JPS6235453B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a high-strength, composite-structure hot-rolled steel sheet with excellent workability. Here, high strength refers to a tensile strength of about 40 to 70 kg/ ^mm2 .
The composite structure refers to a structure consisting of a ferrite phase and a quenched transformed phase (the quenched transformed phase is martensite, bainite, or a mixed structure of both, and may also contain retained austenite). When a rapidly cooled transformed phase is formed, the tensile strength of the steel plate increases and the yield ratio (yield strength/tensile strength) of the steel plate decreases. It is advantageous to make the yield ratio as low as possible (0.7) when press forming steel sheets. Due to its excellent strength and ductility, low-yield-ratio composite steel sheets have recently been adopted as processing materials in the automobile industry and other industries. ℃ or less) has already been proposed (for example, Japanese Patent Publication No. 55-49135,
56-29624, etc.). Therefore, it can be said that this type of technology pattern has already been largely established. However, in these existing inventions, it is generally necessary to obtain a steel plate with a tensile strength of 60 kg/mm class ² , for example.
If the Mn content is 1.3% or more, or if the Mn content is small (1.1 to 1.3%), it is generally necessary to include a Si content of about 1% or more as a steel component (for example, JP-A-56-29624). In addition to increasing costs, there are not necessarily problems with descaling during hot rolling or paint adhesion when used as a product. The present invention significantly reduces these steel components such as Mn and Si by adding a small amount of B, making it possible to obtain a material equivalent to the composite structure steel sheet of the existing invention, thereby solving the above cost problem. This makes it possible to solve problems related to descaling properties or usage characteristics. As a prior invention, the present inventors have already proposed Japanese Patent Application No. 12277-1982 (filed on January 28, 1982, hereinafter referred to as the "prior invention"). We succeeded in reducing the required amount of Mn and in producing a steel sheet with high artificial age hardening properties after processing. In the case of quenching from austenite single phase,
It is known that the addition of B improves the hardenability of steel and reduces the amount of component elements required to obtain the same strength. However, in connection with continuous hot rolling conditions as in the present technology, there are no known results that teach the ingredients and process factors that should obtain a low yield ratio and good ductility. This is because composite-structure steel sheets are obtained by rapid cooling from a state where ferrite and austenite phases coexist, and the influence of B on austenite hardenability from such a two-phase coexistence state is unknown. Not only that, but it is also completely unknown how the two-phase coexistence state in steel containing B changes depending on the continuous hot rolling finishing conditions. The present inventors found that the finish exit temperature is particularly important as a hot rolling finishing condition for obtaining a low yield ratio and good ductility in the steel having the composition range of the present invention containing B, and the temperature range is 790 to 950. ℃, and if hot-rolled with such a finishing exit temperature, rapidly cooled, and low-temperature coiling, the steel of the previous invention can be produced with a significantly lower Mn content than the previous invention. It has been found that a high-strength composite steel sheet can be obtained that has workability equivalent to that of steel sheet and superior artificial age hardening after processing. High artificial age hardenability after processing indicates that the strength of the molded product can be increased by painting and baking treatment after forming the steel plate, and is a desirable characteristic for steel plates used in, for example, automobile parts. To describe the characteristics of the present invention, C 0.05-0.15%,
Mn 0.4% or more and less than 0.7%, Si 0.05-0.9%, Al
0.01~0.1%, B 0.0005~0.006%, N
0.006%, the remainder being substantially Fe, is hot-rolled, the hot-rolled finishing exit temperature is in the temperature range of 790 to 950°C, and the temperature is 450°C or less at an average cooling rate of 30 to 200°C/sec. It is rolled up after reaching the point. The reasons for limiting the technical conditions as described above are as follows. If the C content is less than 0.05%, sufficient tensile strength will not be obtained, and if it exceeds 0.15%, it will be extremely difficult to separate the ferrite phase and austenite phase at the end of hot rolling finishing (before the start of cooling), and the soft ferrite phase will be enriched. However, since it becomes difficult to form a composite tissue,
0.05%C0.15%. If the amount of Mn is less than 0.4%, a sufficient quenched transformation phase cannot be obtained, while if it is more than 0.7%, there will be no problem with the mechanical properties of the obtained steel sheet, but the artificial age hardenability after processing will be reduced. or more and less than 0.7%. The term "artificial age hardenability after processing" is defined as follows. Let Si be the stress required to give 5% tensile strain to a tensile test piece obtained from a steel plate that has reached room temperature after being rolled up,
The test piece given 5% tensile strain was heated to 170°C.
After heating for 20 minutes and cooling to room temperature, a tensile test is performed again, and when the stress required to yield is _S2 , _S2 - _S1 (Kg/ ^mm2 ) is defined as the artificial age hardenability after processing. Si is an element that is effective in improving ductility and has the negative effect of inhibiting descaling and paint adhesion, and the composition range that can compromise both of these aspects is set to 0.05% and 0.9% Si. Al has the effect of denitrification as well as deoxidation, so
B is an essential element for effective use, and its lower limit is 0.01, where these effects are expected.
%, and the upper limit is 0.1 in order to saturate the deoxidizing effect and to suppress the increase in alumina inclusions.
%. If B is less than 0.0005%, the effect of increasing tensile strength and decreasing the yield ratio (based on the effect of promoting rapid cooling transformation phase formation) is small, and if it exceeds 0.006%, these effects are saturated and ductility deteriorates.
%B shall be 0.006%. If N is present in large amounts, it combines with B to form nitrides and nullifies the action of B, so N0.006%
limited to. The reason for limiting the hot rolling finishing exit temperature is that if it deviates from the range of 790 to 950°C, the yield ratio will increase and the ductility will deteriorate. This temperature range was determined from Examples described later. If the cooling rate after finishing is lower than 30°C/sec, no quenched transformation phase will be formed, and if it is higher than 200°C/sec, ductility will deteriorate. In addition, unless the coiling temperature is 450° C. or lower, the quenched transformation phase will not be formed and the second phase will become pearlite, making it impossible to ensure high strength of the steel sheet. Note that the winding temperature should be 450°C or lower, which is the temperature at which winding can be carried out, so there is no specific lower limit specified, but if the winding machine capacity is insufficient, winding at a lower temperature will Care must be taken because the winding of the steel plate becomes loose, which increases surface flaws and deteriorates the shape of the steel plate. Generally, winding is carried out at room temperature or above, and is often carried out at 100°C to 150°C or above, and the present invention may also be carried out in accordance with this. Particularly preferable conditions in the present invention are C 0.06
~0.12%, Mn 0.5% or more and less than 0.7%, Si 0.2~0.7
%, N<0.004%, B 0.002~0.005%, continuous hot rolling and quenching, coiling temperature below 300℃, tensile strength 45~60Kg/mm, low yield ratio of class ² . This is the case when obtaining a steel plate. The present invention will be explained below using examples. Example 1 Analysis values are C 0.095%, Mn 0.66%, Si 0.70
%, P 0.005%, S 0.005%, Al 0.04%, B
0.0021%, N 0.002%, the balance is essentially Fe, and for reference, the corresponding components do not contain B, i.e. the analytical values are C 0.10%, Mn 0.65%, Si
0.71%, P 0.006%, S 0.005%, Al 0.05%,
Using steel with 0.002% N and the remainder essentially Fe, a 25 mm thick steel piece was soaked at 1100°C for 1 hour and heated to 14mm at 1030°C (first pass), then 950°C.
The material was rolled to a thickness of 5.6 mm (2nd pass) and 4.0 mm (3rd pass), and the temperature immediately after the completion of rolling in the 3rd pass was at various temperatures between 750 and 950°C. Then, the temperature was reached to 150°C at an average cooling rate of 60°C/sec, and then slow cooling was performed (20°C/hour) (that is, a simulation experiment was conducted in the case where the winding temperature was 150°C). The drawing shows the steel plate material obtained through the tensile test. From this figure, 0.1%C, 0.65%Mn, 0.7%Si
It can be seen that a significantly low yield ratio cannot be obtained without the addition of B, and that a low yield ratio and high strength can be achieved with the addition of B, and there is no adverse effect on ductility. In the case of steel with this composition, the hot-rolling exit temperature that minimizes the yield ratio and maximizes ductility is 850℃, but after that temperature -20℃
Satisfactory material results are shown in the temperature range of ~+40°C. Example 2 Using each steel shown in Table 1, a continuous hot rolling experiment similar to that in Example 1 was conducted, and the results shown in Table 2 were obtained. In Table 2, "best FT" indicates the hot-rolling finishing exit temperature at which the yield ratio is the lowest and the ductility is the best.

【table】

[Table] Table 2 shows only the results for the best FT.
The pattern of the relationship between mechanical properties and hot-rolling finishing exit temperature is the same for all steels as shown in the drawings, and considering Example 1 and the allowable finishing exit temperature range shown in the drawings and Table 2. The hot rolling finish exit temperature range that provides a satisfactory low yield ratio and ductility is 790 to 950°C in the steel composition range of the present invention. Example 3 Regarding one of the steel plates according to the present invention and a comparative steel with a higher Mn content (described in the previous invention),
Table 3 shows a comparison of the artificial age hardenability after processing. Clearly, the artificial age hardenability after processing is improved in the component steel according to the invention. This is due to the high Mn content due to the reduced Mn content.
This is presumed to be due to a slight increase in the amount of solid solute carbon remaining in the ferrite phase compared to steel containing a large amount of carbon.

【table】

[Table] The steel of the present invention controls the shape of non-metallic inclusions to improve bendability, flange extension properties, etc.
Depending on the content of impurity S, Ca or rare earth element (REM) is added to Ca%/S%>3 or REM%/S.
It is recommended to add such that %>5.

[Brief explanation of the drawing]

The drawing is a chart showing the relationship between hot rolling finishing exit temperature FT and mechanical properties for steel with and without B added.

Claims (1)

[Claims] 1 Contains 0.05 to 0.15% of C, 0.4% to less than 0.7% of Mn, 0.05 to 0.9% of Si, 0.01 to 0.1% of Al, 0.0005 to 0.006% of B, 0.006% of N, and the balance is substantially A steel made of Fe is hot-rolled to 450°C at an average cooling rate of 30-200°C/sec, with a hot-rolled finish temperature in the range of 790 to 950°C.
A method for producing a high-strength composite-structure hot-rolled steel sheet for processing, characterized by rolling it up to the following temperature.