WO2018117507A1 - Low-yield ratio steel sheet having excellent low-temperature toughness and method for manufacturing same - Google Patents

Abstract

An aspect of the present invention relates to a low-yield ratio steel sheet having excellent low-temperature toughness, the steel sheet comprising, in terms of weight%, 0.05-0.1% of C, 0.3-0.7% of Si, 1.0-2.0% of Mn, 0.005-0.04% of Al, 0.04-0.07% of Nb, 0.001-0.02% of Ti, 0.05-0.4% of Cu, 0.1-0.6% of Ni, 0.01-0.08% of Mo, 0.001-0.008% of N, 0.015% or less of P, 0.003% or less of S, and the remainder Fe and unavoidable impurities, wherein a microstructure comprises, by area fraction, 80-92% of ferrite and 8-20% of MA (martensite/austenite mixed structure), the average size of the MA measured for a circle-equivalent diameter being 3㎛ or less.

Description

Low-temperature-resistant steel sheet with excellent low temperature toughness and manufacturing method

The present invention relates to a steel sheet having excellent low temperature toughness and a method of manufacturing the same.

In order to be applicable to shipbuilding and marine structural steels as well as industrial fields requiring molding and seismic characteristics, it is necessary to develop steels having not only low-temperature toughness but also resistance-complexing properties.

Steels with resistance yield ratio not only have excellent formability by increasing the difference between yield strength and tensile strength, but also delay the plastic deformation time until fracture can occur and absorb energy in this process to prevent collapse by external force. can do. In addition, even if there is a deformation, it is possible to repair before collapse, thereby preventing damage to property and life due to damage to the structure.

In order to secure the resistance ratio, the technology of two phase organization of steel was developed. Specifically, the first phase is soft ferrite, and the remaining second phase is martensite, pearlite, or bainite, thereby implementing a resistance ratio.

However, the impact toughness due to the hard two phase and the carbon content is increased for the second phase, so that the toughness of the weld is degraded, thereby causing brittle fracture of the structure at low temperature.

Accordingly, Patent Literature 1 has been developed as a technique for securing both resistance ratio and low temperature impact toughness.

In Patent Document 1, the microstructure includes 2-10 vol% of MA (martensite / austenite mixed structure) and 90 vol% or more of cyclic ferrite, thereby ensuring resistance ratio and excellent low temperature toughness.

According to Patent Document 1, it is possible to implement a yield ratio of about 0.8 but there is an insufficient problem to secure the seismic characteristics can not implement a sufficient resistance yield ratio. Therefore, there is a demand for development of a resistive yield ratio steel sheet excellent in low temperature toughness and a method of manufacturing the same which can ensure a lower yield ratio.

(Prior art document)

(Patent Document 1) Korean Unexamined Patent Publication No. 2013-0076577

One aspect of the present invention is to provide a low-temperature toughness ratio steel sheet and a method of manufacturing the same.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention is by weight, C: 0.05 ~ 0.1%, Si: 0.3 ~ 0.7%, Mn: 1.0 ~ 2.0%, Al: 0.005 ~ 0.04%, Nb: 0.04 ~ 0.07%, Ti: 0.001 ~ 0.02 %, Cu: 0.05-0.4%, Ni: 0.1-0.6%, Mo: 0.01-0.08%, N: 0.001-0.008%, P: 0.015% or less, S: 0.003% or less, including remaining Fe and unavoidable impurities ,

The microstructure includes 80-92% ferrite and 8-20% MA (martensite / austenite mixed structure) as an area fraction, and the MA has excellent low-temperature toughness with an average size of 3 μm or less as measured by its equivalent diameter. It is related to bimetallic steel plate.

In addition, another aspect of the present invention is by weight%, C: 0.05 ~ 0.1%, Si: 0.3 ~ 0.7%, Mn: 1.0 ~ 2.0%, Al: 0.005 ~ 0.04%, Nb: 0.04 ~ 0.07%, Ti: 0.001 to 0.02%, Cu: 0.05 to 0.4%, Ni: 0.1 to 0.6%, Mo: 0.01 to 0.08%, N: 0.001 to 0.008%, P: 0.015% or less, S: 0.003% or less, remaining Fe and unavoidable impurities Heating the slab including 1050 to 1200 ° C;

Hot rolling the heated slab to a finish rolling end temperature of 760 to 850 ° C. to obtain a hot rolled steel sheet;

Cooling the hot rolled steel sheet to 450 ° C. or less at a cooling rate of 5 ° C./s or more; And

After the cooled hot-rolled steel sheet is heated to a temperature range of 850 ~ 960 ℃, the normalizing heat treatment step of maintaining for [1.3t + (10 ~ 30)] minutes; will be.

(T is a value measured in mm of the thickness of the hot rolled steel sheet.)

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. Various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to secure a resistance ratio and excellent low temperature toughness, and in particular, a low resistance ratio of 0.65 or less can be secured, thereby ensuring excellent seismic characteristics as well as formability. Accordingly, it can be applied to industrial fields such as construction, construction, and civil engineering requiring seismic characteristics, and also applicable to shipbuilding and marine structural steels.

1 is a photograph of the microstructure before the normalizing heat treatment of the test number 1 of the invention example.

Figure 2 is a photograph of the microstructure after the normalizing heat treatment of the test number 1 of the invention example.

Figure 3 is a photograph of the microstructure after the normalizing heat treatment of the test No. 9 of the comparative example.

Figure 4 is a photograph of the microstructure after the normalizing heat treatment of the test No. 10 of the comparative example.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

The present inventors are able to secure a yield ratio of about 0.8 in the prior art, but the moldability can be secured to some extent, but it is not enough to realize a sufficient resistance ratio, and it is insufficient to secure seismic characteristics, and solved this problem. In order to study deeply.

As a result, the larger the difference between the hardness of the base material and the second phase to realize the resistance ratio ratio, the more favorable the distribution of MA is more favorable, and in the case of Patent Literature 1, the base material lacks the hardness difference from the MA as the ecuous ferrite, and the MA phase is grain boundary. It has been found that the formation of the structure at the core and the size of the MA are not sufficient to realize sufficient resistance ratio.

Therefore, by using the microstructure of the base material as ferrite and uniformly distributing the fine MA phase in the ferrite grain boundary and the inside of the grain, a resistivity ratio of 0.65 or less can be ensured, and in order to secure such a structure, the structure before the normalizing heat treatment includes bainite. It was confirmed that control was required and the present invention was completed.

Low-temperature-resistant steel sheet with excellent low temperature toughness

Hereinafter, a low-temperature toughness steel sheet excellent in low temperature toughness according to an aspect of the present invention will be described in detail.

In accordance with an aspect of the present invention, the resistive steel sheet having excellent low temperature toughness is wt%, C: 0.05 to 0.1%, Si: 0.3 to 0.7%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.04%, and Nb: 0.04 ~ 0.07%, Ti: 0.001-0.02%, Cu: 0.05-0.4%, Ni: 0.1-0.6%, Mo: 0.01-0.08%, N: 0.001-0.008%, P: 0.015% or less, S: 0.003% or less , Remaining Fe and unavoidable impurities,

The microstructure includes 80-92% ferrite and 8-20% MA (martensite / austenite mixed structure) in an area fraction, and the MA has an average size of 3 µm or less measured in a circular equivalent diameter.

First, the alloy composition of the present invention will be described in detail. The unit of each element content below is weight% unless there is particular notice.

C: 0.05-0.1%

In the present invention, C is an element that causes solid solution strengthening and exists as carbonitride by Nb to secure tensile strength.

When the C content is less than 0.05%, the above effects are insufficient. On the other hand, if the C content is more than 0.1%, the MA is coarse and pearlite is formed, which may degrade the impact characteristics at low temperatures, and it is difficult to secure enough bainite. Therefore, it is preferable that C content is 0.05 to 0.1%.

In addition, the lower limit of the C content may be 0.055%, and the lower limit may be 0.06%. In addition, the more preferable upper limit of C content may be 0.095%, and a more preferable upper limit may be 0.09%.

Si: 0.3 ~ 0.7%

Si serves to deoxidize molten steel by assisting Al and is added to secure yield strength and tensile strength. Moreover, it is an element for controlling the fraction of MA desired by this invention.

When the Si content is less than 0.3%, the above effects are insufficient. On the other hand, when the Si content is more than 0.7%, the impact characteristics may be degraded by coarsening of the MA, and the welding characteristics may be degraded. Therefore, it is preferable that Si content is 0.3 to 0.7%.

In addition, the lower limit of Si content may be 0.35%, and the lower limit may be 0.4%. In addition, the more preferable upper limit of Si content may be 0.65%, and a more preferable upper limit may be 0.6%.

Mn: 1.0-2.0%

Mn contributes greatly to the strength increasing effect by solid solution strengthening and is an element that helps to form bainite.

When the Mn content is less than 1.0%, the above effects are insufficient. On the other hand, if excessively added, the toughness may be reduced due to formation of MnS inclusions and formation of the center portion, so the upper limit is 2.0%. Therefore, it is preferable that Mn content is 1.0 to 2.0%.

In addition, the lower limit of the Mn content may be 1.1%, and the lower limit may be 1.2%. In addition, the more preferable upper limit of Si content may be 1.95%, and a more preferable upper limit may be 1.9%.

Al: 0.005-0.04%

Al needs to be added 0.005% or more as a major deoxidizer of steel. However, when added in excess of 0.04%, the effect is saturated and may cause low temperature toughness by increasing the fraction and size of Al ₂ O ₃ inclusions.

Nb: 0.04-0.07%

Nb is an element that suppresses recrystallization during rolling or cooling by precipitation of solid solution or carbonitride, thereby making the structure fine and increasing the strength. Moreover, it is an element for controlling the fraction of MA desired by this invention.

When the Nb content is less than 0.04%, the above effects are insufficient. On the other hand, if it is more than 0.07% there is a problem that can reduce the toughness of the base material and the toughness after welding.

Ti: 0.001-0.02%

Ti combines with oxygen or nitrogen to form precipitates, thereby inhibiting coarsening of tissues, contributing to miniaturization and improving toughness.

When the Ti content is less than 0.001%, the above effects are insufficient. On the other hand, when the Ti content is more than 0.02%, precipitates are formed coarsely and may cause destruction.

Cu: 0.05 ~ 0.4%

Cu is a component that does not significantly reduce the impact characteristics, and thus improves strength by solid solution and precipitation. In order to sufficiently improve the strength, it should be contained at 0.05% or more, but if the Cu content is more than 0.4%, surface cracks of the steel sheet due to Cu thermal shock may occur.

Ni: 0.1-0.6%

Ni is an element that can improve strength and toughness at the same time as the increase in content is not great, and is an element that helps to form bainite by decreasing the Ar3 temperature.

When the Ni content is less than 0.1%, the above effects are insufficient. On the other hand, when the Ni content is more than 0.6%, the manufacturing cost may increase and the weldability may deteriorate.

Mo: 0.01 ~ 0.08%

Mo is an austenite stabilizing element that affects the amount of MA and plays a large role in improving the strength. It is also an element that prevents the drop in strength during heat treatment and helps to form bainite.

However, since Mo is an expensive alloying element, there is a problem in that manufacturing cost increases when a large amount is added. Therefore, in the present invention, it is intended to secure MA by adding a large amount of Si, Nb and the like, and in the alloy composition of the present invention, Mo may be sufficiently secured by adding 0.01% or more. On the other hand, when the Mo content is more than 0.08%, there is a problem that the manufacturing cost increases and the base material toughness and post-weld toughness may be reduced.

N: 0.001-0.008%

N is an element that forms a precipitate with Ti, Nb, Al and the like to make the austenite structure fine when the slab is heated to help improve strength and toughness. When the N content is less than 0.001%, the above effects are insufficient. On the other hand, when the N content is more than 0.008%, it causes surface cracking at high temperatures, forms precipitates, and the remaining N remains in an atomic state, thereby reducing toughness.

P: 0.015% or less

P may cause grain boundary segregation as impurities and cause the steel to be withdrawn. Therefore, it is important to control the upper limit, and it is preferable to control it to 0.015% or less.

On the other hand, the lower limit of the P content is not particularly limited, but 0% may be excluded.

S: 0.003% or less

S, as an impurity, mainly combines with Mn to form MnS inclusions, which are factors that inhibit low-temperature toughness. Therefore, it is important to control the upper limit, and in order to secure low temperature toughness, it is preferable to control S to 0.003% or less.

On the other hand, the lower limit of the S content is not particularly limited, but 0% may be excluded.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Hereinafter, the microstructure of the resistive steel sheet having excellent low temperature toughness according to an aspect of the present invention will be described in detail.

The microstructure of the resistive steel sheet having excellent low temperature toughness according to an aspect of the present invention includes 80 to 92% ferrite and 8 to 20% MA (martensite / austenite mixed structure) as an area fraction, and the MA is The average size measured by the circular equivalent diameter is 3 micrometers or less. Hereinafter, the fraction of microstructure means an area fraction unless otherwise specified.

Ferrite is to ensure basic toughness and strength, preferably 80% or more. In addition, in order to ensure sufficient MA, the upper limit is preferably 92%. Furthermore, it is preferable that the ferrite does not contain the ecuous ferrite. This is because the Eccentric Ferrari cannot have sufficient resistance ratio because of its small hardness difference from MA.

If the MA is less than 8%, it is difficult to secure a resistance ratio of 0.65 or less. If the MA is more than 20%, the impact toughness may be reduced, and the elongation may be reduced. In addition, when the average size measured by the equivalent circular diameter of the MA exceeds 3㎛, it is difficult to ensure the uniform distribution and resistance ratio of the MA is formed in the grain boundary grains.

Other inevitable phases other than the above-described ferrite and MA may be included and are not excluded. For example, pearlite of 1 area% or less may be included.

In this case, in order to secure excellent resistance ratio characteristics and low temperature toughness, when the 100 μm straight line is drawn on the steel sheet of the present invention, as well as the MA fraction and size, it is preferable that 5 to 13 MAs are disposed on the straight line. That is, a plurality of straight lines are drawn up and down or left and right on a microstructure photograph having a size of 100 μm × 100 μm, and at this time, 5 to 13 MAs may be present on each line. This is because MA, which mainly causes breakage, is present in the grain boundary, and when the above conditions are satisfied, the MA is evenly distributed in the grain boundary and inside the grain, which is advantageous in securing a resistance ratio.

In addition, the ratio of the MA present in the ferrite grains and the MA present in the grain boundary may be 1: 3 to 1:10. The ratio refers to the ratio of the number of MA, and by satisfying the ratio can be uniformly distributed so that the MA present in the ferrite grains becomes 0.5 to 5 area%.

In addition, the ferrite may have an average size of about 20 μm or less as measured by a circular equivalent diameter. This is because when the average size of the ferrite is greater than 20 μm, it may be difficult to secure sufficient toughness and strength.

On the other hand, the steel sheet according to the present invention is normalized (Normalizing) heat treatment and the microstructure of the steel sheet before the normalizing heat treatment may be 50 ~ 90 area% of bainite. Since the microstructure of the steel sheet before the heat treatment is bainite with carbides present therein, it is possible to distribute MA evenly in the grain boundary and the grain boundary after the heat treatment. Therefore, the microstructure of the steel sheet before the heat treatment is preferably 50 to 90 area%. Do.

In addition, the steel sheet according to the present invention has a yield ratio of 0.5 to 0.65, the low temperature impact characteristics at -40 ℃ may be 100J or more. The yield ratio is 0.65 or less, which makes the difference between yield strength and tensile strength not only excellent in formability but also delays plastic deformation until breakage occurs and absorbs energy in this process to prevent collapse by external forces. Can be. Therefore, it can be preferably applied not only to the field of shipbuilding and marine structural steel but also to the industrial field requiring molding and seismic characteristics.

At this time, the yield strength of the steel sheet is 350 ~ 400MPa, tensile strength may be 600MPa or more.

Manufacturing method of resistance composite steel sheet with excellent low temperature toughness

Hereinafter, another aspect of the present invention will be described in detail a method for producing a low-temperature toughness steel sheet excellent in toughness.

Another aspect of the present invention is a method for producing a low-temperature-resistant steel sheet having excellent low-temperature toughness comprises the steps of heating the slab having the above-described alloy composition to 1050 ~ 1200 ℃; Hot rolling the heated slab to a finish rolling end temperature of 760 to 850 ° C. to obtain a hot rolled steel sheet; Cooling the hot rolled steel sheet to 450 ° C. or less at a cooling rate of 5 ° C./s or more; And a normalizing heat treatment step of heating the cooled hot-rolled steel sheet to a temperature range of 850 to 960 ° C., and then maintaining it for [1.3t + (10 to 30)] minutes. However, t is a value measured in mm units of the hot rolled steel sheet.

Slab heating stage

The slab having the alloy composition described above is heated to 1050 ~ 1200 ℃.

If the heating temperature is more than 1200 ℃ austenite grains may be coarsened to lower the toughness, if less than 1050 ℃ Ti, Nb, etc. are not sufficiently dissolved, the strength may be reduced.

Hot rolled stage

The heated slab is hot rolled to a finish rolling end temperature of 760 to 850 ° C. to obtain a hot rolled steel sheet. In general, the rolling temperature of the heat-treated steel is about 850 ~ 1000 ℃ general rolling is applied. However, in the present invention, it is important to form the initial tissue as bainite. Therefore, a controlled rolling process for finishing rolling at low temperature is needed instead of general rolling showing ferrite-pearlite structure.

Re-crystallization rolling during hot rolling is necessary to refine the austenite grain size, and it is advantageous in terms of physical properties as the reduction ratio per pass increases. Unrecrystallized rolling must be completed at a temperature of at least Ar3 of the steel and is at least about 760 ° C. More specifically, the finish rolling end temperature may be defined at 760 to 850 ° C. If the finish rolling finish temperature is higher than 850 ℃, it is difficult to suppress the ferrite-pearlite transformation, and if it is less than 760 ℃ may cause non-uniformity of the microstructure in the thickness direction and to achieve a reduction in the amount of reduction by the load load of the rolling roll May not form tissue. By finishing the finish rolling in the temperature range of 760 to 850 ° C, ferrite-pearlite transformation is suppressed and bainite structure is realized through cooling. The initial structure of bainite is for uniform MA distribution after heat treatment. In the ferrite-pearlite structure, MAs are mainly formed at grain boundaries, while in bainite structures, MAs are formed at both grain boundaries and inside grains.

Cooling stage

The hot rolled steel sheet is cooled to 450 ° C. or less at a cooling rate of 5 ° C./s or more.

Accelerated cooling after hot rolling is very important for the implementation of the target structure of the inventive steel. Bainite should be implemented to form fine and uniform MA. Cooling finish temperature and cooling rate are important factors for bainite formation. If the cooling finish temperature is higher than 450 ℃, the grain size may become coarse and coarse carbide may cause coarse MA to be formed after heat treatment, which may lead to deterioration of toughness and secure more than 50 area% of bainite. Difficult to do

If the cooling rate is less than 5 ℃ / s, the fine structure of the needle-like ferrite or ferrite + pearlite is formed in a large amount may cause a decrease in strength. There is a problem that the yield can be reduced, and it is difficult to secure 50 area% or more of bainite.

At this time, the microstructure of the cooled hot-rolled steel sheet may be 50 ~ 90 area% bainite. Since the microstructure of the steel sheet before the heat treatment is bainite with carbides present therein, it is possible to distribute MA evenly in the grain boundary and the grain boundary after the heat treatment. Therefore, the microstructure of the steel sheet before the heat treatment is preferably 50 to 90 area%. Do.

Normalizing Heat Treatment Step

After the cooled hot rolled steel sheet is heated to a temperature range of 850 ~ 960 ℃, it is maintained for [1.3t + (10 ~ 30)] minutes. T is a value measured in mm units of the hot rolled steel sheet.

If the normalizing temperature is less than 850 ° C or the holding time is less than (1.3t + 10) minutes, the reusability of cementite and pearlite in pearlite and bainite is difficult to re-use, resulting in a decrease in the amount of C employed, resulting in less strength and ultimately remaining The hardened phase remains coarse.

On the other hand, when the normalizing temperature is higher than 960 ° C or the holding time is longer than (1.3t + 30) minutes, all carbides in the bainite grains are moved to the grain boundary or the coarsening of carbides occurs, resulting in the final MA size and uniform distribution. Cannot be formed. In addition, grain growth may occur, resulting in a decrease in strength and deterioration of impact.

Through the following examples will be described the present invention in more detail. However, it is necessary to note that the following examples are intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

To prepare a molten steel having a component composition shown in Table 1 below to prepare a slab using a continuous casting. The slabs were rolled, cooled, and normalized by heat treatment under the conditions shown in Table 2 to prepare 80 mm thick steel sheets.

Table 3 below describes the bainite fraction and mechanical properties of the steel sheet before the normalizing heat treatment.

Table 4 below describes the MA fraction of the steel sheet after the normalizing heat treatment, the average MA size, the number of MAs over 100 μm, and the mechanical properties thereof. In the case of the invention example, except for MA, it was ferrite, and the average grain size of the ferrite was 20 μm or less and was not separately described.

The average MA size is the average size measured by the equivalent diameter, and the number of MAs on a 100 μm line is the number of MAs on each line by drawing 10 straight lines up and down or left and right on a 100 μm X 100 μm microstructure photograph. After measuring the average number was described.

division	Steel grade	C	Si	Mn	P	S	Al	Ni	Mo	Ti	Nb	Cu	N
Invention steel	A	0.081	0.495	1.61	0.01	0.002	0.031	0.15	0.069	0.012	0.047	0.245	0.0037
Invention steel	B	0.078	0.521	1.78	0.01	0.0018	0.026	0.26	0.054	0.013	0.051	0.239	0.0041
Invention steel	C	0.084	0.453	1.75	0.009	0.0019	0.027	0.32	0.048	0.011	0.055	0.256	0.0038
Invention steel	D	0.086	0.535	1.64	0.007	0.0018	0.030	0.25	0.034	0.013	0.049	0.261	0.0034
Comparative steel	E	0.046	0.503	1.69	0.009	0.002	0.011	0.147	0.068	0.013	0.042	0.26	0.0039
Comparative steel	F	0.085	0.11	1.65	0.012	0.002	0.029	0.15	0.068	0.012	0.045	0.246	0.0042
Comparative steel	G	0.084	0.495	1.67	0.009	0.002	0.032	0.147	0.059	0.01	0.021	0.264	0.0036

The unit of each element content in Table 1 is weight%. Invention steels A to D are steel sheets satisfying the component range defined by the present invention, and comparative steels E to G are steel sheets which do not satisfy the component range defined by the present invention. Comparative steel E is less than C content, comparative steel F is less than Si, and comparative steel G is less than Mn content.

division	Exam number	Steel grade	Reheating Temperature (℃)	Finish rolling temperature (℃)	Finish rolling end temperature (℃)	Cooling finish temperature (℃)	Cooling rate (℃ / s)	Normalizing Temperature (℃)	Normalizing time (minutes)
Inventive Example	One	A	1147	798	781	345	10.2	910	121
Inventive Example	2	B	1151	805	798	332	10.3	910	122
Inventive Example	3	C	1142	801	797	362	10.5	910	120
Inventive Example	4	D	1138	788	775	328	10.9	910	122
Comparative example	5	A	1150	965	938	367	12.3	910	121
Comparative example	6	B	1134	794	779	-	-	910	120
Comparative example	7	C	1148	803	785	385	10.2	910	395
Comparative example	8	D	1129	791	780	701	8.3	910	121
Comparative example	9	E	1136	809	802	365	10.3	910	119
Comparative example	10	F	1152	810	799	335	10.1	910	124
Comparative example	11	G	1148	803	786	387	11.6	910	119

division	Exam number	Steel grade	Before normalizing heat treatment
			Bainite (area%)	Yield strength (MPa)	Tensile Strength (MPa)	Yield fee	Elongation (%)
Inventive Example	One	A	66	538	617	0.87	25.4
Inventive Example	2	B	71	524	642	0.82	22.5
Inventive Example	3	C	64	498	624	0.80	23.4
Inventive Example	4	D	80	503	621	0.81	21.5
Comparative example	5	A	8	492	587	0.84	22.5
Comparative example	6	B	0	452	561	0.81	24.2
Comparative example	7	C	64	507	644	0.79	25.1
Comparative example	8	D	7	561	657	0.85	22.4
Comparative example	9	E	One	497	580	0.86	24.3
Comparative example	10	F	31	487	576	0.85	25.7
Comparative example	11	G	27	477	553	0.86	21.7

division	Exam number	Steel grade	After normalizing heat treatment
			MA fraction (area%)	Average MA Size (㎛)	Number of MAs across 100 μm line	Yield strength (MPa)	Tensile Strength (MPa)	Yield fee	Elongation (%)	Impact Toughness (-40 ℃, J)
Inventive Example	One	A	11.8	2.4	9	384	653	0.59	31.2	123
Inventive Example	2	B	12.7	2.3	9	375	642	0.58	32.1	154
Inventive Example	3	C	14.2	2.6	9	381	643	0.59	29.8	114
Inventive Example	4	D	16.5	2.1	8	362	639	0.57	30.8	109
Comparative example	5	A	3.4	6.1	2	371	468	0.79	25.6	58
Comparative example	6	B	2.8	5.6	One	352	448	0.79	25.4	67
Comparative example	7	C	3.5	4.5	One	341	474	0.72	24.9	52
Comparative example	8	D	1.8	3.8	One	365	495	0.74	26.8	34
Comparative example	9	E	0.4	5.0	0	342	438	0.78	26.7	132
Comparative example	10	F	0.8	3.1	2	351	458	0.77	25.8	152
Comparative example	11	G	1.2	5.3	2	348	445	0.78	24.1	117

Inventive examples satisfying both the alloy composition and the manufacturing conditions presented in the present invention can ensure the yield ratio below 0.65, it can be confirmed that the impact toughness of -40 ℃ 100J or more excellent.

In the case of the test Nos. 5, 6, 7, and 8, which are comparative examples, the alloy composition proposed in the present invention was satisfied, but it did not satisfy the manufacturing conditions and did not secure sufficient yield ratio, and the impact toughness of -40 ° C was less than 100J. You can see that it's ten.

In the case of the test Nos. 9 to 11, which are comparative examples, the manufacturing conditions presented in the present invention were satisfied, but the alloy composition did not satisfy the sufficient yield ratio, and the -40 ° C impact toughness was less than 100J. have. In addition, it can be seen that the strength is also inferior due to the insufficient content of C, Si, Nb.

Looking at the invention example of Table 4 it can be seen that the MA fraction is higher than the comparative example. As can be seen from Table 3, by securing a high bainite fraction before the normalizing heat treatment, the grains of the initial bainite structure, carbides in the grains are transformed into fine MA. It can also be seen that the yield ratio is determined by the formation of such fine MAs.

Looking at Figure 1 taken of the microstructure before the normalizing heat treatment of the invention example test number 1 it can be seen that the bainite can be sufficiently secured, and when looking at Figure 2 taken the microstructure after the heat treatment that a fine and uniform MA is formed Able to know.

On the other hand, when looking at the microstructure of the test No. 9 of Comparative Example 9, it can be seen that the polygonal ferrite appears as the main phase and the fraction of MA is significantly lowered because the carbon content is insufficient.

In addition, looking at Figure 4 of the microstructure of the test No. 10 of the comparative example, it can be confirmed that the MA content is reduced because the Si content is less.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

Claims (9)

1. By weight%, C: 0.05-0.1%, Si: 0.3-0.7%, Mn: 1.0-2.0%, Al: 0.005-0.04%, Nb: 0.04-0.07%, Ti: 0.001-0.02%, Cu: 0.05- 0.4%, Ni: 0.1-0.6%, Mo: 0.01-0.08%, N: 0.001-0.008%, P: 0.015% or less, S: 0.003% or less, including the remaining Fe and inevitable impurities,

   The microstructure includes 80-92% ferrite and 8-20% MA (martensite / austenite mixed structure) as an area fraction, and the MA has excellent low-temperature toughness with an average size of 3 μm or less as measured by its equivalent diameter. Bokbi grater.
2. The method of claim 1,

   When a 100-micrometer straight line is drawn to the said steel plate, 5-13 MA which exists in the said linear line among the said MA exists, The low-temperature resistant steel sheet excellent in toughness characterized by the above-mentioned.
3. The method of claim 1,

   The MA is a low-resistance steel sheet excellent in low temperature toughness of the ratio of the MA present in the ferrite grains and the MA present in the grain boundary satisfies 1: 3 ~ 1:10.
4. The method of claim 1,

   The ferrite has excellent low-temperature toughness steel sheet, characterized in that the average size measured by the diameter of the equivalent circle is 20㎛ or less.
5. The method of claim 1,

   The steel sheet is a normalized heat treatment,

   The microstructure of the steel sheet before the normalizing heat treatment is excellent low-temperature toughness steel sheet, characterized in that the bainite is 50 ~ 90 area%.
6. The method of claim 1,

   The steel sheet has a yield ratio of 0.5 to 0.65, the low-temperature toughness excellent steel sheet, characterized in that the low-temperature impact properties at -40 ℃ 100J or more.
7. The method of claim 1,

   Yield strength of the steel sheet is 350 ~ 400MPa, tensile strength is excellent resistance to low-temperature steel sheet, characterized in that 600MPa or more.
8. By weight%, C: 0.05-0.1%, Si: 0.3-0.7%, Mn: 1.0-2.0%, Al: 0.005-0.04%, Nb: 0.04-0.07%, Ti: 0.001-0.02%, Cu: 0.05- Slab containing 0.4%, Ni: 0.1 ~ 0.6%, Mo: 0.01 ~ 0.08%, N: 0.001 ~ 0.008%, P: 0.015% or less, S: 0.003% or less, remaining Fe and unavoidable impurities Heating to;

   Hot rolling the heated slab to a finish rolling end temperature of 760 to 850 ° C. to obtain a hot rolled steel sheet;

   Cooling the hot rolled steel sheet to 450 ° C. or less at a cooling rate of 5 ° C./s or more; And

   After heating the cooled hot-rolled steel sheet to a temperature range of 850 ~ 960 ° C, a normalizing heat treatment step of maintaining for [1.3t + (10 ~ 30)] minutes.

   (T is a value measured in mm of the thickness of the hot rolled steel sheet.)
9. The method of claim 8,

   The microstructure of the cooled hot-rolled steel sheet is a method for producing a low-temperature toughness steel sheet excellent in toughness, characterized in that the bainite is 50 ~ 90 area%.

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