JP5181517B2 - Steel sheet for hot pressing - Google Patents

Description

  The present invention relates to a steel sheet used for a structural part mainly requiring strength of an automobile, in particular, a hot-press steel sheet suitable for manufacturing a structural part by heating in the γ region and hot pressing. About.

  In recent years, there have been great demands for environmental measures and safety measures for automobiles, and the aim is to reduce the weight of vehicles and improve collision safety by increasing the strength of steel plates used. On the other hand, since the workability of steel sheets decreases with increasing strength, for example, as shown in Non-Patent Document 1, when a steel sheet with high hardenability is heated to the γ region and hot pressed. At the same time, attention has recently been focused on a technique for processing a high-strength part by cooling in a mold and quenching to generate a hard martensite phase. Further, in Patent Document 1, although it is a press work in a lower temperature range (specifically, a temperature range of 200 to 850 ° C.), Mo or the like was added to suppress dislocation loss after hot press work. A method of manufacturing a high-strength press-worked part using a steel plate is disclosed.

  However, the method described in Patent Document 1 is a technique for increasing the strength of a machined part by precipitation of dislocations and fine carbides introduced during machining, and the applied temperature during machining is below the transformation point, and the γ region Compared to the technique described in Non-Patent Document 1 that heats up to a low level, the degree of strength increase is small. In addition, in both the technique described in Non-Patent Document 1 and the method described in Patent Document 1, a scale is generated when the steel sheet is heated to the γ region. Therefore, when used as a component, a method such as shot blasting is used. The scale needs to be removed.

Thus, for example, Patent Document 2 discloses a technique for suppressing scale formation during heating by performing zinc-based alloy plating on a steel sheet, and Patent Document 3 by performing aluminum plating on a steel sheet.
Hirose et al., “New Processing Technology for Vehicle Weight Reduction”, Automotive Materials Symposium, Japan Iron and Steel Institute, March 22, 2006, p.31-38 JP 2000-234153 A JP 2003-73774 A JP 2005-139485 JP

  However, the techniques described in Patent Documents 2 and 3 have a problem that it is necessary to perform plating of zinc or aluminum on the steel sheet, resulting in an increase in manufacturing cost.

  The present invention is for hot pressing that can suppress the generation of scale when heated to the γ region without plating zinc or aluminum, or can easily peel the scale during hot pressing even if the scale is generated. An object is to provide a steel sheet.

  The inventors of the present invention have made extensive studies on the generation of scale during hot-pressing of the γ region and the peelability of the scale during hot pressing for hot-press steel sheets with high hardenability containing C and Mn. The following findings were obtained.

  1) When Si is added, scale formation can be greatly suppressed during heating in the γ region.

  2) By controlling the amount of P and reducing the roughness of the steel sheet surface, the peelability of the scale can be improved during hot pressing.

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.5%, Si: 0.5 to 3.0%, Mn: 0.5 to 5.0%, P: 0.02 to 0.5%, S: 0.03 % For hot pressing, containing Al: 2.0% or less, N: 0.01% or less, the balance being Fe and inevitable impurities, and the arithmetic mean roughness Ra of the steel sheet surface being 5 μm or less Provide steel sheet.

  In the hot-press steel sheet of the present invention, Al: 0.5% or more is preferable in order to suppress scale formation.

  Further, in order to improve hardenability, in addition to the above composition, further, in mass%, Cr: 0.05-2.0%, Ni: 0.05-2.0%, Mo: 0.05-2.0%, B: 0.0005-0.0030% It is preferable to contain at least one element selected from among them. Furthermore, in order to increase the strength, in addition to the above composition, at least one selected from Ti: 0.005-0.2%, Nb: 0.005-0.2%, V: 0.005-0.2% in addition to the above composition. These elements can also be contained.

  According to the present invention, scale formation can be suppressed even when heated to the γ region without plating with zinc or aluminum, and even when there is no need to remove scale by a method such as shot blasting or even when it is necessary to remove scale Thus, it is possible to manufacture a steel sheet for hot press work in which the generated scale can be easily peeled off during hot press work.

  Details of the present invention will be described below. The “%” below represents “% by mass” unless otherwise specified.

C: 0.05-0.5%
C is an element effective for increasing the strength because it enhances the hardenability and increases the strength of the martensite after quenching. In order to obtain such an effect, the C content needs to be 0.05% or more, preferably 0.15% or more. On the other hand, an increase in the amount of C causes a decrease in toughness and weldability after quenching, so the amount of C needs to be 0.5% or less, preferably 0.3% or less.

Si: 0.5-3.0%
Si is concentrated at the scale / base metal interface during high-temperature heating such as in the γ region, and scale formation can be greatly suppressed. In order to obtain such an effect, the Si amount needs to be 0.5% or more, preferably 1.0% or more. On the other hand, when a large amount of Si as a ferrite-forming element is added, the hardenability is hindered, which is not preferable for high strength, and cracks occur during hot rolling, so the Si amount is 3.0% or less. It is necessary to make it 2.0% or less.

Mn: 0.5-5.0%
Mn can increase the strength after hot pressing by increasing the hardenability. Moreover, the heating temperature before hot press processing can be reduced by lowering the temperature in the γ region. In order to obtain such an effect, the amount of Mn needs to be 0.5% or more, preferably 1.0%. On the other hand, the addition of a large amount of Mn not only saturates the effect, but also causes a decrease in weldability and an increase in rolling load, so the Mn amount needs to be 5.0% or less, preferably 3.0% or less. .

P: 0.02-0.5%
Since P is concentrated at the scale / base metal interface and improves the peelability of the scale during hot pressing, the amount of P needs to be 0.02% or more, preferably 0.05% or more. On the other hand, the addition of a large amount of P greatly reduces the toughness, so the amount of P needs to be 0.5% or less, preferably 0.1% or less.

S: 0.03% or less
Since S reduces the workability by forming coarse sulfides, the amount of S needs to be 0.03% or less, preferably 0.01% or less.

Al: 2.0% or less
Al is a ferrite-forming element and inhibits hardenability. Therefore, the Al content needs to be 2.0% or less, preferably 1.5% or less. On the other hand, Al can be concentrated at the scale / base metal interface during heating in the γ region, and scale formation can be suppressed, so the Al content is preferably 0.5% or more.

N: 0.01% or less
If N is contained in a large amount, it may cause slab cracking during hot rolling and may cause surface flaws, so the N amount needs to be 0.01% or less, preferably 0.005% or less. .

  The balance is Fe and inevitable impurities, but in order to further increase the strength, the following components can be contained in addition to the above components.

At least one element selected from Cr: 0.05-2.0%, Ni: 0.05-2.0%, Mo: 0.05-2.0%, B: 0.0005-0.0030%
Cr, Ni, Mo, and B can increase the strength after hot pressing by increasing the hardenability. In order to obtain such an effect, it is preferable to add at least one element selected from 0.05% or more of Cr, 0.05% or more of Ni, 0.05% or more of Mo, and 0.0005% or more of B. On the other hand, adding a large amount of Cr, Ni, Mo, B not only saturates the effect, but also causes an increase in rolling load, so Cr, Ni, Mo are each 2.0% or less, B is 0.0030% or less It is preferable to do.

At least one element selected from Ti: 0.005 to 0.2%, Nb: 0.005 to 0.2%, and V: 0.005 to 0.2% .Addition of Ti, Nb, and V to form carbonitride High strength can be achieved. In order to obtain such an effect, it is preferable to add 0.005% or more of at least one element selected from Ti, Nb, and V. On the other hand, addition of a large amount of Ti, Nb, and V not only saturates the effect but also increases the manufacturing cost. Therefore, Ti, Nb, and V are each preferably 0.2% or less.

  Moreover, in order to suppress the production | generation of a scale and to improve the peelability of the scale at the time of a hot press process, it is necessary to adjust the roughness of the steel plate surface as follows.

Arithmetic mean roughness Ra of steel plate surface: 5 μm or less By reducing the roughness of the steel plate surface, the unevenness of the scale / base metal interface can be reduced, suppressing the generation of scale and improving the scale peelability. be able to. In order to obtain such an effect, the arithmetic average roughness Ra of the steel sheet surface is preferably 5 μm or less, more preferably 2 μm or less, and still more preferably 0.2 μm or less. The arithmetic average roughness Ra of the steel sheet surface is a roughness determined according to JIS B 0601 (1994).

  In carrying out the invention, steel of chemical composition corresponding to the intended strength level is melted. As a melting method, a normal converter method, an electric furnace method, or the like can be appropriately applied. The molten steel is cast into a slab, then directly or after cooling and heating, hot rolling is performed, and the scale is removed by pickling. Thereafter, cold rolling or normal cold rolling with a relatively small rolling reduction such as skin pass rolling (also referred to as temper rolling) can be performed as necessary. In that case, the roughness of the steel plate surface can be adjusted using a bride roll or a dull roll. In addition, although the rolling reduction at the time of performing these cold rolling is not specifically limited, What is necessary is just to set it as the rolling reduction of about 0.3 to 80% normally performed. Here, it is preferable to perform cold rolling with a rolling reduction of about 0.3 to less than 10% by temper rolling equipment, and cold rolling with a rolling reduction of 10% or more with normal cold rolling equipment.

  After melting slabs consisting of steel Nos. 1 to 15 having the chemical composition shown in Table 1, heated at 1250 ° C for 1 hour, hot rolled at a finishing temperature of 900 ° C, wound at a winding temperature of 600 ° C and heated It was a sheet. Next, after pickling, the hot-rolled sheet is subjected to the conditions shown in Table 1, that is, as it is, or using a bright roll (B) or dull roll (D), and reduced by a temper rolling facility or a cold rolling facility. Cold rolling was performed at different rates to prepare a sample in which the arithmetic average roughness Ra of the steel sheet surface was adjusted.

  Then, the steel sheet surface was measured at five locations along the direction perpendicular to the rolling in accordance with JIS B 0601 (1994), and the three values excluding the maximum and minimum values were averaged to measure Ra.

In addition, after cutting a specimen of 350 x 80 mm from the sample and heating it in an atmosphere of 900 ° C, which is the γ region for 5 minutes, to generate a scale, punch width 99 mm, die width 103 mm, die depth (Compatible with vertical wall) Perform a single hat press at 95mm, cool to 100 ° C while pressing the die, simulate hot pressing, visually observe scale peeling at the bottom of the hat with small deformation, Evaluation was performed as follows. Here, if the score is 1 or 2, it can be evaluated that the scale peelability is excellent.
Score 1: 100% scale peeling
Score 2: Scale peeling of 99% or more Score 3: Scale peeling of 20% or more and less than 99% Score 4: Scale peeling of less than 20% In addition, a test piece was cut out from the bottom of the hat and cross-section observed using an optical microscope Then, the scale thickness and the cross-sectional hardness Hv of the steel sheet were measured using a Vickers hardness tester (load 4.9 N). In addition, it has been confirmed by a separate test that if the Vickers hardness Hv is 450 or more, it can be sufficiently applied as a structural part of an automobile.

  The results are shown in Table 1. Fig. 1 shows the effect of Si content on the thickness of the scale, and Fig. 2 shows the effect of Si and P content on the peelability of the scale during press working. FIG. 2 summarizes the case where Ra is 5 μm or less.

  As shown in FIG. 1, when the Si amount is 0.5% or more, which is the range of the present invention, the scale thickness is as very small as 5 μm or less, and it can be seen that the generation of scale is suppressed. In addition, as shown in FIG. 2, if the Si amount is 0.5% or more, which is the range of the present invention, and the scale thickness is thinned, and the P amount is 0.02% or more, which is the range of the present invention, the scale peelability score is high. It becomes 2 or less, and it can be seen that the scale peelability is excellent. Furthermore, as in steel No. 5 in Table 1, when Ra exceeds 5 μm with hot rolling, the scale peelability is poor. In addition, when the amount of C and Mn is lower than the range of the present invention as in steel No. 9 and 11, or when the amount of Al is higher than the range of the present invention, the hardness Hv is as small as 425 or less. It is an inappropriate steel plate.

  As described above, by making the chemical composition within the scope of the present invention, it is possible to suppress the generation of scale during heating in the γ region, promote scale peeling during hot pressing, and also have the hardness necessary for structural parts of automobiles. It can be secured.

It is a figure which shows the influence of Si amount which acts on the thickness of a scale. It is a figure which shows the influence of the amount of Si and P which has on the peelability of the scale at the time of press work.

Claims (4)

  In mass%, C: 0.05-0.5%, Si: 0.5-3.0%, Mn: 0.5-5.0%, P: 0.02-0.5%, S: 0.03% or less, Al: 2.0% or less, N: 0.01% or less A steel sheet for hot press working that has a composition comprising Fe and inevitable impurities in the balance, and an arithmetic average roughness Ra of the steel sheet surface of 5 μm or less.   2. The hot-press steel sheet according to claim 1, wherein Al: 0.5% or more.   In addition to the above composition, at least one element selected from Cr: 0.05-2.0%, Ni: 0.05-2.0%, Mo: 0.05-2.0%, B: 0.0005-0.0030% in mass% The steel plate for hot press processing according to claim 1 or 2, comprising:   In addition to the above composition, the composition further comprises at least one element selected from Ti: 0.005-0.2%, Nb: 0.005-0.2%, and V: 0.005-0.2% by mass%. 4. The hot-press steel sheet according to any one of 3 above.

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