KR102223119B1 - Manufacturing method for very thick steel plate and casting slab for very thick steel plate - Google Patents

Abstract

The present invention relates to a method for manufacturing an ultra-thick steel sheet and a cast piece for an ultra-thick steel sheet.
The method of manufacturing an ultra-thick steel sheet according to an embodiment of the present invention, in weight%, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Nb: 0.005 to 0.10%, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less , Preparing a steel material containing the balance iron (Fe) and inevitable impurities; Casting a steel material to produce a cast steel; And rough rolling the cast steel with a rolling reduction rate of 10% or more three or more times; including, casting a steel material to produce a cast steel; a rolling reduction rate of 1.5 to 4.5 mm/min during solidification of the steel material First rolling; And performing a second rolling at a reduction speed of 2 to 4.5 mm/min after solidification of the steel material.

Description

Ultra-thick steel plate manufacturing method and ultra-thick steel plate {MANUFACTURING METHOD FOR VERY THICK STEEL PLATE AND CASTING SLAB FOR VERY THICK STEEL PLATE}

The present invention relates to a method for manufacturing an ultra-thick steel sheet and a cast piece for an ultra-thick steel sheet. More specifically, it relates to a method for manufacturing a high-strength, high-toughness ultra-thick steel sheet having excellent core properties, and a cast plate for an ultra-thick steel sheet.

In general, a steel plate is made by rolling a cast steel made by a continuous casting method. When manufacturing a cast steel, an empty space is generated in the center of the thickness of the cast steel due to the solidification and contraction of the molten steel, and segregation occurs due to the inflow of the solute-concentrated residual steel into the empty space. If segregation remains in the center of the thickness of the cast steel, hydrogen dissolved in the molten steel accumulates in the segregation, causing cracks in the steel plate. In addition, even if hydrogen is not accumulated, the thickness center of the cast steel in which the segregation grains are concentrated has high hardness, so that cracks are easily generated during rolling.

In recent years, for steel materials for industrial/construction machinery, construction/marine structures, and various pressure vessels, the opportunity to use ultra-thick steel plates of 100 mm or more has increased due to the enlargement of facilities. In addition, there is a trend to use rolled materials rather than forged materials in order to reduce the cost of products even in high carbon steels for molds and machine parts. If the central segregation and solidification and shrinkage holes remain in the center of the thickness of the cast steel used in the manufacture of such an ultra-thick steel sheet, it is difficult to solve the problem with a rolling mill that is currently generally used. Therefore, when performing an ultrasonic flaw test on an ultra-thick steel plate, it is often detected as a defect. Several methods have been proposed to solve this problem.

For example, one of the prior art proposes to eliminate the segregation at the center of the thickness of the cast plate by performing a surface pressure reduction of 1 to 25 mm at a solidification solidification rate of 85 to 99%. Another prior art has proposed a method of producing an ultra-thick steel sheet by performing a surface pressure of 1.1 to 2 times the unsolidified thickness in a region of a core solidus ratio of 0.6 or more. However, in order to do this, a large-scale reduction equipment must be installed in the player, and since the reduction must be performed at the end of solidification, segregation cannot be sufficiently removed. In addition, in another prior art, a technique for reducing central segregation by artificially increasing the thickness of the cast steel to 20 to 100 mm during casting, and reducing the central segregation by pressing down the cast steel using a pair of pressing rolls before completion of solidification was proposed. However, the bulging technology for artificially thickening the thickness of the cast steel to 20 to 100 mm may cause problems in the physical properties of the center due to the occurrence of cracks inside the cast steel. In the case of another prior art, a technique was proposed to make a minor segregation in the center of the cast by rolling down the cast with a rolling reduction gradient of 5 to 20 mm/m using a segment. However, the product quality according to the rolling conditions is not specified.

An object of the present invention is to provide a method for manufacturing an ultra-thick steel plate and a cast plate for an ultra-thick steel plate. More specifically, it is intended to provide a method of manufacturing a high-strength, high-toughness ultra-thick steel sheet with excellent core properties and a cast plate for ultra-thick steel sheet.

The method for manufacturing an ultra-thick steel sheet according to an embodiment of the present invention, in weight%, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Nb: 0.005 to 0.10%, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less , Preparing a steel material containing the balance iron (Fe) and inevitable impurities; Casting a steel material to produce a cast steel; And rough-rolling the cast steel with a rolling reduction rate of 10% or more with three or more passes; including, casting a steel material to produce a cast steel; a rolling reduction rate of 1.5 to 4.5 mm/min during solidification of the steel material First rolling; And performing a second rolling at a reduction rate of 2 to 4.5 mm/min after solidification of the steel material.

In the step of preparing a steel material; In, the steel material may further include any one or more of B: 5 to 40 ppm, N: 15 to 150 ppm, Ca: 60 ppm or less, and S: 100 ppm or less.

In the step of manufacturing a cast steel by casting a steel material; In, the casting speed may be 0.6 to 1.8m/min.

In the step of manufacturing a cast steel by casting a steel material; supplying cooling water to the cast steel, but based on the width (w) direction of the cast steel, the edge portion (0 to a) for the amount of cooling water in the central part (section a to b) The ratio of the amount of cooling water in the section and section b to w) is 1.2 or more, a is w/4±w/8, and b may be 3w/4±w/8.

Casting a steel material to produce a cast steel; Thereafter, the step of heating the cast steel to 1050 to 1250 °C; may further include.

In the step of rough rolling the cast steel; In, the rough rolling temperature may be Tnr to 1120 °C.

Coarse rolling of a cast steel; Thereafter, the rough-rolled cast slab is subjected to finishing rolling at Ar3+30 to Tnr°C. It may further include.

Finishing rolling the roughly rolled cast steel; Thereafter, the step of cooling the finished-rolled cast piece further comprises cooling the finished-rolled cast piece; may be ending the cooling at a temperature of 390°C or higher at a cooling rate of 5°C/s or more.

Cast for an ultra-thick steel sheet according to an embodiment of the present invention, by weight, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0 %, Nb: 0.005 to 0.10%, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less, It is a cast iron for an ultra-thick steel sheet containing the balance iron (Fe) and inevitable impurities, and the area of the center segregation and pores per area at the point t/2 based on the thickness (t) direction of the cast steel is 0.031 to 0.087%.

The cast steel may further contain any one or more of B: 5 to 40 ppm, N: 15 to 150 ppm, Ca: 60 ppm or less, and S: 100 ppm or less.

The ultra-thick steel sheet manufactured according to an embodiment of the present invention is a material for extremely thick structures by minimizing pores or segregation in the center of the thickness of the cast steel, and has excellent central properties. In addition, it has high strength characteristics and excellent impact toughness.

The ultra-thick steel sheet manufactured according to an embodiment of the present invention has high impact absorption energy at the 1/2t position of the center of the steel sheet and high impact toughness, and is a high strength, high toughness structural steel sheet without defects caused by ultrasonic flaw detection. .

1 is a conceptual diagram of a continuous casting machine.
2 is a schematic diagram of a gap of a rolling-down roll after solidification as a method for manufacturing a cast steel according to an embodiment of the present invention.
3 is a schematic diagram showing a cross section of a solidification delay portion of a cast steel.
Figure 4 is a photograph showing the final microstructure of the ultra-thick steel sheet manufactured according to an embodiment of the present invention.

In this specification, terms such as first, second and third are used to describe various parts, components, regions, layers, and/or sections, but are not limited thereto. These terms are only used to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

In the present specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless otherwise stated.

In this specification, the terminology used is only for referring to specific embodiments and is not intended to limit the present invention. Singular forms as used herein also include plural forms unless the phrases clearly indicate the opposite. As used in the specification, the meaning of "comprising" specifies a specific characteristic, region, integer, step, action, element and/or component, and the presence of another characteristic, region, integer, step, action, element and/or component, or It does not exclude additions.

In the present specification, the term "combination of these" included in the expression of the Makushi form refers to a mixture or combination of one or more selected from the group consisting of the constituent elements described in the expression of the Makushi form, and the constituent elements It means to include one or more selected from the group consisting of.

In this specification, when a part is referred to as being "on" or "on" another part, it may be directly on or on the other part, or another part may be involved in between. In contrast, when a part is referred to as being "directly above" another part, no other part is interposed between them.

Although not defined differently, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms defined in a commonly used dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal meaning unless defined.

In addition, unless otherwise specified,% means% by weight, and 1 ppm is 0.0001% by weight.

In an embodiment of the present invention, the meaning of further including an additional element means to include the remaining iron (Fe) as much as an additional amount of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art may easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

In general, cast irons are manufactured while molten steel accommodated in a mold is cooled through a cooling table. This is illustrated in FIG. 1. The cast piece 10 to be continuously cast is cooled while passing through at least one segment 20 and proceeds to a subsequent process.

When a cast steel is rolled into a thick steel material, defects of the cast steel remain even after rolling, causing defects. Examples of such defects are central segregation and pores.

Central segregation is caused by the flow of the concentrated solute liquid during solidification when the cast steel is continuously cast. The largest cause of this flow is the residual molten steel flow due to solidification shrinkage near the solidification completion point, and the largest residual liquid flow. Will be affected. That is, when the solute-concentrated residual molten steel is collected in the solidification shrinkage portion near the solidification completion point of the continuous casting process, this becomes central segregation, and if the solidification shrinkage portion is not filled and remains as a space, it becomes center porosity.

When producing extremely thick products, if central segregation and pores remain in the center of the thickness of the cast steel, segregation and pores remain in the center of the thickness of the product even after rolling, and in the case of an extremely thick plate with a thick product, it is inevitably fatal to secure product properties. . Therefore, the pores in the center segregation and empty spaces with higher hardness than the product base material grow into product cracks and are detected as defects in the ultrasonic flaw test.

Therefore, by minimizing the central segregation and pores generated during the playing process, there are no defects in the ultrasonic flaw detection, the shock absorption energy at the center location is high at a low temperature, and a high-strength, high-toughness ultra-thick steel plate is provided that secures a thick product with excellent impact toughness. I want to.

First of all, in order to manufacture an ultra-thick steel sheet having excellent impact toughness, the technology of securing a cast steel with sound internal quality is the most important. Based on this, let's look at an ultra-thick steel material and a method of manufacturing the same that completely compresses the pores remaining in the cast steel after solidification to prevent adverse effects of the product properties due to the pores in advance.

Cast for an ultra-thick steel sheet according to an embodiment of the present invention, by weight, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0 %, Nb: 0.005 to 0.10%, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less, The balance contains iron (Fe) and inevitable impurities, and the area of the center segregation and pores per area at the point t/2 based on the thickness (t) direction of the cast steel is 0.031 to 0.087%.

In addition, any one or more of B: 5 to 40 ppm, N: 15 to 150 ppm, Ca: 60 ppm or less, and S: 100 ppm or less may be further included.

On the other hand, the steel material to be prepared in the method for manufacturing an ultra-thick steel sheet according to an embodiment of the present invention is, by weight, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Nb: 0.005 to 0.10%, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 To 0.2%, P: 0.02% or less, and the balance iron (Fe) and inevitable impurities are included.

In addition, the steel material may further include any one or more of B: 5 to 40 ppm, N: 15 to 150 ppm, Ca: 60 ppm or less, and S: 100 ppm or less.

First, the reasons for limiting the components of cast iron and steel materials will be described.

[Carbon (C): 0.02 to 0.2% by weight]

C is an important element that forms hard bainite and determines the size and fraction of the bainite to be formed, so it is included within an appropriate range. However, if the C content is too high, the toughness decreases, and if the C content is too low, the formation of bainite may be hindered, resulting in a decrease in strength. On the other hand, more specifically, in the case of a plate material used as a ship structural steel, the content of C may be set to 0.06 to 0.16% by weight for better weldability.

[Manganese (Mn): 1.0 to 3.0% by weight]

Mn is a useful element that improves the strength by solid solution strengthening, so it needs to be added in an amount of 1.0% by weight or more. However, if too much is added, the toughness of the weld may be greatly reduced due to an excessive increase in hardenability. More specifically, it may be 1.2 to 3.0% by weight.

[Silicon (Si): 0.01 to 0.8% by weight]

Si is used as a deoxidizing agent and helps to improve strength and toughness. However, if too much is added, low-temperature toughness and weldability may be deteriorated at the same time. On the other hand, if too little is added, the deoxidation effect may be insufficient. More specifically, it may be 0.1 to 0.4% by weight.

[Phosphorus (P): 0.02% by weight or less]

Although P is an element that is advantageous for strength improvement and corrosion resistance, it is advantageous to keep it as low as possible because it can greatly impair impact toughness.

[Sulfur (S): 0.01% by weight or less]

S is an element that greatly impairs impact toughness by forming MnS, so it is advantageous to keep it as low as possible.

[Aluminum (Al): 0.005 to 0.2% by weight]

Al is an element capable of inexpensively deoxidizing molten steel, and may be included in an amount of 0.005% by weight or more in order to exhibit a sufficient effect. However, if too much is added, nozzle clogging may occur during continuous casting.

[Nickel (Ni): 0.01 to 2.0% by weight]

Ni is almost the only element capable of simultaneously improving the strength and toughness of the base material, and may contain 0.01% or more in order to exhibit the effect. However, since Ni is a very expensive element, if too much is added, economical efficiency is remarkably deteriorated, and weldability is also deteriorated.

[Copper (Cu): 0.01 to 1.0% by weight]

Since Cu is an element capable of increasing the strength while minimizing the decrease in toughness of the base material, 0.01% by weight or more may be added to sufficiently obtain the effect, but excessive addition of Cu may significantly impair the product surface quality.

[Niobium (Nb): 0.005 to 0.1% by weight]

Nb is the most important element in the manufacture of TMCP steel, and it precipitates in the form of NbC or Nb(C,N), greatly improving the strength of the base metal and the weld. In addition, Nb dissolved during reheating to a high temperature suppresses recrystallization of austenite and transformation of ferrite or bainite, thereby exhibiting an effect of miniaturizing the structure. Furthermore, in the present invention, the stability of austenite is increased even during cooling after the final rolling, thereby promoting the formation of a hard phase such as bainite even at a low rate of cooling. Therefore, Nb may be added in an amount of 0.005% by weight or more, but if it is excessively added, brittle cracks may appear at the corners of the steel material.

[Titanium (Ti): 0.005 to 0.1% by weight]

Ti suppresses the growth of crystal grains during reheating and greatly improves the low-temperature toughness.More than 0.005% by weight can be added, but excessive addition causes problems such as clogging of the playing nozzle or reduction of low-temperature toughness due to crystallization in the center. I can make it.

[Nitrogen (N): 15 to 150 ppm by weight]

N may be added up to 150 ppm by weight because it increases the strength while greatly reduces the toughness. However, too little N content control can increase the steelmaking load.

[Calcium (Ca): 60 ppm by weight or less]

Ca is mainly used as an element that suppresses non-metallic inclusions of MnS and improves low-temperature toughness. However, excessive Ca addition can react with the oxygen contained in the steel to produce CaO, which is a non-metallic inclusion.

In addition to the above components, the present invention contains Fe and unavoidable impurities. It does not exclude the addition of effective ingredients other than the above ingredients. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal steel manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the steel manufacturing process, all the contents are not specifically mentioned in the present specification.

Hereinafter, a method for manufacturing an extremely thick steel sheet of the present invention will be described in detail. The manufacturing method of the ultra-thick steel sheet of the present invention may consist of a process of manufacturing cast steel-heating slab-rough rolling-finishing rolling-cooling.

That is, preparing a steel material of the above-mentioned components; Casting a steel material to produce a cast steel; And coarse rolling the cast steel with a rolling reduction ratio of 10% or more three or more times; including, and preparing a cast steel of a steel material; a first rolling reduction rate of 1.5 to 4.5 mm/min during solidification of the steel material Rolling; And performing a second rolling at a reduction rate of 2 to 4.5 mm/min after solidification of the steel material.

In addition, casting a steel material to produce a cast plate; Then, it may further include the step of heating the cast plate.

In addition, after the step of rough rolling the cast steel, the step of finishing the roughly rolled cast steel; may further include.

In addition, the step of finishing rolling the roughly rolled cast steel; Thereafter, cooling the finished-rolled cast steel; may further include.

Detailed conditions for each process are as follows.

[Cast casting production]

First, a steel material satisfying the above-described composition is prepared, and the steel material is cast to produce a cast iron.

At this time, the cast piece may have a thickness of 200 mm or more. More specifically, it may have a thickness of 300 mm or more.

At this time, the step of first rolling at a rolling reduction rate of 1.5 to 4.5 mm/min during solidification of the steel material and a second rolling step at a rolling reduction rate of 2 to 4.5 mm/min after solidification of the steel material. That is, during the production of the cast steel, using the segment 20 as shown in FIG. 1, before the solidification of the cast steel is completed, as shown in FIG. 2, it is reduced at a reduction rate of 1.5 to 4.5 mm/min, and after the solidification is completed, 2 to 4.5 mm using a segment device. It is reduced to /min to manufacture a slab that minimizes central segregation and porosity.

Here, the rolling reduction rate can be calculated by the rolling reduction amount, the casting speed, and the length of the segment, as shown in Equation 1 below.

[Equation 1]

Rolling speed (mm/min) = Rolling amount (mm) x Casting speed (m/min) / Segment length (m)

If the rolling reduction rate before the completion of solidification in the first rolling step is too small and the rolling reduction amount is too small, it is difficult to remove the central segregation and pores, and when the rolling reduction rate is too large after the solidification in the second rolling step is completed, the rolling productivity is too large. Considering the drop and segment equipment load, large equipment remodeling costs such as enlargement of equipment or change of segment type are required.

More specifically, the speed of the first rolling step may be 1.5 to 3.5 mm/min. In addition, more specifically, the speed of the second rolling step may be 2.5 to 4 mm/min.

Meanwhile, the casting speed may be 0.6 to 1.8 m/min. The above speed is appropriate when considering the productivity of cast iron and minimization of central segregation. If the casting speed is too slow, the performance productivity is lowered, and if the casting speed is too fast, internal cracks easily occur between the columnar crystals due to bulging between the rolls or the like. More specifically, the casting speed may be 0.6 to 1.0 m/min.

On the other hand, cooling water is supplied to the cast steel, but the amount of cooling water in the rim (0 to a section and b to w section) relative to the amount of cooling water in the central part (section a to b) based on the width (w) direction of the cast steel The ratio of is 1.2 or more, a is w/4±w/8, and b may be 3w/4±w/8.

In general, in solidifying the cast steel, solidification does not occur uniformly in the width direction as shown in FIG. 3. If the cast steel is pressed down when such non-uniform solidification occurs, the solute-concentrated molten steel is collected in the solidification delay part, and thus the width of the cast steel In order to reduce the non-uniform solidification delay in the width direction, the amount of cooling water at the edge can be increased compared to the central part of the cast steel in order to reduce the non-uniform solidification delay part in the width direction due to the non-uniform solidification in the direction of the product.

Here, the central part of the cast iron is a section of w/4±w/8 to 3w/4±w/8, and the rim part is 0 to w/4±w/8 and 3w/4± It can be defined as a w/8 to w section.

The amount of cooling water in the edge portion relative to the amount of cooling water in the central portion may be more specifically 1.3 or more, and more specifically 1.5 or more.

[Casting heating]

Next, it may further include the step of heating the manufactured cast.

In this case, the heating temperature may be 1050 to 1250°C. It is to solidify any one or more carbonitrides of Ti and Nb formed during casting. That is, in order to sufficiently dissolve any one or more carbonitrides of Ti and Nb, it may be preferable to heat to 1050° C. or higher. However, there is a concern that austenite coarsens when heated to too high a temperature. More specifically, heating may be reheating.

[Coarse Rolling]

Next, the heated cast iron is roughly rolled. The reason for rough rolling is to adjust its shape.

At this time, the extraction temperature of the heated cast iron may be maintained between 1050 and 1120 °C. In particular, when a high alloy is added, maintaining the temperature between 1050 and 1080° C. has the advantage of reducing product surface cracking during subsequent rolling, and simultaneously improving impact toughness by low-temperature extraction.

On the other hand, the rough rolling temperature may be Tnr to 1120 ℃. Tnr temperature is the temperature at which recrystallization of austenite stops. It will be possible to obtain the effect of reducing the size of the austenite and the destruction of the columnar structure formed during casting by rolling and the compression of the pores.

On the other hand, the number of passes with a reduction ratio of 10% or more is three or more. In other words, in order to improve the impact toughness of the center of the extremely thick material, a reduction force of 10% or more per rough rolling pass is rolled at least three times to transmit the reduction force to the center to minimize pores or segregation in the center, so that defects are not detected by ultrasonic flaw detection. In addition, it is possible to secure a product exceeding 80t with excellent impact toughness with an impact absorbing energy of 100J or more at the t/2 position at -40℃.

[Fine rolling]

Next, the rough-rolled cast steel may be fine-rolled. Finish rolling is to introduce a non-uniform microstructure into the austenite structure of the roughly rolled cast steel.

At this time, the finishing rolling temperature may be Ar3+30 to Tnr ℃. The Tnr temperature is the temperature at which recrystallization of austenite stops, and the Ar3 temperature is the start temperature of ferrite transformation. In this case, it is possible to suppress transformation of the cornerstone ferrite, which greatly decreases the impact toughness, before the start of cooling.

[Cooling after rolling]

Next, the finished rolled cast steel can be cooled.

In this case, cooling may be terminated at a temperature of 390°C or higher at a cooling rate of 5°C/s or higher.

As shown in FIG. 4, the microstructure of the steel material of the present invention may be 10 to 50%, more specifically 15 to 40%, and the remainder of the soft ferrite formed from the initial austenite grain boundary as shown in FIG. 4.

If the cooling rate is too low, there is a possibility that the area fraction of the soft ferrite becomes 50% or more and the tensile strength becomes less than 570 Mpa. In addition, when the cooling end temperature is too low, the ferrite fraction becomes 10% or less, and it may be difficult to obtain an impact toughness of 100 J or more at a low temperature.

Hereinafter, the cast piece for an extremely thick steel plate of the present invention will be described.

[Center segregation and pore area]

The area of central segregation and pores per area at point t/2 based on the thickness (t) direction of the cast slab is 0.031 to 0.087%. The smaller the area of the center segregation and pores is, the better the required quality can be obtained.If it is too large, it requires a long time heating to alleviate the central segregation in the heating and rolling process, and the number of passes with a reduction ratio of 10% or more to compress the pores There is a drawback that needs to be increased further. More specifically, the area of the cast steel may be 74.5mm x 32.5mm.

Hereinafter, the microstructure and mechanical properties of the ultra-thick steel sheet manufactured by the present invention will be described.

[Fine organization]

The microstructure of the steel sheet includes ferrite and bainite, and the volume fraction of ferrite may be 10 to 50%. More specifically, it may be 15 to 40%. The rest may be bainite.

[The tensile strength]

The tensile strength of the steel sheet may be 570 Mpa or more.

[Shock absorption energy]

The shock absorption energy at the point t/2 based on the thickness (t) direction of the steel sheet may be 100J or more at -40°C.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

[Example]

Experiment and evaluation

After rolling and cooling the slabs satisfying the component system shown in Table 1 to meet the manufacturing conditions shown in Table 2 below, yield strength, tensile strength, ferrite area fraction and yield ratio were measured and shown in Table 3 below.

Table 1 is a table showing steel grades that satisfy the component system according to the present invention and steel grades that do not. In Table 1, the unit of the content of each element is% by weight or ppm (indicated by *). In addition, the Tnr temperature means the temperature at which recrystallization of austenite stops, the Ar3 temperature means the ferrite transformation start temperature, and the unit is °C.

River number C Si Mn P S Al Ni Cu Ti Nb N* ^* Ca* Tnr Ar3 Invention Steel A 0.04 0.28 1.4 0.008 0.002 0.026 0.01 0.01 0.017 0.03 37 20 954 719 Invention Steel B 0.07 0.32 1.9 0.009 0.002 0.027 0.45 0.17 0.015 0.02 20 14 950 722 Inventive Steel C 0.11 0.31 1.4 0.008 0.002 0.026 1.01 0.01 0.017 0.03 37 20 948 732 Inventive Strength D 0.18 0.27 1.7 0.010 0.001 0.032 0.18 0.20 0.015 0.03 29 18 958 705 Comparative Strength E 0.01 0.25 1.3 0.015 0.003 0.025 0.01 0.02 0.018 0.03 34 22 924 777 Comparative Steel F 0.21 0.32 0.5 0.014 0.004 0.027 0.20 0.20 0.017 0.03 35 22 968 715 Comparative Strong G 0.14 0.33 3.2 0.013 0.002 0.031 0.02 0.03 0.017 0.02 36 17 898 601

Referring to Table 1, steel grades A to D are steel grades that satisfy the component system according to the present invention, and steel grades E to G do not satisfy the component system according to the present invention. More specifically, for steel grade E and steel grade F, the C content is less than 0.02% by weight, which is the lowest limit of the content of C (0.02% by weight to 0.2% by weight) according to the present invention, and is more than 0.2% by weight, which is the uppermost limit, and steel grade G Is greater than 3.0% by weight, which is the upper limit of Mn according to the invention. In addition, referring to Table 1, the austenite recrystallization end temperature (Tnr) and the Ar3 transformation temperature are different for each steel type.

In addition, Table 2 is a steel slab composed of the components of Table 1 by casting, rolling, and cooling in the form shown in Table 2 to manufacture a steel material.

At this time, the first rolling speed and the second rolling speed were calculated using Equation 1 below.

[Equation 1]

Rolling speed (mm/min) = Rolling amount (mm) x Casting speed (m/min) / Segment length (m)

In addition, the segment length in the first rolling step and the second rolling step was calculated as 2 m.

No. Cast iron manufacturing conditions Rough rolling conditions Finish rolling conditions Cooling condition Remark Steel grade number 1st rolling (before solidification)
Reduction
(mm) 1st rolling speed (mm/min) 2nd rolling (after solidification) rolling reduction
(mm) 2nd rolling speed (mm/min) casting
speed
(m/min) coagulation
Quantity of delay part/
Central volume Reheat
extraction
Temperature
( ^o C) Rough rolling
End
Temperature
( ^o C) Reduction rate
10%
Number of passes Rolled
Initiate
Temperature
( ^o C) Rolled
End
Temperature
( ^o C) Cooling
speed
( ^o C/s) Cooling
End
Temperature
( ^o C) The present invention
Whether the control conditions are satisfied Steel grade A A-1 8 2.8 6 2.1 0.7 1.5 1080 990 3 910 780 7 415 O A-2 5 2 10 4 0.8 1.8 1090 1000 3 920 790 7 430 O A-3 5 2 6 2.4 0.8 1.5 1090 970 3 895 780 6 440 O A-4 5 1.75 0 0 0.7 1.8 1080 980 4 920 780 7 410 X Steel grade B B-1 8 2.8 10 3.5 0.7 1.5 1070 990 3 948 808 7 420 O B-2 5 2 10 4 0.8 1.0 1075 995 2 933 793 8 435 X B-3 3 1.2 8 3.2 0.8 1.5 1130 1030 3 923 783 9 425 X B-4 15 5.25 0 0 0.7 1.8 1105 1025 4 945 815 6 410 X Steel grade C C-1 8 2.8 6 2.1 0.7 1.5 1110 970 3 913 773 8 430 O C-2 5 2 10 4 0.8 1.8 1095 960 3 928 798 6 417 O C-3 5 2 6 2.4 0.8 1.5 1085 985 3 938 838 8 395 O C-4 20 7 0 0 0.7 1.0 1105 965 2 820 780 7 412 X Steel grade D D-1 8 2.8 6 2.1 0.7 1.5 1075 965 3 914 774 7 433 O D-2 10 4 10 4 0.8 1.8 1065 1025 3 889 749 9 422 O D-3 5 2 6 2.4 0.8 1.5 1115 995 3 879 739 6 420 O D-4 10 3.5 0 0 0.7 1.8 1095 1015 2 820 780 7 442 X Steel grade E E-1 8 2.8 6 2.1 0.7 1.5 1077 1005 3 875 835 8 427 X E-2 10 4 10 4 0.8 1.8 1087 985 3 855 815 9 396 X E-3 18 6.3 0 0 0.7 1.5 1125 950 3 845 825 8 380 X Steel grade F F-1 8 2.8 6 2.1 0.7 1.5 1135 995 3 963 823 9 415 X F-2 10 4 10 4 0.8 1.3 1105 1045 4 943 803 7 429 X F-3 5 2 10 4 0.8 1.5 1110 980 3 933 793 6 395 X Steel grade G G-1 8 2.8 6 2.1 0.7 1.5 1087 970 3 793 714 5 410 X G-2 10 4 10 4 0.8 1.8 1077 1000 4 783 704 6 390 X G-3 5 2 10 4 0.8 2 1119 990 3 801 723 8 410 X

For the steel manufactured as shown in Table 2, as shown in Table 3, the yield strength (YS), tensile strength (TS), impact absorption energy (CVN) at the 1/2t position at -40°C, the presence of CTOD, and measured by ultrasonic waves. It was judged whether it was defective.

Steel grade number Product thickness YS TS CVN
(@-40 ^o C) CTOD
Union Whether the ultrasound is defective Whether the steel characteristics condition of the present invention is satisfied Steel grade A A-1 80 497 625 280 O O O A-2 90 474 587 251 O O O A-3 80 471 592 175 O O O A-4 80 534 656 61 X X X Poor impact toughness, poor CTOD and ultrasound
(Insufficient amount of reduction after solidification) Steel grade B B-1 90 486 590 234 O O O B-2 100 481 582 79 X O X Impact toughness, less than CTOD
(Quantity of coagulation delay part and
Insufficient number of coercion) B-3 90 531 587 55 X X X Impact toughness, lack of CTOD and poor ultrasound (lack of pressure reduction before coagulation and
Extraction temperature exceeded) B-4 90 521 576 97 O O X Insufficient impact toughness (lack of reduction after solidification) Steel grade C C-1 80 502 637 249 O O O C-2 100 481 589 122 O O O C-3 80 525 591 156 O O O C-4 80 591 663 36 O O X Insufficient impact toughness (lack of reduction after solidification, amount of solidification delayed part, and number of downward pressure reduction) Steel grade D D-1 90 471 596 193 O O O D-2 100 482 599 143 O O O D-3 90 489 603 176 O O O D-4 90 561 668 86 O X X Insufficient impact toughness (lack of reduction and number of reductions after solidification) Steel grade E E-1 80 445 511 256 O O X under-strength
(Less than C content) E-2 100 451 502 198 O O X under-strength
(Less than C content) E-3 90 455 532 223 O O X Insufficient strength (less C content) Steel grade F F-1 80 529 677 83 X O X Impact toughness, less than CTOD (exceeding C content) F-2 100 501 686 55 X O X Impact toughness, less than CTOD (exceeding C content) F-3 90 499 659 77 X O X Impact toughness, less than CTOD (exceeding C content) Steel grade G G-1 80 491 597 88 O O X Insufficient impact toughness
(Exceeding Mn content) G-2 100 497 585 31 O O X Insufficient impact toughness
(Exceeding Mn content) G-3 90 533 615 75 O O X Insufficient impact toughness (exceeding Mn content)

Experiment result

Referring to Table 1 to Table 3, among steel grades A to G, the component system of the present invention, the casting speed, the rolling reduction before and after solidification of the cast steel, the cooling water injection amount at the edge of the cast steel relative to the cooling water injection amount at the center of the cast steel, and reheating. A-1, A-2, A-3, and B satisfying the conditions of extraction temperature, rough rolling end temperature, rolling reduction ratio of 10% or more, finish rolling start temperature, finish rolling end temperature, cooling rate, and cooling end temperature -1, C-1, C-2, C-3, D-1, D-2, D-3 have a tensile strength of 570 MPa or more, and an impact toughness of 100 J or more at the t/2 position at -40°C. It was judged that it was possible to secure and at the same time manufacture a stable product without defects when detected through CTOD and ultrasound.

However, in the case of B-3 with a rolling reduction before solidification of the cast steel less than 5mm, it was not possible to reduce the central segregation before solidification of the cast steel, so the shock absorption energy at the t/2 position at -40℃ was less than 100J, as well as defects in CTOD and ultrasonic inspection. A-4, B-4, C-4, and D-4, in which there is no reduction after solidification of the cast steel, has an impact absorption energy of less than 100J at the t/2 position at -40℃ due to the effect of pores after solidification of the cast steel, as well as sometimes CTOD. And ultrasonic flaw detection also occurred. In addition, B-2 is the case where the amount of coolant injection at the edge of the cast iron is insufficient and the number of passes of 10% or more of the rough rolling reduction ratio is less than 3 times relative to the amount of coolant injection at the center of the cast steel, and the shock absorption energy at the t/2 position at -40°C is Not only was it less than 100J, but also CTOD and ultrasonic flaw defects occurred.

In addition, steel grade E with a C content of less than 0.02% by weight was rejected because the tensile strength was less than the required level of 570 MPa, and steel grade F with a C content of more than 0.02% by weight significantly decreased the impact toughness as the tensile strength increased. At the same time, a CTOD failure occurred. Finally, in the case of steel grade G, it was found that it was impossible to secure impact toughness due to the excess of Mn content.

The present invention is not limited to the above embodiments, but may be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present invention pertains, other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that it can be implemented with Therefore, it should be understood that the embodiments described above are illustrative and non-limiting in all respects.

10: Cast iron
20: segment
30: secondary cooling nozzle

Claims (10)

In% by weight, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Nb: 0.005 to 0.10%, V: 0.005 to Steel material containing 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less, balance iron (Fe) and unavoidable impurities Preparing a;
Casting the steel material to produce a cast steel; And
Including; the step of rough rolling the cast steel with a reduction ratio of 10% or more in three or more passes,
Casting the steel material to produce a cast plate; The,
First rolling at a rolling reduction speed of 1.5 to 4.5 mm/min during solidification of the steel material; And
After solidification of the steel material, comprising the step of a second rolling at a reduction rate of 2 to 4.5 mm/min,
During solidification of the steel material, the rolling reduction in the first rolling step at a rolling reduction rate of 1.5 to 4.5 mm/min is more than 3 mm and less than 15 mm.
The method of claim 1,
Preparing the steel material; In,
The steel material is B: 5 to 40ppm, N: 15 to 150ppm, Ca: 60ppm or less, and S: an ultra-thick steel sheet manufacturing method further comprising any one or more of 100ppm or less.
The method of claim 1,
Casting the steel material to produce a cast plate; In,
Casting speed is 0.6 to 1.8m / min ultra-thick steel sheet manufacturing method.
According to 1,
Casting the steel material to produce a cast plate; In,
Supplying cooling water to the cast iron,
Based on the width (w) direction of the cast steel, the ratio of the amount of cooling water in the rim (section 0 to a and section b to w) to the amount of cooling water in the central part (section a to b) is 1.2 or more Manufacturing method.
(A is w/4±w/8, and b is 3w/4±w/8.)
The method of claim 1,
Casting the steel material to produce a cast steel; after,
Heating the cast steel to 1050 to 1250 °C; further comprising
Ultra-thick steel sheet manufacturing method.
The method of claim 1,
Rough rolling the cast steel; in,
The rough rolling temperature is Tnr to 1120 ℃ method for producing an ultra-thick steel sheet.
The method of claim 1,
Rough rolling the cast steel; after,
The method of manufacturing an ultra-thick steel sheet further comprising: finishing the rough-rolled cast steel at Ar3+30 to Tnr°C.
The method of claim 7,
Finishing rolling the roughly rolled cast steel; after,
Cooling the fine-rolled cast piece; further comprising,
Cooling the finished rolled cast steel; The,
Ultra-thick steel sheet manufacturing method to terminate the cooling at a temperature of 390 ℃ or more at a cooling rate of 5 ℃ / s or more.
In% by weight, C: 0.02 to 0.2%, Mn: 1.0 to 3.0%, Si: 0.01 to 0.8%, Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Nb: 0.005 to 0.10%, V: 0.005 to For extremely thick steel sheets containing 0.3%, Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.2%, P: 0.02% or less, balance iron (Fe) and inevitable impurities It's a masterpiece,
A cast piece for an ultra-thick steel plate having a center segregation and pore area of 0.031 to 0.087% per area at a point t/2 based on the thickness (t) direction of the cast piece.
The method of claim 9,
The above cast iron,
B: 5 to 40 ppm, N: 15 to 150 ppm, Ca: 60 ppm or less, and S: cast iron for an extremely thick steel sheet further comprising any one or more of 100 ppm or less.

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