the same time, the tensile strength is increased, but the effect is not as great as that on the yield point. Therefore, fine-grained steel is characterized by a high yield ratio (ratio of yield strength to tensile strength), which is considered to be disadvantageous for thin plates that require formability. However, the high strength required for structural members should mean high yield stress, and a large yield ratio means that work hardening during plastic deformation is small, so if the total elongation is sufficient, it will actually be easier to form. In some cases it may even be desirable. By the way, in the ultra-fine grained steel obtained by the present invention, transformation occurs in a very short time, so the distance that carbon travels due to diffusion is small, and therefore large carbides are difficult to form. In fact, in Fig. 4c, it is difficult to recognize a pearlite-like structure even though the metamorphosis has been completed. When the thickness of the carbide is thin, the slip lines during deformation have an increased chance of penetrating the entire thickness without intersecting inside the carbide, making it difficult to generate cracks and increasing the total elongation of the steel material. This is the reason why drawability increases when the lamella spacing is reduced in high carbon pearlite steel. Figure 3 compares the strength-ductility balance of the fine-grained DP steel of the present invention with that of the conventional DP steel, and it can be seen that although it is inferior to the conventional DP steel, it is at a higher level than the precipitation strengthened material. . Next, let's talk about cooling. In the present invention, two-phase separation, which was caused by components and controlled cooling in the conventional method, is caused by processing strain, so by the end of rolling, the structure has already become a mixed structure of ferrite and austenite, so controlled cooling is not necessary. It is not necessary, as long as there is a sufficient cooling rate for the austenite to become a hardened structure. A higher cooling rate is more desirable in terms of suppressing the growth of crystal grains, but a cooling rate of 20°C/sec is sufficient unless the steel is an ultra-low carbon steel. A higher cooling rate is better in order to prevent coarsening of carbides, but since coarsening of carbides is synonymous with reduction of excess carbon in ferrite, this effect is canceled out. As described above, according to the present invention, no particular cooling control is required, which is advantageous in terms of actual production. The reasons for limiting the constituent elements of the present invention will be described below. Ingredients: In principle, 0.02% or more of C is required to obtain a quenched phase, so the lower limit is set at 0.02%, and 0.2% because if it exceeds 0.2%, the orientation of DP steel will disappear from the viewpoint of workability. was set as the upper limit. Si has the effect of promoting two-phase separation and improving the strength-ductility balance, but if it is too large, the processing rate required for the desired amount of transformation becomes high and control becomes difficult, so the upper limit should be set at 1%. desirable. Mn adjusts the transformation point, makes deformation-induced transformation more likely to occur, and contributes to grain refinement by preventing rapid grain growth of deformation-induced ferrite, and also contributes to ultra-fine ferrite by improving hardenability. This is effective in forming a hardened structure consisting of martensite or martensite and bainite in the other parts. However, if it is added in excess of 1.5%, the transformation temperature will drop, making it difficult to form fine grained ferrite on the surface, so it was set at 1.5% or less. P and S are elements that are normally contained in steel to some extent, but if they are contained in large amounts, they impair the ductility and toughness of steel. However, the amount contained in ordinary steel, P0.03
% or less, S0.02% or less does not significantly affect the essence of the present invention, so the amount is not particularly limited. Although some amount of N is contained in steel as an impurity element, the amount is usually about 0.002 to 0.01%, and within this range it does not significantly affect the properties of the steel of the present invention. Note that when the amount of N is less than 0.002%, deformation-induced transformation occurs more easily than in the present invention, and when it exceeds 0.01%, it becomes slightly less likely to occur, especially when elements such as Al and Ti are included. Al is usually contained in steel to some extent for deoxidation, but if it is normally contained at 0.1% or less, it generally does not have a large effect on the properties of the steel of the present invention. Ratio of ferrite phase: If the fine-grained ferrite phase is less than 50%, the strength will be high but the workability will be poor, making it impractical. If it is more than 70%, the reinforcing effect of the second phase will be small, so the ratio should be reduced to 50-70%. limited to less than Preferred conditions for manufacturing the steel plate of the present invention will be described. Rolling temperature: When the rolling temperature exceeds Ar ₃ +100℃, transformation due to processing does not occur, and when the rolling temperature is lower than Ar ₁ +50℃, large ferrite grains are formed and mixed grains are formed, and the existing ferrite is processed and ductility decreases. Put it away. Therefore, the temperature range is set to Ar ₃ +100°C to _{Ar 1} +50°C. Reduction ratio: If the total reduction ratio is less than 60%, the finished plate will have mixed grains and the workability will decrease, so 60% was set as the lower limit. 1
The larger the rolling reduction ratio of the passes, the smaller the ferrite grains become, and a rolling reduction ratio of 40% or more is preferable, but multi-pass rolling may be used as long as the time between passes is sufficiently short. However, in the case of multi-pass rolling, the cumulative reduction rate at which the effect appears is 1
This is larger than in the case of pass large reduction processing, and a cumulative reduction rate of 50% or more in passes within 2 seconds is required. Therefore, it is possible to adjust the ferrite grain size and the amount of ferrite by setting the cumulative rolling reduction rate of 1 pass or 2 passes or more to 50% or more, and by selecting the cumulative rolling reduction rate. Ferrite metamorphosis must not progress beyond 95%. Rolling time: 1 or 2 intermediate passes in case of multi-pass rolling
Processing in which the cumulative reduction rate is 50% or more in passes or more is 2
Must be done within seconds. Otherwise, the processing effect will be lost due to recovery between passes, the ferrite will become coarse, and the workability of the finished plate will deteriorate. Therefore, the machining time for one pass or two or more passes in which the cumulative reduction rate is 50% or more is set to within 2 seconds. In order to make the grains of ferrite ultra-fine, it is particularly effective to increase the rolling reduction in the later stages of multi-pass rolling, and at this time, the shorter the time between passes, the more the cumulative effect of processing strain will be exerted. For example, when the final pass or two or more passes of multi-pass rolling are performed at a cumulative reduction rate of 50% or more within 1 second, the ferrite grain size becomes ultra-fine, 4 microns or less. From this point of view, continuous hot rolling using a tandem mill is suitable for rolling. Cooling rate: The lower limit of the cooling rate is 20°C/sec, which is necessary for the austenite that the steel has at the end of rolling to become a quenched structure. Example Carbon steel having the components shown in Table 1 was subjected to continuous hot rolling under the rolling and cooling conditions shown in Table 2. A in Table 2
- D are the steels of the present invention, and each has a strength depending on the ratio of the quenched phase, and high tensile strength steels of 70 to 80 kg/mm ² class are produced. The yield ratio (YR) of the steel of the present invention is slightly higher than that of conventional composition-adjusted + cooling-controlled dual-phase steel, but lower than that of ordinary rolled material, showing the characteristics of dual-phase steel. In Table 2, E to I are comparative steels, E and G cannot obtain a hardened structure due to slow cooling rate, F has a large rolling reduction and transformation ends before cooling starts, and H has a hardened structure. Because the temperature was low, the transformation ended during rolling, so none of them became duplex steel. Comparative steel F does not have a two-phase structure, but has high strength because of its fine crystal grains. However, the yield ratio is high due to fine grain reinforcement. In case I, the cumulative reduction rate within 2 seconds is less than 50%, and the ferrite grain size is large. In J, the finishing temperature is high, so the ferrite transformation does not proceed sufficiently, and there are many quenched phases, and although the strength is high, the ductility is on the same level as conventional materials. The above results are shown in FIG. In the figure, the circle mark indicates the steel of the present invention, and the mark x indicates the comparative steel. As described above, according to the present invention, it is possible to inexpensively provide a high-strength steel sheet with a dual-phase structure as hot-rolled, without adding any special ingredients or requiring controlled cooling, simply by adjusting the rolling temperature and reduction rate. . High strength, but high yield ratio due to fine grain reinforcement. In case of I, the cumulative reduction rate within 2 seconds is less than 50% and the ferrite grain size is large. In J, the finishing temperature is high, so the ferrite transformation does not proceed sufficiently, and there are many quenched phases, and although the strength is high, the ductility is on the same level as conventional materials. The above results are shown in FIG. In the figure, the circle mark indicates the steel of the present invention, and the mark x indicates the comparative steel. According to the present invention as described above, it is possible to produce a duplex steel plate as hot-rolled by simply adjusting the rolling temperature and rolling reduction ratio without adding any special ingredients or requiring controlled cooling. It can be made with a wide range of ingredients. In other words, dual-phase structure high-strength steel sheets, which were conventionally expensive to manufacture, can now be manufactured at low cost.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows the relationship between the proportion of the quenched second phase and the tensile strength (0.1C-0.5Si-1.5Mn steel, Nα =
11.5), Figure 2 shows 0.1C-0.5Si-1.0Mn steel (Ar ₃ :
Figure 3 shows the relationship between rolling reduction and transformation rate when a steel (793°C) is rolled for one pass at a temperature near the Ar ₃ transformation point and then rapidly cooled (water-cooled) immediately after rolling. A diagram showing a comparison of the strength-ductility balance of steel manufactured by the conventional method and comparison steel (marked with a circle),
Figure 4 is a micrograph showing the structure of 0.12C-0.48Si-0.5Mn steel processed at 850℃ at various reduction rates and water-cooled immediately after processing, where a is 80% reduction and b
is the case where the rolling reduction rate is 87%, and c is the case where the rolling reduction rate is 95%.

Claims (1)

[Claims] 1% by weight: C: 0.02-0.2%, Si: 1% or less,
Mn: 1.5% or less, the balance consists of Fe and unavoidable impurities, and the volume ratio is 50 to 70 deformation-induced equiaxed ferrite crystal grains surrounded by large-angle grain boundaries with an average grain size of 5 μm or less.
A two-phase high-strength hot-rolled steel sheet containing less than % martensite and a hardened structure of martensite or martensite and bainite.