JPH0726150B2 - Manufacturing method of tempered high-strength steel sheet with excellent weldability and brittle fracture propagation stopping properties - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics, and more specifically, a tank,
Tensile strength 60kgf / mm ² used for bridges, penstocks, etc.
The present invention relates to a method for producing a heat-treated high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics.

[Prior art] Conventionally, tensile strength has been applied to tanks, bridges, penstocks, etc.
60~80kgf / mm ² class of tempered high tensile steel plates have been used.

These steel sheets have Mn, Ni, and
Since it contains a large amount of alloying elements such as Cr and Mo, and in order to prevent welding cold cracking, preheating at a temperature of about 100 ° C was required, and weldability was not sufficient.

Therefore, the C content, which has the most adverse effect on the weld crack resistance, is significantly lower than that of the conventional steel, and the strength decrease due to the reduction of the C content is compensated by utilizing the effect of improving the hardenability of a small amount of B. An excellent tempered high-strength steel sheet has been developed and is already in wide use.

However, when the C content is reduced in the heat-treated high-strength steel sheet, the cause is not yet clear, but there is a problem that the brittle fracture propagation stopping property is not good as compared with the conventional steel having a large C content.

In order to improve this brittle fracture propagation stopping property, it is effective to contain Ni that significantly improves toughness,
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
There is a problem that weldability deteriorates due to an increase of + V / 10 + 5B and further increases in cost, which is not preferable.

From this conventional technical problem, by adjusting the manufacturing conditions rather than adjusting the contained components and component ratios of steel,
Techniques for improving the brittle fracture propagation stopping property have been developed (see Japanese Patent Publication Nos. 62-001456 and 63-058906).

Thus, the invention of Japanese Patent Publication No. 63-058906 discloses that the structure is a mixed structure of ferrite and fine dispersed martensite by direct quenching after controlled rolling, whereby a high plastic deformability of ferrite and a mixed structure of ferrite are obtained. It is a technology that stops fracture by utilizing high work hardening performance, but since this technology produces ferrite, 60 kgf /
There is a problem that it is difficult to secure a strength of mm ² or more.

Further, the invention of Japanese Patent Publication No. 63-058906 discloses that by direct quenching after controlled rolling, a martensite structure in which the surface layer structure is elongated is obtained, thereby ensuring good toughness at each position in the plate thickness direction and brittle fracture. This is a technique to improve the propagation stop property, but this technique cannot control the strict control of controlled rolling.If the reduction is excessive at low temperature, the central part of the plate thickness will be insufficiently quenched and the upper bainite will be generated. The toughness deteriorates. On the contrary, if the reduction at low temperature is too small, the surface layer portion does not have an elongated structure, and the toughness of the surface layer portion deteriorates.

[Problems to be Solved by the Invention] The present invention has been made in view of various problems in the above-described conventional method for producing a steel having improved weldability and brittle fracture propagation stopping characteristics, that is, the present inventor. Is a brittle fracture propagation arresting property after conducting extensive research and improvement in order to improve the brittle fracture propagation arresting property of the tempered high-strength steel sheet with a tensile strength of 60 to 80 kgf / mm ² class of low C steel. It was found that as a cause of deterioration, the austenite grains become coarse due to the low C content, and the surface layer portion having a high cooling rate during quenching becomes a coarse grain martensite structure and the toughness deteriorates.

Therefore, the present inventor has studied a technique for forming a mixed structure of fine grain martensite + lower bainite having a good toughness in the surface layer with the components and the component ratio of the low C steel, and as a result, the heat treatment method has been conventionally used. Since reheating quenching and tempering are performed, direct quenching and tempering after hot rolling are performed.
We have found that it is effective to perform appropriate control of heating / rolling conditions, which is the previous stage, and by quantifying the appropriate conditions, we can improve the weldability and brittle fracture propagation stop properties with excellent heat treatment. We have developed a method for manufacturing tensile steel sheets.

[Means for Solving the Problems] The method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics according to the present invention is (1) C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt% , Mn 0.4
~ 1.5wt%, Cr 0.05 ~ 1.0wt%, Mo 0.05 ~ 1.0wt%, V 0.01 ~ 0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, and Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt%, and the balance Fe and Steel consisting of unavoidable impurities, heating temperature ≤ 1250 ℃, total rolling reduction ≥ 75%, and finishing temperature T
Hot rolling was completed under the condition that F (° C) satisfies the following formula 0.063 (0.333TF + 70.4C-300) ² + 0.10TF-151C-229 ≦ -130 (1) (C is the carbon amount in wt%) After that, direct quenching is performed from the temperature of the austenite region, and then tempering is performed at a temperature of less than the Ac ₁ point. (2) C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, Mn 0.4
~ 1.5wt%, Cr 0.05 ~ 1.0wt%, Mo 0.05 ~ 1.0wt%, V 0.01 ~ 0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Nb 0.005 to
A steel containing 0.1 wt% and satisfying Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% with the balance Fe and unavoidable impurities
Heating temperature at a temperature of 50 ° C or less and at a temperature at which Nb carbonitride is completely dissolved, total rolling reduction ≧ 75%, and finishing temperature TF (° C) is 0.063 (0.333TF + 70.4C- 300) ² + 0.067TF-151C-203 ≤-130 (2) (C is the carbon content of wt%) After the hot rolling is finished, quenching is performed directly from the temperature in the austenite region, and then, The second invention is a method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics, which is characterized by performing tempering at a temperature below the Ac ₁ point, and (3) C 0.04 to 1.10 wt% , Si 0.01 to 0.50 wt%, Mn 0.4
~ 1.5wt%, Cr 0.05 ~ 1.0wt%, Mo 0.05 ~ 1.0wt%, V 0.01 ~ 0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Cu 0.05 to 0.50 wt%, Ni 0.05 to 1.5 wt%, Ti 0.005 to 0.0
Contains 1 or 2 or more selected from 3 wt% and Ca 0.001 to 0.005 wt%, and satisfies Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% Then, the steel consisting of the balance Fe and unavoidable impurities is heated at a temperature of ≤1250 ° C, a total reduction of ≥75%, and a finishing temperature T
Hot rolling was completed under the condition that F (° C) satisfies the following formula 0.063 (0.333TF + 70.4C-300) ² + 0.10TF-151C-229 ≦ -130 (1) (C is the carbon amount in wt%) After that, direct quenching is performed from the temperature of the austenite region, and then tempering is performed at a temperature of less than the Ac ₁ point. (4) C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, Mn 0.4
~ 1.5wt%, Cr 0.05 ~ 1.0wt%, Mo 0.05 ~ 1.0wt%, V 0.01 ~ 0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Nb 0.005 to
0.1 wt%, Cu 0.05 to 0.50 wt%, Ni 0.05 to 1.5 wt%, Ti 0.005 to 0.0
Contains 1 or 2 or more selected from 3 wt% and Ca 0.001 to 0.005 wt%, and satisfies Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% Steel containing the balance Fe and unavoidable impurities
Heating temperature at a temperature of 50 ° C or less and at a temperature at which Nb carbonitride is completely dissolved, total rolling reduction ≧ 75%, and finishing temperature TF (° C) is 0.063 (0.333TF + 70.4C- 300) ² + 0.067TF-151C-203 ≤-130 (2) (C is the carbon content of wt%) After the hot rolling is finished, quenching is performed directly from the temperature in the austenite region, and then, A fourth aspect of the present invention is a method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics, which is characterized by performing tempering at a temperature of less than Ac ₁ point.

The method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping properties according to the present invention will be described in detail below.

First, the contained components and component ratios of steel used in the method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping properties according to the present invention will be described.

C is an element necessary to secure the strength as a high-strength steel plate. If the content is less than 0.04 wt%, the tensile strength is 60 kgf / mm.
It is difficult to secure ² or more, and if it exceeds 0.10 wt%, the weld crack resistance will be impaired.
Therefore, the C content is 0.04 to 0.10 wt%.

Si is an element effective for deoxidation, and if the content is less than 0.01 wt%, this effect is small, and if it exceeds 0.50 wt% excessively, weldability and toughness are deteriorated. Therefore, Si
The content is 0.01 to 0.50 wt%.

Mn is an element necessary for improving the hardenability and ensuring the strength inside the plate thickness. If the content is less than 0.4 wt%, such effects cannot be sufficiently secured, and if it exceeds 1.5 wt%. If it is contained excessively, the weldability and toughness deteriorate.
Therefore, the Mn content is 0.4 to 1.5 wt%.

Cr is an element effective in improving the hardenability, and if the content is less than 0.05 wt%, this effect cannot be fully exerted, and if it exceeds 1.0 wt%, the weldability is increased. Will hurt you. Therefore, the Cr content is 0.05 to 1.
0 wt%

Mo is an element that enhances the hardenability and increases the temper softening resistance. If the content is less than 0.05 wt%, this effect is not sufficient, and if it exceeds 1.0 wt%, the weldability is impaired. And expensive. Therefore, the Mo content is
0.05 to 1.0 wt%

V is an element that increases the hardenability and increases the resistance to temper softening when it is contained in a small amount. If the content is less than 0.01 wt%, such an effect is not sufficient, and 0.1 wt%
If it is contained in excess of 10%, the toughness deteriorates. Therefore, the V content is 0.01 to 0.1 wt%.

B is an element that significantly enhances the hardenability by containing a trace amount. If the content is less than 0.0005 wt%, a sufficient effect cannot be obtained, and if it exceeds 0.005 wt%, the hardenability is improved. The effect is saturated, and the toughness deteriorates due to the precipitation of the B compound. Therefore, the B content is 0.0005.
~ 0.005 wt%

Al is a deoxidizing element, and is an element having an effect of fixing N as AlN in order to secure the hardenability effect of B. If the content is less than 0.01 wt%, such an effect is small, and , If the content exceeds 0.1 wt%, the toughness deteriorates. Therefore, the Al content is 0.01 to 0.1 wt%.

Nb is an element effective for refining and spreading of austenite grains through the effect of suppressing recrystallization during rolling.
If it is less than 0.005 wt%, such effect is small, and if it is 0.1
Excessive content exceeding wt% deteriorates toughness. Yotsutte
The Nb content is 0.005-0.1 wt%.

In addition to the components described above, Cu, Ni, a fixed element of Ni and N, which is effective as a hardenability-improving element depending on the strength level and plate thickness, and Ca, which is a morphology controlling element of inclusions, are selected. It is possible to contain one kind or two or more kinds.

Cu is an element effective in increasing strength by solid solution strengthening and precipitation strengthening, and if the content is less than 0.05 wt%, such effects cannot be sufficiently exhibited, and if it exceeds 0.50 wt%, it becomes excessive. If contained, hot workability deteriorates, and surface cracks are likely to occur. Therefore, the Cu content is 0.05 to 0.50 wt%.

Ni is an element that improves hardenability and toughness of the base metal, and if the content is less than 0.05 wt%, such an effect is not sufficient, and if it exceeds 1.5 wt%, it is excessively contained. And scale flaws are more likely to occur, further increasing the cost. Therefore, the Ni content is 0.05 to 1.5 wt%
And

Ti is a carbonitride forming element, and is an element effective for fixing N as TiN in order to stabilize the effect of improving the hardenability of B. When the content is less than 0.005 wt%, The effect is not sufficient, and if it is contained in excess of 0.03 wt%, the toughness deteriorates. Therefore, the Ti content is 0.005 to 0.03 wt%.

Ca has a spheroidizing effect on non-metallic inclusions and is an element effective in reducing anisotropy. If the content is less than 0.001 wt%, such an effect is not sufficient, and if it exceeds 0.005 wt%. If added too much, non-metallic inclusions increase and the toughness deteriorates. Therefore, the Ca content is 0.001 to 0.005 wt%.

Next, the manufacturing conditions of the method for manufacturing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping properties according to the present invention will be described.

When the steel does not contain Nb of the components and component ratios described above, it is heated to a temperature of 1250 ° C or lower because austenite grains grow abnormally at a temperature higher than this temperature, and sufficient reduction is applied. This is because the austenite grains do not become fine and the toughness becomes poor.

In addition, the Nb of the steel of the content components and component ratios described above
When Nb is contained, the temperature at which the carbonitride of Nb completely forms a solid solution at 1250 ° C or lower (changes depending on the Nb, C, and N contents, but according to Irvine et al., [Nb (wt%)] × [C + 12 / 14N]
≤2.26-6770 / T) can be evaluated as Nb in steel.
If the carbonitride remains coarse, the effect of suppressing recrystallization of austenite during rolling by Nb cannot be sufficiently obtained, and
This is because the toughness becomes poor.

The reason why the total rolling reduction is set to 75% or more is that if the rolling reduction is less than this, the coarse austenite grains during heating are not sufficiently refined in the central portion of the plate thickness, resulting in poor toughness.

Further, the reason why the finishing temperature TF (° C.) is set to the formula (1) or the formula (2) as described in the claims will be described.

Fig. 1 shows the NDT temperature (one measure of the brittle fracture propagation stopping property) in the drop weight test when the plate was rolled under various rolling conditions and then directly quenched and tempered to a tensile strength of approximately 68 kgf / mm ^2. FIG. 3 is a diagram showing a relationship between vTrs (fracture surface transition temperature in a Charpy impact test) on a surface of 3 mm. The components and ratios of the steel used in Fig. 1 are C 0.
07〜0.10wt% -Si 0.25wt% -Mn 1.25wt% -Cr 0.15wt
% -Mo 0.10wt% -Nb 0-0.015wt% -V 0.037wt% -B
0.001wt% -Pcm 0.16-0.19wt%, heating temperature: 1200
℃, total reduction: 80%, finishing temperature: 1000 ~ 840 ℃, quenching:
Direct quenching, tempering: Temperature adjustment targeting TS = 68 kgf / mm ² , test piece: JIS No. 4 (5 mm subsize).

From FIG. 1, there is a good correlation between the NDT temperature and the surface vTrs, and in order to improve the brittle fracture propagation stopping property,
Clearly, there is a need to improve the toughness of the surface.

As the factors that govern this toughness, as-quenched hardness (an index of the quenched structure) and austenite grain size are important, and the relationship between these and toughness was quantified in the case of the surface portion. As a result, the surface vTrs can be accurately estimated by the following equation using the as-quenched Vickers hardness HVQ, and the average width d (cm) and C content (wt%) of the austenite grains in the crack propagation direction. Was found (see FIG. 2).

vTrs = 0.063 {HVQ-0.83 ( 1120C + 270)} 2 -66.7logd -1/2 -68 (℃) (3) HVQ: quenching remains Vickers hardness d: average width of the crack advancing direction of the austenite grains C: carbon content Amount (wt%) From this formula, for VTrs, there is an optimum value of HVQ for each C content, and if it is higher or lower than that value, toughness deteriorates, and d is reduced. It can be seen that the toughness becomes better.

Next, the heating temperature, the rolling reduction, the finishing temperature and the C content were selected as the factors controlling HVQ and d in this equation, and the relationship between these and HVQ and d was quantified.

As a result, the heating temperature is 1250 ° C or lower (when Nb is contained, it is 1250 ° C or lower, and the temperature at which the Nb carbonitride is completely dissolved), and as long as the total rolling reduction is 75% or more. , HVQ and d are determined only by the rolling finish temperature TF, and can be expressed by the following equations (see FIGS. 3 and 4).

In this FIG. 3, HVQ = 0.333TF + 1000C−81 (4) TF: rolling finishing temperature (° C.) C: carbon content (wt%).

In Fig. 4, logd- ¹ / ² = -1.5TF / 1000 + 2.26C + 2.42 (5) (Nb-free) logd- ¹ / ² = TF / 1000 + 2.26C + 2.03 (5 ') (Nb-containing) TF: rolling finishing temperature (° C) C: carbon content (wt%) Fig. 4 (a) does not contain Nb (shown by the above formula (5)),
In Fig. 4 (b), the Nb content is 0.015 wt% (the above formula (5 ')
Indicate. ).

In order to secure the brittle fracture propagation stopping property equivalent to or higher than that of the conventional steel having a relatively large C content, it is necessary to set the NDT temperature to -50 ° C or lower, and in order to satisfy this, FIG. It can be seen that it is necessary to keep the surface vTrs at -130 ° C or lower. In the inequality vTrs ≤ −130 of the equation (3),
By substituting the expressions (5) and (5 ′) for rearranging, the expressions (1) and (2) described in the claims are obtained.

For the reason explained above, the finishing temperature TF is defined by the formulas (1) and (2) in the claims.

Further, the direct quenching from the temperature of the austenite region, heating, from the fine grained austenite structure containing a large number of bainite transformation nuclei such as the deformation zone obtained by the control of the rolling conditions, tough fine martensite and This is because the mixed structure of the lower bainite is generated on the surface portion.

The reason why the direct quenching is performed from the austenite region is that if quenching is performed at a temperature lower than that, it is difficult to secure ferrite with a strength of 60 kgf / mm ² or more.

Further to the tempering temperature and the temperature of the Ac less than ₁ point is transformed into tissue partially austenite at Ac ₁ point or higher, ferrite and upper bainite is generated after tempering, because results in a significant decrease in strength Is.

[Example] An example of a method for producing a heat-treated high-strength steel excellent in weldability and brittle fracture propagation stopping property according to the present invention will be described.

Example A slab is manufactured by smelting steel having the contained components and component ratios shown in Table 1 by a conventional method, and this slab is rolled into various plate thicknesses under the heating conditions and rolling conditions shown in Table 2. After performing the heat treatment shown in Table 2, a tensile test and an impact test were performed, and the weldability was evaluated by an oblique Y-type weld cracking test.

Moreover, the brittle fracture propagation stopping property was evaluated by a drop weight test or a double tensile test.

Table 3 shows these results.

As is clear from Table 3, all of the steel sheets A to G produced by the method for producing a heat-treated high-strength steel having excellent weldability and brittle fracture propagation stopping characteristics according to the present invention have 60 kgf / mm ² It has excellent tensile strength and good weldability with a crack prevention preheating temperature of 25 ° C or higher.

Furthermore, Kca value at NDT temperature of -60 ℃ or less and 0 ℃ in drop weight test It has the excellent brittle fracture propagation stopping properties described above.

Compared with the steel sheet produced by the method for producing a heat-treated high-strength steel having excellent weldability and brittle fracture propagation stopping characteristics according to the present invention, the comparative steel H has a Pcm value that is too high, so the weldability is insufficient. Comparative steels I to M do not have appropriate rolling conditions,
The brittle fracture propagation stopping property is not sufficient.

[Effects of the Invention] As described above, since the method for producing a heat-treated high-strength steel excellent in weldability and brittle fracture propagation stopping characteristics according to the present invention has the above-described configuration, the weldability and brittle fracture propagation stopping characteristics are thus obtained. It has an effect that it is possible to effectively produce a heat-treated high-strength steel sheet having excellent low C tensile strength of 60 to 80 kgf / mm ² class.

[Brief description of drawings] Fig. 1 is a diagram showing the relationship between vTrs (° C) and NDT temperature (° C) on a surface of 3 mm, and Fig. 2 is the measured vTrs (° C) and calculated vTrs of the surface.
Fig. 3 shows the relationship with (℃), and Fig. 3 shows the rolling finish temperature TF.
Diagram showing the relationship between (℃) and DQ hardness (HVQ), No. 4
Figures (a) and (b) are diagrams showing the relationship between the rolling finish temperature TF (° C) and logd ¹² (cm ¹² ) when Nb is not contained and when Nb is contained.

Claims (4)

[Claims] 1. C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, M
n 0.4-1.5wt%, Cr 0.05-1.0wt%, Mo 0.05-1.0wt%, V 0.01-0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, and Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt%, and the balance Fe and Steel consisting of unavoidable impurities, heating temperature ≤ 1250 ℃, total rolling reduction ≥ 75%, and finishing temperature T
Hot rolling was completed under the condition that F (° C) satisfies the following formula 0.063 (0.333TF + 70.4C-300) ² + 0.10TF-151C-229 ≦ -130 (1) (C is the carbon amount in wt%) Then, a method for producing a heat-treated high-strength steel sheet having excellent weldability and brittle fracture propagation stopping properties, characterized by performing direct quenching from a temperature in the austenite region and then tempering at a temperature of less than the Ac ₁ point. 2. C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, M
n 0.4-1.5wt%, Cr 0.05-1.0wt%, Mo 0.05-1.0wt%, V 0.01-0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Nb 0.005 to
A steel containing 0.1 wt% and satisfying Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% with the balance Fe and unavoidable impurities
Heating temperature at a temperature of 50 ° C or less and at a temperature at which Nb carbonitride is completely dissolved, total rolling reduction ≧ 75%, and finishing temperature TF (° C) is 0.063 (0.333TF + 70.4C- 300) ² + 0.067TF-151C-203 ≤-130 (2) (C is the carbon content of wt%) After the hot rolling is finished, quenching is performed directly from the temperature in the austenite region, and then, A method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics, characterized by performing tempering at a temperature below the Ac ₁ point. 3. C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, M
n 0.4-1.5wt%, Cr 0.05-1.0wt%, Mo 0.05-1.0wt%, V 0.01-0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Cu 0.05 to 0.50 wt%, Ni 0.05 to 1.5 wt%, Ti 0.005 to 0.0
Contains 1 or 2 or more selected from 3 wt% and Ca 0.001 to 0.005 wt%, and satisfies Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% Then, the steel consisting of the balance Fe and unavoidable impurities is heated at a temperature of ≤1250 ° C, a total reduction of ≥75%, and a finishing temperature T
Hot rolling was completed under the condition that F (° C) satisfies the following formula 0.063 (0.333TF + 70.4C-300) ² + 0.10TF-151C-229 ≦ -130 (1) (C is the carbon amount in wt%) Then, a method for producing a heat-treated high-strength steel sheet having excellent weldability and brittle fracture propagation stopping properties, characterized by performing direct quenching from a temperature in the austenite region and then tempering at a temperature of less than the Ac ₁ point. 4. C 0.04 to 1.10 wt%, Si 0.01 to 0.50 wt%, M
n 0.4-1.5wt%, Cr 0.05-1.0wt%, Mo 0.05-1.0wt%, V 0.01-0.1wt
%, B 0.0005 to 0.005 wt%, Al 0.01 to 0.1 wt%, Nb 0.005 to
0.1 wt%, Cu 0.05 to 0.50 wt%, Ni 0.05 to 1.5 wt%, Ti 0.005 to 0.0
Contains 1 or 2 or more selected from 3 wt% and Ca 0.001 to 0.005 wt%, and satisfies Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.22wt% Steel containing the balance Fe and unavoidable impurities
Heating temperature at a temperature of 50 ° C or less and at a temperature at which Nb carbonitride is completely dissolved, total rolling reduction ≧ 75%, and finishing temperature TF (° C) is 0.063 (0.333TF + 70.4C- 300) ² + 0.067TF-151C-203 ≤-130 (2) (C is the carbon content of wt%) After the hot rolling is finished, quenching is performed directly from the temperature in the austenite region, and then, A method for producing a tempered high-strength steel sheet having excellent weldability and brittle fracture propagation stopping characteristics, characterized by performing tempering at a temperature below the Ac ₁ point.