KR20120070739A - High strength steel sheet having excellent elongation and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high strength steel sheet having excellent elongation and a method for manufacturing the same are provided to obtain the balance of tensile strength and elongation(TS×El) of 20000MPa% or more by controlling components and manufacturing conditions. CONSTITUTION: A high strength steel sheet having excellent elongation comprises 0.07-0.2wt.% of C, 2.0wt.%(0 is exclusive) or less of SI, 3.0-7.0wt.% of Mn, 0.03wt.% or less(0 is exclusive) of P, 0.015wt.% or less(0 is exclusive) of S, 1.3-3.0wt.% of AI, 0.02 wt.% or less(0 is exclusive) of N, and the remaining amount of Fe and inevitable impurities. The steel sheet has a microstructure comprising 10-40 area% of austenite and the remaining amount of ferrite.

Description

High strength steel sheet with excellent elongation and manufacturing method thereof {HIGH STRENGTH STEEL SHEET HAVING EXCELLENT ELONGATION AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high strength steel sheet excellent in elongation and a manufacturing method thereof, and more particularly to a high strength steel sheet excellent in elongation which can be used in automobile structural members and building materials and the like.

High-strength steel has been used in various applications such as automobile bodies, and recently, transformed tissue steels have been developed which are relatively excellent in formability compared to solid solution hardened steels.

The transformation tissue steel is represented by so-called DP (Dual Phase) steel, Transformation Induced Plasticity (TRIP) steel, and CP (Complex Phase) steel, and these transformation tissue steels have mechanical properties depending on the type and fraction of the parent phase and the second phase, respectively. That is, the level of tensile strength and elongation will vary. However, it is well known that these metamorphic steels rarely have a balance between tensile strength and elongation (TS X El) of more than 20,000.

Meanwhile, in addition to the metamorphic steel, there are TWIP (Twinning Induced Plasticity) steels that form austenite single phase by adding a large amount of C and Mn in steel. In the case of the TWIP steel, the balance between tensile strength and elongation (TS X El) is 50,000 MPa. By showing a value of more than%, it shows extremely excellent material properties.

However, in general, in order to manufacture TWIP steel, when the content of C is 0.4% by weight, the content of Mn is about 25% by weight or more, and the content of Mn is about 20% by weight or more when the content of C is 0.6% by weight. Otherwise, austenite, which causes twinning in the mother phase, is not stably secured, and epsilon martensite (ε) of HCP structure and martensite (α ') of BCT structure are formed, which is extremely detrimental to workability. A large amount of austenite stabilizing element must be added so that austenite can be present. As described above, the TWIP steel to which the alloying component is added in a large amount is not only difficult to manufacture the casting and rolling processes due to the problems resulting from the alloying component but also has a disadvantage of high manufacturing cost due to a large increase in alloy cost.

Therefore, many attempts have been made in view of improving the workability of the DP and TRIP steels and lowering the high manufacturing cost of the TWIP steels. In addition, in order to stabilize residual austenite, a method of forming lath austenite between plate-like martensite (Q & P, Quenching & Partitioning Process), and forming lath-like martensite before annealing to form residual austenite Methods for stabilizing (TAM, Trip Aided Annealed Martensite) have been studied.

However, all of these methods have a problem in weldability because they basically use steel having a C content of 0.3% or more, and it is very difficult to produce commercially since a separate facility investment for making martensite on a lath is required. There is.

The present invention is to provide a high-strength steel sheet having excellent elongation at which the balance (TS × El) of tensile strength and elongation has a value of 20,000 MPa?% Or more by controlling the composition components and manufacturing conditions.

In the present invention, by weight%, C: 0.07 ~ 0.2%, Si: 2.0% or less (excluding 0), Mn: 3.0 ~ 7.0%, P: 0.03% or less (excluding 0), S: 0.015% or less (0 Silver), Al: 1.3 ~ 3.0%, N: 0.02% or less (excluding 0), balance Fe and other unavoidable impurities, and the microstructure consists of 10-40% austenite and balance ferrite in an area fraction. Provides a high strength steel sheet excellent in elongation.

The steel sheet may further include at least one selected from the group consisting of Ti: 0.005 to 0.3%, Nb: 0.005 to 0.3%, and V: 0.005 to 0.3%, and Mo: 0.001 to 0.3% or Ni: 0.001 to It may further comprise one or two of 0.3%, and may further comprise B: 0.0001 ~ 0.01%.

The steel sheet preferably has a tensile strength of 780 MPa or more, preferably an elongation of 20% or more, and a tensile strength and elongation balance (TS × El) of 20,000 MPa?% Or more.

The steel sheet may be a hot rolled steel sheet, a cold rolled steel sheet or a galvanized steel sheet, wherein the galvanized steel sheet may be a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet.

In the present invention, by weight%, C: 0.07 ~ 0.2%, Si: 2.0% or less (excluding 0), Mn: 3.0 ~ 7.0%, P: 0.03% or less (excluding 0), S: 0.015% or less (0 Silver), Al: 1.3-3.0%, N: 0.02% or less (excluding 0), reheating the slab composed of the balance Fe and other unavoidable impurities at 1100 ~ 1300 ℃; A hot rolling step of manufacturing the steel sheet by hot rolling the reheated slab to a finish hot rolling temperature of 800 to 1000 ° C .; A winding step of winding the hot rolled steel sheet at 760 ° C. or less; A cold rolling step of cold rolling the wound steel sheet; And an annealing step of annealing and heat-treating the cold rolled steel sheet at 730 to 800 ° C.

The slab may further include one or more selected from the group consisting of Ti: 0.005 to 0.3%, Nb: 0.005 to 0.3%, and V: 0.005 to 0.3%, and Mo: 0.001 to 0.3% or Ni: 0.001 to It may further comprise one or two of 0.3%, and may further comprise B: 0.0001 ~ 0.01%.

In addition, after the winding step may further comprise the step of heat treatment at 500 ~ 700 ℃, before the cold rolling step may be carried out pickling. After the annealing step, zinc plating may be performed, and in this case, the zinc plating is preferably hot dip galvanizing or alloyed hot dip galvanizing.

According to the present invention can provide a steel sheet having excellent ductility and at the same time have a tensile strength similar to the conventional AHSS (Advanced High Strength Steel) TRIP, DP, CP steel.

Hereinafter, the present invention will be described in detail.

C: 0.07-0.2% (% means% by weight)

The content of carbon (C) is preferably 0.07 to 0.2%. Since C is necessary for stabilizing residual austenite and securing strength, it is preferable to add 0.07% or more. However, since the weldability is inferior when the content exceeds 0.2%, the content range of the C is preferably 0.07 to 0.2%.

Si: 2.0% or less (excluding 0)

The content of silicon (Si) is preferably 2.0% or less (excluding 0). Si is an element that inhibits precipitation of carbides in ferrite, promotes the concentration of carbon into austenite, and consequently stabilizes residual austenite. However, when exceeding 2%, since cold rolling property worsens, it is preferable to limit the upper limit to 2%.

Mn: 3.0-7.0%

The content of manganese (Mn) is preferably 3.0 to 7.0%. Mn is well known as an element that suppresses ferrite formation and facilitates austenite formation. When Mn is less than 3%, it forms TRIP steel, which makes it difficult to secure abnormal tissues of the present invention and exceeds 7%. Since the process cost can be increased due to the excessive amount of alloy input during the converter operation, the content range of the Mn is preferably set to 3.0 ~ 7.0%.

P: 0.03% or less (excluding 0)

The content of phosphorus (P) is preferably 0.03% or less (excluding 0). P is an impurity element, and if its content exceeds 0.03%, the weldability is lowered and the risk of brittleness of steel is increased. Therefore, the upper limit thereof is preferably limited to 0.03%.

S: 0.015% or less (excluding 0)

The content of sulfur (S) is preferably 0.015% or less (excluding 0). S is an impurity element in steel similar to P and is an element that inhibits the ductility and weldability of the steel sheet. When the content exceeds 0.015%, the ductility and weldability of the steel sheet are highly likely to be impaired. Therefore, the upper limit thereof is preferably limited to 0.015%.

Al: 1.3 ~ 3.0%

The content of aluminum (Al) is preferably 1.3 to 3.0%. The Al is an alloy element for enlarging the ferrite region, and has the advantage of increasing the annealing temperature during the manufacture of the ideal tissue steel using the continuous annealing as in the present invention, and it is easy to connect with other steel species during the continuous work of the annealing furnace. There is this. However, when the Al is less than 1.3%, the effect of increasing the annealing temperature is small, and there is a disadvantage that it is difficult to secure abnormal tissue, and when the Al exceeds 3%, there is a disadvantage that the strength drops sharply, the content range of Al It is preferable to set it as 1.3 to 3.0%.

N: 0.02% or less (excluding 0)

The content of nitrogen (N) is preferably 0.02% or less (excluding 0). N is a component that has an effective function of stabilizing austenite, but if it exceeds 0.02%, the risk of brittleness is greatly increased, so the upper limit is preferably limited to 0.02%.

The steel sheet of the present invention may further include at least one selected from the group consisting of Ti: 0.005 to 0.3%, Nb: 0.005 to 0.3%, and V: 0.005 to 0.3% in addition to the above components. Ti, Nb, and V are elements effective for increasing the strength of steel sheets and miniaturizing particle diameters. When the content of Ti, Nb and V is less than 0.005%, it is difficult to secure the above effects, and when the content exceeds 0.3%, ductility may be greatly reduced due to an increase in manufacturing cost and excessive precipitates.

In addition, Mo: 0.001 ~ 0.3% or Ni: 0.001 ~ 0.3% may further include one or two, wherein Mo is effective in inhibiting pearlite formation and austenite stabilization, the Ni is effective in austenite stabilization. There is. If the content of Mo or Ni is less than 0.001%, it is difficult to secure the above effects, and if the content is more than 0.3%, the manufacturing cost is rapidly increased.

In addition, it may further include B: 0.0001 ~ 0.01%, the B has the advantage of suppressing the formation of ferrite, there is an advantage of suppressing the formation of additional ferrite during cooling after the abnormal reverse annealing. If the content of B is less than 0.0001%, it is difficult to secure the same effect as above. If the content exceeds 0.01%, there is a problem in that ferrite formation is promoted by precipitation of Fe ₂₃ (C, B) ₆ .

In the steel sheet proposed by the present invention, the microstructure is preferably composed of 10 to 40% of austenite and the balance of ferrite. If the austenite structure is less than 10%, it is not easy to secure ductility, and if it exceeds 40%, an excessive amount of Mn must be added, thereby increasing the process cost.

In addition, the steel sheet of the present invention preferably has a tensile strength of 780 MPa or more, and the elongation is preferably 20% or more. In addition, it is possible to secure a tensile strength and elongation balance (TS × El) of 20,000 MPa?% Or more.

The steel sheet of the present invention described above may be a hot rolled steel sheet, a cold rolled steel sheet or a galvanized steel sheet, wherein the galvanized steel sheet may be a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet.

Hereinafter, the manufacturing method of this invention is demonstrated.

The slab that satisfies the above composition and range is reheated at 1100-1300 ° C. If the reheating temperature is less than 1100 ° C., the hot rolling load may increase rapidly. If the reheating temperature is higher than 1300 ° C., the amount of surface scale may increase, leading to material loss.

The reheated slab is hot rolled to a finish hot rolling temperature of 800 ~ 1000 ℃ to produce a steel sheet. If the finish rolling temperature is less than 800 ℃ there is a problem that the rolling load is greatly increased, if it exceeds 1000 ℃ may be the amount of scale on the surface. The steel of the present invention can be single-phase or abnormal-phase rolling depending on the component range.

After the hot rolling, the hot rolled steel sheet is wound at 760 ° C. or less. When the coiling temperature exceeds 760 ° C., an oxide film on the surface of the steel sheet may be excessively generated, causing defects. Since the said winding can be performed also at normal temperature, it does not specifically limit about the minimum.

After the winding process, it is also possible to add a step of heat treatment at 500 ~ 700 ℃, which can be lowered through the heat treatment, the strength of the hot rolled plate, through this, it can reduce the load during cold rolling. If the heat treatment temperature is less than 500 ℃ less effect of the strength is lowered, if it exceeds 700 ℃ may cause the steel sheets to stick to each other, leading to surface defects, thereby reducing productivity.

Thereafter, the cold rolled steel sheet is subjected to cold rolling. Pickling may be performed before the cold rolling, but may be performed by a method conventional in the art. By the annealing heat treatment of the cold-rolled steel sheet at 730 ~ 800 ℃, the cold rolled steel sheet proposed by the present invention can be produced, if the annealing temperature during the continuous annealing is less than 730 ℃ reverse transformation into austenite and recrystallization It may be difficult to secure the elongation due to insufficient reasons, and when it exceeds 800 ° C., there is a problem that it is difficult to secure the abnormal tissue steel, which is the object of the present invention, by an austenite single-phase annealing.

After the annealing heat treatment, the steel sheet may be galvanized. The zinc plating may be hot dip galvanized or galvanized zinc.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only examples for describing the present invention in more detail, and do not limit the scope of the present invention.

(Example)

The steel having the composition shown in Table 1 was vacuum-dissolved in 34 kg ingot, and then made into slabs through sizing rolling. After holding the slab at a temperature of 1200 ° C. for 1 hour, after finishing rolling at 900 ° C., it was charged into a furnace preheated to 600 ° C., and then retained for 1 hour to simulate hot rolling. After pickling and 50% cold rolling, the mechanical properties of the specimens heat-treated at the annealing temperature shown in Table 2 and the microstructures are shown in Table 2 below. However, the results for the microstructure are described only for specimens subjected to annealing at 770 ° C.

division Chemical composition (% by weight) C Si Mn S P Al N Etc Comparative Steel 1 0.057 1.62 6.37 0.0050 0.010 2.29 0.0038 - Inventive Steel 1 0.113 0.09 5.22 0.0052 0.011 2.84 0.0033 - Comparative Steel 2 0.104 1.58 6.28 0.0060 0.010 1.04 0.0050 - Inventive Steel 2 0.105 1.58 6.23 0.0048 0.012 2.23 0.0030 - Comparative Steel 3 0.152 1.59 4.13 0.0045 0.011 1.06 0.0045 - Invention Steel 3 0.156 1.63 4.14 0.0047 0.012 2.26 0.0028 - Comparative Steel 4 0.156 1.60 4.15 0.0038 0.010 3.47 0.0021 - Inventive Steel 4 0.158 0.09 5.17 0.0044 0.011 2.83 0.0029 - Comparative Steel 5 0.154 0.08 4.97 0.0032 0.012 3.74 0.0022 - Comparative Steel 6 0.154 1.60 6.29 0.0059 0.011 1.06 0.0048 - Inventive Steel 5 0.153 1.56 6.11 0.0054 0.010 2.28 0.0037 - Comparative Steel 7 0.154 1.58 6.09 0.0033 0.010 3.41 0.0022 - Inventive Steel 6 0.159 1.55 6.21 0.0045 0.011 2.08 0.0021 Ti: 0.03 Inventive Steel 7 0.148 1.59 5.89 0.0034 0.011 2.18 0.0034 Nb: 0.02 Inventive Steel 8 0.151 1.55 6.31 0.0026 0.012 2.12 0.0031 V: 0.02 Inventive Steel 9 0.145 1.57 6.01 0.0044 0.010 2.31 0.0033 Mo: 0.12 Inventive Steel 10 0.148 1.53 5.91 0.0047 0.010 2.08 0.0047 Ni: 0.09 Inventive Steel 11 0.152 1.49 6.13 0.0051 0.011 2.13 0.0027 B: 0.002

division Annealing Temperature
(℃) YS
(MPa) TS
(MPa) Total el
(%) TS X El
(MPa?%) Austenitic fraction
(area%) Steel grade Specimen No. Comparative Steel 1 Comparative Example 1 770 717 974 16.1 15681 13.1 Comparative Example 2 800 584 1001 11.6 11612 - Inventive Steel 1 Comparative Example 3 710 816 821 14.6 11987 - Inventory 1 740 765 877 27.5 24118 - Inventive Example 2 770 707 976 21.2 20691 18.7 Comparative Steel 2 Comparative Example 4 770 523 1529 7.9 12079 3.6 Comparative Example 5 800 848 1599 7.2 11513 - Inventive Steel 2 Inventory 3 740 937 1033 27.5 28408 - Honorable 4 770 865 1116 20.5 22878 19.3 Comparative Steel 3 Comparative Example 6 770 420 1293 12.8 16550 4.1 Comparative Example 7 800 616 1466 10.8 15833 - Invention Steel 3 Comparative Example 8 710 690 805 20.9 16825 - Inventory 5 740 701 824 30.9 25462 - Inventory 6 770 677 894 28.9 25837 18.4 Honorable 7 800 626 1044 20.9 21820 - Comparative Steel 4 Comparative Example 9 740 669 809 18.8 15209 - Comparative Example 10 770 919 767 32.2 24697 8.7 Inventive Steel 4 Comparative Example 11 710 993 883 11.1 9801 - Inventive Example 8 740 871 929 27.5 25548 - Proposition 9 770 807 1050 22.1 23205 21.5 Comparative Steel 5 Comparative Example 12 740 747 776 21.7 16839 - Comparative Steel 6 Comparative Example 13 770 513 1684 9.9 16672 8.4 Comparative Example 14 800 793 1748 6 10488 - Inventive Steel 5 Inventory 10 740 956 1042 30.5 31781 - Inventive Example 11 770 1069 1389 28.8 40003 26.3 Inventive Example 12 800 794 1293 21.8 28187 - Comparative Steel 7 Comparative Example 15 740 846 901 20.6 18561 - Inventive Steel 6 Inventive Example 13 770 1089 1401 26.8 37547 25.1 Inventive Steel 7 Inventive Example 14 770 1099 1399 25.6 35814 24.6 Inventive Steel 8 Inventive Example 15 770 1055 1375 24.6 33825 22.1 Inventive Steel 9 Inventive Example 16 770 1049 1415 30.2 42733 26.8 Inventive Steel 10 Inventive Example 17 770 1049 1400 29.1 40740 25.5 Inventive Steel 11 Inventive Example 18 770 1064 1395 27.8 38781 24.3

As can be seen in Tables 1 and 2, Inventive Examples 1 to 18 manufactured in accordance with the present invention have excellent tensile strength and elongation by securing an austenite in an appropriate range, and the tensile strength × elongation balance of these products is also excellent. can confirm.

On the other hand, Comparative Examples 1 and 2, although meeting the annealing conditions of the present invention, contains less carbon, failing to secure a balance of tensile strength x elongation of a certain level or more.

Comparative Examples 3 to 15 In addition, it does not satisfy any one of the composition components and ranges and annealing conditions of the present invention, it can be seen that the mechanical properties are low.

Claims (17)

By weight%, C: 0.07 ~ 0.2%, Si: 2.0% or less (excluding 0), Mn: 3.0 ~ 7.0%, P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0) , Al: 1.3 ~ 3.0%, N: 0.02% or less (excluding 0), balance Fe and other unavoidable impurities,
The microstructure is a high strength steel sheet having excellent elongation of 10 to 40% of austenite and residual ferrite in an area fraction.
The high strength steel sheet having excellent elongation according to claim 1, wherein the steel sheet further comprises one or more selected from the group consisting of Ti: 0.005 to 0.3%, Nb: 0.005 to 0.3%, and V: 0.005 to 0.3%.
The high strength steel sheet according to claim 1, wherein the steel sheet further comprises one or two of Mo: 0.001 to 0.3% or Ni: 0.001 to 0.3%.
The high strength steel sheet having excellent elongation according to claim 1, wherein the steel sheet further comprises B: 0.0001 to 0.01%. The high strength steel sheet having excellent elongation according to claim 1, wherein the steel sheet has a tensile strength of 780 MPa or more.
The high strength steel sheet having excellent elongation according to claim 1, wherein the steel sheet has an elongation of 20% or more.
The high strength steel sheet according to claim 1, wherein the steel sheet has an excellent elongation of 20,000 MPa?% Or more in terms of tensile strength and elongation balance (TS × El).
The high strength steel sheet having excellent elongation according to any one of claims 1 to 7, wherein the steel sheet is a hot rolled steel sheet, a cold rolled steel sheet, or a galvanized steel sheet.
The high strength steel sheet having excellent elongation according to claim 8, wherein the galvanized steel sheet is a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet.
By weight%, C: 0.07 ~ 0.2%, Si: 2.0% or less (excluding 0), Mn: 3.0 ~ 7.0%, P: 0.03% or less (excluding 0), S: 0.015% or less (excluding 0) Reheating the slab of Al: 1.3-3.0%, N: 0.02% or less (excluding 0), residual Fe and other unavoidable impurities at 1100-1300 ° C .;
A hot rolling step of manufacturing the steel sheet by hot rolling the reheated slab to a finish hot rolling temperature of 800 to 1000 ° C .;
A winding step of winding the hot rolled steel sheet at 760 ° C. or less;
A cold rolling step of cold rolling the wound steel sheet; And
The method of manufacturing a high strength steel sheet having excellent elongation comprising the annealing step of annealing the cold-rolled steel sheet at 730 ~ 800 ℃.
The method of claim 10, wherein the slab further comprises one or more selected from the group consisting of Ti: 0.005 to 0.3%, Nb: 0.005 to 0.3%, and V: 0.005 to 0.3%. .
The method of claim 10, wherein the slab further comprises one or two of Mo: 0.001 to 0.3% or Ni: 0.001 to 0.3%. The method of manufacturing a high strength steel sheet having excellent elongation according to claim 10, wherein the slab further comprises B: 0.0001 to 0.01%.
The method of manufacturing a high strength steel sheet having excellent elongation according to claim 10, further comprising a heat treatment at 500 to 700 ° C. after the winding step.
The method of manufacturing a high strength steel sheet having excellent elongation according to claim 10, wherein pickling is performed before the cold rolling step.
The manufacturing method of the high strength steel plate as described in any one of Claims 10-15 excellent in the elongation rate which zinc-plats after the annealing step.
The method of claim 16, wherein the zinc plating is hot dip galvanizing or alloying hot dip galvanizing.