JPS647130B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for manufacturing high-tensile steel materials used for automobile strength members, wheels, welded steel pipe materials, structural members, etc. (Prior art) Among the various methods for strengthening steel, grain refinement is known as the only method to increase toughness as well as strength, and is particularly effective in improving the material quality of steel materials used as hot-rolled. It is an important technique that must be considered first in most cases when measuring. What has been achieved industrially with conventional grain refining technology is as small as 4
It is about ~6μ. This is usually carried out using a method called controlled rolling, which is a technique for strongly rolling steel containing alloying elements such as Nb at relatively low temperatures. In this case, Nb needs to be in solid solution as rolled, so heating is performed at a high temperature of 1200°C or higher before rolling to dissolve Nb, and then finish rolling is performed at a low temperature of 800°C or lower. Because it is done with
This method has industrial disadvantages, such as waiting for the temperature of the steel plate to drop, resulting in a significant drop in production efficiency, and a significant increase in deformation resistance during rolling, resulting in a heavy load on the rolling mill. In addition, various methods have been proposed, such as heating and rolling in a low temperature range or forced cooling after rolling, but in all cases the particle size remains within the above-mentioned range. Recently, carbon steel that does not contain special elements has a grain size of 3~
Method for obtaining ultrafine grains of 4μ or less
123823), but this method requires a large reduction rate during rolling and is subject to equipment limitations. (Problems to be Solved by the Invention) The present invention is capable of producing high-quality, high-strength steel material consisting of an ultrafine ferrite structure with a grain size of 3 to 4 μm or less at a low cost without reducing productivity using ordinary hot rolling equipment. With the goal. Furthermore, an ultra-fine ferrite structure is one in which the individual crystal grain size is 3 to 4 μ or less (grain size number 13 or more).
refers to a structure in which ferrite accounts for 60% or more by volume. (Means for solving the problems) The gist of the present invention is as follows. That is, by weight, it contains C: 0.05-0.2% Si: 0.01-1.0% Mn: 0.3-2.0%, and further contains one or more of Ti, Nb, and Ta, respectively.
After heating steel containing 0.01 to 0.1% Ac to a temperature of ₃ points or more and 1100℃ or less, hot rolling is performed in the cooling process, and at that time, a total reduction of 50% or more is applied within 15 seconds in the final stage of rolling. Add and adjust the finishing temperature.
A method for producing ultrafine-grained, low-alloy hot-rolled high-strength steel, characterized by cooling at Ar ₃ points to Ar ₃ +100°C at a cooling rate of 10°C/s or more within 5 seconds after the end of hot rolling. The content of the present invention will be explained in detail below. The gist of the present invention is to cause a suitable amount of fine precipitates to be present in the steel at the start of hot rolling, and to induce ferrite transformation using these precipitates as transformation nuclei during rolling. Conventionally, it has been known that if precipitates are present when steel transforms from high-temperature austenite to ferrite by cooling, these precipitates become transformation nuclei in areas other than grain boundaries. On the other hand, the present inventors have previously shown that when a large reduction is applied to carbon steel just before the transformation from austenite to ferrite, ferrite transformation is induced and fine ferrite grains are formed. When there is an appropriate amount of , deformation-induced transformation easily occurs,
It was found that an ultrafine ferrite structure can be formed even if the pass rolling reduction is relatively small. Figure 2 is 0.15C
-1.5Mn steel with various amounts of Nb added,
The ferrite grain size after rolling with varying heating temperatures and rolling reduction ratios is shown, and it can be seen that the microstructure becomes fine even at low rolling reductions when Nb is precipitated by low-temperature heating before rolling. The structure of the present invention will be explained below, but first, the steel components will be explained. C is the main component that affects the structure and material quality of carbon steel, but if it is less than 0.05%, high strength cannot be obtained;
If it exceeds this value, transformation due to processing will be insufficient, and weldability and workability will also decrease. Therefore, 0.05%≦C≦
Limited to 0.2%. Addition of Mn is desirable as it improves the strength-toughness balance and also has the effect of refining the structure, but its effect will not appear if it is less than 0.3%, and if it exceeds 2%, the transformation point will drop too much and ferrite transformation will occur. It becomes insufficient. Therefore, it was limited to 0.3%≦Mn≦2.0%. Adding Si in an amount of 0.01% or more improves the strength-ductility balance of the steel sheet, so it is desirable to add Si.
If it exceeds %, weldability may be impaired.
It was limited to 0.01%≦Si≦1.0%. Ti, Nb and Ta are carbides (and carbonitrides)
Since it forms a transformation nucleus, it is an essential element in the present invention. If the total amount of one or more of these added is less than 0.01%, the amount of precipitates will be small and the effect will be small. Moreover, if it exceeds 0.1%, the size of the precipitates becomes inappropriate and the effect is reduced. Therefore, Ti, Nb
The total addition amount of one or more of Ta and Ta was limited to 0.01% to 0.1%. Special elements other than those listed above (such as V) are effective if they precipitate in appropriate amounts at the start of finish rolling, but Ti,
Since it does not show the same effect as Nb and Ta, it is not particularly limited. However, it may be added as a supplement. The formation temperature of precipitates is low, and elements (Mo
etc.) have no effect. Next, the manufacturing method of the present invention will be explained. In the present invention, the heating temperature of the slab, which is the rolled material, needs to be low. In conventional controlled rolling,
Because special elements such as Nb must be dissolved in the steel at the start of rolling, the slab is heated to a high temperature (~1250°C). However, in the present invention, precipitates must be present in the steel at the start of finish rolling. Once the special elements are solid-solved in the slab, some of them will precipitate during rough rolling, but this will not be enough to achieve the effects of the present invention, and on the contrary, the solid-solute elements will suppress the transformation. The ferrite grain size is not ultra-fine. Added special elements (Ti, Nb and Ta)
The slab heating temperature must not exceed 1100°C in order for the slab to be sufficiently analyzed at the start of finish rolling. Therefore, the slab heating temperature was limited to 1100℃ or less. Further, from the spirit of the present invention, it is a matter of course that the heating temperature is equal to or higher than Ac ₃ (heating transformation point). The heated slab is hot-rolled, but when the rolling temperature drops below Ar ₃ (cooling transformation point), ferrite is generated without further processing, but since the grain size of such ferrite is large,
When such a structure is subjected to processing, recovery is slow and ductility and toughness are significantly impaired. On the other hand, ferrite induced by processing is ultrafine and easily recovers and recrystallizes even if it is further processed after its formation. When the rolling temperature is high, the amount of ferrite induced by processing is small, and obtaining a sufficient amount requires an unrealizable reduction rate. In order to obtain a sufficient amount of fine ferrite in practical rolling equipment, the finishing temperature is
It is desirable that the rolling temperature be below Ar ₃ +100°C.
Figure 1 shows the effects of slab heating temperature and processing temperature on the ferrite structure of steel containing 0.047% Nb.
It can be seen that in the temperature range from 730°C (Ar ₃ points) to about 850°C, the ferrite particle size number is large and the amount of ferrite transformation is large. For the above reasons, the finishing temperature of rolling
The temperature was limited to Ar ₃ to Ar ₃ +100°C. Note that when the effect of the special element on the Ar ₃ transformation point is not necessarily clear, the calculated value of the following formula that does not include the term of the special element may be used as Ar ₃ for convenience. Ar ₃ =901-325C+33Si-92Mn (℃)
(The amount of components is wt%) Next, the rolling conditions in the above temperature range will be explained. The method of obtaining an ultra-fine ferrite structure using carbon steel that does not contain special elements, which the present inventors previously disclosed, is basically rolling with a large reduction ratio, and when one pass large reduction is replaced by multiple passes. required a short interpass time so that the next pass could be performed before the effects of the previous pass disappeared. According to the present invention, these requirements are greatly relaxed due to the addition of special elements, and as shown in Figure 3, if rolling is performed at a cumulative reduction rate of 50% or more within 15 seconds from the final pass, the grain size number will be 13.5. It is possible to obtain an ultra-fine structure of grain size or larger (grain size of 3μ or less). Figure 3 shows C: 0.14%, Mn: 1.5%,
The relationship between the cumulative reduction rate within 15 seconds and the ferrite grain size of steel containing 0.047% Nb was rolled at a heating temperature of 1000°C and a finishing temperature of 780°C, and water-cooled at 20°C/s to 400°C or less. It is something that If the inter-pass time is long and it takes 15 seconds or more for two or three passes of rolling, the rolling reduction ratio for one pass must be increased, making it impractical. Therefore, the rolling conditions were limited to applying a total reduction of 50% or more within 15 seconds at the final rolling stage. The shorter the inter-pass time and the larger the rolling reduction, the smaller the grain size. Next, cooling conditions will be explained. The steel of the present invention has an ultrafine ferrite structure immediately after processing, but in some cases, grain growth may occur due to slow cooling after processing. Further, if a certain proportion of austenite remains immediately after processing, ferrite grows during cooling. In such cases, coarsening can be prevented by performing rapid cooling. At this time, if the product was left in a high temperature range for 5 seconds or more after rolling, even if it was rapidly cooled afterwards, there would be no effect of preventing the above-mentioned coarsening, so the time after rolling until the start of water cooling was limited to 5 seconds or less. Figure 4 shows C: 0.14%, Mn: 1.5%, Nb: 0.047
% steel was heated at a temperature of 1000°C and the rolling finishing temperature was
This figure shows the relationship between the elapsed time after rolling at 780°C and a total reduction rate of 85% and the ferrite particle size number, and it can be seen from the figure that water cooling must be started within 5 seconds as mentioned above. . Further, since there is no effect of preventing coarsening unless the cooling rate is 10°C/s or more, the cooling rate was set to be 10°C/s or more. The above quenching naturally strengthens the ferrite, and is also effective in increasing the strength by converting the austenite remaining after processing into a reinforcing structure such as bainite and martensite. It goes without saying that the higher the cooling rate is at C/s or higher, the more effective it is. (Example) Using five types of steels having the components shown in Table 1, continuous hot rolling was performed under the conditions shown in Table 3 according to the finish rolling pass schedule shown in Table 2. Steel A is comparative material, steel B to E
has components within the scope of this invention. The finishing rolling schedule f is normal rolling with a normal rolling reduction in each pass, and the schedule g is 5-pass rolling with a large rolling reduction in the later passes and the first pass being an empty pass. The sample number in Table 3 does not contain any special elements, so
The amount of ferrite produced at the end of rolling was insufficient,
The structure after cooling becomes mainly bainite.
The trial number is Nb because the heating temperature is high.
The structure is still mainly bainite because Ti is dissolved in solid solution and suppresses ferrite transformation. In the trial sample, the finishing temperature was high, so the amount of ferrite produced was not sufficient, and the particle size of ferrite was not reduced to that extent. Since the finishing temperature of the trial sample was below Ar ₃ , the ferrite grain size was fine, but it became processed ferrite and its elongation deteriorated. indicates the conventional structure and material due to the normal process of comparative materials. Other sample numbers (marked with *) are within the scope of the present invention and have very fine particle size numbers of 14.5 to 15.5.
Exhibits excellent strength-ductility balance. In Figure 5, the product of strength (TS) and elongation (E) listed in Table 3 is plotted against the particle size number (the numbers in the table correspond to the trial numbers in Table 3). The superiority of fine steel is clear. In addition, the material quality is slightly better when the rolling reduction ratio is large, but even at a normal rolling reduction ratio, the ferrite grains are ultra-fine and the material quality is comparable to the best one. (Effects of the Invention) According to the present invention, it has become possible to manufacture and provide high-strength thin steel sheets of excellent material with a tensile strength of 60 kg/mm ² or more at low cost without reducing productivity using ordinary rolling equipment.

【table】

【table】

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【table】 [Brief explanation of the drawing]

Figure 1 shows 0.15C-1.5Mn steel with various Nb additions at heating temperatures of 1250°C and 1000°C.
The ferrite particle size is shown when rolling and cooling is performed by changing the rolling reduction rate from 50 to 85% within 15 seconds including the final pass. Figure 2 shows the effects of heating temperature and rolling temperature on the ferrite structure after rolling of 0.14C-1.5Mn-0.047Nb steel. Figure 3 is 0.14C―1.5Mn―
Cumulative rolling reduction and ferrite grain size within 15 seconds including the final pass for 0.047Nb steel rolled at a heating temperature of 1000°C and a finishing temperature of 780°C, then water-cooled at a cooling rate of 20°C/s to below 400°C. shows the relationship between Figure 4 shows 0.14C-1.5Mn-0.047Nb steel heated to a temperature of 1000.
℃, finishing temperature 780℃, total reduction rate of 2 passes 85%,
The relationship between the elapsed time after processing and the ferrite particle size is shown for those processed with an interpass time of 2 seconds and quenched in water for a predetermined time after processing. FIG. 5 shows the relationship between ferrite grain size and material quality (strength x elongation) for steel with a ferrite-based structure among Examples.

Claims (1)

[Claims] 1. Contains, by weight, C: 0.05-0.2% Si: 0.01-1.0% Mn: 0.3-2.0%, and further contains one or more of Ti, Nb, and Ta, respectively.
After heating steel containing 0.01 to 0.1% Ac to a temperature of ₃ points or more and 1100℃ or less, hot rolling is performed in the cooling process, and at that time, a total reduction of 50% or more is applied within 15 seconds in the final stage of rolling. Add and adjust the finishing temperature.
A method for producing ultrafine-grained, low-alloy hot-rolled high-strength steel, characterized by cooling at Ar ₃ points to Ar ₃ +100°C at a cooling rate of 10°C/s or more within 5 seconds after the end of hot rolling.