WO2023113327A1 - Hot rolled steel sheet for ground reinforcement and steel pipe for ground reinforcement, and manufacturing methods thereof - Google Patents

Abstract

The present invention relates to: a hot rolled steel sheet for ground reinforcement and a steel pipe for ground reinforcement, which have excellent strength and formability; and manufacturing methods thereof.

Description

Hot-rolled steel sheet and steel pipe for ground reinforcement and their manufacturing method

The present invention relates to a hot-rolled steel sheet and steel pipe for ground reinforcement having excellent strength and formability and a manufacturing method thereof.

As facilities such as underground tunnels for undergrounding roads, underground transit centers, and underground shopping centers increase, the need for ground reinforcing materials that are the basis of the facilities is increasing. Accordingly, the steel pipe standard for ground reinforcement (KS D 3872) was newly enacted. Steel pipes for ground reinforcement used for ground structure reinforcement in civil engineering and construction must satisfy YS: 800 MPa, TS: 860 MPa, EL: 10% or more in the longitudinal direction. In order to satisfy the corresponding physical properties, hot-rolled steel sheet requires YS: 700 MPa or more, TS: 750 MPa or more, and formability of EL: 15% or more. Although it is easy to obtain high strength by using a low-temperature structure such as bainite or martensite, the structure has the disadvantage of impairing the physical properties of the welded part by causing a softening of the welded part due to a slow cooling rate after welding.

On the other hand, in order to manufacture a small-diameter steel pipe suitable for ground reinforcement, formability must be excellent because it undergoes a lot of work hardening during pipe manufacturing due to the small diameter of the steel pipe, and the microstructure must be uniform to enable pipe manufacturing without shape defects. do.

One aspect of the present invention is to provide a hot-rolled steel sheet and steel pipe for ground reinforcement with excellent strength and formability and a manufacturing method thereof.

One embodiment of the present invention, in weight%, C: 0.05 ~ 0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5 ~ 1.9%, Ti: 0.05 ~ 0.15%, Nb: 0.03 ~ 0.1% , Mo: 0.03~0.1%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), the rest is Fe and unavoidable impurities It contains, satisfies the following relational expressions 1 and 2, has a microstructure containing 90% or more of ferrite in area%, the crystal grains of the ferrite have an average size of 15 μm or less, and Ti, Nb, Mo alone or 0.05% by weight or more of carbides contained in a composite, and the carbides provide a hot-rolled steel sheet for ground reinforcement having excellent strength and formability having an average size of 20 nm or less.

[Relational Expression 1] 0.002 ≤ (Ti/48 + Mo/96 + Nb/93) ≤ 0.004

[Relational Expression 2] 0.002 ≤ (C/12) - (Ti/48 + Mo/96 + Nb/93) ≤ 0.006

(However, in the relational expressions 1 and 2, the content of each alloy element means % by weight.)

Another embodiment of the present invention provides a steel pipe for ground reinforcement having excellent strength and formability manufactured using the hot-rolled steel sheet.

Another embodiment of the present invention is, in weight percent, C: 0.05 to 0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5 to 1.9%, Ti: 0.05 to 0.15%, Nb: 0.03 to 0.1 %, Mo: 0.03~0.1%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), the rest is Fe and unavoidable Reheating a steel slab containing impurities and satisfying the following relations 1 and 2 in a temperature range of 1150 to 1300 ° C; Obtaining a hot-rolled steel sheet by finish hot-rolling the reheated steel slab at a temperature range of 800 to 950° C.; and winding the hot-rolled steel sheet in a temperature range of 550 to 700° C.

[Relational Expression 1] 0.002 ≤ (Ti/48 + Mo/96 + Nb/93) ≤ 0.004

[Relational Expression 2] 0.002 ≤ (C/12) - (Ti/48 + Mo/96 + Nb/93) ≤ 0.006

(However, in the relational expressions 1 and 2, the content of each alloy element means % by weight.)

Another embodiment of the present invention provides a method for manufacturing a steel pipe for ground reinforcement with excellent strength and formability, including the step of obtaining a steel pipe by manufacturing a hot-rolled steel sheet manufactured by the above manufacturing method.

According to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet and steel pipe for ground reinforcement with excellent strength and formability and a manufacturing method thereof.

Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below is % by weight.

C: 0.05 to 0.1%

C is added to form carbides with Ti, Nb, and Mo as well as the solid solution strengthening effect, and is an element for securing tensile strength. In order to obtain the above effect, it is preferable to add 0.05% or more. However, if it exceeds 0.1%, carbide coarsening occurs and precipitation strengthening effect cannot be sufficiently secured, and the pearlite fraction in the microstructure increases, making it impossible to secure 90% or more ferrite to be obtained in the present invention. Therefore, the content of C is preferably in the range of 0.05 to 0.1%. The lower limit of the C content is more preferably 0.06%, even more preferably 0.065%, and most preferably 0.07%. The upper limit of the C content is more preferably 0.09%, even more preferably 0.085%, and most preferably 0.08%.

Si: 0.1% or less (excluding 0%)

Si is not only useful for deoxidation of steel, but also effective for securing strength through solid solution strengthening. However, when the Si content exceeds 0.1%, silicon oxide is formed, which makes plating difficult. Therefore, the Si content is preferably 0.1% or less. The Si content is more preferably 0.08% or less, more preferably 0.065% or less, and most preferably 0.05% or less.

Mn: 1.5 to 1.9%

Mn is added to secure the solid solution strengthening effect and the hardenability of the welded part during cooling after welding. In order to obtain the above effects, it is preferable that 1.5% or more is added. However, if it exceeds 1.9%, Mn segregation increases and defects and material deviations may occur during continuous casting. Therefore, the Mn content is preferably in the range of 1.5 to 1.9%. The lower limit of the Mn content is more preferably 1.55%, even more preferably 1.6%, and most preferably 1.65%. The upper limit of the Mn content is more preferably 1.85%, even more preferably 1.8%, and most preferably 1.75%.

Ti: 0.05 to 0.15%

Ti is added for precipitation strengthening effect and grain coarsening suppression. When the content of Ti is less than 0.05%, it is difficult to obtain high strength targeted in the present invention, and when it exceeds 0.15%, coarse carbides are formed, making precipitation hardening ineffective. Therefore, the Ti content is preferably in the range of 0.05 to 0.15%. The lower limit of the Ti content is more preferably 0.07%, even more preferably 0.08%, and most preferably 0.09%. The upper limit of the Ti content is more preferably 0.14%, more preferably 0.13%, and most preferably 0.12%.

Nb: 0.03~0.1%

Nb is added to obtain a finer grain size by inhibiting recrystallization during hot rolling in addition to the precipitation strengthening effect. When the content of Nb is less than 0.03%, it is difficult to obtain a sufficient precipitation strengthening effect, and when it exceeds 0.1%, strength may decrease due to the formation of coarse precipitates. Therefore, the Nb content is preferably in the range of 0.03 to 0.1%. The lower limit of the Nb content is more preferably 0.035%, even more preferably 0.038%, and most preferably 0.04%. The upper limit of the Nb content is more preferably 0.08%, more preferably 0.07%, and most preferably 0.06%.

Mo: 0.03~0.1%

Mo is added to suppress precipitate growth. In addition, by delaying the formation of ferrite so that ferrite is formed at a low temperature, it contributes to crystal grain refinement. When the content of Mo is less than 0.03%, it may be difficult to sufficiently obtain the above effect. On the other hand, if it exceeds 0.1%, economic feasibility may decrease. Therefore, the Mo content is preferably in the range of 0.03 to 0.1%. The lower limit of the Mo content is more preferably 0.035%, more preferably 0.04%, and most preferably 0.045%. The upper limit of the Mo content is more preferably 0.09%, even more preferably 0.08%, and most preferably 0.07%.

P: 0.02% or less (excluding 0%)

Since P, as an impurity element, is segregated at grain boundaries and deteriorates toughness, it is preferable not to include P as much as possible, and in the present invention, the upper limit is limited to 0.02%. The P content is more preferably 0.018% or less, even more preferably 0.017% or less, and most preferably 0.015% or less.

S: 0.02% or less (excluding 0%)

S is a major element that forms MnS as an impurity element, and since toughness is lowered due to the formation of coarse MnS, its content is limited to 0.02% or less in the present invention. The S content is more preferably 0.015% or less, even more preferably 0.01% or less, and most preferably 0.005% or less.

N: 0.01% or less (excluding 0%)

N is an impurity element, and when its content exceeds 0.01%, it reacts with Ti and Nb at high temperatures to form nitrides. there is. Therefore, the N content is preferably 0.01% or less. The N content is more preferably 0.008% or less, even more preferably 0.007% or less, and most preferably 0.006% or less.

In addition, the hot-rolled steel sheet of the present invention preferably satisfies the following relational expressions 1 and 2.

[Relational Expression 1] 0.002 ≤ (Ti/48 + Mo/96 + Nb/93) ≤ 0.004

The relational expression 1 is a parameter for improving strength by controlling the contents of Ti, Mo, and Nb, which are precipitation hardening elements. When the value of (Ti/48 + Mo/96 + Nb/93) is less than 0.002, the amount of precipitates is too small to be effective in improving strength. On the other hand, when the value of (Ti/48 + Mo/96 + Nb/93) exceeds 0.004, an effective precipitation hardening effect may not be obtained due to coarsening of the precipitate.

[Relational Expression 2] 0.002 ≤ (C/12) - (Ti/48 + Mo/96 + Nb/93) ≤ 0.006

The relational expression 2 is a parameter representing the content of C used for the solid solution strengthening effect excluding the content of C used for the precipitation hardening effect. When the value of (C/12) - (Ti/48 + Mo/96 + Nb/93) is less than 0.002, the high strength targeted by the present invention cannot be achieved because the ferrite phase cannot obtain sufficient strength. On the other hand, when the value of (C/12) - (Ti/48 + Mo/96 + Nb/93) exceeds 0.006, the remaining C content excessively increases, making the precipitate easily coarse to obtain the target strength. However, since the fraction of pearlite increases, it is difficult to obtain ferrite of 90% or more, which is the target of the present invention. In addition, as the formation of pearlite is promoted at the center of the steel sheet where the cooling rate is relatively slow, a large difference in hardness between the surface and the inside occurs. Due to this, a difference in hardness occurs in the thickness direction in the steel pipe as well. In addition, during rolling processing, a specific part receives a lot of processing, which may cause problems such as shape defects and cracks. Rolling refers to a process of forming protrusions on the surface of a steel pipe.

In addition to the above-described steel composition, the rest may include Fe and unavoidable impurities. Inevitable impurities can be unintentionally mixed in the normal steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. Further, the present invention does not entirely exclude the addition of other compositions than the aforementioned steel composition.

The hot-rolled steel sheet of the present invention preferably has a microstructure containing 90% or more of ferrite in terms of area%. In the present invention, it is important to secure 90% or more of ferrite in order to secure excellent formability. The microstructure of the present invention is theoretically preferably a single phase of ferrite, but one or more of pearlite, retained austenite, bainite and martensite may be formed inevitably in the manufacturing process. However, when the low-temperature transformation phase such as bainite or martensite increases, formability deteriorates. In addition, when pearlite is present on the surface of the steel pipe, cracks are likely to occur during rolling due to hard cementite. Therefore, it is preferable that the remaining tissue is as small as possible. The fraction of the ferrite is more preferably 93% or more, and even more preferably 95% or more. Meanwhile, the ferrite may be one or more of polygonal ferrite, bainitic ferrite, and acicular ferrite.

At this time, the crystal grains of the ferrite preferably have an average size of 15 μm or less. When the crystal grain size of the ferrite exceeds 15 μm, sufficient strength may not be obtained due to grain coarsening. The crystal grain size of the ferrite is more preferably 12 μm or less, and even more preferably 10 μm or less.

The hot-rolled steel sheet of the present invention preferably contains 0.05% by weight or more of carbides containing Ti, Nb, and Mo alone or in combination, and the carbides preferably have an average size of 20 nm or less. In this way, by forming a large amount of fine carbides having an average size of 20 nm or less at 0.05% by weight or more, an excellent strength improvement effect can be obtained without destruction of carbides. The fraction of the carbide is more preferably 0.07% by weight or more, and even more preferably 0.08% by weight or more. The average size of the carbide is more preferably 15 nm or less, and even more preferably 10 nm or less. On the other hand, in the present invention, the more the carbide is formed, the more advantageous it is, so the upper limit is not particularly limited, but considering the contents of Ti, Nb, and Mo included in the steel, it is difficult to exceed 0.2% by weight.

The hot-rolled steel sheet of the present invention provided as described above has yield strength (YS): 700 MPa or more, tensile strength (TS): 750 MPa or more, and elongation (EL): 15% or more, so that excellent strength and formability can be secured.

Meanwhile, in the present invention, a steel pipe manufactured using the hot-rolled steel sheet may be provided.

The steel pipe of the present invention has yield strength (YS): 800 MPa or more, tensile strength (TS): 860 MPa or more, and elongation (EL): 10% or more, so that excellent strength and formability can be secured.

On the other hand, it is necessary to uniformly deform in the thickness direction of the steel pipe during rolling processing so that defects due to stress concentration do not occur during processing and stable rolling processing is possible. There are advantages to processing. The hardness deviation is from each hardness value measured at 0.5 mm, t / 4, t / 2 points (t: steel pipe thickness) in the thickness direction from the surface of the steel pipe [(maximum hardness value-minimum hardness value) / maximum hardness value ×100].

Hereinafter, a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described.

First, a steel slab satisfying the aforementioned alloy composition and relational expressions 1 and 2 is reheated in a temperature range of 1150 to 1300 ° C. Reheating the steel slab at a temperature range of 1150 to 1300 ° C is to make the alloy composition and microstructure uniform. When the reheating temperature is less than 1150 ° C., the precipitate formed on the slab is not dissolved, so that the optimum precipitation strengthening effect cannot be obtained in the subsequent process. In addition, when the reheating temperature exceeds 1300 ° C., excessive crystal grain growth occurs, making it difficult to secure the target material and quality. Therefore, the reheating temperature of the steel slab is preferably in the range of 1150 to 1300 °C. The lower limit of the reheating temperature is more preferably 1170°C, even more preferably 1180°C, and most preferably 1200°C. The upper limit of the reheating temperature is more preferably 1290°C, even more preferably 1270°C, and most preferably 1250°C.

Thereafter, the reheated steel slab is finished hot-rolled at a temperature range of 800 to 950° C. to obtain a hot-rolled steel sheet. When the finish hot rolling temperature is less than 800 ° C, a portion of austenite is transformed into ferrite, and the finally obtained crystal grain size becomes non-uniform, and when it exceeds 950 ° C, scale defects and the like may occur. Therefore, the finish hot rolling temperature preferably has a range of 800 ~ 950 ℃. The lower limit of the finish hot rolling temperature is more preferably 820°C, even more preferably 825°C, and most preferably 850°C. The upper limit of the finish hot rolling temperature is more preferably 940°C, even more preferably 920°C, and most preferably 900°C.

Thereafter, the hot-rolled steel sheet is wound in a temperature range of 550 to 700 °C. If the coiling temperature is less than 550 ° C., the microstructure desired by the present invention cannot be obtained, and sufficient precipitation strengthening effect cannot be obtained because carbide formation is not sufficient. I can't get up and get the intensity I'm aiming for. Therefore, the winding temperature is preferably in the range of 550 ~ 700 ℃. The lower limit of the coiling temperature is more preferably 600°C, even more preferably 620°C, and most preferably 640°C. The upper limit of the coiling temperature is more preferably 680°C, even more preferably 665°C, and most preferably 650°C. Meanwhile, cooling from the finish hot rolling to winding may be performed on a run-out table.

Meanwhile, in the present invention, after obtaining a hot-rolled steel sheet through the above process, a steel pipe may be obtained by forming the hot-rolled steel sheet into a pipe. At this time, electric resistance welding (ERW) or the like may be used as a welding method when manufacturing the pipe. In addition, the present invention may further include the step of rolling the steel pipe after the step of obtaining the steel pipe.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for exemplifying the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After preparing a steel slab having an alloy composition shown in Table 1, reheating, finish hot rolling, and winding under the conditions shown in Table 2 below were performed to prepare a hot-rolled steel sheet having a thickness of 2.8 to 6 mm. Thereafter, the manufactured hot-rolled steel sheet was manufactured to manufacture a steel pipe. At this time, roll forming and electric resistance welding, which are common electrically welded steel pipe manufacturing methods, were used in the above pipe manufacturing, and the pipe manufacturing condition (t/D) according to the thickness (t) of the material and the diameter (D) of the steel pipe was applied at 14% or more. did The microstructure, precipitates, and mechanical properties were measured for the hot-rolled steel sheet and the steel pipe prepared as described above, and hardness was additionally measured for each position in the thickness direction of the steel pipe, and the results are shown in Tables 3 and 4 below.

The type and fraction of microstructures were measured using an optical microscope (OM). In addition, the ferrite grain size was measured using the circular intercept method of ASTM E 112 after photographing using a scanning electron microscope (SEM).

The size of the precipitate was measured by collecting the precipitate from the specimen using the carbon replica method and using a transmission electron microscope (TEM). The fraction of the precipitate was obtained by measuring the contents of Ti, Nb, and Mo using the residue extraction method.

Yield strength (YS), tensile strength (TS) and elongation (EL) were measured by performing a tensile test according to the tensile test method of KS B 0802 standard. In the case of hot-rolled steel sheet, a No. 5 test piece of the KS B 0801 standard was machined in the longitudinal direction according to the hot-rolling direction. In addition, in the case of the steel pipe, a tensile test was performed using a No. 11 test piece of the KS B 0801 standard.

The hardness at each position in the thickness direction was measured with a 1 kg load using a Vickers hardness tester at 0.5 mm, t/4, and t/2 points (t: steel pipe thickness) in the thickness direction from the outer surface of the steel pipe. At this time, 5 points were measured at each location and the average value was obtained. In addition, the hardness deviation described in Table 3 below was calculated as [(maximum hardness value-minimum hardness value) / maximum hardness value × 100] from each hardness value measured for each location.

Steel grade No.	Alloy composition (% by weight)
	C	Si	Mn	P	S	Ti	Nb	Mo	N	Equation 1	Equation 2
invention steel 1	0.0699	0.012	1.668	0.0138	0.0038	0.0929	0.041	0.049	0.003	0.00289	0.00294
invention steel 2	0.0989	0.015	1.692	0.0134	0.0032	0.0895	0.038	0.051	0.003	0.00280	0.00544
invention steel 3	0.058	0.013	1.702	0.0138	0.0028	0.086	0.041	0.054	0.003	0.00280	0.00204
Invention Steel 4	0.071	0.015	1.892	0.0132	0.0042	0.103	0.04	0.052	0.003	0.00312	0.00280
invention steel 5	0.074	0.013	1.525	0.0133	0.0042	0.0989	0.038	0.054	0.003	0.00303	0.00314
invention steel 6	0.0846	0.012	1.725	0.0133	0.003	0.141	0.05	0.044	0.003	0.00393	0.00312
invention steel 7	0.0862	0.012	1.742	0.0135	0.0038	0.052	0.052	0.041	0.003	0.00207	0.00511
invention steel 8	0.0634	0.013	1.696	0.0138	0.0041	0.069	0.0951	0.064	0.004	0.00313	0.00216
invention steel 9	0.0653	0.014	1.667	0.0145	0.0042	0.082	0.032	0.065	0.004	0.00273	0.00271
invention steel 10	0.068	0.022	1.712	0.0175	0.0035	0.1011	0.043	0.092	0.003	0.00353	0.00214
Invention Steel 11	0.068	0.022	1.712	0.0175	0.0035	0.1011	0.043	0.033	0.003	0.00291	0.00275
comparative steel 1	0.068	0.012	1.702	0.0151	0.0048	0.048	0.018	0.042	0.004	0.00163	0.00404
comparative steel 2	0.072	0.013	1.682	0.0131	0.0035	0.16	0.041	0.049	0.003	0.00428	0.00172
comparative lecture 3	0.0702	0.015	1.435	0.0111	0.0042	0.1011	0.04	0.058	0.003	0.00314	0.00281
comparative lecture 4	0.073	0.014	1.698	0.011	0.0042	0.0956	0.12	0.051	0.003	0.00381	0.00274
comparative steel 5	0.043	0.013	1.703	0.0113	0.0034	0.102	0.041	0.053	0.003	0.00312	0.00047
comparative steel 6	0.115	0.018	1.688	0.0142	0.0044	0.1011	0.04	0.048	0.003	0.00304	0.00655
comparative steel 7	0.0989	0.013	1.172	0.0113	0.0033	0.062	0.035	0.032	0.004	0.00200	0.00624
comparative river 8	0.082	0.014	1.55	0.0135	0.0042	0.123	0.072	0.082	0.003	0.00419	0.00264
[Equation 1] (Ti/48 + Mo/96 + Nb/93) [Equation 2] (C/12) - (Ti/48 + Mo/96 + Nb/93)

division	Steel grade No.	Reheat temperature (℃)	Finish hot rolling temperature (℃)	Winding temperature (℃)
Invention example 1	invention steel 1	1214	892	612
Invention example 2	invention steel 1	1156	888	614
Invention example 3	invention steel 1	1208	806	624
Invention example 4	invention steel 1	1231	945	631
Invention Example 5	invention steel 1	1210	878	552
Example 6	invention steel 1	1212	869	692
Comparative Example 1	invention steel 1	1121	891	614
Comparative Example 2	invention steel 1	1213	789	621
Comparative Example 3	invention steel 1	1215	972	622
Comparative Example 4	invention steel 1	1208	885	518
Comparative Example 5	invention steel 1	1209	886	711
Example 7	invention steel 2	1210	895	615
Invention Example 8	invention steel 3	1205	900	612
Inventive Example 9	Invention Steel 4	1214	895	613
Inventive Example 10	invention steel 5	1222	873	624
Inventive Example 11	invention steel 6	1205	865	618
Inventive Example 12	invention steel 7	1198	857	614
Inventive Example 13	invention steel 8	1202	858	611
Inventive Example 14	invention steel 9	1195	868	608
Inventive Example 15	invention steel 10	1194	884	603
Inventive Example 16	Invention Steel 11	1213	868	603
Comparative Example 6	comparative steel 1	1211	890	620
Comparative Example 7	comparative steel 2	1201	912	608
Comparative Example 8	comparative lecture 3	1205	968	612
Comparative Example 9	comparative lecture 4	1211	867	608
Comparative Example 10	comparative steel 5	1205	901	621
Comparative Example 11	comparative steel 6	1209	895	612
Comparative Example 12	comparative steel 7	1210	876	618
Comparative Example 13	comparative river 8	1205	865	582

division	microstructure			Carbides containing Ti, Nb, Mo alone or in combination
	ferrite (area%)	balance (area%)	Ferrite grain size (㎛)	Fraction (% by weight)	average size (nm)
Invention example 1	98.9	P:1.1	8.8	0.107	5.6
Invention example 2	99.0	P:1.0	8.5	0.095	5.5
Invention example 3	97.2	P:2.8	6.7	0.123	10.8
Invention example 4	95.9	P:4.1	13.5	0.122	6.2
Invention example 5	92.8	B:6.1, M:1.1	7.8	0.076	4.8
Example 6	90.3	P: 9.7	7.9	0.128	7.5
Comparative Example 1	98.8	P:1.2	8.3	0.046	6.2
Comparative Example 2	97.5	P:2.5	6.5	0.102	28.4
Comparative Example 3	98.2	P:1.8	15.3	0.093	6.8
Comparative Example 4	86.3	B:12.5, M:1.2	8.0	0.044	5.5
Comparative Example 5	86.8	P:13.2	10.3	0.106	23.1
Example 7	91.2	P:8.8	8.8	0.111	14.2
Invention Example 8	99.2	P:0.8	9.2	0.092	4.8
Inventive Example 9	99.0	P:1.0	9.8	0.087	7.0
Inventive Example 10	98.5	P:1.5	9.7	0.114	4.5
Inventive Example 11	98.0	P:2.0	7.5	0.141	18.2
Inventive Example 12	98.2	P:1.8	8.3	0.072	5.9
Inventive Example 13	98.1	P:1.9	5.2	0.115	6.8
Inventive Example 14	99.5	P:0.5	9.2	0.106	7.2
Inventive Example 15	96.8	B:2.1, RA:1.1	8.5	0.110	3.8
Inventive Example 16	98.2	P:1.8	9.8	0.101	6.7
Comparative Example 6	97.8	P:2.2	10.2	0.046	6.5
Comparative Example 7	97.5	P:2.5	8.1	0.156	52.5
Comparative Example 8	99.5	P:0.5	14.6	0.049	9.8
Comparative Example 9	98.2	P:1.8	4.8	0.131	51.3
Comparative Example 10	99.2	P:0.8	9.2	0.059	6.6
Comparative Example 11	87.7	P:12.3	9.5	0.113	22.1
Comparative Example 12	90.5	P:9.5	7.5	0.047	6.7
Comparative Example 13	94.8	P:5.2	6.9	0.175	48.2
P: pearlite, RA: retained austenite, B: bainite, M: martensite

division	hot rolled steel			steel pipe
	YS (MPa)	TS (MPa)	EL (%)	YS (MPa)	TS (MPa)	EL (%)	Hardness (Hv) by location in the thickness direction			Hardness Deviation (%)
							0.5mm from surface	t/4	t/2
Invention example 1	733	793	22.4	867	922	14	267	288	296	10
Invention Example 2	712	769	23.1	825	882	14	273	275	297	8
Invention Example 3	717	760	20.3	835	878	13	268	274	282	5
Invention Example 4	734	797	21.5	872	922	14	272	290	288	6
Invention Example 5	715	762	22.4	847	890	14	268	275	276	3
Example 6	712	775	21.8	821	894	14	268	279	288	7
Comparative Example 1	679	735	23.3	789	852	15	257	267	264	4
Comparative Example 2	651	743	24.1	774	848	15	260	265	267	3
Comparative Example 3	671	735	22.3	788	842	14	251	263	265	5
Comparative Example 4	672	752	21.1	781	862	14	255	272	277	8
Comparative Example 5	685	760	19.1	792	876	13	255	273	276	8
Invention Example 7	711	798	20.1	838	932	14	273	291	294	7
Invention Example 8	705	760	24.4	824	877	15	268	275	272	3
Inventive Example 9	736	807	21.4	843	911	14	271	285	278	5
Inventive Example 10	711	761	19.4	829	875	15	268	273	268	2
Inventive Example 11	781	853	18.4	935	1012	13	298	315	311	5
Inventive Example 12	706	759	23.1	816	861	14	254	265	268	5
Inventive Example 13	738	792	19.5	880	906	13	272	288	292	7
Inventive Example 14	711	761	21.5	836	878	14	260	272	276	6
Inventive Example 15	754	812	22.3	868	911	15	271	284	284	5
Inventive Example 16	714	772	22.3	851	885	14	264	275	272	4
Comparative Example 6	626	728	24.6	745	844	17	255	264	268	5
Comparative Example 7	679	746	23.2	786	856	16	257	269	266	4
Comparative Example 8	692	767	23.1	782	815	15	252	268	275	8
Comparative Example 9	675	772	24.0	781	878	16	265	274	285	7
Comparative Example 10	682	745	28.2	782	848	20	255	268	274	7
Comparative Example 11	684	788	18.3	782	887	13	258	315	278	18
Comparative Example 12	648	736	24.8	756	848	17	251	304	272	17
Comparative Example 13	681	742	20.2	787	851	14	259	272	268	5

As can be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 16 satisfying the alloy composition, relational expressions 1 and 2 and manufacturing conditions proposed by the present invention, the microstructure and precipitate to be obtained by the present invention are secured In addition to having excellent mechanical properties, it can be seen that the variation of hardness for each position in the thickness direction of the steel pipe is also at a low level.

In the case of Comparative Example 1, the alloy composition of the present invention was satisfied, but due to the low reheating temperature of the steel slab, the precipitation hardening element was not re-dissolved, so a sufficient precipitation hardening effect was not obtained. For this reason, it was not possible to secure high strength, which is the target of the present invention.

In the case of Comparative Example 2, the alloy composition of the present invention was satisfied, but due to the formation of coarse precipitates during rolling according to the low finish hot rolling temperature, sufficient precipitation strengthening effect was not obtained. For this reason, it was not possible to secure high strength, which is the target of the present invention.

In the case of Comparative Example 3, the alloy composition of the present invention was satisfied, but the crystal grains were coarsened as the finish hot rolling temperature was high, so that the high strength targeted by the present invention was not secured.

In the case of Comparative Example 4, the alloy composition of the present invention is satisfied, but due to the low coiling temperature, the microstructure to be obtained by the present invention is not obtained, and sufficient precipitation strengthening effect is not obtained. could not be obtained.

In the case of Comparative Example 5, the alloy composition of the present invention was satisfied, but as the coiling temperature was high, coarse carbides were formed, so that the high strength targeted by the present invention was not secured.

In the case of Comparative Example 6, since the content of Ti and Nb was low, sufficient precipitation strengthening effect was not obtained, and thus high strength, which is the target of the present invention, was not secured.

In the case of Comparative Example 7, the high strength targeted by the present invention was not secured due to the formation of coarse carbides as the Ti content was high.

In the case of Comparative Example 8, the solid solution strengthening effect was low according to the low Mn content, and the high strength targeted by the present invention was not secured because a sufficient precipitation strengthening effect was not obtained due to a change in the phase transformation condition during cooling.

In the case of Comparative Example 9, the high strength targeted by the present invention was not secured due to the formation of coarse carbides as the Nb content was high.

In the case of Comparative Example 10, as well as not satisfying the relational expression 2, the high strength targeted by the present invention was not secured due to the low C content.

In the case of Comparative Example 11, as the C content increased, the dissolved C content increased and the pearlite content increased, so that the microstructure to be obtained by the present invention was not obtained, and the relational expression 2 was not satisfied. It can be seen that not only was not secured, but also the deviation of hardness for each position in the thickness direction of the steel pipe also increased.

In the case of Comparative Example 12, since relational expression 2 was not satisfied, it was not possible to obtain an appropriate balance between the precipitate and solid solution C proposed by the present invention, so that high strength was not secured, and the deviation of hardness by position in the thickness direction of the steel pipe increased. Able to know.

In the case of Comparative Example 13, since the relational expression 1 was not satisfied, a sufficient precipitation strengthening effect was not obtained, and thus high strength, which is the target of the present invention, was not secured.

Claims (9)

1. In % by weight, C: 0.05 to 0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5 to 1.9%, Ti: 0.05 to 0.15%, Nb: 0.03 to 0.1%, Mo: 0.03 to 0.1% , P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), the remainder including Fe and unavoidable impurities,

   Satisfies the following relations 1 and 2,

   In terms of area%, it has a microstructure containing more than 90% ferrite,

   The crystal grains of the ferrite have an average size of 15 μm or less,

   Containing 0.05% by weight or more of carbides containing Ti, Nb, Mo alone or in combination,

   The carbide is a hot-rolled steel sheet for ground reinforcement having an average size of 20 nm or less.

   [Relational Expression 1] 0.002 ≤ (Ti/48 + Mo/96 + Nb/93) ≤ 0.004

   [Relational Expression 2] 0.002 ≤ (C/12) - (Ti/48 + Mo/96 + Nb/93) ≤ 0.006

   (However, in the relational expressions 1 and 2, the content of each alloy element means % by weight.)
2. The method of claim 1,

   The microstructure is a hot-rolled steel sheet for ground reinforcement containing at least one of pearlite, retained austenite, bainite and martensite as a remainder structure.
3. The method of claim 1,

   The hot-rolled steel sheet has yield strength (YS): 700 MPa or more, tensile strength (TS): 750 MPa or more, and elongation (EL): hot-rolled steel sheet for ground reinforcement of 15% or more.
4. A steel pipe for ground reinforcement manufactured using the hot-rolled steel sheet according to any one of claims 1 to 3.
5. The method of claim 4,

   The steel pipe is a steel pipe for ground reinforcement having a yield strength (YS): 800 MPa or more, tensile strength (TS): 860 MPa or more, and elongation (EL): 10% or more.
6. The method of claim 4,

   The steel pipe is a steel pipe for ground reinforcement having a hardness deviation of 15% or less.

   (However, the hardness deviation is from each hardness value measured at 0.5 mm, t / 4, t / 2 points (t: steel pipe thickness) in the thickness direction from the surface of the steel pipe [(maximum hardness value-minimum hardness value) / It is defined as the maximum hardness value × 100].)
7. In % by weight, C: 0.05 to 0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5 to 1.9%, Ti: 0.05 to 0.15%, Nb: 0.03 to 0.1%, Mo: 0.03 to 0.1% , P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), the remainder including Fe and unavoidable impurities, the following relational expression 1 and reheating the steel slabs satisfying 2 in a temperature range of 1150 to 1300° C.;

   Obtaining a hot-rolled steel sheet by finish hot-rolling the reheated steel slab at a temperature range of 800 to 950° C.; and

   Method for manufacturing a hot-rolled steel sheet for ground reinforcement comprising the step of winding the hot-rolled steel sheet in a temperature range of 550 to 700 ° C.

   [Relational Expression 1] 0.002 ≤ (Ti/48 + Mo/96 + Nb/93) ≤ 0.004

   [Relational Expression 2] 0.002 ≤ (C/12) - (Ti/48 + Mo/96 + Nb/93) ≤ 0.006

   (However, in the relational expressions 1 and 2, the content of each alloy element means % by weight.)
8. A method for manufacturing a steel pipe for ground reinforcement comprising the step of obtaining a steel pipe by manufacturing a hot-rolled steel sheet manufactured by the manufacturing method according to claim 7.
9. The method of claim 8,

   After the step of obtaining the steel pipe, the manufacturing method of the steel pipe for ground reinforcement further comprising the step of rolling the steel pipe.