JPH0362501B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for producing a thick steel plate having excellent properties as cast. [Prior Art and Problems] In the manufacturing technology of thick steel plates, there has been a strong trend toward omitting, simplifying, and directing processes in order to reduce manufacturing costs. One manifestation of this is the idea of directly linking casting and hot rolling by omitting the reheating process, and even omitting hot rolling by casting into slabs of the thickness of the product. However, it is difficult to incorporate the material to be rolled into a thick steel plate as cast. Prior austenite grain boundaries are usually the preferred nucleation sites for ferrite transformation. Therefore, the transformed structure from the coarse solidified austenite grains as cast is such that the prior austenite grain boundaries become coarse acylular ferrite, and the interior of the prior austenite grains also becomes considerably coarse ferrite. The properties of steel sheets having such a structure are generally not very good, and in particular they tend to be poor in low-temperature toughness. [Means for solving the problems] The present invention eliminates the drawbacks of the conventional method as described above,
This is a method for producing thick steel plates with excellent properties as cast, and the gist of this method is that C: 0.03-0.25%, Si: 0.01-0.5%, Mn: 0.3-0.
1.8%, Ti: 0.004~0.04%, N: 0.002~0.008%,
Furthermore, if necessary, Al: ≦0.05%, Cu: ≦1.5%,
Ni:≦5%, Cr:≦1%, Mo:≦1%, Nb:
Steel containing one or more of ≦0.2%, V:≦0.5%, B:≦0.0025%, with the balance consisting of iron and unavoidable impurities is cast into a thick steel plate, and the coarse austenite grains as cast are cast. During cooling after solidification, temperature is increased from 800℃ to below 600℃ by 2℃/
It is characterized by cooling at a cooling rate of not less than 1 second and not more than 50°C/second. The present invention will be explained in detail below. The metallurgical difference between the prior art and the present invention is the behavior of ferrite transformation. Conventionally, it has been said that prior austenite grain boundaries are the preferential nucleation site for ferrite transformation. For this reason, many technologies to date have aimed at refining austenite grains by rolling in a recrystallized region, and then flattening the austenite grains by rolling in a non-recrystallized region to increase the austenite grain boundary area per unit volume. However, in as-cast steel plates, the austenite grains are coarse and the grain boundary area is small, and the transformed ferrite naturally becomes coarse. In the present invention, instead of the ferrite transformation from the austenite grain boundaries as described above, ferrite transformation is realized from within the austenite grains. What is important here is the existence of sites for generating ferrite from within the austenite grains and the process conditions for generating ferrite from these sites. First, the reasons for limiting the components of the steel of the present invention will be described. C is an essential element for strengthening steel, and if it is less than 0.03%, the required strength cannot be obtained;
If it exceeds 0.25%, the toughness of the base metal and weld zone will be impaired, so it was limited to 0.03% or more and 0.25% or less. Si is effective as a deoxidizing element and a steel-strengthening element, and is added in an amount of 0.01% or more, but if it exceeds 0.5%, it causes deterioration in workability and impairs the surface quality of the steel sheet. Mn is effective in strengthening steel, but if it is less than 0.3%, it has no effect, and if it exceeds 1.8%, workability deteriorates. Ti is an important element in the present invention. That is, Ti oxides and nitrides generated during the solidification and cooling process act effectively as ferrite generation nuclei within the austenite grains during transformation. This effect is lost when Ti addition is less than 0.004%.
Addition of more than 0.04% causes Ti oxides and nitrides to solidify.
Coagulation and coarsening occur during cooling, impairing the effect of Ti addition. N is an element that forms TiN and works effectively as described above if added at 0.002% or more, but 0.008
If it exceeds 0.0%, TiN not only aggregates and coarsens, but also impairs the toughness of the weld, so it should be set at 0.002% or more and 0.008% or less. Al is added as a deoxidizing element, but its effect is saturated if it exceeds 0.05%, so the upper limit is limited to 0.05%. Cu is effective in improving the corrosion resistance and strength of steel, but
Excessive addition may cause hot cracking of the weld metal.
1.5% or less. Although Ni increases the strength and low-temperature toughness of steel, it is difficult to obtain an economical effect if it is added excessively, so it should be kept at 5% or less. Cr, Mo, and B improve the hardenability of steel and are effective in stabilizing the structure within prior austenite grains, which is characteristic of the present invention. However, excessive addition causes hot cracking during transformation, so Cr and Mo are each kept at 1% or less, and B at 0.0025% or less. In the present invention, Nb and V precipitate as fine carbonitrides during the cooling process after rolling and increase the strength of the steel, but excessive addition impairs low-temperature toughness, so they are limited to 0.2% or less and 0.5% or less, respectively. Next, we will talk about the manufacturing method. In the present invention, after casting molten steel that satisfies the above-mentioned components, the temperature is lowered to 800°C during cooling.
Cool down to 600°C or less at a cooling rate of 2°C/second or more and 50°C/second or less. During this cooling process, fine ferrite is transformed from within the austenite grains. Here, the transformation nuclei are Ti oxides and nitrides, as well as composite compounds of these and MnS. If the cooling rate is too high below 800℃, the structure will become coarse bainite or martensite,
On the other hand, if it is too small, a coarse ferrite/pearlite structure will result, making it impossible to generate fine ferrite within austenite grains, which is the object of the present invention. For this reason, the cooling rate from 800°C to 600°C or less is limited to 2°C/second or more and 50°C/second or less. [Example] Table 2 shows the results of experiments conducted on the invention steel and comparative steel having the components shown in Table 1. Note that the underlined items in the table do not meet the conditions of the present invention. In Table 1, Steels A to E are steels of the present invention.
Steel F is a comparison steel to which no Ti is added.
Steel G is a comparison steel with excess Ti and N. Steels 1 to 6 in Table 2 are the steels of the present invention and exhibit excellent strength and toughness. In No. 7, the cooling rate below 800 to 600°C was too fast, and the structure became coarse bainite, resulting in significantly poor toughness. On the other hand, Sample No. 8 has a high water-cooling stop temperature and a too slow cooling rate, so the structure is a coarse ferrite-pearlite mainly caused by ferrite transformation from prior austenite grain boundaries, and its toughness is poor. Similar to No. 7, No. 9 had a cooling rate that was too fast, so the toughness was extremely poor. 10 is the case where Ti is not added. Although ferrite transformation from within the prior austenite grains is recognized in the structure at this time, the ability to generate ferrite is low and the toughness level is inferior compared to the steel of the present invention. In addition, in the case of no Ti addition, the ferrite transformation nuclei from within the prior austenite grains are Si and
These are Mn oxide and MnS. In No. 11, Ti and N are excessive, and the Ti nitrides and oxides that should become the core of ferrite transformation aggregate and become coarse before transformation, and no longer play their role. Therefore, the structure is coarse ferrite/pearlite and has poor toughness. As mentioned above, samples 7 to 11 do not have satisfactory toughness. As described above, if even one of the conditions deviates from the scope of the present invention, the object of the present invention will not be achieved.

【table】

[Table] [Effects of the Invention] As described above, according to the present invention, by generating fine ferrite from within the grains of coarse solidified austenite grains, it is possible to produce thick steel plates with excellent properties even as cast. Since it can be manufactured, it is extremely useful industrially.

Claims (1)

[Claims] 1% by weight: C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.8%, Ti: 0.004 to 0.04%, N: 0.002 to 0.008%, balance iron and unavoidable impurities. The steel is cast into a thick steel plate to produce coarse austenite grains as cast, and during cooling after solidification, it is
A method for producing a thick steel plate having excellent properties as cast, characterized by cooling the steel plate to 600°C or less at a cooling rate of 2°C/second or more and 50°C/second or less. 2 In weight%, C: 0.3-1.25%, Si: 0.01-0.5%, Mn: 0.3-1.8%, Ti: 0.004-0.04%, N: 0.002-0.008%, and Al: ≦0.05%, Cu: ≦1.5%. , Ni: ≦5%, Cr: ≦1%, Mo: ≦1%, Nb: ≦0.2%, V: ≦0.5%, B: ≦0.0025%, containing one or more of the following, the balance being iron. Steel containing unavoidable impurities is cast into a thick steel plate to produce coarse austenite grains as cast, and during cooling after solidification, the temperature is lowered from 800°C to 600°C at 2°C/second or more and 50°C/second. A method for producing a thick steel plate having excellent properties as cast, characterized by cooling at the following cooling rate.