JP2006169591A - Non-heat treated steel plate with high yield strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-heat treated steel plate having high yield strength, with respect to a non-heat treated type steel plate having high yield strength and being useful for large-sized structures, such as building structures and bridges. <P>SOLUTION: The non-heat treated steel plate contains, by mass, 0.01 to 0.08% C, 0.05 to 0.5% Si, 0.5 to 2.5% Mn, 0.01 to 0.07% Al, 0.05 to 1.5% Cr, 0.05 to 1% Mo, 0.005 to 0.03% Ti, 0.0005 to 0.003% B and 0.002 to 0.008% N and also has a metallic structure composed of bainite in which island martensite is controlled to ≤1% (including 0%) by area ratio. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-tempered steel sheet, and more preferably to a steel sheet having high yield strength useful for large structures such as building structures and bridges.

  Steel sheets used for large structures as described above are required to have high toughness in addition to high strength. In order to produce a steel sheet satisfying these characteristics, it is common to perform quenching and tempering treatment after rolling. However, this method requires a reheating process after rolling, which increases the construction period and costs. Therefore, there is a demand for a method of manufacturing a steel sheet that omits the reheating treatment to shorten the construction period and reduce the cost. Control rolling is a technique that has seen remarkable development in recent years. In this technology, the crystal grains are refined by appropriately controlling the temperature and rolling reduction during rolling, and the strength and toughness of the steel are ensured as it is rolled. Such steel is generally called non-tempered steel.

By the way, high yield strength is required for large structural steel plates used as materials for bridges and the like. Therefore, the present inventors have proposed the technique of Patent Document 1 as a non-tempered steel sheet having increased toughness of the base material and increased yield strength. In this technique, the toughness and the high yield strength of the steel sheet can be achieved to some extent by strictly controlling the solid solution amount of Nb and B in the steel component composition. However, the yield strength achieved by Patent Document 1 is about 450 to 500 MPa, and further higher yield strength is desired.
JP 2004-149839 A ([Claims], [0006], [0052], etc.)

  This invention is made | formed in view of such a condition, The objective is to provide the non-tempered steel plate with high yield strength.

  The inventors of the present invention have intensively studied the technique of Patent Document 1 with the aim of further increasing the yield strength of the steel sheet. As a result, the steel composition is strictly defined, and the metal structure of the steel sheet is found to be bainite that suppresses the formation of island martensite. completed.

  That is, the non-tempered steel sheet having high yield strength according to the present invention is in mass%, C: 0.01 to 0.08%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.5%, Al: 0.01-0.07%, Cr: 0.05-1.5%, Mo: 0.05-1%, Ti: 0.005-0.03%, B: 0 .0005 to 0.003% and N: 0.002 to 0.008%, and the metal structure is bainite in which island-like martensite is suppressed to 1% or less (including 0%) in terms of area ratio. Has a summary.

Furthermore, as other elements,
(1) Nb: 0.005 to 0.03%,
(2) V: 0.005 to 0.05%,
(3) Cu: 0.05-3%,
(4) Ni: 0.05-3%,
(5) Group consisting of Zr: 0.0005-0.005%, Mg: 0.0003-0.005%, Ca: 0.0005-0.005%, and REM: 0.0003-0.003% One or more elements selected from
Etc. are preferably included. The balance is Fe and inevitable impurities.

  According to the present invention, a steel sheet having a high yield strength is obtained without tempering treatment by strictly defining the component composition of steel and making the metallographic structure of the steel sheet bainite with suppressed generation of island martensite. Can provide.

  The present inventors examined the above-mentioned Patent Document 1 aiming at further increasing the yield strength of the steel sheet. As a result, the component composition of steel is strictly defined, and if the metal structure of the steel sheet is made bainite, it becomes high strength (TS), and if the amount of island martensite formed in the bainite structure is reduced, the yield strength is increased. It was found that (higher YS) can be realized. First, how the present invention was completed will be described.

  The present inventors prepared several steel sheets having the same tensile strength (TS) by using bainite as the metal structure of the steel sheet, and measured the yield strength (YS) of each steel sheet. As a result, there was a variation in YS even if the steel plates had the same level of TS. Therefore, the present inventors examined the factors governing the YS of the steel sheet. YS of the steel sheet is related to the abundance of island martensite (hereinafter, island martensite may be referred to as MA) in the bainite structure. I found out that This will be described with reference to the drawings.

  FIG. 1 is a graph showing the relationship between the amount of MA produced in steel and the yield strength in steel that satisfies the component conditions of the steel of the present invention. FIG. 1 shows the change in YS when the amount of MA produced in steel is changed. The metal structure in steel is bainite, and island martensite is generated in the bainite structure. The TS at point A is 701 MPa, the TS at point B is 668 MPa, the TS at point C is 618 MPa, and TS can be said to be at the same level (JIS standard 60 kilo steel level).

  As can be seen from FIG. 1, YS tends to improve when the amount of MA produced in steel decreases, and when the amount of produced MA is 1% or less, YS is 500 MPa or more. On the other hand, the steel sheet in which many island martensites are generated in the bainite structure has a low YS. The present inventors consider that YS is lowered because dislocations are introduced at high density around the island martensite, and this dislocation lowers YS.

  As described above, in order to increase the yield strength of the non-tempered steel sheet, the amount of MA produced in the bainite structure may be reduced. Hereinafter, the present invention will be described in detail.

  First, the metal structure of the steel sheet according to the present invention will be described. The metal structure of the steel sheet of the present invention is bainite in which the formation of island martensite (MA) is suppressed to an extremely low level. By setting it as such a bainite structure, a high strength (TS) steel plate is obtained. That is, the metal structure of the present invention means that when the entire metal structure is 100% in area ratio, the structure other than bainite (for example, ferrite) is suppressed to less than 20% in area ratio. Means a structure mainly composed of bainite with 80% or more (preferably 90% or more).

  And in this invention, by suppressing the production | generation of MA among metal structures, it can prevent that a dislocation | rearrangement is introduce | transduced with high density around MA, and can raise the YS of a steel plate. Moreover, the toughness of the base material can be improved by suppressing the formation of MA.

  The MA (island martensite) is 1% or less, preferably 0.8% or less, and more preferably 0.5% or less, when the area of the entire metal structure is 100%.

  The metal structure of the steel sheet is observed with an optical microscope after the sample is subjected to nital corrosion, and the area ratio of bainite is calculated when the entire metal structure is 100%. The observation magnification may be 100 times. The observation position of the metal structure is a depth t / 4 position where the thickness of the steel sheet is t. The number of observation points is three, and the average area ratio of the bainite structure is calculated.

  The area ratio of MA is calculated by observing with an optical microscope after the sample is subjected to repeller corrosion, and calculating the area ratio of MA when the entire metal structure is 100%. The observation magnification may be 1000 times. The observation position of the metal structure is a depth t / 4 position where the thickness of the steel sheet is t. The number of observation points is three, and the area ratio of MA is calculated on average.

  Next, the component composition of the steel plate of the present invention will be described. The steel sheet of the present invention is in mass%, C: 0.01 to 0.08%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.5%, Al: 0.01 to 0. 0.07%, Cr: 0.05-1.5%, Mo: 0.05-1%, Ti: 0.005-0.03%, B: 0.0005-0.003%, and N: 0 It is important to include 0.002 to 0.008%. The reason for specifying such a range is as follows.

C: 0.01 to 0.08%
C is an important element for ensuring the strength (TS and YS) of the base material, and must be contained at least 0.01%. Preferably it is 0.015% or more, More preferably, it is 0.025% or more. However, if it exceeds 0.08%, the amount of C diffusion increases, and it becomes easy to locally concentrate C during the austenite transformation. Therefore, the amount of MA produced increases and YS cannot be increased. Further, when cooled after hot rolling, bainite is not generated and martensite is easily generated, and satisfactory base material toughness cannot be obtained. Therefore, in the present invention, the C content is set to 0.08% or less. Preferably it is 0.07% or less, More preferably, it is 0.055 or less.

Si: 0.05-0.5%
Si is an element that suppresses the amount of MA produced and increases the yield strength. Si is also an element useful as a deoxidizer, and it is necessary to contain 0.05% or more in order to effectively exhibit such an action. Preferably it is 0.1% or more. However, if over 0.5% is included, the toughness of the base metal is lowered, so the upper limit is made 0.5%. More preferably, it is 0.4% or less, More preferably, it is 0.3% or less.

Mn: 0.5 to 2.5%
Mn is an element that enhances hardenability and contributes to high strength (high TS and high YS) of the steel sheet. Moreover, since it accelerates and cools after hot rolling so that it may mention later in this invention, a bainite transformation can be accelerated | stimulated by improving hardenability and the spreading | diffusion of C can be suppressed. Therefore, the concentration of C can be suppressed, the generation of MA can be suppressed, and the yield strength of the steel sheet can be further increased. Mn is an element that also has the effect of increasing the base material toughness by refining bainite. In order to exhibit such an action effectively, it is necessary to contain 0.5% or more of Mn. Preferably it is 1% or more, More preferably, it is 1.2% or more. However, if the Mn content is excessive and exceeds 2.5%, the hardenability becomes too high and the base metal toughness is significantly deteriorated, so it should be suppressed to 2.5% or less. Preferably it is 2.25% or less.

Al: 0.01 to 0.07%
Al is an element useful as a deoxidizer. Moreover, Al has the effect | action which further improves the hardenability at the time of the accelerated cooling after the rolling by B by using together with B. By increasing the hardenability, the production of MA is suppressed and the yield strength of the steel sheet can be further increased as in the case of Mn. In order to exhibit such an action effectively, it is necessary to contain 0.01% or more. Preferably it is 0.020% or more. However, if the content exceeds 0.07%, the amount of alumina-based nonmetallic inclusions increases, the cleanliness is lowered, and the base material toughness is deteriorated. More preferably, it is good to set it as 0.06% or less.

Cr: 0.05 to 1.5%
Cr is an element that improves the hardenability and improves the strength (TS and YS) of the steel sheet. Further, by increasing the hardenability, the amount of MA produced can be reduced as in the case of Mn, which contributes to high yield strength. In addition, it is an element that promotes the refinement of bainite and contributes to the improvement of the base material toughness. In order to exhibit such an action effectively, it is necessary to contain 0.05% or more. Preferably it is 0.4% or more, More preferably, it is 0.7% or more. However, if it exceeds 1.5%, the weld crack resistance of the HAZ part particularly when performing high heat input welding is likely to deteriorate, so it is set to 1.5% or less. Preferably it is 1.2% or less.

Mo: 0.05 to 1%
Mo is an element that enhances hardenability and, like Cr, increases the strength (TS and YS) of the steel sheet and contributes to higher YS. Moreover, by containing together with B, the hardenability at the time of cooling after rolling is controlled, and the balance between strength (TS) and toughness of the base material can be optimized. In order to exert such an effect, it is necessary to contain 0.05% or more. A preferred lower limit is 0.10%. However, if excessively contained, the weld crack resistance of the HAZ part deteriorates, so 1% is made the upper limit. Preferably it is 0.8% or less, More preferably, it is 0.5% or less.

Ti: 0.005 to 0.03%
Ti is easy to combine with N to form a nitride, and is an element useful for fixing N in steel, resulting in an increase in solute B and solute Nb. As a result, the hardenability is improved and the strength (TS and YS) of the steel sheet can be increased. Ti, like Mn, also contributes to higher yield strength by suppressing the formation of MA. Further, Ti has the effect of suppressing the coarsening of the γ grains and preventing the deterioration of the base material toughness. In order to exhibit such an action effectively, it is necessary to contain 0.005% or more. Preferably it is 0.01% or more. However, if it exceeds 0.03% and is contained excessively, the toughness of the base metal is reduced instead. Preferably it is 0.02% or less.

B: 0.0005 to 0.003%
B is an element that enhances hardenability and improves the strength (TS and YS) of the steel sheet. Further, by increasing the hardenability, the amount of MA produced can be reduced as in the case of Mn, and the yield strength can be further increased. Furthermore, by containing together with Mo, the hardenability at the time of accelerated cooling after rolling is controlled, and the balance between strength (TS) and toughness of the base material is optimized. In order to exhibit such an action effectively, B must be contained in an amount of 0.0005% or more. Preferably it is 0.001% or more. However, if the content exceeds 0.003%, the toughness of the base metal deteriorates, so the B content should be suppressed to 0.003% or less. Preferably it is 0.0025% or less.

N: 0.002 to 0.008%
N combines with Al and Ti to form nitrides, and effectively acts to improve the toughness of the base metal by refining the structure. In order to exert such an action effectively, it is necessary to contain 0.002% or more. Preferably it is 0.003% or more. However, if N is contained excessively, the toughness of the weld metal is deteriorated or the hardenability of the base material is lowered, so the N content needs to be suppressed to 0.008% or less. Preferably it is 0.007% or less, More preferably, it is 0.006% or less. It is recommended that the N content be as low as possible.

The steel plate according to the present invention contains the above components, and may further contain other elements as necessary. For example,
(1) Nb: 0.005 to 0.03%,
(2) V: 0.005 to 0.05%,
(3) Cu: 0.05-3%,
(4) Ni: 0.05-3%,
(5) Group consisting of Zr: 0.0005-0.005%, Mg: 0.0003-0.005%, Ca: 0.0005-0.005%, and REM: 0.0003-0.003% One or more elements selected from
Etc. can be included. Further, although other elements may be contained, the balance is usually Fe and inevitable impurities (for example, P and S). The amounts of P and S allowed as inevitable impurities are, for example, P: 0.020% or less (including 0%) and S: 0.010% or less (including 0%).

Nb: 0.005 to 0.03%
Nb is an element that increases the hardenability at the time of accelerated cooling after rolling by containing it together with Mo and B, and can increase the strength (TS and YS) of the steel sheet. Moreover, since yield of MA can be reduced with the improvement of hardenability, high yield strength can be achieved. Furthermore, by containing together with B, the hardenability during accelerated cooling after rolling is controlled, and the balance between the strength (TS) and toughness of the base material can be optimized. In order to exert such an effect effectively, it is recommended to contain 0.005% or more. More preferably, it is 0.01% or more. However, if the content exceeds 0.03%, the bainite becomes coarse and the toughness of the base material decreases, so the upper limit is preferably set to 0.03%. More preferably, it is 0.025% or less.

V: 0.005-0.05%
V is an element that effectively acts to increase the strength (TS and YS) of the base material. In order to exhibit such an action effectively, it is preferable to contain 0.005% or more. More preferably, it is 0.01% or more. However, if the content is excessive, the high heat input HAZ toughness may decrease, so the upper limit is made 0.05%. A more preferable upper limit is 0.04% or less.

Cu: 0.05 to 3%
Cu is an element that effectively acts to increase the strength (TS and YS) of the base material by solid solution strengthening and precipitation strengthening. In order to exhibit such an action effectively, it is desirable to make it contain 0.05% or more. More preferably, it is 0.2% or more. However, if excessively contained, the high heat input HAZ toughness may decrease, so the content is made 3% or less. More preferably, it is 1% or less, More preferably, it is 0.6% or less, Most preferably, it is 0.5% or less.

Ni: 0.05-3%
Ni is an element that effectively acts to improve the base material toughness. In order to exhibit such an action effectively, it is preferable to contain 0.05% or more. More preferably, it is 0.2% or more. However, since an excessive addition exceeding 3% tends to cause scale soot, the upper limit is preferably made 3%. More preferably, it is 2% or less, More preferably, it is 1.5% or less.

1 selected from the group consisting of Zr: 0.0005 to 0.005%, Mg: 0.0003 to 0.005%, Ca: 0.0005 to 0.005%, REM: 0.0003 to 0.003% More than the seed elements Zr, Mg, and Ca are formed by depositing low-melting point oxides such as ZrO ₂ , MgO, and CaO in the prior austenite grains during cooling after high heat input welding and transforming them into bainite using them as nuclei. To refine. This is an element that complicates the fracture path and improves the high heat input welding HAZ toughness. On the other hand, Ca and REM are elements that act effectively to improve the drawing and toughness of the S segregation part present in the center of the plate thickness by combining with a small amount of S present in the steel to form a sulfide. .

  Zr is 0.0005% or more (particularly 0.001% or more), 0.005% or less (particularly 0.003% or less), and Mg is 0.0003% or more in order to effectively exert such an action. In particular, 0.001% or more), 0.005% or less (particularly 0.004% or less), Ca is 0.0005% or more (particularly 0.001% or more), 0.005% or less (particularly 0 0.004% or less) and REM are preferably contained in an amount of 0.0003% or more (particularly 0.001% or more) or 0.003% or less (particularly 0.01% or less).

P: 0.020% or less (including 0%)
P is an element that affects the base material toughness, and if it exceeds 0.020%, the base material toughness deteriorates remarkably, so the upper limit is made 0.020%. More preferably, it is good to restrain to 0.010% or less.

S: 0.010% or less (including 0%)
S, like P, is an element that affects the base material toughness. If the content exceeds 0.010%, coarse sulfides are generated to deteriorate the base material toughness. %. More preferably, it is desirable to suppress it to 0.005% or less.

  The steel plate according to the present invention can be manufactured, for example, as follows. That is, it can be manufactured by appropriately controlled rolling and then appropriately accelerated cooling.

  The heating temperature at the time of controlled rolling (hot rolling) is about 1000 to 1150 ° C. If the steel piece is heated to 1000 ° C. or higher, Nb and B can be dissolved, and excessive refinement of the austenite grain size can be suppressed, so that the hardenability can be improved. For this reason, local concentration of C due to diffusion of C can be prevented, and generation of MA can be suppressed and high yield strength can be increased. More preferably, it is desirable to heat to 1050 ° C. or higher. However, if the heating temperature exceeds 1150 ° C., the γ grains become coarse and the base material toughness deteriorates, so the heating temperature is set to 1150 ° C. or lower. More preferably, it is 1100 degrees C or less.

  Regarding the rolling conditions, the heated steel slab may be rolled at an arbitrary reduction amount, but the reduction amount from 860 ° C. to the rolling end temperature is preferably 45 to 70%. By suppressing the reduction in the temperature range from 860 ° C. to the end temperature of rolling to 70% or less, it is possible to prevent excessive refinement due to recrystallization of austenite grains and introduction of excessive strain into the austenite grains. Improves. Therefore, local concentration of C due to diffusion of C can be prevented, and generation of MA can be suppressed and high yield strength can be increased. Also, if the amount of reduction in this temperature range becomes excessive, the work transformation is promoted and ferrite precipitates and does not become a bainite-based structure, and it becomes difficult to ensure the base material strength (TS), so the upper limit of the amount of reduction is 70% is preferable. More preferably, it is 65% or less. However, if the amount of reduction is too small, the bainite structure becomes coarse and the base material toughness decreases, so the amount of reduction is 45% or more. More preferably, it is 50% or more.

  The amount of rolling reduction is the ratio of the difference between the plate thickness at 860 ° C. and the plate thickness at the rolling end temperature with respect to the plate thickness at 860 ° C. That is, the amount of reduction in the temperature range from 860 ° C. to the rolling end temperature can be calculated by the following equation (1).

  Reduction amount (%) = [(plate thickness at 860 ° C.−plate thickness at rolling end temperature) / plate thickness at 860 ° C.] × 100 (1)

  The rolling end temperature is preferably 770 to 700 ° C. By setting the rolling end temperature to 700 ° C. or higher, it is possible to prevent excessive strain from being introduced into the austenite grains, so that the hardenability is improved. Therefore, local concentration of C due to diffusion of C can be prevented, and generation of MA can be suppressed and high yield strength can be increased. When the temperature is lower than 700 ° C., ferrite transformation is promoted and ferrite is precipitated, so that the metal structure does not become bainite and it is difficult to ensure the base material strength (TS). Therefore, it is recommended that the rolling end temperature be 700 ° C or higher. More preferably, it is 710 degreeC or more. The upper limit of the rolling end temperature is 770 ° C. By finishing the rolling at 770 ° C. or less, refinement of the bainite structure is promoted, and the base material toughness can be ensured. Preferably it is 750 degrees C or less.

  The accelerated cooling after the controlled rolling is performed from the rolling end temperature to 400 ° C., for example, in the range of 3 ° C./sec or more. By performing accelerated cooling after hot rolling, it is possible to prevent C from being concentrated due to C diffusion during austenite transformation, so that MA formation can be suppressed. Therefore, the yield strength can be increased. If a large amount of MA is generated, the cooling rate may be increased. More preferably, it is 5 ° C./sec or more, and further preferably 7 ° C./sec or more.

  This will be described with reference to the drawings. FIG. 2 shows a case where the steel satisfying the component conditions of the steel of the present invention has a heating temperature during hot rolling of 1100 ° C., a reduction amount from 860 ° C. to the rolling end temperature of 60%, and a rolling end temperature of 720 ° C. It is a graph which shows the amount of MA production | generation when changing the cooling rate after hot rolling. As is apparent from FIG. 2, when the cooling rate after hot rolling is increased, the amount of MA produced tends to decrease.

  In the production method of the present invention, steel plates having various thicknesses can be produced. For example, a high yield strength can be achieved at a thickness of 20 to 100 mm (particularly 25 to 80 mm).

  The steel sheet according to the present invention is useful as a material for large structures such as building structures and bridges.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steel having the composition shown in Table 1 below (the balance is Fe and inevitable impurities) is melted by a normal method to form a slab, then heated to 1000 to 1150 ° C., and rolling is finished at the temperature shown in Table 2 below. A steel plate was produced. At this time, the amount of reduction in the temperature range from 860 ° C. to the rolling end temperature was 45 to 70%. Next, from the end of rolling to 400 ° C., cooling was performed at a cooling rate shown in Table 2 below. Table 2 below shows the heating temperature, the amount of reduction in the temperature range from 860 ° C. to the rolling end temperature, the rolling end temperature, the cooling rate from the end of rolling to 400 ° C., and the thickness of the steel sheet.

  The area ratio of MA was calculated by the following procedure for each obtained steel plate. About each obtained steel plate, the sample was extract | collected from 1/4 position of plate | board thickness, the repeller corrosion was carried out, and it observed with the optical microscope (1000 times magnification). The number of observation visual fields was three, and the area ratio with respect to the entire metal structure was calculated. The results are shown in Table 2 below.

  Moreover, the metal structure (remainder) of each obtained steel plate was sampled from a 1/4 position of the plate thickness, was subjected to Nital corrosion, and observed with an optical microscope (100 times magnification). The number of observation visual fields was three. The results are shown in Table 2 below. In Table 2, B means bainite as referred to in the present invention, and means a structure in which bainite is generated in an area ratio of 80% or more when the entire metal structure is 100%. However, the structure (for example, ferrite etc.) other than bainite is included to such an extent that the effect of the present invention is not impaired. On the other hand, B + F means a structure in which ferrite is generated in an area ratio of 20% or more as a structure other than bainite when the entire metal structure is 100%. Since ferrite is generated in an area ratio of 20% or more, strength (TS and YS) cannot be secured.

Next, a tensile test and an impact test were performed on each of the obtained steel plates, and the base material properties [tensile strength (TS), 0.2% proof stress (YS) and toughness] were evaluated in the following manner.
(1) Tensile test: A JIS No. 4 test piece was sampled from the position of 1/4 of the thickness of each steel plate and subjected to a tensile test. In the tensile test, tensile strength and 0.2% proof stress were measured. The tensile strength (TS) was 570 MPa or more, and the 0.2% proof stress (YS) was 500 MPa or more.
(2) Impact test: A JIS No. 4 test piece was taken from the position of 1/4 of the thickness of each steel plate, subjected to a Charpy impact test to determine the ductile fracture surface ratio, and the fracture surface transition temperature was calculated. The case where the fracture surface transition temperature (vTrs) was −60 ° C. or lower was regarded as acceptable (good base material toughness).

  It can be considered from Table 2 as follows. No. 1 to 10 are steel plates that satisfy the requirements defined in the present invention, and have high strength (TS) and high yield strength (YS). Moreover, the impact properties (base material toughness) are also good. On the other hand, no. 11 to 20 are steel plates that do not satisfy the requirements defined in the present invention, and the yield strength (YS) is particularly low.

It is a graph which shows the relationship between MA production | generation amount in steel and yield strength in steel which satisfies the component conditions of this invention steel. It is a graph which shows MA production | generation amount when changing the cooling rate after hot rolling about steel which satisfies the component conditions of this invention steel.

Claims (7)

% By mass
C: 0.01 to 0.08%,
Si: 0.05 to 0.5%,
Mn: 0.5 to 2.5%
Al: 0.01 to 0.07%,
Cr: 0.05 to 1.5%,
Mo: 0.05 to 1%
Ti: 0.005 to 0.03%,
B: 0.0005-0.003%, and N: 0.002-0.008%,
A non-tempered steel sheet having a high yield strength, wherein the metal structure is bainite in which island-like martensite is suppressed to 1% or less (including 0%) in terms of area ratio.   The non-tempered steel sheet according to claim 1, further comprising Nb: 0.005 to 0.03% as another element.   Furthermore, V: 0.005-0.05% is contained as another element, The non-tempered steel plate of Claim 1 or 2.   Furthermore, Cu: 0.05-3% is contained as another element, The non-tempered steel plate in any one of Claims 1-3.   Furthermore, Ni: 0.05-3% is contained as another element, The non-tempered steel plate in any one of Claims 1-4. Furthermore, as other elements,
Zr: 0.0005 to 0.005%,
Mg: 0.0003 to 0.005%,
The non-tempered steel sheet according to any one of claims 1 to 5, which contains at least one selected from the group consisting of Ca: 0.0005 to 0.005% and REM: 0.0003 to 0.003%.   The non-tempered steel sheet according to any one of claims 1 to 6, wherein the balance is Fe and inevitable impurities.

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