Rashi #1: Iron from the north is harder than other iron. They (Resha'im of Yisrael) are compared to iron - "Holchei Rachil Nechoshes u'Varzel" (6:28). It says about Yirmeyah "I made you... an iron pillar and copper wall" (1:18). His [iron] is stronger than theirs, of Nebuchadnetzar will come from the north against them. Will their iron shatter yours?!

Rashi #2: Pharaoh Necho comes to help you. He is strong like iron. Will he shatter Nebuchadnetzar, who is iron from the north and copper?! Targum Yonasan is like this Perush.

Radak: "Ha'Yaro'a" is like "Tero'em b'Shevet Barzel" (Tehilim 2:9), i.e. breaking. Will iron break iron? [Yes,] if iron is mixed with copper; that is strong iron. I.e. they think that they can stand against Nebuchadnetzar. [They err]; even if they are iron, he is iron mixed with copper.... It says mi'Tzafon, for Bavel is to the north of Eretz Yisrael - "mi'Tzafon Tipasach ha'Ra'ah" (1:14).

Malbim: The previous two verses interrupted Yirmeyah's Nevu'ah. He asked Hashem (he thought that the decree includes him), and Hashem answered him (Yirmeyah's ed will be good). Now it returns to the Nevu'ah - "I will give their remnant to the sword" (verse 9). They relied on Egypt. Before the Churban, they thought that Pharaoh Necho will help them, but he did not help them. Also after the Churban they went to Egypt [to evade Nebuchadnetzar], and Nebuchadnetzar's sword caught them there. Pharaoh is regular iron. Will he shatter Nebuchadnetzar, who is iron from the north (a magnet, which always points to the north)? It rules over iron, and pulls it! With this iron is copper, i.e. help from the awesome nations that he rules over, which are strong like copper.