WO2024111984A1 - Steel material having low manganese content and method of manufacturing same - Google Patents

Abstract

The present invention relates to a steel material for general structures which is widely used throughout the industry and a method of manufacturing same, and more specifically, relates to a steel material used for industrial machines, construction, and various materials for parts, and a method of manufacturing same.

Description

Low manganese steel and its manufacturing method

The present invention relates to general structural steels and their manufacturing methods that are widely used throughout the industry, and more specifically, to steels and their manufacturing methods used in industrial machinery, construction, and various parts materials.

General structural steel with a yield strength of 235 MPa and a tensile strength of 400 MPa is the most representative general purpose material and is widely used not only domestically but also overseas. In particular, general structural steels are widely used in the industrial machinery and parts materials industry, which require structures that can be bolted without welding, simple shapes, and a normal level of dimensional accuracy.

For these general structural steels, standards are established for each country, and products that meet these standards are manufactured and sold on the market by each steel company. The most representative standards for general structural steel include Japan's JIS, America's ASTM, and Korea's KS. Typically, steel manufacturers produce and manage their products somewhat more strictly than the quality assurance range, such as ingredients and materials, within these standards. However, there are no restrictions on production and sales of items that are not specified or guaranteed in the relevant standards. For example, in the case of SS (Steel Structure), a general structural steel according to the JIS standard, there are no guarantee items related to weldability, so there are no restrictions on the addition content of alloying elements such as C and Mn within the standard. On the other hand, in the case of SM (Steel for Marine), a general steel for welded structures, not only the alloy elements but also the upper limits of carbon equivalent (Ceq.) and weld crack susceptibility index (Pcm.) are specified to ensure functional differentiation from SS materials. is making it clear.

(Patent Document 1) Japanese Patent Publication No. 2005-301241

(Patent Document 2) Japanese Patent Publication No. 2008-067400

According to one aspect of the present invention, a general structural steel plate that satisfies the yield strength (YS) of 235 MPa or more and the tensile strength (TS) of 400 MPa or more, and can innovatively reduce the manufacturing cost by adding a very low manganese content, and a manufacturing method thereof. We would like to provide.

The object of the present invention is not limited to the above-described content. Anyone skilled in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the overall content of the present invention specification.

One aspect of the present invention is,

In weight percent, carbon (C): 0.15-0.25%, silicon (Si): 0.05-0.5%, manganese (Mn): 0.15% or less (excluding 0%), phosphorus (P): 0.03% or less (0%) (excluding), sulfur (S): 0.01% or less (excluding 0%), aluminum (Al): 0.05% or less (excluding 0%), including the balance Fe and other inevitable impurities,

It is composed of a dual phase structure of ferrite and pearlite,

Provided is a low manganese steel that satisfies the following equations 1 and 2 and has a thickness of 40 mm or less.

[Relationship 1]

[C]/[Mn] > 1.5

(In Equation 1 above, [C] and [Mn] each represent the weight percent content of the elements in parentheses.)

[Relational Expression 2]

[Cr] + [Mo] + [Cu] + [Ni] < 0.05

(In Equation 2 above, [Cr], [Mo], [Cu], and [Ni] each represent the weight percent content of the elements in parentheses.)

The low manganese steel may include 70 to 90% ferrite and 10 to 30% pearlite in terms of area percent as a microstructure.

The low manganese steel may have a yield strength of 235 MPa or more and a tensile strength of 400 MPa or more.

The low manganese steel may have an elongation of 30% or more.

The average grain size of the ferrite may be 30 to 50 μm.

Another aspect of the present invention is,

In weight percent, carbon (C): 0.15-0.25%, silicon (Si): 0.05-0.5%, manganese (Mn): 0.15% or less (excluding 0%), phosphorus (P): 0.03% or less (0%) (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), aluminum (Al): 0.05% or less (excluding 0%), including the balance Fe and other inevitable impurities, and the following equations 1 and 2 Reheating the satisfactory steel slab at 1050-1180°C;

Rough rolling the reheated steel slab at 950 to 1050°C; and

Obtaining a hot-rolled steel sheet with a thickness of 40 mm or less by hot-rolling the rough-rolled slab at 800 to 950 ° C.;

It provides a method for manufacturing low manganese steel, including.

[Relational Expression 1]

[C]/[Mn] > 1.5

(In Equation 1 above, [C] and [Mn] each represent the weight percent content of the elements in parentheses.)

[Relational Expression 2]

[Cr] + [Mo] + [Cu] + [Ni] < 0.05

(In Equation 2 above, [Cr], [Mo], [Cu], and [Ni] each represent the weight percent content of the elements in parentheses.)

During the hot rolling, descaling can be performed to minimize the thickness of the surface scale layer by spraying high-pressure water of 100 bar or more on the surface of the steel sheet.

According to one aspect of the present invention, while meeting the yield strength (YS) of 235 MPa or more and the tensile strength (TS) of 400 MPa or more, the manufacturing cost can be innovatively reduced by adding a very low manganese content (can be reduced by more than 50% compared to the existing product). ) General structural thick plate steel and its manufacturing method can be provided.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Figure 1 shows a photograph taken with an optical microscope of the cross section of a steel material obtained from Inventive Example 7 of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

Meanwhile, the terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary. Additionally, the meaning of “including” used in the specification is to specify a configuration and not to exclude the presence or addition of another configuration.

The present inventors have conducted extensive research and experiments in an attempt to reduce innovative manufacturing costs on general structural steel with a yield strength of 235 MPa or more and a tensile strength of 400 MPa or more that are widely used around the world. As a result, by not intentionally adding manganese (Mn), one of the five major elements of steel, or limiting it as much as possible, while controlling the amount of C and Si added, it is possible to meet the guarantee items within international standards such as JIS and ASTM. was confirmed, and the present invention was completed.

The manufacturing cost-saving general structural thick plate low manganese steel according to the present invention has, in weight percent, carbon (C): 0.15 to 0.25%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 0.15% or less ( 0% excluded), Phosphorus (P): 0.03% or less (0% excluded), Sulfur (S): 0.01% or less (0% excluded), Aluminum (Al): 0.05% or less (0% excluded) , the balance contains Fe and other inevitable impurities.

Hereinafter, the reason for adding components and limiting the content of the low manganese steel according to the present invention will be explained in detail. At this time, when indicating the content of each element in this specification, unless otherwise specified, it indicates weight%.

Carbon (C): 0.15-0.25%

Carbon (C) is the most effective element that can secure the strength of steel, and must be added in an appropriate amount to secure strength above a certain level without a separate cooling process after hot rolling. In order to sufficiently secure the above-mentioned effects, it is desirable to add C at 0.15% or more. However, if the C content exceeds 0.25%, not only may the toughness of the steel deteriorate excessively, but there is a problem that cracks may occur during reheating. Therefore, in the present invention, it is preferable to control the C content to 0.15 to 0.25%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the C content may be 0.21%, 0.215%, or the upper limit of the C content may be 0.245%.

Silicon (Si): 0.05~0.5%

Silicon (Si) is effective in deoxidation during the steelmaking process, but is a major element that combines with oxygen at high temperatures to create scale on the surface of steel. In addition to the deoxidation effect, it is preferable to add 0.05% or more of Si to control scale generated during heating and hot rolling. However, if the Si content exceeds 0.3%, not only will the surface scale of the final product become excessively thick, but it may also cause defects such as indentation flaws, which is undesirable. Therefore, in the present invention, it is preferable to control the Si content to 0.05 to 0.5%. Meanwhile, in terms of further improving the above-mentioned effect, more preferably, the lower limit of the Si content may be 0.10%, or the upper limit of the Si content may be 0.30%.

Manganese (Mn): 0.15% or less (excluding 0%)

Manganese (Mn) is a representative hardenability element that suppresses the formation of ferrite and improves the strength of steel by effectively increasing hardenability by lowering the Ar3 temperature. Mn preferentially combines with S in molten steel to suppress the formation of iron pyrite (FeS ₂ ), which makes grain boundaries vulnerable during hot rolling. However, since manganese (Mn) accounts for a significant portion of the manufacturing cost of general structural steel, adding more than necessary reduces the price competitiveness of the product. Therefore, in the present invention, in order to secure a tensile strength of 400 MPa or more and dramatically reduce manufacturing costs, the Mn content is controlled to 0.15% or less. However, in terms of further improving the above-mentioned effect, preferably, the upper limit of the Mn content may be 0.134%. Meanwhile, the present invention is a technology that intentionally does not add Mn, one of the five major elements of steel, or limits it as much as possible in order to reduce manufacturing costs. In the present invention, the lower the Mn content, the more desirable it is, so the lower limit is set separately. It is not limited. However, considering the fact that Mn is inevitably present even if it is not intentionally added, the lower limit of the Mn content excludes 0% (i.e., exceeds 0%), and more preferably, the lower limit of the Mn content is 0.03%. You can.

Phosphorus (P): 0.03% or less (excluding 0%)

Phosphorus (P) is an element that is inevitably contained in steel, and is an element that inhibits the toughness of steel. Therefore, in order to prevent deterioration of the toughness of steel, it is desirable to control the content of P as low as possible to 0.03% or less, but excluding 0% in consideration of the unavoidable content level. Meanwhile, to further improve the above-described effect, the upper limit of the P content may be 0.015%.

Sulfur (S): 0.01% or less (excluding 0%)

Sulfur (S) is an element that inhibits the toughness of steel by forming MnS inclusions in steel. Therefore, it is desirable to control the S content as low as possible to 0.01% or less, but considering the unavoidable content level, 0% is excluded. Meanwhile, in terms of further improving the above-mentioned effect, preferably, the upper limit of the S content may be 0.005%.

Aluminum (Al): 0.05% or less (excluding 0%)

Aluminum (Al) is a deoxidizing agent for steel and is an effective element in lowering the oxygen content in molten steel. If the Al content exceeds 0.05%, it is undesirable because the cleanliness of the steel is impaired. Therefore, in the present invention, it is preferable to control the Al content to 0.05% or less, and 0% is excluded in consideration of the increase in load and manufacturing cost during the steelmaking process. Meanwhile, in terms of further improving the above-mentioned effect, preferably, the upper limit of the Al content may be 0.03%, or the lower limit of the Al content may be 0.01%.

Meanwhile, in the low manganese steel according to one aspect of the present invention, it is required to control the contents of C and Mn to satisfy the following relational equation 1 in order to secure strength that can guarantee specifications while reducing manufacturing costs.

[Relationship 1]

[C]/[Mn] > 1.5

(In Equation 1 above, [C] and [Mn] each represent the weight percent content of the elements in parentheses.)

If the value of [C]/[Mn] defined in relational equation 1 is 1.5 or less, the Mn content is relatively large, which may be advantageous in securing strength, but as a result, the manufacturing cost increases and price competitiveness is weakened. Accordingly, in the present invention, it was confirmed that by controlling the value of [C]/[Mn] to exceed 1.5, the desired level of strength can be secured and the manufacturing cost can be dramatically reduced. Meanwhile, in terms of improving the above-described effects, the value of [C]/[Mn] may preferably be 1.6 or more, and more preferably 1.61 or more.

In addition, in the low manganese steel according to one aspect of the present invention, the upper limit values of alloy elements of Cr, Mo, Cu, and Ni need to be managed so as to satisfy the following relational equation 2 in order to reduce manufacturing costs.

[Relational Expression 2]

[Cr] + [Mo] + [Cu] + [Ni] < 0.05

(In Equation 2 above, [Cr], [Mo], [Cu], and [Ni] each represent the weight percent content of the elements in parentheses.)

If the value of [Cr] + [Mo] + [Cu] + [Ni] defined in equation 2 above is 0.05 or more, it can be determined that the corresponding element was intentionally added to the molten steel. In this case, a value much larger than the actual standard guarantee lower limit can be obtained, which may be advantageous in securing strength, but there is a problem in that the elongation is less than the target value or the manufacturing cost is increased as a result, resulting in poor price competitiveness. Meanwhile, in terms of improving the above-described effect, more preferably, the value of [Cr] + [Mo] + [Cu] + [Ni] can be set to 0.045 or less, and most preferably can be set to 0.042 or less. there is.

Meanwhile, although not particularly limited, according to one aspect of the present invention, considering that at least one of Cr, Mo, Cu and Ni is inevitably included, at least one of Cr, Mo, Cu and Ni is 0 It may be included in excess of %. Or, according to another aspect, Cr, Mo, Cu, and Ni may all exist in amounts exceeding 0% in the steel material.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be mixed due to raw materials or environmental variables, so this cannot be ruled out. Since these impurities are known to anyone skilled in the normal steel manufacturing process, all of them are not specifically mentioned in this specification.

Meanwhile, according to one aspect of the present invention, the microstructure of the above-described low manganese steel is preferably composed of a dual phase structure of ferrite and pearlite. By meeting the above-mentioned microstructure, it is possible to obtain C-Mn-based as-rolled steel with a yield strength (YS) of 235 MPa or more and a tensile strength (TS) of 400 MPa or more.

Meanwhile, although not particularly limited, according to one aspect of the present invention, the low manganese steel may include ferrite: 70 to 90% and pearlite: 10 to 30% as a microstructure and area percent. Among the microstructures, if ferrite is less than 70%, pearlite increases relatively according to the lever rule, which is advantageous in increasing strength, but may cause problems with ductility or deterioration. On the other hand, if ferrite exceeds 90% of the microstructure, ductility is good, but yield strength and tensile strength may not reach the target lower limit. In addition, in the microstructure, if pearlite is less than 10%, the ferrite fraction becomes too large according to the lever rule described above, which may be advantageous for ductility, but may cause a problem in which securing the target strength is difficult. On the other hand, if pearlite exceeds 30% of the microstructure, strength increases, but ductility significantly decreases, which may cause problems such as bending and deterioration of processability.

Meanwhile, although not particularly limited, according to one aspect of the present invention, the average grain size of the ferrite may be 30 to 50 μm. If the average grain size of the ferrite is less than 30㎛, it is advantageous to increase strength, but ductility may deteriorate. On the other hand, if the average grain size of the ferrite exceeds 50㎛, ductility is excellent, but the yield strength may not reach the target lower limit.

Meanwhile, as a low-cost general structural steel, the above-mentioned low manganese steel is limited to thick plates (hot rolled steel plates) with a thickness of 40 mm or less. Heavy plate steel for general structures is used in industrial sites in a wide variety of thicknesses, from 6 mm to extremely thick materials exceeding 100 mm. At this time, in order for the manufacturer to match the thickness of the final product, the slab is hot-rolled at an appropriate temperature. Due to the nature of inexpensive general structural steel, it is air-cooled after hot rolling without separate water cooling, so the physical properties such as strength of the product are determined only by the composition and rolling amount. becomes dependent on In other words, when slabs of the same composition are hot rolled at the same temperature, the thinner the thickness, the relatively greater amount of strain accumulated inside the material, increasing the strength, and vice versa, the strength decreases.

Therefore, in the present invention, it is possible to secure the desired level of strength in consideration of the total amount of deformation accumulated inside the steel, and as a low-cost general structural steel, the thickness of the low manganese steel is limited to 40 mm or less. Meanwhile, there is no separate limitation on the minimum thickness, but as an example considering the fact that it is used as a general structural thick plate, the lower limit of the thickness may be 6 mm.

In addition, according to one aspect of the present invention, the low manganese steel has a yield strength (YS) of 235 MPa or more (preferably 235 MPa or more and 300 MPa or less), and/or a tensile strength (TS) of 400 MPa or more (preferably is 400 MPa or more and 500 MPa or less), and/or the elongation (El) is 30% or more (preferably 30% or more and 40% or less, more preferably 30% or more and 35% or less). By meeting this requirement, it can be used as a general structural steel in various industrial fields, so it is not particularly limited, but can be used particularly suitably as a floor plate.

Hereinafter, the method for manufacturing low manganese steel according to the present invention will be described in detail. However, the manufacturing method of low manganese steel according to the present invention does not necessarily mean that it must be manufactured by the following manufacturing method.

Reheating stages of slabs

First, a steel slab meeting the above-mentioned composition is reheated at 1050-1180°C. At this time, in the composition of the steel slab, the explanation for the composition of the steel described above applies equally to the reason for adding each component and the reason for limiting the content.

On the other hand, if the reheating temperature of the steel slab is less than 1050°C, the casting structure undergoes reverse transformation and complete austenitization cannot be achieved. On the other hand, if the reheating temperature of the steel slab exceeds 1180°C, there is a risk that the austenite grains will become excessively coarse and an uneven structure may be formed. In addition, as scale grows excessively in the heating furnace, there is a risk that it may not be completely removed before hot rolling. Therefore, in the present invention, it is preferable that the reheating temperature of the steel slab is performed at 1050 to 1180°C. In terms of further improving the above-mentioned effect, more preferably, the lower limit of the reheating temperature may be 1100°C, and the upper limit of the reheating temperature may be 1150°C.

Rough rolling stage

Subsequently, the reheated steel slab is subjected to rough rolling at 950 to 1050°C. If the rough rolling temperature is less than 950°C, the rolling load increases and the pressure is relatively weak, so that the strain is not sufficiently transmitted to the center of the slab in the thickness direction, and there is a risk that defects such as voids may not be removed. On the other hand, if the rough rolling temperature exceeds 1050°C, recrystallization occurs simultaneously with rolling and then grains grow, and there is a risk that the initial austenite grains become excessively coarse. More preferably, in terms of improving the above-mentioned effect, the lower limit of the rough rolling temperature may be 970°C, or the upper limit of the rough rolling temperature may be 1030°C.

hot rolling stage

Next, the rough-rolled slab is hot-rolled at a temperature range of 800 to 950° C. to manufacture a hot-rolled steel sheet with a thickness of 40 mm or less. If the finishing temperature of the hot rolling is less than 800°C, there is a risk that the microstructure may become non-homogeneous due to dual-phase reverse rolling, and the rolling shape is deteriorated, leading to a problem of poor sheetability. From a strength standpoint, if the rolling temperature is too low, the energy value accumulated inside the material increases, which has the effect of refining the particles, but the productivity is relatively low, making it unsuitable for mass production. On the other hand, when the finishing temperature of the hot rolling exceeds 950°C, there is an advantage in that rolling productivity increases and plateability improves, but the driving force is large enough to allow ferrite grains to grow while the sheet is cooled to room temperature after the end of rolling, resulting in yield strength. And there is a risk that the tensile strength may become inferior. More preferably, in terms of improving the above-mentioned effect, the lower limit of the finishing temperature of the hot rolling may be 820°C, or the upper limit of the finishing temperature of the hot rolling may be 941°C.

Descaling step

Meanwhile, in the present invention, de-scaling is performed to minimize the surface scale layer thickness by spraying high-pressure water on the surface of the steel sheet during the above-described hot rolling. This is a process applied during hot rolling of thick steel plates. Although high-pressure water is sprayed directly on the surface of the steel plate, it is different from accelerated cooling, which intentionally cools the center of the material, and is manufactured as-rolled. There is no problem in applying it to general structural steel. Since the greater the pressure of the sprayed water, the better the scale removal effect, the pressure of the high-pressure water applied to descaling in the present invention is not particularly limited. However, if the lower limit of the pressure of the sprayed water is at least 100 bar or more, there will be no problem in removing scale generated within the rolling temperature range described above.

cooling step

Meanwhile, although not particularly limited, after the hot rolling, air cooling may be performed on the obtained hot rolled steel sheet, and at this time, the average cooling rate may be 1 to 55°C/s.

The hot rolled steel sheet of the present invention manufactured as described above may be a thick steel sheet having a thickness of 40 mm or less. This is because, in the case of thick steel plates exceeding 40 mm in thickness, it may be difficult to secure a yield strength of 235 MPa or more through ordinary hot rolling alone.

Meanwhile, in the case of steel sheets with a thickness of less than 40 mm, the amount of reduction accumulated to the final thickness is sufficiently large without being greatly affected by the thickness of the slab used during hot rolling, so austenite can be effectively refined in a high temperature region. , As a result, it is possible to secure the target room temperature yield strength and tensile strength. Therefore, in the present invention, taking this into consideration, it is more preferable to set the thickness of the low-cost general structural thick steel plate to a range of 40 mm or less.

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

(Example)

After preparing a steel slab having the alloy composition shown in Table 1 below, steel slab heating, rough rolling, and hot rolling were performed on the steel slab under the conditions shown in Table 2 below. Subsequently, the obtained hot-rolled steel sheet was cooled to 5°C/s or less to produce a final thick steel sheet. Meanwhile, during the hot rolling, high-pressure water of 200 bar was sprayed on the surface of the steel sheet to descale it.

Yield, tensile strength, and elongation were measured for the thick steel plate, and the results are shown in Table 3 below. At this time, the tensile test is the average of the values obtained by processing a plate-shaped specimen with a gage length of 50 mm and testing it three times at room temperature.

In addition, in order to confirm the effect of cost reduction, the alloy cost for each component used in each comparative example and invention example was calculated in won/ton units. At this time, when the alloy cost for Invention Examples 1 to 3 using Invention Steel 1 was defined as '1', the relative alloy cost compared to Invention Steel 1 was calculated and shown in Table 3 below.

division	Alloy composition [weight%] (remaining Fe and impurities)										Relationship 1	Relation 2
	C	Si	Mn	P	S	Al	Cr	Ni	Cu	Mo
Comparison lecture 1	0.160	0.14	0.112	0.016	0.005	0.03	-	-	-	-	1.43	0.000
Comparison lecture 2	0.214	0.18	0.296	0.014	0.007	0.02	0.32	0.008	0.007	0.004	0.70	0.339
Comparison lecture 3	0.191	0.25	0.480	0.014	0.003	0.03	0.03	0.03	0.02	0.004	0.40	0.084
Comparison lecture 4	0.262	1.32	0.160	0.012	0.002	0.02	0.02	0.01	0.01	0.003	1.64	0.043
Comparison lecture 5	0.451	0.03	0.129	0.008	0.001	0.03	0.01	-	-	0.027	3.50	0.037
Comparative lecture 6	0.084	0.25	0.055	0.015	0.004	0.03	-	0.01	0.01	0.002	1.53	0.022
Comparison lecture 7	0.233	0.17	0.086	0.009	0.003	0.02	1.16	0.32	-	0.156	2.71	1.636
Invention lecture 1	0.216	0.23	0.134	0.007	0.004	0.02	0.02	0.01	0.01	0.002	1.61	0.042
Invention lecture 2	0.243	0.14	0.093	0.013	0.004	0.02	0.01	0.01	0.01	0.002	2.61	0.032
Invention lecture 3	0.225	0.25	0.065	0.011	0.005	0.03	0.01	0.01	0.01	0.002	3.46	0.032

division	Steel grade No.	Slab reheat temperature [℃]	Rough rolling temperature [℃]	Hot rolling finishing temperature [℃]	thickness [mm]
Comparative Example 1	Comparison lecture 1	1146	968	912	20
Comparative Example 2		1127	995	898	15
Comparative Example 3		1133	876	789	30
Comparative Example 4	Comparison lecture 2	1145	1043	900	16
Comparative Example 5		1160	967	915	19
Comparative Example 6		1124	985	932	25
Comparative Example 7	Comparison lecture 3	1143	1006	887	12
Comparative Example 8		1155	988	913	25
Comparative Example 9		1129	974	926	40
Invention Example 1	Invention lecture 1	1130	1029	899	40
Invention Example 2		1127	1011	897	25
Invention Example 3		1136	1025	906	19
Invention Example 4	Invention lecture 2	1129	992	893	10
Comparative Example 10		1133	1048	982	40
Invention Example 5		1125	1023	941	25
Invention Example 6	Invention lecture 3	1126	1012	940	25
Invention Example 7		1123	987	906	40
Comparative Example 11		1134	961	920	80
Comparative Example 12	Comparison lecture 4	1117	996	912	25
Comparative Example 13	Comparison lecture 5	1141	1014	931	30
Comparative Example 14	Comparative lecture 6	1130	1017	942	25
Comparative Example 15	Comparison lecture 7	1127	1022	925	25

division	Steel grade No.	yield strength [MPa]	tensile strength [MPa]	elongation [%]	cost comparison	note
Comparative Example 1	Comparison lecture 1	229	396	31	0.63	YS, TS insufficient
Comparative Example 2		227	394	32		YS, TS insufficient
Comparative Example 3		232	400	32		YS is below
Comparative Example 4	Comparison lecture 2	293	437	31	2.42	High manufacturing cost
Comparative Example 5		290	435	32
Comparative Example 6		288	434	32
Comparative Example 7	Comparison lecture 3	277	446	32	2.08
Comparative Example 8		278	442	32
Comparative Example 9		275	439	32
Invention Example 1	Invention lecture 1	268	431	32	One	Cost reduction possible
Invention Example 2		263	428	32
Invention Example 3		259	422	33
Invention Example 4	Invention lecture 2	264	430	32	0.75
Comparative Example 10		228	411	33		YS is below
Invention Example 5		257	427	32		Cost reduction possible
Invention Example 6	Invention lecture 3	262	429	32	0.94
Invention Example 7		260	429	32
Comparative Example 11		204	406	34		YS is below
Comparative Example 12	Comparison lecture 4	381	563	27	2.66	Below EL
Comparative Example 13	Comparison lecture 5	305	524	28	1.03	Below EL
Comparative Example 14	Comparative lecture 6	223	376	36	0.88	YS, TS insufficient
Comparative Example 15	Comparison lecture 7	325	563	23	11.56	Below EL

As shown in Tables 1 to 3, it was confirmed that the target yield strength, tensile strength and elongation could be secured in the case of invention examples 1 to 7 that satisfied the alloy composition and manufacturing conditions proposed in the present invention.

In particular, a microstructure photograph of the cross section of the steel material obtained from Inventive Example 7 taken with an optical microscope is shown in Figure 1. In the case of Inventive Example 7, unlike the comparative examples described later, both yield strength and tensile strength were satisfied, and not only was the Mn content extremely low at less than 0.10%, but the total content of Cr, Mo, Cu, and Ni was also the lowest. , it was confirmed that manufacturing costs could be dramatically reduced.

On the other hand, in invention examples 1 to 7, in terms of area%, it has a microstructure containing 70 to 90% ferrite and 10 to 30% pearlite, and the average grain size of ferrite satisfies the range of 30 to 50 μm. Confirmed. In addition, in the case of invention examples 1 to 7, it was confirmed that compared to the comparative examples, it was possible to secure the desired yield strength, tensile strength and elongation, and the manufacturing cost was very low in the range of 0.75 to 1, resulting in a cost reduction effect. .

On the other hand, in Comparative Examples 1 to 9, where the alloy composition was outside the scope of the present invention, the desired properties could not be obtained. Specifically, in Comparative Examples 1 to 3, one or more characteristics of yield strength and tensile strength fell below the standard value. In addition, Comparative Examples 4 to 9 required excessive amounts of Cr and Mn to secure the desired yield strength and tensile strength, and the manufacturing cost was too high, ranging from 2.08 to 2.42.

In addition, although the alloy composition was within the scope of the present invention, the yield strength of Comparative Examples 10 to 11 in which the manufacturing process conditions were outside the scope of the present invention also fell below the standard value. Specifically, Comparative Example 10 was a case where the finishing temperature of hot rolling was excessively high, and strength decreased due to coarse grain size. In addition, in Comparative Example 11, the thickness of the final product was too thick, the strain energy accumulated inside the material was too low, and the particle refinement effect was insufficient, resulting in insufficient strength.

In addition, Comparative Examples 12 and 13 did not meet the carbon content and/or manganese content specified in the present invention and did not satisfy relational expression 2, so it was confirmed that cost reduction was not possible due to insufficient elongation and high manufacturing cost. did.

Even though Comparative Example 14 satisfies the relational equations 1 and 2 and the manufacturing conditions specified in the present invention, the carbon content suggested in the present invention is insufficient, and the yield strength and tensile strength fall below the standard values.

In Comparative Example 15, it was confirmed that even though the alloy composition and manufacturing conditions specified in the present invention were met, Equation 2 was not satisfied, so the elongation was below the standard value, and the manufacturing cost was also very high, making it impossible to reduce costs.

As described above, the detailed description of the present invention has described preferred embodiments of the present invention, but those skilled in the art can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described later, but also by their equivalents.

Claims (7)

1. In weight percent, carbon (C): 0.15-0.25%, silicon (Si): 0.05-0.5%, manganese (Mn): 0.15% or less (excluding 0%), phosphorus (P): 0.03% or less (0%) (excluding), sulfur (S): 0.01% or less (excluding 0%), aluminum (Al): 0.05% or less (excluding 0%), including the balance Fe and other inevitable impurities,

   It is composed of a dual phase structure of ferrite and pearlite,

   A low manganese steel that satisfies the following equations 1 and 2 and has a thickness of 40 mm or less.

   [Relationship 1]

   [C]/[Mn] > 1.5

   (In Equation 1 above, [C] and [Mn] each represent the weight percent content of the elements in parentheses.)

   [Relational Expression 2]

   [Cr] + [Mo] + [Cu] + [Ni] < 0.05

   (In Equation 2 above, [Cr], [Mo], [Cu], and [Ni] each represent the weight percent content of the elements in parentheses.)
2. In claim 1,

   A low manganese steel containing, as a microstructure, by area %, ferrite: 70-90%, and pearlite: 10-30%.
3. In claim 1,

   Low manganese steel with a yield strength of 235 MPa or more and a tensile strength of 400 MPa or more.
4. In claim 3,

   Low manganese steel with an elongation of 30% or more.
5. In claim 1,

   A low manganese steel where the average grain size of the ferrite is 30 to 50 μm.
6. In weight percent, carbon (C): 0.15-0.25%, silicon (Si): 0.05-0.5%, manganese (Mn): 0.15% or less (excluding 0%), phosphorus (P): 0.03% or less (0%) (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), aluminum (Al): 0.05% or less (excluding 0%), including the balance Fe and other inevitable impurities, and the following equations 1 and 2 Reheating the satisfactory steel slab at 1050-1180°C;

   Rough rolling the reheated steel slab at 950 to 1050°C; and

   Obtaining a hot-rolled steel sheet with a thickness of 40 mm or less by hot-rolling the rough-rolled slab at 800 to 950 ° C.;

   Method for manufacturing low manganese steel, including.

   [Relational Expression 1]

   [C]/[Mn] > 1.5

   (In the above relational equation 1, [C] and [Mn] each represent the weight percent content of the elements in parentheses.)

   [Relational Expression 2]

   [Cr] + [Mo] + [Cu] + [Ni] < 0.05

   (In Equation 2 above, [Cr], [Mo], [Cu], and [Ni] each represent the weight percent content of the elements in parentheses.)
7. In claim 6,

   A method of manufacturing low manganese steel, in which descaling is performed to minimize the thickness of the surface scale layer by spraying high-pressure water of 100 bar or more on the surface of the steel sheet during the hot rolling.

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