JPS5945729B2 - Manufacturing method of hot rolled steel for warm working - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing hot rolled steel materials suitable for warm working.

In recent years, the industry has been considering the use of warm processing, and warm pressing of plate materials is a typical example.

The purpose of warm working can be broadly classified as follows.
(b) Part 1 is aimed at reducing the load or power and improving workability during processing of steel materials, by increasing the material temperature almost uniformly during processing, reducing deformation resistance during processing and improving ductility at the same time. (hereinafter referred to as the uniform heating method).

(Example) Part 2 is to increase the difference in deformation resistance within the processed material by locally heating the steel material, thereby preventing breakage during processing or preventing local thinning (reduction in plate thickness). (hereinafter referred to as the local heating method).

Hot rolled steel materials have been developed for cold working, but there has been little development for hot working.

The present invention provides a method for manufacturing hot-rolled steel materials that have properties suitable for warm working steel materials. %, N=0.0020-0.015%, and Nb=0.01-0010%, V=0.01-002
0%, Ti=0.03~0530%, the killed steel whose main constituents are one or more of Ti=0.03~0530% is solution treated, hot rolled, and then heated to 500°C at an average cooling rate of 10℃/see or higher. A method for producing hot rolled steel for warm working, which is characterized by forced cooling to a temperature range below ℃.

In addition, warm working as referred to in the present invention refers to actively heating the steel material,
Processing should be carried out in a temperature range below the Al point, and after actively heating the steel material to a temperature range below the Al point, the steel material should be cooled to at least a temperature range below the blue brittle region (approximately 300 degrees Celsius or below) before processing. also means The steel of the present invention has the following characteristics.

(b) In the uniform heating method, which aims to reduce the load during processing,
Normally, warm working is carried out in a temperature range higher than the blue brittle range, but when working in a normal hot rolled carbon steel material in such a high temperature range, even if the forming load can be reduced, the formed product after processing cooling will deteriorate. It had the disadvantage that the strength was lower than that when cold worked.

However, even when the steel of the present invention is warm-worked in a high temperature range of about 500 to 600°C, there is almost no decrease in strength after processing and cooling, and in fact, it may even be able to maintain a higher strength than when cold-worked.

Furthermore, if the heating and holding time is shortened, the above characteristics can be maintained at temperatures below the A1 point. (Example) When conventional steel is not subjected to any working strain, its tensile strength does not increase even if it is heated, even though the strength at room temperature increases due to aging and the yield point increases.However, in the steel of the present invention, the working strain Even if it is zero, both the yield point and tensile strength increase after heating and cooling.

Therefore, when the steel of the present invention is warm worked, even if there is a part where the strain is zero, the strength of that part increases, so the strength of the entire molded product increases regardless of the amount of processing strain caused by warm working. This feature is exhibited. By heating and then cooling the local part of the workpiece before processing,
It is possible to increase the strength of that part and make a difference in deformation resistance during processing.

Therefore, this difference in deformation resistance can be effectively used to prevent breakage or local thinning of the meat. (→ Also, the steel of the present invention has a sufficiently large increase in strength (particularly tensile strength) in the blue brittle region, so it can be heated locally to a temperature in the blue brittle region and warm-worked to prevent breakage or breakage. Localized muscle thinning can also be prevented.

Although the characteristics of the steel of the present invention have been explained above, the heating temperature during warm working should be below the A1 point.

If the steel is heated above the A1 point, there is a risk that the working strain will recover even in the steel of the present invention, and the strength after warm working will decrease, and it is also unfavorable from the viewpoint of toughness after working. The preferred processing temperature is 200-650°C. In addition, the heating temperature when applying local heating before processing to give a strength difference after cooling is 200 to 650.
°C is preferred. The heating time does not need to be particularly limited at low temperatures, but is preferably relatively long (about several tens of minutes) and short (about several minutes) at high temperatures.

The hot-rolled steel material that exhibits the above-mentioned remarkable effects is manufactured in the following manner. First, the chemical composition of the steel of the present invention is C=
0.02~・0.15%, Mn=over 0.90~2,50
%, N-0.0020-0.015% and Nb=0.
01-0.10%, V=0.01-0.20%, Ti=
One or more of 0.03 to 0.30% is used as a main component, and the reasons for this limitation are as follows.

C and N are set at C≧0.02% in order to ensure the strength of the base metal and to ensure that the strength after cooling hardly decreases compared to cold working even when warm worked in a temperature range above the blue brittle range. , N≧0
．． It is necessary to set it to 0020%. However, C is 0.15%
When it becomes too much, it becomes difficult to dissolve NbC and TiC,
and warm workability deteriorates. Upper limit of N 0.015%
was determined from the viewpoint of weldability. The preferred range is C=0.
03-0.13%, N=0.0020-0.010%. Furthermore, in the present invention, Nb=0 as a precipitation hardening element.
．． 01-0.10%, V=0.01-0.20%, Ti
It is necessary to add one or more types from 0.03 to 0.30%.

These lower limits are such that even if warm worked in a temperature range above the blue brittle range, the strength after cooling will hardly decrease compared to cold working, and that the strength after cooling will be increased even by mere heating. It was determined as the amount necessary to achieve this goal. Moreover, when these upper limits are exceeded, the effect of addition is not only saturated, but also the weldability is deteriorated and the steel becomes expensive. Preferred ranges are Nb=0.01-0.07% and V=0.0, respectively.
3 to 0.15%, Ti=0.05 to 0.25%.
Further, when adding Ti, it is preferable that Ti/C<4. In addition to deoxidizing, Mn is required to be 0.15% or more to prevent hot embrittlement, but in the present invention it is 0.90%.
Super-contain.

The reason for this is that in addition to improving the strength and toughness of the steel, it also improves the toughness and ductility in the high temperature range during warm working, thereby further improving warm workability. Moreover, if it exceeds 2.50%, the steel becomes expensive and warm workability deteriorates. A preferable amount of Mn is more than 0.95 to 2.30%. Furthermore, the present invention is manufactured as killed steel, and the reason for this is to improve the alloy yield of Nb, V, and Ti and to provide excellent workability.

In this case, it is preferable to add AI=0.01 to 0.10%. In addition, other elements may be added as appropriate so as not to impair the features of the present invention or to supplement them.

For example, in order to improve weather resistance and corrosion resistance, Ni, Cu, and Cr are removed.
．． It is also effective to change the sulfide form by adding 5% or less, or by adding small amounts of Zr, Ca, REM, etc. to further improve workability. Further, in order to increase the strength, 1.0% or less of Si or 0.5% or less of MO may be added. Reducing other impurity elements such as P and S is also effective in improving warm workability. Steel having the above-mentioned chemical composition is formed into a steel billet by a conventional method and then hot rolled into a predetermined size and shape.

In order to bring out the features of the present invention, it is necessary to add N to the steel after rolling.
l) It is necessary to make the carbonitrides of V and Ti as fine as possible, and to keep at least a portion of Nb, V and Ti in solid solution.

For this purpose, it is necessary to solutionize the carbonitrides of Nb, V, and Ti before starting hot rolling. This solution treatment is accomplished by heating and holding the steel at a high temperature or by directly rolling a continuously cast steel billet. The required minimum solution temperature when heating and holding at a high temperature varies depending on the amount of C, N, etc. in the steel, but it is necessary to heat the steel to at least 1100° C. or higher. Hot rolling is carried out above the Arg point as usual, and then in the present invention forced cooling is forced to a temperature range of 500°C or below at an average cooling rate of 10°C/See or above. The reason is that part of the carbonitrides of Nb, V, and Ti are finely precipitated, and the rest is dissolved in solid solution Nb, V, and Ti.
, even if the warm working temperature becomes higher, there is no decrease in strength after cooling due to the presence of Ti in the steel, and even if the working strain is zero, the strength after cooling increases due to heating. It comes to have characteristics. In this case, after rolling, it is necessary to forcefully cool the product to a temperature range of 500°C or lower, preferably 450°C or lower. Also, the average cooling rate in this forced cooling is 10°C/See or more (more preferably 15°C/See)
℃/Sec or more and 80℃/Sec or less). In the production of the steel products of the present invention, after forced cooling, they may be air-cooled to room temperature as is, or they may be wound into coils and slowly cooled to room temperature, but it is more industrially valuable to wind them into coils using a hot strip mill. It's a dog. In addition, the above-mentioned forced cooling end temperature is preferably 200'C or higher, and industrially 30'C or higher is recommended.
The characteristics of the present invention can be fully exhibited even at temperatures above 0'C. Examples of the present invention will be described below.

Steel having the chemical components shown in Table 1 was melted in a converter to form a slab, and then hot-rolled steel sheets with a thickness of 2.7 mm were produced in a hot strip mill according to the hot rolling conditions shown in Table 1.

Note that the forced cooling was performed by forcing the strip to cool on a hot run table. In Table 1, steel type A is comparative steel, and steel types B, C, and D are steels of the present invention. Table 2 shows the strength at room temperature after processing the obtained steel plates under various conditions.

The processing conditions were as rolled (reference value), 10% tensile strain applied at room temperature, 10% tensile strain applied warm (after heating at 600°C for 30 minutes), and heated at 600°C for 30 minutes (no strain). is. By comparing the strength after warm straining and after cold straining, it can be seen whether the strength after warm working has increased or decreased compared to the strength after cold working. In Table 2, comparative steel A has lower strength when warm worked than when cold worked, whereas inventive steels B, C, and D all have lower strength than when cold worked. Rather, the strength after warm working is improved, and the characteristic of high strength after warm working, which is one of the effects of the present invention, is clearly manifested.
In Table 2, the as-rolled strength and heating at 600°C for 30 minutes (
By comparing the strengths (without strain), we can see the usefulness for warm processing that utilizes the strength difference due to local heating.
Comparative steel A had a slightly increased yield point due to local heating, whereas inventive steel C. In case D, both yield point and tensile strength were significantly improved by local heating. It can be seen that the steel of the present invention is also effective in the local heating method. Table 3 shows the effect of warm deep drawing in terms of the improvement in the critical drawing ratio.

In the warm deep drawing process, a portion of the blank (disc) corresponding to the bottom of the punch was locally heated to about 400°C, and then deep drawing was performed. As can be seen from Table 3, Comparative Steel A has a critical drawing ratio of 0.0 after warm working compared to cold working. While the steel of the present invention only improves i0 by 0.20 or more, it can be seen that the steel of the present invention is also excellent in the local heating method.

The present invention has been described in detail above, and when applied to warm working applications, the steel of the present invention has properties that cannot be obtained with conventional hot-rolled carbon steel, and its industrial value is great. .

Claims (1)

[Claims] 1 C=0.02 to 0.15%, Mn=more than 0.90 to 2
．． 50%, N=0.0020-0.015%, and N
b=0.01~0.10%, V=0.01~0.20%
, a killed steel containing one or more of Ti=0.03 to 0.30% as a main component is solution treated, hot rolled, and then heated to a temperature of 50° C. at an average cooling rate of 10° C./sec or more.
．． A method for producing hot rolled steel for warm working, characterized by forced cooling to a temperature range of 0°C or lower.