KR101298699B1 - High strength steel and method for manufacturing the same - Google Patents

Abstract

The present invention is 0.14 wt% to 0.16 wt% of carbon, 0.13 wt% to 0.18 wt% of silicon, 1.15 wt% to 1.20 wt% of manganese, 0.03 wt% or less of phosphorus, 0.015 wt% or less of sulfur, 0.05 wt% to 0.15 wt% of nickel High strength comprising, up to 0.15% chromium, 0.35% to 0.45% copper, 0.07% to 0.09% vanadium, 0.02% to 0.03% niobium, 0.010% to 0.013% nitrogen, and balance iron It relates to a steel material and a method of manufacturing the same.
According to the present invention, it is possible to reduce the cost of steel by lowering the content of silicon and manganese, and not including titanium and aluminum, and 20% tensile strength of 600MPa or more through optimization of rolling conditions such as control of the component system, heating conditions and cooling conditions. It is possible to provide a high strength steel having an elongation above.

Description

High strength steel and method for manufacturing the same

The present invention relates to a high strength steel and a method for manufacturing the same, and more particularly, to a high strength steel exhibiting high tensile strength and high elongation characteristics and a method for manufacturing the same.

As the facilities of houses, plants, etc. become higher, steel materials are required to have high strength. In addition, the total weight of the steel material during the construction of the facility is continuously decreasing, and accordingly, there is a greater demand for increasing the strength of the steel itself. In addition, there is a demand for a steel that is resistant to impact that exhibits high elongation along with the strength of the steel.

The technical structure described above is a background technique for assisting the understanding of the present invention, and does not mean the prior art widely known in the technical field to which the present invention belongs.

It is an object of the present invention to provide a high-strength steel and a method of manufacturing the same, which can realize high tensile strength and high elongation while reducing material costs.

High-strength steel according to an aspect of the present invention is 0.14% to 0.16% by weight of carbon, 0.13% to 0.18% by weight of silicon, 1.15% to 1.20% by weight of manganese, 0.03% by weight or less of phosphorus, 0.015% or less of sulfur, nickel 0.05 wt% to 0.15 wt%, chromium 0.15 wt% or less, copper 0.35 wt% to 0.45 wt%, vanadium 0.07 wt% to 0.09 wt%, niobium 0.02 wt% to 0.03 wt%, nitrogen 0.010 wt% to 0.013 wt% and Contains the balance of iron.

The steel may comprise 0 wt% titanium.

The steel may include 0 wt% aluminum.

The steel may exhibit a tensile strength of 600 MPa or more and an elongation of 20% or more.

High-strength steel manufacturing method according to another aspect of the present invention is 0.14% to 0.16% by weight of carbon, 0.13% to 0.18% by weight of silicon, 1.15% to 1.20% by weight of manganese, 0.03% by weight or less of sulfur, 0.015% by weight or less of sulfur , 0.05 wt% to 0.15 wt% nickel, 0.15 wt% or less chromium, 0.35 wt% to 0.45 wt% copper, 0.07 wt% to 0.09 wt% vanadium, 0.02 wt% to 0.03 wt% niobium, 0.010 wt% to 0.013 wt% nitrogen Reheating the material comprising% and the balance of iron at 1,150 ° C. to 1,250 ° C .; Extracting the material from a heating furnace and rolling; And cooling the material such that the temperature of the material is 650 ° C to 700 ° C.

In the step of reheating the material at 1,150 ° C to 1,250 ° C, the heating time may be 1 hour to 3 hours.

In the step of rolling after extracting the material from the heating furnace, the rolling end temperature may be 1,000 ℃ to 1,050 ℃.

In the step of extracting the material from the heating furnace and rolling, the total reduction ratio of the material may be 65% to 80%.

In the step of cooling the material such that the temperature of the material is 650 ° C to 700 ° C, the cooling may be performed by water cooling.

According to the present invention, it is possible to reduce the cost of steel by lowering the content of silicon and manganese, and not including titanium and aluminum, and 20% tensile strength of 600MPa or more through optimization of rolling conditions such as control of the component system, heating conditions and cooling conditions. Steel materials having the above elongation can be provided.

1 is a process flow diagram of a method of manufacturing high strength steel according to an embodiment of the present invention.
2 is a perspective view of a hot rolling apparatus according to an embodiment of the present invention.

Hereinafter, an embodiment of a high strength steel and a method of manufacturing the same according to the present invention. The terms to be described below are terms defined in consideration of functions in the present invention, and may be changed according to a user's or operator's intention or custom. Therefore, definitions of these terms should be made based on the contents throughout this specification.

The present invention is 0.14 wt% to 0.16 wt% of carbon, 0.13 wt% to 0.18 wt% of silicon, 1.15 wt% to 1.20 wt% of manganese, 0.03 wt% or less of phosphorus, 0.015 wt% or less of sulfur, 0.05 wt% to 0.15 wt% of nickel High strength comprising, up to 0.15% chromium, 0.35% to 0.45% copper, 0.07% to 0.09% vanadium, 0.02% to 0.03% niobium, 0.010% to 0.013% nitrogen, and balance iron It relates to a steel material and a method of manufacturing the same.

The present invention reduces the amount of silicon and manganese, and does not include titanium and aluminum, it is possible to reduce the cost of steel, by adjusting the composition of the steel and control the cooling conditions, such as tensile strength of more than 600MPa (TS: Tensile Strength ), It is possible to provide a high strength low alloy steel exhibiting an elongation (EL) of 20% or more.

Hereinafter, the limited range of the component system which comprises the high strength steel material of this invention, and the reason for limitation are demonstrated.

(1) Carbon (C): 0.14% to 0.16% by weight

Carbon may be added to ensure strength and toughness of the structural steel. Increasing the carbon content lowers the A ₁ and A ₃ transformation temperature and increases the quenching hardness to improve the quenching hardness, but increases the possibility of deformation during quenching. Carbon combines with elements such as iron, chromium, molybdenum and vanadium to form carbides to improve strength and hardness. When the content of carbon is less than 0.14% by weight, the effect of improving strength and hardness is insignificant, and when the content of carbon is more than 0.16% by weight, the toughness is worsened and the deformation is more likely to occur during quenching.

(2) Silicon (Si): 0.13% to 0.18% by weight

Silicon is a ferrite stabilizing element and an element that improves the activity of carbon. During heat treatment, it helps the carbon migration in the cementite of pearlite structure to reduce the carbon content in the structure to improve toughness and ductility. Silicon may also be added as a deoxidizer to remove oxygen in the steel during the steelmaking process. In addition, silicon can increase the strength by the solid solution strengthening effect. The higher the content, the higher the strength, but if it is added less than 0.13% by weight, deoxidation effect may occur, and when it exceeds 0.18% by weight, the toughness may be deteriorated, so it is preferably included in the content range. That is, the present invention can realize a tensile strength of 600MPa or more while having a low content of silicon.

(3) Manganese (Mn): 1.15 wt% to 1.20 wt%

Manganese can be added as an element to enhance strength by solid solution strengthening. In addition, the rolling region can be enlarged by lowering the A ₃ temperature with the austenite stabilizing element, and the grains by rolling can be refined to improve strength and toughness. When added to less than 1.15% by weight manganese may reduce the effect of contributing to the improvement of strength, and when added in excess of 1.20% by weight can be weakened the toughness of the weld when quenching cracks or steel used. Therefore, manganese is preferably added 1.15% by weight to 1.20% by weight. That is, the present invention can realize a tensile strength of 600MPa or more while having a low content of manganese.

(4) Phosphorus (P): 0.03 wt% or less

Phosphorus is an ingredient that is excellent for excellent solid solution effect and corrosion resistance. When added in excess of 0.03% by weight can combine with iron to form Fe ₃ P. This compound segregates at the grain boundaries, does not homogenize even when annealing, reduces the impact resistance and promotes tempering brittleness. Therefore, it is preferably included in 0.03% by weight or less.

(5) Sulfur (S): 0.015% by weight or less

Sulfur generally combines with manganese, zinc, titanium and molybdenum to improve the machinability of the steel. When it exceeds 0.015% by weight, it may cause the formation of sulfide-based inclusions such as manganese sulfide (MnS) to cause cracking during hot or cold rolling, and may act as an element to lower the impact value by lowering the Charpy impact absorption energy. Therefore, it is preferably included 0.15% by weight or less.

(6) Nickel (Ni): 0.05% to 0.15% by weight

Nickel can be refined and dissolved in austenite and ferrite to strengthen the matrix. Thereby, the strength and toughness of steel materials can be improved. Nickel is effective in improving toughness when added at 0.05 wt% or more, but is preferably included within 0.15 wt% because it is an expensive element and may cause brittleness when added in excess. More preferably about 0.10% by weight.

(7) Chromium (Cr): 0.15% by weight or less

Chromium generally improves the corrosion, oxidation and emulsification of steels. However, when added excessively, a nonmagnetic weak phase called σ phase may appear, so it is preferably included at 0.15% by weight or less.

(8) Copper (Cu): 0.35% to 0.45% by weight

Copper is a trap element that can remain in steel and is an impurity that cannot be completely removed in the steelmaking process. Copper is an austenite stabilizing element that lowers the transformation temperature to increase quenchability. In addition, it is dissolved in ferrite may increase the strength and hardness, but may also act to lower the elongation. When the addition of less than 0.35% by weight, the strength and hardness synergistic effect is insignificant, and the addition of more than 0.45% by weight may reduce the elongation and surface quality of the steel is preferably 0.35% to 0.45% by weight.

(9) Vanadium (V): 0.07% to 0.09% by weight

Vanadium may combine with carbon during cooling to form VC, contributing to strengthening precipitation and suppressing grain growth. Vanadium increases temper resistance, improving overall mechanical properties such as strength and toughness. When a small amount is added, the effect of improving the mechanical properties is insignificant, and when the excessive amount is added, the curing ability is reduced, so that it is preferably included 0.07% to 0.09% by weight.

(10) Niobium (Nb): 0.02 wt% to 0.03 wt%

Niobium may be added as an element to precipitate in the form of NbC or NbCN to improve the strength of the base material. By suppressing grain growth during rolling to refine grains, it is possible to induce toughness improvement and precipitation strengthening effect after rolling cooling. When added below 0.02% by weight, the effect is negligible, and when added in excess of 0.03% by weight, brittle cracks can be induced, and it is preferable to add 0.02% to 0.03% by weight.

(11) Nitrogen (N): 0.010 wt% to 0.013 wt%

Nitrogen is an invasive element like carbon and has a fast diffusion rate in steel. Nitrogen increases the tensile strength and yield strength of steel, and forms nitrides in combination with other alloying elements, thereby contributing to the refinement of austenite grains. However, it is preferable that the excess nitrogen impairs the high temperature toughness, so it is included at 0.013% by weight or less, and the lower limit is preferably limited to 0.010% by weight because the control of the nitrogen content increases the steelmaking load.

The high strength steel of the present invention includes the above-mentioned components and includes iron (Fe) in the remainder. In addition, although elements that are inevitably included may be mixed, these elements are elements that are inevitably mixed in raw materials, materials, manufacturing facilities, and the like. In addition, the high strength steel according to the present invention does not include titanium and aluminum. On the other hand, while explaining the content of the component (alloy element) constituting the high-strength steel of the present invention, when expressed by weight% or less, the component may not be included but may be inevitably included in the limitation of the steelmaking process.

Titanium has a strong affinity with oxygen, nitrogen, carbon, sulfur and the like and is therefore commonly used as a deoxidizer, a denitrifier, and a tanning agent. In addition, titanium, like niobium, is a ferrite stabilizing element and an element that refines crystal grains. Aluminum may be added as a deoxidizer component and is effective in refining grains of steel by precipitating AlN, a nitride. In addition, it is effective in the prevention of high temperature oxidation and emulsification resistance, it is possible to perform the function of keeping the dissolved oxygen amount low by lowering the dissolved oxygen amount in the steel. The present invention can realize high tensile strength and high elongation without adding titanium and aluminum, and can reduce the cost of steel.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of a high-strength steel and a method for manufacturing the same. In this process, the thicknesses of the lines and the sizes of the components shown in the drawings may be exaggerated for clarity and convenience of explanation.

1 is a process flow diagram of a method of manufacturing a high strength steel according to an embodiment of the present invention, Figure 2 is a perspective view of a hot rolling apparatus according to an embodiment of the present invention.

Referring to Figure 1, first the material is (re) heated at 1,150 ℃ to 1,250 ℃ (S110). When the heating temperature is lower than 1,150 ° C the rolling load may be large, and when the heating temperature is higher than 1,250 ° C it may be difficult to secure the strength by coarsening austenite grains. In the present invention, the term 'material' refers to a material such as an ingot, slab, bar, strip, and the like. The shape of the material is not constant and its thickness, length, etc. may be changed during hot rolling.

The heating time of the furnace may be about 1 hour to 3 hours, preferably about 2 hours. Rolling load can be reduced in the above temperature range, and it is advantageous to secure strength by preventing coarsening of grains. The components segregated during casting by heating are reused and extracted in a heating furnace.

Next, the raw material extracted from the heating furnace is rolled (S120). There is no limitation on the total rolling reduction during rolling, but 65% to 80% is more advantageous for securing strength and improving elongation.

At this time, the temperature at the end of the rolling, that is, the rolling end temperature can be set to 1,000 ℃ to 1,050 ℃. That is, it is possible to suppress the generation of the load of the rolling mill by eliminating the control rolling in which the rolling is performed at less than 1,000 ℃, it is possible to implement a high-strength steel showing a high tensile strength in a general rolling equipment.

Next, the finished material is cooled to 650 ℃ to 700 ℃ (S130). By cooling, some of the supercooled structure such as pearlite, bainite, etc. are generated inside the steel having the ferrite structure, and the generated supercooled structure has a higher strength than the general ferrite-pearlite structure, thereby improving the strength. The temperature is based on the surface temperature of the material.

When the cooling end temperature is less than 650 ℃, bainite and martensite structure is formed to increase the strength, but it is confirmed that the elongation and low temperature impact value is sharply reduced. In addition, when cooling end temperature is higher than 700 degreeC, since cooling is complete | finished by temperature more than phase transformation temperature, it will have a ferrite-pearlite structure and intensity | strength will fall.

In addition, there is no restriction | limiting in the cooling method of a raw material. For example, water cooling, air cooling, and the like may be used, but are more effective in terms of quenching process time and subcooled tissue generation using water cooling. That is, it can be cooled by accelerated cooling, for example, it is preferable to quench the rolled material at a cooling rate of 5 ° C./sec to 50 ° C./sec and then air or slow cool.

The present invention is sufficient to include the rolling process, and is not limited to the process conditions such as the number of passes during the rolling, the number of scalers applied.

2, the rolling apparatus according to an embodiment of the present invention is a heating furnace 102, sizing press 104, rough rolling mill 106, edge heater 108, descaler 110, finishing mill ( 112, the runout table 114, the cooling unit 116, or the winder 118 may be included.

In general, the material S before being charged into the heating furnace 102 may be a slab transferred from a performance factory or a crushing plant, and may be referred to as a bar after rough rolling and a strip after finishing rolling.

The heating furnace 102 is a reheating furnace for hot rolling the material S, and heavy fuel oil, natural gas, coke gas, and the like may be used as a fuel used in the heating furnace 102. As described above, the heating temperature in the heating furnace 102 is preferably 1,150 ° C to 1,250 ° C, and is preferably heated for 1 to 3 hours. The heating furnace 102 may include a preheating zone, a heating zone, and a cracking zone along the advancing direction of the material S, and may further include a charging zone before the preheating zone. In the preheating zone, the material S is heated to a low temperature, in the heating zone, the heating temperature is raised to reach the target temperature, and in the cracking zone, the temperature is uniformly distributed in all parts of the material S. have. The charging zone or the preheating zone may play a role of preventing rupture, cracking, and cracking in the material S due to rapid temperature rise.

The sizing press 104 may be a width rolling mill which reduces the width variation in the length direction of the material S and rolls it to a predetermined width according to the demand of the final consumer.

Roughing mill (106, Roughing Mill) can be rolled to the appropriate thickness and width required in finishing rolling. The movement of the raw material S from the entry side to the exit side of the rough-rolling machine 106 or the movement of the raw material S from the exit side to the entrance side is called a pass, and these passes can be performed a plurality of times. By using the recuperation phenomenon, it is possible to set a waiting time for reducing the temperature gradient of the material (S).

The edge heater 108 may be installed to prevent a temperature drop of the edge portion of the material S, and the descaler 110 may remove the scale of the surface of the material S with high pressure water.

The finishing mill 112 is an apparatus which manufactures a steel plate in final shape, such as thickness and width required by a customer or a cold rolling process. The temperature at the end of rolling in the finishing mill 112 may be set to 1,000 ° C to 1,050 ° C, and may be set to a total reduction ratio of 65% to 80%.

The material S passed through the finishing mill 112 may be water cooled to 650 ° C. to 700 ° C. by the coolant from the cooling unit 116 while passing through the runout table 114, and then the winder 118. Can be wound by.

The rolling device described above is just one embodiment and some of the devices constituting the rolling device may be omitted, and other additional devices may be further included. For example, a descaler may be added before, after or inside the roughing mill. In another example, a descaler may be present in the furnace to remove scale created on the hot surface of the workpiece. As another example, it may further include an edger (Edger Mill) for equalizing the width deviation caused by the sizing press. In addition, it is attached for convenience of the name of the above-mentioned apparatus, and other names may be used.

< Example  And Comparative example >

Tensile strength, yield strength and elongation were evaluated by preparing the material of the embodiment having the above-described component system and the comparable component system, and the component system is shown in Table 1 below and the results of the characteristic evaluation in Table 2.

Specifically, the materials having the compositions of Examples 1 to 3, Comparative Example 1 and Comparative Example 2 were reheated at 1,200 ° C. for 2 hours. After reheating, the material was transferred directly to the rolling stand without waiting, and then 65% to 80% was rolled in the rolling mill, and a total number of passes of the rolling mill were applied. After rolling, water was cooled until the surface temperature of the raw material was about 650 ° C to 700 ° C.

C Si Mn P S Cu Ni Cr Mo Nb V N Example 1 0.15 0.16 1.18 0.03 0.01 0.40 0.10 0.10 0.02 0.02 0.08 0.01 Example 2 0.15 0.14 1.15 0.03 0.01 0.40 0.10 0.10 0.02 0.02 0.07 0.01 Example 3 0.15 0.18 1.20 0.03 0.01 0.40 0.10 0.10 0.02 0.02 0.09 0.01 Comparative Example 1 0.15 0.10 1.00 0.03 0.01 0.40 0.10 0.10 0.02 0.02 0.05 0.01 Comparative Example 2 0.15 0.23 1.30 0.03 0.01 0.40 0.10 0.10 0.02 0.02 0.10 0.01

Tensile Strength (MPa) Yield strength (MPa) Elongation (%) Example 1 604 466 21 Example 2 600 451 22 Example 3 621 473 20 Comparative Example 1 532 402 26 Comparative Example 2 642 489 16

As shown in Table 2, 0.14 wt% to 0.16 wt% carbon, 0.13 wt% to 0.18 wt% silicon, 1.15 wt% to 1.20 wt% manganese, 0.03 wt% or less phosphorus, 0.015 wt% or less sulfur, 0.05 wt% nickel % To 0.15% by weight, chromium 0.15% by weight or less, 0.35% to 0.45% copper, 0.07% to 0.09% vanadium, 0.02% to 0.03% niobium, 0.010% to 0.013% nitrogen, and the balance In Examples 1 to 3 including iron, a tensile strength of 600 MPa or more, an elongation of 20% or more, and a yield strength of 450 MPa or more are shown.

However, in Comparative Example 1 having a silicon content of 0.10% by weight, a manganese content of 1.00%, and a vanadium content of 0.05% by weight, the elongation is excellent, but the tensile strength is very low, and the silicon content is 0.23% by weight, manganese. In Comparative Example 2 having a content of 1.30% by weight and a vanadium content of 0.10% by weight, the tensile strength is excellent but the elongation is very low as 16%.

As such, the present invention can lower the silicon, manganese content, and do not include titanium and aluminum to reduce the material cost of the steel, tensile strength of 600MPa or more through optimization of the rolling conditions, such as control of the component system and heating conditions, cooling conditions, It is possible to provide a high strength structural steel exhibiting a yield strength of 450 MPa or more and an elongation of 20% or more.

Although described with reference to one embodiment of the present invention, this is only exemplary and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. In addition, the above-described rolling apparatus is only one embodiment and the technical spirit of the present invention can be applied to other rolling apparatus. Accordingly, the true scope of the present invention should be determined by the following claims.

102: heating furnace 104: sizing press
106: roughing mill 108: edge heater
110: descaler 112: finishing mill
114: runout table 116: cooling unit
118: winder

Claims (9)

0.14 wt% to 0.16 wt% carbon, 0.13 wt% to 0.18 wt% silicon, 1.15 wt% to 1.20 wt% manganese, 0.03 wt% phosphorus, 0.01 to 0.015 wt% sulfur, 0.05 wt% to 0.15 wt% nickel, chromium 0.10 To 0.15% by weight, 0.35% to 0.45% copper, 0.07% to 0.09% vanadium, 0.02% to 0.03% niobium, 0.010% to 0.013% nitrogen, and the balance of iron,
High strength steel with tensile strength of 600 MPa or more and elongation of 20% or more.
delete delete delete 0.14 wt% to 0.16 wt% carbon, 0.13 wt% to 0.18 wt% silicon, 1.15 wt% to 1.20 wt% manganese, 0.03 wt% phosphorus, 0.01 to 0.015 wt% sulfur, 0.05 wt% to 0.15 wt% nickel, chromium 0.10 To 0.15 wt%, 0.35 wt% to 0.45 wt% copper, 0.07 wt% to 0.09 wt% vanadium, 0.02 wt% to 0.03 wt% niobium, 0.010 wt% to 0.013 wt% nitrogen, and the balance iron
Reheating at 1,150 ° C. to 1,250 ° C .;
Extracting the material from a heating furnace and rolling; And
Cooling the material so that the temperature of the material is 650 ° C to 700 ° C;
As a high strength steel manufacturing method comprising a,
Cooling the material comprises quenching the material at 5 ° C./sec to 50 ° C./sec, followed by slow cooling,
The high strength steel is a high strength steel manufacturing method showing a tensile strength of 600MPa or more, 20% or more elongation.
The method of claim 5,
In the step of reheating the material at 1,150 ℃ to 1,250 ℃, the heating time is 1 hour to 3 hours.
The method of claim 5,
Extracting the material in a heating furnace and then rolling, wherein the rolling finish temperature is 1,000 ℃ to 1,050 ℃ manufacturing method of high strength steel.
The method of claim 5,
Extracting the material in a heating furnace and then rolling, wherein the total rolling reduction of the material is 65% to 80%.
The method of claim 5,
The quenching is a high strength steel manufacturing method carried out by water cooling.

Images